Presentation
22 year-old member of the armed forces. Sudden onset left hip pain and inability to weight bear during a football match. No tackle or other trauma. O/E shorted externally rotated left hip.
Patient Data
Large unilocular cyst in the neck of the left neck of femur extending into the greater trochanter. The bony margin on the anterolateral aspect of the cyst is thinned to 2mm.
Intracaspular neck of femur fracture with multiple small fracture fragments, one of which is lying in the dependent position of the cyst.
Case Discussion
This young patient fractured his hip with little or minimal trauma. This should immediately alert the clinician to an underlying problem, suggesting a pathological fracture.
The femur is the second commonest site for a unicameral bone cyst after the humerus. They are typically identified in early life and are more common in males.
The fallen fragment sign is said to be pathognomonic for a simple (unicameral) bone cyst following a pathological fracture. The reason being the cyst is filled with serosanginous fluid, which is non inhibiting to the denser bone fragment falling through it into the dependent position.
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