Pathology
An understanding of PFPS and its therapy starts with basic biomechanical concepts related to the knee joint. The knee is composed of two joints: the articulation between the tibia and femur (tibiofemoral joint) and that between the patella and the femur (patellofemoral joint). Tracking disorders occur at the patellofemoral joint, so our discussion of biomechanics will focus on this area.

The primary function of the patella is to improve the angle of pull of the quadriceps muscles. Without the patella, the quadriceps group is in a poor position to generate the strong contraction forces that are necessary for knee extension against resistance that is encountered every day in common activities, such as climbing stairs or getting up from a squatting position.

The patella is embedded within the tendon that attaches the quadriceps to the tibia. Since it is embedded in the tendon, the patella moves superiorly along the line of pull created by the quadriceps. The quadriceps group does not pull in a straight superior direction, but in a slight diagonal. This is because the quadriceps (with the exception of the rectus femoris) originate on the femur. The femur has a natural varus (turned inward) angulation, so the quadriceps pull along this diagonal line. The degree to which this pull deviates from a straight vertical line is called the Q (quadriceps) angle.

The Q angle is determined by the intersection of two lines. The first line connects the tibial tuberosity with the midpoint of the patella. The second line connects the anterior superior iliac spine (ASIS) with the midpoint of the patella. Most individuals have some degree of femoral varus angulation, so it is normal for the quadriceps to pull the patella laterally to some degree. Sources disagree on the how much of a Q angle is too much, but the majority indicate a Q angle greater than 15 degrees for females and greater than 10 degrees for males is excessive.

The patella has a ridge down the middle of it, and this ridge must fit in the groove between the two femoral condyles (see Figure 3). As the knee is extended, the patella moves in a superior direction and glides between the two femoral condyles. As the knee is flexed, the patella moves inferiorly. During movement, the ridge on the underside of the patella must stay centered between the femoral condyles. If it does not stay centered, a patellar tracking disorder could result.

The pain of PFPS may come from several sources. When the patella does not move correctly between the ridges created by the femoral condyles, excessive friction on the underside of the patella occurs. This friction may eventually cause the articular cartilage to soften and wear down - a condition called chondromalacia patellae.

Other structures around the knee joint, such as the quadriceps retinaculum and the fibrous joint capsule, are richly innervated and may also be a source of pain. The medial and lateral sides of the patellar tendon have fibrous continuity with the joint capsule, and it is likely that excessive stress on the tendinous fibers may then pull on the capsule. Due to the rich innervation of tissues in this area, it doesn’t take much tensile force to register pain sensations.

The distal portion of the vastus medialis is referred to as the vastus medialis obliquus (VMO), because its fibers mostly run in an oblique direction. It has been speculated that the primary function of the VMO is to offset the tendency of the other quadriceps to pull the patella in a lateral direction. Strength imbalances between the VMO and the other quadriceps muscles have frequently been implicated as causing the pain of PFPS.

References