Genesis of the ball-and-socket ankle.

Ball-and-socket ankle is a rare deformity associated with such pre-existing conditions as congenital shortening of the lower limb, coalition of tarsal bones, absent digital rays and aplasia or hypoplasia of the fibula. We have observed seven patients with this deformity for an average of six years from initial examination at 20 days to 3.5 years. Arthrography showed that the configuration of the ankle was apparently normal in patients under 10 months of age and that a ball-and-socket joint develops by four to five years of age, possibly in compensation for the loss of inversion and eversion caused by tarsal coalition. Ball-and-socket joint is therefore probably not congenital, but is an acquired deformity secondary to various pre-existing congenital conditions.