Transient Patellar Dislocation

Clinical History:
This is a 13-year-old male who presented for initial evaluation of his left knee, after having a non-contact injury during athletics at school one week prior. He was unsure of the position of his knee at the time of injury. The patient localized the pain to the medial aspect of his knee. He ambulates with difficulty using a single crutch.

Transient Patellar Dislocation

MR Technique:
3.0 T scanner (Siemens Verio) using an 8-channel knee coil. Image 1: axial T2-weighted image (TR/TE 833/23 ms, scan time 3 min 45 sec, slice thickness 3 mm); Image 2: coronal STIR (TR/TE 5250/34 ms, scan time 3 min 53 sec, slice thickness 3 mm); Image 3: coronal STIR (TR/TE 5250/34 ms, scan time 3 min 53 sec, thickness 3 mm).

Imaging Findings:
There is a partial tear of the medial patellar retinaculum with preservation of the patellar and quadriceps tendon, as well as the prepatellar retinacula; a large lipohemarthrosis is seen (Image 1). A moderate-sized bone contusion is seen in the anterolateral femoral condyle (Image 2) and medial aspect of the patella (Image 3). A nondisplaced fracture of the infero-medial pole of the patella is present, which extends to the articular margin (Image 3).