Early post-operative performance of an anatomically designed hybrid thread interference screw for ACL reconstruction: A comparative study

Although the anterior cruciate ligament (ACL) reconstruction using interference screw is a well-accepted surgical procedure, patients still suffer graft failure in the initial rehabilitation phase. Graft fixation stability of a newly designed anatomical hybrid thread tapered interference screw (AHTTIS) was compared with a conventional standard one (CSIS) by conducting in-vitro mechanical tests. According to the CSIS manufacturer’s instruction, eight tapered bone tunnels, matching AHTTIS geometry, and eight straight cylindrical tunnels were drilled in artificial bone blocks. Bovine tendon grafts were fixed using AHTTIS and CSIS in their corresponding bone tunnels. Each graft was subjected to a pre-load, a dynamic cyclic sinusoidal tensile load, and a tensile pullout load. Aggregate graft displacement relative to the bone tunnel under dynamic cyclic load, load corresponding to a 3 mm displacement; load and displacement at yield and ultimate failure of the graft; and graft-bone-screw stiffness resulted from pullout tests were recorded. There were no significant differences between recorded parameters of the two groups, except the graft displacements relative to the bone tunnel at ultimate failure load (8.35 ± 3.03 mm versus 5.28 ± 1.37 mm) and at the yield point (7.73 ± 3.02 mm versus 4.59 ± 1.25 mm) (P < 0.05), which were both significantly greater for the AHTTIS group. Considering all tests were made in the worst-case scenario of fixation of an interference screw, i.e., the employed synthetic bone just mimicked cancellous bone, graft fixation with the AHTTIS is practical. However, in the case of employing high strain rate loads on AHTTIS, further considerations are needed to reduce the side effect of slippage of the graft.