Intended for healthcare professionals

Endgames Spot Diagnosis

A painful swollen knee after a fall

BMJ 2017; 356 doi: (Published 12 January 2017) Cite this as: BMJ 2017;356:i6772
  1. Sheena Seewoonarain, orthopaedic registrar1,
  2. Chris Servant, consultant orthopaedic surgeon2
  1. 1Western Sussex Hospitals NHS Trust, Spitalfield Lane, Chichester, UK
  2. 2The Ipswich Hospital NHS Trust, Heath Road, Ipswich, Suffolk, UK

Correspondence to: sseewoonarain{at}

A 27 year old man stumbled down a step and twisted his left knee. He felt immediate severe pain in the knee and was unable to weight bear. On examination he had a swollen knee with a limited, painful range of movement. There was no obvious neurovascular deficit. Plain radiographs of the left knee were taken (fig 1). What is the diagnosis?


Dislocation of the knee (tibiofemoral joint).


The radiographs show a loss of congruence between the articular surfaces of the tibia and femur. The tibia is translated posteriorly and laterally relative to the femur (a posterolateral rotatory knee dislocation). There is a bone fragment (fig 2, arrow) secondary to avulsion of the tibial attachment of the posterior cruciate ligament.

Fig 2
Fig 2

Bone fragment (arrow) secondary to avulsion of the tibial attachment of the posterior cruciate ligament

Dislocation of the knee is an uncommon but serious injury that is considered an orthopaedic emergency due to the risk of severe morbidity. It is associated with multiple ligamentous injuries and neurovascular compromise, primarily to the common peroneal nerve and the popliteal artery.12

The most common cause is a high energy injury, but cases involving ultra low velocity injuries in obese patients have been described.2 Knee dislocations often reduce spontaneously and subsequent radiographs can appear normal. Although the knee can be painful and swollen, a joint effusion might not be present due to capsular damage and extravasation of fluid. Therefore these injuries may be missed or underestimated.1


  • Knee dislocations are an orthopaedic emergency that may be missed. A high clinical suspicion must be maintained, therefore, with careful review of any imaging modalities used.

  • Patient consent obtained.

  • Competing interests: All authors have read and understood BMJ policy on declaration of interests and declare the following: none.


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