The Influence of Screw Geometry on Hamstring Tendon Interference Fit Fixation

Abstract

We used a standardized model of calf tibial bone to investigate the influence of screw diameter and length on interference fit fixation of a three-stranded semitendinosus tendon graft for anterior cruciate ligament reconstruction. 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 10 specimens each: group 1, screw diameter equaled graft diameter and screw length was 23 mm; group 2, screw diameter equaled graft diameter plus 1 mm and screw length was 23 mm; group 3, screw diameter equaled graft diameter and screw length was 28 mm. The mean pull-out forces in 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 significantly higher than those from group 1. These results indicate that screw geometry has a significant influence on hamstring tendon interference fit fixation. Increasing screw length improves fixation strength more than oversizing the screw diameter. This is important, especially for increasing tibial fixation strength because the tibial graft fixation site has been considered to be the weak link of such a reconstruction.

RETRACTED ARTICLE: Graft fixation alternatives in anterior cruciate ligament reconstruction

“…Interference screws may provide the advantage of rigid aperture fixation (fixation at the native ligament footprint adjacent to the articular surface), which increases knee stability and graft isometry and avoids suture stretch and graft-tunnel motion [28] Early fixation techniques for soft-tissue grafts were limited to distal, indirect fixation (suspensory fixation), which are hindered by inferior stiffness, the windshield-wiper (anterior/posterior), and bungee cord effects (superior/inferior), which may lead to delayed biologic incorporation and tunnel enlargement. When distal (suspensory) fixation is used, a complete filling of the tunnel with the graft may prevent this graft-tunnel motion.…”

“…This may be due to bone plug length, which is limited versus soft-tissue length, which is unlimited, within the tunnel. Also, the ability of screw threads to interdigitate in the graft, or ''grab'' the graft, is greater with cancellous bone than softtissue grafts [28]. Whereas the interference screw works by compression with a soft-tissue graft, both compression and interdigitation are used with a bone plug.…”

“…With a soft-tissue graft, screw diameter should approximate that of the osseous tunnel to ensure adequate strength [50]. When using a softtissue graft, Weiler et al [28] recommended a screw diameter 1 mm larger than the graft diameter, especially at the tibial site, or a longer screw, 28 mm rather than 23 mm, in a hamstring graft. This is based on the fact that a screw with a diameter 1 mm larger than the graft diameter has a significantly greater pullout strength than a screw with a diameter equal to the graft with a semitendinosus tendon [28].…”

“…When using a softtissue graft, Weiler et al [28] recommended a screw diameter 1 mm larger than the graft diameter, especially at the tibial site, or a longer screw, 28 mm rather than 23 mm, in a hamstring graft. This is based on the fact that a screw with a diameter 1 mm larger than the graft diameter has a significantly greater pullout strength than a screw with a diameter equal to the graft with a semitendinosus tendon [28]. Because of concern for graft laceration, the sharp threads of metallic interference screws used for bone plug fixation were blunted in subsequent models, allowing for use with soft-tissue grafts [30].…”

“…Insertion torque has been positively correlated with pullout strength in the laboratory [28][29][30][31]. Insertion torque may be altered by increasing screw diameter, decreasing gap size, and performing tunnel dilation.…”