Acute lateral patellar dislocation at MR imaging: injury patterns of medial patellar soft-tissue restraints and osteochondral injuries of the inferomedial patella.
OBJECTIVE
To assess magnetic resonance (MR) imaging findings after acute lateral patellar dislocation (LPD) with emphasis on the medial patella restraints and to describe a medial patellar impaction deformity.


METHODS
Knee MR images obtained within 8 weeks after LPD were evaluated for medial retinacular and medial patellofemoral ligament (MPFL) disruption, vastus medialis obliquus (VMO) edema and/or elevation, and other derangements. One hundred patients with no evidence of prior LPD were evaluated as controls. The Student t test was used for statistical comparisons.


RESULTS
Eighty-two examinations were performed in 81 patients with LPD (mean age, 20 years; age range, 9-57 years). Seventy-six percent (62 of 82 examinations) showed medial retinacular disruption at its patellar insertion; 30% (25 of 82), at its midsubstance. The MPFL femoral origin was identified in 87% (71 of 82); of these, 49% (35 of 71) showed injury. Forty-eight percent (39 of 82) showed more than one site of injury to the medial stabilizers; 45% (37 of 82) showed edema or hemorrhage at the inferior VMO. Mean VMO elevation in the coronal plane of the adductor tendon was 2.2 cm, with a range of 0.6-4.5 cm (in control subjects, 0.9 cm; range, 0.1-2.5 cm; P <.001). At the inferomedial patella, 70% (57 of 82) of LPD examinations showed osteochondral injury and 44% (36 of 82) showed concave impaction deformity (0 of 100 control subjects). Other examination findings in LPDs included contusions of the lateral femoral condyle (66 [80%] of 82 examinations) or medial patella (50 [61%] of 82), intraarticular bodies (12 [15%] of 82), effusion (45 [55%] of 82), medial collateral injury (nine [11%] of 82), and meniscal tear (nine [11%] of 82).


CONCLUSIONS
Injury to the medial retinaculum, MPFL, and VMO may be identified at MR imaging after acute LPD. Concave impaction deformity of the inferomedial patella is a specific sign of prior LPD.
Elias
David A
DA
Department of Medical Imaging, Mount Sinai Hospital and the University Health Network, University of Toronto, 600 University Ave, Toronto, Ontario, Canada M5G 1X5.
White
Lawrence M
LM
Fithian
Donald C
DC
D000328:Adult; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D004204:Joint Dislocations / Q000175:diagnosis*; D008279:Magnetic Resonance Imaging; D008297:Male; D010329:Patella / Q000293:injuries*; D017847:Patellar Ligament / Q000293:injuries*