Explanation
The identification of a fallen fragment sign confirms the diagnosis of a benign Unicameral Bone Cyst, precluding the need for acute biopsy or aggressive tumor resection. According to the standard orthopaedic treatment algorithm, the absolute first line of management for a fractured UBC in the upper extremity is non-operative conservative immobilization (using a broad arm sling or cast). This allows the pathological fracture to consolidate. Immediate surgical or percutaneous intervention is contraindicated as it disrupts the acute fracture hematoma and risks further injury to the adjacent physis. In approximately 15% to 20% of pediatric cases, the osteogenic rush and remodeling associated with fracture healing will lead to spontaneous obliteration of the underlying cyst. Interventions such as aspiration, injection of methylprednisolone (the Scaglietti method), bone marrow aspirate concentrate (BMAC), or curettage and grafting are strictly reserved for cysts that remain persistent or symptomatic only after the acute fracture has fully united.

Reference:
1. Herring, J. A. (2020). Tachdjian's Pediatric Orthopaedics: From the Texas Scottish Rite Hospital for Children (6th ed.). Elsevier.
2. Canale, S. T., & Beaty, J. H. (2012). Campbell's Operative Orthopaedics (12th ed.). Mosby.