Introduction
The patella (lat. : shell; kneecap) is a triangular, flat bone disc, which is located in front of the knee joint and plays a major role in its function. The kneecap is embedded in the tendon of the large thigh muscle (Musculus quadriceps femoris) and acts as a placeholder between the tendon and the knee joint as well as a deflection pulley, so that the extension of the lever arm results in an optimal transfer of traction from the thigh muscle to the lower leg.
To keep the patella in its central position in front of the knee joint, it is stabilized towards the middle of the body (medially) and towards the side (laterally) by collateral ligaments, from which the surrounding muscles are guided in their bony gliding groove of the thigh. If, however, there are slight instabilities in the ligament-muscle apparatus that secures the kneecap, this can cause the kneecap to leave its actual gliding groove and shift laterally. This so-called patellal lateralization leads to the consequence that, on the one hand, the transmission of tractive force is impaired and, on the other hand, an increased contact pressure of the patella on the gliding groove is created, which leads to an excessive and uneven load on the kneecap. This results in increased abrasion of the articular cartilage at the back of the kneecap, especially on the outer surface, which can develop into a painful cartilage disease (Chondropathia patellae) or even complete, partial cartilage wear (patella arthrosis).
Causes
The reason for the positional deviation of the patella from its slide bearing is usually a fault in one of the systems stabilizing it. One possible cause may be a bony misalignment of the patella or the gliding channel, so that, for example, deviations in shape impede physiological gliding. On the other hand, the reason for patellal lateralization can be instability in the collateral ligaments – especially the medial collateral ligament – so that they can no longer adequately guide the patella and it shifts to the side.
This may be the case, for example, after injuries or trauma, but congenital ligament instability may also be the reason. The third possible cause is an imbalance in the muscles surrounding the patella. If the outer muscle parts of the thigh predominate and the inner thigh muscles (Musculus vastus medialis) are relatively weaker, the patella tends to be pushed or pulled more to the side against the outer part of the sliding bearing.