X-ray of patellar dislocation and subluxation

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Author: Mikael Häggström [notes 1]

Evaluation

In obvious patellar dislocation, make a rough estimation of the distance compared to normal position.

In borderline cases of subluxation, preferably apply the following measurements:

  • The lateral patellofemoral angle, formed by:[1]
  • A line connecting the most anterior points of the medial and lateral facets of the trochlea.
  • A tangent to the lateral facet of the patella.
With the knee in 20° flexed, this angle should normally open laterally.[1]
  • The patellofemoral index is the ratio between the thickness of the medial joint space and the lateral joint space (L). With the knee 20° flexed, it should measure 1.6 or less.

Also, look for any resultant fractures of the patella.

Notes

  1. For a full list of contributors, see article history. Creators of images are attributed at the image description pages, seen by clicking on the images. See Radlines:Authorship for details.

References

  1. 1.0 1.1 Saggin, Paulo Renato Fernandes; Saggin, Jose Idílio; Dejour, David (2012). "Imaging in Patellofemoral Instability ". Sports Medicine and Arthroscopy Review 20 (3): 145–151. doi:10.1097/JSA.0b013e3182553cfe. ISSN 1062-8592.