chapter 4:imaging the patellofemoral joint


This subject is special enough to require a small separate section. Until the individual is 4 or 5 years of age, the patella remains wholly cartilaginous and thus radiologically invisible. This is important in diagnosing some of the early disorders affecting this joint. The action of abnormal stress on a patella is particularly difficult to evaluate, unlike many joints, until approximately age 11 years. The normal patellofemoral joint passes through three radiologic stages.

Birth to 5 Years

The patellar ossification center has not yet appeared, and the patella is radiologically invisible. The trochlea will, however, show the sulcus, but usually less markedly than it actually is (Fig. 4.27).

Five to Eleven Years

The ossific nucleus appears (Fig. 4.28) and gradually increases in size, attaining more or less final adult appearance by the end of this time. The trochlea likewise reveals more and more of its form as the cartilage anlage ossifies. During this period, little can be appreciated of the relationship between the two bones because of the large volume that remains radiologically invisible. The shape and orientation of the ossific nucleus is directed by the forces applied to the patella. Equilibrated forces result in the ossific nucleus appearing in the center of the patella. Eccentric forces may result in dis­placement (usually laterally) of the ossific nucleus. We do not know yet whether most dysplastic forms are present at birth or formed during the courses of development. Taking experience from the hip, one could surmise that it is a little of the first and a lot of the second.

After 11 Years

The adult form is now recognizable (Fig. 4.29), but in the earlier part of this period, the joint line is thicker than it will be later because not all of the cartilaginous anlage has ossified. By the end of puberty (14‑18 years), the joint line will have assumed its per­manent adult form, which now will be changed only by disease.



Inside Chapter 4: