Knee and shoulder fractures: association of fracture detection and marrow edema on MR images with mechanism of injury.

PURPOSE: To assess the value of magnetic resonance (MR) imaging for enabling the classification of fracture mechanisms and to compare marrow edema caused by compressive forces with that caused by tensile forces.
MATERIALS AND METHODS: In 62 knees or shoulders, 78 fractures were identified prospectively on MR images or plain radiographs; compressive or tensile forces were determined retrospectively to have caused impaction or distraction fractures, respectively. Edema was measured on T1-weighted images. Frequencies of fracture detection on MR images or plain radiographs were compared.
RESULTS: Forty-nine (63%) and 29 (37%) fractures were attributed to compressive and tensile forces, respectively. Edema measured 31 mm +/- 10 in impaction fractures and 2.5 mm +/- 2.4 in distraction fractures (P < .001). MR images and plain radiographs of 63 fractures were compared. On MR images, distraction fractures, including four of seven Segond fractures, were missed more often than impaction fractures (P < .008). Fractures overlooked on MR images were associated with less edema (P < .003).
CONCLUSION: On MR images, impaction fractures demonstrate prominent marrow edema, and distraction fractures demonstrate minimal edema. Impaction fractures are more often missed on plain radiographs, and distraction fractures are more often missed on MR images. Segond fractures should be suspected if MR images show lateral capsular ligamentous injury in the knee; in these cases, evaluation with plain radiography is warranted.