Biomechanical evaluation of medial malleolus fractures treated with headless compression screws

The gold standard for fixation of medial malleolus fractures has yet to be determined. Most agree the best results of displaced unstable ankle fractures are with open reduction and internal fixation. Hardware irritation necessitating screw removal is a known complication. An alternative fixation method of medial malleolar fractures has been described using headless compression screws. There are currently no biomechanical studies that assess the stability of this method. The purpose of the study is to complete a biomechanical evaluation of partially threaded cancellous screws (PT) and headless compression screws (HC) in an external rotation ankle fracture model.

Composite polyurethane sawbone models of tibia were obtained. A custom jig was created to ensure identical osteotomies. The models were fixed with either two partially threaded cancellous screws or two headless compression screws. The models were fitted into the Material Test System (MTS) machine and the force transducer was programed to apply axial offset. The two constructs were loaded until 5 mm of displacement occurred, our defined point of failure. The amount of force (Newtons) necessary to create fracture line displacement was recorded for each model.

The axial rotational force to create facture displacement between 1 mm and 3 mm, between 1 mm and 5 mm, and between 3 mm and 5 mm were statistically significant (p < 0.05) for both models. The mean axial rotational force measured in Newtons at 1 mm, 3 mm, and 5 mm of displacement for the PT was 149.32, 244.19 and 477.76 respectively. The mean axial rotational force measured in Newtons at 1 mm, 3 mm, and 5 mm of displacement for the HC was 152.05, 224.07, and 498.31 respectively.

No statistically significant difference was found between the biomechanical properties of partially threaded cancellous screws and headless compression screws used in the fixation of medial malleolus fractures. These results support HC screws as a viable alternative in a biomechanical model of medial malleolus fractures.