Linda M. McGrady

Biomechanical evaluation of anterior thoracolumbar spinal instrumentation.

Study Design A biomechanical study was designed to assess relative construct stabilities of modern anterior thorecolumbar instrumentations in a calf spine model with an anterior and middle column defect. Objectives The purpose is to compare the biomechanical stability of various anterior fixation devices in an unstable calf spine model. Summary of Background Data Modern types of anterior thoracolumbar instrumentations evolved to either rods or plates. Biomechanical properties and comparative studies of these instrumentations are lacking. Methods Twenty fresh calf spines (L2-L5) were used for the biomechanical tests. L2-L5) vertebrae were used to attach the loading and base frames, respectively. Specimens underwent nondestructive biomechanical tests performed using a three-dimensional motion measuring system. In each specimen, three different cases were tested; Intact spine, anterior fixation with an interbody graft after total discectomy and end-plate excision of L3-L4 disc, anterior fixation only with out the graft. Four antieror fixation, University Anteior Plating System, the Kaneda device, the Z-plate, and Taxes Scottish Rite Hospital system were used. Each device was tested on Five specimens. A polymethyl methacrylate block was grafting, and a fixation device was implanted with axial compression. Rotational angles of the L3-L4 segment stabilized by a fixation device and graft were normalized by the corresponding angles of the intact specimen to study the overall stabilizing effects. Results With the interbody graft and fixation devices, all showed significant stabilizing effects in flexion, extension, and lateral bending. All devices restored axial rotation stability to intact specimen, but only the Kaneda device restored the torsional stability beyond the intact specimen. No statistical differences in stabilizing effects in axial rotation were found between any of the tested devices. When the graft was removed,. the Kaneda device significantly decreased the motions in all directions compared with the intact motions whereas the University plato decreased the motions in flexion, extension, and lateral bending. The Texas Scottish Rite Hospital system was found to reduce the flexion and lateral bending motions significantly, and Z-plate decreased lateral bending motions only. Stablizing effects of the interbody graft were significant in lateral bendings for all devices. Additionally, the significant stabilizing role of the graft was noted in flexion and extension in Z-plate only. The graft did not significantly reduce the axial rotation motion in any instrumentations. Conclusions Modern anterior instrumentations for the thoracolumbar spine, such as the Kaneda device, Texas Scottish Rite Hospital system, Z-plate, and University plate, restored the stability in all motions when an interbody graft was inserted. The stability of fixalion devices revealed that the Kaneda device is the best, particularly in restoring the torsioral stability. The information on the relative stability provided by different instrumentations should help the spine surgeon in choosing the appropriate instrumentation for the particular circumstance.

Biomechanical evaluation of anterior and posterior fixations in an unstable calf spine model

STUDY DESIGN Fresh calf lumbar spines were used to perform flexibility tests in multiple loading directions to compare the stabilizing effects of anterior and posterior rigid instrumentations. OBJECTIVE To compare the biomechanical flexibility of anterior and posterior instrumentation constructs using an unstable calf spine model. SUMMARY OF BACKGROUND DATA Unstable burst fractures of the thoracolumbar spine can be managed anteriorly or posteriorly. Controversy persists, however, on the merit of anterior fixation versus that of posterior fixation in terms of how much stability can be achieved. METHODS Fifteen fresh calf spines (L2-L5) were loaded with pure unconstrained moments in flexion, extension, axial rotation, and lateral bending directions. After removal of L3-L4 disc and endplates to create an 1.5-cm anterior and middle column defect, testing was performed on five specimens after anterior Kaneda rod fixation, anterior University Plate fixation, or posterior ISOLA pedicle screw fixation (AcroMed, Cleveland, OH). Testing was repeated after inserting a polymethylmethacrylate block to stimulate an interbody anterior graft with instrumentation. RESULTS All fixation devices provided a significant stabilizing effect in flexion and lateral bending. In extension, all constructs except ISOLA (AcroMed) without graft were stiffer than the intact specimen. In axial rotation with no graft, only the Kaneda device significantly reduced the flexibility from that of the intact specimen. The interbody graft provided additional rigidity to the ISOLA (AcroMed) instrumentation construct in flexion and extension and to the Kaneda construct in lateral bending. There was no significant effect of grafting in axial rotation. CONCLUSIONS A short, transpedicular instrumentation, such as ISOLA (AcroMed), provided less rigid fixation in flexion and extension without the anterior structural graft. The Kaneda rod and University plate with grafting provided a significant stabilizing effect in all directions compared with the intact specimen. When no graft was inserted, the Kaneda device was more effective in preventing axial rotation than the other devices. In lateral bending, the University plate provided more rigid fixation than the Kaneda device without grafting.

Strength of anterior vertebral screw fixation in relationship to bone mineral density

A biomechanical study was performed to investigate the relationship among the pullout strength of anterior vertebral screw fixation, bone mineral density (BMD) of the vertebral body, screw insertion torque, and width of the vertebral body. BMD was measured using a dual energy x-ray absorptiometry unit. Screw insertion torque and width of the lumbar vertebrae were also measured before the pullout test. Pullout strength was significantly correlated with BMD (r = 0.85) and screw insertion torque (r = 0.47), but not with width of the vertebral body. Multiple regression analyses demonstrated better correlation between pullout strength and the other parameters (r = 0.886). A stepwise regression analysis showed that BMD is the most significant predictor of the pullout strength followed by width of the vertebral body, whereas screw insertion torque is not a significant predictor. Furthermore, the mean pullout strength of the grade I osteoporosis group was significantly greater than that of grade II or III osteoporosis group.

The Effect of Dynamic Femoroacetabular Impingement on Pubic Symphysis Motion A Cadaveric Study

Background: A link between femoroacetabular impingement and athletic pubalgia has been reported clinically. One proposed origin of athletic pubalgia is secondary to repetitive loading of the pubic symphysis, leading to instability and parasymphyseal tendon and ligament injury. Hypothesis/Purpose: The purpose of this study was to investigate the effect of simulated femoral-based femoroacetabular impingement on rotational motion at the pubic symphysis. The authors hypothesize that the presence of a cam lesion leads to increased relative symphyseal motion. Study Design: Controlled laboratory study. Methods: Twelve hips from 6 fresh-frozen human cadaveric pelvises were used to simulate cam-type femoroacetabular impingement. The hips were held in a custom jig and maximally internally rotated at 90° of flexion and neutral adduction. Three-dimensional motion of the pubic symphysis was measured by a motion-tracking system for 2 states: native and simulated cam. Load-displacement plots were generated between the internal rotational torque applied to the hip and the responding motion in 3 anatomic planes of the pubic symphysis. Results: As the hip was internally rotated, the motion at the pubic symphysis increased proportionally with the degrees of the rotation as well as the applied torque measured at the distal femur for both states. The primary rotation of the symphysis was in the transverse plane and on average accounted for more than 60% of the total rotation. This primary motion caused the anterior aspect of the symphyseal joint to open or widen, whereas the posterior aspect narrowed. At the torque level of 18.0 N·m, the mean transverse rotation in degrees was 0.89° ± 0.35° for the native state and 1.20° ± 0.41° for cam state. The difference between cam and the native groups was statistically significant (P < .03). Conclusion: Dynamic femoroacetabular impingement as caused by the presence of a cam lesion causes increased rotational motion at the pubic symphysis. Clinical Relevance: Repetitive loading of the symphysis by cam impingement is thought to lead to increased symphyseal motion, which is one possible precursor to athletic pubalgia.

Does anterior plating of the cervical spine predispose to adjacent segment changes

Study Design. In a human cadaveric model, the effects of plate supplementation on the mechanical behaviors of adjacent segments were investigated. Objectives. The objective was to determine the effects of anterior cervical fusion and plating on the adjacent segments. Summary of Background Data. Increases in intradiscal pressure and intervertebral motion at adjacent segments have been reported in the lumbar spine following an instrumented fusion. It is unclear if the same phenomenon presents in the cervical spine. Methods. Seven human cadaveric cervical spines (C2–T1) were used, and fusion of the C5–C6 segment was chosen for the purpose of this study. Two miniature pressure transducers were implanted within each adjacent disc. Flexion, extension, lateral bending, and torsion loads up to 2.5 Nm were applied to the intact spine, and following each of the two procedures, anterior discectomy and grafted fusion, and anterior plating of the C5–C6 motion segment. Results. At the surgical level, a significant increase in segmental stiffness was observed after plating in all directions. Following the grafted fusion, there were no statistically significant changes at the superior adjacent segment, and there was a 13.7% increase in axial rotation in the inferior adjacent segment. Once anterior plating was applied, slight increase (<12%) over the intact spines was noted in lateral bending in both adjacent segments. However, there was no significant difference between the grafted fusion and anterior plated fusion at either adjacent segment. At both adjacent disc levels, the differences in intradiscal pressures between grafted fusion and plated fusion were less than 30% in all directions, and none of these differences was statistically significant. Conclusions. Intradiscal pressures and intervertebral motion at the adjacent levels are not significantly affected by the instrumented anterior fusion. The clinically observed degenerative change at adjacent segments in the cervical spine is more likely to be attributed to natural progression of the spondylotic process as opposed to biomechanical effect of the instrumentation or fusion.

Intradiscal pressure and kinematic behavior of lumbar spine after bilateral laminotomy and laminectomy

BACKGROUND CONTEXT Bilateral laminotomy has been proposed as an alternative to laminectomy for decompression of lumbar spinal stenosis. Preservation of the posterior midline ligaments with laminotomy is presumed to maintain spinal segment stability. There have been no previous studies that directly compare the amount of destabilization and increase in disc pressures between the two procedures. PURPOSE To quantify spinal segmental instability caused by bilateral laminotomy and laminectomy, and to compare the central and peripheral intradiscal pressures after the two procedures. STUDY DESIGN/SETTING Mechanical testing of the lumbar motion segments of calf spines. METHODS Nine fresh calf spines were tested under flexion, extension, lateral bending and axial rotation, intact first, then after laminotomy and laminectomy at the level of L4-L5. Four miniature pressure transducers were implanted in the central and peripheral disc at L4-L5 to measure intradiscal pressures. Three-dimensional motion was measured with motion analysis system. RESULTS Comparing with bilateral laminotomy, laminectomy showed significant increase in segmental motion at the surgical level in flexion (16%, p<.05), extension (14%, p<.04) and right axial rotation (23%, p<.03). In flexion, the stress at the anterior annulus increased a nonsignificant 20% after laminotomy, but significant 130% after laminectomy (p<.02). In the intact spine, the posterolateral annulus experienced the highest stress with lateral bending to the same side when compared with other loading directions. This stress remained unchanged after laminotomy but increased 9% after laminectomy (p<.06). In rotation, axial intradiscal stresses were evenly distributed and unchanged after each procedure. CONCLUSIONS Laminectomy causes more destabilization of a spinal motion segment than laminotomy and significantly increases disc stress in the anterior annulus.

A noninvasive, three-dimensional spinal motion analysis method.

Study Design. A three‐dimensional, noninvasive motion analysis method was developed by monitoring the orientation of the principal axes of each vertebra. Objectives. To develop a method of performing three‐dimensional, noninvasive motion analysis of the spine using computed tomography data. Summary of Background Data. The concept of using principal axes of the moment of inertia tensor to measure the orientation and position of a rigid body has been applied to the wrist and subtalar joints, but has not yet been applied to the spine. Methods. Scans were taken of two isolated vertebrae in various known positions. Centroids, area, moments, and product of inertia of each scan were determined using a commercial program. Custom software combined data using the parallel axis theorem to give three‐dimensional data for each vertebra. Changes in the centroid and principal axes were used to calculate translation and rotation, respectively. Results. The system accuracy was within 1.0° in rotation and 1.0 mm in translation. Some errors occurred in minor motions when a smaller number of scans were used. System resolution was 0.43 mm. Conclusions. A system has been developed capable of calculating three‐dimensional spinal motion based on measurements of a series of computed tomography images. The system has an accuracy similar to that of current motion analysis methods, but future studies will be necessary to apply this system in vivo.

The ideal amount of lumbar foraminal distraction for pedicle screw instrumentation.

Study Design A cadaveric study was performed to analyze the changes of foraminal dimensions of the lumbar spine and also to estimate the amount of foraminal distraction, achieved with pedicle screw instrumentation, that will create the greatest improvement in foraminal dimensions. Objectives To establish the ideal amount of lumbar foraminal distraction using pedicle screw instrumentation and to investigate the changes of adjacent segments and lumbar lordosis with distraction maneuvers. Summary of Background Data In patients undergoing fusions and instrumentation for foraminal stenosis, distraction of the lumbar spine may improve the dimensions of the intervertebral foramen. Results of previous studies have shown that anterior interbody distraction of the lumbar spine significantly increased the space in the lumbar foramen. No previous study has assessed the ideal amount of distraction, changes in the adjacent segments, or the effect on lumbar lordosis using posterior pedicle screw instrumentation. Methods Six fresh frozen human cadaveric spines from T5 to the sacrum were used. The ISOLA instrumentation system (Acromed Inc., Cleveland, OH) was applied using pedicle screws at L4, L5, and S1. L4‐L5 and L5‐S1 rods were contoured to the same degree of lumbar lordosis as in the specimen. The spine was potted in the upright position with the sacrum secured in the mold, and an axial load of 40 pounds was applied to the specimen. Radiographs were used to measure the posterior disc height, foraminal height, foraminal area, and lumbar lordosis. Two mm, 4 mm, 6 mm, 8 mm, and 10 mm of single level distraction were added to L4‐L5 and L5‐S1. The dimensions of the specimen were digitized and calculated after each distraction. Results The posterior disc height, foraminal height, and foraminal area increased with distraction. The greatest incremental change in the foraminal area was noted at 6 mm of distraction. The L4‐L5 foraminal area increased 22.6% at 6 mm of L4‐L5 distraction, and the L5‐S1 foraminal area increased 39.2% at 6 mm of L5‐S1 distraction. With distraction above 6 mm, the foraminal change plateaued. The greatest decrease in lumbar lordosis was only 4.6° and occurred at 10 mm of distraction. There was no significant effect of distraction on the foramina of adjacent levels. Conclusion Posterior distraction using pedicle screws increased foraminal dimensions. The ideal amount of distraction on the rod, or the minimal amount of distraction necessary for improving foraminal dimensions, may be 6 mm using pedicle screw instrumentation.

Prediction of Fatigue Screw Loosening in Anterior Spinal Fixation using Dual Energy X-ray Absorptiometry

Study Design A biomechanical study was performed to investigate a relation between the bone mineral density of the vertebral body and the number of loading cycles to induce fatigue loosening of an anterior vertebral screw. Objectives The Objective of this study was to investigate the potential usefulness of dual energy x-ray absorptiometry for measuring bone mineral density of the vertebral body in predicting the fatigue loosening of the anterior vertebral screw. Summary of Background Data Loosening of the vertebral body screw is a well known failure in spinal in strumentation, and is more commonly observed than pullout failure. The relation between bone mineral density and pullout strength of the screw has been investigated previously, but no studies are available on the fatigue loosening in anterior spinal fixation. Methods Bone mineral density was measured using dual energy x-ray absorptiometry and the screw loosening was produced by a cyclic loading in the cephalad-caudal direction. Screw loosening was defined as 1mm displacement of the screw relative to bone, and the number of loading cycles to induce the screw loosening was obtained and statistically correlated with bone mineral density. Results There was a positive correlation between the number of loading cycles to induce screw loosening and bone mineral density (R=0.8, P < 0.01). The average number of loading cycles to induce screw loosening was significantly less for specimens with bone mineral density < 0.45 g/cm2 compared to those with bone mineral density < 0.45 g/cm2. Conclusion These findings suggest that bone mineral density may be a good predictor of anterior vertebral screw loosening. Bone mineral density <0.45 g/cm2 may be critical value for loosening of the anterior vertebral body screw. However further biomechanical and clinical studies are required before using this threshold value clinically.

In vitro evaluation of shoulder external rotation after a Bankart reconstruction.

The Bankart procedure is a commonly performed an terior shoulder reconstruction that may shorten the anterior capsule. In this study, biomechanical testing was performed to evaluate the effect of the classic Bankart reconstruction on external rotation of the shoulder. The procedure was performed on six fresh- frozen cadaveric specimens that were free of intraar ticular pathologic changes. With the arm in 0° of ab duction, resisting torques about the vertical axis were measured while the humerus was externally rotated. The intact specimens demonstrated a nonlinear load displacement behavior in which little resisting torque was measured until a moderate amount of external rotation was applied. Shortening the capsule by 2 mm resulted in a 40% decrease in the low-tension zone of external rotation, and shortening the capsule an addi tional 5 mm resulted in a 67% decrease in the low- tension zone. This study demonstrates that the normal shoulder capsule is quite sensitive to anterior capsular shortening and suggests that care must be exercised when performing a Bankart reconstruction to avoid overtightening this structure.