Radiological and anatomical evaluation of the atlantoaxial transarticular screw fixation technique

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✓ Sixty-one patients treated with C1–2 transarticular screw fixation for spinal instability participated in a detailed clinical and radiological study to determine outcome and clarify potential hazards. The most common condition was rheumatoid arthritis (37 patients) followed by traumatic instability (15 patients). Twenty-one of these patients (onethird) underwent either surgical revision for a previously failed posterior fusion technique or a combined anteroposterior procedure. Eleven patients underwent transoral odontoidectomy and excision of the arch of C-1 prior to posterior surgery. No patient died, but there were five vertebral artery (VA) injuries and one temporary cranial nerve palsy. Screw malposition (14% of placements) was comparable to another large series reported by Grob, et al. There were five broken screws, and all were associated with incorrect placement.

Anatomical measurements were made on 25 axis bones. In 20% the VA groove on one side was large enough to reduce the width of the C-2 pedicle, thus preventing the safe passage of a 3.5-mm diameter screw.

In addition to the obvious dangers in patients with damaged or deficient atlantoaxial lateral mass, the following risk factors were identified in this series: 1) incomplete reduction prior to screw placement, accounting for two-thirds of screw complications and all five VA injuries; 2) previous transoral surgery with removal of the anterior tubercle or the arch of the atlas, thus obliterating an important fluoroscopic landmark; and 3) failure to appreciate the size of the VA in the axis pedicle and lateral mass. A low trajectory with screw placement below the atlas tubercle was found in patients with VA laceration.

The technique that was associated with an 87% fusion rate requires detailed computerized tomography scanning prior to surgery, very careful attention to local anatomy, and nearly complete atlantoaxial reduction during surgery.

Article Information

Address reprint requests to: H. Alan Crockard, F.R.C.S., The Department of Surgical Neurology, The National Hospital for Neurology and Neurosurgery, Queen Square, London WC1N 3BG, England.

© AANS, except where prohibited by US copyright law.

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Figures

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    Drawings showing the C-2 vertebra (A, top view; B, frontal view) and the measured anatomical parameters in this study.

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    A: Postoperative lateral plain radiograph showing a fractured right screw. Note persistent anterior and rotatory displacement of C-1 on C-2. B: Immediate postoperative axial CT scan showing the left screw transiting the VA groove (left arrow). The right screw (right arrow) fractured 2 weeks later. C and D: Anteroposterior and lateral conventional left vertebral angiograms obtained in the same patient showing occlusion of the artery by a malpositioned screw (arrows); note slimming of the artery caused by low flow and the developing collateral vessels.

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    Postoperative plain radiographs of the C1–2 joint. A and B: Lateral and anteroposterior transoral views, respectively, showing a fractured right screw at the joint line (black arrow) in a patient in whom the left screw was positioned too laterally (white arrowhead). Note incomplete reduction of C1–2 displacement. C: Coronal CT reconstruction through the C1–2 segment showing the right screw encroaching on the course of the VA (white arrowhead). D: Axial CT slice taken through the C1–2 joint showing the point of fracture of the left screw and the malpositioned right screw (white arrowheads).

  • View in gallery

    Anatomical and radiological appearance of an enlarged VA groove. A: Photograph depicting anatomy; C-2 viewed from below showing large VA groove nearly eroding most of the C-2 lateral mass and pedicle (left curved arrow). The right pedicle is normal (right curved arrow). In these circumstances transarticular screw fixation would not be recommended. B: Spine CT reformat showing enlarged left VA groove (1).

  • View in gallery

    Drawings depicting a lateral view of the C1–2 joint. A: Drawing showing normal position with normal screw trajectory. B: Drawing showing the effect of incompletely reduced displacement and the segment in full flexion: with screw trajectory aiming to the anterior tubercle of C-1, the screw will be low enough to transect the VA underneath the C-2 lateral mass. A VA injury would be inevitable in these circumstances.

  • View in gallery

    A: Sagittal CT paramedian reconstruction through the lateral mass of the C1–2 joint, with the VA groove (white arrow) seen underneath the C-2 lateral mass. B: Artist's rendition of the same CT scan showing the relation of the VA groove (curved arrow) to the screw trajectory (dotted lines).

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