chapter 1: normal anatomy

Embryology

"You made all the delicate, inner parts of my body, and knit them together in my mother's womb. Thank you for making me so wonderfully complex! It is amazing to think about. Your workmanship is marvelous—and how well 1 know it!"

—Psalm 139:13 14 (Living Bible).

According to Walmsley (1) and later confirmed by Gray and Gardner (2), the patella's anlage is distinguishable within the quadriceps condensation by close aggregation of rounded cells in the 20 mm embryo or at 7.5 weeks of gestation (Fig. 1.1). By 30 mm, it is clearly a cartilaginous anlage (1 3). In the embryo, the knee develops in a position of 90° flexion, before motor units exist to induce movement (1). This means that the patella initially conforms to the distal aspect of the femoral condyles that will articulate with the tibial plateaus instance. The patella acquires a free articular surface by the formation of a primitive joint plate in common with the distal femur. The subsequent mechanical behavior of the patella molds it and determines, to some extent, its ultimate shape.

The patella increases in relative size up to the sixth month of fetal life (1), after which it increases at the same rate as the other bones of the lower extremity. Initially, the medial and lateral patellar facets are equal in size. However, by the 192 mm stage (23 weeks' gestation), the patella has already acquired lateral facet predominance, a characteristic of the adult patella and one that plays a key role in understanding disorders of this joint.

Development of the trochlear surface of the femur, the surface that articulates only with the patella, is particularly fascinating. Several authors (1, 4, 5) are in agreement that very early in fetal life, and before movement has occurred, the general adult and human form of the femoral trochlea has been achieved. Langer (5) puts this as early as the 24 mm (8 weeks' gestation) stage. At this stage, the lateral facet extends more proximally, is more prominent anteriorly, and has greater transverse width; in short, all of the essential characteristics of the adult. This is even more striking because this development is not in contact with or in response to the patella but to the quadriceps mechanism.

It would appear that the form of the trochlea is primary and genetically determined. Furthermore, the general forms of both parts of the patellofemoral joint are determined before they are in use. There is, however, at least one exception to this that may be important. Ficat and Hungerford examined 10 stillborn fetuses and did not identify a ridge separating the medial facets, a characteristic of adult patellae. This ridge has been present in several postmortem knee specimens from children between 3 and 8 years of age, which have become available. It would appear that this subdivision of the medial facet is a secondary characteristic, presumably in response to the forces and constraints applied during early life. There is little doubt that form follows function and that the final shape of both the patella and the femoral trochlea will be modified by use.

        

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