Ace Your Orthopaedic Oncology Structured Oral Exam: Non-Ossifying Fibroma
Key Takeaway
For anyone wondering about Ace Your Orthopaedic Oncology Structured Oral Exam: Non-Ossifying Fibroma, A non-ossifying fibroma is a common, asymptomatic developmental lesion typically found incidentally in children's long bones, characterized by a well-demarcated, lucent metaphyseal appearance. As discussed in an orthopaedic oncology structured context, these lesions often resolve by adulthood. Treatment usually involves observation and surveillance radiography, with diagnosis confirmation through a bone tumour MDT advised.
A 9-year-old child presents with a painless lump on the distal tibia. Radiographs show a well-circumscribed, eccentrically located, lucent metaphyseal lesion with a sclerotic rim. As an orthopaedic surgeon, what are the most likely differential diagnoses, and what is your assessment of the lesion shown below?
Candidate: "Based on the radiographic appearance—eccentric, metaphyseal location, lucency with a sclerotic border in a child—the most likely diagnosis is a Non-Ossifying Fibroma (NOF). The differential includes a Unicameral Bone Cyst (UBC), Aneurysmal Bone Cyst (ABC), and Osteofibrous Dysplasia. I would assess stability, symptom status, and lesion size relative to the cortical diameter to determine if intervention is required."
Candidates often jump immediately to suggesting surgery (curettage/bone graft) without characterizing the lesion or acknowledging that most NOFs are incidental, self-limiting, and require only observation. They also frequently fail to mention the "50% cortical involvement" or "longitudinal dimension" criteria that govern the threshold for prophylactic surgery.
A high-scoring answer is structured as follows:
- Diagnosis: Confidently identify as NOF based on the classic "bubbly" lucency and sclerotic rim.
- Differential: Categorize by morphology: UBC (central, fluid), ABC (expansile, fluid-fluid levels on MRI), and Osteofibrous Dysplasia (tibia-predominant, epithelial components).
- Management Criteria: State that if the lesion involves >50% of the cortical diameter, spans >33mm in length, or causes pain/pathological fracture, then curettage and bone grafting is the gold standard. If none of these exist, reassure and observe radiographically until skeletal maturity.
The patient returns 6 months later with a history of a sudden 'pop' and sharp pain in the distal tibia while playing football. Radiographs show a transverse fracture through the lesion. How does this alter your management plan?
Candidate: "This is a pathological fracture through a pre-existing lesion. My priority is to stabilize the fracture while simultaneously treating the lesion. I would perform open curettage to clear the fibrous tissue, bone grafting (auto or allograft) to fill the void, and provide internal fixation—such as a plate or intramedullary device—to protect the construct until union."
Candidates who suggest waiting for the fracture to heal before performing the curettage. This leads to persistent, weakened bone and higher rates of non-union or delayed union. Failing to mention internal fixation is also a critical error, as the defect created by curettage significantly weakens the bone structurally.
The perfect answer addresses the lesion and the mechanical failure simultaneously. Emphasize that curettage *without* internal fixation in a weight-bearing long bone following a pathological fracture is an inappropriate construct. Mention that the curetted material must be sent for histopathology to confirm the diagnosis, as a fracture can obscure or alter the appearance of benign lesions.
