Odontoid Fractures: Anterior Odontoid Screw Fixation

Albert J.M. Yee, M.D., MSc, FRCSC
Assistant Professor, Department of Surgery
University of Toronto

Fractures of the odontoid account for approximately 20% of all cervical fractures with many (~70%) being Type II fractures (ie. fracture crossing the base of the odontoid process at the junction with the axis body)1. The optimal treatment of Type II fractures remains controversial and evidence suggests no standards or guidelines, but rather presents options in care2-4. The decision as to whether or not to operate and if so, what type of procedure to perform, is subject to much discussion.

The bimodal pattern of patient age (young or old) presenting with these injuries that result from either high or low energy trauma is important to consider when planning treatment5,6. An overwhelming goal of surgery in spinal fracture care is to restore mechanical stability. In odontoid fractures, surgical stability can be achieved by either fracture fixation of the C2 body to the odontoid process or by arthrodesis of the C1-C2 motion segment. The desire to maintain cervical motion is attractive, thus odontoid screw fixation has been increasingly reported. Lessening or obviating the need for halo immobilization is desirable and additionally there is no need for autologous bone graft harvest. Fracture healing rates of this procedure has been demonstrated, in observational studies, to be comparable to atlantoaxial fusion rates with a similar complication rate between procedures, although patient numbers in these studies are small and randomized studies are lacking7-9.

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Figure 1a
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Figure 1b

 

 

 

 

 

 

Figure 1 demonstrates a Computed Tomography scan coronal (Fig. 1A) and sagittal (Fig. 1B) reformatted images of a 26-year-old female with a displaced Type II odontoid fracture following a high energy motor-vehicle accident.

Patient selection for any surgical procedure is paramount and this consideration in odontoid fracture care is mandatory. Issues relating to fracture configuration, the size of the remaining ‘peg' in achieving distal fixation, the need for and ease of fracture reduction and bone density are all variables to consider in the ability to properly place screw(s) with sufficient fixation to encourage fracture healing (Figures 1 and 2). While odontoid screw fixation may be an attractive option to many patients presenting with Type II odontoid fractures, its practical use may be tempered by the aforementioned factors as the procedure does present some technical challenges, even in experienced hands. The risk of fracture non-union can be significant, particularly in some displaced Type II odontoid fractures. The ability to achieve proper fracture re-alignment, and the ability to achieve screw fixation that satisfactorily stabilizes the fracture adhering to AO principles in lag screw fixation, will influence surgical success. C1-C2 fusion is often an easier procedure to perform technically, while acknowledging that the procedure significantly reduces neck motion, particularly in rotation, by about 50%.

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Figure 2a
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Figure 2b
















Figure 2 demonstrates a plain lateral (Fig. 2A) and open-mouth odontoid AP (Fig. 2B) view of the same patient 9 months following a fracture reduction that was required under image guidance under general anaesthesia followed by anterior odontoid screw fixation with two AO small fragment terminally threaded lag screws.

Several critical steps in odontoid fixation warrant discussion. Patient head and neck position need to be considered in the context of anaesthesia, fracture reduction, and steps to optimize spinal precautions. Reductions performed with the patient awake provide the opportunity to clinically monitor neurologic status. Reduction and adjustments to neck position under anaesthesia, provide less immediate feedback while attempting to properly reduce a fracture and facilitate the necessary insertional screw trajectory. The potential expertise and availability of intraoperative neuro-monitoring needs to be considered and coordinated in advance of surgery as should discussions with anaesthesia regarding intubation strategies. Often the head and neck need to be positioned in some extension to facilitate exposure to the inferior edge of C2. If fracture reduction is lost, for example with a posteriorly displaced fracture configuration, less extension should be utilized until provisionary fixation can be achieved. Real-time intra-operative imaging is necessary throughout the procedure.

The procedure is often performed with biplanar fluoroscopy which mandates modifications to conventional operating room tables' head support. Gardner Well's Tongs or halo traction can maintain in-line C-spine control using a radiolucent frame extension from the table to the patient's head and a low profile frame extension will facilitate ease in positioning fluoroscopic equipment. The ability to have two C-arms in the operating room can facilitate the procedure although more advanced intra-operative imaging and 3D technology may be available at certain centres. The anterior neck needs to be widely draped realizing that the exposure is through a standard left or right anterior Smith-Robinson approach to the cervical spine but with a skin incision near C5-6. Thus, the appropriate trajectory for the drill and screw can be obtained allowing placement in the anterior inferior portion of the C2 vertebral body. This will require cephalad retractors that expose the retropharyngeal space up to the level of C2-3. Exposure of the inferior bony portion of C2 is performed with efforts to minimize disruption to the C2-3 disc. It is important to facilitate seating of a screw so that it is almost countersunk at C2/3 to minimize prominence of hardware that may cause post-surgical dysphagia. Starting 2-3mm lateral to the midline of C2 also helps in this regard and will allow placement of either one or two 3.5mm screws that can engage bone on the far side of the fracture line. Outcomes do not appear to be that different when considering using one or two screws10-11.

Attention to screw position - engaging as much of the bone in the remaining odontoid peg, and efforts to facilitate fracture compression using lag techniques are important to achieve. There does not appear to be significant differences biomechanically between one versus two screws in load-to failure stability. Internal fixation with one or two screws appears to provide approximately 50% of the initial strength of an unfractured odontoid11. Two screws may help control rotational stability, although it may be a challenge getting one, let alone two, screws in good position. Nine millimeters appears to be a critical diameter for the placement of two 3.5 cortical screws12. A cannulated screw system can facilitate insertion although there is some risk to inadvertent advancement of the wire. This can be addressed by the insertion of two 1.25mm Kirschner wires, removing one and overdrilling the path of the removed wire using a 2.5mm drill bit. The near fragment can be overdrilled with a 3.5mm drill bit, tapping, and subsequent insertion of a partially threaded screw with or without a washer. In relatively osteopenic bone, tapping to the far fragment may not be necessary, although in healthier bone, this may be important in order to facilitate engagement of the far fragment to achieve fracture compression.

There are limited opportunities to ensure good screw fixation into the odontoid process. If not performed properly, there may also be difficulty in achieving fracture compression as there is a tendency in tap and/or screw advancement to push the process away in a distractive manner given the limited size of the fragment. Smaller 2.7mm screws could be considered in anatomically smaller odontoids. The use of Herbert screws has been reported, although some consideration needs to be given as to how much torque should be applied to obtain adequate fracture compression but avoid thread stripping, while attempting to seat the terminal threads into the body of C2. Complications such as neurologic sequelae and loss of fracture fixation have been described although many of these relate to early usage and inappropriate indications for this technique. Relative and absolute contraindications to consider in this technique include: 1) an irreducible fracture pattern, 2) a fracture with involvement and comminution of the atlantoaxial joints, 3) a long oblique fracture line particularly if positioned from anterior-caudal to posterior-cranial, 4) fracture involvement of the anterior-caudal C2 region with comminution, 5) concomitant unstable C1 ring fractures, 5) pathological fractures, and 6) osteopenia.

The use of screw fixation in established odontoid fracture non-unions is also controversial. In summary, the main advantage to odontoid screw fixation is the preservation of cervical motion which fusion surgery does not permit. In experienced hands, this technique can facilitate fracture healing and provide early spinal stability, thereby enhancing the recovery and rehabilitation for select patients.

References

  1. Anderson L.D. and D'Alonzo R.T. JBJS(Am) 56A: 1663-74, 1974.
  2. Ochoa G., Injury, 36 Suppl 2: B54-64, 2005.
  3. Ziai W.C. and Hurlbert R.J., Can J Neurol Sci, 27(4): 294-301, 2000.
  4. Julien T.D. et al, Neurosurg Focus 8(6), e1, 2000.
  5. Bednar D.A. et al, J Spinal Disord 8(2): 166-169, 1995.
  6. Koivikko M.P. et al, JBJS(Br) 86(8): 1146-1151, 2004.
  7. Marchesi D.G., Orthopaedics 20(10):911-96, 1997.
  8. McCullen G.M. and Garfin S.R., Spine 25(5):643-652, 2000.
  9. Chang K.W. et al, J Sinal Disord; 7(1):62-69, 1994.
  10. Jenkins J.D. et al, J Neurosurg; 89(3): 366-370, 1998.
  11. Sasso R. et al, Spine 19(14): 1950-53, 1993.
  12. Nucci R.C. et al. Spine; 20(3):264-70, 1995.

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