Effect of implant design and endplate preparation on the compressive strength of interbody fusion constructs

Study Design. A human cadaveric study on the compressive strength of different lumbar interbody fusion implants and endplate preparation techniques was performed. Objectives. To assess the axial compressive strength of an implant with peripheral endplate contact as opposed to full surface contact, and to assess whether removal of the central bony endplate affects the axial compressive strength. Summary of Background Data. The compressive strength of interbody fusion constructs has been compared between implants and bone grafts. Neither implant design nor endplate preparation has been shown to affect strength. Removal of the central bony endplate for bone grafts was noted to improve graft incorporation but also to facilitate subsidence. Methods. A total of 44 vertebrae were tested in four experimental groups by combining two interbody implants (full-surface vs peripheral surface support) with two endplate preparation techniques (intact bony endplate vs removal of the central bony endplate). Specimens were tested to ultimate compressive failure using a 50 N/second ramped load. Yield strength and ultimate compressive strength were compared between groups using two-factor analysis of covariance. A P value less than 0.05 was considered significant, Stepwise linear regressions assessed the predictive power of age, bone mineral content, and the implant's normalized endplate coverage on yield strength and ultimate compressive strength. Results. Neither implant design nor endplate preparation technique affected yield strength or ultimate compressive strength. Age, bone mineral content, and the normalized endplate coverage were strong predictors of yield strength (P < 0.0001; r(2) = 0.459) and ultimate compressive strength (P < 0.0001; r(2) = 0.510). Conclusions. An implant with only peripheral support resting on the apophyseal ring offers axial mechanical strength similar to that of an implant with full support. Neither supplementary struts nor a solid implant face has any additional mechanical advantage, but reduces graft-host contact area. Removal of the central bony endplate is recommended because it does not affect the compressive strength and promotes graft incorporation.