Which condition can lead to a ball-in-socket type ankle joint?

• A. Middle facet coalition
• B. Posterior facet fracture
• C. Anterior talar process
• D. Achilles tendinopathy

Answer: (A)

The ball-in-socket appearance at the ankle is produced by a coalition at the middle facet of the subtalar joint (talocalcaneal coalition). When the talus and calcaneus fuse along the middle facet, the normally rounded talar head articulates against a socket-like surface formed by the fused calcaneal facet. This fusion creates a ball-and-socket configuration and markedly reduces subtalar motion, often presenting as a rigid flatfoot in adolescence. The other options don’t create this joint pattern: a posterior facet fracture disrupts the joint surface without fusion and a ball-and-socket appearance isn’t produced; an anterior talar process is simply a bony projection; Achilles tendinopathy involves the tendon, not the subtalar joint.