Is just one screw really enough? Single- versus double-screw in the medial malleolus in supination-external rotation ankle fractures: A comparative biomechanical study using partially threaded cancellous screws

Objective: The aim of this study is to investigate stiffness and the maximum load to failure values of single- and double -screw fixation of oblique medial malleolus fractures using partially threaded cancellous screws. Our hypothesis is that single -screw fixation of medial malleolus fractures after SER injuries provides similar stiffness when compared with double -screw fixation. Design: Biomechanical study. Methods: Twelve composite polyurethane synthetic right distal tibiae were used in the experiment. Oblique fractures of the medial malleolus were created with a band saw using a custom-made osteotomy guide to standardize the cuts in all models. Bone models were randomly separated into two groups and fixed with one (n = 6) or two (n = 6) 4.0 mm partially threaded cancellous screws placed perpendicular to the fracture line. These were tested by applying an offset axial tension at 10 mm/minute up to maximum load displacement, defined as subsidence of the medial malleolus fragment. Maximum load to failure was determined for the groups at the point where the curve ceased to be linear and suffered an inflection. Force versus displacement curves were obtained and recorded. The student's t -test for independent samples was used to compare stiffness (N / mm) and maximum load (N) between experimental groups, with a p value of < 0.05. Results: There were no significant differences in stiffness (p = 0.290) and maximum load (p = 0.191) among the two fixation constructs. Mean stiffness was 62.26 (+/- SD 21.11) N/mm for double -screw fixation group and 48.24 (+/- SD 22.40) N/mm for single -screw fixation group. Mean maximum load was 387.83 (+/- SD 115.78) N for double -screw fixation group and 306.64 (+/- SD 81.97) N for single -screw fixation group. Conclusion: Fixation with one 4.0 mm partially threaded cancellous screw was not shown to be biomechanically inferior to fixation with two 4.0 mm partially threaded cancellous screws in an oblique fracture of the medial malleolus, supporting previous clinical studies that have shown that one screw is sufficient for fractures of the medial malleolus.