Dens or Odontoid Fractures

Odontoid fractures are fractures of dens or odontoid process of axis vertebra or C2. Dens is a strong, tooth-like process projecting upwards from the body of the axis.

Fractures of the axis make up 27% of all cervical spine injuries. Fractures of the odontoid process are the most common subtype of axis fractures (41%). Odontoid fractures account for 10-15% of all cervical fractures.

These fractures occur in young patients and the elderly in a bimodal fashion. Whereas in young patients the fractures are as a result of blunt trauma to head leading to cervical hyperflexion or hyperextension, in elderly people these occur by simple fall. Elderly people also have higher morbidity and mortality.

Anatomy of Odontoid Process or Dens

Anatomy of axis vertebra

Axis vertebra has an odontoid process or dens and body. The cartilaginous junction between the dens and vertebral body that does not fuse until 6 years of age and should be kept in mind so as not to misdiagnose it as fracture.

Range of Motion of Cervical Spine

Occipital-C1-C2 ligamentous stability is provided by the odontoid process and its supporting ligaments. Transverse ligament limits the anterior translation of the atlas. Both apical ligaments and alar ligaments limit rotation of the upper cervical spine

The odontoid is derived from mesenchyme of the first cervical vertebra. During development, it becomes separated from the atlas and fuses with the axis.

The apex, or tip, of the odontoid, is derived from the most caudal occipital sclerotome, or proatlas.

This separate ossification center, ossiculum terminale, appears at age 3 years and fuses by age 12 years.

Blood Supply of Odontoid

Vertebral and carotid arteries form the main source of blood supply to odontoid. Anterior ascending artery and posterior ascending artery are branches of vertebral artery beginning at the level of C3. They ascend anterior and posterior to the odontoid and meet superiorly to form an apical arcade.

Cleft perforators come from of the extracranial internal carotid artery and supply the superior portion of the odontoid.

The apex of the odontoid is supplied by branches of the internal carotid artery and base is supplied from branches of the vertebral artery.

Os Odontoideum

This condition was earlier thought to be a failure of fusion at the base of the odontoid but new evidence has suggested the likelihood of residual old traumatic process. An os odontoduem can be divided into two main types

• Orthotopic: the normal position with a wide gap between C2 and os)
• Dystopic: displaced

It could be confused with odontoid fractures and the treatment is mostly observation

Fracture Pattern

The displacement of the fragment could be anterior (hyperflexion) or posterior (hyperextension). Anterior displacement is associated with transverse ligament failure and atlantoaxial instability.

Posterior displacement is caused by direct impact from the anterior arch of atlas during hyperextension.

Anderson and D’Alonzo Classification of Odontoid Fractures

Type I  Odontoid Fractures

These type of odontoid fractures involve the tip of dens at the insertion of the alar ligament. These are usually stable fractures but may be associated with atlanto-occipital dislocation. These constitute about 5% of odontoid fractures. Generally, it is a stable fracture.

Type II Odontoid Fractures

These are the most common odontoid fractures and account for 60% of the cases. Fractures occur at the base. These fractures are associated with a higher rate of nonunion.

 Type III Odontoid Fractures

Accounts for 30% of the fractures. Fractures occur through the body of C2 and do not actually involve dens. These are unstable fracture as the atlas and occiput can now move together as a unit

Clinical Presentation

Patients present with neck pain that worsens with the motion of the neck.  Other findings may be

• Motor power loss ranging to quadriplegia
• Feeling of the instability of head on the spine
• Dysphagia may be present when associated with a large retropharyngeal hematoma

Imaging

Xrays

AP, lateral, open-mouth odontoid view of the cervical spine is commonly done in injury to the upper cervical spine and odontoid fractures. Fracture pattern is best seen on open-mouth odontoid.

Flexion-extension radiographs are important to diagnose occipitocervical instability in Type I odontoid fractures and Os odontoideum.

Instability defined as atlanto-dens-interval > 10mm and < 13mm space available for cord.

CT/MRI

CT scan would be best for fracture delineation and to assess the stability of the fracture pattern. CT angiogram is required to determine the location of vertebral artery prior to posterior instrumentation procedures. MRI is indicated if the neural loss is there.

Treatment of Dens Fractures

Os Odontoideum patients require observation in most of the cases. Type I fractures are managed by cervical orthosis. Type II in young patients can be managed in halo brace if risk factors for nonunion are not present. Otherwise, they should be operated. In elderly patients who are not candidates for surgery, type II fractures are managed with cervical orthoses.

Type III fractures are treated with Cervical Orthosis

Nonoperative Treatment

Observation

Os odontoideum and in cases with no neurologic symptoms or instability.

Hard Cervical Orthosis

These are worn for 6-12 weeks in patients with type I and type II in elderly who are not surgical candidates. The union may not occur in these cases but a fibrous union provides enough stability.

Halo Vest Immobilization

It is done in young patients with no risk factors for nonunion. This is also worn for 6-12 weeks.

Surgical Treatment

Posterior C1-C2 fusion

This involves fusion of first and second vertebra posteriorly. This treatment is considered in patients with

• Type II fractures with risk factors for nonunion.
• Type II/III fracture nonunions
• Os odontoideum with neurologic deficits or instability
• Type I with atlantooccipital instability (extremely rare)

Anterior Odontoid Osteosynthesis

This involves the surgical fixation of the fracture. This is done in patients with type II fractures with risk factors for nonunion and acceptable alignment and minimal displacement provided fracture pattern allow proper screw placement. It is associated with higher failure rates than posterior C1-2 fusion

Transoral Odontoidectomy

Odontoid is removed when there is severe posterior displacement of dens with spinal cord compression and neurologic deficits

Image Credit: http://www.jaaos.org/content/18/7/383/F1.large.jpg