chapter 2: Biomechanics of the Patellofemoral Joint

Functions of the Patella

Some authors believe that the patella is not very important in extensor mechanism me¬chanics (1 3) and, therefore, readily recommend patellectomy. Others, on the contrary, attribute to the patella a more prominent role (4 8), recommending its preservation whenever possible.

Perhaps the patella's most important function is facilitating extension of the knee by increasing the distance of the extensor apparatus from the axis of flexion and extension of the knee. Throughout the entire range of motion, the patella increases the force of extension by as much as 50% (9). Hyaline cartilage, with its very low compressive stiffness and coefficient of friction (10), is indispensable for transmitting the quadriceps force around the distal femur to the tibia. This function is underlined by the existence of a superior patella in the tendon of the intermedius muscle in certain mammals who function with the knee in extreme flexion (11).

The patella acts as a guide for the quadriceps tendon in centralizing the divergent input from the four muscles of the quadriceps, transmitting these forces to the patellar tendon. This decreases the possibility of dislocation of the extensor apparatus and controls the capsular tension of the knee. The patella also protects the cartilage of the trochlea as well as the condyles by acting as a bony shield.

Most authors seem to have taken for granted an essential function of the patella, the loss of which results in patients seeking treatment. One of the capital characteristics of hyaline cartilage is its absence of a nerve supply. Healthy cartilage allows the transmission of forces to subchondral and cancellous bone in such a way that the pain threshold of the richly innervated bone is not surpassed. It is also well known that tendons are capable of withstanding great tensile loads, but not high friction or compression. The presence of the patella in the extensor apparatus protects the tendon from friction and permits the extensor apparatus to tolerate high compressive loads.

Finally, the patella plays a role in the aesthetic appearance of the knee. This can be appreciated in the patellectomized knee in which the flattened ends of the condyles are easily visible with the knee flexed. Of all these functions, the most important role of the patella is in extension of the knee. Patellectomy results in weakened extension of the knee or even incomplete knee extension. Some muscle atrophy inevitably follows patellectomy despite sustained and intensive physical therapy.


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