Forcefully engaging rods into tulips with gap discrepancy leading to pedicle screw loosening-a biomechanical analysis using long porcine spine segments

BACKGROUND CONTEXT

Long-segment pedicle screw instrumentation is widely used to treat complex spinal disorders. Rods are routinely precontoured to maximize assistance on the correcting side of the deformity, but there often exists a residual gap discrepancy between the precontoured rods and screw tulips. No previous research has investigated the diminished pullout strength of the most proximal or distal pedicle screw resulting from a mismatched rod in long-segment pedicle screw instrumentation.

PURPOSE

The present study aimed to investigate the decreased pullout force of pedicle screws affected by the gap discrepancy when forcefully engaging a mismatched rod into a tulip in a normal-density porcine spine.

STUDY DESIGN

The pedicle screw fixation strength under axial pullout force was compared among three different gap discrepancies between rods and tulips using long porcine spine segments.

METHODS

Twelve porcine lumbar vertebrae (L3-L6) were implanted with pedicle screws and rods. Screws on one side had no gap between the tulip and rod (0-mm group), while the most proximal screw on the other side had an intentional gap of 3 mm (3-mm group) or 6 mm (6-mm group). Three hours after forcefully engaging the rod into the tulips at room temperature, the set screws in all specimens were loosened, and each specimen was dissected into individual vertebrae for subsequent pullout testing.

RESULTS

The control group exhibited significantly greater pullout strength (1987.68 ± 126.80 N) than the groups from different rod-tulip configurations (p<.05), with significantly greater strength in the 3-mm group (945.62 ± 97.43 N) than the 6-mm group (655.30 ± 194.49 N) (p<.05). Only 47.6% and 33.0% of the pullout strength was retained in the 3-mm and 6-mm groups, respectively, compared to the control group.

CONCLUSIONS

Gap discrepancies between rods and tulips can significantly reduce pedicle screw pullout strength, with a correlation between decreased strength and increased gaps. Surgeons should avoid forcefully engaging mismatched rods and consider well-fitted contoured rods in spinal surgery to minimize the risk of screw loosening.

CLINICAL SIGNIFICANCE

The gap discrepancy between rod and tulip significantly affected pullout strength, with greater gaps leading to reduced strength. Forcefully engaging mismatched rods into tulips in degenerative spinal surgery should be avoided to minimize the risk of early screw pullout.

Influence of various pilot hole profiles on pedicle screw fixation strength in minimally invasive and traditional spinal surgery: a comparative biomechanical study

Despite advancements in pedicle screw design and surgical techniques, the standard steps for inserting pedicle screws still need to follow a set of fixed procedures. The first step, known as establishing a pilot hole, also referred to as a pre-drilled hole, is crucial for ensuring screw insertion accuracy. In different surgical approaches, such as minimally invasive or traditional surgery, the method of creating pilot holes varies, resulting in different pilot hole profiles, including variations in size and shape. The aim of this study is to evaluate the biomechanical properties of different pilot hole profiles corresponding to various surgical approaches. Commercially available synthetic L4 vertebrae with a density of 0.16 g/cc were utilized as substitutes for human bone. Four different pilot hole profiles were created using a 3.0 mm cylindrical bone biopsy needle, 3.6 mm cylindrical drill, 3.2-5.0 mm conical drill, and 3.2-5.0 mm conical curette for simulating various minimally invasive and traditional spinal surgeries. Two frequently employed screw shapes, namely, cylindrical and conical, were selected. Following specimen preparation, screw pullout tests were performed using a material test machine, and statistical analysis was applied to compare the mean maximal pullout strength of each configuration. Conical and cylindrical screws in these four pilot hole configurations showed similar trends, with the mean maximal pullout strength ranking from high to low as follows: 3.0 mm cylindrical biopsy needle, 3.6 mm cylindrical drill bit, 3.2-5.0 mm conical curette, and 3.2-5.0 mm conical drill bit. Conical screws generally exhibited a greater mean maximal pullout strength than cylindrical screws in three of the four different pilot hole configurations. In the groups with conical pilot holes, created with a 3.2-5.0 mm drill bit and 3.2-5.0 mm curette, both conical screws exhibited a greater mean maximal pullout strength than did cylindrical screws. The strength of this study lies in its comprehensive comparison of the impact of various pilot hole profiles commonly used in clinical procedures on screw fixation stability, a topic rarely reported in the literature. Our results demonstrated that pilot holes created for minimally invasive surgery using image-guided techniques exhibit superior pullout strength compared to those utilized in traditional surgery. Therefore, we recommend prioritizing minimally invasive surgery when screw implantation is anticipated to be difficult or there is a specific need for stronger screw fixation. When opting for traditional surgery, image-guided methods may help establish smaller pilot holes and increase screw fixation strength.

Finite element analysis of optimized novel additively manufactured non-articulating prostheses for cervical total disc replacement

Ball-and-socket designs of cervical total disc replacement (TDR) have been popular in recent years despite the disadvantages of polyethylene wear, heterotrophic ossification, increased facet contact force, and implant subsidence. In this study, a non-articulating, additively manufactured hybrid TDR with an ultra-high molecular weight polyethylene core and polycarbonate urethane (PCU) fiber jacket, was designed to mimic the motion of normal discs. A finite element (FE) study was conducted to optimize the lattice structure and assess the biomechanical performance of this new generation TDR with an intact disc and a commercial ball-and-socket Baguera^®C TDR (Spineart SA, Geneva, Switzerland) on an intact C5-6 cervical spinal model. The lattice structure of the PCU fiber was constructed using the Tesseract or the Cross structures from the IntraLattice model in the Rhino software (McNeel North America, Seattle, WA) to create the hybrid I and hybrid II groups, respectively. The circumferential area of the PCU fiber was divided into three regions (anterior, lateral and posterior), and the cellular structures were adjusted. Optimal cellular distributions and structures were A2L5P2 in the hybrid I and A2L7P3 in the hybrid II groups. All but one of the maximum von Mises stresses were within the yield strength of the PCU material. The range of motions, facet joint stress, C6 vertebral superior endplate stress and path of instantaneous center of rotation of the hybrid I and II groups were closer to those of the intact group than those of the Baguera^®C group under 100 N follower load and pure moment of 1.5 Nm in four different planar motions. Restoration of normal cervical spinal kinematics and prevention of implant subsidence could be observed from the FE analysis results. Superior stress distribution in the PCU fiber and core in the hybrid II group revealed that the Cross lattice structure of a PCU fiber jacket could be a choice for a next-generation TDR. This promising outcome suggests the feasibility of implanting an additively manufactured multi-material artificial disc that allows for better physiological motion than the current ball-and-socket design.

Biomechanical evaluation of pedicle screw stability after 360-degree turnback from full insertion: effects of screw shape, pilot hole profile and bone density

Intraoperative pedicle screw depth adjustment after initial insertion, including both forward and backward adjustments, is sometimes necessary to facilitate rod application and ensure that the screw is in the correct position, which is determined by intraoperative fluoroscopy. Adjusting the screw with forward turns has no negative influence on the screw fixation stability; however, screw turnback may weaken the fixation stability. The aim of this study is to evaluate the biomechanical properties of screw turnback and demonstrate the reduction in the fixation stability after the screw is turned 360° from its full insertion position. Commercially available synthetic closed-cell polyurethane foams with three different densities simulating various degrees of bone density were utilized as substitutes for human bone. Two different screw shapes (cylindrical and conical) together with two different pilot hole profiles (cylindrical and conical) were tested. Following specimen preparation, screw pullout tests were conducted using a material test machine. The mean maximal pullout strength between full insertion and 360-degree turnback from full insertion in each setting was statistically analyzed. The mean maximal pullout strength after 360-degree turnback from full insertion was generally lower than that at full insertion. The reduced mean maximal pullout strength after turnback increased with decreasing bone density. Conical screws had significantly lower pullout strength after 360-degree turnback than cylindrical screws. The mean maximal pullout strength was reduced by up to approximately 27% after 360-degree turnback when using a conical screw in a low bone density specimen. Additionally, specimens treated with a conical pilot hole presented a less reduction in pullout strength after screw turnback as compared to those with a cylindrical pilot hole. The strength of our study was that we systematically investigated the effects of various bone densities and screw shapes on screw stability after turnback, which has rarely been reported in the literature. Our study suggests that pedicle screw turnback after full insertion should be reduced in spinal surgeries, particularly procedures that use conical screws in osteoporotic bone. Pedicle screw secured with a conical pilot hole might be beneficial for screw adjustment.

The role of counter-torque holders in tightening of pedicle screw-rod constructs: a biomechanical study in a porcine model

BACKGROUND CONTEXT

Pedicle screw-rod assembly procedures following pedicle screw insertion include contouring and placing rods into screw tulips, introducing set screws into the tulip along the screw thread, applying a counter-torque holder and tightening all the set screws clockwise. Even if an appropriate pedicle screw is implanted, screw dislodgement after tightening of the tulip and set screw is not uncommon. Pedicle wall violation resulting from excessive rotational force due to inadequate use of a counter-torque holder might be the reason. However, the strain change in the pedicle during tulip-set screw tightening and the role of counter-torque have never been investigated.

PURPOSE

This study determined differences in the strain change in the outer and inner pedicle walls during tulip-set screw tightening; additionally, the influence of counter-torque on pedicle wall violation was elucidated.

STUDY DESIGN

A controlled biomechanical study; the strain values of outer and inner pedicle walls in cadaveric porcine L4-L5 vertebrae during tulip-set screw tightening with or without a counter-torque holder were measured.

METHODS

Twelve L4-L5 fresh-frozen porcine lumbar vertebrae were implanted with screw-rod constructs; the set screw was randomly locked into the tulip in the right L5, right L4, left L5 and left L4 testing groups. The maximal values from eight strain gauges (P-R-O: outer cortex of right pedicle in proximal vertebra; P-R-I: inner cortex of right pedicle in proximal vertebra; D-R-O: outer cortex of right pedicle in distal vertebra; D-R-I: inner cortex of right pedicle in distal vertebra; P-L-O: outer cortex of left pedicle in proximal vertebra; P-L-I: inner cortex of left pedicle in proximal vertebra; D-L-O: outer cortex of left pedicle in distal vertebra; D-L-I: outer cortex of left pedicle in proximal vertebra) for each specimen during tightening to 12 Nm were measured.

RESULTS

The maximal strain values of the ipsilateral strain gauges in all testing groups were almost significantly higher when a counter-torque holder was not used than when one was used. The strain values in the adjacent pedicle of specimens without a counter-torque holder were significantly increased: P-R-O and P-R-I in the right L5 group; D-R-I in the right L4 group; P-L-I and P-L-O in the left L5 group; D-L-O and D-L-I in the left L4 group.

CONCLUSIONS

The constraint effect of counter-torque during tulip-set screw tightening is necessary. Clockwise rotational force with a fragile lateral pedicle wall suggests that caution is required when using a counter-torque holder to tighten the right L5 and left L4 constructs.

CLINICAL SIGNIFICANCE

A counter-torque holder is important during tulip-set screw tightening; improper use may lead to adjacent pedicle wall violation, sequentially resulting in pedicle screw loosening.

Biomechanical Comparison of Different Numbers and Configurations of Cross-Links in Long-Segment Spinal Fixation-An Experimental Study in a Porcine Model

STUDY DESIGN

Biomechanical study.

OBJECTIVE

Cross-links are a type of common clinical spinal instrumentation. However, the effects of the position and number of cross-links have never been investigated in long-segment spinal fixation, and the variables have not been optimized. We conducted an in vitro biomechanical study by using a porcine long-segment spinal model with 5 different crosslink configurations to determine the optimal construct for clinical practice.

METHODS

Five modalities with paired segmental screws from T15-L5 were tested in 20 porcine spines. The spines without cross-links composed the control group, Group A; those with a single cross-link from L2-3 composed Group B; those with 2 cross-links from L1-2 and L3-4 composed Group C; those with 2 cross-links from T15-L1 and L4-5 composed Group D; and those with 3 cross-links from T15-L1, L2-3 and L4-5 composed Group E. Spinal stiffnesses in flexion, extension, lateral bending, and axial rotation were compared among 5 different cross-link configurations in 5-level porcine spinal units.

RESULTS

Flexional, extensional and lateral bending stiffnesses did not significantly change with an increasing number of cross-links or positions in the construct. Axial stiffness was significantly increased with 2 cross-links compared to one (P < 0.05) and with placement more distant from the center of the long spinal fixation construct (P < 0.05).

CONCLUSIONS

Two cross-links individually placed proximal and distal from the center of a construct is an optimal and efficient configuration to achieve biomechanical stability in non-rigid lumbar spines undergoing long-level fixation.

Biomechanical comparison of pedicle screw fixation strength among three different screw trajectories using single vertebrae and one-level functional spinal unit

Three key factors are responsible for the biomechanical performance of pedicle screw fixation: screw mechanical characteristics, bone quality and insertion techniques. To the best of the authors' knowledge, no study has directly compared the biomechanical performance among three trajectories, i.e., the traditional trajectory (TT), modified trajectory (MT) and cortical bone trajectory (CBT), in a porcine model. This study compared the pullout strength and insertion torque of three trajectory methods in single vertebrae, the pullout strength and fixation stiffness including flexion, extension, and lateral bending in a one-level instrumented functional spinal unit (FSU) that mimics the in vivo configuration were clarified. A total of 18 single vertebrae and 18 FSUs were randomly assigned into three screw insertion methods (n = 6 in each trajectory group). In the TT group, the screw converged from its entry point, passed completely inside the pedicle, was parallel to the superior endplate, was located in the superior third of the vertebral body and reached to at least the anterior third of the vertebral body. In the MT group, the convergent angle was similar to that of the TT method but directed caudally to the anterior inferior margin of the vertebral body. The results of insertion torque and pullout strength in single vertebrae were analyzed; in addition, the stiffness and pullout strength in the one-level FSU were also investigated. This study demonstrated that, in single vertebrae, the insertion torque was significantly higher in CBT groups than in TT and MT groups (p < 0.05). The maximal pullout strength was significantly higher in MT groups than in TT and CBT groups (p < 0.05). There was no significant difference in stiffness in the three motions among all groups. The maximal pullout strength in FSUs of MT and CBT groups were significantly higher than the TT groups (p < 0.05). We concluded that either MT or CBT provides better biomechanical performance than TT in single vertebrae or FSUs. The lack of significance of stiffness in FSUs among three methods suggested that MT or CBT could be a reasonable alternative to TT if the traditional trajectory was not feasible.

Quantitative imaging of ultrasound backscattered signals with information entropy for bone microstructure characterization

Osteoporosis is a critical problem during aging. Ultrasound signals backscattered from bone contain information associated with microstructures. This study proposed using entropy imaging to collect the information in bone microstructures as a possible solution for ultrasound bone tissue characterization. Bone phantoms with different pounds per cubic foot (PCF) were used for ultrasound scanning by using single-element transducers of 1 (nonfocused) and 3.5 MHz (nonfocused and focused). Clinical measurements were also performed on lumbar vertebrae (L3 spinal segment) in participants with different ages (n = 34) and postmenopausal women with low or moderate-to-high risk of osteoporosis (n = 50; identified using the Osteoporosis Self-Assessment Tool for Taiwan). The signals backscattered from the bone phantoms and subjects were acquired for ultrasound entropy imaging by using sliding window processing. The independent t-test, one-way analysis of variance, Spearman correlation coefficient r_s, and the receiver operating characteristic (ROC) curve were used for statistical analysis. The results indicated that ultrasound entropy imaging revealed changes in bone microstructures. Using the 3.5-MHz focused ultrasound, small-window entropy imaging (side length: one pulse length of the transducer) was found to have high performance and sensitivity in detecting variation among the PCFs (r_s = − 0.83; p < 0.05). Small-window entropy imaging also performed well in discriminating young and old participants (p < 0.05) and postmenopausal women with low versus moderate-to-high osteoporosis risk (the area under the ROC curve = 0.80; cut-off value = 2.65; accuracy = 86.00%; sensitivity = 71.43%; specificity = 88.37%). Ultrasound small-window entropy imaging has great potential in bone tissue characterization and osteoporosis assessment.

Biomechanical comparison of pedicle screw fixation strength in synthetic bones: Effects of screw shape, core/thread profile and cement augmentation

Pedicle screw loosening resulting from insufficient bone-screw interfacial holding power is not uncommon. The screw shape and thread profile are considered important factors of the screw fixation strength. This work investigated the difference in pullout strength between conical and cylindrical screws with three different thread designs. The effects of the thread profiles on the screw fixation strength of cannulated screws with or without cement augmentation in osteoporotic bone were also evaluated. Commercially available artificial standard L4 vertebrae and low-density polyurethane foam blocks were used as substitutes for healthy vertebrae and osteoporotic bones, respectively. The screw pullout strengths of nine screw systems were investigated (six in each). These systems included the combination of three different screw shapes (solid/cylindrical, solid/conical and cannulated/cylindrical) with three different thread profiles (fine-thread, coarse-thread and dual-core/dual-thread). Solid screws were designed for the cementless screw fixation of vertebrae using the standard samples, whereas cannulated screws were designed for the cemented screw fixation of osteoporotic bone using low-density test blocks. Following specimen preparation, a screw pullout test was conducted using a material test machine, and the maximal screw pullout strength was compared among the groups. This study demonstrated that, in healthy vertebrae, both the conical and dual-core/dual-thread designs can improve pullout strength. A combination of the conical and dual-core/dual-thread designs may achieve optimal postoperative screw stability. However, in osteoporotic bone, the thread profile have little impact on the screw fixation strength when pedicle screws are fixed with cement augmentation. Cement augmentation is the most important factor contributing to screw pullout fixation strength as compared to screw designs.

Biomechanical study of the fixation stability of broken pedicle screws and subsequent strategies

Pedicles are often broken when screws are inserted into hard pedicles with small diameters or when the diameter of the screw itself is inadequate. However, there is a lack of biomechanical literature that addresses screw loosening as a result of broken pedicles or the resulting salvage of those screws. We performed a novel in vitro study to compare the pullout strength of screws between intact pedicles and two different types of broken pedicles; strategies to prevent screw loosening were also compared. Thirty L4 Sawbones were designed to represent intact pedicles, semi-pedicles, and nonpedicles and were prepared for screw insertion. Three sizes of polyaxial screws (diameter × length dimensions of 6.0 mm × 45 mm, 6.0 mm × 50 mm and 6.5 mm × 45 mm) were independently and randomly distributed into the intact-pedicle group (IP group, n = 30), the semi-pedicle group (SP group, n = 15), or the nonpedicle group (NP group, n = 15). The experiments were conducted across a minimum of five trials for each of the chosen screw sizes. We then analyzed the results of the imaging, pullout testing, and embedded bone volume. Any fractures or defects of the vertebrae caused by screw insertion were excluded from the study. The appropriate screw trajectory and insertional depth were confirmed using axial and sagittal X-ray imaging prior to screw pullout testing. A pullout strength of only 41% to 45% for the SP group and 29% to 39% for the NP group was retained following a broken pedicle. The use of longer or larger-diameter screws appears to be an effective salvaging procedure for the NP group (p < 0.05). The embedded bone volume percentage analysis indicated that, compared to the IP group, 68% to 76% of effective bone embedded into the screw threads in the SP group, and 58% to 65% in the NP group. There was no direct correlation between the pullout strength and the embedded bone volume; however, less effective embedded bone volume was associated with lower pullout strength. This study describes the evolution of the well-established screw pullout test being applied to the broken pedicle Sawbone testing model. The pedicle plays an important role in determining the pullout strength of a screw. However, a salvage procedure that utilizes a longer or larger-diameter screw might be a reliable clinical approach to address broken pedicles.

BIOMECHANICAL EVALUATION OF LOW-MODULUS BONE CEMENT FOR ENHANCING APPLICABILITY IN VERTEBROPLASTY — AN EXPERIMENTAL STUDY IN PORCINE MODEL

Polymethylmethacrylate (PMMA) bone cement has been widely used in vertebroplasty to treat osteoporotic vertebral compression fracture. However, the high compression stiffness of PMMA is suspected to induce adjacent vertebral fracture following vertebroplasty. In the current study, modified low-modulus cement was prepared by combining PMMA with castor oil to solve this problem. The percentage of height recovery and compression stiffness of vertebral bodies was compared after injection of standard PMMA or low-modulus cement. This study aims to investigate whether low-modulus cement is as effective as standard PMMA for storing the initial vertebral height; while lowering the compression stiffness in treatment of osteoporotic vertebral compression fractures. A total of 20 fresh porcine lumbar vertebrae were assigned into two groups (10 per group): standard and low-modulus. All specimens received a four-week decalcification to mimic human osteoporotic vertebrae. The standard and low-modulus groups received a simulated compression fracture followed by treatment of standard and low-modulus cement augmentation, respectively. The low-modulus cement was prepared by combining standard PMMA with 15% weight fractions of castor oil. For all the 20 specimens, vertebral compression fracture was created by reducing the vertebral height of 25% using a material testing machine. The compression stiffness determined from the creation of compression fracture was defined as the intact group (20 specimens). The fractured vertebrae were then treated with standard and low-modulus cement augmentation. The vertebral height was measured pre- and post-treatment, and the percentage of vertebral height recovery was compared between two cementing groups. Following cement augmentation, axial compression test was conducted to compare compression stiffness among three groups. The results indicated that there is no significant difference in percentage of vertebral height between standard (83.42[Formula: see text][Formula: see text][Formula: see text]11.60%) and low-modulus (88.50[Formula: see text][Formula: see text][Formula: see text]6.15%) groups ([Formula: see text]). Moreover, the compression stiffnesses were 1166.49[Formula: see text][Formula: see text][Formula: see text]392.91 N/mm, 1795.85[Formula: see text][Formula: see text][Formula: see text]247.45[Formula: see text]N/mm and 1362.57[Formula: see text][Formula: see text][Formula: see text]236.92[Formula: see text]N/mm for intact, standard and low-modulus groups, respectively. There is significant difference among three groups ([Formula: see text]). We concluded that the modified low-modulus cement is as effective as standard PMMA for storing the initial vertebral height while lowering the compression stiffness in treatment of osteoporotic vertebral compression fractures. These reduce the risks of adjacent vertebral body fracture following vertebroplasty.

Application of two-parameter scoliometer values for predicting scoliotic Cobb angle

BACKGROUND

Adolescent idiopathic scoliosis, in which obvious curves are visible in radiographic images, is also seen in combination with lumps in the back. These lumps contribute to inclination, which can be measured by a scoliometer. To the authors' knowledge, there are no previous formulas combining thoracic and lumbar scoliometer values simultaneously to predict thoracic and lumbar Cobb angles, respectively. This study aimed to create more accurate two-parameter mathematical formulas for predicting thoracic and lumbar Cobb angles.

METHODS

Between Dec. 2012 and Jan. 2013, patients diagnosed with idiopathic scoliosis in an outpatient clinic were enrolled. The maximal trunk rotations at the thoracic and lumbar regions were recorded with a scoliometer. Right asymmetry hump was deemed positive (+), and left asymmetry hump was deemed negative (-). The Cobb angles were measured with a Picture Archiving and Communication System. Statistical analysis included Pearson's correlation coefficient, multivariate regression and Bland-Atman analysis.

RESULTS

One-hundred and one patients were enrolled in our study. The average thoracic curve (TC) was 23.3 ± 1.8°, while the average lumbar curve (LC) was - 23.3 ± 1.4°. The thoracic inclination (TI) and lumbar inclination (LI) were 4.5 ± 0.7 and - 5.9 ± 0.6, respectively. The one-parameter formula for the thoracic curve was TC = 2.0 TI + 14.3 (r = 0.813); for the lumbar curve, it was LC = 0.9 LI - 16.9 (r = 0.409). By multivariate regression, the two-parameter formulas for the thoracic and lumbar curves were TC = 2.6 TI - 1.4 LI (r = 0.931) and LC = - 1.5 TI + 2.0 LI (r = 0.874), respectively. The two-parameter formulas were more accurate than the one-parameter formulas.

CONCLUSIONS

Based on the results of these two-parameter formulas for thoracic and lumbar curves, the Cobb angles can be predicted more accurately by the readings of the scoliometer. Physicians and other healthcare practitioners can thus evaluate patients with scoliosis more precisely than before with a scoliometer.

Characterization of a novel caudal vertebral interbody fusion in a rat tail model: An implication for future material and mechanical testing

Background

Of the proposed animal interbody fusion models, rat caudal discs have gained popularity in disc research due to their strong resemblance to human discs with respect to geometry, composition and mechanical properties. The purpose of this study is to demonstrate an efficient, repeatable and easily accessible animal model of interbody fusion for future research into mechanical testing and graft materials.

Methods

Twelve 12-week-old female Sprague–Dawley (SD) rats underwent caudal interbody fusion of the third and fourth coccygeal vertebrae of the tail. Serial radiological evaluation, and histological evaluation and manual palpation after sacrifice were performed to assess the fusion quality. Mechanical testing of functional units (FUs) of non-operated and operated segments was compared using a three-point bending test.

Results

At postoperative 12 weeks, callus formation was observed at the fusion sites in all rats, with the mean radiological evaluations of 2.75/3 according to the Bransford classification. Newly formed bone tissue was also observed in all rats with the mean histological score of 5.85/7, according to the Emery grading system. No palpable gaps and obvious change of bending stiffness was observed in the operated segments. The mean bending stiffness of the FUs was statistically higher than that of the control FUs (26.57 ± 6.71 N/mm vs. 12.45 ± 3.21 N/mm, p < 0.01).

Conclusion

The rat caudal disc interbody fusion model proved to be an efficient, repeatable and easily accessible model. Future research into adjuvant treatments like growth factor injection and alternative fusion materials under conditions of osteoporosis using this model would be worthwhile.

Effects of Strontium Ranelate on Spinal Interbody Fusion Surgery in an Osteoporotic Rat Model

Osteoporosis is a bone disease that afflicts millions of people around the world, and a variety of spinal integrity issues, such as degenerative spinal stenosis and spondylolisthesis, are frequently concomitant with osteoporosis and are sometimes treated with spinal interbody fusion surgery. Previous studies have demonstrated the efficacy of strontium ranelate (SrR) treatment of osteoporosis in improving bone strength, promoting bone remodeling, and reducing the risk of fractures, but its effects on interbody fusion surgery have not been adequately investigated. SrR-treated rats subjected to interbody fusion surgery exhibited significantly higher lumbar vertebral bone mineral density after 12 weeks of treatment than rats subjected to the same surgery but not treated with SrR. Furthermore, histological and radiographic assessments showed that a greater amount of newly formed bone tissue was present and that better fusion union occurred in the SrR-treated rats than in the untreated rats. Taken together, these results show significant differences in bone mineral density, PINP level, histological score, SrR content and mechanical testing, which demonstrate a relatively moderate effect of SrR treatment on bone strength and remodeling in the specific context of recovery after an interbody fusion surgery, and suggest the potential of SrR treatment as an effective adjunct to spinal interbody fusion surgery for human patients.

Elution and Mechanical Strength of Vancomycin-Loaded Bone Cement: In Vitro Study of the Influence of Brand Combination

Antibiotic-loaded bone cement (ALBC) is widely used in orthopaedic surgery for both prevention and treatment of infection. Little is known about the effect of different brand combinations of antibiotic and bone cement on the elution profile and mechanical strength of ALBC. Standardized specimens that consisted of one of the 4 brands of bone cement and one of the 3 brands of vancomycin were fashioned, producing 12 combinations of ALBC. Two dosages of vancomycin in 40g bone cement were used to represent the high (4g vancomycin) and low (1g vancomycin) dose groups. Concentrations of vancomycin elution from ALBC was measured for up to 336 hours. The ultimate compression strength was tested at axial compression using a material testing machine before and after elution. In both high-dose and low-dose groups, Lyo-Vancin in PALACOS bone cement resulted in the highest cumulative elution and Vanco in Simplex P bone cement resulted in the lowest elution (458% and 65% higher in high- and low-dose groups, respectively). The mechanical strength was not significantly compromised in all groups with low dose vancomycin (range: 70.31 ± 2.74 MPa to 87.28 ± 8.26MPa after elution). However, with the addition of high dose vancomycin, there was a mixed amount of reduction in the ultimate compression strength after cement aging, ranging from 5% (Vanco in Simplex P, 81.10 ± 0.48 MPa after elution) to 38% (Sterile vancomycin in CMW, 60.94 ± 5.74 MPa after elution). We concluded that the selection of brands of vancomycin and bone cement has a great impact on the release efficacy and mechanical strength of ALBC.

Modified fixations for distal femur fractures following total knee arthroplasty: a biomechanical and clinical relevance study

PURPOSE

Distal femur fractures adjacent to total knee arthroplasty are a rare yet complex problem. Recently, extramedullary locking plate and retrograde intramedullary nail fixations have become popular options, but the complication rates associated with these procedures are 15-20 %. Modified fixations were assessed in an effort to reduce complications from unstable periprosthetic fractures.

METHODS

Using experimental and finite element methods, this study compared the construct behaviours of a locking plate, a retrograde intramedullary nail, and their modifications (a spiral-blade supplemented in an intramedullary nail or a locking plate/allograft hybrid) when subjected to various fracture types, locations, loading conditions, and bony strength. The implanted models were used to assess construct stiffness, fracture micromotion, and implant stress under different osteoporotic conditions. Finally, we collected 40 cases for radiological analysis to indicate the appropriate procedure for treating periprosthetic fractures following total knee arthroplasty.

RESULTS

Regardless of the fracture type, femoral constructs fixed with a conventional or spiral-blade supplemented intramedullary nail exhibited higher axial but lower torsional stiffness than those fixed with a locking plate. Torsional deformation occurred if the lower-positioned fracture had no medial support. The locking plate/allograft construct exhibited the highest stiffness and the least micromotion. A review of 40 clinical cases confirmed the above findings regarding the locking plate/allograft construct.

CONCLUSION

The spiral-blade supplement of retrograde intramedullary nail and locking plate/allograft modified constructs significantly stabilizes the unstable fractured gaps. The locking plate/allograft is recommended for the periprosthetic fractures with deficient bone stock and severe osteoporosis to improve alignment and healing potentials.

A Biomechanical Comparison of Expansive Pedicle Screws for Severe Osteoporosis: The Effects of Screw Design and Cement Augmentation

Expansive pedicle screws significantly improve fixation strength in osteoporotic spines. However, the previous literature does not adequately address the effects of the number of lengthwise slits and the extent of screw expansion on the strength of the bone/screw interface when expansive screws are used with or without cement augmentation. Herein, four designs for expansive pedicle screws with different numbers of lengthwise slits and different screw expansion levels were evaluated. Synthetic bones simulating severe osteoporosis were used to provide a comparative platform for each screw design. The prepared specimens were then tested for axial pullout failure. Regardless of screw design, screws with cement augmentation demonstrated significantly higher pullout strength than pedicle screws without cement augmentation (p < 0.001). For screws without cement augmentation, solid screws exhibited the lowest pullout strength compared to the four expansive groups (p < 0.01). No significant differences in pullout strength were observed between the expansive screws with different designs (p > 0.05). Taken together, our results show that pedicle screws combined with cement augmentation may greatly increase screw fixation regardless of screws with or without expansion. An increase in both the number of slits and the extent of screw expansion had little impact on the screw-anchoring strength. Cement augmentation is the most influential factor for improving screw pullout strength.

Preliminary biomechanical study of different acetabular reinforcement devices for acetabular reconstruction

Background

Acetabular reinforcement devices (ARDs) are frequently used as load-sharing devices to allow allograft incorporation in revision hip arthroplasty with massive acetabular bone loss. The key to a successful reconstruction is robust fixation of the device to the host acetabulum. Interlocking fixation is expected to improve the initial stability of the postoperative construct. However, all commercially available ARDs are designed with non-locking fixation. This study investigates the efficacy of standard ARDs modified with locking screw mechanisms for improving stability in acetabular reconstruction.

Methods

Three types of ARDs were examined to evaluate the postoperative compression and angular stability: i) standard commercial ARDs, ii) standard ARDs modified with monoaxial and iii) standard ARDs modified with polyaxial locking screw mechanisms. All ARDs were implanted into osteomized synthetic pelvis with pelvic discontinuity. Axial compression and torsion tests were then performed using a servohydraulic material testing machine that measured load (angle) versus displacement (torque). Initial stability was compared among the groups.

Results

Equipping ARDs with interlocking mechanisms effectively improved the initial stability at the device/bone interface compared to standard non-locked ARDs. In both compression and torsion experiments, the monoaxial interlocking construct demonstrated the highest construct stiffness (672.6 ± 84.1 N/mm in compression and 13.3 ± 1.0 N·m/degree in torsion), whereas the non-locked construct had the lowest construct stiffness (381.4 ± 117.2 N/mm in compression and 6.9 ± 2.1 N·m/degree in torsion) (P < 0.05).

Conclusions

Our study demonstrates the potential benefit of adding a locking mechanism to an ARD. Polyaxial ARDs provide the surgeon with more flexibility in placing the screws at the cost of reduced mechanical performance. This in vitro study provides a preliminary evaluation of biomechanical performance for ARDs with or without interlocking mechanisms, actual clinical trial deserves to be further investigated in future studies.

Anterior transfer of tibialis posterior tendon for treating drop foot: Technique of enforcing tendon implantation to improve success rate

An absolutely convincing technique of anterior transfer of the tibialis posterior (TP) tendon for treating drop foot has not been developed. Thirty-seven consecutive adult patients with drop foot owing to deep peroneal nerve injury were treated with bone-to-bone TP tendon transfer. The TP tendon with a small bony attachment was procured from the undersurface of the navicula and then transferred through a tunnel of the interosseous membrane. The navicular attachment was implanted in the tunnel of the navicula or intermediate cuneiform. Cancellous bone graft procured from the distal tibial metaphysis was packed into the tunnel inlet. Side-to-side tendon suturing was performed between the TP tendon and tibialis anterior tendon. Thirty-one patients were followed for a mean of 2.8 years (range, 1.2-4.8 years), and all achieved satisfactory outcome for the ankle. All patients achieved a normal gait after one year and at the latest follow-up.

CONCLUSIONS

The described technique may provide a high success rate. This surgical technique is not complex, and complications are few.

Liquid antibiotics in bone cement: an effective way to improve the efficiency of antibiotic release in antibiotic loaded bone cement

Objectives

The objective of this study was to compare the elution characteristics, antimicrobial activity and mechanical properties of antibiotic-loaded bone cement (ALBC) loaded with powdered antibiotic, powdered antibiotic with inert filler (xylitol), or liquid antibiotic, particularly focusing on vancomycin and amphotericin B.

Methods

Cement specimens loaded with 2 g of vancomycin or amphotericin B powder (powder group), 2 g of antibiotic powder and 2 g of xylitol (xylitol group) or 12 ml of antibiotic solution containing 2 g of antibiotic (liquid group) were tested.

Results

Vancomycin elution was enhanced by 234% in the liquid group and by 12% in the xylitol group compared with the powder group. Amphotericin B elution was enhanced by 265% in the liquid group and by 65% in the xylitol group compared with the powder group. Based on the disk-diffusion assay, the eluate samples of vancomycin-loaded ALBC of the liquid group exhibited a significantly larger inhibitory zone than samples of the powder or the xylitol group. Regarding the ALBCs loaded with amphotericin B, only the eluate samples of the liquid group exhibited a clear inhibitory zone, which was not observed in either the xylitol or the powder groups. The ultimate compressive strength was significantly reduced in specimens containing liquid antibiotics.

Conclusions

Adding vancomycin or amphotericin B antibiotic powder in distilled water before mixing with bone cement can significantly improve the efficiency of antibiotic release than can loading ALBC with the same dose of antibiotic powder. This simple and effective method for preparation of ALBCs can significantly improve the efficiency of antibiotic release in ALBCs.

Cite this article: Bone Joint Res 2014;3:246–51.

The effects of necrotic lesion size and orientation of the femoral component on stress alterations in the proximal femur in hip resurfacing - a finite element simulation

Background

Due to the advantages of its bone-conserving nature, hip resurface arthroplasty (HRA) has recently gained the interest of orthopedic surgeons for the treatment of young and active patients who have osteonerosis of the femoral head. However, in long-term follow-up studies after HRA, narrowing of the femoral neck has often been found, which may lead to fracture. This phenomenon has been attributed to the stress alteration (stress shielding). Studies addressing the effects of necrotic size and the orientation of the implant on stress alterations are lacking.

Methods

Computed tomography images of a standard composite femur were used to create a three-dimensional finite-element (FE) intact femur model. Based on the intact model, FE models simulating four different levels of necrotic regions (0°, 60°, 100°, 115°) and three different implant insertion angles (varus 10°, neutral, valgus 10°) were created. The von Mises stress distributions and the displacement of the stem tip of each model were analyzed and compared for loading conditions that simulated a single-legged stance.

Results

Stress shielding occurred at the femoral neck after HRA. More severe stress shielding and an increased displacement of the stem tip were found for femoral heads that had a wider necrotic lesion. From a biomechanics perspective, the results were consistent with clinical evidence of femoral neck narrowing after HRA. In addition, a varus orientation of the implant resulted in a larger displacement of the stem tip, which could lead to an increased risk of implant loosening.

Conclusions

A femoral head with a wide necrotic lesion combined with a varus orientation of the prosthesis increases the risk of femoral neck narrowing and implant loosening following HRA.

Biomechanical comparison of different combinations of hook and screw in one spine motion unit--an experiment in porcine model

BACKGROUND

The biomechanical performance of the hooks and screws in spinal posterior instrumentation is not well-characterized. Screw-bone interface failure at the uppermost and lowermost vertebrae is not uncommon. Some have advocated for the use of supplement hooks to prevent screw loosening. However, studies describing methods for combined hook and screw systems that fully address the benefits of these systems are lacking. Thus, the choice of which implant to use in a given case is often based solely on a surgeon's experience instead of on the biomechanical features and advantages of each device.

METHODS

We conducted a biomechanical comparison of devices instrumented with different combinations of hooks and screws. Thirty-six fresh low thoracic porcine spines were assigned to three groups (12 per group) according to the configuration used for of fixation: (1) pedicle screw; (2) lamina hook and (3) combination of pedicle screw and lamina hook. Axial pullout tests backward on transverse plane in the direction normal to the rods were performed using a material testing machine and a specially designed grip with self-aligned function.

RESULTS

The pullout force for the pedicle screws group was significantly greater than for the hooks and the combination (p < 0.05). However, no significant difference was found between the hooks and the combination (p > 0.05).

CONCLUSIONS

Pedicle screws achieve the maximal pullout strength for spinal posterior instrumentation.

Biomechanical comparison of different combinations of hook and screw in one spine motion unit--an experiment in porcine model

Background

The biomechanical performance of the hooks and screws in spinal posterior instrumentation is not well-characterized. Screw-bone interface failure at the uppermost and lowermost vertebrae is not uncommon. Some have advocated for the use of supplement hooks to prevent screw loosening. However, studies describing methods for combined hook and screw systems that fully address the benefits of these systems are lacking. Thus, the choice of which implant to use in a given case is often based solely on a surgeon’s experience instead of on the biomechanical features and advantages of each device.

Methods

We conducted a biomechanical comparison of devices instrumented with different combinations of hooks and screws. Thirty-six fresh low thoracic porcine spines were assigned to three groups (12 per group) according to the configuration used for of fixation: (1) pedicle screw; (2) lamina hook and (3) combination of pedicle screw and lamina hook. Axial pullout tests backward on transverse plane in the direction normal to the rods were performed using a material testing machine and a specially designed grip with self-aligned function.

Results

The pullout force for the pedicle screws group was significantly greater than for the hooks and the combination (p < 0.05). However, no significant difference was found between the hooks and the combination (p > 0.05).

Conclusions

Pedicle screws achieve the maximal pullout strength for spinal posterior instrumentation.

Hypothermic manipulation of bone cement can extend the handling time during vertebroplasty

Background

Polymethylmethacrylate (PMMA) is commonly used for clinical applications. However, the short handling time increases the probability of a surgeon missing the crucial period in which the cement maintains its ideal viscosity for a successful injection. The aim of this article was to illustrate the effects a reduction in temperature would have on the cement handling time during percutaneous vertebroplasty.

Methods

The injectability of bone cement was assessed using a cement compressor. By twisting the compressor, the piston transmits its axial load to the plunger, which then pumps the bone cement out. The experiments were categorized based on the different types of hypothermic manipulation that were used. In group I (room temperature, sham group), the syringes were kept at 22°C after mixing the bone cement. In group 2 (precooling the bone cement and the container), the PMMA powder and liquid, as well as the beaker, spatula, and syringe, were stored in the refrigerator (4°C) overnight before mixing. In group 3 (ice bath cooling), the syringes were immediately submerged in ice water after mixing the bone cement at room temperature.

Results

The average liquid time, paste time, and handling time were 5.1 ± 0.7, 3.4 ± 0.3, and 8.5 ± 0.8 min, respectively, for group 1; 9.4 ± 1.1, 5.8 ± 0.5, and 15.2 ± 1.2 min, respectively, for group 2; and 83.8 ± 5.2, 28.8 ± 6.9, and 112.5 ± 11.3 min, respectively, for group 3. The liquid and paste times could be increased through different cooling methods. In addition, the liquid time (i.e. waiting time) for ice bath cooling was longer than for that of the precooling method (p < 0.05).

Conclusions

Both precooling (i.e. lowering the initial temperature) and ice bath cooling (i.e. lowering the surrounding temperature) can effectively slow polymerization. Precooling is easy for clinical applications, while ice bath cooling might be more suitable for multiple-level vertebroplasty. Clinicians can take advantage of the improved injectability without any increased cost.

Study the effect of polymerization temperature in the release of antibiotic from bone cement

Antibiotic-loaded bone cement is widely used to treat musculoskeletal infections. Here we tested whether the curing temperature affected pore size of the cement and antibiotic release. Bone cement containing vancomycin was cured at 3 temperatures (50, 25 and 0°C). The solidified vancomycin-loaded cement was stored for 1-week at 25 and 5°C and then assayed for antibiotic release. We found that a significantly higher proportion of vancomycin was released from cement cured at 0°C versus that cured at 50°C, and that lower storage temperature also increased antibiotic release.

Retrograde nailing of a femoral supracondyle

Because standard femoral supracondylar nails have certain disadvantages, they are often replaced by traditional femoral or tibial locked nails. The purpose of this study was to make a biomechanical comparison between both types of traditional locked nails to determine which technique was more suitable for treating unstable femoral supracondylar fractures. Fourteen left Sawbones femurs (Pacific Research Laboratories, Vashon, Washington) were osteotomized in the femoral supracondylar area. One centimeter of the medial cortex in the proximal fragment was obliquely removed to simulate an unstable fracture without shortening. Seven specimens were treated with traditional retrograde dynamic femoral locked nails, and the other 7 with traditional retrograde dynamic tibial locked nails. All specimens were tested with a servohydraulic materials testing machine to compare their relative stability. Static compression, dynamic cyclic compression, and static compression to failure were tested. An extensometer was used to measure the displacement of fragments. Displacement between the fragments increased following the increment in loads in both nails. The load-displacement curve was nearly linear up to 1000 N for both nails. The femoral nail had a greater stiffness compared with the tibial nail at 100 and 200 N (P=.02 and P=.04, respectively) in static compression and at 700 to 1000 N (P=.01 in each case) in dynamic cyclic compression, as well as larger loads in static compression to failure (8663 vs 7547 N, respectively; P<.001). Clinically, a traditional femoral locked nail may be more suitable to replace a standard femoral supracondylar nail in a retrograde fashion to treat an unstable femoral supracondylar fracture.

Cement leakage causes potential thermal injury in vertebroplasty

BACKGROUND

Percutaneous vertebroplasty by injecting PMMA bone cement into the fractured vertebrae has been widely accepted in treatment of spinal compression fracture. However, the exothermic polymerization of bone cement may cause osseous or neural tissue injury. This study is thus designed to evaluate the potential risk of thermal damage in percutaneous vertebroplasty.

METHOD

Twelve porcine vertebrae were immersed in 37°C saline for the experiment. In the first stage of the study, vertebroplasty without cement leakage (control group, n = 6) was simulated. The anterior cortex, foramen, posterior cortex and the center of the vertebral body were selected for temperature measurement. Parameters including peak temperature and duration above 45°C were recorded. In the second stage, a model (n = 6) simulating bone cement leaking into the spinal canal was designed. The methods for temperature measurement were identical to those used in the first stage.

RESULTS

In Stage 1 of the study (vertebroplasty of the porcine vertebral body in the absence of cement leakage), the average maximal temperature at the anterior cortex was 42.4 ± 2.2°C; at the neural foramen 39.5 ± 2.1°C; at the posterior cortex 40.0 ± 2.5°C and at the vertebral center, 68.1 ± 3.4°C. The average time interval above 45°C was 0 seconds at the anterior cortex; at the neural foramen, 0 seconds; at the posterior cortex, 0 seconds and at the vertebral center, 223 seconds. Thus, except at the core of the bone cement, temperatures around the vertebral body did not exceed 45°C. In Stage 2 of the study (cement leakage model), the average maximal temperature at the anterior cortex was 42.7 ± 2.4°C; at the neural foramen, 41.1 ± 0.4°C; at the posterior cortex, 59.1 ± 7.6°C and at the vertebral center, 77.3 ± 5.7°C. The average time interval above 45°C at the anterior cortex was 0 seconds; at the neural foramen, 0 seconds; at the posterior cortex, 329.3 seconds and at the vertebral center, 393.2 seconds. Based on these results, temperatures exceeded 45°C at the posterior cortex and at the vertebral center.

CONCLUSIONS

The results indicated that, for bone cement confined within the vertebra, curing temperatures do not directly cause thermal injury to the nearby soft tissue. If bone cement leaks into the spinal canal, the exothermic reaction at the posterior cortex might result in thermal injury to the neural tissue.

Pullout strength of pedicle screws with cement augmentation in severe osteoporosis: a comparative study between cannulated screws with cement injection and solid screws with cement pre-filling

Background

Pedicle screws with PMMA cement augmentation have been shown to significantly improve the fixation strength in a severely osteoporotic spine. However, the efficacy of screw fixation for different cement augmentation techniques, namely solid screws with retrograde cement pre-filling versus cannulated screws with cement injection through perforation, remains unknown. This study aimed to determine the difference in pullout strength between conical and cylindrical screws based on the aforementioned cement augmentation techniques. The potential loss of fixation upon partial screw removal after screw insertion was also examined.

Method

The Taguchi method with an L₈ array was employed to determine the significance of design factors. Conical and cylindrical pedicle screws with solid or cannulated designs were installed using two different screw augmentation techniques: solid screws with retrograde cement pre-filling and cannulated screws with cement injection through perforation. Uniform synthetic bones (test block) simulating severe osteoporosis were used to provide a platform for each screw design and cement augmentation technique. Pedicle screws at full insertion and after a 360-degree back-out from full insertion were then tested for axial pullout failure using a mechanical testing machine.

Results

The results revealed the following 1) Regardless of the screw outer geometry (conical or cylindrical), solid screws with retrograde cement pre-filling exhibited significantly higher pullout strength than did cannulated screws with cement injection through perforation (p = 0.0129 for conical screws; p = 0.005 for cylindrical screws). 2) For a given cement augmentation technique (screws without cement augmentation, cannulated screws with cement injection or solid screws with cement pre-filling), no significant difference in pullout strength was found between conical and cylindrical screws (p >0.05). 3) Cement infiltration into the open cell of the test block led to the formation of a cement/bone composite structure. Observations of the failed specimens indicated that failure occurred at the composite/bone interface, whereas the composite remained well bonded to the screws. This result implies that the screw/composite interfacial strength was much higher than the composite/bone interfacial strength. 4) The back-out of the screw by 360 degrees from full insertion did not decrease the pullout strength in any of the studied cases. 5) Generally, larger standard deviations were found for the screw back-out cases, implying that the results of full insertion cases are more repeatable than those of the back-out cases.

Conclusions

Solid screws with retrograde cement pre-filling offer improved initial fixation strength when compared to that of cannulated screws with cement injection through perforation for both the conically and cylindrically shaped screw. Our results also suggest that the fixation screws can be backed out by 360 degrees for intra-operative adjustment without the loss of fixation strength.

Repair of large cranial defects by hBMP-2 expressing bone marrow stromal cells: comparison between alginate and collagen type I systems

Despite a wide range of available sources for bone repair, significant limitations persist. To bioengineer bone, we have previously transferred adenovirus-mediated human BMP-2 gene into autologous bone marrow stromal cells (MSC). We have successfully repaired large, full thickness, cranial defects using this approach. We report now the effectiveness of various hydrogels as the scaffold for this type of bone regeneration, comparing specifically alginate with Type I collagen. Cultured MSC of miniature swine were infected with BMP-2 or beta-gal adenovirus 7 days before implantation. These cells were mixed with alginate, ultrapure alginate, alginate-RGD, or type I collagen to fabricate the MSC/biomaterial constructs. The results of cranial bone regeneration were assessed by gross examination, histology, 3D CT, and biomechanical tests at 6 weeks and 3 months after implantation. We found that the BMP-2 MSC/collagen type I construct, but not the beta-gal control, effectively achieved nearly complete repair of the cranial defects. No bone regeneration was observed with the other hydrogels. Biomechanical testing showed that the new bone strength was very close and only slightly inferior to that of normal cranial bone. Controlling for the integration of stem cells and ex vivo gene transfer, the alginate scaffolds has a significant negative impact on the success of the construct. Our study demonstrates better bone regeneration by collagen type I over alginate. This may have therapeutic implications for tissue engineered bone repair.

Effect of lag-screw positions on modes of fixation failure in elderly patients with unstable intertrochanteric fractures of the femur

PURPOSE

To investigate the effect of lag-screw positions in the femoral head on modes of fixation failure.

METHODS

591 patients aged 65 years or older underwent sliding compression screw fixation for type A(1) (n=249) and type A(2) (n=342) intertrochanteric fractures of the femur after low-energy injuries. There were 18 cases of fixation failure; in 13 (group 1) the lag screw was placed in the central-central area, and in 5 (group 2) in the inferior 1/3-central area. Clinical variables of the 2 groups were compared.

RESULTS

All 18 cases of fixation failure were actually type A(23) and misinterpreted as type A(22) fractures. In 13 cases, failure was attributable to cut-out of the lag screws at the superolateral edge of the femoral head. In 3 cases, failure was attributable to telescoping and more than 2 cm shortening of the femur. In one case, failure was due to penetration of the lag screw into the acetabulum. In another case, failure was due to plate loosening with breakage of cortical screws. In the 13 cases with cut-out of the lag screw (group 1), the lag screw was placed in the central-central area (p<0.001). In the remaining 5 cases with no cut-out (group 2), the lag screw was placed in the inferior 1/3-central area (p<0.01). Complications occurred significantly earlier in group 1 than in group 2 patients (2.6 vs. 4.6 months, p=0.02). The tip-apex distance was significantly shorter in group 1 than in group 2 patients (19 vs. 30 mm, p<0.001).

CONCLUSION

Misinterpretation of type A(23) fractures as type A(22) may lead to fixation failure. Operative treatment should not be delayed once complications occur. The positions of the lag screw may affect the mode of fixation failure.

The healing of critical-sized femoral segmental bone defects in rabbits using baculovirus-engineered mesenchymal stem cells

Management of massive segmental bone defects remains a challenging clinical problem and bone marrow-derived mesenchymal stem cells (BMSCs) hold promise for bone regeneration. To explore whether BMSCs engineered by baculovirus (an emerging gene delivery vector) can heal large bone defects, New Zealand White (NZW) rabbit BMSCs were transduced with the BMP2-expressing baculovirus or VEGF-expressing baculovirus, and co-implanted into critical-sized (10mm) femoral segmental defects in NZW rabbits. X-ray analysis revealed that the baculovirus-engineered BMSCs not only bridged the defects at as early as week 2, but also healed the defects in 100% of rabbits (13/13) at week 4. The osteogenic metabolism, as monitored by positron emission tomography (PET) also suggested the completion of bone healing at week 8. When compared with other control groups, the BMP2/VEGF-expressing BMSCs remarkably enhanced the segmental bone repair and mechanical properties, as evidenced by micro-computed tomography (microCT), histochemical staining and biomechanical testing. The ameliorated bone healing concurred with the augmented angiogenesis. These data demonstrated, that BMSCs engineered to express BMP2 and VEGF accelerate the repair of large femoral bone defects and improve the quality of the regenerated bone, which paves an avenue to utilizing baculovirus as a vector for BMSCs modification and regenerative medicine.

Pullout strength for cannulated pedicle screws with bone cement augmentation in severely osteoporotic bone: influences of radial hole and pilot hole tapping

BACKGROUND

Pedicle screw fixation in a severely osteoporotic spine remains a challenge for orthopedic surgeons. The previous literature does not adequately address the effects of radial holes for cannulated screws with cement injection and pilot hole tapping on the bone/screw interfacial strength.

METHODS

Specially designed cannulated pedicle screws, with or without radial holes, were installed in tapped and untapped pilot holes and then injected with cement. A uniform synthetic bone (test block) was used to provide a platform for each screw design. Specimens with inserted screws were then tested for axial pullout failure.

FINDINGS

(1) Cannulated screws with cement augmentation significantly increased the pullout strength in comparison to solid screws. Additionally, the amount of cement exuded from the cannulated screws increased with an increasing number of radial holes, leading to an increase in the average ultimate pullout strength for cannulated screws with a large number of radial holes. (2) Radiological examination indicated that the cement was exuded from the most proximal holes at the very beginning of its flow path, whereas no cement exudation was found at the remaining distal holes. (3) Cement exudation from the holes of cannulated screws into the open cell of the test block led to a composite (cement/bone) structure at the area of cement exudation. Observations of the failed specimens indicated that failure occurred at the composite/bone interface, while the composite was well bonded to the screws. This implies that the screw/composite interfacial strength was much higher than the composite/bone interfacial strength. (4) Tapping pilot holes decreased the pullout strength of the screws. Generally, larger standard deviations were found for the tapped cases, implying that untapped cases results are more repeatable than tapped cases results.

INTERPRETATION

Cannulated pedicle screws with radial holes combined with PMMA cement augmentation but without tapping may be a viable clinical option for achieving fixation in severely osteoporotic bone.

Reconstruction interlocking nails for ipsilateral femoral neck and shaft fractures: biomechanical analysis of effect of supplementary cannulated screw on intracapsular femoral neck fracture

BACKGROUND

To stabilize the femoral head in ipsilateral femoral neck and shaft fractures, one cannulated screw was supplemented in front of the reconstruction interlocking nail (recon nail).

METHODS

Twenty-eight left sawbone femurs were divided into two groups. The 14 femurs in each group were osteotomized with subcapital or transcervical fractures. The shafts of all femurs were also osteotomized, and 1cm of the distal segmental cortex was excised. Next, all the fractures were treated with static recon nails to concomitantly stabilize both femoral neck and shaft fractures. Additionally, seven subcapital fractures and seven transcervical fractures were supplemented with one cannulated screw in front of the recon nails. All specimens were tested with a Material Testing System machine to investigate the relative stability during uniaxial cyclic compression.

FINDINGS

The subcapital and transcervical fractures supplemented with one cannulated screw could bear more loads than those without screw supplementation (P<0.01 and P<0.01, respectively at any testing interval). Moreover, in the case of the subcapital fractures, the single supplemented cannulated screw could reduce the displacement at 500 N by 24% (P< 0.001). On the other hand, in the case of the transcervical fractures, one supplemented cannulated screw could reduce the displacement at 500 N by 4% (P=0.003).

INTERPRETATION

From the biomechanical viewpoint, the subcapital fractures in combined fractures are recommended to supplement with one cannulated screw in front of the recon nails to eliminate complications associated with stabilization.

Biomechanical optimization of different fixation modes for a proximal femoral L-osteotomy

BACKGROUND

Numerous proposed surgical techniques have had minimal success in managing greater trochanter overgrowth secondary to retarded growth of the femoral capital epiphysis. For reconstruction of residual hip deformities, a novel type of proximal femur L-osteotomy was performed with satisfactory results. Although the clinical outcome was good, the biomechanical characteristics of the femur after such an osteotomy have not been clearly elucidated. Therefore, this study presents a three dimensional finite element analysis designed to understand the mechanical characteristics of the femur after the L-osteotomy.

METHODS

A patient with left hip dysplasia was recruited as the study model for L-osteotomy. The normal right hip was used as a reference for performing the corrective surgery. Four FEA models were constructed using different numbers of fixation screws but the same osteotomy lengths together with four FEA models with the same number of fixation screws but different osteotomy lengths. The von Mises stress distributions and femoral head displacements were analyzed and compared.

RESULTS

The results revealed the following: 1). The fixation devices (plate and screws) sustained most of the external loading, and the peak value of von Mises stress on the fixation screws decreased with an increasing number of screws. 2). Additional screws are more beneficial on the proximal segment than on the distal segment for improving the stability of the postoperative femur. 3). The extent of osteotomy should be limited because local stress might be concentrated in the femoral neck region with increasing length of the L-osteotomy.

CONCLUSION

Additional screw placement on the proximal segment improves stability in the postoperative femur. The cobra-type plate with additional screw holes in the proximal area might improve the effectiveness of L-osteotomies.

Bone marrow mesenchymal stem cells form ectopic woven bone in vivo through endochondral bone formation

Autologous vascularized bone grafts, allografts, and biocompatible artificial bone substitutes each have their shortcomings. Bones regenerated using recombinant human bone morphogenetic proteins, demineralized bone powder, or combinations of these are generally small and do not meet the need. The current trend is to use tissue engineering approaches with bone marrow mesenchymal stem cells (MSCs) to generate bones of a desired size and shape. A suspension of osteogenically induced MSCs (CD11a-, CD29+, CD44+) was added to 2% alginate, gelled by mixing this combination with calcium sulfate (CaSO(4) 0.2 g/mL), and injected into the subcutaneous pocket in the dorsal aspect of nude mice. Cells of various concentrations (0, 10, 50, and 70 million/mL) were used. These implanted constructs were harvested at predetermined times up to 30 weeks for histology. The doubling time of bovine MSCs is 3.75 +/- 1.96 days and the proliferation is rapid. Histological evaluation revealed signs of endochondrosis with woven bone deposition. The equilibrium modulus increased with time in vivo, though less than that of normal tissue. Implants seeded with 70 million cells/mL for 6 months resulted in the best formation of equilibrium modulus. This approach has several advantages: (i) obtaining MSCs is associated with low donor morbidity; (ii) MSCs proliferate rapidly in vitro, and a large number of viable cells can be obtained; and (iii) the MSC/alginate constructs can develop into bone-like nodules with high cell viability. Such a system may be useful in large-scale production of bony implants or in the repair of bony defects. The fact that endochondral bone formation led to woven bone suggests its potential feasibility in regional cell therapy.

Liquid gentamicin and vancomycin in bone cement: a potentially more cost-effective regimen

This study investigated the use of liquid gentamicin, a much less costly antibiotic (<1/20 the price of tobramycin) with a broad antimicrobial spectrum, alone and in combination with vancomycin in bone cement. Standardized cement specimens loaded with 480 mg of liquid gentamicin, 4 g of powdered vancomycin, or both antibiotics were tested for elution characteristics, bioactivity, compressive strength, and porosity. Vancomycin elution was enhanced by 146% with the addition of gentamicin liquid, and gentamicin elution was enhanced by 45% when combined with vancomycin. Bioassay confirmed the bactericidal activity of the released antibiotics. Adding liquid gentamicin increased porosity, whereas adding vancomycin did not. Compressive strength decreased by 13%, 37%, and 45% in specimens containing vancomycin, liquid gentamicin, and both antibiotics, respectively. Despite inferior mechanical properties, the temporary nature of cement beads and spacers makes the liquid gentamicin-vancomycin mixture a potentially more cost-effective regimen in bone cement to treat musculoskeletal infections.

Thermal damage potential during hip resurfacing in osteonecrosis of the femoral head: an experimental study

Hip resurfacing arthroplasty has become an attractive treatment option for young, active patients with femoral head necrosis. However, little information is available about the potential thermal damage to the remaining femoral head when a cemented component is used. We used an experimental model to measure the temperature profile at the cement-bone interface during hip resurfacing. We compared four simulated lesion sizes-15, 25, 33, and 50%-of the femoral head, and a control group with no cystic lesion. Temperatures were measured with the specimens in a 37 degrees C saline bath or with copious pulsed lavage. With specimens tested in the bath, peak temperatures were higher, and durations of temperatures above 50 degrees C were longer, in femoral heads with necrotic lesions (88.8 +/- 7.5 degrees C; 17.6 +/- 1.1 min for a 15% lesion; 96.2 +/- 7.2 degrees C; 22.86 +/- 1.3 min for a 25% lesion; 99.7 (c) 200 +/- 9.4 degrees C; 28.6 +/- 2.0 min for a 33% lesion; and 97.2 +/- 4.2 degrees C; 35.6 +/- 2.4 min for a 50% lesion) than those in the control group (65.8 +/- 4.9 degrees C; 10.0 +/- 1.3 min). The larger the cement-filled cysts, the longer the temperatures remained above 50 degrees C. Although copious lavage reduced the temperature profile in each group, the temperatures remained above 50 degrees C for 7 to 17 min in specimens with necrotic cysts. The measured temperatures during surface replacement are sufficiently high in magnitude and long in duration to cause thermal damage to the remaining bone in femoral heads with preexisting necrotic lesions. Hip resurfacing for femoral head necrosis should be performed with caution.

Biomechanical comparison of a new stand-alone anterior lumbar interbody fusion cage with established fixation techniques - a three-dimensional finite element analysis

Background

Initial promise of a stand-alone interbody fusion cage to treat chronic back pain and restore disc height has not been realized. In some instances, a posterior spinal fixation has been used to enhance stability and increase fusion rate. In this manuscript, a new stand-alone cage is compared with conventional fixation methods based on the finite element analysis, with a focus on investigating cage-bone interface mechanics and stress distribution on the adjacent tissues.

Methods

Three trapezoid 8° interbody fusion cage models (dual paralleled cages, a single large cage, or a two-part cage consisting of a trapezoid box and threaded cylinder) were created with or without pedicle screws fixation to investigate the relative importance of the screws on the spinal segmental response. The contact stress on the facet joint, slip displacement of the cage on the endplate, and rotational angle of the upper vertebra were measured under different loading conditions.

Results

Simulation results demonstrated less facet stress and slip displacement with the maximal contact on the cage-bone interface. A stand-alone two-part cage had good slip behavior under compression, flexion, extension, lateral bending and torsion, as compared with the other two interbody cages, even with the additional posterior fixation. However, the two-part cage had the lowest rotational angles under flexion and torsion, but had no differences under extension and lateral bending.

Conclusion

The biomechanical benefit of a stand-alone two-part fusion cage can be justified. This device provided the stability required for interbody fusion, which supports clinical trials of the cage as an alternative to circumferential fixations.

Biomechanical comparison of lumbar spine instability between laminectomy and bilateral laminotomy for spinal stenosis syndrome - an experimental study in porcine model

Background

The association of lumbar spine instability between laminectomy and laminotomy has been clinically studied, but the corresponding in vitro biomechanical studies have not been reported. We investigated the hypothesis that the integrity of the posterior complex (spinous process-interspinous ligament-spinous process) plays an important role on the postoperative spinal stability in decompressive surgery.

Methods

Eight porcine lumbar spine specimens were studied. Each specimen was tested intact and after two decompression procedures. All posterior components were preserved in Group A (Intact). In Group B (Bilateral laminotomy), the inferior margin of L4 lamina and superior margin of L5 lamina were removed, but the L4–L5 supraspinous ligament was preserved. Fenestrations were made on both sides. In Group C (Laminectomy) the lamina and spinous processes of lower L4 and upper L5 were removed. Ligamentum flavum and supraspinous ligament of L4–L5 were removed. A hydraulic testing machine was used to generate an increasing moment up to 8400 N-mm in flexion and extension. Intervertebral displacement at decompressive level L4–L5 was measured by extensometer

Results

The results indicated that, under extension motion, intervertebral displacement between the specimen in intact form and at two different decompression levels did not significantly differ (P > 0.05). However, under flexion motion, intervertebral displacement of the laminectomy specimens at decompression level L4–L5 was statistically greater than in intact or bilateral laminotomy specimens (P = 0.0000963 and P = 0.000418, respectively). No difference was found between intact and bilateral laminotomy groups. (P > 0.05).

Conclusion

We concluded that a lumbar spine with posterior complex integrity is less likely to develop segment instability than a lumbar spine with a destroyed anchoring point for supraspinous ligament.

The position of the bipolar cup reflects the direction of the hip contact force acting on it

We radiographically measured the bipolar cup position to analyze the direction of joint force acting on the bipolar cup. The abduction angle of the bipolar cup was measured in each radiograph taken immediately and at six 12 weeks and yearly after the operation. Radiographs in patients with weight bearing were also investigated. The results indicated that the abduction angle of the bipolar cup was 24.1 degrees +/- 11.2 degrees immediate postoperatively and was 16.2 degrees +/- 5.1 degrees at 6 weeks, 16.1 degrees +/- 5.1 degrees at 3 months, and 16.2 degrees +/- 5.1 degrees at 1 year. The cup abduction angles with weight bearing were not different from those without weight bearing and were 15.9 degrees +/- 4.9 degrees , 16.2 degrees +/- 4.4 degrees , and 16.1 degrees +/- 4.7 degrees on the supine, double-legged stance, and single-legged stance radiographs, respectively. Because the position of the bipolar cup reflects the direction of loads pivoting on it, the direction of the joint force in the frontal plane acting on the bipolar prosthesis is about 16 degrees to vertical.

Plating treatment for tibial plateau fractures: a biomechanical comparison of buttress and tension band positions

INTRODUCTION

The load tolerance of conventional plate treatment for bicondylar tibial plateau fractures with the buttress and the tension band positions was compared from biomechanical viewpoints.

MATERIALS AND METHODS

Fourteen left synthetic tibiae divided into two groups were tested. T-plates in the buttress group were placed on the medial tibial subcondyle and T-plates in the tension band group were placed on the lateral tibial subcondyle. All specimens were evaluated with a Material Testing System (MTS) machine. A linear variance displacement transducer gauge was placed on the medial tibial subcondyle. The anatomic axis of the synthetic femur and the tibia was maintained at 6 degrees varus in the frontal plane. The MTS actuator was set to increase displacement at 3 mm/min. All specimens were evaluated to failure. The relative motion and the ultimate failure load (UFL) between both groups were compared.

RESULTS

The buttress group had significantly less displacement than the tension band group following the incrementally increased loading (p < 0.001). At failure, the buttress group also had a higher UFL (p < 0.001) and less displacement (p = 0.009) than the tension band group.

CONCLUSION

All medial tibial condylar fragments achieved improved stabilization from the medial aspect of the proximal tibia by conventional plates. When conventional plates cannot be placed medially due to skin ailments, lateral placement of conventional plates may have the insufficient stability. Protected weight bearing should be followed strictly.

The effect of necrotic lesion size and rotational degree on the stress reduction in transtrochanteric rotational osteotomy for femoral head osteonecrosis--a three-dimensional finite-element simulation

BACKGROUND

The prospect for success in the efficacy of osteotomy for precollapse stage of femoral head osteonecrosis depends on the ability to predict reliably the stress changes derived from specific osteotomies. A three-dimensional finite-element analysis was thus designed to compute necrotic femoral head stress changes with different extent of necrocrosis that accompany anterior or posterior rotational osteotomies.

METHOD

Computed tomography images of a standard composite femur were used to create the three-dimensional finite-element intact femur model. Based on the intact model, 27 models simulating three different levels of necrotic region together with nine different rotational osteotomies were created. The von Mises stress distributions of each model were analyzed and compared for a loading condition simulating single-legged stance.

FINDINGS

(1) The stress reduction in anterior rotational osteotomy is more effective as compared to that of the posterior rotational osteotomy for various necrotic lesion sizes. (2) Von Mises stress on the necrotic zone decreased with increasing rotational angle. The decreasing rate was higher for the femoral head with a narrow lesion. (3) Femoral head with a wider necrotic lesion had a higher risk for developing collapse due to high local stress on the surface of necrotic region; whereas the necrotic region tended to expand in size instead of collapse for femoral head with a narrow lesion due to high local stress on the interface between necrotic region and healthy bone.

INTERPRETATION

Transtrochanteric rotational osteotomy is a technically demanding procedure and associated with high complication risks, a more scrupulous planning including the finite-element analysis should be considered before doing surgery in clinical subjects.

Precooling of the femoral canal enhances shear strength at the cement-prosthesis interface and reduces the polymerization temperature

Preheating of the femoral stem in total hip arthroplasty improves the cement-prosthesis bond by decreasing the interfacial porosity. The main concern, however, is the potential thermal osteonecrosis because of an increased polymerization temperature. In this study, the effects of femoral canal precooling on the characteristics of the cement-stem interface were evaluated in an experimental model for three test conditions: precooling of the femoral canal, preheating of the stem (44 degrees C), and a control in which stems were inserted at room temperature without thermal manipulation of the implant, cement, or bone. Compared to the control group, precooling of the femoral canal and preheating of the stem had similar effects on the cement-stem interface, with greater interfacial shear strength and a reduced porosity. Femoral canal precooling also produced a lower temperature at the cement-bone interface. No difference was found in the ultimate compressive strength of bone cement for the three preparation conditions. Based on this laboratory model, precooling of the femoral canal could improve shear strength and porosity at the stem-cement interface, minimize thermal injury, and maintain the mechanical strength of the cement.

A biomechanical comparison of unlocked or locked reamed intramedullary nails in the treatment of mid-third simple transverse femoral shaft fractures

BACKGROUND

Despite that a mid-third simple transverse femoral shaft fracture has been traditionally treated with an unlocked reamed intramedullary nail, recently a static locked reamed intramedullary nail has been favored by some orthopedists to avoid missing extended fracture lines. A prospective comparison of both nails was conducted to investigate the superiority between the nails from biomechanical viewpoints.

METHODS

Seven pairs of fresh healthy cadaver femora underwent mid-third transverse osteotomy. Consequently, all seven left femora were stabilized using Kuntscher nails and all seven right femora, static Russell-Taylor locked nails. Finally, all 14 femora were tested using a Material Testing System (MTS) machine to investigate the sustained mechanical loads, the maximal failure load, and the relative fragment displacement.

RESULTS

The locked nails produced the larger mechanical loads (p = 0.02). The unlocked nails had the larger fragment displacement (p = 0.02) and the higher maximal failure load (6090 verse 5590 newtons, p = 0.02). All tests ended due to basal neck oblique fractures.

CONCLUSIONS

Biomechanically, an unlocked nail, being a load-sharing device, is superior to a static locked nail, being a load-bearing device, in the treatment of a mid-third simple transverse femoral shaft fracture. Clinically, careful investigation of the fracture types and adequate selection of the nail type should be performed in treating these types of fractures.

The effect of interspinous ligament integrity on adjacent segment instability after lumbar instrumentation and laminectomy--an experimental study in porcine model

Eight fresh porcine lumbar spines received a posterior instrumentation at L4-L5 using pedicle screw-rod system. Each specimen was tested utilizing laminectomies of varying extent. Group A (Integrity) preserved the spinous process and interspinous ligament; Group B (Partial laminectomy) removed the inferior portion of L4 spinous process and preserved the interspinous ligament of L3-L4; Group C (Complete laminectomy) removed the entire L4 spinous process. Hydraulic testing machine was used to generate an increasing moment up to 8400 N mm in flexion and extension. The intervertebral displacement on the superior adjacent disc between L3-L4 was measured using an extensometer. Under extension, no significant difference in the intervertebral displacement was observed among three different models of laminectomy. However, under flexion, the intervertebral displacement on adjacent disc with complete laminectomy was statistically larger than those of integrity and partial laminectomies (P=0.000976 and P=0.0363, respectively). No difference was found between integrity and partial laminectomy groups (P>0.05). This study implies that an instrumented spine with integrity of posterior complex is less likely to develop adjacent instability than a spine with destruction of the anchoring point for supraspinous ligament.

Early administration of hyperbaric oxygen therapy in distraction osteogenesis--a quantitative study in New Zealand rabbits

BACKGROUND

We investigated the effect of hyperbaric oxygen (HBO) therapy on the early phase of tibial lengthening in our established rabbit model.

METHODS

Twenty-four male rabbits (six per group) underwent right tibial lengthening by 5 mm. Group 1 then underwent 2.5 atmospheres of absolute hyperbaric oxygenation for 2 hours daily for 6 weeks postoperatively; group 2, for early 5 weeks (weeks 1-5), group 3, for late 5 weeks (weeks 2-6), and group 4 had no HBO therapy. Bone mineral density (BMD) was measured before surgery and weekly thereafter from weeks 2 through 6. The mechanical strengths of the lengthened tibias were measured.

RESULTS

Significantly higher mean %BMDs were obtained for groups 1 and 2 compared with groups 3 and 4. There was no difference in the mean %BMD between groups 1 and 2 (p > 0.05). The results were similar for mean percentage maximal torque; group 1 had the maximum torque, followed sequentially by groups 2 though 4.

CONCLUSION

The study results suggest that early and full-term administration of HBO therapy on tibial lengthening may achieve better benefits.

Effect of hyperbaric oxygen therapy on patellar tendinopathy in a rabbit model

BACKGROUND

Hyperbaric oxygen therapy is a method for augmenting oxygen availability to tissues. This study investigated the effect of hyperbaric oxygen therapy on the collagenase-induced tendinopathy in the rabbit patellar tendon.

METHODS

In this study, 13 rabbits were treated by ultrasound-guided injection of 0.025 mL collagenase into the patellar tendon at the right knee, with the left knee serving as a control condition. The rabbits were randomly divided into two groups. After tendinopathy had been confirmed by histologic examination 3 weeks after treatment, hyperbaric oxygen therapy was initiated for group 1. The hyperbaric oxygen therapy involved 30 daily sessions of 2.5 ATA for 120 minutes starting 6 weeks after treatment. The rabbits in group 2 were put in normobaric room air. Both groups were killed 10 weeks after treatment. Histologic examinations as well as mechanical and biochemical tests were performed after the animals were killed.

RESULTS

The ultimate tensile load in the tendon that had hyperbaric oxygen therapy was 34.8% greater than that in the control tendon 10 weeks after treatment (p < 0.05). Hydroxyproline concentrations increased 82.2% simultaneously in the tendons that had hyperbaric oxygen therapy, as compared with the concentrations in the control tendons (p < 0.05). However, no statistical difference was found between the two groups in terms of pyridinoline concentration at the 10th week (p > 0.05). The histologic examination demonstrated an increase in blastlike tenocytes in group 1, with more mature phenotype, more organized collagen matrix, absence of myxoid degeneration, and increased vascularity at the 10th week, as compared with the control knee.

CONCLUSIONS

The results validate the effectiveness of hyperbaric oxygen therapy in the treatment of tendinopathy. Hyperbaric oxygen therapy may increase collagen synthesis and collagen cross-link formation during the early healing process.

The degrees to which transtrochanteric rotational osteotomy moves the region of osteonecrotic femoral head out of the weight-bearing area as evaluated by computer simulation

BACKGROUND

Transtrochanteric rotational osteotomy is a technical demanding procedure. Currently, the pre-operative planning of the transtrochanteric rotational osteotomy is mostly based on X-ray images. The surgeons would need to reconstruct the three-dimensional structure of the femoral head and the necrosis in their mind. This study develops a simulation platform using computer models based on the computed tomography images of the femoral head to evaluate the degree to which transtrochanteric rotational osteotomy moves the region of osteonecrotic femoral head out of the weight-bearing area in stance and gait cycle conditions. Based on this simulation procedure, the surgeons would be better informed before the surgery and the indication can be carefully assessed.

METHOD

A case with osteonecrosis involving 15% of the femoral head was recruited. Virtual models with the same size lesion but at different locations were devised. Computer models were created using SolidWorks 2000 CAD software. The area ratio of weight-bearing zone occupied by the necrotic lesion on two conditions, stance and gait cycle, were measured after surgery simulations.

FINDINGS

For the specific case and virtual models devised in this study, computer simulation showed the following two findings: (1) The degrees needed to move the necrosis out of the weight-bearing zone in stance were less by anterior rotational osteotomy as compared to that of posterior rotational osteotomy. However, the necrotic region would still overlap with the weight-bearing area during gait cycle. (2) Because the degrees allowed for posterior rotation were less restricted than anterior rotation, posterior rotational osteotomies were often more effective to move the necrotic region out of the weight-bearing area during gait cycle.

INTERPRETATION

The computer simulation platform by registering actual CT images is a useful tool to assess the direction and degrees needed for transtrochanteric rotational osteotomy. Although the results indicated that anterior rotational osteotomy was more effective to move the necrosis out of the weight-bearing zone in stance for models devised in this study, in circumstances where the necrotic region located at various locale, considering the limitation of anterior rotation inherited with the risk of vascular compromise, it might be more beneficial to perform posterior rotation osteotomy in taking account of gait cycle.

Biomechanical comparison of newly designed stemless prosthesis and conventional hip prosthesis--an experimental study

Local bone loss after implantation of traditional stem-type prostheses remains an unsolved problem during the long-term application of total hip replacement. The stress shielding effect and osteolysis were thought to be the two main factors that result in local bone loss after prosthesis implantation. A newly designed stemless cervico-trochanteric (C-T) prosthesis was thus developed to reduce stress shielding and osteolysis caused by the implantation of conventional stem-type prosthesis. Eight synthetic femora were implanted with C-T and porous coated anatomic (PCA) prostheses. Under 2,000-Newton load, the surface strains of proximal femora were compared between the intact, PCA press-fit femora and the C-T implanted femora with three different fixation modes: two-screw fixation, three-screw fixation, and three-screw combined with cement fixation. The results revealed that stress shielding in the C-T implanted femora was significantly eliminated compared to that of the PCA implanted femora (p<0.01). No statistical difference in strain magnitude was found for the C-T implanted femora among the three different fixation modes (p>0.1). The C-T implanted femur has more physiological strain distribution. Moreover, from the C-T prosthetic characteristic design, the localized osteolysis would be also reduced due to the overall coverage of neck-trochanteric area. The newly designed C-T prosthesis may be a useful alternative to the traditional stem-type prosthesis in the future.

The effect of sagittal alignment on adjacent joint mobility after lumbar instrumentation--a biomechanical study of lumbar vertebrae in a porcine model

BACKGROUND

The mechanisms and changes in range of motion of neighboring mobile segment (adjacent level) after the instrumented posterior stabilization are not completely understood. This study aims to investigate the effect of sagittal alignment on the adjacent joint mobility after lumbar instrumentation.

METHODS

Eight fresh porcine lumbar spines were instrumented with pedicle screw implants from L2 to L4. Each specimen was tested in three different sagittal alignments. Group A were instrumented in lordotic alignment (lordosis 20 degrees ), Group B in straight alignment (lordosis 0 degrees ), and Group C in kyphotic alignment (kyphosis 20 degrees ). Hydraulic testing machine was used to generate an increasing moment in flexion and extension respectively for each specimen. The vertebral displacement of the disc between L1-L2 and L4-L5 were measured simultaneously with an extensometer.

FINDINGS

There were no significant differences in vertebral displacement between the three different sagittal alignments in both the superior and inferior adjacent segments under extension motion. However, under flexion motion, the vertebral displacement on the superior adjacent segment (L1-L2) with kyphotic alignment was statistically larger than that of the straight and lordotic alignments (P = 0.0198 and P = 0.000473 respectively), and no differences were found between the three different sagittal alignments on the inferior adjacent segment (L4-L5).

INTERPRETATION

The iatrogenically produced kyphotic lumbar spine by posterior instrumentation might cause larger adjacent joint mobility on the superior adjacent joint as compared to the instrumented lordotic lumbar spine. This study implies that an instrumented spine in lordosis is less likely to develop adjacent instability than a kyphotic spine.