When we leave the limb for a moment to itself, we notice that there is a remarkable change in the point of incidence of the axis of the leg upon the foot. The tibia and upper fragment of the fibula, although really remain ing in their natural position, appear driven in wards, while the foot is rotated outwards.
The changes of direction of the leg and foot are such, that if the axis of the leg were pro longed inferiorly, instead of falling on the astragalus, it would leave this bone, and con sequently the whole foot, more or less on its outer side; hence the impossibility patients experience of bearing upon the foot , which only presents its inner edge to the ground.
This change is a necessary and constant effect of the displacement of the foot, when the fibula ceases to support it on the outer side, and when the peronai muscles begin to con tract. The foot and external malleolus which make part of one system, move in one direc tion; the tibia and upper fragment of the fibula move, or, to speak perhaps more correctly, remain, in another. The centre of this new motion is no longer in the articulation, but, in an oblique line, passing through the joint, and extending from the malleolus internus to the point of fracture of the fibula: this line is well expressed in Jig. 51, representing the frac ture of the fibula, and taken from the engrav ing which accompanies the work of Pott.
The retiring angle seen (fig.51, 52, a) in this partial luxation of the tibia inwards, on the outer part of the articulation, and the pro jecting one ( b) existing at the inner, consti tute the most striking features of the accident ; these angles correspond exactly to the extremi ties of the line above-mentioned, in the direc tion of which the weight of the body acts, when the foot being turned outwards this line may be seen to traverse the leg obliquely from the lower part of the upper fragment of the broken fibula to the malleolus internus.
We cannot omit to notice also, that there is in all these cases a remarkable rotation of the whole foot on its long axis, in such a direction that the upper surface of the astragalus looks obliquely upwards and inwards, (fig. 52, c,) the inner edge of the foot is turned downwards, the sole inclined outwards, the outer edge raised, and the dorsum turned directly upwards. The extent of this rotatory motion is besides always proportioned to the displacement out wards; both are attributable to the same causes, viz. the weight of the body, and the action of the peronmi muscles, when the patient has at tempted to walk after the fracture has occurred.
It is on these combined movements when not corrected by a proper mode of treatment, that the deformity of the foot, and all the consequent difficulties in walking, depend.
Complete luxation of the tibia inwards from the astragalus, complicated with a simplefrac .ture of the fibula.—This is a very severe, and, fortunately, a very rare accident. In alluding to it, Dupuytren says,'"' that " the foot is not only susceptible of being carried outwards, but also upwards at the same time ;" a double displacement, which he had observed to occur only once in 200 cases of fractures of the fibula treated in the Hotel Dieu for fifteen years, " but the case was so marked," he says, " that in future it cannot be mistaken or passed over in silence." It cannot occur unless the fibula is fractured ; for this condition is indis pensable to any displacement of the foot in wards or outwards; it requires besides a com plete laceration of the short thick ligaments placed between the tibia and fibula, the strength of which is such that, in most experiments on the dead subject, they resist more powerfully than the structure of the bones themselves.
It was as a consequence of the fracture of the fibula and a rupture of these ligaments, that, in the case alluded to, the astragalus was seen dislocated outwards, and then drawn up on the outer side of the tibia. In short, the astragalus, the malleolus externus, and the foot, which formed but one system of parts firmly connected, were drawn first to the outer side of the leg, and then two inches upwards on the tibia.
A carpenter, aged fifty-four years, was ad mitted into the Hotel Dieu, in February, 1816. His right leg presented all the signs of fracture of the fibula at its inferior part, such as devia tion and rotation of the foot outwards, promi nence of the tibia, and of the internal malleolus inwards, depression and crepitation above the outer ankle ; but that which most attracted the attention was, 1st, the shortening of the limb, and, 2dly, the enormous increase in breadth of the space which should naturally intervene be tween the two malleoli. The sinking down of the lowest part of the tibia, even to the level of the sole of the foot, where the projection of the internal malleolus could be felt, the elevation of the astragalus, of the peroneal malleolus, and the whole of the foot along the external surface of the tibia, even to two inches, were all symp toms quite unusual in fracture of the fibula, and left no doubt that the ligaments which stretched inferiorly from this bone to the tibia had been lacerated, and that the foot, yielding to a violent effort from within outwards, and from below upwards, had been luxated in these directions, and had carried with it the peroneal inalleolus. This then is evidently a case of complete dislocation of the tibia inwards, or, as the French writers would call it, a luxation of the foot outwards and upwards.