Heterotopic Ossification of the Distal Portion of Biceps Femoris

Heterotopic ossification is the formation of mature lamellar bone in soft tissue. We report a very unusual case of heterotopic ossification arising in the distal portion of the biceps femoris muscle. A 29-year-old woman presented with knee pain after playing golf. She had had no apparent history of trauma. Radiography of the knee showed a calcified mass adjacent to the lateral side of her knee joint. The radiological diagnosis was calcification of the lateral collateral ligament. The pain soon disappeared without treatment. At the age of 45 years, she complained of a growing mass in her left knee that was occasionally painful. Computed tomography and magnetic resonance imaging demonstrated a well matured ossified mass in the lateral side of her left knee. Radiologically and macroscopically, it was found to be in continuity with the distal part of biceps femoris. The mass was excised en bloc. Microscopically, the lesion was mainly composed of well-matured lamellar bone with bone marrow and islands of cartilage showing enchondral ossification. No apparent zoning was found. Cellular atypia was not observed. Extraskeletal osteosarcoma was ruled out. The mass was diagnosed as a heterotopic ossification arising from the distal part of biceps femoris.