The origin of femoral trochelar dysplasia: comparative anatomy, evolution,and growth of the patellofemoral joint

We performed a comparative analysis of the femoropatellar morphology examin ing the evolutionary aspects to search for the origin of trochelar dysplasi a. Trochlear dysplasia is frequent in the human population and, when associ ated with morphological and positional abnormalities of the patella, can le ad to patellar pain syndrome in minor cases or patellar dislocation in seve re cases. There is no strict relationship between the observed anomalies an d clinical expression. The shape of the articular surfaces is variable in m ammals depending on their type of locomotion: unguligrade, digitigrade, pla ntigrade. In greater apes, the femoral diaphysis is straight and the trochl ea is flat and symmetrical. The patella does not tend to dislocate laterall y since the knee under load is always flexed. In human adults, the femoral diaphysis has a valgus obliquity angle of 8 degrees to 10 degrees. Conseque ntly, the trochlea has a deepened sulcus and an elevated lateral lip, avoid ing lateral patellar dislocation, especially during initial knee flexion. I n the human newborn, the femoral diaphysis is vertical. As the child starts walking, the femoral obliquity angle develops between 1 and 7 years of age , inducing a secondary valgus of the extensor apparatus. This obliquity doe s not develop in non-walking children. Fossil femurs of australopithecus de monstrate that a high obliquity angle had appeared more than 3 million year s ago, but also exhibit a poorly deepened trochlea and a slight elevation o f the lateral facet. At 1.8 million years, the fossils have the oblique dia physis, the strongly deepened sulcus and the strongly elevated lateral face t. The obliquity angle of the femoral diaphysis is the leading feature whic h initiated the later modifications of the patellofemoral joint that over 3 million years were never inscribed in the human genoma. Lateral trochleal lip and deep sulcus are features that were first acquired, then once select ed, genetically assimilated, and now appear on the fetal cartilaginous epip hysis.