A search was made for impingement of the femoral part upon the patella on femoropatellar axial views made at a 90 degree angle of flexion in 81 cases of unicondylar arthroplasty (46 medial and 35 lateral). The patients were followed up during a period ranging from 5 to 10 years. 16 cases of impingement between the condyle part and the patella were found. Radiologically, this resulted in a notch on the patella. Clinical symptoms were usually moderate. The anomaly most often found at the onset of this complication was faulty positioning of the condyle part. The complication was more frequently observed in lateral femorotibial arthroplasty (10 cases out of 35) than in medial one-compartment arthroplasty (6 cases out of 46). This variation can be explained on the basis of morphological differences between the medial and lateral sides of the trochleocondylar junction. A study carried out using anatomical specimens, X-rays and scanner sections was used to define the differences.