Anatomic significance of magnetic resonance imaging findings in hip fracture

This study was done to determine the histologic etiology for the linear black signal seen on magnetic resonance imaging at the site of proximal femoral fractures. No previous satisfactory explanation for this black line exists in the literature. To determine the pathologic etiology of this linear black signal, the magnetic resonance imaging appearances of osteotomies and fractures of proximal femora were correlated with gross and histologic appearances in 12 freshly frozen porcine hips. In phase 1, a unilateral femoral neck osteotomy was performed in 1 hip, and a sham operation was performed on the contralateral hip. The radiologist was able to easily determine the site and side of the osteotomy using magnetic resonance imaging. In phase 2, the remaining 8 femoral specimens were stressed using the 1125 Instron Materials Testing Machine in a compression mode. In the 8 specimens, 4 fractures were created that were visible on magnetic resonance imaging and plain radiographs; 2 fractures were apparent only on magnetic resonance images, and 2 specimens did not have fractures identified by plain radiographs or magnetic resonance images. Gross and microscopic analysis of the specimens confirmed the magnetic resonance imaging findings in all 8 cases (6 fractures and 2 nonfractures). Histologic evaluation showed trabecular impaction at the 6 fracture sites corresponding to the linear black signal on magnetic resonance images. No edema or hemorrhage was present in these postmortem specimens. The authors conclude that the low signal band seen with proximal femoral fractures is produced by impaction of the trabecular bone and is not the result of perifracture edema or hemorrhage.