COMPARISON OF MECHANICAL STABILITY OF THREE-SCREW FIXATION METHODS: IN SAW BONE

Introduction: Fracture of cortical long bone can be treated with open reduction and internal fixation. Although the lag-screw technique would provide a stronger compression across the fracture site, this may not translate directly into a significantly better mechanical stability of the construct. In narrow long bone such as the metacarpus, it may be technically difficult to over-drill the near cortex. In addition, the fracture configuration as well as the presence of soft tissue attachment and neurovascular bundle in such smaller bones may give rise to technical difficulty in fixation with all the screws from the same side. Hypothesis: We therefore propose to test the hypothesis that there is no difference in the mechanical stability of the construct, in terms of three points bending and axial loading, between fixation with lag screw (L) and cortical screw (1C) in long bone oblique fracture. Secondly, we proposed that fixation with alternate cortical screws from both sides of the cortices (2C) may confer a stronger mechanical stability than fixation with all screws from one side (1C). Method: A 12 cm long oblique osteotomy was created along the shaft of tibial saw bone to produce an artificial long oblique fracture. The two fragments were held together and precompressed similarly with reduction clamp. Lag screw technique (L), unilateral (1C) and bilateral (2C) simple cortical screws techniques were used to fix the fracture with a total of four 4.5mm cortical screw in each construct. The test piece was placed into position and compressed under displacement control using a Bose ElectroForce ® 3300 Series test instrument (Bose Corporation ElectroForce Systems Group, Minnesota USA). The displacement was applied at a rate of 0.42mm/s. The displacement and load were measured every 0.0586 seconds and recorded using Win-Test Software (Bose Corporation ElectroForce Systems Group, Minnesota USA). Result: In three-points bending, fixation with unilateral cortical screws (1C) provided the highest mean stiffness values of the three techniques with an average of 27.72 N/mm (standard deviation STD 4.90 N/mm). The lag screw technique (L) had slightly lower mean stiffness values with an average of 26.29 N/mm (STD 3.46 N/ mm). Fixation with opposing screws (2C) had a lowest mean stiffness averaging 18.31 N/mm (STD 14.49 N/ mm). In axial compression, unilateral cortical screws (1C) provided the highest average stiffness at 290.33 N/mm (STD 89.84 N/mm) The opposing pre-compression technique had an average stiffness of 241.24 N/ mm (STD 121.30 N/mm) and the lag screw technique 198.94 N/mm (STD 58.33N/mm). Conclusion: In conclusion, fixation with unilateral cortical screws (1C) provided a better mechanical stability than lag screw. Fixation with bilateral cortical screws (2C) did not provide a more stable construct than unilateral screw fixation.