Ron N. Alkalay

Prevention of postlaminectomy epidural fibrosis using bioelastic materials.

Study Design. The use of elastic protein-based polymers for the prevention of epidural fibrosis following lumbar spine laminectomy was investigated in a rabbit model. Objectives. To determine the safety and efficacy of two bioelastic polymers in matrix and gel forms as interpositional materials in preventing postlaminectomy epidural fibrosis. Summary of Background Data. Postlaminectomy epidural fibrosis complicates revision spine surgery and is implicated in cases of “failed back syndrome.” Materials employed as mechanical barriers to limit tethering of neural elements by the fibrosis tissue have met with little success. A recent family of protein-based polymers, previously reported to prevent postoperative scarring and adhesions, may hold promise in treating this condition. Methods. Sixteen female New Zealand White rabbits underwent laminectomy at L4 and L6. Two polymer compositions, each in membrane and gel forms, were implanted at a randomly assigned level in four rabbits each, with the remaining level serving as an internal control. The animals were killed at 8 weeks, and qualitative and quantitative histology and gross pathologic examination were performed for both the control and the experimental sites to assess the polymers’ efficacy in preventing dorsal epidural fibrosis. Results. The use of the polymers caused no adverse effects. Compared to the control sites, both polymers in either gel or membrane form significantly reduced the formation of epidural fibrosis and its area of contact with the dura postlaminectomy. However, no significant difference in efficacy was detected between either the polymers or their respective forms in preventing epidural fibrosis. Conclusions. The selected compositions of biosynthetic, bioelastic polymers were safe and effective in the limiting the direct contact and consequent tethering of the underlying neural elements by the postlaminectomy epidural fibrosis in rabbits.

In vivo evaluation of calcium sulfate as a bone graft substitute for lumbar spinal fusion

BACKGROUND CONTEXT Posterolateral fusions of the lumbar spine have nonunion rates as high as 35%. The availability of autologous bone to promote fusion is limited, particularly for multilevel fusions. Bone substitutes have been proposed to augment or replace autologous bone for spinal surgery. Calcium sulfate offers high porosity, osteoconductivity, and high resorption rate. This material has been used successfully for treatment of long bone defects but has not been investigated as a bone graft substitute for spinal fusions. PURPOSE To determine whether the use of calcium sulfate granules in conjunction with an implantable electrical stimulator is a safe and effective means of attaining spinal fusion. STUDY DESIGN/SETTING A rabbit lumbar fusion model was used to assess a calcium sulfate bone graft substitute in combination with electrical stimulation for spinal fusion. METHODS Thirty-six adult New Zealand White female rabbits were divided into three groups. Each group underwent a single-level (L5-L6) fusion, receiving 3.0 cc calcium sulfate granules with bone marrow aspirate from the iliac crest. Group 1 had no electrical stimulator applied. Groups 2 and 3 received a 40-microA (Group 2) or a 100-microA (Group 3) implantable electrical stimulator. The animals were sacrificed at 8 weeks, and the rabbit spines were subjected to radiographic assessment, manual palpation, and mechanical testing. RESULTS Two rabbits died postoperatively. The radiographic assessment revealed no fusions occurred at the adjacent nonoperated control levels (L4-L5). There were no fusions observed within Group 1, containing the calcium sulfate and bone marrow aspirate alone. The sites with the implantable stimulators showed a dose-dependent increase in fusion stiffness. However, no fusion mass in Group 2 or 3 was graded as bilaterally complete. CONCLUSION This study found that calcium sulfate as a bone graft substitute was unsuccessful in promoting spine fusion in a rabbit model. There was radiographic evidence of rapid resorption of the calcium sulfate within 4 weeks after surgery. The use of electrical stimulation created a dose-dependent increase in mechanical competence of the bony mass. However, the addition of direct current (DC) current did not significantly alter fusion rates with calcium sulfate used as the bone graft substitute in this model.

Preventing distal pullout of posterior spine instrumentation in thoracic hyperkyphosis: a biomechanical analysis.

Study Design An in vitro biomechanical study. Objective Compare the mechanical behavior of 5 different constructs used to terminate dual-rod posterior spinal instrumentation in resisting forward flexion moment. Summary of Background Data Failure of the distal fixation construct can be a significant problem for patients undergoing surgical treatment for thoracic hyperkyphosis. We hypothesize that augmenting distal pedicle screws with infralaminar hooks or sublaminar cables significantly increases the strength and stiffness of these constructs. Methods Thirty-seven thoracolumbar (T12 to L2) calf spines were implanted with 5 configurations of distal constructs: (1) infralaminar hooks, (2) sublaminar cables, (3) pedicle screws, (4) pedicle screws+infralaminar hooks, and (5) pedicle screws+sublaminar cables. Progressive bending moment was applied to each construct until failure. The mode of failure was noted and the constructs stiffness and failure load determined from the load-displacement curves. Results Bone density and vertebral dimensions were equivalent among the groups (F=0.1 to 0.9, P>0.05). One-way analysis of covariance (adjusted for differences in density and vertebral dimension) demonstrated that all of the screw-constructs (screw, screw+hook, and screw+cable) exhibited significantly higher stiffness and ultimate failure loads compared with either sublaminar hook or cable alone (P<0.05). The screw+hook constructs (109±11 Nm/mm) were significantly stiffer than either screws alone (88±17 Nm/mm) or screw+cable (98±13 Nm/mm) constructs, P<0.05. Screw+cable construct exhibited significantly higher failure load (1336±328 N) compared with screw constructs (1102±256 N, P<0.05), whereas not statistically different from the screw+hook construct (1220±75 N). The cable and hook constructs failed by laminar fracture, screw construct failed in uniaxial shear (pullout), whereas the screws+(hooks or wires) failed by fracture of caudal vertebral body. Conclusions Posterior dual rod constructs fixed distally using pedicle screws were stiffer and stronger in resisting forward flexion compared with cables or hooks alone. Augmenting these screws with either infralaminar hooks or sublaminar cables provided additional resistance to failure.

The Effect of Cement Augmentation on the Geometry and Structural Response of Recovered Osteopenic Vertebrae : An Anterior-Wedge Fracture Model

Study Design. The efficacy of cement augmentation in restoring the geometry and structural competence of failed thoracic and lumbar human vertebrae under mechanical loads was studied. Objectives. To quantify whether cement augmentation restores and maintains the geometry and structural competence of failed osteopenic vertebrae and to assess the contribution of vertebral geometry to the achieved augmentation. Summary of Background Data. Cement augmentation of failed vertebrae was clinically shown to alleviate significant pain and functional impairments associated with vertebral fragility fractures. However, the procedure’s efficacy in restoring the structural response of the failed vertebrae and maintaining the achieved geometry under functional loads remains unclear. Methods. Nineteen thoracic and lumbar human vertebrae were tested to failure under compression-flexion loading. The vertebrae were allowed to recover, were retested to failure, augmented with Polymethylmethacrylate and again retested to failure. Repeated measures analysis was used to compare the change in vertebral geometry and structural response, defined as the multiplanar force and moment response of the vertebra to the imposed deformation, at each of the test stages. Linear regression was used to assess the role of the geometry of the failed vertebrae in affecting the outcome of augmentation. Results. Augmentation significantly increased the compressive (228%) and flexion (118%) strength of the failed vertebrae and achieved a significant, albeit partial, restoration of vertebral geometry. However, the structural response of the failed vertebrae was markedly altered, whereas under applied loads, the achieved height restoration was significantly diminished. Although the geometry of the fractured vertebral body was associated with the degree of restoration of the vertebral body afteraugmentation, it was not correlated with the change in the structural parameters. Conclusion. Augmentation increases the structural competence of failed vertebrae and to a degree, restores their geometry. However, the structural response of the augmented vertebrae was significantly modified. Furthermore, the augmented vertebrae were unable to maintain the degree of geometry restoration under load.

The effects of design and positioning of carbon fiber lumbar interbody cages and their subsidence in vertebral bodies.

Study Design A biomechanical study using human cadaveric lumbar spines. Objectives To determine the strength and stiffness of 3 carbon fiber cage designs in axial compression. To assess the effects of bone mineral density (BMD) on vertebral endplate failure with respect to the different cage patterns. Summary of Background Data Unilateral transforaminal approaches are gaining popularity compared with posterolateral lumbar interbody fusion. With differences in the inherent strengths of each quadrant of the endplate, the effect of different cage designs and their location on the endplate may affect subsidence and fusion success. Methods BMD measurements were obtained from 30 human spinal segments from L3 to L5. Discectomies were performed and cages were placed on the cephalad endplate of each vertebra in 3 configurations: 2 small posterolateral rectangular cages; 1 small anterior banana cage; and 1 small central rectangular cage. Each segment was tested under compression until endplate failure was recorded. Two-way analysis of variance was used to test for the effects of cage design on cage subsidence and endplate failure. Analysis of covariance was conducted to test for the effects of age, BMD, and vertebral levels on the failure load and stiffness for each cage design. Results Cage design was not significant in affecting failure force across the endplate. There were insignificant differences comparing stiffness in compression for the 3 different cage placements patterns. Low BMD adversely affected failure force and construct stiffness across all 3 cage patterns. Conclusions Cage design and position do not significantly affect failure of the construct or stiffness in compression across the endplate. BMD significantly affects both failure forces and stiffness but is not dependent on the positioning or design of the cage.

Effect of the metastatic defect on the structural response and failure process of human vertebrae: an experimental study.

BACKGROUND Pathologic vertebral fractures are associated with intractable pain, loss of function and high morbidity in patients with metastatic spine disease. However, the failure mechanisms of vertebrae with lytic defects and the failed vertebraes ability to retain load carrying capacity remain unclear. METHODS Eighteen human thoracic and lumbar vertebrae with simulated uncontained bone defects were tested under compression-bending loads to failure. Failure was defined as 50% reduction in vertebral body height. The vertebrae were allowed to recover under load and re-tested to failure using the initial criteria. Repeated measure ANOVA was used to test for changes in strength and stiffness parameters. FINDINGS Vertebral failure occurred via buckling and fracture of the cortex around the defect, followed by collapse of the defect region. Compared to the intact vertebrae, the failed vertebrae exhibited a significant loss in compressive strength (59%, p<0.001), stiffness (53%, p<0.05) and flexion (70%, p<0.01) strength. Significant reduction in anterior-posterior shear (strength (63%, p<0.01) and stiffness (67%, p<0.01)) and lateral bending strength (134%, p<0.05) were similarly recorded. In the intact vertebrae, apart from flexion strength (r(2)=0.63), both compressive and anterior-posterior shear strengths were weakly correlated with their stiffness parameters (r(2)=0.24 and r(2)=0.31). By contrast, in the failed vertebrae, these parameters were strongly correlated, (r(2)=0.91, r(2)=0.86, and r(2)=0.92, p<0.001 respectively). INTERPRETATION Failure of the vertebral cortex at the defect site dominated the initiation and progression of vertebral failure with the vertebrae failing via a consolidation process of the vertebral bone. Once failed, the vertebrae showed remarkable loss of load carrying capacity.

MR diffusion is sensitive to mechanical loading in human intervertebral disks ex vivo

To use T2 and diffusion MR to determine the change in the mechanical function of human disks with increased degenerative state.

The effects of design and configuration on the biomechanical response of an internal spinal fixator

Abstract This study examines the biomechanical performance of an internal spinal fixator and the effects of specific design features under a range of loading modes. The commercial device was mounted on plastic vertebrae in a corpectomy injury model and attached by a series of experimental jigs to an appropriate material testing machine and tested under axial compression, torsion and flexion and extension moments. Results from the torsional tests indicated that increasing the clamp tightening torque from 5 to 15 N m significantly increased the rigidity of the fixation system. The inclusion of the transverse elements resulted in a significant increase in the torsional stiffness, with the increase largely overriding the effect of clamp tightening torque. By contrast, under compressive and both flexion and extension loads, neither of the design features of the fixator had a marked effect on the overall measured stiffness of the system. However, under extension loads, there were specific interactions between the two design parameters. The present study clearly indicates the need for the optimization of the design of the clamps and for alternative configurations of the transverse elements to enhance their performance under sagittal loads.

Mechanical assessment of the effects of metastatic lytic defect on the structural response of human thoracolumbar spine.

To investigate the effects of a clinical lytic defect on the structural response of human thoracolumbar functional spinal unit. A novel CT‐compatible mechanical test system was used to image the deformation of a T12‐L1 motion segment and measure the change in strain response under compressive loads ranging from 50 to 750 N. A lytic lesion (LM) with cortex involvement (33% by volume) was introduced to the upper vertebral body and the CT experiments were repeated. Finite element models, established from the CT volumes, were used to investigate the defects effects on the structural response and the state of principal and shear stresses within the affected and adjacent vertebrae. The lytic lesion resulted in severe loss of the vertebral structural competence, resulting in significant, non‐linear, and asymmetric increase in the experimentally measured strains and computed stresses within both vertebrae (p < 0.01). At the cortex, the tensile strains were significantly increased, while compressive strains significantly decreased, (p < 0.05). Both the vertebral bone and cortex regions adjacent to the defect showed significant increase in computed compressive, tensile, and shear stresses (p < 0.01). Changes in stress and strain distribution within the affected and adjacent vertebral bone and the experimentally observed bulging and buckling of the vertebral cortices suggested that initiation of catastrophic vertebral failure may occur under load magnitudes encountered in daily living. Although the effect of LM on the global deformation of the spine was well‐predicted, our results show that FE predictions of local strain changes must be carefully assessed for clinical relevance.

The effect of screw head design on rod derotation in the correction of thoracolumbar spinal deformity: laboratory investigation.

OBJECT The use of fixed-axis pedicle screws for correction of thoracolumbar deformity in adult surgery is demanding because of the challenge of assembling the bent rod to the screw in order to achieve curve correction. Polyaxial screw designs, providing increased degrees of freedom at the screw-rod interface, were reported to be insufficient in achieving correction of thoracic deformity in the axial plane. Using a multisegment bovine calf spine model, this study investigated the ability of a new uniplanar screw design to achieve derotation correction of the vertebrae and maintain a degree of correction comparable to that of fixed-axis and polyaxial screw designs. METHODS Eighteen calf thoracolumbar spine segments from T-6 to L-1 (n = 6 per screw design) underwent bilateral facetectomies at the T9-11 levels and were instrumented bilaterally with pedicle screws and rods. To assess the efficacy of each screw design in imparting rotational correction, each instrumented level was tested under applied torsional moments designed to simulate the motion applied during derotation surgery. Once rotation was achieved, the whole spine was tested to assess the overall stiffness of the construct. RESULTS The fixed-axis construct showed increased efficacy in imparting rotation compared with the uniplanar (115% increase, p > 0.05) and polyaxial (210% increase, p < 0.05) constructs. Uniplanar screws showed a 21% increase in torsional stiffness compared with the polyaxial screws, but this difference was not statistically significant. CONCLUSIONS The design of screw heads plays a significant role in affecting the rotation of the vertebrae during the derotation procedure. Uniplanar screws may have the advantage of maintaining construct stiffness after derotation.