Describe the occurence, clinical features and diagnostics.
Occurrence: uncommon but often associated with a posterior hip dislocation following a dashboard injury
Local swelling and ecchymosis
Hip x-ray (AP with internal rotation and lateral view; should include the proximal thigh) : abnormal trabecular pattern, cortical defects, shortening and angulation of the femoral neck 
MRI if findings are unclear or if an occult fracture is suspected