It should be remembered that the tibia alone articulates with the femur, and therefore bears the whole Weight of the body, while the fibula supplements it, particularly in resisting lateral and torsion forces, and assists in forming the ankle-joint by the external malleolus. The tibia is thickest in front, in the region of the crest, and about the centre of the bone. The narrowest part is at the junction of the middle and lower thirds, measuring about 1 inch in transverse diameter, while the average diameter is 1¾ inches, and farther, at this point, internally, two columns of spongy matter, one occupying the upper two-thirds, and the other the lower one-third, meet, and thus this point is the weakest, and it is here that fractures of the tibia from indirect violence generally occur.
Fracture of the tibia alone is generally due to direct violence, affects the lower one-third most frequently, and when transverse often presents no displacement, the fibula acting as a splint, and occasionally enabling the patient to walk.
The nutrient foramen of the tibia is the largest in the body, and enters the posterior surface of the bone in its upper third, whence it is directed downwards in the compact shaft for about 2 inches before entering the medulla. The shaft of the tibia is very frequently affected by acute osteomyelitis, which, if it succeeds in plugging the nutrient artery, in addition to stripping the periosteum, may cause death of the shaft from cutting off the blood-supply. In operating on the tibia for osteomyelitis the disease is easily attacked through the superficial broad internal aspect. The shaft of the tibia is also a frequent seat of syphilitic nodes, a chronic periostitis being set up, which results in the formation of a characteristic spindle - shaped osseous node. In some cases gummatous changes take place, the skin breaks down, and deep ulcers form, while the bone may become softened and bend.
Rickets attacks the tibia and also the fibula at an early stage, various deformities being produced. Perhaps the most common is a curve, with the convexity directed forwards, and inwards or outwards, occurring about the junction of middle and lower third, the malleolus touching the ground in severe cases. A general curving of the tibia, with the convexity outwards, occurs frequently in genu varum, and almost the whole deformity in some of these cases lies in the tibia, and not in the femur. A cuneiform osteotomy is generally performed to rectify the more acute tibial curves, a wedge being removed from the convexity of the curve. The lower epiphysis of the tibia, which includes the malleolus and the fibular facet, may be separated by injury. It unites with the shaft about the eighteenth year. The malleolus may be broken by violence, and its tip is frequently torn off in Pott's fracture of the fibula.
The shaft of the fibula is narrow, but strong, compact, and somewhat elastic, presenting several sharp projecting ridges, which act as flanges, and give the bone in section a shape not altogether unlike that of a girder. As in addition the bone is covered by muscles, it is not often broken alone, despite its exposed position, save in its lower one-fourth. The fracture which occurs in that position is due to indirect violence, is called Pott's fracture, and is one of the most frequent and important fractures in the body. As it is associated with dislocation of the ankle, it will be considered under that region. Other fractures of the fibula alone are generally due to direct violence, are transverse, and, the tibia acting as a splint, produce little or no displacement. Where both bones are broken by indirect violence, the tibia breaks about the junction of the middle and lower one-third, the line of fracture running obliquely downwards, forwards, and inwards, while the fibula breaks higher up. This fracture is very apt to become compound, not only because the crest of the tibia is sharp and very superficial, but because both the lower end of the upper fragment tends to project forwards, and also the upper end of the lower fragment, being tilted by the tendo Achillis pulling the heel backwards and upwards. The weight of the foot also assists in producing this displacement of the lower fragment, and in addition it generally produces slight eversión. The superficial position of the tibia also favours comminution in fractures by direct violence.
A spiral fracture of the tibia, due to torsion, sometimes occurs at the junction of the middle and lower third, running obliquely down and inwards, the upper fragment presenting a V-shaped extremity (Gosselin), while in some cases there is in addition a vertical fracture, extending down through the lower fragment to the ankle, and the fibula may break at a higher level.
Amputation of the leg used to be performed at a point a hand's breadth beneath the knee-joint (seat 0f election), the stump left by this operation being short, and enabling the patient to walk by kneeling on a pin-leg. Nowadays, speaking generally, the stump should be left as long as possible, the forms of amputation used being adapted to circumstances. Amputation at the seat of election, or middle third, may be performed by long anterior and short posterior flaps, or by a single large external flap, which may be made to contain the anterior tibial artery (Farabceuf). Amputation in the middle or lower third may be done by a large posterior flap ; and Teak's amputation, in which a long anterior flap, four times the length of the posterior, is taken, and then turned over on itself, so that the scar is quite away from the apex of the stump, may be performed in the lower third. It is generally well to bevel the cut end of the tibia so as to remove the sharp tip of the crest, and in amputations at the upper extremity of the leg some recommend the removal of the upper end of the fibula. Owing to the fibula lying behind the plane of the tibia, it is sometimes possible in amputating to run the knife from within outwards behind the tibia, but between it and the fibula, and so get it jammed.