chapter 2: Biomechanics of the Patellofemoral Joint
In full extension, the patella articulates with the supratrochlear fat pad. Normal knee valgus creates an angle between the line of pull of the quadriceps and the patellar tendon, the so‑called "Q angle" (Fig. 2.8). Hvid and Andersen (38) showed that the Q angle correlates with internal hip rotation. The "screw home" mechanism of the tibiofemoral joint in terminal extension (whereby the tibia rotates externally in relation to the femur) further lateralizes the tibial tubercle. With the quadriceps contracted and the knee fully extended, the patella is free from the confines of the femoral trochlea. The pull of the quadriceps then produces a valgus vector (Fig. 2.9), which is resisted by the medial patellar retinaculum (Fig. 2.10) (32) and the vastus medialis. It is dangerous, however, to focus too much on the Q angle. The clinician must recognize that this is only one of many factors affecting patellar balance. An increased Q angle provides no direct correlation with patellofemoral pain, any more than correcting a high Q angle assures consistent relief of pain.
With the knee fully extended and the quadriceps flaccid, the distal patella rests at the proximal trochlea. Setting the quadriceps produces 8 to 10 mm of proximal movement of the patella. This proximal movement is limited by the patellar tendon. In most cases, the proximal movement has a definite lateral component, although Moller et al (39) found no difference in vastus medialis and lateralis contraction by electromyography in patients with symptomatic subluxation and chondromalacia. Stein et al (40) stated, however, that the patella moves medially with normal ambulation, and this is consistent with clinical observations. Hefzy and Yang (41) also noted medial translation of the patella in the first 40° of knee flexion.
Observing the course of flexion of the patella, beginning from a position of forced full extension, can bring out some of the dynamics associated with various pathologic conditions. Van Eijden et al (42) pointed out that there is a linear relationship between the angle of knee flexion and movement of the patella and patellar tendon with respect to the tibia. During the first 20° of flexion, the tibia derotates. This significantly decreases the Q angle and also decreases the lateral vector. The patella is drawn into the trochlea, and the first articular contact is made by 10° knee flexion. The patella enters the trochlea from a slightly lateral position. This course of patellar movement can be followed nicely with the help of serial computerized tomographic (CT) or magnetic resonance imaging (MRI) slices of the patella during progressive knee flexion. From 20 to 30° of flexion, the patella becomes more prominent as it is lifted away from the axis of rotation of the knee by the prominence of the femoral trochlea (4). Beyond 30°, the patella begins to settle into the deepening trochlear groove. Instability beyond 30° is less common, and many patellofemoral pain problems are associated with abnormal patellar tracking in the first 30° of knee flexion. Also, if static alignment of the patella is excessively tilted to the lateral side, flexion of the knee will cause posterior movement of the iliotibial band, and the lateral retinaculum will experience abnormally elevated tension as the patella is drawn into the trochlea by medial retinacular pull (Fig. 2.11).
Many factors, including geometric characteristics of contact surfaces (20), are brought into play to maintain relatively constant unit loads. Cartilage thickness and subchondral bone (43) quality will affect the patellar response to stress. Analytical stereophotogrammetry (44, 45) now permits cartilage thickness determinations in vitro to an accuracy of 90 microns. Rotation of the femur (46) during knee extension also affects patellar tracking. The whole complex system of adaptation to maintain constant unit load, therefore, must be considered to understand biomechanical function of the patellofemoral joint. Perhaps newer techniques, such as stereophotogrammetry (47), will permit us to understand better the complex interaction between knee kinematics and patellofemoral contact pressure. In any meaningful model, however, retinacular forces (48) must be considered.
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