A Biomechanical Comparison of Shape Design and Positioning of Transforaminal Lumbar Interbody Fusion Cages

STUDY DESIGN Cadaveric biomechanical analysis. OBJECTIVE The aim of this study was to compare three interbody cage shapes and their position within the interbody space with regards to construct stability for transforaminal lumbar interbody fusion. METHODS Twenty L2-L3 and L4-L5 lumbar motion segments from fresh cadavers were potted in polymethyl methacrylate and subjected to testing with a materials testing machine before and after unilateral facetectomy, diskectomy, and interbody cage insertion. The three cage types were kidney-shaped, articulated, and straight bullet-shaped. Each cage type was placed in a common anatomic area within the interbody space before testing: kidney, center; kidney, anterior; articulated, center; articulated, anterior; bullet, center; bullet, lateral. Load-deformation curves were generated for axial compression, flexion, extension, right bending, left bending, right torsion, and left torsion. Finally, load to failure was tested. RESULTS For all applied loads, there was a statistically significant decrease in the slope of the load-displacement curves for instrumented specimens compared with the intact state (p < 0.05) with the exception of right axial torsion (p = 0.062). Among all instrumented groups, there was no statistically significant difference in stiffness for any of the loading conditions or load to failure. CONCLUSIONS Our results failed to show a clearly superior cage shape design or location within the interbody space for use in transforaminal lumbar interbody fusion.