Explanation
Epidemiologically, Unicameral Bone Cysts are most frequently identified in the first two decades of life, with a strong male predilection (ratio of 2:1 to 3:1). They predominantly affect the proximal humerus (50-65%) and the proximal femur (25-30%). Because the cyst slowly thins the endosteal cortex from within, it does not induce a periosteal reaction and is typically entirely painless and asymptomatic in its un-fractured state. Consequently, incidental discovery is relatively rare. The vast majority—ranging from 50% to 70% in robust orthopaedic literature—first come to clinical attention only after a minor, low-energy torsional or direct trauma results in a structural cortical breach. Therefore, the pathological fracture, and by extension the potential for a fallen fragment sign, is the classic initial presenting feature rather than a late-stage complication.

Reference:
1. Campanacci, M. (1999). Bone and Soft Tissue Tumors: Clinical Features, Imaging, Pathology and Treatment (2nd ed.). Springer.
2. Dormans, J. P., & Drummond, D. S. (1994). Pediatric hematology and oncology clinics: Bone tumors. Orthopedic Clinics of North America, 25(3).