Black tendon-identifying a rare autosomal recessive disorder: Intraoperative diagnosis of alkaptonuria: A case report.

Tendon injuries in the lower extremities are common in physically active individuals, although spontaneous Achilles tendon ruptures are linked to oral corticosteroid or fluoroquinolone use. Such ruptures are typically due to degenerative changes within the tendon and frequently occur during sudden dorsiflexion of a plantar-flexed foot. Bilateral spontaneous ruptures are especially prevalent in patients undergoing long-term corticosteroid therapy. Here, we present a rare case of bilateral spontaneous calcaneal avulsion in an otherwise healthy woman, ultimately diagnosed with alkaptonuria. This case underscores the importance of considering metabolic disorders in atraumatic tendon ruptures and highlights the diagnostic value of intraoperative findings.