INFERIOR TIBIO-FIBULAR ARTICULATION.
—This articulation is intimately connected with that of the ankle, from which, although anatomically distinct, it cannot virtually be separated.
The tibia and the fibula, at the lower part of the leg, are closely connected for a consider able portion of their extent. The tibia pre sents, on its external aspect and inferiorly, a triangular-shaped surface, two inches in verti cal height, and concave from side to side : superiorly, or towards the apex of this space, it presents a rough and scabrous surface ; but inferiorly it is smooth and encrusted, in the recent state, with articular cartilage. The inner surface of the lower end of the fibula is of similar shape, but convex ; it is rough su periorly, and smooth inferiorly. Here the two bones form an arthrodial articulation.
a. The cartilage, which in this situation in vests the opposed surfaces of the tibia and fibula, is continuous with that which covers the inferior surface of the tibia. It is also lined by—b.—synovial membrane prolonged upwards from the ankle joint, and which forms a small cul-de-sac in the tibio-fibular articulation. The rough irregular surfaces, on the bones above the line of reflexion of the synovial membrane, have the fibres of a strong inter osseous ligament implanted into them.
c. The ligaments of the inferior tibio-fibular articulation are three in number ; 1. an ante rior, 2. a posterior, and 3. an interosseous.
1, Anterior tibio-jibular ligament.—The fibres of this ligament pursue a direction downwards and outwards, from the anterior margin of the small articulating surface on the tibia to the outer malleolus ; and as the lower margin of this ligament projects below the level of the tibia, it deepens somewhat the cavity for the reception of the astragalus. The tendon of the peronmus tertius muscle covers this liga ment in front.
2. Posterior tibio-fibular ligament.—This is a strong, round, fibro-cartilaginous cord, which passes from one malleolus to the other in an arched manner, having a concavity directed downwards, and connected with the posterior ligament of the ankle joint, and a convexity which adheres uniformly to the posterior arti cular margins of the tibia and fibula. This
ligament not only connects the two bones to each other, but it also, like circumferential fibro-cartilages elsewhere, serves the purpose of deepening the mortice-shaped cavity of the ankle joint which it borders. It likewise pre vents the immediate contact of the osseous surfaces in forced extension of the foot, being interposed between the bones as an elastic cushion.
3. Interosseous tibio-fibular is composed of short transverse bands firmly implanted, at right angles, into the opposed rough surfaces on the bones already described. Superiorly, the fibres of this ligament extend nearly as far as the lower margin of the inter osseous nzenzbrane, (separated from it by a small interval, through which passes a branch of the fibular artery), whilst inferiorly they are limited by the direct contact of the two bones of the leg, which they serve to bind firmly together. In order to exhibit this structure, either of two methods may be adopted ; the bones of the leg may be sawn across about their centres, and then forcibly torn asunder, (in this way the ligament may be seen, and its powers of resistance appreciated), or the ligament may be exhibited in situ, by making a vertical, transverse, section of both bones, traversing the two malleoli, and also the joint of the ankle.* Mechanism of the tibio-fibular articulations. —The movements of the fibula on the tibia are extremely limited ; this is in accordance with the general plan on which the skeleton of the lower extremity is formed, its use being to serve as an organ of support, and of locomo tion, only. The bones of the leg are connected together by the intervention of ligaments, (not consolidated together as in the arrangement met with in a few of the Mammalia), and thus a greater degree of elasticity is obtained without any sacrifice of strength ; and it may be pre sumed that the slight degree of yielding and of gliding motion, which is permitted in the tibio fibular articulations, may occasionally serve to diffuse, and to lessen the intensity of shocks applied to the lower extremity, and may thus diminish its liability to injury, especially to fracture.