A knee injury is any form of damage caused to either the bony aspects of the knee joint or its accompanying ligament, tendon, or cartilage structures. There are an almost unlimited variety of ways in which the knee either absorbs the forces of movement, such as those occurring in any running or jumping activity, or in which it comes into sometimes violent collision with other objects. It is for this reason that the nature and the severity of knee injuries is wide ranging.
The knee joint acts a hinge, flexing and extending in concert with the muscles of the upper and lower leg. The knee function is essential to the ability of the body to propel itself forward, backward, upward, or in a lateral direction. The knee is a rather small mechanism relative to both the mass of the body that it assists in support, as well as in proportion to the forces exerted on it in movement. The size of the joint, coupled with the very limited rotational movement permitted the joint, increase its natural vulnerability to injury.
The skeletal components of the knee joint are the femur (thigh bone), the tibia and fibula (the bones of the lower leg that extend to the ankle), and the patella (kneecap), which is the protective cap positioned over the front of the joint. Two meniscuses, which is a cushioning cartilage, are positioned between the femur and the head of the tibia and fibula. The joint is effectively stabilized by a network of ligaments connecting the joint bones, of which the anterior cruciate (ACL), the medial collateral (MCL), and the posterior cruciate (PCL) are most subject to sports injury. The patellar tendon is a connective tissue situated below the kneecap.
Knee injuries occur as a result of three distinct types of mechanisms: overuse injuries, which are often related to either structural imbalances or postural factors; twisting or explosive movements that do not involve any external contact; and contact to the knee joint that causes structural damage. In some cases where a twisting or explosive movement is coincidental with the delivery of a significant force to the knee, the outcome will often be a catastrophic knee injury.
Overuse knee injuries occur most typically in sports involving intense repetitive motion. Distance runners are particularly vulnerable to such injuries, as the forces generated are often magnified in one knee or the other due to imbalances in the athlete's leg length or pelvic and hip joint alignment, which results in an unequal distribution of forces. Over time, the knee cartilage can become worn and, as the knee loses some of its absorbent quality, running can produce pain. This condition is often addressed by the use of an orthotic, an insert in the running shoe that stabilizes gait.
Another common knee injury that arises in basketball and similar sports in which the athlete seeks to be explosive in movement is an irritation of the patellar tendon. This form of tendonitis is known as "jumper's knee." The repeated extension and contraction of the tendon in the jumping motion can cause it irritation through contact with the adjacent bone. In an extreme case, where the tendon is weakened through tendonitis, it may spontaneously rupture as the athlete attempts to jump.
Serious knee injuries will sometimes occur without the knee being subjected to the force of external contact. When an athlete is moving across an uneven surface, if a foot becomes lodged in a hole or depression, the knee may be violently twisted in a direction opposite to that in which the athlete was moving. These forces can cause a variety of injuries to the joint, ranging from a dislocation of the knee to a sprain or tear of one of the ligament structures. Similar injuries occur on playing surfaces where the athlete's foot sticks without warning to the surface. The effect is similar to the foot becoming caught in an uneven surface, as the knee tends to be moving in a forward direction when the sticking mechanism twists it in an opposite manner. The earlier versions of the artificial surfaces used in football and baseball facilities in the United States were notorious for these serious non-contact injuries. This mechanism has also been the cause, although less frequently, of similar basketball knee injuries. The type of footwear worn by the athlete, particularly cleat length in field sports, is also a contributing factor to the degree of adhesion between the athlete and the surface.
The most dramatic knee injuries arise when the knee joint is the subject of a direct blow. The severity of the injury is not solely determined by the amount of force delivered to the knee; the angle of the impact, the position of the knee at impact, and any prior injury to the joint, particularly the ligament structures, are important factors. As a general rule, a blow to the side of the knee places greater stress on the ligaments.
A blow delivered to the kneecap from a head-on position can result in a number of consequences. The least serious of these is a hyperextension of the knee joint, where the joint is forced past the 180½ line that represents the line of the leg when the knee is fully extended. A severe hyperextension may also cause ligament damage. A blow of this nature, if sufficiently forceful, may also separate the natural connection of the femur and the tibia and fibula structure, resulting in a dislocation. When the joint is dislocated, there is usually significant ligament damage as well; these events often require surgery. Depending on the nature of the blow, the patella may sustain a fracture, often a consequence of an unprotected fall onto a hard surface.
A blow delivered laterally to the side of the knee as the foot is planted will often result in severe sprain or complete tear of the anterior cruciate and medial collateral ligaments. The knee has no power to absorb or redirect forces that are applied from that direction, which results in the ligament tear. A tear of this structure is a serious limitation on the structural integrity and the movement of the joint, and reconstructive surgery coupled with significant rehabilitation is invariably required if the athlete seeks to return to the particular sport.