A water-borne adhesive modeled after the sandcastle glue of P. californica

Polyacrylate glue protein analogs of the glue secreted by Phragmatopoma californica, a marine polycheate, were synthesized with phosphate, primary amine, and catechol sidechains with molar ratios similar to the natural glue proteins. Aqueous mixtures of the mimetic polyelectrolytes condensed into liquid complex coacervates around neutral pH. Wet cortical bone specimens bonded with the coacervates, oxidatively crosslinked through catechol sidechains, had bond strengths nearly 40% of the strength of a commercial cyanoacrylate. The unique material properties of complex coacervates may be ideal for development of clinically useful adhesives and other biomaterials.

Local and global subaxial cervical spine biomechanics after single-level fusion or cervical arthroplasty

An experimental in vitro biomechanical study was conducted on human cadaveric spines to evaluate the motion segment (C4-C5) and global subaxial cervical spine motion after placement of a cervical arthroplasty device (Altia TDI,Amedica, Salt Lake City, UT) as compared to both the intact spine and a single-level fusion. Six specimens (C2-C7) were tested in flexion/extension, lateral bending, and axial rotation under a +/- 1.5 Nm moment with a 100 N axial follower load. Following the intact spine was tested; the cervical arthroplasty device was implanted at C4-C5 and tested. Then, a fusion using lateral mass fixation and an anterior plate was simulated and tested. Stiffness and range of motion (ROM) data were calculated. The ROM of the C4-C5 motion segment with the arthroplasty device was similar to that of the intact spine in flexion/extension and slightly less in lateral bending and rotation, while the fusion construct allowed significantly less motion in all directions. The fusion construct caused broader effects of increasing motion in the remaining segments of the subaxial cervical spine, whereas the TDI did not alter the adjacent and remote motion segments. The fusion construct was also far stiffer in all motion planes than the intact motion segment and the TDI, while the artificial disc treated level was slightly stiffer than the intact segment. The Altia TDI allows for a magnitude of motion similar to that of the intact spine at the treated and adjacent levels in the in vitro setting.

A cadaver knee simulator to evaluate the biomechanics of rectus femoris transfer

A cadaver knee simulator has been developed to model surgical transfer of the rectus femoris. The simulator allows knee specimens six degrees of freedom and is capable of modeling both the swing and stance phases of human gait. Experiments were conducted using a mechanical hinge analog of the knee to verify that time, flexion angle, and knee extension force measurements recorded when using the simulator were not influenced by its design or operation. A ballistic double pendulum model was used to model the swing phase of gait, and the contributions of hip and ankle torques and hamstrings cocontraction were included when modeling the stance phase of gait. When modeling swing, range of motion and time to peak knee flexion in swing for the hinge knee were similar to those of in vivo test subjects. Measurements of hinge knee extension force when modeling stance under various biomechanical conditions matched those predicted using an analytical model. Future studies using cadaver knee specimens will apply techniques described in this paper to further our understanding of changes in knee biomechanics caused by rectus femoris transfer surgery.

Biomechanical analysis of bicondylar tibial plateau fixation: how does lateral locking plate fixation compare to dual plate fixation?

OBJECTIVES

This study is designed to test the comparative strength of lateral-only locked plating to medial and lateral nonlocked plating in a cadaveric model of a bicondylar proximal tibial plateau fracture.

METHODS

Ten matched pairs of human cadaveric proximal tibia specimens were used for biomechanical testing. Cyclic loading using a materials testing device simulated initial range of motion and load bearing following surgical repair. Subsidence of the medial and the lateral condyles was measured following 10,000 cycles from 100N to 1,000N; the maximum load to failure on the medial condyle for both plate constructs was also measured.

RESULTS

On the lateral side, dual plating (DP) allowed an average of 0.68 +/- 0.14 mm of subsidence, compared with 1.03 +/- 0.27 mm for the fixed-angle plate (FAP) (P = 0.077). On the medial side, DP allowed an average of 0.78 +/- 0.15 mm of subsidence, compared with 1.51 +/- 0.32 mm for the FAP (P = 0.045). No significant difference was found in the maximal load to medial condyle fixation failure between either plating construct (P = 0.204).

CONCLUSIONS

The results of this study demonstrate that dual-plate fixation allows less subsidence in this bicondylar tibial plateau cadaveric model when compared to isolated locked lateral plates. This may raise concerns about the widespread use of isolated lateral locked plate constructs in bicondylar tibial plateau fractures.

Effect of lateral meniscal allograft sizing on contact mechanics of the lateral tibial plateau: an experimental study in human cadaveric knee joints

BACKGROUND

A mismatch of the original lateral meniscus and a lateral meniscus allograft by inaccurate preoperative radiographic sizing can have significant consequences on ultimate function.

HYPOTHESIS

The size of a lateral meniscal allograft affects the contact mechanics of the femoral condyle on the tibial plateau.

STUDY DESIGN

Controlled laboratory study.

METHODS

Four right and 2 left knees were tested as intact joints, after meniscectomy, and after replantation with the original menisci and 16 right or 9 left human, fresh-frozen lateral meniscal allografts, respectively. The allografts were allocated into 7 groups according to their outer and inner anteroposterior and mediolateral diameters. Biomechanical testing was performed as compressive loadings with constrained motions in extension and 30 degrees of flexion. Measurements were done with Fuji pressure-sensitive films for contact parameters of the direct femorotibial and meniscotibial contact.

RESULTS

Oversized lateral meniscal allografts led to greater forces across the articular cartilage, whereas undersized allografts resulted in normal forces across the articular cartilage but greater forces across the meniscus. Two undersized transplants failed. Most of the contact parameters of allografts 10% smaller or larger than the original menisci were in the range of the intact knees. The knees after meniscectomy showed greater forces of the direct femorotibial contact areas than did the intact knees and the knees with the replanted original menisci. The contact mechanics of the knees with the replanted original menisci were close to normal.

CONCLUSION

The size of a lateral meniscal allograft has a significant effect on the contact mechanics of the tibial plateau.

CLINICAL RELEVANCE

Preoperative radiographic sizing needs to be performed precisely to identify a suitable lateral meniscal allograft. A mismatch may be the reason for failure of the allograft or subsequent development of degenerative changes. A mismatch on graft selection of less than 10% of the size of the original meniscus may be acceptable.

Biomechanical analysis of distal femur fracture fixation: fixed-angle screw-plate construct versus condylar blade plate

OBJECTIVE

The objective of this study is to establish the relative strength of fixation of a locking distal femoral plate compared with the condylar blade plate.

METHODS

Eight matched pairs of fresh-frozen cadaveric femurs were selected and evaluated for bone density. A gap osteotomy model was used to simulate an OTA/AO A3 comminuted distal femur fracture. One femur of each pair was fixed with the blade plate; the other, with a locking plate. After 100 N preload and 10,000 cycles between 100 N and 1000 N, total displacement of each specimen was assessed. After completion of cyclic loading, maximum load to failure was tested.

RESULTS

Significantly greater subsidence (total axial displacement) occurred with the blade plate (1.70 +/- 0.45 mm; range, 1.21-2.48 mm) than with the locking plate fixation (1.04 +/- 0.33 mm; range, 0.67-1.60 mm) after cyclic loading (P = 0.03). In load-to-failure testing, force absorbed by the locking plate before failure (9085 +/- 1585 N; range, 7269-11,850 N) was significantly greater than the load tolerated by the blade plate construct (5591 +/- 945 N; range, 3546-6684 N; P = 0.001). Variability in bone mineral density did not affect the findings (fixed angle distal femoral plate r = 0.1563; condylar blade plate r = 0.0796).

CONCLUSIONS

The locking screw-plate construct proved stronger than the blade plate in both cyclic loading and ultimate strength in biomechanical testing of a simulated A3 distal femur fracture. Although differences were small, the biomechanical performance of the locking plate construct over the blade plate may lend credence to use of the locking plate versus the blade plate in the fixation of comminuted distal femur fractures.

Static versus dynamic loading in the mechanical modulation of vertebral growth

STUDY DESIGN

Measures of absolute and relative growth modulation were used to determine the effects of static and dynamic asymmetric loading of vertebrae in the rat tail.

OBJECTIVES

To quantify the differences between static and dynamic asymmetric loading in vertebral bone growth modulation.

SUMMARY OF BACKGROUND DATA

The creation and correction of vertebral wedge deformities have been previously described in a rat-tail model using static loading. The effects of dynamic loading on growth modulation in the spine have not been characterized.

METHODS

A total of 36 immature Sprague-Dawley rats were divided among four different groups: static loading (n = 12, 0.0 Hz), dynamic loading (n = 12, 1.0 Hz), sham operated (n = 6), and growth controls (n = 6). An external fixator was placed across the sixth and eighth caudal vertebrae as the unviolated seventh caudal vertebra was evaluated for growth modulation. Static or dynamic asymmetric loads were applied at a loading magnitude of 55% body weight. After 3 weeks of loading, growth modulation was assessed using radiographic measurements of vertebral wedge angles and vertebral body heights.

RESULTS

The dynamically loaded rats had a final average wedge deformity of 15.2+/- 6.4 degrees, which was significantly greater than the statically loaded rats whose final deformity averaged 10.3 degrees +/- 3.7 degrees (P < 0.03). The deformity in both groups was statistically greater than the sham-operated (1.1+/- 2.0 degrees) and growth control rats (0.0+/- 1.0 degrees) (P < 0.001). The longitudinal growth was significantly lower on the concavity compared with the convexity in both the dynamically (0.34 +/- 0.23 mm vs. 0.86 +/- 0.23 mm) and statically (0.46 +/- 0.19 mm vs. 0.83 +/- 0.32 mm) loaded rats (P < 0.001). These growth rates were significantly less than the sham operated and growth control rats (P < 0.001).

CONCLUSIONS

A variety of fusionless scoliosis implant strategies have been proposed that use both rigid and flexible implants to modulate vertebral bone growth. The results from this study demonstrate that dynamic loading of the vertebrae provides the greatest growth modulation potential.

The influence of collagen fiber orientation and other histocompositional characteristics on the mechanical properties of equine cortical bone

This study examined relative influences of predominant collagen fiber orientation (CFO), mineralization (% ash), and other microstructural characteristics on the mechanical properties of equine cortical bone. Using strain-mode-specific (S-M-S) testing (compression testing of bone habitually loaded in compression; tension testing of bone habitually loaded in tension), the relative mechanical importance of CFO and other material characteristics were examined in equine third metacarpals (MC3s). This model was chosen since it had a consistent non-uniform strain distribution estimated by finite element analysis (FEA) near mid-diaphysis of a thoroughbred horse, net tension in the dorsal/lateral cortices and net compression in the palmar/medial cortices. Bone specimens from regions habitually loaded in tension or compression were: (1) tested to failure in both axial compression and tension in order to contrast S-M-S vs non-S-M-S behavior, and (2) analyzed for CFO, % ash, porosity, fractional area of secondary osteonal bone, osteon cross-sectional area, and population densities of secondary osteons and osteocyte lacunae. Multivariate multiple regression analyses revealed that in S-M-S compression testing, CFO most strongly influenced total energy (pre-yield elastic energy plus post-yield plastic energy); in S-M-S tension testing CFO most strongly influenced post-yield energy and total energy. CFO was less important in explaining S-M-S elastic modulus, and yield and ultimate stress. Therefore, in S-M-S loading CFO appears to be important in influencing energy absorption, whereas the other characteristics have a more dominant influence in elastic modulus, pre-yield behavior and strength. These data generally support the hypothesis that differentially affecting S-M-S energy absorption may be an important consequence of regional histocompositional heterogeneity in the equine MC3. Data inconsistent with the hypothesis, including the lack of highly longitudinal collagen in the dorsal-lateral ;tension' region, paradoxical histologic organization in some locations, and lack of significantly improved S-M-S properties in some locations, might reflect the absence of a similar habitual strain distribution in all bones. An alternative strain distribution based on in vivo strain measurements, without FEA, on non-Thoroughbreds showing net compression along the dorsal-palmar axis might be more characteristic of the habitual loading of some of the bones that we examined. In turn, some inconsistencies might also reflect the complex torsion/bending loading regime that the MC3 sustains when the animal undergoes a variety of gaits and activities, which may be representative of only a portion of our animals, again reflecting the possibility that not all of the bones examined had similar habitual loading histories.

Mechanical bond strength of the cement-tibial component interface in total knee arthroplasty

The purpose of this study was to mechanically test the cement-tibial component interface using titanium and cobalt-chrome sample prostheses with several commercially available surface textures. The results of this study indicate that the type of metal substrate and surface preparation of contemporary tibial baseplates may influence the strength of the metal-cement interface and as such influence tibial component survival. The results indicate that, in general, metal-cement interface strength increases with increasing surface roughness and common surface treatments such as AlO2 grit-blasting (Ra = 6.76 microm) produce interface strengths similar to plasma-spray, porous-coated specimens. Macrosurfaced tibial components, although comparable in tension, may be vulnerable to metal-cement interface failure with rotational loading.

Anterior cervical fixation: analysis of load-sharing and stability with use of static and dynamic plates

BACKGROUND

Anterior plates provide stability following decompression and fusion of the cervical spine. Various plate designs have emerged, and they include static plates with fixed-angle screws, rotationally dynamic plates that allow the screws to toggle in the plate, and translationally dynamic plates that allow the screws to both toggle and translate vertically. The goal of this study was to document the effects of plate design following a single-level corpectomy and placement of a full-length strut graft and the effects following 10% subsidence of the graft.

METHODS

A total of twenty-one cadaveric cervical spines (C2-T1) were randomized into three treatment groups and were tested for initial range of motion. A C5 corpectomy was performed, reconstruction was done with a full-length interbody spacer containing a load-cell, and an anterior cervical plate was applied. Load-sharing data were recorded with incremental axial loads. The range of motion was measured with +/- 2.5 Nm of torque in flexion-extension, lateral bending, and axial rotation. Then, the total length of the interbody spacer was reduced by 10% to simulate subsidence, and load-sharing and the range of motion were retested.

RESULTS

With the full-length interbody spacer, there were no significant differences in the abilities of the constructs to share load or limit motion. Following shortening of the interbody spacer, the static plate construct lost nearly 70% of its load-sharing capability, while neither of the dynamic plate constructs lost load-sharing capabilities. Also, the static plate construct allowed significantly more motion in flexion-extension following simulated subsidence than did either of the dynamic plate constructs (p < 0.05).

CONCLUSIONS

Although all of the tested anterior cervical plating systems provide similar load-sharing and stiffness following initial placement of the interbody spacer, the static plate system lost its ability to share load and limit motion following simulated subsidence of the interbody spacer. Both dynamic plate systems maintained load-sharing and stiffness despite simulated subsidence.

CLINICAL RELEVANCE

This study provides an improved understanding of the immediate performance of anterior cervical fusion surgery with plate fixation.

Measuring contact area, force, and pressure for bioengineering applications: Using Fuji Film and TekScan systems

The goal of this study was to compare the TekScan I-Scan Pressure Measurement System with two methods of analysis involving the Fuji Film Prescale Pressure Measuring System in estimating area, force and pressure. Fuji Film and TekScan sensors were alternately placed between a cylindrical peg and a finely ground steel base plate, and compressed with known forces. All Fuji stains were digitally scanned and analyzed. The Erase method of Fuji Film analysis consisted of manually removing portions of the image judged by the user to be outside the perimeter of the stain. The second method of Fuji Film analysis, termed the Threshold method, used the threshold tool to analyze only those pixels that were stained from loading. The TekScan system utilized special matrix-based sensors interfaced with a Windows compatible desktop computer that was equipped with specialized data acquisition hardware and analysis software. The data from this study did not support the hypothesis that all three methods would have accuracies within +/-5% of a known value, when estimating area, force and pressure. Specifically, the TekScan system was found to be more accurate than either of the Fuji Film methods when estimating area and pressure.

Creation of an experimental idiopathic-type scoliosis in an immature goat model using a flexible posterior asymmetric tether

STUDY DESIGN

Longitudinal follow-up of animals after a surgically initiated scoliosis.

OBJECTIVE

To create a progressive, structural, idiopathic-type, lordoscoliotic curve convex to the right in the thoracic spine of the immature goat using a flexible posterior asymmetric tether with minimal disruption of the spinal elements along the curve.

SUMMARY OF BACKGROUND DATA

Our previous work created an experimental scoliosis model using a rigid posterior asymmetric tether to study the safety and efficacy of fusionless scoliosis treatments. Posterior asymmetric tethers, whether rigid or flexible, represent the most reliable method of creating an experimental deformity that approximates idiopathic scoliosis. Although our initial rigid model was unique in creating progressive structural scolioses without violation of the essential spinal elements along the curve, there were a number of shortcomings associated with the model. These included substantially stiff curves and unpredictability of curve progression.

METHODS

Scoliosis was created in 24 Spanish Cross X female goats (age, 6-8 weeks; weight, 8-12 kg) using a braided synthetic ligament as a left posterior asymmetric tether from T5 to L1. Convex rib resection and concave rib tethering from ribs 8 to 13 were performed without disruption of the spinal elements before tensioning of the posterior tether. All goats were followed over an 8-week period with serial radiographs to document progression of the deformity. At the end of 8 weeks, the 20 goats with progressive curves were randomized into treatment groups for a separate study. However, 6 of these 20 remained untreated in the subsequent study and, therefore, were followed for an additional 12 to 16 weeks.

RESULTS

There were two deaths in the early postoperative period due to pulmonary complications. Of the remaining 22 goats, 20 (91%) developed progressive, structural, idiopathic-type, lordoscoliotic curves convex to the right in the thoracic spine. Initial scoliosis after tethering measured 55.4 degrees on average (range, 37 degrees-75 degrees) and progressed to 74.4 degrees on average (range, 42 degrees-93 degrees) over 8 weeks. The average progression of 19.0 degrees (range, 5 degrees-33 degrees) was statistically significant (P < 0.001). The average initial lordosis after tethering measured -18.9 degrees (range, -13 degrees to -27 degrees) and progressed to -40.7 degrees on average (range, -28 degrees to -56 degrees) over 8 weeks. The average progression in lordosis of -21.8 degrees (range, -5 degrees to -43 degrees) was significant (P < 0.001).

CONCLUSIONS

This study demonstrated the effectiveness of a flexible posterior asymmetric tether in creating idiopathic-type deformities in a shorter tethering period than previously described. With substantial remaining spinal growth after the 8-week tethering period and preservation of the essential spinal elements in an undisturbed state, this model is suitable for the study of scoliosis progression and various fusionless scoliosis treatment methods.

Mechanical modulation of vertebral growth in the fusionless treatment of progressive scoliosis in an experimental model

STUDY DESIGN

Wedging of apical spinal segments was measured during creation and correction of an experimental scoliosis in a goat model.

OBJECTIVES

To create and correct apical vertebral wedge deformities in a progressive experimental scoliosis model by purely mechanical means.

SUMMARY OF BACKGROUND DATA

The creation and correction of vertebral wedge deformities has been previously described in a rat tail model using external fixation.

METHODS

Experimental scoliosis was created in 14 goats using a posterior asymmetric tether with convex rib resection and concave rib tethering. After a period of up to 13 weeks, all tethers were removed and goats were randomized into treated (n = 8) and untreated (n = 6) groups. Treated goats underwent anterior thoracic stapling with four shape memory alloy staples along the convexity of the maximal curvature. Goats were followed for an additional 7 to 13 weeks during treatment. Serial radiographs were used to document progression or correction of the maximal scoliotic deformity as well as to measure the wedging of the apical spinal segment (two adjacent vertebrae and the intervening disc).

RESULTS

During the tethering period, all goats achieved a progressive, structural, lordoscoliotic curve of significant magnitude (mean: 61 degrees, range: 49 to 73 degrees). Wedging of the apical spinal segment measured 11.1 degrees at the beginning and 22.4 degrees at the end of the tethering period. The increase in apical spinal segment wedging of +11.3 degrees (10.7 degrees vertebral/0.6-degree disc) was significant (P = 0.001). During the treatment period, the scoliosis in the stapled goats measured 56.8 degrees at the beginning and 43.4 degrees at the end for an average correction of -13.4 degrees (range: 0 to-22 degrees) (P = 0.001), whereas the untreated goats measured 67.0 degrees at the beginning and 59.8 degrees at the end for an average correction of -7.2 degrees (range: +7 to -21 degrees) (P = 0.19). Additionally, wedging of the apical spinal segment in the stapled goats measured 22.5 degrees at the beginning and 20.3 degrees at the end for an average correction of -2.2 degrees (-0.6 degrees vertebral/-1.6-degree disc); wedging of the apical vertebral segment in the untreated goats measured 22.3 degrees at the beginning and 25.8 degrees at the end of the treatment period for an average progression of +3.5 degrees (3.5 degrees vertebral/0.0-degree disc). The difference in apical spinal segment correction versus progression in the stapled (-2.2 degrees) versus control (+3.5 degrees) goats was significant (P < 0.05).

CONCLUSIONS

This study demonstrates the ability to create wedge deformities at the apex of an experimental scoliosis in a large animal model and to control the progression of these deformities using anterior thoracic staples.

Biological effects of calcium sulfate as a bone graft substitute in ovine metaphyseal defects

Calcium sulfate has been used as a bone graft substitute in many fields, from dentistry to orthopedics. However, the results of many studies have yielded inconclusive results. In the present study, a sheep model was used with tibial and femoral metaphyseal defects to determine whether calcium sulfate was as effective as autograft and allograft in promoting new bone formation in a critical size defect. Medical-grade calcium sulfate pellets, autograft bone, allograft bone, or nothing was used to fill the metaphyseal defects. The sheep were allowed to heal for 12 weeks. Sagittal sections from the bones were analyzed with high-resolution contact radiographs, backscattered electron microscopy, and light microscopy. The volume fractions of bone within the defect perimeter were determined, and the histologic quality of the bone was observed. The volume fraction of new bone in the autograft, calcium sulfate, and allograft were not statistically different, but all were significantly different than the untreated control. The majority of the calcium sulfate had been resorbed at 12 weeks, and the histologic quality of the bone appeared similar to the autograft-treated bone. Calcium sulfate appears to be a useful biocompatible bone graft substitute that yields results similar to autograft bone in sheep metaphyseal defects.

Three-dimensional analysis of 2 fusionless scoliosis treatments: a flexible ligament tether versus a rigid-shape memory alloy staple

STUDY DESIGN

Experimental scoliosis was created and subsequently corrected in goats. The 3-dimensional (3-D) effects of the treatments were analyzed.

OBJECTIVE

To analyze the 3-D effect of 2 different fusionless scoliosis treatment techniques on an experimental idiopathic-type scoliosis using plain radiographs and computerized tomography.

SUMMARY OF BACKGROUND DATA

Scoliosis is a complex 3-D spinal deformity with limited treatment options. By preserving growth, motion, and function of the spine, fusionless scoliosis surgery provides theoretical advantages over current forms of treatment.

METHODS

Scoliosis was created in 24 Spanish cross-X female goats using a flexible, left posterior asymmetric tether from the T5 to L1 laminae, with convex rib resection and concave rib tethering from T8 to T13. After 8 weeks of posterior tethering, goats were randomized into 3 treatment groups: group 1, no treatment; group 2, anterior-shape memory alloy staple; and group 3, anterior ligament tether with bone anchor. The 6 levels of maximal curvature were instrumented in groups 2 and 3. All goats were observed for an additional 12-16 weeks. Serial radiographs and computerized tomography were used to document progression/correction of coronal, sagittal, and transverse plane deformities throughout the study.

RESULTS

There were 20 goats that had progressive, structural, idiopathic-type, lordoscoliotic curves convex to the right in the thoracic spine over the 8-week tethering period. An overall deformity score equaling the sum of the scoliosis, lordosis, and axial rotation measurements was calculated for each goat at 3 times.

CONCLUSION

The data in this study show the ability of a ligament tether attached to a bone anchor to correct scoliosis modestly in the coronal plane, but not in the sagittal or transverse plane. In addition, although a significant decrease in the deformity score was shown initially in this group (P < 0.001), the effect was lost over time. The final deformity in the bone anchor/ligament tether group wassignificantly less than either the stapled or untreated groups (P < 0.03). Further study is warranted to provide a better understanding of the 3-D effects of fusionless scoliosis treatments.

Less invasive posterior fixation method following transforaminal lumbar interbody fusion: a biomechanical analysis

BACKGROUND CONTEXT

Current surgical trends increasingly emphasize the minimization of surgical exposure and tissue morbidity. Previous research questioned the ability of unilateral pedicle screw instrumentation to adequately stabilize posterior fusion constructs. No study to date has addressed the effects of reduced posterior instrumentation mass on interbody construct techniques. Unilateral surgical exposure for transforaminal lumbar interbody fusion (TLIF) allows ipsilateral pedicle screw placement. Theoretically, percutanous contralateral facet screw placement could provide supplemental construct support without additional surgical exposure.

PURPOSE

Identify the biomechanical effects of reduced spinal fusion instrumentation mass on interbody construct stability.

STUDY DESIGN

An in vitro biomechanical study using human lumbar spines comparing stability of TLIF constructs augmented by: (1) bilateral pedicle screw fixation, (2) unilateral pedicle screw fixation, or (3) a novel unilateral pedicle screw fixation supplemented with contralateral facet screw construct.

METHODS

Seven fresh frozen human cadaveric specimens were tested in random construct order in flexion/extension, lateral bending, and axial rotation using +/-5.0 Nm torques and 50 N axial compressive loads. Analysis of torque rotation curves determined construct stability. Using paired statistical methods, comparison of construct stiffness and total range of motion within each specimen were performed using the Wilcoxon signed ranks test with a Holm-Sidák multiple comparison procedure (alpha=0.05).

RESULTS

In flexion/extension, lateral bending, and axial rotation, there were no measurable differences in either stiffness or range of motion between the standard bilateral pedicle screw and the novel construct after TLIF. After TLIF, the unilateral pedicle screw construct provided only half of the improvement in stiffness compared with bilateral or novel constructs and allows for significant off-axis rotational motions, which could be detrimental to stability and the promotion for fusion.

CONCLUSIONS

All tested TLIF constructs with posterior instrumentation decreased segmental range of motion and increased segmental stiffness. While placing unilateral posterior instrumentation decreases overall implant bulk and dissection, it allows for significantly increased segmental range of motion, less stiffness, and produces off-axis movement. The technique of contralateral facet screw placement provides the surgical advantages of unilateral pedicle screw placement with stability comparable to TLIF with bilateral pedicle screws.

Biomechanical analysis of a new concept: an add-on dynamic extension plate for adjacent-level anterior cervical fusion

STUDY DESIGN

A biomechanical study on a pneumatically controlled 7-axis spine simulator using Delron and human cadaveric spine models.

OBJECTIVES

To compare the biomechanical properties of an anterior cervical extension plate with comparable-length anterior cervical plates.

SUMMARY OF BACKGROUND DATA

Adjacent-level anterior cervical surgery next to a previously plated fusion can be technically challenging because of scarring from the previous surgery and the extensive exposure potentially required. An extension plate that attaches to an existing cervical plate could make adjacent-level cervical surgery less problematic.

METHODS

Flexibility and load sharing were tested in 18 delron models and 10 cadaveric spines. An extension plate (E-plate, Aesculap, Tüttlingen, Germany) at C5-C6 was attached to a single-level plate (ABC plate, Aesculap) at C4-C5 and compared with a two-level plate (ABC plate) spanning C4-C6. Segmental motion was monitored with optical tracking, and interbody graft load was measured with a load cell. Statistical analysis was performed with a Student's t test'.

RESULTS

In the delron models, both constructs displayed virtually 100% load sharing for the full interbody graft and subsidence models, and range of motion in flexion-extension or axial rotation was not statistically significantly different. The failure mode for the extension plate was lateral bending. In the cadaveric spines, load sharing, range of motion, and stiffness were not statistically significantly different between constructs.

CONCLUSIONS

This extension plate appears to be biomechanically equivalent to the ABC cervical plates with which it was compared in this study.

Superficial versus deep transfer of the posterior tibialis tendon

Transfer of the posterior tibialis tendon to the dorsum of the foot is a commonly performed procedure in conditions that weaken ankle dorsiflexors and evertors, resulting in equinovarus foot deformities. When transferring the tendon, surgeons have the choice of routing the tendon deep to the extensor retinaculum or superficial to it. This study compares the biomechanics of these two routing methods. Seven cadaveric lower limbs were tested by applying known forces to the transferred posterior tibialis tendon. Resultant kinematics indicated that passing superficial to the retinaculum resulted in a significantly more efficient motion than transfer deep to the retinaculum.

Functional trade-offs in the limb bones of dogs selected for running versus fighting

The physical demands of rapid and economical running differ from the demands of fighting in ways that may prevent the simultaneous evolution of optimal performance in these two behaviors. Here, we test an hypothesis of functional trade-off in limb bones by measuring mechanical properties of limb bones in two breeds of domestic dog (Canis lupus familiaris L.) that have undergone intense artificial selection for running (greyhound) and fighting (pit bull) performance. The bones were loaded to fracture in three-point static bending. To correct for the effect of shear, we estimated the shear stress in the cross section and added energy due to shear stress to the tensile energy. The proximal limb bones of the pit bulls differed from those of the greyhounds in having relatively larger second moments of area of mid-diaphyseal cross sections and in having more circular cross-sectional shape. The pit bulls exhibited lower stresses at yield, had lower elastic moduli and failed at much higher levels of work. The stiffness of the tissue of the humerus, radius, femur and tibia was 1.5-2.4-fold greater in the greyhounds than in the pit bulls. These bones from the pit bulls absorbed 1.9-2.6-fold more energy before failure than did those of the greyhounds. These differences between breeds were not observed in the long bones of the feet, metacarpals and metatarsals. Nevertheless, the results of this analysis suggest that selection for high-speed running is associated with the evolution of relatively stiff, brittle limb bones, whereas selection for fighting performance leads to the evolution of limb bones with relatively high resistance to failure.

The efficacy and integrity of shape memory alloy staples and bone anchors with ligament tethers in the fusionless treatment of experimental scoliosis

BACKGROUND

Scoliosis is a complex three-dimensional deformity with limited treatment options. Current treatments present potential problems that may be addressed with use of fusionless techniques for the correction of scoliosis. However, there are few data comparing the efficacy of different fusionless implant strategies in controlling scoliosis or on the integrity of rigid compared with flexible devices in an in vivo setting over time. The objective of this study was to compare the efficacy and integrity of rigid and flexible anterior thoracic tethers used to treat experimental scoliosis.

METHODS

Experimental scoliosis was created in twenty-four Spanish Cross-X female goats and was subsequently treated with either anterior shape memory alloy staples or anterior ligament tethers attached to bone anchors. Serial radiographs were analyzed to determine the efficacy of the implants in controlling scoliosis progression as well as the integrity of the implants at study completion. After the goats were killed, the implants were analyzed with use of three quantitative indices of implant integrity and implant pullout testing.

RESULTS

Over the treatment period, scoliosis progressed from 77.3 degrees to 94.3 degrees in the goats treated with staples and was corrected from 73.4 degrees to 69.9 degrees in the goats treated with bone anchors, with loosening of eighteen of forty-two staples (two of the eighteen dislodged) and evidence of drift in two of forty-nine anchors. Histologic sections revealed a consistent halo of fibrous tissue around the staple tines but well-fixed bone anchors at all sites. Pullout testing demonstrated that bone anchors had greater strength than staples initially and at the study completion, with an increase in bone anchor fixation over the course of the study.

CONCLUSIONS

In this scoliosis model, the flexible ligament tethers attached to bone anchors demonstrated greater efficacy and integrity than the more rigid shape memory alloy staples.

Fusionless scoliosis correction using a shape memory alloy staple in the anterior thoracic spine of the immature goat

STUDY DESIGN

Experimental scoliosis was created in goats and then treated using anterior thoracic stapling.

OBJECTIVE

To correct, without fusion, a progressive idiopathic-type scoliotic deformity in an immature goat model using a shape memory alloy staple.

SUMMARY OF BACKGROUND DATA

Fusionless scoliosis treatment techniques, using minimally invasive approaches to the anterior thoracic spine, provide theoretical advantages over currently available forms of treatment.

METHODS

Experimental scoliosis was created in 40 goats using a posterior asymmetric tether with convex rib resection and concave rib tethering for a period of up to 15 weeks. Twenty-seven goats with progressive deformities were used for subsequent study and randomized into 4 treatment groups: group I, anterior thoracic stapling with removal of the posterior tether; group II, removal of the posterior tether only; group III, anterior thoracic stapling with persistent posterior tethering; and group IV, persistent posterior tethering with no treatment. The treatment period lasted an additional 6 to 14 weeks. Staple backout was graded radiographically. After killing the goats, histology and disc biochemistry analyses were conducted.

RESULTS

The goats in group I corrected from an initial 57 degrees of curvature to 43 degrees over the duration of the treatment period. Group II goats, which served as a control for group I, corrected from 67 degrees to 60 degrees during the treatment period. Group III goats demonstrated a modest correction from 65 degrees to 63 degrees with the stapling procedure, whereas group IV goats (controls for group III) progressed from 55 degrees to 67 degrees with a persistent posterior tether during the treatment period. The difference between the correction in group III and progression in group IV was statistically significant (P = 0.002). Complications were limited to partial staple backout in 27% of 56 staples.

CONCLUSIONS

The results of this study support the efficacy of an anterior thoracic staple in correcting moderately severe scoliosis and halting the progression of more malignant scoliosis without fusion in a goat model.

Determining relevance of a weight-bearing ovine model for bone ingrowth assessment

A weight-bearing ovine model was used to quantify cancellous bone ingrowth and remodeling in porous-coated implants over 6, 12, and 24 weeks in situ. The null hypothesis for the investigation was that there would be no significant difference between the amount of cancellous bone ingrowth and rate of remodeling in this ovine model compared to a reported human bilateral implant model. Bone ingrowth progressed from 20.1 +/- 8.2% at 6 weeks in situ to 23.8 +/- 7.9% at 12 weeks, and 30 +/- 5.1% at 24 weeks. Fluorochrome analysis demonstrated a mineral apposition rate of 1.07 +/- 0.28 microm/day for bone at the porous-coating interface, whereas host bone remodeling at 0.89 +/- 0.23 microm/day. Histological analysis showed no adverse tissue or inflammatory response. The null hypothesis was supported in that regression analysis demonstrated that the amount of cancellous bone ingrowth over time (p = 0.545) and mineral apposition rate over time (p = 0.089) in this ovine model was not significantly different than reported human bilateral knee data. The results of this study appear to validate the ovine model for use in understanding skeletal attachment of porous-coated implants to cancellous bone in humans.

Experimental scoliosis in an immature goat model: a method that creates idiopathic-type deformity with minimal violation of the spinal elements along the curve

STUDY DESIGN

Experimental scoliosis was created in an immature goat model.

OBJECTIVES

To create a progressive, structural, idiopathic-type, lordoscoliotic curve convex to the right in the thoracic spine of the immature goat while maintaining the anterior and posterior elements of the spine along the maximal curve in a pristine state.

SUMMARY OF BACKGROUND DATA

Progressive scoliotic curves in a small animal model have been created using a posterior asymmetric tether. However, attempts in larger animal models have had less success and typically required violation of the spinal elements.

METHODS

Scoliosis was created in 40 Spanish Cross X female goats (age 1-2 months, weight 8-12 kg) using a left posterior asymmetric tether from T5 to L1. Convex rib resection and concave rib tethering from T8 to T13 were performed before compression across the rigid construct. Goats were followed over a 6- to 15-week period with serial radiographs to document progression of the deformity.

RESULTS

Of the 40 goats that underwent posterior assymetric tethering with rib procedures, 7 (18%) encountered substantial complications (five deaths and two neurologic injuries). Of the 33 available for analysis, 27 goats (82%) developed progressive, structural, idiopathic-type, lordoscoliotic curves in convex to the right in the thoracic spine. All curves demonstrated characteristic radiographic features of idiopathic scoliosis including significant displacement of the apical vertebra from the midline, wedging of both the vertebral bodies and discs, rotation, and decreased flexibility. Initial scoliosis after posterior asymmetric tethering measured 42 degrees on average (range 33-50 degrees ) and progressed to 60 degrees on average (range 44-73 degrees ) over 6 to 15 weeks. The average progression of +18 degrees (range 6-37 degrees ) was statistically significant (P < 0.001).

CONCLUSIONS

This study establishes an experimental model for scoliosis that creates progressive, structural, idiopathic-type, lordoscoliotic curves convex to the right in the thoracic spine of the immature goat with high statistical significance (P < 0.001). In addition, this method of experimental scoliosis creation avoids violation of the spinal elements throughout the maximal portion of the curve providing an ideal opportunity subsequent study of the deformity.

Effects of initial graft tension on knee stability after anterior cruciate ligament reconstruction using hamstring tendons: a cadaver study

PURPOSE

Tension degradation within hamstring grafts and anterior knee laxity were analyzed in a cadaveric anterior cruciate ligament (ACL) reconstruction model undergoing cyclic motion. It was hypothesized that suture fixation of a hamstring graft would lose tension during cycling initially and then stabilize, and that anterior knee laxity would increase as tension was lost. Hamstring grafts fixed under 3 different loads were evaluated to determine how initial graft tension affected knee laxity after cyclic motion.

TYPE OF STUDY

Cadaveric biomechanical analysis.

METHODS

Eighteen pairs of fresh-frozen hamstring tendons were tested on 2 cadaveric knees undergoing ACL reconstruction. The hamstring pairs were separated equally and randomly into one of 3 tension groups: 68 N (15 lb), 45 N (10 lb), and 23 N (5 lb). The loads were applied to the graft at 30 degrees of flexion, and the grafts were secured to the tibia with a suture and post technique. The knee was then cycled 1,000 times using an Instron machine (Instron, Canton, MA) through a range of motion between 0 degrees to 90 degrees. Constant monitoring and recording of graft tension was performed. A KT-1000 (Medmetrics, San Diego, CA) was performed (1) on the intact knee, (2) after ACL excision, (3) after ACL reconstruction and initial graft fixation, and (4) at the completion of the 1,000 cycles. An analysis of variance test was used to evaluate data.

RESULTS

The tension within the grafts after 1,000 cycles decreased to 34.5 N (7.6 lb), 16.8 N (3.7 lb), and 15.4 N (3.4 lb) from the preloads of 68, 45, and 23 N, respectively (P <.05 in all cases). This represented an average decrease of 50.2% of the initial tension after 1,000 cycles. Manual-maximum KT testing of the intact knees was 5.8 +/- 0.3 mm, and after ACL excision was 13.2 +/- 0.9 mm. KT testing revealed 6.0 +/- 0.9 mm, 8.1 +/- 1.9 mm, and 8.9 +/- 1.1 mm of anterior translation after fixation in the tension groups of 68, 45, and 23 N, respectively. After 1,000 cycles, the translation increased to 7.8 +/- 1.0 mm, 10.5 +/- 1.9 mm, and 10.3 +/- 1.5 mm, respectively.

CONCLUSIONS

This study showed that initial graft tension decreases with cyclic loading, resulting in increased knee laxity. To restore anterior translation to within 3 mm of the native ACL condition after cyclic loading, approximately 68 N of initial tension is required using this fixation technique.