A biomechanical study comparing the compressive forces generated by a conventional 4.5 AO/ASIF cortical lag screw with a differentially pitched cortical compression screw.

The aim of this study was to compare the interfragmentary compression generated across a simulated femoral fracture model by a conventional 4.5 mm AO/ASIF cortical lag screw with a differentially pitched cortical compression screw. A 45-degree osteotomy was made in a whole bone composite femoral shaft, this was internally fixed with either a conventional 4.5 mm AO/ASIF cortical lag screw or the differentially pitched cortical screw and the compressive force generated at the fracture site measured on an Instron 8874 Axial/Torsion Servohydraulic Testing System. The mean interfragmentary compression generated by the differentially pitched screw was 81.4% of that generated by the 4.5 mm AO/ASIF cortical lag screw. The 4.5 mm AO/ASIF cortical screw produces a steep rise in compression per turn of the screw. The screw based on the differential pitch design creates a more gradual increase to peak compression. The resistance to torque was greater for the AO screw than for the differential pitch screw. Maximal interfragmentary compression is achieved within 4 180 degrees turns after the head engages the near cortex for the 4.5 mm AO/ASIF screw but required 5 180 degrees turns for the differentially pitched screw. Interfragmentary compression is achievable in cortical bone using differential pitch technology. A differentially pitched screw offers obvious advantages over a conventional screw allowing independent placement of lag screw and neutralisation plate, without needing additional exposure of the fracture site, limiting the insult to local fracture biology. It is proposed as an adjunct to osteosynthesis in long bone fractures.