Anomalies of ossification in the posterolateral femoral condyle: assessment by MRI

Background. Anomalies of ossification in the lower femoral epiphysis are of ten radiographically indistinguishable from juvenile osteochondritis dissec ans. Objective. To clarify the MRI characteristics of the anomalies of ossificat ion in the posterolateral femoral condyle that distinguish it from juvenile osteochondritis dissecans. Materials and methods. We retrospectively exami ned the medical records, plain radiographs (n = 4), MRI (n = 4) and follow- up MRI (n = 2) of four boys (age 8-11 years) with anomalies of ossification in the posterolateral femoral condyle. Results. Plain radiography showed symmetrical marginal irregularity of the posterolateral femoral condyles of both knees. These lesions were asymptoma tic, and the areas of irregular radiographic appearances reduced in size or disappeared without treatment within a mean observation period of 3.5 mont hs. MRI showed a clearly demarcated low-intensity islet with the same signa l intensity as subchondral bone (which was considered to be an accessory os sification nucleus) in a high-signal area in which the signal intensity was equal to that of normal articular cartilage. The areas observed as radiolu cent zones on plain radiography were visualised at the same signal intensit y as articular cartilage, and were continuous with articular cartilage on M RI; thus they were regarded as uncalcified cartilage. These MR findings are different from MR images of osteochondritis dissecans. Conclusions. MRI is considered to be the most effective non-invasive diagno stic method for these two conditions.