Patellar instability

What is it?

The patella is a sesamoid bone (i.e. a bone present inside the tendon) that is located anteriorly in the knee, moves in the femoral trochlea and intervenes in flex-extension movements favoring the function of the quadriceps muscle that is its main motor.
It is defined as unstable when it tends to come out of its sliding seat. A dislocation occurs when the patella exits laterally from the femoral trochlea. Instability may be present due to congenital anatomical factors or as a result of traumatic phenomena.

Causes and risk factors

Causes of patella dislocation from the femoral trochlea can be congenital or traumatic.
Congenital anomalies depend on anatomical alterations of the knee that predispose the patella to dislocation. The most important are the too high position of the patella, dysplasia of the femoral trochlea that is too flattened shape, defects of the anatomical axis such as valgus knee.
Patella dislocation can also occur as a result of discursive trauma to the knee or direct trauma to the internal region resulting in external dislocation.
Following the dislocation episode, a situation of relapsing instability of the patella can occur, which predisposes to further dislocations.

Which are the symptoms?

Instability is felt by the patient as a feeling of weakness of the knee, accompanied by occasional episodes of collapse of the lower limb involved.
Patella dislocation instead is an acute event characterized by the external translation of the patella that can spontaneously return to its seat, or if it remains in an external position, a reduction maneuver may be necessary.
In the following hours the knee tends to swell and become stiff in extension.
Swelling and functional impotence often resolve after a few days of rest.
Other anatomical structures of the knee such as the menisci, cruciate ligaments, and bone may also be involved and present injuries in the event of trauma.

How is it diagnosed?

The diagnosis of instability is based on a clinical examination to identify predisposing anatomic factors and any injuries caused by dislocation episodes. An accurate history with the account of the episodes of sagging and dislocation is fundamentally important. 
X-rays with specific projections are useful to observe the conformation of the patella and femoral trochlea and their relationship.
In order to obtain a definitive diagnosis, it is fundamental to have an MRI scan that allows a better study of the ligaments, the articular cartilage as well as allows to observe possible bone detachments.

How is it treated?

Following the first episode of dislocation the treatment of choice is usually conservative and is based on a few days of rest, application of local ice, walking with crutches, use of a brace.
Subsequently it is useful to undertake a rehabilitative path to gradually recover the complete mobility of the knee and perform strengthening exercises and re-education of the quadriceps muscle.
In cases of repeated dislocations, surgical treatment may be necessary to restore the stability of the patellofemoral joint. It is possible to surgically reconstruct the medial patellofemoral ligament, which is frequently injured. 
It may be necessary to translate the patella to improve motion by performing a transposition of the anterior tibial tuberosity on which the patellar ligament inserts.

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