When he patella (kneecap) - this small bone in front of the knee that moves up and down in the femoral groove (trochlea) - lies in place, then one can walk, run, stand, sit, and generally move around easily. When, however, it is displaced from the specific groove, the knee joint loses its proper functionality.
First, “slipping out of its normal position” means that the ligaments on the inner and outer aspect of it, which hold it onto the femoral groove, have been injured. Thus, the patella becomes dislocated towards the outside, deviating from its normal course. This means that this injury may include concomitant lesions in the muscles and ligaments.

Epidemiological studies have shown that patellar dislocation accounts for 3% of all knee injuries. It is considered to be a “sports and adolescent injury” with high rates at the ages of 15-19. For example, in the United States, as shown in a study published in The Journal of Knee surgery in 2012, 51.9% of dislocations occurred during a sporting activity - the highest rates were recorded for basketball (18.2%) soccer (6.9%), and American football (6.3%) players. It is also a common dance injury, representing 9% of the cases.

Contrary to patellar subluxation, which is a chronic problem and is attributed to the anatomical predisposition, its dislocation is usually acute and traumatic. Actually, it is the second most common cause of traumatic haemarthrosis of the knee. The most common injury mechanisms are abrupt turning of the torso with quick changes of the knee flexion angle (as is the case during a dance figure), or direct knee collision.
Symptoms include knee pain which increases when the patient moves, but also when they sit, stiffness, swelling, joint weakness regarding the bearing of body weight, while the knee slides sideways and the joint deformity is visible. Repetitive subluxations or dislocations render the knee unstable: The patient feels much more pain and recurrences are frequent, while their condition may lead to osteoarthritis.

Patellar subluxation has to be promptly treated at the Emergency Room, because a reduction is required. However, especially if we are talking about children and adolescents, monitoring by an orthopaedist is essential. During the examination, the doctor shall obtain a complete personal and family history, they shall request a detailed description of the injury mechanism, as well as of the symptoms. Thence, they shall examine the range of motion and sensitivity of the knee. X-rays will show the relationship between the patella and the femoral groove, while an MRI (which is not actually required for the diagnosis of the dislocation) shall respond to whether any other concomitant lesions are observed, or not.
Patellar subluxation causes an injury to the ligaments that hold it in place. Consequently, repeated injuries render it increasingly unstable, so it is much easier for it to be dislocated. Especially if there are any anatomical knee abnormalities, then recurrences are more frequent.
Surgical treatment is recommended for cases of instability of the patellofemoral joint. Depending on the causes, an arthroscopic reconstruction of the ligaments and the soft tissues of the knee can be performed. Cases where recurrent dislocations are attributed to anatomical / bone abnormalities require more complicated surgical procedures.

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