Patellar dislocation

Illustration showing the patella dislocating to the lateral (outer) side with an associated tear of the medial patello-femoral ligament (MPFL).

What happens during a patellar dislocation?

In the first dislocation event the patella usually dislocates to the lateral (outer) side.

This is due to the anatomy of the region. At the same time other structures may be injured, such as the medial patellofemoral ligament which may be torn, and there may also be a small fracture on the medial side of the patella and some associated bone bruising.

"....First patellar dislocation is a common injury of the knee, involving often adolescents and the active population. The consequences of the first episode can be various and potentially disabling. Among these, acute patellar dislocation can often result in recurrent patellar instability....."

Erica Bulgheroni et al

Soccer, gymnastics, and dancing are typical injuries, where outward rotational forces and valgus stress when flexing a knee may drive the patella out of the sulcus. In rare cases, dislocation may be due to a direct blow to the medial aspect of the patella.

Jaakko Sinikumpu et al

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What predisposes to patellar dislocation?

A number of conditions predispose a person to patellar dislocation:

patella alta - when the kneecap rides too high
trochlear dysplasia - when the underlying groove is abnormal
increased TT-TG distance - when the anatomy leads to increased sideways force on the patella
patellar tilt - when tissue imbalance leads to a tilted patella, and tightening of the tissues at the side of the knee aggravated it

Once there has been a dislocation event, and particularly if there are underlying anatomical factors, the patella may become chronically unstable.

Acute patellar dislocation ...."is the result of an indirect force on the knee joint, which leads to valgus and external rotation of the tibia relative to the femur. PD is unlikely to occur on a knee with normal patellofemoral joint (PFJ) anatomy....."
Usually there is "....an acute injury of the ligamentous medial patellar stabilizers in the background of factors predisposing to patellar instability....

Panagiotis Samelis et al

"....The percentage of recurrent patellar dislocation after the first episode varies from 15 to 80%,....."

Erica Bulgheroni et al

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Why can't I bend my knee after dislocating it?

During a patellar dislocation, the kneecap jumps out of the underlying groove in which it usually glides. If the restraining MPFL is torn, it may not return to the groove after the event, but may remain caught on the other side with the knee bent, and attempts to straighten the knee will be very painful.

An anaesthetic with muscle relaxation may be needed to allow the clinician to return the kneecap to the groove, which will allow the leg to be straight again.

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Can a dislocated knee be permanent?

With proper management at the time, it should always be possible to return the kneecap to the groove.

A delay in treatment with poor underlying anatomy may lead to a permanent disfigurement.

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Management of patellar dislocation

A first event may be managed conservatively, with a period of immobilisation, after which sports may be limited for some months.

If there is a subsequent event, the patient is usually subjected to a full investigation of any underlying causes and surgery may be planned. This may include a reconstruction of the MPFL. In patients where there is a marked problem with the anatomy, such as trochlear dysplasia, corrective surgery may be undertaken.

"....there is no evidence that bone abnormalities should be corrected after the first episode of patella dislocation, although in rare cases of severe associated anatomic patellofemoral deformities, procedures like tibial tuberosity transpositions and/or trochleoplasty can be taken into consideration and associated to MPFL repair/reconstruction...."

Steve Wolfe et al

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