Odontoid Screw Fixation
Christopher T. Martin
Keith W. Michael
John M. Rhee
Radiologic Assessment—Key Factors to Consider on X-ray, MRI, CT
Anteroposterior (AP), lateral, and open mouth odontoid views, in addition to a fine-cut CT scan, are standard preoperative imaging modalities.
Patients with a short neck and a barrel chest, kyphotic alignment, or inability to extend the neck may have an impossible screw trajectory due to interference from the chest wall. Assess the patient clinically and assess the scout CT views. Draw out the trajectory and where your instruments will need to lie and verify that the chest wall will not interfere (Figure 32-1).
Figure 32-1 ▪ Scout CT view shows that the trajectory of an odontoid screw, and thus the instruments needed to place it, would just barely clear the sternum if the patient were positioned like this (yellow arrow). Ideally, the patient would be positioned in more extension for surgery, which would then allow for more clearance away from the sternum assuming that the fracture remains reasonably reduced in that position.
In those with significant osteopenia or osteoporosis, we perform a posterior C1-2 fusion rather than odontoid screw fixation.
Fracture patterns that extend from posterior-superior to anterior-inferior (Figure 32-2A) are a relative contraindication, as the obliquity of the fracture line does not facilitate compression with screw placement, or the screw will obliquely cross the fracture line. Posterior-inferior to superior-anterior fracture lines (Figure 32-2B) are more amenable to odontoid screw fixation, since the screw will traverse relatively perpendicularly across the fracture and compress it.
Figure 32-2 ▪ Fracture patterns that extend from posterior-superior to anterior-inferior (A) are a relative contraindication, as the obliquity of the fracture line does not facilitate compression with screw placement, or the screw will obliquely cross the fracture line. Posterior-inferior to superior-anterior fracture lines (B) are more amenable to odontoid screw fixation, since the screw will traverse relatively perpendicularly across the fracture and compress it.
Fractures in which the tip is displaced posteriorly may require relative flexion of the neck for reduction, which then may hinder access for screw insertion.
Chronic nonunions are unlikely to heal with odontoid screw fixation and may be better treated with C1-2 posterior arthrodesis.
Irreducible canal compromise is a relative contraindication. Most acute fractures can be reduced with traction and intraoperative manipulation. However, if the fracture is irreducible it may be advisable to perform C1-2 posterior arthrodesis.
Every planned case of odontoid screw fixation should be consented and prepared for a posterior C1-2 fusion in case the fracture cannot be appropriately reduced, adequate fluoroscopic visualization is not achieved, or the required screw trajectory cannot be obtained in the reduced position.
Special Equipment
Self-retaining cervical retractors
K-wire driver and cannulated drill/screw system
Positioning
The patient is supine on a radiolucent table with a bump under the shoulders to allow maximal head extension, which assists with obtaining appropriate screw trajectory and with fracture reduction.
The arms are tucked at the sides and well padded. Pillows are placed beneath the knees and the pressure points are well padded.
Gardner-Wells tongs with 10 lb of traction can be applied to keep the head stable and assist with fracture reduction.
Biplanar fluoroscopy is critical to appropriate localization and screw placement, with open mouth AP and lateral views (Figure 32-3).
Figure 32-3 ▪ Biplanar fluoroscopy is critical to appropriate localization and screw placement, with open mouth anteroposterior and lateral views.
It is important not to proceed with incision until the positioning is perfect, the fracture is reduced, and usable biplanar views are readily obtainable.
AP view is taken prior to incision with adequate visualization of the odontoid (outlined by red arrow, Figure 32-4A).Related posts:
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