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Neuroradiology Case of the Week

Case 232

Chandler Shyu, MD and P-L Westesson, MD, PhD, DDS

Clinical Presentation: An 85-year-old female presented to the ED with neck pain after a fall. Cervical spine radiograph and CT were performed to evaluate for a cervical spine fracture.

Imaging Findings: C-spine series demonstrated a linear lucency through the odontoid base on both lateral and open mouth views. There was also prevertebral soft tissue swelling (Figs. 1A&B). It is difficult to see the fracture lines on axial CT images (not shown) when scanning plane is parallel to the horizontal oriented fracture line. However, coronal and sagittal reformat nicely demonstrate a transverse fracture line through the base of the dens with minimal displacement (Figs. 2A&B).

Diagnosis: Odontoid fracture type II

Discussion: Odontoid fractures are the most common upper cervical spine fractures.  The mechanism of injury is usually flexion loading resulting in anterior subluxation of the dens. However a small percentage of caused by hyperextension (e. g.  forward fall onto forehead) which results in posterior displacement of odontoid fragment.
      The Anderson and D’Alonso classification is the most commonly used classification system which describes three distinct fracture patterns based on stability of injury.

• Type I: oblique avulsion type fracture through the tip of the odontoid. The alar ligament which connects the odontoid to the occiput is disrupted. This is the least common type of odontoid fractures (5%). It is also considered a stable injury and managed conservatively with cervical collar.
• Type II: transverse fracture through the base of the odontoid. This is the most common type comprising about 60% of dens fractures. This is considered an unstable fracture because blood supply is usually compromised in a type II fracture. If not treated surgically, non-union occurs in 30-50% of cases, especially in the elderly population.
• Type III: fracture at odontoid base which extends into with extension into C2 vertebra body. This is the second most common fracture type accounting for about 30% of cases. Type III dens fracture is also considered a stable injury and can be managed conservatively with immobilization up to 12 weeks.

Differential consideration includes os odontoideum which is a congenital non-union of a secondary ossification center. The distinguishing feature is that os have well-corticated margins while acute fractures should have sharp radiolucent margins.

Key Points:

• Usual mechanism is hyperflexion trauma.
• Type II is most commonly seen and represents an unstable fracture that requires surgical fixation. Coronal/sagittal CT reformat are critical for detection of horizontally oriented type II fractures.
• Type I and III are less common and are considered stable injuries.
• Distinguish non-union ossicles from acute fracture by presence of well-corticated or sharp lucent margin

References:

1. Greenspan A . Orthopedic Radiology: A Practical Approach . 3rd Ed., Philadelphia: Lippincott Williams & Wilkins, 2000; 351-352.
2. Duke Orthopaedics presents Wheeless’ Textbook of Orthopaedics. Dens fracture. [http://www.wheelessonline.com/ortho/dens_fracture]
3. Yousem D. Neuroradiology: The Requisites. 2nd Ed., Philadelphia: Mosby. 203; 844-846. 
4. Boyarsky I, Godorov G. C2 fractures. Emedicine, March 15, 2005. [http://www.emedicine.com/orthoped/topic597.htm]