Biodegradable Interference Screw Fixation Exhibits Pull-Out Force and Stiffness Similar to Titanium Screws

Recently, increased interest in biodegradable interference screws for bone-tendon-bone graft fixation has led to numerous screws becoming available. The implants are made from different polymers and have different designs, which might influence their mechanical properties. Several studies have reported a wide range of mechanical results for these screws using different biomechanical models. The aim of the present study is to compare reliable biomechanical data for six different biodegradable interference screws, consisting of five different polymers, with a conventional titanium screw in a standardized model. Seventy proximal calf tibias were used to determine maximal pull-out force, stiffness of fixation, and insertion torque for interference screw fixation of bone-tendon-bone grafts. Additionally, maximal torque at failure was determined. Data were analyzed with respect to aspects of screw design, such as drive and thread shape. Five of the six biodegradable screws provided initial pull-out force and stiffness of fixation comparable with that of a conventional titanium screw. Torque at failure can be greatly increased by adapting the drive design to the mechanical properties of the polymeric raw material. A correlation between pull-out force and thread height indicates that fixation rigidity depends on screw design, even in a biodegradable implant.