Explanation
This clinical scenario describes a 'suspended fragment' or 'trapdoor sign,' which must be differentiated from a true fallen fragment sign. When a pathological fracture breaches a solid benign bone lesion, the fractured cortical fragment cannot fall freely. In Fibrous Dysplasia, the medullary cavity is replaced by a dense, gritty, fibro-osseous matrix characterized histologically by irregular, woven bone spicules lacking osteoblastic rimming (the classic 'Chinese character' pattern). When the cortex fractures, the bony fragment is caught and held in place by this thick, cellular fibrous stroma, causing it to appear suspended or hinged (trapdoor) on the radiograph. Solid lesions such as Enchondromas, Non-Ossifying Fibromas, and Fibrous Dysplasia will exhibit this tethering effect, firmly ruling out a simple fluid-filled cavity.

Reference:
1. Greenspan, A., & Jundt, G. (2015). Orthopedic Imaging: A Practical Approach (6th ed.). Wolters Kluwer.
2. Stacy, G. S., et al. (2006). Pitfalls in the radiographic diagnosis of bone tumors. Radiographics, 26(4), 1005-1026.