Comparison of pullout strength for seven- and nine-millimeter diameter interference screw size as used in anterior cruciate ligament reconstruction.

This study compares biomechanical properties of 7- and 9-mm diameter screws providing interference fixation in anterior cruciate ligament reconstruction. Sixteen pairs of fresh-frozen bovine knees were evaluated. Uniaxial load to failure was performed at a deformation rate of 30 mm/s along the mechanical axis of the ligament graft with the knees secured at 45 degrees of flexion in a custom jig. A video analyzer was used to measure ligament strain and bone-to-bone deformation. Ultimate force, deformation, and failure mode were recorded and compared. The 7-mm screws provided 98% yield strength, and 95% ultimate force compared with the 9-mm screws. The average femoral pullout strength was 1161 +/- 93 N in the 7-mm group and 1198 +/- 142 N in the 9-mm group. Failure mode was similar in both groups. Clinically, the usage of 7-mm screws may reduce iatrogenic injuries to the patellar tendon graft compared with larger screws. This study shows that the biomechanical advantages of 9-mm screws compared with 7-mm screws are minimal.