Odontoid Screw Fixation

10.1055/b-0034-84470

Odontoid Screw Fixation

Curtis A. Dickman and Volker K. H. Sonntag

Anterior screw fixation of the odontoid process is clinically useful for the internal fixation of unstable type II odontoid fractures.1–8 A variety of other methods has been used for the internal fixation of unstable dens fractures. These other atlantoaxial fixation techniques, however, sacrifice all C1-C2 motion. The major advantage of the odontoid screw technique is that it fixates the odontoid fracture directly yet completely preserves normal C1-C2 motion. This chapter reviews the clinical techniques for odontoid screw fixation.

Indications

The Anderson and D′Alonzo9 classification system divides odontoid fractures into three subtypes. The system is based on fracture morphology and healing capabilities, and both factors are used to guide treatment. Type I fractures involve the tip of the dens. Type II fractures involve the neck of the dens. Type III fractures extend from the base of the dens into the body of C2 ( Fig. 39.1 ). Odontoid screw fixation is designed primarily for treating unstable type II dens fractures because of their morphology and their predisposition to nonunion. Most odontoid fractures (type I, nondisplaced type II, and type III) heal satisfactorily with a cervical orthosis. Widely displaced type II odontoid fractures (>6 mm), however, are predisposed to nonunion when treated nonoperatively.6,9–14

Odontoid screw fixation is reserved for patients with widely displaced type II fractures (6 mm or more), fracture nonunions, or patients who are unable to wear a halo brace ( Table 39.1 ). The probability of a type II fracture uniting when treated with an orthosis primarily depends on the extent of bone displacement.6,9–14 Nondisplaced fractures or fractures displaced less than 6 mm have a 90% chance of healing with an orthosis.10,14 In comparison, type II fractures displaced 6 mm or more have 78% chance of nonunion when treated with an orthosis.10 Type II fractures with extensive bone comminution at the base of the dens (type IIA fractures) also have a high risk for nonunion and should be considered for early surgical fixation ( Fig. 39.2 ).15

**Indications for Odontoid Screw Fixation**
Type II odontoid fractures
Acute, widely displaced (6 mm or more dens dislocation)
Subacute, unstable (alignment not maintained with orthoses)
Chronic nonunion Inability to wear halo brace for a displaced fracture
Type III odontoid fractures*
Shallow fractures if nonunion develops or if unstable with an orthosis
*Acute type III fractures can usually be managed nonoperatively.

If there are no medical contraindications to surgery, odontoid screw fixation can be considered for the initial treatment of fractures that are at a high risk for nonunion. Surgery also may be considered for patients with a displaced dens fracture who object to wearing a halo brace, those who cannot tolerate a halo brace (i.e., because of cosmetics, psychological concerns, or multiple skull fractures), or those who prefer to have surgery rather than to wear a halo brace.

Odontoid fractures that fail treatment with an orthosis require surgery. Orthotic failures include recurrent subluxations of the dens, an inability to restore satisfactory C1-C2 alignment, and nonunions. If the dens heals in a malaligned position, chronic spinal cord compression and neurological deficits may develop. When the alignment of the dens cannot be maintained satisfactorily with a halo brace, surgical fixation should be considered. All patients treated with an orthosis require meticulous follow-up to detect nonunion and persistent atlantoaxial instability.

Anderson and D′Alonzo9 classification of odontoid fractures. Type I fractures are through the apex of the dens. Type II fractures extend across the neck of the dens. Type III fractures extend into the body of C2. (Reprinted with permission from Barrow Neurological Institute.)

Type IIA fractures have extensive comminution and are at high risk for nonunion when treated with an orthosis.15 (From Hadley M, Browner C, Liu S, et al. New subtype of acute odontoid fractures (type IIA). Neurosurgery 1988;22(1):67–71. Reprinted with permission.)

Contraindications

There are a variety of contraindications to odontoid screw fixation ( Table 39.2 ): severe osteoporosis, fractures that extend deeply into the body of the dens (type III), or extensively comminuted bone fragments that have weakened the bone and will not hold a screw securely. If the transverse ligament is disrupted, an odontoid screw will not restore C1-C2 stability even if the dens is fixated solidly.16 Odontoid screw fixation cannot be performed if C1-C2 alignment cannot be restored. Alignment is best restored preoperatively. Intraoperative realignment of the dens can be achieved with direct methods (i.e., manipulation of C1, C2, or both with instruments) or by altering the position of the head. All realignment maneuvers are monitored directly with fluoroscopy. Large chests, short necks, and fixed cervical flexion deformities prohibit the proper trajectory of the screws parallel to the anterior surface of the spine. Os odontoideum and chronic nonunions are associated with poor outcomes when treated with odontoid screws. Os odontoideum has sclerotic bone, which does not heal satisfactorily. Chronic nonunions have a higher nonunion rate than acute fractures because the fibrous tissue that forms between the fracture surfaces interferes with bone healing.

**Contraindications for Odontoid Screw Fixation**
Severe osteoporosis
Extensive bone comminution
Disrupted transverse atlantal ligament
Os odontoideum
Type III odontoid fractures (with deep extension into C2 body)
Inability to restore C1-C2 alignment
Inability to achieve the appropriate trajectory for screw insertion
Short neck
Large barrel chest, large breasts
Fixed cervical flexion deformity
Inability to extend patient′s head and neck
Chronic nonunions of type II odontoid fractures (relative contraindication)

Preoperative Radiographic Evaluation

Preoperative thin-section computed tomography (CT) is required to evaluate the bone architecture and to assess abnormalities that may preclude screw placement, (e.g., an aberrant course of the vertebral artery or extensive concurrent cervical fractures). Magnetic resonance imaging (MRI) of the cervical ligaments can be used to detect a disrupted transverse atlantal ligament preoperatively.16,17 The relationship of an odontoid fracture associated with a disrupted transverse ligament has been reported in several cases.16 Odontoid screw fixation is contraindicated when this disruption occurs because the screw will not restore atlantoaxial stability.

Surgical Procedure

Operating Room and Patient Positioning

Precise imaging using simultaneous anteroposterior (AP) open-mouth and lateral plane fluoroscopy is mandatory to visualize C2 intraoperatively so that the screw trajectory can be guided accurately. Fluoroscopy is an essential aspect of performing this operation and is achieved with two separate C-arms. The fracture must be reduced to anatomical alignment by positioning the head or by intraoperative manipulation before the screw is inserted. The screws compress the adjacent bone fragments together but will not restore alignment if the dens is malaligned horizontally. A malaligned fixation must be avoided because it can cause neural compression.

Odontoid screw fixation is performed using an anterior cervical operative exposure while the patient is supine. The patient′s head and neck are extended to provide the proper screw trajectory into the tip of the dens ( Fig. 39.3 ). Extension is performed carefully under fluoroscopic monitoring to avoid distraction or subluxation of the fracture. Extension is needed to position the drill and screw trajectory parallel to the anterior surface of the cervical spine so that the screw can be aimed into the tip of the dens. The proper screw trajectory cannot be achieved in individuals with barrel-shaped large chests, short necks, or fixed cervical flexion deformities. Instead of odontoid screw fixation, such patients should undergo a posterior C1-C2 fixation.

**(A)** Patients are positioned supine with the head and neck extended to facilitate placing the screw into the tip of the dens. A transverse incision is made over the C4-C5 interspace to provide horizontal trajectory, parallel to the anterior surface of the spine. The Apfelbaum retraction system (Aesculap, San Francisco, CA) and the screw trajectory are depicted *(insert).* **(B)** The proper screw trajectory cannot be achieved in patients with a short neck or a barrel chest.20 (Reprinted with permission from Barrow Neurological Institute.)

Skull fixation with a Mayfield head holder interferes with positioning the C-arms and with obtaining the intraoperative radiographs. The head is best positioned on a radiolucent headrest, supported on a doughnut. Enough space must be maintained to position the two C-arms around the patient′s head. Alternatively, the patient can be kept in a halo brace until screw fixation is achieved. To allow AP open-mouth fluoroscopy, the mouth is propped open widely with cloth, gauze, or other radiolucent materials.

Patient positioning and setup of the C-arms are critical for the success of this procedure ( Fig. 39.4 ). These maneuvers often take longer than the surgical insertion of the screw. The AP C-arm is positioned above the head of the operating table. The lateral C-arm advances from the patient′s left side. The anesthesia machines are positioned on the patient′s left, near the thorax. The surgeon is on the patient′s right side.

Ideally, the dens should be realigned preoperatively, and the alignment should be verified after the patient is positioned on the operating table. However, it is not always possible to restore C1-C2 alignment preoperatively. When the dens is displaced posteriorly, alignment may be restored intraoperatively by directly pushing the C2 body posteriorly with a curette. When the dens is displaced anteriorly, alignment can be restored by extending the patient′s head and by positioning C1 and the dens more posteriorly.

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