Although the patellofemoral (PF) articulation is an integral part of the knee, the two areas are separated in this text because of the differences in the mechanisms and onset of injury. Injury to the patellofemoral articulation is the result of overuse, congenital malalignment, structural insufficiency, impaired motor control, or trauma. The mechanics of the PF joint are interdependent with the foot, ankle, knee, and hip. Examination of individuals with nonacuteonset patellofemoral pathology should include the entire lower extremity, lumbar region, and gait biomechanics.
Lying within the patellar tendon, the patella is the largest sesamoid bone in the body. The patella's anatomical design allows for increased mechanical efficiency of the quadriceps muscle group, protection of the anterior portion of the knee joint, and the absorption and transmission of the joint reaction forces. In the frontal and sagittal planes, the patella is triangular. In the frontal plane, the superior portion is wider than its inferior apex; in the sagittal plane, it is marked with an anterior, nonarticulating surface and a narrower posterior articulating surface (Fig. 11-1).
The left patella. (A) Anterior view identifying the poles; (B) Posterior view identifying the of the articular surfaces and facets. The lateral and medial facets may be conceptualized as having superior, middle, and inferior articular surfaces. The odd facet has no such subdivisions.
Articulations and Ligamentous Support
The patella's articular surface has three distinct facets: the medial, lateral, and odd facets, which create seven unique articular surfaces.1 Each facet is covered with up to a 5-mm thickness of hyaline cartilage, thicker than the femur's articular cartilage. The medial and lateral facets have superior, middle, and inferior articular surfaces. The odd facet, lying medial to the medial facet, has no articular subdivisions (see Figure 11-1B).
At birth, the patella is cartilaginous, and it ossifies between 3 and 6 years of age. When separate ossification centers fail to join, a bipartite patella may result, reducing the efficiency of the extensor mechanism. The superior lateral border is the most common site, with up to 50% of the cases occurring bilaterally.2,3
The mechanics of the patellofemoral joint differ when weight bearing and non–weight bearing. When non–weight bearing, the patella moves on the femur. During weight bearing, the femur moves under the patella. This relationship further explains the need to examine the hip (which controls movement of the femur) in individuals with patellofemoral pain.4
During open-chain knee flexion and extension, the patella tracks within the femoral trochlear groove, the area between the two femoral condyles lined with articular cartilage. As the knee moves from flexion into extension, the patella tracks medially within the range of 45 to 18 degrees. The patella then ...