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

ABSTRACT

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.