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 (one-third) 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.

Transarticular screws at the C1 to C2 level of the cervical spine provide rigid fixation, but there is a danger of injury to a vertebral artery. The risk is related to the technical skill of the surgeon and to variations in local anatomy. We studied the grooves for the vertebral artery in 50 dry specimens of the second cervical vertebra (C2). They were often asymmetrical, and in 11 specimens one of the grooves was deep enough to reduce the internal height of the lateral mass at the point of fixation to < or =2.1 mm, and the width of the pedicle on the inferior surface of C2 to < or =2 mm. In such specimens, the placement of a transarticular screw would put the vertebral artery at extreme risk, and there is not enough bone to allow adequate fixation. Before any decision is made concerning the type of fixation to be used at C2 we recommend that a thin CT section be made at the appropriate angle to show both the depth and any asymmetry of the grooves for the vertebral artery.

Transarticular screws at the C1 to C2 level of the cervical spine provide rigid fixation, but there is a danger of injury to a vertebral artery. The risk is related to the technical skill of the surgeon and to variations in local anatomy.We studied the grooves for the vertebral artery in 50 dry specimens of the second cervical vertebra (C2). They were often asymmetrical, and in 11 specimens one of the grooves was deep enough to reduce the internal height of the lateral mass at the point of fixation to ≤2.1 mm, and the width of the pedicle on the inferior surface of C2 to ≤2 mm. In such specimens, the placement of a transarticular screw would put the vertebral artery at extreme risk, and there is not enough bone to allow adequate fixation.Before any decision is made concerning the type of fixation to be used at C2 we recommend that a thin CT section be made at the appropriate angle to show both the depth and any asymmetry of the grooves for the vertebral artery. [Br] 1997;79-B:820-3. Received 6 January 1997; Accepted after revision 28 April 1997 The anatomy of the second cervical vertebra (C2) has been described in detail. J Bone Joint Surg MATERIAL AND METHODSWe obtained 50 dry specimens of the second cervical vertebra for physical assessment and anatomical measurement from the Departments of Anatomy of the Royal College of Surgeons of England and University College, London. We measured 11 parameters, three angular and eight linear dimensions. The anatomical measurements focused on the pedicle, the lateral mass, the groove of the vertebral artery and the transverse foramen. Paired structures were measured bilaterally. The angular measurements were made using a standard goniometer accurate to 1° and the linear measurements using a PC-linked electronic calliper accurate to 0.01 mm with a facility for direct downloading of the data. 13 The calliper had a depth gauge which was used to measure accurately the vertebral body, the height of the dens and the depth of the vertebral growth. The mean, the standard deviation, and the range were calculated for all 50 specimens amounting to a total of 100 measurements for each observation. Right to left asymmetry was also analysed by the Spearman correlation and the paired samples by Student's t-test and the Wilcoxon matched-pairs signed-rank test. All mean values were expressed as the mean with confidence intervals (CI) of 95%, unless otherwise stated in the text. We made the following measurements: the height of the pedicle as measured from its superior surface to the inferior surface within the transverse foramen, its width measured from the internal surface of the pedicle to its external surface at the level of the transverse foramen, the median angle of the pedicle which is the angle between a line through the axis of the pedicle and the mid-sagittal line,

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Biocompatible osteoconductive polymer acts as a good "spacer" that reduces graft collapse and intersegmental kyphosis. However, it did not show any radiologic evidence of biodegradation or incorporation during the follow-up period of 24 months.