The patella or kneecap is a small oval bone that is part of the extensor mechanism of the knee.
The patellofemoral joint (PFJ) comprises the patella and the end of the femur. With knee movement, the kneecap slides in a femoral groove (the trochlear). The quadriceps muscles are attached above via the quadriceps tendon. At the lower end, the patellar tendon attaches it to the tibia bone. The function of the patella is to enhance quadriceps strength. It also protects the knee from to some extent from a front on direct knock.
Dislocation of the patella
Dislocation of the patella is a moderately common problem encountered with the knee. Stability of the patella is dependent on three main factors: 1. the conformity of the two bones; 2 the medial patellofemoral ligament (MPFL); and 3. the quadriceps muscles, especially the vastus medialis. When it contracts, the vastus medialis helps pull the patella in a medial direction.
When the patella dislocates, it pops out laterally or to the outside of the knee. It usually occurs with a twisting injury particularly with the femur twisting inwards. The dislocation can be a complete or partial (a so-called subluxation). The patella may either remain dislocated or spontaneously reduce. It can be often reduced immediately by gently straightening the leg. If it remains dislocated, it can be painful as the adjacent muscles often go into spasm. Sometimes the patella will be reduced at a Hospital Emergency Department.
Treatment for a dislocated patella
If this is your first patella dislocation, you will need X-Rays of you knee. X-Rays will confirm that the patella is reduced and may also demonstrate fractures that may occur. Fractures occur in up to 30% of patients with a patella dislocation. Sometimes these fractures displace and produce a loose body in the knee. If this happens, surgery is required to either reattach the fragment or remove if it is small or fragmented.
A brace provides comfort following a patella dislocation, by immobilising the knee. Crutches are usually required, but putting weight through the leg is encouraged. Prolonged bracing, however, can lead to excessive knee stiffness and muscle wasting. Therefore the support should be removed for periods to allow range-of-movement exercises as comfort allows, often after two to five days. Progressively, the brace is removed for more extended periods. When a person can bear all their weight without fear of their knee collapsing, the support stopped. Most people will benefit from seeing a physiotherapist to help them in the early phase but also especially later to facilitate a strengthening and rehabilitation program. It may take from six to 12 weeks for the knee to get back to normal.
Recurrent patella dislocations
The most common complication of a patella dislocation is further episodes of patella instability or recurrent dislocations. We performed a study at Geelong Hospital many years ago that showed the recurrence rate was 68%. For females under the age of 15, it was even higher. Initially, these should be managed with rehabilitation and avoiding activities that are putting the knee at risk.
When is surgery required?
If the patella continues to be unstable, then surgery may be required. Recurrent patellar instability may be due to several anatomic or biomechanical reasons. The main factor is that tearing of the medial patellofemoral ligament means that it is no longer functioning. Some people also have generalised ligament laxity that puts them at risk. The kneecap can even be sitting higher than usual (patella alta). Occasional the trochlear groove is so shallow that it fails to provide a bony restraint; a condition called trochlear dysplasia. Finally, the leg can be “knock-kneed” or have an abnormal rotational profile that mechanically predisposes to instability.
Further imaging will determine the type of patellofemoral reconstruction required. These include plain X-Rays to assess patella height and the depth of the trochlear groove. An MRI will give useful information about the competence of the MPFL and can determine the degree of damage to the patellar articular cartilage. A CT scan may also provide information about any alignment issues.
A reconstruction of the medial patellofemoral ligament is the main operation performed. Typically a hamstring tendon is used for this. If the patella sits too high, then it can be lowered by a tibial tuberosity osteotomy. Finally, if the trochlear groove is excessively shallow, it can be deepened.
Dr Brown has lectured on recurrent and acute patellofemoral dislocation, and has successfully treated many patients with this complaint.