The knee is a frequently injured joint, with its ligaments, menisci (a thin fibrous cartilage between the surfaces of some joints), and patellofemoral (knee) joint vulnerable to acute and repetitive use damage.

Most knee injuries require exercise training for rehabilitation, and some require surgery as well.

Predisposing factors to knee injury include the following:

1. Lower extremity malalignment (e.g. Q angle abnormalities, flat feet);
2. Limb length discrepancy;
3. Muscular imbalance and weakness.
4. Inflexibility;
5. Previous injury;
6. Inadequate proprioception;
7. Joint instability;
8. Playing surface and equipment problems; and
9. Slight predominance in females (particularly for patellofemoral problems).

Ligamentous sprains and tears are common in the knee, particularly in athletes. Because of its structure and insertion points, the anterior cruciate ligament (ACL) is more frequently injured compared with the posterior cruciate ligament (PCL). Classically, the ACL is injured when external rotation of the tibia is coupled with a valgus force on the knee (e.g. direct force from the lateral side of the knee, planting the foot and twisting the knee). Ligamentous sprains and tears are common in the knee, particularly in athletes.

The menisciare also frequently injured, particularly in athletes. The medial meniscus is more frequently torn than the lateral meniscus, due in part to its attachment to the medial collateral ligament. The menisci are poorly innervated (supplied with nerves) and relatively avascular (lack of blood vessels); thus, they are not very pain sensitive and are slow to heal following injury. The “terrible triad” is a traumatic sports injury in which the ACL, medial collateral ligament, and medial meniscus are damaged simultaneously

Patellofemoral pain syndrome is a common disorder in young athletes (particularly females) that produces anterior knee pain. Often, patellofemoral pain syndrome is caused by an off-center line of pull of the patella, which irritates the joint surfaces and retinaculum of the knee. An off-center pull of the patella can result from insufficiency muscular imbalance during knee extension and from excessive varus and valgus stresses (a deformity involving oblique displacement of part of a limb towards/away from the midline, respectively) from Q angles outside of the normal range of 13° to 18°.