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 bony mortise is the most important factor in stabilising the ankle when it is loaded. Putting weight on the ankle makes the talus fit closely into the mortise, and the shape of the bones produces stability. As it does so, it may move 1-2mm laterally, finding the "best fit" position. Recent work shows that some of the variation in stability of the normal ankle can be accounted for by variation in the size and shape of the talus.
Without axial loading, the ligaments become more important for stability, especially in plantar flexion. The anterior talofibular ligament is most important in plantar flexion. The calcaneofibular, posterior talofibular and tibiofibular ligaments are more important in plantigrade/dorsiflexion - the data are less clear for this position. The superficial deltoid ligament resists valgus strain and also contributes to stabilising the midfoot in early stance phase. The deep deltoid acts as a check-rein on the talus, resisting external rotation and translation - this is a crucial function in most ankle fractures.
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?