Tibial Interference Screw Positioning Relative to the Bone Plug in ACL Reconstruction: A Biomechanical Comparison of Cortical Versus Cancellous-Sided Placement

The biomechanical strength of a bone-patellar tendon-bone graft in the tibia may vary depending on whether the interference screw abuts the cancellous vs the cortical surface of the bone plug. In a porcine model 10x20-mm bone-patellar tendon-bone grafts were prepared and fixed in a 10-mm diameter tibial tunnel using a 9x25-mm titanium interference screw. the screw was positioned on the cancellous surface of the graft in group A vs the cortical side of the graft in group B (n=14). Specimens underwent precycling cyclic loading and load-to-failure testing. The mean ultimate failure load was 493 +/- 245 N for group A vs 304 +/- 145 N for group B (P=.008). Sixty-nine percent of specimens in group A survived 1000 cycles of load testing compared with 21% of specimens in group 13. Forty-three percent of specimens in group B sustained intratendinous failure adjacent to the bone plug compared with 15% of specimens in group A. Orientation of the tibial interference screw along the cancellous vs the cortical side of the graft results in superior cyclic loading and ultimate failure load characteristics. Additionally screw placement along the cortical side may weaken the tendon interface and lead to tendon failure under load. This study indicates that placement of the tibial interference screw along the cancellous side of the graft is biomechanically favorable. However the clinical ramifications of these findings are not clear.