Dislocation of the patella (kneecap) can be a distressing event, and management is focused both on repairing damage and on identifying underlying anatomical factors that predispose to the injury.


How can you tell if your kneecap is dislocated?

dislocation of the patella with torn MPFL

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Dislocation tends to follow a direct blow or an episode like landing from a jump with the knee extended. If the kneecap suddenly dislocates, but then relocates itself again, despite the acute pain at the time there may be no residual deformity and it is not uncommon for a patient to be unsure what exactly happened, although the incident may be followed by acute tenderness and possibly swelling. If however the incident results in the kneecap staying dislocated, the deformity will be obvious, with the knee held in flexion usually with an unsightly lump on the outside of the knee as well the pain and tenderness and possibly skin bruising. Even if the patella has re-located, bone bruising or small fractures may be evident on X-ray of the medial patella and lateral femur.


Can you pop a dislocated kneecap back into place?

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A dislocated patella may spontaneously slip back into the groove when the knee is straightened. Patients who have experienced dislocations before may know to get someone to help them by maintaining medial pressure on the kneecap while slowly extending the knee. It may slip back just as the knee reaches full extension, which is where the groove is shallowest.

VIDEO - Reducing a dislocated patella

Irreducible Lateral Patellar Dislocation: A Case Report and Literature Review Grewal B, Elliott D, Daniele L and Reidy J. Ochsner J. 2016 Summer; 16(2): 180–184.


What complications can occur from dislocation of the kneecap?

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With the first dislocation episode, the medial patellofemoral ligament MPFL) may become torn. There may be bone bruising on the outer (lateral) part of the groove on the femur and matching bone bruising on the inner (medial) part of the patella. It is also possible for the lower pole of the patella to fracture, and even for a small fragment to fall into the joint cavity.

Management of Acute Patellar Dislocation: A Case Report. Enix DE, Sudkamp K, Scali F, Keating R and Welk A. J Chiropr Med. 2015 Sep; 14(3): 212–219.


What causes a kneecap to dislocate?

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The dislocation may be traumatic from a blow to the side of the kneecap. It may also be spontaneous when perhaps landing from a jump with the knees extended when there are underlying anatomical factors that predispose to dislocation, such as knock-knees (valgus deformity), a high-riding kneecap (patella alta), a deformity at the top of the groove (trochlear dysplasia), tight lateral retinaculum with patellar tilt, or rotational deformities at the top of the femur near the hip (femoral anteversion) or a rotational deformity of the shaft of the tibia). Also the groove may line up poorly with the attachment of the patellar tendon (high TT-TG distance).

Another set of problems that may predispose to kneecap dislocation are the hypermobility syndromes and connective tissue disorders, especially Ehlers Danlos Syndrome.

Predicting Risk of Recurrent Patellar Dislocation. Parikh SN, Lykissas MG and Gkiatas I. Curr Rev Musculoskelet Med. 2018 Jun; 11(2): 253–260.

Ehlers-Danlos Syndrome in Orthopaedics. Shirley ED, DeMaio M and Bodurtha J. Sports Health. 2012 Sep; 4(5): 394–403.


How long does it take to recover from a dislocated kneecap?

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Sometimes the patient experiences just the one episode, and it never recurs. Usually there is a period of immobilisation in a brace or knee immobiliser at the first incident and the knee settles down in three weeks or so, unless there is blood in the joint or an underlying fracture. 

When there are underlying anatomical factors causing the patellar instability a dislocation event may just be an episode on a continuum, so the patient may have been accustomed to experiencing occasional subluxation episodes before the first dislocation event, and the first event may be followed by the problem becoming recurrent, particularly if the MPFL is now damaged in addition to the primary anatomical problems.


When is surgery appropriate after patellar dislocation?

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The first dislocation event is usually managed without surgery, with the patella manipulated back into place and the knee immobilised. Surgery at the first event may be appropriate if the surgeon is unable to do a closed reduction or if there are fracture fragments that need to be stabilised. Looking ahead, dislocation of the patella may become recurrent, and the patient may reach a point of no longer trusting the knee and request that something surgical is done to stabilise the situation. Most surgeons would recommend that the MPFL be repaired if underlying anatomical issues are mild, but surgeons experienced in patellofemoral surgery would also consider improving any problem with the underlying anatomy, such as doing a trochleoplasty for dysplasia, and also do a full reconstruction of the MPFL.

Modern management of patellar instability. Rhee S-J, Pavlou G, Oakley J, Barlow D and Haddad F. Int Orthop. 2012 Dec; 36(12): 2447–2456.


How successful is surgery for chronic kneecap dislocation?

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Simple MPFL reconstruction is likely to eventually fail if there is significant problems with the underlying anatomy, and such patients should be referred to a specialist in patellofemoral surgery.

Avoiding Complications with MPFL Reconstruction. Smith MK, Werner BC and Diduch DR. Curr Rev Musculoskelet Med. 2018 Jun; 11(2): 241–252.


eBook on patellar instability

Factors contributing to an Unstable Patella - ebook