The contribution of the three columns of the spine to rotational stability. A biomechanical model

Lateral-Opening Injection Tool Used in Percutaneous Vertebroplasty to Treat Asymptomatic Osteoporotic Vertebral Burst Fractures: A Retrospective Study

OBJECTIVE

The adequate management of asymptomatic osteoporotic vertebral burst fractures (OVBFs) was still controversial. Percutaneous vertebroplasty (PVP) could achieve quick recovery with minor trauma, but there were certain safety problems by traditional bone cement injection method. Thus, the aim of this study was to assess the efficacy of lateral-opening injection tool used in PVP treating patients with asymptomatic OVBFs.

METHODS

This was a retrospective study of OVBFs treated in our institute from March 2016 to March 2020. A total of 66 patients (mean age 72.10 ± 7.98 years, with 21 men and 45 women) who were diagnosed with acute asymptomatic OVBFs with mild spinal canal compromise were treated with PVP by using a lateral-opening injection tool. Two puncture needles were simultaneously placed transpedicularly in the fractured vertebra, and the inner core was removed, and the lateral-opening injection tool was inserted. The adjustment of lateral hole was to improve the distribution height of bone cement and avoid the entry of bone cement into the posterior wall of vertebral body. Related clinical outcomes and images were assessed, including back pain (visual analog scale [VAS]), vertebral height ratio (fractured vertebral height/average adjacent nonfractured vertebral height), kyphosis Cobb angle, union of the fractured vertebral posterior wall, distribution of bone cement, surgical data, and complications.

RESULTS

The average follow-up time of all cases was 21.23 ± 9.35 months. The mean amount of bone cement was 3.28 ± 0.35 ml in the vertebrae and the mean operative time was 34.02 ± 5.23 min. There were 60 cases of bone cement that contacted the upper and lower endplates on at least one side. There was no cement leakage into the spinal canal or fracture displacement of the posterior wall of the vertebral body in all cases. The VAS scores were 3.78 ± 0.42 at 1 day postoperatively and 0.53 ± 0.40 at the last follow-up, significantly lower than 8.40 ± 0.48 preoperatively (p < 0.05). The average height ratio of anterior, middle, and posterior vertebral body after operation increased compared with that pre-operation (p < 0.05), and the postoperative kyphosis angle decreased (p < 0.05). At 6 months follow-up, there was no significant height loss of the vertebral body. Computed tomography examination 3 months postoperatively showed that the fracture of posterior vertebral wall healed well in all cases. There were seven cases of bone cement leakage without clinical symptoms and two adjacent vertebral fractures caused by falling. There were no cases of deep vein embolism, lower limb muscle atrophy, pneumonia, decubitus.

CONCLUSION

The lateral opening tool can be safely and effectively used in the PVP treatment on asymptomatic OVBFs with mild spinal canal compromise.

REVIEW OF MCCORMACK CLASSIFICATION FOR THORACOLUMBAR SEGMENT FRACTURES

ABSTRACT Objective: The aim of this study was to evaluate the success and complications rates of thoracolumbar fractures with a score equal or superior to 7 according to the load sharing classification (McCormack), surgically treated with short posterior fixation at three points. Methods: We evaluated 40 patients with thoracolumbar fractures and score by the load sharing classification greater than or equal to 7, treated exclusively by surgery by short posterior fixation. We assessed epidemiological data, increased kyphosis, loss of vertebral body height and complications at 2-year follow-up. Results: Although there was a statistically significant increase in kyphosis and a decrease in vertebral height, there was no clinical repercussion. Conclusion: We conclude that the McCormack classification is not a fundamental predictor for indication of anterior approach complementary to the short posterior fixation.

A Biomechanical Assessment of Kyphoplasty as a Stand-Alone Treatment in a Human Cadaveric Burst Fracture Model

STUDY DESIGN

In vitro biomechanics study.

OBJECTIVE

To determine whether kyphoplasty is an adequate stand-alone treatment for restoring biomechanical stability in the spine after experiencing high-energy vertebral burst fractures.

SUMMARY OF BACKGROUND DATA

Kyphoplasty in the treatment of high-energy vertebral burst fractures has been shown by previous studies to significantly improve stiffness when used in conjunction with pedicle screw instrumentation. However, it is not known whether kyphoplasty as a stand-alone treatment may be an acceptable method for restoring biomechanical stability of a spinal motion segment post-burst fracture while allowing flexibility of the motion segment through the intervertebral discs.

METHODS

Young cadaveric spines (15-50 yr old; 3 males and 1 female; bone mineral density 0.27-0.31 gHA/cm) were divided into motion segments consisting of 3 intact vertebrae separated by 2 intervertebral discs (T11-L1 and L2-L4). Mechanical testing in axial, flexion/extension, lateral bending, and torsion was performed on each specimen in an intact state, after an experimentally simulated burst fracture and postkyphoplasty. Computed tomography was used to confirm the burst fractures and quantify cement placement.

RESULTS

Between the intact and burst-fractured states significant decreases in stiffness were seen in all loading modes (63%-69%). Burst fracture increased the average angulation of the vertebral endplates 147% and decreased vertebral body height by an average of 40%. Postkyphoplasty, only small recoveries in stiffness were seen in axial, flexion/extension, and lateral bending (4%-12%), with no improvement in torsional stiffness. Large angular deformations (85%) and height loss (31%) remained postkyphoplasty as compared with the intact state.

CONCLUSION

Lack of overall improvement in biomechanical stiffness indicates failure of kyphoplasty to sufficiently restore stability as a stand-alone treatment after high-energy burst fracture. The lack of stability can be explained by an inability to biomechanically repair the compromised intervertebral discs.

LEVEL OF EVIDENCE

3.

Cement augmentation in vertebral burst fractures

As a result of axial compression, traumatic vertebral burst fractures disrupt the anterior column, leading to segmental instability and cord compression. In situations with diminished anterior column support, pedicle screw fixation alone may lead to delayed kyphosis, nonunion, and hardware failure. Vertebroplasty and kyphoplasty (balloon-assisted vertebroplasty) have been used in an effort to provide anterior column support in traumatic burst fractures. Cited advantages are providing immediate stability, improving pain, and reducing hardware malfunction. When used in isolation or in combination with posterior instrumentation, these techniques theoretically allow for improved fracture reduction and maintenance of spinal alignment while avoiding the complications and morbidity of anterior approaches. Complications associated with cement use (leakage, systemic effects) are similar to those seen in the treatment of osteoporotic compression fractures; however, extreme caution must be used in fractures with a disrupted posterior wall.

Anterior release generates more thoracic rotation than posterior osteotomy: a biomechanical study of human cadaver spines

STUDY DESIGN

Biomechanical testing of human cadaveric spines.

OBJECTIVE

To determine the effect of anterior and posterior anatomic structures on the rotational stability of the thoracic spine.

SUMMARY OF BACKGROUND DATA

Historically, large and/or stiff spinal deformities were treated with anterior release to facilitate correction. However, anterior release increases risks and requires a 2-part procedure. Recently, large or rigid deformities have been treated with a single posterior procedure using pedicle screws and spinal osteotomies. No study has yet evaluated the effect of anterior release or posterior osteotomy on thoracic spinal column rotation.

METHODS

Thoracolumbar spines were obtained from cadavers and segmented into upper, middle, and lower specimens. Specimens were cyclically loaded with a ±5 N·m moment in axial rotation for 10 cycles. Specimens were tested intact and then retested after sectioning or removal of each structure to simulate those removed during anterior release and posterior osteotomy. The total increases in axial rotation after posterior and anterior resections were calculated using a 3-dimensional motion capture camera system. For each ligament resection, the absolute and percent change in degrees of rotation was calculated from comparison with the intact specimen. The median data points were compared to account for outliers.

RESULTS

Resection of anterior structures was more efficacious than resection of posterior structures. An 8.8% to 71.9% increase in the amount of axial rotation was achieved by a posterior release, whereas resection of anterior structures led to a 141% to 288% increase in rotation. The differences between the anterior and posterior resections at all levels tested (T2-T3, T6-T7, and T10-T11) were significant (P < 0.05).

CONCLUSION

Anterior release generated significantly more thoracic rotation than posterior osteotomy in biomechanical testing of human cadaver spines.

LEVEL OF EVIDENCE

N/A.

Kyphoplasty as an alternative treatment of traumatic thoracolumbar burst fractures Magerl type A3

INTRODUCTION

Traumatic thoracolumbar spine fractures are frequently classified as burst fractures Magerl type A3. There still are many controversies regarding the treatment of this fracture. The therapeutic spectrum ranges from conservative to invasive operative methods with attendant morbidities. The minimal-invasive technique of kyphoplasty has established itself as a common treatment of osteoporotic vertebral compression fractures and is associated with a low complication rate. The aim of this study is to evaluate the functional and radiological results after kyphoplasty of traumatic thoracolumbar burst fractures.

PATIENTS AND METHODS

Patients with traumatic thoracolumbar fractures type A3.1, A3.2 and A3.3, who were treated with kyphoplasty, were included in this study. The clinical outcome was measured at follow up with a neurological assessment, the visual analogue pain scale (VAS), the Oswestry Disability Score (ODI) and the SF-36 Health Survey. The radiological measurements, performed on preoperative, postoperative and follow up radiographs, included the sagittal index, the wedge angle and the modified Cobb angle of Daniaux.

RESULTS

26 patients with 23 A3.1, one A3.2 and five A3.3 fractures were treated between 2004 and 2007, including five patients with multiple vertebral fractures. At follow up the Oswestry Disability Score (26.2%) and the SF-36 score (60.1%) assessed a moderately limitation of functional outcome and quality of life without any neurological deficits. Radiological measurements showed a postoperative height restoration and reduction of kyphosis, but at follow up a secondary loss of correction except in five cases. Six minor ventrocranial cement leakages without further clinical consequence were observed.

CONCLUSIONS

The present study showed that kyphoplasty is a safe and feasible method for the treatment of burst fractures. It allowed the correction of the kyphosis, stabilisation of the facture, pain reduction and early mobilisation.

Biomechanical evaluation of a posterolateral lumbar disc arthroplasty device: an in vitro human cadaveric model

STUDY DESIGN

This in vitro analysis defines biomechanical properties of the Triumph Lumbar Disc (TLD) for posterolateral lumbar disc arthroplasty.

OBJECTIVES

To quantify segmental kinematics afforded by the TLD, determine whether intervertebral positional changes of the device affect spinal kinematics, and compare kinematics following TLD reconstruction with historical CHARITE data.

SUMMARY OF BACKGROUND DATA

As an alternative to arthrodesis, total disc arthroplasty serves to restore the biomechanical properties of the spine. However, there are kinematic and clinical concerns regarding posterior lumbar column destabilization and reconstruction using motion-preserving technology.

METHODS

Seven human lumbosacral spines were biomechanically evaluated under the following conditions: (1) Intact; (2) Unilateral facetectomy; (3) Facetectomy and discectomy; (4) TLD-posterior; (5) TLD-central; (6) TLD-anterior. Centers of intervertebral rotation (COR) and intradiscal pressures were compared at the operative/adjacent levels.

RESULTS

In axial rotation, L4-L5 range of motion (ROM) increased to 110%+/-15.3% following unilateral facetectomy and 134.3%+/-26.9% following facet/discectomy. Implanting the TLD in 3 positions-posterior, central, and anterior-demonstrated no significant differences in ROM (P>0.05). Trends were similar in flexion/extension and lateral bending. The neutral zone increased under the facet/discectomy condition and TLD reconstructions compared to the intact condition (P<0.05). CORs were dispersed at the operative level following destabilization. TLD in posterior and central positions restored loci to near the intact condition, whereas the anterior position produced more diffuse CORs. Adjacent level intradiscal pressures decreased in flexion-extension following destabilization and reconstruction.

CONCLUSION

Axial rotation ROM following TLD reconstruction (central position) was 131.5% of intact, compared to historical controls for the CHARITE device (162.0%). This indicates that lumbar disc arthroplasty performed using a posterolateral approach may offer biomechanical advantages over the anterior approach. Moreover, this approach may minimize surgical complications and allow simultaneous decompression of the neural elements. Intervertebral positional changes of the TLD were not shown to affect spinal kinematics.

Thoracolumbar burst fractures: a systematic review of management

The management of thoracolumbar burst fractures remains challenging. Ideally, it should effectively correct the deformity, induce neurological recovery, allow early mobilization and return to work, and be associated with minimal risk of complication. This article reviews the related studies reporting their clinical data for the management of thoracolumbar burst fractures, discusses the most suitable approach in cases such as these, highlights specific treatment recommendations, and proposes a treatment algorithm. Using PubMed and Scopus databases to search the term thoracolumbar burst fractures, abstracts and original articles in English investigating the treatment of thoracolumbar burst fractures were searched and analyzed.

THORACOLUMBAR BURST FRACTURE: LOAD SHARING CLASSIFICATION AND POSTERIOR INSTRUMENTATION FAILURE

Objectives: To assess the relationship between sagittal collapse, posterior instrumentation failure and the McCormack classification among patients with thoracolumbar burst fractures who had undergone posterior arthrodesis more than one level above and one level below the fracture. Methods: This was a retrospective review based on the medical records and radiographs of 26 patients who underwent operations between January 1990 and December 2006. The parameters assessed were: worsening of kyphosis (Cobb), neurological dysfunction (Frankel) and failure of surgical treatment (implant breakage or loosening). Results: No relationship was found between sagittal collapse and the scoring in the McCormack classification (r = 0.221; P = 0.322). There was also no relationship between failure of the posterior implant and the scoring in this classification (p = 0.85). Conclusion: Our findings suggest that this classification is not applicable to patients who underwent arthrodesis more than one level above and one level below the fracture.

Axial plane lumbar responses after anterior selective thoracic fusion for main thoracic adolescent idiopathic scoliosis

Study Design

A retrospective radiographic study.

Purpose

To evaluate the axial plane lumbar responses after anterior selective thoracic fusion (STF) in patients with main thoracic adolescent idiopathic scoliosis (MT-AIS).

Overview of Literature

Anterior scoliosis surgery induces more MT derotation through disc preparation than posterior surgery.

Methods

Twenty-eight MT-AIS patients treated with STF were evaluated after a minimum follow-up (FU) of 2 years. The MT and lumbar coronal angles, as well as the MT and lumbar rotational angles at the most rotated vertebrae were measured.

Results

At the last FU, the MT coronal correction and derotation rates were 65% and 41%, respectively. The lumbar coronal correction rate was 61% but there was minimal lumbar derotation (2%). Nine cases were decompensated (coronal balance >10 mm). After surgery, the compensated and decompensated groups showed similar MT coronal and axial correction rates. During the FU, the MT and lumbar apecies rotated in the same direction (r=0.443). In addition, significant MT derotation occurred in the decompensated group with increasing lumbar rotational correction loss. At the last FU, while the MT coronal correction was similar between the two groups, there was more MT derotation in the decompensated group. Furthermore, the MT rotational change was strongly associated with the coronal C7 plumb line position (r=0.728).

Conclusions

After anterior STF in patients with MT-AIS, the final MT derotation is strongly associated with the coronal C7 plumb line position. During the FU, the excessive MT derotation in the decompensated group was attributed to excessive lumbar rotational correction loss.

Direct reduction of thoracolumbar burst fractures by means of balloon kyphoplasty with calcium phosphate and stabilization with pedicle-screw instrumentation and fusion

STUDY DESIGN

Prospective consecutive series.

OBJECTIVE

To evaluate the outcomes of the treatment of acute thoracolumbar burst fractures by transpedicular balloon kyphoplasty with calcium phosphate cement and posterior instrumented fusion.

SUMMARY OF BACKGROUND DATA

In the surgical treatment of thoracolumbar fractures, the major problem after posterior correction and transpedicular instrumentation is failure to support the anterior spinal column, leading to the loss of correction and instrumentation failure.

METHODS

Twenty-three consecutive patients with an average age of 48 years, who sustained thoracolumbar A3-type burst fracture with or without neurologic deficit were included in this prospective study. Twenty-one of 23 patients had single fractures and 2 had each one additional A1 compression contiguous fracture. On admission 5 (26%) of 23 patients had neurologic lesion (5 incomplete, 1 complete). Bilateral transpedicular balloon kyphoplasty was performed with quick hardening calcium phosphate cement to reduce segmental kyphosis and restore vertebral body height and supplementary pedicle-screw instrumentation [long including 4 vertebrae for T9-L1 fractures and short (3 vertebrae) for L2-L4 fractures]. Gardner kyphosis angle, anterior and posterior vertebral body height ratio, and spinal canal encroachment were calculated before to after surgery.

RESULTS

All 23 patients were operated within 2 days after admission and were followed for at least 24 months after index surgery. Operating time and blood loss averaged 70 minutes and 250 cc, respectively. The 5 patients with incomplete neurologic lesions improved by at least 1 American Spine Injury Association grade, whereas no neurologic deterioration was observed in any case. Overall sagittal alignment was improved from an average preoperative 16 degrees to 1 degrees kyphosis at final follow-up observation. The anterior vertebral body height ratio improved from 0.6 before surgery to 0.9 (P < 0.001) after surgery, whereas posterior vertebral body height was improved from 0.95 to 1 (P < 0.01). Spinal canal encroachment was reduced from an average 32% before surgery to 20% after surgery. No differences in preoperative values and postoperative changes in radiographic parameters between short and long group were shown. Cement leakage was observed in 4 cases: 3 anterior to vertebral body and 1 into the disc without sequela. In the last computed tomography evaluation, there was shown a continuity between calcium phosphate and cancellous vertebral body bone. Posterolateral radiological fusion was achieved within 6 to 8 months after index operation. There was no instrumentation failure or measurable loss of sagittal curve and vertebral height correction in any group of patients.

CONCLUSION

Balloon kyphoplasty with calcium phosphate cement secured with posterior long and short fixation in the thoracolumbar and lumbar spine, respectively, provided excellent immediate reduction of post-traumatic segmental kyphosis and significant spinal canal clearance and restored vertebral body height in the fracture level in an equal amount both in the short and the long instrumentation.

Effects of dorsal versus ventral shear loads on the rotational stability of the thoracic spine: a biomechanical porcine and human cadaveric study

STUDY DESIGN

A biomechanical in vitro study on porcine and human spinal segments.

OBJECTIVE

To investigate axial rotational stability of the thoracic spine under dorsal and ventral shear loads.

SUMMARY OF BACKGROUND DATA

Idiopathic scoliosis is a condition restricted exclusively to humans. An important difference between humans and other vertebrates is the fact that humans ambulate in a fully erect position. It has been demonstrated that certain parts of the human spine, more specifically the dorsally inclined lower thoracic and high lumbar parts, are subject to dorsally directed shear loads. It has been hypothesized that these dorsal shear loads reduce the rotational stability of the spine, thereby increasing the risk to initiate idiopathic scoliosis.

METHODS

Fourteen porcine and 14 human thoracic functional spinal units (FSUs) with intact costotransverse and costovertebral articulations were used for biomechanical testing. In both dorsal and ventral directions, shear loads were applied to the upper vertebra of the FSU in the midsagittal plane (centrally), and at 1 cm to the right and to the left (eccentrically), resulting in a rotary moment. Vertebral rotation was measured at 3 incremental loads by an automated optoelectronic 3-dimensional (3D) movement registration system.

RESULTS

The results of this study showed that eccentrically applied shear loads induce vertebral rotation in human as well as in porcine spinal segments. At the mid-thoracic and lower thoracic levels, significantly more vertebral rotation occurred under dorsal shear loads than under ventral shear loads.

CONCLUSION

These data show that, in humans and in quadrupeds, the thoracic spine is less rotationally stable under dorsal shear loads than under ventral shear loads.

Treatment of acute thoracolumbar burst fractures with kyphoplasty and short pedicle screw fixation: Transpedicular intracorporeal grafting with calcium phosphate: A prospective study

BACKGROUND

In the surgical treatment of thoracolumbar fractures, the major problem after posterior correction and transpedicular instrumentation is failure to support the anterior spinal column, leading to loss of correction and instrumentation failure with associated complaints. We conducted this prospective study to evaluate the outcome of the treatment of acute thoracolumbar burst fractures by transpedicular balloon kyphoplasty, grafting with calcium phosphate cement and short pedicle screw fixation plus fusion.

MATERIALS AND METHODS

Twenty-three consecutive patients of thoracolumbar (T(9) to L(4)) burst fracture with or without neurologic deficit with an average age of 43 years, were included in this prospective study. Twenty-one from the 23 patients had single burst fracture while the remaining two patients had a burst fracture and additionally an adjacent A1-type fracture. On admission six (26%) out of 23 patients had neurological deficit (five incomplete, one complete). Bilateral transpedicular balloon kyphoplasty with liquid calcium phosphate to reduce segmental kyphosis and restore vertebral body height and short (three vertebrae) pedicle screw instrumentation with posterolateral fusion was performed. Gardner kyphosis angle, anterior and posterior vertebral body height ratio and spinal canal encroachment were calculated pre- to postoperatively.

RESULTS

All 23 patients were operated within two days after admission and were followed for at least 12 months after index surgery. Operating time and blood loss averaged 45 min and 60 cc respectively. The five patients with incomplete neurological lesions improved by at least one ASIA grade, while no neurological deterioration was observed in any case. The VAS and SF-36 (Role physical and Bodily pain domains) were significantly improved postoperatively. Overall sagittal alignment was improved from an average preoperative 16° to one degree kyphosis at final followup observation. The anterior vertebral body height ratio improved from 0.6 preoperatively to 0.9 (P<0.001) postoperatively, while posterior vertebral body height improved from 0.95 to 1 (P<0.01). Spinal canal encroachment was reduced from an average 32% preoperatively to 20% postoperatively. Cement leakage was observed in four cases (three anterior to vertebral body and one into the disc without sequalae). In the last CT evaluation, there was a continuity between calcium phosphate and cancellous vertebral body bone. Posterolateral radiological fusion was achieved within six months after index operation. There was no instrumentation failure or measurable loss of sagittal curve and vertebral height correction in any group of patients.

CONCLUSIONS

Balloon kyphoplasty with calcium phosphate cement secured with posterior short fixation in the thoracolumbar spine provided excellent immediate reduction of posttraumatic segmental kyphosis and significant spinal canal clearance and restored vertebral body height in the fracture level.

Musculoskeletal injuries in tennis

Tennis is popular in the United States, with millions participating in the sport. Unlike many other sports, young, middle-aged, and elderly persons are actively playing tennis. Most injuries that are seen are secondary to overuse. Faulty stroke mechanics are often involved in the development of specific injuries seen in tennis players. Fortunately, most injuries respond to conservative treatment and do not require surgery. Most players are able to return to their previous level of activity after appropriate treatment.

Is fusion necessary for surgically treated burst fractures of the thoracolumbar and lumbar spine?: a prospective, randomized study

STUDY DESIGN

A prospective clinical trial was conducted.

OBJECTIVES

To compare the results of fusion versus nonfusion for surgically treated burst fractures of the thoracolumbar and lumbar spine.

SUMMARY OF BACKGROUND DATA

The operative results of surgically treated burst fractures with short segmental fixation have been well documented. There is no report comparing the results of fusion and nonfusion.

METHODS

Fifty-eight patients were included in this study, with the inclusion criteria as follows: neurologically intact spine with a kyphotic angle > or = 20 degrees, decreased vertebral body height > or = 50% or a canal compromise > or = 50%, incomplete neurologic deficit with a canal compromise 50%, complete neurologic deficit, and multilevel spinal injury or multiple traumas. All patients were randomly assigned to fusion or nonfusion groups, and operative treatment with posterior reduction and instrumentation was carried out. Posterior fusion with autogenous bone graft was performed for the fusion group (n = 30), and no fusion procedure was done for the nonfusion group (n = 28). The average follow-up period was 41 months (range, 24-71 months).

RESULTS

The average loss of kyphotic angle was not statistically significant between these 2 groups. The radiographic parameters were statistically significantly better in the nonfusion group, including angular change in the flexion-extension lateral view (4.8 degrees vs. 1.0 degrees), lost correction of decreased vertebral body height (3.6% vs. 8.3%), intraoperative estimated blood loss (303 mL vs. 572 mL), and operative time (162 minutes vs. 224 minutes). The scores on the low back outcome scale were not statistically significant for these 2 groups.

CONCLUSIONS

The short-term results of short segmental fixation without fusion for surgically treated burst fractures of the thoracolumbar spine were satisfactory. The advantages of instrumentation without fusion are the elimination of donor site complications, saving more motion segments, and reducing blood loss and operative time.

Kyphoplasty: Vertebral Augmentation for Compression Fractures

Osteoporosis has devastating consequences for individuals and society in terms of suffering, disability, and increased health care expenses. Early diagnosis and treatment of compression fractures with kyphoplasty allow restoration of normal anatomy as much as possible. Vertebral augmentations by kyphoplasty are efficacious treatments for osteoporotic compression fractures. Kyphoplasty minimizes the risk for cement leakage by compacting the cancellous bone to the periphery, sealing off the fracture clefts,and creating a cavity into which cement is poured. This technique may prevent propagation of further fractures by reducing the col-lapsed vertebral bodies toward its native height, normalizing the sagittal spinal alignment and the force transmission along the spine.

The use of interbody cage devices for spinal deformity: a biomechanical perspective

Spinal instrumentation has revolutionized the treatment of spinal deformities and offers a plethora of techniques and designs to surgically treat deformity conditions. The authors address the biomechanical properties afforded by various posterior and anterior spinal instrumentation mechanisms, with and without intervertebral reconstruction, and the principles associated with optimal reconstruction techniques. The integration of multiple strategies can improve anterior and posterior construct stiffness in the treatment of spinal deformities. Structural interbody support probably is the best method to minimize longitudinal rod and screw-bone interface strain. Moreover, anterior load-bearing structural grafts and interbody devices have been shown to increase construct stiffness, decrease the incidence posterior implant failure, permit the use of smaller diameter longitudinal rods, and may enhance the rate of successful spinal arthrodesis. From a biomechanical standpoint, treatment of medium to high-grade spondylolisthesis with stand-alone interbody or transvertebral cages, in the absence of supplemental posterior fixation, is contraindicated. Collectively, the included studies reinforce the principles of load sharing between the anterior and posterior spinal columns and affirm the biomechanical dominance of anterior column support in circumferential spinal arthrodesis.

Correlation and quantification of projected 2-dimensional radiographic images with actual 3-dimensional Y-axis vertebral rotations

BACKGROUND

Historically, measurement of 2-dimensional (2-D) radiographic images on the anteroposterior radiograph has been made to assess 3-dimensional (3-D) y-axis vertebral rotations.

OBJECTIVES

To correlate and quantify measurements of the projected 2-D radiographic image with the degree of 3-D y-axis rotation.

STUDY DESIGN

A computer model was positioned in a simulated x-ray beam. Points of model contact with the simulated beam were projected onto a line in the neutral position and the first 7 degrees of both positive and negative y-axis rotation using two different axes of rotation. A larger model, a shape-altered model, and a decreased source-object-distance model were also studied.

RESULTS

3-D y-axis rotation of vertebrae causes an off-center displacement of the 2-D projected lamina junction in relation to the projected vertebral body. The magnitude of displacement increases with increasing degrees of rotation. In our model, no clinically significant difference was found in the amount of the projected off-center displacement of the lamina junction between either of our two chosen axes of rotation. However, significant differences in the projected offset were found between vertebrae with the same degree of rotation as a result of changes in vertebral shape, size, and positioning. The projected lamina junction off-centering at a given rotation is quantified for our model.

CONCLUSION

Use of millimetric measurement of the projected lamina offset on the anteroposterior radiograph is an inaccurate method for the assessment of the degree of 3-D y-axis vertebral rotation.

Mechanical loading on the low back in three methods of refuse collecting

The mechanical loading on the low back was studied in three different current methods of refuse collecting: in polythene bags, two-wheeled mini-containers and large four-wheeled containers. To this end the most prominent activities of each collecting method were performed in a laboratory. On the basis of movement analysis, force measurements and biomechanical modelling, spinal compressive and shear forces were estimated. From these forces and from the frequency of activities during the working day (assessed in a preliminary field study) the low-back stress in each collecting method was evaluated. In the bag-method, peak forces when throwing the bags ranged from 3341 to 5179 N (average compression) and from 284 to 673 N (shear) among the different conditions studied. The act of picking up bags also showed rather high forces (exceeding the NIOSH limit for disc compression in most cases). The frequency of exposure to these forces in the field is rather high (workers pick up and throw on average 807 times each day). The mini-container method compares favourably to the bags method. Peak compressive and shear force in tilting/pushing and pulling mini-containers ranged from 1657 to 2654 N and from 123 to 248 N respectively. Also, the frequency of stressful events in the field is lower in this method. In the large container method extremely high peak forces (e.g. compression ranged from 4991 to 5810 N) were observed in the task of putting the empty container back from street level to sidewalk level (surmounting the kerb). The frequency of activities like pushing, pulling and lifting the large container in the field is much lower compared with activities in the other methods. On the basis of the frequency and magnitude of spinal forces it was concluded that the mini-containers should be preferred to the bags. If kerbs are removed at container places and tasks are performed by two instead of a single person, the large container method would form another good alternative to the stressful task of collecting refuse in bags.

Distraction and compression loads enhance spine torsional stiffness

We have assessed the degree to which compression and distraction forces applied to the spine of the magnitude achievable through instrumentation systems increase torsional stiffness, thereby possibly enhancing the environment for arthrodesis. A functional relationship between spine torsional stiffness and axial compression and distraction forces is described. To establish this relationship whole human thoracolumbar spines as well as individual motion segments were tested in torsion under a variety of axial loading conditions consistent with forces applied clinically. These studies indicate that applying axial loads to the whole thoracolumbar spine through the action of compression or distraction increases the stiffness of the spine in torsion. Compression and distraction forces increased the torsional stiffness of thoracic segments, but only compression forces were found to significantly increase the stiffness of lumbar segments. The soft tissue structures of both the anterior and the posterior columns have the ability to increase torsional stiffness with axial loading.

An anatomic basis for spinal instability: a porcine trauma model

To determine the anatomic basis for spinal instabilities, 16 porcine cervical spine specimens were subjected to a well-defined sagittal plane trauma. The multidirectional instability of each specimen was measured before and after trauma. Detailed anatomic dissections were performed on each traumatized specimen to quantitate the extent of injury to several distinct anatomic structures and columns. Multiple regression models were constructed to determine which anatomic structures and columns correlated best with each multidirectional instability. Flexion instability correlated best with injury to the interspinous/supraspinous ligaments and the ligamentum flavum. Extension instability correlated best with anterior longitudinal ligament and pedicle injury. Axial rotation instability correlated best with anterior disc-end-plate and capsular ligament injuries, while lateral bending instability correlated best with posterior disc-end-plate injuries. Anterior column injuries correlated best with extension, axial rotation, and lateral bending instabilities, while posterior column injuries correlated best with flexion instability. Finally, individual anatomic structural injuries had higher correlations with multidirectional instabilities than did the injuries defined by the anatomic columns.