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Neuroradiology Case of the Week

Case 253

Brian Sorensen, MD, and P-L Westesson, MD, PhD, DDS

Clinical Presentation: A 51-year-old male presents with neck pain and headache following a motor vehicle collision.

Imaging Findings: Sagittal and coronal reformatted CT images demonstrate a well-corticated ossification immediately superior to the odontoid process of axis. No other osseous abnormalities are present. Prevertebral soft tissues are normal. 

Diagnosis: Ossiculum terminale

Discussion: Odontoid deformities include two commonly missed distinguished entities: os odontoideum and ossiculum terminale. Both anomalies denote a dens separation. The two are distinguished by etiology and location. Most authors agree that os odontoideum is a sequela of odontoid synchondral fracture prior to union at age 5-7. The disruption occurs at the cartilaginous plate between dens and the body of axis, leaving an ossicle posterior to the transverse atlantal ligament (TAL). In contrast, ossiculum terminale represents congenital nonunion of the odontoid body from a terminal ossicle located above the TAL. Thus, atlantoaxial instability is more common with os odontoideum.
     Two additional entities, nonunion of dens fracture occurring after age 7 and nontraumatic unfused odontoid synchondrosis, may be distinguished from os odontoideum. The smooth, well-corticated ovoid ossicle representing os odontoideum is separated from the body of axis by a large gap which begins above the superior facets of C2. Conversely, the gap seen with fracture nonunion is typically narrow. Atlantoaxial translatory motion associated with os odontoideum is not a feature of physiologically unfused odontoid synchondrosis.

References:

1. Fassett DR, McCall T, Brockmeyer DL. Odontoid synchondrosis fractures in children. Neurosurg Focus. 2006 Feb 15;20(2):E7. [Medline]
2. Liang CL, Lui CC, Lu K, Lee TC, Chen HJ. Atlantoaxial stability in ossiculum terminale. J Neurosurg. 2001 Jul;95(1 Suppl):119-21. [Medline]
3. Ross JS, Brant-Zawadski M, Chen MZ, Moor KR. Diagnostic Imaging: Spine, 1st ed. Altona: Amirsys Inc; 2004.
4. Truumees E. Os odontoideum. Emedicine, March 18, 2005. http://www.emedicine.com/orthoped/topic424.htm.