Influence of Thread Design on Bioabsorbable Interference Screw Insertion Torque During Retrograde Fixation of a Soft-Tissue Graft in Synthetic Bone

Purpose: Bioabsorbable interference screw design may influence biomechanical characteristics. This study compared the insertion torque and load at failure characteristics of 2 types of screws during retrograde fixation of a soft-tissue graft. Type of Study: Biomechanical study. Methods: Eight matched pairs of doubled 100-mm long tibialis anterior allografts were prepared and fixed in appropriately sized tunnels created in 10 lb/ft3 (0.16 g/cm3) dense synthetic bone blocks using screws of similar length and root and thread diameter designed with either a large buttress thread with a smaller taper or small buttress thread with a larger taper. Insertion torque was measured at one-third, two-thirds, and full screw insertion. After the graft fixation constructs were mounted in a servohydraulic-testing device with the loading axis aligned directly with the tunnel and preloaded to 25 N, they were cycled 3 times from 0 to 50 N, and then subjected to a 20 mm/minute traction force to failure. Results: All constructs failed by graft slippage past the screw. Mean maximum load at failure (360.5 ± 68 N v 341.6 ± 58 N, P = .2) and stiffness (63.6 ± 16 N/mm2v 66.4 ± 14 N/mm2, P = .89) was similar between constructs fixed with a large buttress thread small-taper screw and small buttress thread large-taper screw, respectively. The small buttress thread screw with a large taper displayed greater mean insertion torque at one-third insertion (4.1 ± 0.57 in-lb v 3.2 ± 0.49 in-lb, P = .03), whereas the large buttress thread screw with a small taper displayed greater mean insertion torque at full insertion (11.1 ± 0.74 in-lb v 9.4 ± 1.3 in-lb, P = .012). Mean differences were not observed at two-thirds screw insertion (P = .12). Conclusions: Large buttress thread small-taper screws displayed biomechanical fixation characteristics comparable to small buttress thread large-taper screws. Clinical Relevance: Given reports of superior screw-graft-bone tunnel contact area, these biomechanical results suggest that use of a large buttress screw with a small taper may be preferable for retrograde soft-tissue graft fixation.