chapter 2: Biomechanics of the Patellofemoral Joint

Law of Valgus

This is an aspect of the physiology and biomechanics of the patellofemoral joint that is so important for understanding both normal and abnormal function of the knee that we feel compelled to treat it as a separate section and elevate it to the level of a "law." The physiologic valgus of the lower extremity is a characteristic only of those animals who walk on their hind legs, being most pronounced in man. Bringing the lower extremity to the midline reduces the work required for maintaining balance. A quadruped normally, whether walking or running, is supported by one extremity on each side of his body. The physiologic valgus in the biped is then important and necessary. However, it has its effect on the knee.

The physiologic valgus per se would not necessarily dictate that the forces would have a valgus vector except for the fact that the muscles follow the longitudinal axis of the femur, giving a Q angle facing laterally. From the soft tissue point of view, the lateral stabilizers are considerably stronger and more fibrous than the medial counterpart. It is not exactly clear why this should be so, but it is anatomically evident. We do not mean to imply that we find these facts to represent a morphologic failure. In fact, the patellofemoral joint is a remarkable design to fulfill the demands imposed on it. It also in many ways appears to be a priori, because the design is present in the fetus long before there is any apparent mechanical need for it. However, understanding valgus helps the understanding of nearly all of the pathophysiology involved in this joint and also how one might restore the delicate balance that is compatible with symptom‑free function.

Morphologically, the "law of valgus" finds expression in the predominant lateral trochlear surface. This predominance is evidenced both in size and anterior projection. Also, the most frequently discovered patellar form (Wiberg Type II) has a definitely larger lateral facet than medial facet. Rather than consider this a dysplastic form, it seems to be one that is normal for the joint and capable of properly modulating the forces that pass through it. The lateral soft tissue elements reinforced by the fascia lata are balanced by similar medial soft tissue stabilizers. The congruence of the contact surfaces offers further stability, which, however, decreases in terminal extension. The orientation of the lateral trochlear facet offers further impediment to lateralization. This is the stage in which disorders of the patellofemoral joint will be played. We will see the tremendous importance of this setting as each of the acts unfolds.


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