The influence of screw geometry on hamstring tendon interference fit fixation

We used a standardized model of calf tibial bone to investigate the influen ce of screw diameter and length on interference fit fixation of a three-str anded semitendinosus tendon graft for anterior cruciate ligament reconstruc tion. Biodegradable poly-(L-lactide) interference screws with a diameter of 7, 8, and 9 mm and a length of 23 and 28 mm were used, We examined results in three groups of 19 specimens each: group 1, screw diameter equaled graf t diameter and screw length was 23 mm; group 2, screw diameter equaled graf t diameter plus 1 mm and screw length was 23 mm; group 3, screw diameter eq ualed graft diameter and screw length was 28 mm. The mean pull-out forces i n groups 1, 2, and 3 were 367.2 +/- 78 N, 479.1 +/- 111.1 N, and 537.4 +/- 139.1 N, respectively. The force data from groups 2 and 3 were significantl y higher than those from group 1. These results indicate that screw geometr y has a significant influence on hamstring tendon interference fit fixation . Increasing screw length improves fixation strength more than oversizing t he screw diameter, This is important, especially for increasing tibial fixa tion strength because the tibial graft fixation site has been considered to be the weak link of such a reconstruction.