For the purpose of simplicity, the ankle joint may be divided into two compartments by an imaginary line passing transversely through the center of the malleoli. The posterior compartment is formed by the posterior surface of the trochlea, the anterior being formed by the relaxation of the anterior ligament over the neck of the astragalus.
In thirteen experiments on thirteen ankle joints of the cadaver, it seemed to be proved that the posterior external approach was a reliable approach and a safe one, as far as encountering important structures is concerned. The joint cavity is easily accessible by this route, and with the foot dorsi-flexed to a right angle, evacuation of the fluid was complete.