[Comparative study of a series of normal knees and a series of knees with patellar instability].

In studying the long-term follow-up of 39 cases of patellofemoral instability operated in our department for recurrent dislocation, we also analyzed their morphological appearance on X-ray. We compared the data given by Maldague and Malghem, Dejour and Walch for the pathological conditions of our cases with the same number of normal knees. This comparative analysis showed that measuring the bulging of the upper part of the trochlea and the depth of its groove on a lateral view is a reliable method to evaluate patellar instability. The classical axial view gives less useful information to quantify the instability. However, this axial view shows a particular aspect in the cases of severe instability: the fulcrum of the trochlea is displaced inwards while the inner trochlear facet is smaller or even absent. This precise radiological examination and the complete clinical examination with an analysis of the "morphotype" (Lerat) are necessary in the screening, diagnosis and treatment of severe patellofemoral instability.