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 differ ent lumbar interbody fusion implants and endplate preparation techniques wa s performed. Objectives. To assess the axial compressive strength of an implant with per ipheral 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 co nstructs has been compared between implants and bone grafts. Neither implan t design nor endplate preparation has been shown to affect strength. Remova l of the central bony endplate for bone grafts was noted to improve graft i ncorporation 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 suppo rt) with two endplate preparation techniques (intact bony endplate vs remov al of the central bony endplate). Specimens were tested to ultimate compres sive failure using a 50 N/second ramped load. Yield strength and ultimate c ompressive strength were compared between groups using two-factor analysis of covariance. A P value less than 0.05 was considered significant, Stepwis e linear regressions assessed the predictive power of age, bone mineral con tent, 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 stre ngth (P < 0.0001; r(2) = 0.459) and ultimate compressive strength (P < 0.00 01; r(2) = 0.510). Conclusions. An implant with only peripheral support resting on the apophys eal ring offers axial mechanical strength similar to that of an implant wit h full support. Neither supplementary struts nor a solid implant face has a ny additional mechanical advantage, but reduces graft-host contact area. Re moval of the central bony endplate is recommended because it does not affec t the compressive strength and promotes graft incorporation.