Maximum Compressive Forces of Rescue Screws with Varying Diameters and Thread Patterns in Synthetic Osteoporotic and Normal Bone Models.

Abstract

Objectives: "Rescue screws" are used to address screw stripping during fracture fixation. Bone screw fasteners (BSF; Osteocentric Technologies) have a thread pattern that may enhance purchase. This study compared maximum compressive forces of traditional buttress screws (TBS; Stryker) and BSF in rescue/lag modes using synthetic models of osteoporotic and normal bone.

Methods: 2.7-mm TBS screws were placed in lag mode and the maximum compression force prior to stripping was measured. Rescue screws, varied by screw type and diameter, were placed in lag mode through previously stripped lag holes, using appropriate glide holes in the proximal cortex and the maximum compression force prior to stripping was measured. Force differences were compared between the stripped 2.7-mm TBS screws, and each rescue screws (3.5-mm TBS, 2.7-mm BSF, and 3.5-mm BSF). Trials were conducted on two synthetic-layered biomechanical models, a synthetic 20-10 pound-per-cubic-foot (PCF) osteoporotic model and a 40-30 PCF normal bone density model.

Results: 2.7-mm rescue BSF produced similar compression to the original 2.7-mm TBS lag screw (BSF-501.0 N vs TBS-577.0 N, p=0.441) in normal bone, but lower compression in osteoporotic bone (61.0 N vs 104.0 N, p=0.001). 3.5-mm TBS and BSF rescue screws increased the amount of compression in osteoporotic bone (3.5-mm TBS vs 2.7-mm BSF: 136.0 N (16.4) vs 61.0 N (14.5); 3.5mm BSF vs 2.7-mm BSF: 198.4 N (4.2) vs 61.0 N (14.5) p<0.001) and in normal bone (3.5-mm TBS vs 2.7-mm BSF: 968.0 N (30.3) vs 501.0 N (197.7); 3.5-mm BSF vs 2.7-mm BSF: 1136.0 N (51.2) vs 501.0 N (197.7) p<0.001) versus 2.7-mm rescue screws.3.5-mm rescue BSF produced more compression than 3.5-mm TBS (198.4 N (4.2) vs 136.0 N (16.3), p<0.001) in osteoporotic bone, but not in normal bone (1136.0 N (51.2) vs 968.0 N (30.3), p=0.106).

Conclusions: After stripping of a 2.7-mm Traditional Buttress Screw (TBS), 3.5-mm rescue TBS and Bone screw fasteners (BSF) generated more compressive force than the original lag screws in both normal and osteoporotic surrogate bone. 2.7-mm rescue BSF provided similar compression as the original lag screw in normal bone, but not osteoporotic bone. Depending on bone quality, surgeons can use different rescue screws to obtain similar or more fracture compression and construct stability.