Patella: Structure, Function & Diseases

The functionality of joint movements does not proceed as simply as it usually seems in everyday life. With regard to the knee joint, nature resorted to a trick to facilitate the muscle movements of the thigh. From there, it set up an additional bone that moves in a sliding manner: the kneecap.

What is the kneecap?

The kneecap is a bone that is disc-shaped as well as roundly triangular in shape and lies in front of the knee joint. In technical language, it is also called the patella. When the knee joint is bent, the muscles of the front thigh must exert force on the lower leg. To do this, the knee joint needs a kind of spacer, the function of which is taken over by the kneecap and thus acts as a force redirector. Since the kneecap is embedded in a sliding groove and is thus movable, it also sits loosely over the knee joint, i.e. it can be moved easily from the outside. Because of this, the kneecap is very delicate and therefore particularly susceptible to injury and fracture.

Anatomy and structure

The attachment of the kneecap is at the top of the thigh muscle and at the bottom from its tendon on the tibia. However, since a tendon must always be associated with a muscle, the patellar tendon is not a tendon according to the professional definition, but rather a ligament that connects the patella to the tibia. In order to be able to ensure movement of the patellar tendon during power transmission of the thigh muscles, it is embedded in a sliding groove in which it can slide back and forth during bending and stretching of the leg. There is a layer of cartilage on its posterior surface.

Function and task

In order for the force of the thigh muscles to act on the tibia, a movable spacer is needed to make bending and extending the leg possible at all. The patella assumes this function and can slide about 10 cm in its gliding groove. In doing so, it serves as a lever arm to reduce the amount of force required. It is thus a physical force redirector and increases the distance of the force vector of the anterior thigh muscle from the center of rotation of the knee joint. Because the patella is embedded in a tendon and acts as an extended lever arm for it, it is often referred to as the sesamoid bone. Furthermore, the patella protects the tendon, which originates from the thigh muscles, from damage caused by excessive pressure loads. In order for the kneecap to slide at all, there are two bursae at the knee joint to reduce pressure and friction between the tendon, muscle, bone and skin.

Diseases and ailments

A common complaint of the kneecap is patellar dislocation, where it has jumped out of its gliding path. This can cause the capsular ligaments to be torn. Once the kneecap has popped out of its glide path, there is a risk that it could happen again and again. A common disease of the kneecap is patellar osteoarthritis. The causes of this disease are mostly constitutional, for example, when the kneecap is malpositioned, the cartilage is of poor quality, or there are general leg malpositions (knock knees, bow legs). If the patella runs too far out in the gliding groove, this is called patal lateralization, which leads to an asymmetrical load on the patella. Causes can be malpositions of the gliding groove or the patella itself or muscle weakness. In children and adolescents, growth-related overloading of the patella can occur, which decreases again in adulthood and can lead to discomfort, especially when climbing stairs. In addition, one of the bursae at the knee joint can become inflamed due to a skin injury, which can lead to a knee infection or even blood poisoning. In this case, the affected bursa must be removed. Athletes often suffer from overuse of the tendon in which the kneecap is inserted, which can lead to degenerative disease of the kneecap. If external force is applied to the kneecap, it may cause a fracture, which may entail, for example, the consequential damage of the kneecap jumping out of the sliding path more easily, which could subsequently happen again and again.