Subchondral bone overgrowth in the presence of full-thickness cartilage defects in the knee.

Between October 2000 and December 2003, 252 autologous chondrocyte implants were performed in 183 patients. Eighty lesions showed overgrowth of the subchondral bone plate under the chondral lesion, this was termed a "bone boss." Thirty-seven were on the medial femoral condyle (MFC), 18 on the lateral femoral condyle (LFC), 21 in the trochlea and 4 on the patella. There was a statistically significant association between the LFC and "bone boss." The lesions showing this finding were of a larger area (3.4 cm2 and 2.8 cm2 respectively, p=0.006), and had more diffuse chondral changes than lesions without. The patients with a "bone boss" had a tendency to longer duration of symptoms (85.3 months and 64.3 months respectively, p=0.089). The "bone bosses" were resected back to the level of the surrounding subchondral bone prior to implantation. Radiological and clinical follow-up showed no statistical difference between the two groups. A discussion of the possible aetiology of the "bone boss" is made.