This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
Fractures of the bones of the leg are most often due to direct violence, but sometimes to indirect. The tibia is rarely broken alone, but either it or the fibula may be fractured by a direct blow. On account of the tibia being subcutaneous these fractures are frequently compound. The shafts of the bones, being of compact tissue, are usually broken obliquely. When the fibula is broken above its lower fourth there is usually little displacement because the attached muscles hold it in place.
Fractures of the tibia whether accompanied or not by fracture of the fibula most often occur at the junction of the middle and the lower thirds. The line of fracture is downward, forward, and inward. The displacement of the lower fragment is backward, upward and slightly outward. It is produced mainly by the muscles of the calf pulling on the tendo calcaneus (Achillis). The upper fragment is pulled forward by the quadriceps fem-oris (Fig. 564).
The difficulty usually encountered in treatment is a persistent projecting forward of the upper fragment with a drawing up and turning outward of the lower fragment and foot. The displacing action of the tendo calcaneus (Achillis) is more powerful than that of the quadriceps. On this account the first attempt at correction should be to place the leg in the "Pott's position." This consists in flexing the knee to a right angle and placing the leg on its outer side. This relaxes the gastrocnemius and plantans and is sufficient in some cases to allow of the displacement being remedied. If this fails extension may be tried or tenotomy of the tendo calcaneus should be done and the fragments will at once come into good position.
Fig. 564. - Fracture of the tibia with displacement of the upper fragment forward and lower fragment backward and upward.
Woolsey has pointed out that the weight of the foot tends to its outward displacement but another reason is that the insertion of the tendo calcaneus is not beneath the middle of the ankle-joint but more towards its outer side, so that when it contracts it carries the foot outward. The flexor and extensor muscles of the leg balance each other, but the peronei muscles on the outer side have no additional corresponding opponents on the inner side; hence another reason for displacement, of the foot and lower fragment outward.