Dysplasia of the trochlea was studied on a strict profile image of the knee. 1305 radiographs were analysed corresponding to several patello-femoral conditions (major and subjective patello-femoral osteoarthritis) and to control subjects. Two criteria are defined: the depth and the eminence of the trochlea. The depth was often minimal in instabilities that give a radiological characteristic image of intersection between the trochlear end line and the lateral condyle. This picture had a great diagnostic value for patellar instability. The eminence represented the owerhang of the trochlear end line in relation to the anterior cortex of the femur. By measuring it, it was possible to establish a statistically significant variability of the degree of instability on one hand, between instabilities, patellar syndromes and controles on the other hand. The intersection sign and the trochlear eminence in isolated patello-femoral osteoarthritis allows the direct filiation between dysplasia of the trochlea and osteoarthritis to be confirmed. Taking into account the size of the eminence, the hollowing out type trochleoplasties appears more fiable than raising of the lateral edge.