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Patient 3: 19-year-old man with acute knee pain and swelling and a 2-year history of recurrent similar symptoms
  1. A-L Cheng1,
  2. C King2,
  3. R Lloyd-Smith2,
  4. G Andrews2,
  5. B B Forster2
  1. 1University of Saskatchewan, Saskatchewan, Canada
  2. 2University of British Columbia, Vancouver, British Columbia, Canada
  1. Correspondence to Dr Bruce B Forster, Department of Radiology, UBC Hospital, 2211 Wesbrook Mall, Vancouver, British Columbia, Canada V6T 285; bruce.forster{at}vch.ca

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Answer

See page 950 for the question.

Diagnosis

Patellar dislocation.

Findings

The plain radiographs demonstrate a small fragment of bone adjacent to the lateral femoral condyle on both the skyline and the anterior–posterior (AP) radiographs (fig 3, showing the fragment on the skyline view). On the skyline view there is also an osteochondral defect near the junction of the patellar apex and the lateral facet (arrow). The MRI images (fig 4a) show marrow oedema in the medial and inferior patella and in the lateral femoral condyle. There is oedema in the soft tissues adjacent to the medial patellar retinaculum, and the medial patellar retinaculum is wavy and less well visualised than normal (fig 4b). Findings are consistent with recent patellar dislocation.

Figure 3

Skyline view of the patella. Arrow demonstrates osteochondral fragment (same as fig 1B with arrow).

Figure 4

(A,B) MRI scan of the patient’s knee (axial proton density with fat saturation). (A) Same as fig 2a with arrows; (B) same as fig 2b with arrows.

The patient was started on a rehabilitation programme.

Discussion

Acute patellar dislocation is a common injury, especially in the second to third decades of life.1 Patients who have an episode of patellar dislocation are often not conscious of the transient dislocation, making clinical diagnosis difficult. …

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