The muscular anatomy that provides dynamic stability to the knee is most easily divided into quadrants corresponding to their locations of anterior, posterior, lateral, or medial. These structures enable joint motion to occur and also provide dynamic protection to supporting structures of the tibiofemoral joint, including ligaments and menisci.
The quadriceps muscle group makes up the largest portion of the anterior compartment of the knee and consists of four muscles: rectus femoris, vastus medialis, vastus lateralis, and vastus intermedialis (see figure 4.7). These muscles form a common patellar tendon innervated by the femoral nerve.
The most anterior of the quadriceps muscles is the rectus femoris, originating from the anterior inferior iliac spine and the superior rim of the acetabulum (Cox and Cooper 1994). The three other heads border the rectus femoris distally. The rectus femoris, as a two-joint muscle, performs both hip flexion and knee extension. Lack of flexibility of the rectus femoris can contribute to abnormal patellar tracking.
The vastus lateralis, the largest head of the quadriceps muscles, originates on the anterior inferior greater trochanter, the intertrochanteric line, the lateral lip of the linea aspera, and the intermuscular septum. The fibers run in a 12 to 15º lateral direction to the femur, with a portion of the distal attachment terminating into the lateral retinaculum (Gray 1973; Lieb and Perry 1968). Dominance of the vastus lateralis along with tightness in the lateral retinaculum can result in excessive lateral displacement of the patella.
The vastus medialis originates at the lower end of the anterior intertrochanteric line. The vastus medialis also originates from the linear aspera and intermuscular septum, with a division that originates from the medial supracondylar line and adductor longus and adductor magnus tendon. The distal portion of the vastus medialis, the vastus medialis oblique (VMO), has fibers that run in a 60 to 65° medial direction to the femur (Gray 1973; Lieb and Perry 1968). Together with the vastus medialis longus, which has a fiber direction 15 to 18° medial to the femur, its primary function is to maintain dynamic patellar alignment. Swelling and pain can occur at the VMO because of its oblique fiber direction, which opposes the Q-angle alignment.
The vastus intermedialis originates on the anterior mediolateral surface of the femoral diaphysis. Its fibers run almost entirely in a vertical direction and contribute to extension of the knee.
The four quadriceps muscles converge into the superior pole of the patella and form the quadriceps tendon. Continuing distally, the patellar tendon extends from the inferior patellar pole to the tibial tuberosity. The tendon is widest at the apex of the patella and tapers slightly as it attaches into the tibial tuberosity. On average, the patellar tendon is 5 to 6 cm (2 to 2.4 in.) long and 7 mm thick (Cox and Cooper 1994; Fulkerson and Hungerford 1990a). However, patellar tendon length is actually a function of the height of the patella itself. The patellar tendon morphology may have ramifications for use as a graft source for ACL reconstruction.
The quadriceps function antagonistically to the hamstrings in an eccentric mode to control knee flexion. In this mode of muscular contraction, the quadriceps absorb compressive forces and decelerate the weighted extremity. The fiber direction of the vastus medialis oblique (VMO) serves to control patellar tracking through varying degrees of knee motion. It is critical to maintain dynamic balance of the quadriceps to limit the dominance of lateral structures.