The effect of interbody fusion cage design on the stability of the instrumented spine in response to cyclic loading: an experimental study.

BACKGROUND CONTEXT: In the lumbar spine, end plate preparation for the interbody fusion cages may critically affect the cage's long-term performance. This study investigated the effect of the interbody cage design on the compliance and cage subsidence of instrumented spines under cyclic compression. PURPOSE: We aimed to quantify the role of cage geometry and bone density on the stability of the spinal construct in response to cyclic compressive loads. STUDY DESIGN: Changes in the cage-bone interface and the effect of bone density on these changes were evaluated in a human cadaveric model for three intervertebral cage designs. METHODS: The intervertebral space of 27 functional cadaveric spinal units was instrumented with bilateral linear cages, single anterior conformal cages, or single unilateral oblique cages. Once augmented with a pedicle screw fixation system, the instrumented spine unit was tested under cyclic compression loads (400-1,200 N) to 20,000 cycles at a rate of 2 Hz. Compliance of the cage-bone interface and cage subsidence was computed. Two-way repeated multivariate analysis of variance was used to test the effects of cage design and bone density on the compliance and subsidence of the cages. RESULTS: The anterior conformal shaped cage showed reduced interface stiffness (p<.01) and higher hysteresis (p<.01) and subsidence rate (10%-30%) than the bilateral linear and unilateral oblique-shaped cages. Bone density was not associated with the initial compliance of the cage-bone interface or the rate of cage subsidence. Higher bone density did decrease the rate of reduction in cage-bone interface stiffness under higher cyclic loads for the anterior conformal shaped and unilateral oblique cages. CONCLUSIONS: Cage design and position significantly affected the degradation of the cage-bone interface under cyclic loading. Comparisons of subsidence rate between the different cage designs suggest the peripheral location of the cages, using the stronger peripheral subchondral bone of the apophyseal ring, to be advantageous in preventing the subsidence and failure of the cage-bone interface.