and lateral meniscus (fibrous cartilage within a joint) respectively. In the healthy knee, the roll and glide of the femur produce approximately 140 degrees of flexion and extension. Although flexion and extension are the dominant motions at the tibiofemoral articulations, internal rotation and external rotation are also produced at these articulations. The amount of internal and external rotation is a function of flexion and extension. Rotation is absent when the knee is in full extension, but there can be up to 30 degrees of internal rotation and 45 degrees of external rotation when the knee is in 90 degrees of flexion.¹⁵

The patellofemoral articulation is created by the patella and the femur. The posterior surface of the patella articulates with the trochlear groove of the femur. The trochlear groove is a U-shaped concavity located between the two condyles of the femur.9 The patella is a sesamoid bone, which is bone that has developed within a tendon. In this case, the tendon that envelopes the patella is the quadriceps tendon. The patella’s primary function is to act as a modified pulley mechanism that changes the direction of the force vector of the quadriceps. This change in the line of pull increases the momentum, and consequently the amount of torque, giving the quadriceps a mechanical advantage. Another function of the patella is to protect the anterior knee.¹⁵

The knee joints contain two C-shaped fibrocartilage structures, medial and lateral menisci, that are attached to the medial and lateral tibial plateaus respectively (Figures 3 & 4). The two menisci help stabilize the joint by deepening the articular surface of the tibia. They also aid in the absorption of shock and the transmission of force by increasing the articular surface area, produce synovial fluid (a source of nutrients and lubrication to the joint), and help prevent the condyles of the femur from articulating directly on the tibial plateaus, which protects against friction wear of the femur and tibia.¹⁵

The condyles of the femur and the tibial plateaus are covered with a hyaline cartilage called articular cartilage (Figures 1 & 2). The primary function of articular cartilage is to absorb shock in the joint.¹⁹ The articular cartilage does this by absorbing and discharging synovial fluid as pressure changes within the joint.²

The anterior and posterior cruciate ligaments are thick fibrous connective tissue structures that help guide the knee during motion (Figure 4). The anterior cruciate ligament attaches to the tibia at the anterior intercondylar portion of the tibial plateau; it travels upward and backward and attaches to the femur at the intercondylar fossa (an anatomical pit, groove, or depression). The anterior cruciate ligament prevents excessive anterior translation of the tibia with respect to the femur. The posterior cruciate ligament attaches to the tibia at the posterior proximal tibial shaft and attaches to the femur at the posterior intercondylar fossa. The posterior cruciate ligament prevents excessive posterior translation of the tibia with respect to the femur.²

The medial and lateral collateral ligaments also help guide the knee during motion (Figures 3 & 4). The medial collateral ligament attaches to the tibia at the medial proximal tibial shaft and attaches to the femur at the medial epicondyle (any of several prominences on the outer part of a long bone). The medial collateral ligament helps protect against excessive valgus forces on the knee. The lateral collateral ligament attaches to the proximal head of the fibula and attaches to the femur on the lateral epicondyle. The lateral collateral ligament provides protection from excessive varus forces on the knee.²

The articular capsule of the knee is irregular in that it does not completely envelope the joint like most other synovial joints. It is covered with a synovial membrane that produces synovial fluid to lubricate and provide nutrients for the joint structures.¹⁵

The knee joint is crossed by 12 muscles that stabilize the joint and produce the anatomical motions of the joint. These 12 muscles can be divided into three groups: the quadriceps femoris, the hamstring, and the unclassified group. The quadriceps femoris group is comprised of the rectus femoris, vastus intermedius, vastus lateralis, and the vastus medialis (Figure 5). This muscle group is responsible for knee extension.¹⁵ The vastus medialis, however, has an important function in providing a medial force on the patella that counterbalances the lateral components of force generated by the remaining three quadriceps muscles. The dynamic medio-lateral equilibrium created by the vastus medialis helps to maintain patellar tracking.¹⁵ The rectus femoris also assists in hip flexion because the proximal attachment is on the anterior inferior iliac spine.¹⁶