The AO Foundation advocates the use of partially threaded lag screws in the fixation of fractures of the medial malleolus. However, their threads often bypass the radiodense physeal scar of the distal tibia, possibly failing to obtain more secure purchase and better compression of the fracture.

We therefore hypothesised that the partially threaded screws commonly used to fix a medial malleolar fracture often provide suboptimal compression as a result of bypassing the physeal scar, and proposed that better compression of the fracture may be achieved with shorter partially threaded screws or fully threaded screws whose threads engage the physeal scar.

We analysed compression at the fracture site in human cadaver medial malleoli treated with either 30 mm or 45 mm long partially threaded screws or 45 mm fully threaded screws. The median compression at the fracture site achieved with 30 mm partially threaded screws (0.95 kg/cm² (interquartile range (IQR) 0.8 to 1.2) and 45 mm fully threaded screws (1.0 kg/cm² (IQR 0.7 to 2.8)) was significantly higher than that achieved with 45 mm partially threaded screws (0.6 kg/cm² (IQR 0.2 to 0.9)) (p = 0.04 and p < 0.001, respectively). The fully threaded screws and the 30mm partially threaded screws were seen to engage the physeal scar under an image intensifier in each case.

The results support the use of 30 mm partially threaded or 45 mm fully threaded screws that engage the physeal scar rather than longer partially threaded screws that do not. A 45 mm fully threaded screw may in practice offer additional benefit over 30 mm partially threaded screws in increasing the thread count in the denser paraphyseal region.

Cite this article: Bone Joint J 2013;95-B:1662–6.

Conventionally, medial malleolus fractures are treated surgically with anatomical reduction and internal fixation using screws. There seems to be no consensus, backed by scientific study on the optimal screw characteristics in the literature.

We retrospectively examined case notes and radiographs of 48 consecutive patients taken from our trauma database (21 male, 27 female) with an average age of 50 years (range 16-85) who had undergone medial malleolus fracture fixation with screws at the Royal Free Hospital, London between January 2009 and June 2010.

The most commonly used screw was the AO 4.0 mm diameter cancellous partially-threaded screw in 40, 45 and 50 mm lengths (40 mm n = 28, 45 mm n = 26, 50 mm n = 23) with the threads passing beyond the physeal scar in all cases. Incomplete reduction defined as > 1mm fracture displacement was observed on post-operative x-rays in 12 out of 48 cases (25%), all of which relied on partially-threaded screw fixation. In 5 cases where AO 4.0 mm diameter fully-threaded screws engaging the physeal scar had been used, no loss of reduction was observed.

This unusual, occasional use of fully-threaded screws prompted us to investigate further using a porcine model and adapted pedo-barographic transducer. We compared pressures generated within the fracture site using AO 4.0 mm partially-threaded cannulated screws, 4.0 mm partially-threaded cancellous screws and 4.0 mm fully-threaded cancellous screws.

Fully-threaded cancellous 4.0 mm diameter screws generated almost 3 times the compression of a partially-threaded cancellous screw with superior stability at the fracture. Partially-threaded screws quickly lost purchase, compression and stability particularly when they were cannulated. We also observed that screw thread purchase seemed enhanced in the physeal region.

We conclude that fully-threaded cancellous 4.0 mm AO screws are superior to longer partially-threaded screws and that use of cannulated 4.0 mm partially-threaded screws should be avoided in fixation of medial malleolus fractures.