Heterotopic ossification after total knee arthroplasty. 54/615 cases after 1-6 years' follow-up.

We found heterotopic ossifications in 54 (9%) of 615 cases after total knee arthroplasty. The largest ossifications were located in the anterior distal femur. In 12 cases smaller ossifications were found in other knee regions. The development of heterotopic ossification showed a positive correlation with hypertrophic arthrosis and a negative correlation with rheumatoid arthritis. We propose a new 3-grade classification which refers only to the anterior distal femoral region. Grade III heterotopic ossifications occurred in 4 patients (4 knees) who had clinical symptoms; 2 were successfully reoperated with removal of the ossifications. Prophylaxis should be considered in patients with marked hypertrophic arthrosis or marked periosteal damage to the anterior distal femur.