A biomechanical study comparing a raft of 3.5mm cortical screws with 6.5mm cancellous screws in depressed tibial plateau fractures

There has been a recent trend towards using a raft of small diameter 3.5mm cortical screws for supporting depressed tibial plateau fractures (Schatzker type III). Our aim was to compare the biomechanical properties of a raft of 3.5mm cortical screws with that of 6.5mm cancellous screws in a synthetic bone model. Ten rigid polyurethane foam (sawbone) blocks, with a density simulating osteoporotic bone and ten blocks with a density simulating normal density bone were obtained. A Schatzker type III fracture was created in each block. The fracture fragments were then elevated and supported using two 6.5mm cancellous screws in ten blocks and four 3.5mm cortical screws in the remaining. The fractures were loaded using a Lloyd testing machine. The mean force needed to produce a depression of 5mm was 700.8N with the four-screw construct and 512.4N with the two-screw construct in the osteoporotic model. This difference was highly statistically significant (p=0.009). The mean force required to produce the same depression was 1878.2N with the two-screw construct and 1938.2N with the four-screw construct in the non-osteoporotic model. Though the difference was not statistically significant (p=0.42), an increased fragmentation of the synthetic bone fragments was noticed with the two-screw construct but not with the four-screw construct. A raft of four 3.5mm cortical screws is biomechanically stronger than two 6.5mm cancellous screws in resisting axial compression in osteoporotic bone.