ABSTRACT
BACKGROUND: Osteochondroma is one of the most common benign bone tumors, and it may cause bone and joint deformities and limited range of motion of an adjacent joint. The pes anserinus region is one of the most frequent sites of osteochondroma, but knee locking caused by osteochondromas in the pes anserinus region is extremely rare. CASE SUMMARY: We describe a 13-year-old Japanese girl's extra-articular knee locking that occurred when the semitendinosus tendon got caught in osteochondroma that had developed in the pes anserinus region. The osteochondroma was surgically resected. The postoperative outcome has been excellent, with no recurrence of knee locking or tumor one-year post-surgery. CONCLUSION: When a young person develops knee locking, the possibility of extra-articular as well as intra-articular locking should be considered. Osteochondroma, one of the causes of extra-articular locking, can be treated with surgery with good postoperative results.