Para osteo arthropathy--ectopic ossification of healing tendon about the rodent ankle joint: histologic and type V collagen changes.

Ectopic bone formation, or para osteo arthropathy (POA), is commonly seen in humans following lesions to the central nervous system. The condition also occurs after severe trauma, burns, or tetanus, and, rarely, after poliomyelitis or cauda equina lesions. The lack of a suitable laboratory animal model has hampered study of its etiology and treatment. The lesion occurs just above the rodent ankle joint 7 weeks after repair of the severed achilles tendon. Tendon repair using a single silk suture resulted in ectopic calcification in 22 of 23 animals. Ectopic ossification, with extensive mineralization, occurred in 11 animals, 5 of which showed bone marrow. Ectopic mineralization was moderate in 8 others, minimal in 4, and absent in one. Collagen typing thrice during healing indicated that type V collagen increased by one-third at 10 days, doubled at 20 days, and returned to near normal amounts at 50 days. Since type V collagen is a major component of the nonfibrous collagen of blood vessels, increased amounts of type V collagen are consistent with the presence of many new blood vessels in the granulation tissue at 10 and 20 days. Vascularity is less dense at 50 days when ectopic mineralization and new bone formation are observed. These findings suggest that POA and the degree of maturation of healing tendon may be linked to reduced amounts of type V collagen. It is concluded that the sutured tendon of the rodent ankle joint offers an animal model by which to study ectopic bone formation, or para osteo arthropathy.