Stability is a very important concept in ankle fractures. The normal stabilisation mechanisms of the ankle are preserved in many fractures, so they will not displace under normal loading. The "precautions" that are often taken in managing ankle fractures (such as plaster casts, avoidance of weightbearing and check radiography) are unnecessary in stable fractures, and those that have been stabilised by surgery.
The most important structure in the fractured ankle is the deep deltoid (or tibiotalar) ligament. This is a short, strong ligament which lies deep and rather posterior in the ankle and can be seen at arthros- copy. It should not be confused with the superficial deltoid ligament which lies more anteriorly and is seen during fixation of the medial malleolus, and which has little influence on ankle fracture stability (Michelson 1996). The deep deltoid acts as a check-rein which prevents abnormal movement of the talus, even if the lateral malleolus is displaced (Michelson 1996) or the syndesmosis is torn (Boden 1989). If the deep deltoid ligament (DDL) is intact, axial loading of the ankle results in the talus moving slightly laterally and becoming fully congruent with the plafond articular surface, which then acts as an additional stabiliser (Michelson 1990, 1996). Therefore, if the DDL is intact, weightbearing will make the ankle more stable, not less, and will not result in talar displacement.
However, if the DDL is torn or the medial malleolus is fractured, the talus is no longer strongly attached to the tibia and the talus can displace laterally. Yablon (1977) described how the talus “follows the fibula” in the unstable ankle. However, it is important to recognise that this only happens when the DDL is detached from the tibia. When the DDL is intact, fibular displacement does not cause talar displacement. In fact, Michelson (1992) and Harper (1995) showed that in stable ankle fractures with fibular displacement, the lateral malleolus is actually congruent with the talus while the proximal fibula rotates abnormally; the apparent external rotation is due to internal rotation of the proximal fragment by muscle action. In fact, as we will see later, even a partially torn DDL will usually restrict talar movement enough to provide stability for functional treatment.
The syndesmotic ligaments, especially the posterior tibiofibular ligament, are important secondary stabilisers. Even if the syndesmosis is torn, abnormal talar movement will not occur in the presence of an intact DDL. Although both main ankle fracture classifications (Lauge-Hansen and AO-Weber, see below) predict that supra-syndesmotic fractures always have a DDL rupture, this is not always the case in real fractures (Hermans 2011).
However, if both DDL and syndesmosis are torn (or medial and posterior malleolar fractures are present), the ankle mortise can become extremely unstable. Burns (1993) found that this combination produced a 39% reduction in tibiotalar contact area and a 42% increase on contact pressure. Syndesmotic injuries are usually accompanied by an interosseous membrane tear and a fracture of the fibula above the syndesmosis. However, the interosseous membrane tear may extend above the fibular fracture, so the injury complex may be more unstable than would be suggested by the level of the fibular fracture (Nielsen 2004).
Unfortunately, the large body of work on ankle fracture stability that has accumulated in the last 15 years is largely ignored in treating patients; the care that most patients get is based on older research taken out of context.
So there is basic science and clinical evidence that some fractures might have an intact deep deltoid ligament and behave in a stable manner. They would have an undisplaced mortise, even if there was slight displacement of the fibula. How common are such fractures, and are they clinically as benign as the above evidence suggests?