Effect of screw length on bioabsorbable interference screw fixation in a tibial bone tunnel

Initial tibial fixation strength is the weak link after anterior cruciate l igament reconstruction with a quadrupled hamstring tendon graft fixed with bioabsorbable interference screws. The purpose of this study was to determi ne the biomechanical differences between 28-mm and tapered 35-mm interferen ce screws for tibial fixation of a soft tissue graft in 16 young cadaveric tibias. Failure mode, displacement before failure, and ultimate failure loa d were tested with a testing machine aligned with the tibial tunnel to simu late a worst-case scenario. The mode of failure was graft slippage past the screw in all but one of the specimens. The mean maximum load at failure of the 28-mm screw was 594.9 +/- 141.0 N, with mean displacement at failure o f 10.97 +/-2.20 mm. The mean maximum load at failure of the 35-mm screw was 824.9 +/- 124.3 N, with a mean displacement to failure of 14.38 +/-2.15 mm . The 38% difference in mean maximal load at failure was significant. Impor tant variables in hamstring tendon graft fixation within a bone tunnel incl ude bone mineral density, dilatation, gap size, screw placement, and screw width and length. Attention to these variables will help to provide secure graft fixation during biologic incorporation throughout the rehabilitation period.