Radiological evidence of union of a long bone fracture is conventionally accepted when bridging callus is seen across:

• A One of the four cortices on two views
• B Two of the four cortices on two views
• C Three of the four cortices on two views ✓
• D All four cortices on two views

Explanation

Union is assessed on anteroposterior and lateral radiographs, where each view shows two cortices, giving four in total. Bridging of three of the four cortices is the widely used radiographic criterion for union, correlating well with clinical union. Requiring all four cortices is overly strict and delays diagnosis, while one or two cortices bridged does not reliably exclude motion at the fracture site.

Reference: Apley's System of Orthopaedics and Fractures, 10th ed.

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