Casey K. Lee

Lumbosacral spinal fusion. A biomechanical study.

The effects of spinal fusion on fused segment and the adjacent, unfused segments play a significant role in the clinical effectiveness of spinal fusion for low-back pain with or without sciatica. Much of the information on this important subject is derived from clinical impressions. The purpose of this biomechanical study is to investigate the altered kinematics and biomechanics of the three different types of spinal fusion (posterior, bilateral-lateral, and anterior) on the adjacent, unfused segment as well as within the fused segment and to investigate their clinical implications. Sixteen fresh human cadaver lumbosacral spines were tested under a simulated physiologic loading condition. The test specimens included three motion segments, L3-4, L4-5, and L5-S1. To study the mechanics of the lumbar spine under combined compression and bending loads, a special apparatus was designed. These loads were applied by an MTS machine through two sets of pulley systems. The loads, as well as displacement data from both actuators, were recorded. A video camera system was utilized to record the kinematics of the spinal motion segment. The unfused specimen was tested first, and the fused specimen then was retested under the identical loading conditions. A total of 16 spine specimens were tested and evaluated--five posterior, four bilateral-lateral, and seven anterior fusions. All types of fusion resulted in increased bending and axial stiffnesses. Overall, anterior fusion provided the largest increase in stiffness, followed by bilateral-lateral fusion and posterior fusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of a prosthetic intervertebral disc.

This article is a preliminary report of a 10-year investigation of the development of an intervertebral disc prosthesis. Spinal fusion is a method for the treatment of chronic, disabling low-back pain that does not respond to nonoperative treatments. Spinal fusion, however, has various adverse effects, and the results of spinal fusion are often unpredictable. The goal of this research project was to develop disc prostheses that have mechanical properties very similar to those of natural, normal discs. Two types of disc prosthesis, one with fiber-reinforced polyurethane and the other with multicomponent, non-fiber-reinforced polymers (C-Flex), have been designed and manufactured. The fiber-reinforced disc was made of polyurethane end-plates with A100 hardness, a homogenous nucleus with A40, and 12 layers of multidirectional (0, +45°), fiber-reinforced anulus with A40 polyurethane. The design and modeling of the multicomponent polymers (non-fiber-reinforced) was made of C-Flex endplates with A90 hardness, a nucleus with A35 occupying 35% of the volume, and an anulus with 70A. Mechanical testing of these disc prostheses demonstrated similar mechanical properties to those of natural, normal discs.

Biomechanics of lumbosacral spinal fusion in combined compression-torsion loads.

The current study investigates the stabilizing effects of three different types of spinal fusion to the juxta-free motion segments and to the fused segment of the lumbosacral spine under combined compression-torsion loads. Sixteen fresh human cadaver lumbosacral spines were tested under a simulated physiologic loading condition. The relative movements of the motion segments, as well as the angular rotations and the center of rotation were then computed and analyzed. The average torsional stiffness of the unfused three-motion segment was found to be 2.35 nm/ degree. After fusion, the torsional stiffness did not increase significantly. Under the compression-torsional load, the anterior and bilateral-lateral fusions provided adequate stabilizing effect on the fused segment. The posterior fusion provided the least amount of stabilizing effect. These findings are similar to the results of the compression- bending experiment. Whereas the compressionbending loads produced significantly increased stress at the juxta-free segments, the compression-torsional loads did not produce any significant amount of increase in torsional stress at the juxta-free segments.

Quantitative assessment of back strength using isokinetic testing.

The overall objective of this research is to relate disorders of the lumbar spine to the mechanics of the system. An isokinetic-isometric testing procedure was designed, and groups of normal subjects and patients with back pain were tested. The procedure allows sensitive detection of muscle weakness specific to some part of the range of motion or some functional contraction speed. A biomechanical analysis was performed on several parameters of back strength assessment to develop performance indexes that can be used in establishing screening modalities. Maximum torque and trunk angles are different in normal and patient populations.

Prevention of postlaminectomy scar formation.

An animal experimental study was performed to investigate prevention of scar formation under lumbar laminectomy by using new biodegradable interposing materials—polylactic acid (PLA) foam and membrane. The experimental animals consisted of 32 dogs, 16 control and 16 experimental. The experimental surgery consisted of L5 or L6 complete laminectomy and covering of the laminectomy defect with the experimental materials. The same procedure but without the covering of the laminectomy defect was performed on the control group animals. Animals were sacrificed at varying intervals (2-52 weeks) and the lumbar spines were evaluated with histologic preparations. The PLA membrane is found to be a promising material for prevention of scar tissue extension and adhesion after laminectomy but has a problem of marginal fitting. PLA foam is found to behave as a scaffold for scar tissue extension and adhesion onto the nerve. Other foamy materials such as gelatin foam or avitane are probably behaving similarly, causing scar tissue extension and adhesion. The new materials were found to be completely biocompatible and slowly biodegradable. A combined use of posteriorly convexed stiff PLA membrane and marginal gap filler with PLA foam may provide solutions for both prevention of scar tissue extension and adhesion and prevention of postlaminectomy spinal stenosis.

Isokinetic evaluation of trunk muscles.

The purpose of this study is to Identify those individuals at risk who have weak trunk muscles and are prone to back pain. The overall thrust of this research is to develop a quantitative method to assess dynamic strength of the trunk muscles. Two unique isokinetic strength testing units in sitting and standing postures have been designed. Three groups of volunteers were tested at different times and places. Quantitative measurements of the maximum strength, fatigue behavior of the abdominal and paraspinal muscles, maximum strength in different age groups and the role of the illopsoas muscle have been performed. The study shows that women have lower maximum strength but equal or better fatigue endurance than men. There is a significant change in maximum strength with age. The maximum abdominal strength change with age showed a bimodal distribution. The illopsoas muscle approximately doubled the maximum back strength in flexion. The test in the sitting posture was tolerated better than the test in the standing posture. Isokinetic back strength testing in the sitting posture was found to be effective and safe.

The Mechanical Properties of the Canine Lumbar disc and Motion Segment

A study was initiated to measure the mechanical properties of the canine lumbar spine disc and motion segment at two specific levels. Compressive stiffness was determined to be 717.8 N/mm at L2–3 and 949.0 N/mm at L5–6. Torsional stiffness was found to be 1.04 Nm/deg at L2–3 and 1.72 Nm/deg at L5–6. These data were then compared to human lumbar spine disc and motion segment properties that have been reported in the literature. After normalizing for size differences, the canine lumbar disc showed a similar axial modulus (14.03 MPa for L2–3 and 16.30 MPa for L5–6) and a significantly higher torsional modulus (30.80 MPa for L2–3 and 26.17 MPa for L5–6) when compared to human values. The relative contributions of ligaments, posterior elements, and intervertebral disc to overall stability of the motion segment was found to be similar in canines and humans. As has been shown in human spine research, the posterior elements including the facet joints were found to be significant structures in providing torsional rigidity of the canine spine.

A biomechanical study of a cervical spine stabilization device: Roy-Camille plates.

Study Design Three‐hole Roy‐Camille posterior plates (Howmedica, Inc., Rutherford, NJ) were used to fix severely destabilized fresh cadaveric cervical spines. Fixed spine constructs were tested mechanically in flexion‐extension and torsion, and the results were compared with the same characteristics in the intact spine before destabilization. Stainless steel and titanium plates and screws were evaluated. Objectives To determine if the application of Roy‐Camille posterior plates provided suitably strong and rigid fixation of a severe, surgically created three‐column instability. Summary of Background Data Controversy still remains regarding the exclusive use of posterior cervical plating in the face of three‐column instability. Posterior plating has been evaluated biomechanically in severely destabilized calf spines; however, posterior plating of similarly destabilized human cadaveric cervical spines using the Roy‐Camille system has not been examined. Methods The authors chose to test the main motions of the neck (flexion, extension, and torsion) in the intact and the plated state using a servohydraulic materials testing system. Testing the surgically altered spine before fixation proved to be futile because of drastic instability, which is characteristic of the chosen defect. Once fixed, the spines were tested, and the rigidity of the constructs were compared with that of the intact state. Strength and failure mechanisms were evaluated. Results The rigidity of the plated spine constructs surpassed that of the intact spines; the stainless steel and titanium systems were mechanically equivalent. Thus, application of the Roy‐Camille plates of either type dramatically reduced the motion of the unstable spine. Strength of the fixed spine constructs was limited by screw pull‐out at theoretically predictable levels of force. Conclusions Posterior application of Roy‐Camille plates can fix cervical spines with severe destabilizing defects rigidly. Screw pull‐out of the most proximal or distal screw was always the mechanism of failure.

FINITE-ELEMENT MODELING OF THE SYNTHETIC INTERVERTEBRAL DISC

An ideal and realistic finite-element analysis should include several factors, such as geometry (shape and size), the relative volumes of the nucleus to the anulus f ibrosus, the fiber volume fraction within the anulus, and material properties. In this study, a finite-element analysis model for the disc that included these factors was developed. In addition, synthetic disc prostheses with the same specifications of geometry and material properties were manufactured. The developed model was found to be reliable and accurate when finite-element analysis results were compared with experimental synthetic disc data. The model also matched the mechanical behavior of the natural disc. This finite-element analysis model provides a tool to study the mechanical behavior of discs with varying degrees and types of injury and degeneration.

Tension band wiring-bone grafting for spondylolysis and spondylolisthesis. A clinical and biomechanical study.

Patients with spondylolysis or spondylolisthesis with persistent symptoms often require surgical treatment. The purpose of this article is to present a new surgical technique and clinical results of 13 patients with symptomatic spondylolysis/spondylolisthesis who were treated with tension band wiring (intra- or intersegmental) and bone grafting techniques. This article also presents the results of biomechanical effects of these tension band wiring methods on canine lumbar spines with experimental spondylolytlc defects. Thirteen adult patients, three with spondylolysis, and ten with spondylolytlc spondylolisthesis, were treated with intrasegmental or intersegmental wiring with bone grafting technique, and the clinical results were evaluated at the mean follow-up period of 20 months. Patients with spondylolysis were treated with intrasegmental wiring (transverse processes to the splnous process of the same segment) with bone grafting at the lytic defect. Patients with spondylolytlc spondylolisthesis were treated with intersegmental wiring (transverse process of the segment with defect to the spinous process of the same segment and to the spinous process of the segment below) with bone grafting to the defect and one-level fusion. All 13 patients had a solid spinal fusion and/or healing of the defect at the follow-up evaluation. Eleven had excellent clinical results; one a good, and one a fair result. The results of the biomechanical study showed that the experimental spondylolytic defect produced a significant decrease in bending stiffness (flexion-extension), and the wiring techniques (both the intra- and intersegment) increased the bending stiffness to that of the normal intact spinal segment.