The effect of disc degeneration on anterior shear translation in the lumbar spine.

Many pathologies involving disc degeneration are treated with surgery and spinal implants. It is important to understand how the spine behaves mechanically as a function of disc degeneration. Shear loading is especially relevant in the natural and surgically stabilized lumbar spine. The objective of our study was to determine the effect of disc degeneration on anterior translation of the lumbar spine under shear loading. We tested 30 human cadaveric functional spinal units (L3-4 and L4-5) in anterior shear loading. First, the specimens were imaged in a 1.5 T magnetic resonance scanner. The discs were graded according to the Pfirrmann classification. The specimens were then loaded up to 250 N in anterior shear with an axial compression force of 300 N. Motion of the vertebrae was captured with an optoelectronic camera system. Inter- and intra-observer reliability for disc grading was determined (Cohen's and Fleiss' Kappa), and a non-parametric test was performed on the translation data to characterize the effect of disc degeneration on this parameter. We found fair to moderate agreement between and within observers for the disc grading. We found no significant effect of disc degeneration on anterior shear translation (Kruskal-Wallis ANOVA). Our results indicate that disc degeneration, as classified with the Pfirrmann scale, does not predict lumbar spinal motion in shear.

[Case-control study on therapeutic effects between unilateral decompression via fenestration under Quadrant retractor and open decompression technique with fusion and internal fixation for the treatment of lumbar spinal stenosis].

OBJECTIVE: To compare therapeutic effects between unilateral decompression technique only and open decompression technique with fusion and internal fixation for the treatment of lumbar spinal stenosis. METHODS: From March 2008 to February 2011, 82 patients with lumbar spinal stenosis were treated with operations, and divided into two groups. There were 13 males and 19 females in group A, with a mean age of (56.31±4.31) years old. The patients in group A were treated with unilateral decompression via fenestration technique only, including 23 patients obtaining single level decompression and 9 patients obtaining two levels decompression. In group B, there were 18 males and 32 females, with a mean age of (57.53±4.28) years old. The patients in group B were treated with open decompressive technique with fusion and internal fixation, including 38 patients obtaining single level decompression and 12 patients obtaining two levels decompression. The VAS of back pain and leg pain, ODI were recorded before and after surgery to evaluate low back pain,leg pain and walking tolerance. RESULTS: All the patients were followed up, and the duration ranged from 10.9 to 43.4 months,with a mean of 32.8 months. There were no differences in age, stenosis level, VAS of back and leg pain and ODI before surgery between two groups. Compared with the corresponding ones in group B, the operation time, blood loss, hospitalization time,recovery time of routine daily life and finacial expenditure of patients were all shorter or less in group A. There was no statistically difference in complications between two groups. CONCLUSION: "Unilateral decompression via fenestration technique" is a less invasive and more effective decompressive technique for degenerative spinal stenosis without posterior elements damage. It has advantages in operation time, blood loss, hospitalization time, recovery to daily life and financial expenditure. When controlling the operative indications strictly, the technique could be an important procedure for surgical treatment of degenerative spinal stenosis, especially in the elderly population.

Load transfer characteristics between posterior spinal implants and the lumbar spine under anterior shear loading: an in vitro investigation.

STUDY DESIGN: A biomechanical human cadaveric study. OBJECTIVE: To determine the percentage of shear force supported by posterior lumbar spinal devices of varying stiffnesses under anterior shear loading in a degenerative spondylolisthesis model. SUMMARY OF BACKGROUND DATA: Clinical studies have demonstrated beneficial results of posterior arthrodesis for the treatment of degenerative spinal conditions with instability. Novel spinal implants are designed to correct and maintain spinal alignment, share load with the spine, and minimize adjacent level stresses. The optimal stiffness of these spinal systems is unknown. To our knowledge, low-stiffness posterior instrumentation has not been tested under an anterior shear force, a highly relevant force to be neutralized in the clinical case of degenerative spondylolisthesis. METHODS: The effects of implant stiffness and specimen condition on implant load and intervertebral motion were assessed in a biomechanical study. Fifteen human cadaveric lumbar functional spinal units were tested under a static 300 N axial compression force and a cyclic anterior shear force (5-250 N). Implants (high-stiffness [HSI]: ø 5.5-mm titanium, medium-stiffness [MSI]: ø 6.35 × 7.2-mm oblong PEEK, and low-stiffness [LSI]: ø 5.5-mm round PEEK) instrumented with strain gauges were used to calculate loads and were tested in each of 3 specimen conditions simulating degenerative changes: intact, facet instability, and disc instability. Intervertebral motions were measured with a motion capture system. RESULTS: As predicted, implants supported a significantly greater shear force as the specimen was progressively destabilized. Mean implant loads as a percent of the applied shear force in order of increasing specimen destabilization for the HSI were 43%, 67%, and 76%; mean implant loads for the MSI were 32%, 56%, and 77%; and mean implant loads for the LSI were 18%, 35%, and 50%. Anterior translations increased with decreasing implant stiffness and increasing specimen destabilization. CONCLUSION: Implant shear stiffness significantly affected the load sharing between the implant and the natural spine in anterior shear ex vivo. Low-stiffness implants transferred significantly greater loads to the spine. This study supports the importance of load-sharing behavior when designing new implants.