Primary traumatic patellar dislocation.

Chun-Hao Tsai, Chin-Jung Hsu, Chih-Hung Hung and Horng-Chaung Hsu. Journal of Orthopaedic Surgery and Research 2012; 7:21[ online link to full paper ]

This is the editor's interpretation of a paper published in the orthopaedic literature in 2012 - our attempt to make relevant medical articles accessible to lay readers. If you wish to read the original it is easy to ask your librarian to obtain a reprint for you from any medical library.

The authors of this paper are discussing 'Primary traumatic patellar dislocation'. What they mean by this is the very first event in which a particular knee patient experiences an injury resulting in dislocation of the patella (kneecap).


Incidence of primary traumatic patellar dislocation

In their article the authors highlight that most patients suffering a primary traumatic patellar dislocation are under the age of 20 and that the dislocation most often occurs during a sporting activity. Women are more prone to this injury than men.

They note that "[a]cute traumatic patellar dislocation is the second most common cause of traumatic hemarthrosis of the knee and it accounts for approximately 3% of all knee injuries". Their emphasis on the words 'acute' and 'traumatic' mean that they are talking about those sudden events where force was applied to the knee, and damage was done to the kneecap or the soft tissue structures that normally keep it stable (hence the bleed or 'haemarthrosis'). In other words they are distinguishing this group from patients whose kneecaps have always been lax or who are now lax from a previous injury.


Mechanism of Injury

The authors advise that for a blow or stress to result in patellar dislocation generally the knee is both bent and in a knock-knee'd position (flexion with valgus), and the force usually tears the MPFL ligament (medial patellofemoral ligament) and the fibrous network of the medial retinaculum, resulting in bleeding inside the joint. In a quarter of these patients a chunk of bone (osteochondral fracture) may also be knocked off the back of the patella on the medial side.


History and Examination

The authors inform that:

  • the doctor needs to ask about any family history of lax joints or patellar dislocation.
  • If the knee is tense with fluid, then this should be syringed out (aspirated) to improve comfort but also to make the examination easier and to take better X-ray images. Any fatty globules in the aspirate would indicate the likelihood of there being a fracture.
  • The doctor should check if there is general joint laxity, examine the laxity of the patella on the good side, and evaluate both knees for problems of poor alignment.
  • In trying to determine if the patient might heal adequately without surgery, the doctor should assess how apprehensive the patient is when the patella is pushed over to the lateral side, and how tender the tissues are to the inner side of the knee, and also if there appear to be any soft tissue deficiency in the retinaculum, the part of the quads muscles known as the VMO (vastus medialis obliquus) and also the 'adductor mechanism'.




In the authors' opinion the most useful X-ray views include - 45° flexion Merchant view, 45° flexion weight-bearing view, and 30° lateral view. These are best done once any blood has been aspirated so that the knee can bend properly.

CT Scan

They advise that a CT scan is useful in evaluating any predisposing factors that might have contributed to the dislocation - patellofemoral mal-alignment, patellar tilt, trochlear dysplasia and any twisting deforming of the long bones. It should also reveal any damage to the cartilage surfaces of patella and underlying femur bone.

MRI Scan

They list the following as characteristic MRI findings after a patellar dislocation:

  • fluid in the joint cavity (effusion)
  • bruising on patella and femur - on the medial patellar facet at the back of the patella where it rode over the edge of the groove, and on the lateral part of the goove itself
  • possibly bony an actual bone/cartilage defect in the same area

If you refer to the original article (link above) you will see some excellent images.

MRI is also particularly useful in determining the nature and extent of the soft tissue damage to the stabilisers on the medial side of the patella - the MPFL ligament, the medial retinaculum and the VMO.


Treatment and Outcomes

According to the authors, more than 50% of people continue to have some problem with their kneecap after this first traumatic event, which may include eventual arthritis at the back of the patella, and up to 40% can have a recurrence of the dislocation. This implies that proper treatment is essential after this first event to prevent  or minimise these sequelae, but unfortunately the published literature is not that clear about the best management at this stage - whether to treat 'conservatively' (which means without surgery) or whether to operate and try to improve stability. 

They recommend that if an osteochondral fracture is suspected on imaging, then arthroscopy should be carried out to confirm it, and if it is a big break - more than 10% of the joint surface - then 'open' surgery should be performed to try and fix the fragment back. Other than that the immediate route forward is not clear from published studies. The authors quote another study by Stefancin and Parker (ref 1) who reviewed 70 articles and concluded that it was best initially to follow a conservative route for first-time traumatic patellar dislocation, unless there are clinical, radiographic, CT, and/or MRI findings of cartilage injury, osteochondral fractures, or large defects of the soft tissue medial stabilisers.

After reviewing all the literature the authors suggest that - after reduction of the dislocation - patients should be immobilised initially for comfort for 3–4 weeks (preferably via a splint supporting the back of the knee) to allow immediate weight bearing as tolerated on crutches. Then they advocate early mobilisation to keep the cartilage healthy. As soon as comfort allows, the patients can move into a patella-stabilising brace (they suggest On-Track brace and the Patellar Tracking Orthosis (PTO)), and continue with resisted closed-chain exercises and passive range of motion in the brace.


When surgery is indicated

If surgery is indicated either from the initial findings, or if there is recurrent instability, the main procedures involve stabilisation of the torn medial tissues - especially MPFL and VMO, and plication of the medial retinaculum.


1. Stefancin JJ, Parker RD: First-time traumatic patellar dislocation: a systematic review. Clin Orthop Relat Res. 2007, 455: 93-101.