The effects of a semiconstrained integrated artificial disc on zygapophyseal joint pressure and displacement.

STUDY DESIGN: Measurement of zygapophyseal joint pressure and displacement was performed after placement of a semiconstrained integrated artificial disc (SIAD) in a cadaver model. OBJECTIVE: To understand the likelihood of accelerated zygapophyseal joints degeneration as a result of the implant. SUMMARY OF BACKGROUND DATA: A SIAD has been developed to treat lumbar spondylosis secondary to segmental disc degeneration and spinal stenosis. The SIAD replaces the stenotic segment's disc. Previous studies have demonstrated that nonconstrained artificial disc (NAD) replacements fail to achieve their optimal long-term outcomes likely because of significantly increased zygapophyseal joint pressure and displacement at the implanted level. Moreover, clinical studies have reported an increased incidence of zygapophyseal joint degeneration after lumbar disc replacement. METHODS: Eight cadaver lumbar specimens (L2-L5) were loaded in flexion, neutral, extension, left bend, and right rotation. Zygapophyseal joint pressure and displacement were measured during each of the 5 positions at each of the 3 levels with the ratio of deformation calculated under the different loads. An artificial disc was placed at the L3-L4 level, and the measurements were repeated. RESULTS: After L3-L4 segment implantation, the pressure in the zygapophyseal joint at operative segment was not significantly changed by SIAD and NAD implantation in axial compression and flexion, compared with physiological disc. Notable differences in zygapophyseal joint pressure between the SIAD and NAD were identified at the operative level in extension, left bend, and right rotation. The adjacent-level effect of NAD was significantly greater than that of SIAD. The ratio of deformation difference between the 2 discs was increased by load experienced in extension, flexion, left bend, and right rotation. CONCLUSION: The SIAD provided a superior biomechanical milieu for zygapophyseal joints at the implanted and adjacent levels compared with NAD, which may avoid the acceleration of postoperative zygapophyseal joint degeneration. LEVEL OF EVIDENCE: 1.

Effects of sagittal endplate shape on lumbar segmental mobility as evaluated by kinetic magnetic resonance imaging.

STUDY DESIGN: Retrospective analysis using kinetic magnetic resonance imaging. OBJECTIVE: To investigate relationships between vertebral endplate remodeling, Modic changes, disc degeneration, and lumbar segmental mobility. SUMMARY OF BACKGROUND DATA: Previous studies have shown that disc degeneration and vertebral endplate Modic changes are associated with differences in spinal motion, however, the effects of vertebral endplate morphology on lumbar segmental motion have not been fully investigated. METHODS: A total of 420 patients underwent kinetic magnetic resonance imaging of 2100 lumbar motion segments. Sagittal endplate shapes (concave, flat, irregular), Modic changes (types, 0-3), and disc degeneration (grade, I-V) were assessed along with translational and angular motion of vertebral segments in flexion, extension, and neutral positions. RESULTS: The most common findings were concave endplate shape (63.24%), type 2 Modic change (71.79%), and grade II disc degeneration (40.33%). Flat, irregular endplates were more common at L1-L2, L4-L5, and L5-S1 than L2-L3 and L3-L4. Types 1, 2, and 3 Modic changes increased in frequency according to endplate shape: concave less than flat less than irregular. Type 0 was observed to decrease with the change of endplate shape from flat to concave to irregular. Vertebral levels with irregular endplates had more disc generation than those with flat; levels with flat endplates had significantly more disc degeneration than those with concave. Translational motion of the lumbar segment was greatest at levels with irregular endplates and decreased at those with flat and then concaves endplates. Angular motion was least at levels with irregular endplates and increased at levels with flat, then concave endplates. CONCLUSION: The degree of pathogenic lumbar segmental motion is associated with remodeling of the sagittal endplate. Endplate remodeling may occur as an adaptation to restrain abnormal movement of the lumbar segment. LEVEL OF EVIDENCE: N/A.