The Effects of Design and Positioning of Carbon Fiber Lumbar Interbody Cages and Their Subsidence in Vertebral Bodies
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STUDY DESIGN: A biomechanical study using human cadaveric lumbar spines. OBJECTIVES: To determine the strength and stiffness of 3 carbon fiber cage designs in axial compression. To assess the effects of bone mineral density (BMD) on vertebral endplate failure with respect to the different cage patterns. SUMMARY OF BACKGROUND DATA: Unilateral transforaminal approaches are gaining popularity compared with posterolateral lumbar interbody fusion. With differences in the inherent strengths of each quadrant of the endplate, the effect of different cage designs and their location on the endplate may affect subsidence and fusion success. METHODS: BMD measurements were obtained from 30 human spinal segments from L3 to L5. Discectomies were performed and cages were placed on the cephalad endplate of each vertebra in 3 configurations: 2 small posterolateral rectangular cages; 1 small anterior banana cage; and 1 small central rectangular cage. Each segment was tested under compression until endplate failure was recorded. Two-way analysis of variance was used to test for the effects of cage design on cage subsidence and endplate failure. Analysis of covariance was conducted to test for the effects of age, BMD, and vertebral levels on the failure load and stiffness for each cage design. RESULTS: Cage design was not significant in affecting failure force across the endplate. There were insignificant differences comparing stiffness in compression for the 3 different cage placements patterns. Low BMD adversely affected failure force and construct stiffness across all 3 cage patterns. CONCLUSIONS: Cage design and position do not significantly affect failure of the construct or stiffness in compression across the endplate. BMD significantly affects both failure forces and stiffness but is not dependent on the positioning or design of the cage.
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