A nondisplaced acetabular fracture is being assessed for nonoperative management. Roof arc angles are measured on the AP, obturator oblique, and iliac oblique radiographs. Which measurement threshold supports nonoperative treatment in a congruous hip?
- A Roof arc angle of at least 30 degrees on all three views
- B Roof arc angle of at least 45 degrees on all three views ✓
- C Roof arc angle of at least 60 degrees on the AP view only
- D Roof arc angle of at least 20 degrees on any single view
Correct answer: B. Roof arc angle of at least 45 degrees on all three views
Explanation
Roof arc angles measure the intact weight bearing dome of the acetabulum. An angle of 45 degrees or more on the AP, obturator oblique, and iliac oblique views indicates that the superior dome is uninvolved, and with a congruous hip and intact subchondral arc on CT this supports nonoperative management with protected weight bearing. Note that roof arcs cannot be applied to posterior wall fractures, where wall size and hip stability decide treatment instead.
Reference: Rockwood and Green's Fractures in Adults, 9th ed.
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Written and medically reviewed by the StethoPrep medical team.