With the advent of standing CT and MRI scans, there is increasing interest in establishing the role and usefulness of these investigations. When ordering a standing investigation, we assume that 100% of body weight is loaded through the limb, but most machines have handlebars for support and some have seats to allow patients the opportunity to sit. The aim of this study was to evaluate the amount of load going through the lower limbs in various positions supported and unsupported, to explore the range and variation in measurements obtained. Following ethics and local IRB approval, 40 healthy volunteers were asked to stand on an electronic weighing scales and be measured for height. They were then asked to stand on an identical electronic weighing scale on the PedCAT standing CT. Their weight was measured single and double leg stance, with the hands supported and unsupported on the side bars. The subjects were then asked to sit with a single and then both legs on the scale.Aims:
Methods:
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:
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.
