## Why Jefferson fracture with transverse ligament rupture is right The rule of Spence states that combined lateral overhang of the C1 lateral masses **B** beyond the C2 superior articular facets greater than 7 mm indicates a Jefferson fracture—an axial-loading burst fracture of the C1 ring with transverse ligament rupture. The clinical presentation (diving into shallow water causing axial loading to the crown of the head), acute torticollis, and radiographic finding of 9 mm lateral overhang (>7 mm threshold) are pathognomonic for this injury. The lateral masses of C1 are forced outward, disrupting the transverse ligament that normally stabilizes the dens. This is the classic mechanism and imaging finding for Jefferson fracture (Sutton Radiology 7e Ch 49). ## Why each distractor is wrong - **Odontoid fracture Type II with anterior displacement**: Type II odontoid fractures occur at the base of the dens and present with dens displacement, not lateral mass overhang. The rule of Spence specifically assesses C1 lateral mass alignment relative to C2, not dens position. Type II fractures are classified by Anderson-D'Alonzo grading and have a high non-union rate (~30%), but the imaging finding described (lateral overhang >7 mm) is not characteristic. - **Atlantoaxial rotatory subluxation (Grisel syndrome)**: Grisel syndrome (rotatory subluxation) typically occurs in children after upper respiratory infection and presents with asymmetry of the dens-to-lateral-mass distances on open-mouth view. The key finding is asymmetric positioning of the dens relative to the two lateral masses **B**, not symmetric lateral overhang of both masses. Rotatory subluxation is classified by Fielding-Hawkins criteria based on anterior displacement, not lateral overhang. - **Basilar invagination with dens projection above McGregor line**: Basilar invagination is a chronic condition where the dens projects above anatomic reference lines (McGregor, Chamberlain, or McRae lines) and is associated with rheumatoid arthritis, Paget disease, osteogenesis imperfecta, or achondroplasia. It does not present acutely after trauma and is not characterized by lateral mass overhang. The clinical scenario (acute diving injury) and imaging finding (lateral overhang) are incompatible with basilar invagination. **High-Yield:** Rule of Spence: combined lateral overhang of C1 lateral masses >7 mm = Jefferson fracture with transverse ligament rupture; mechanism = axial loading (diving, fall on crown of head). [cite: Sutton Radiology 7e Ch 49; Maheshwari Orthopedics 10e]
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.