Abstract
Introduction: C1 lateral mass screw fixation offers a powerful alternative biomechanical fixation for upper cervical disorders. The anatomical constraints to this fixation have not been described yet and are essential to ensure avoidance of neurovascular damage.
Methods: 50 patients (including 5 patients with rheumatoid arthritis) underwent upper cervical CT scans. Analysis of these CT scans involved use of calibrated scan measurements to identify the midpoint of the posterior lateral mass, the dimensions of the lateral mass, the direction of optimum screw passage, the position of the vertebral foramen at C1 and the ideal entry point for lateral mass screw fixation.
Results: The average length of screw within the lateral mass was 20 mm with 13.5mm of screw not in bone, behind the lateral mass, but necessary to allow rod placement posteriorly adjacent to other fixation points. The safest entry point was directly beneath the medial edge of the lamina origin. The ideal direction of screw angulation is parallel with the posterior arch, in the saggital plane. This entrypoint was on average 8.8 mm from the vertebral artery foramen laterally and 5.8 mm from the medial aspect of the lateral mass. Vertical space available for sublaminar screw placement was 3mm or less in 9% of lateral masses.
Discussion: C1 lateral mass screws are best placed beneath the lamina origin, parallel with the arch in the saggital plane using an entrypoint in line with the medial edge of the lamina origin. An entry point under the midpoint of the lamina origin, or passing through the lamina at its attachment to the lateral mass, is likely to damage the vertebral artery in a significant proportion of cases.
The abstracts were prepared by I. B. McPhee. Correspondence should be addressed to the Spine Society of Australia Secretariat, The Adelaide Centre for Spinal Research, Institute of Medical and Veterinary Science, PO Box 14, Rundle Mall, Adelaide SA 5000, Australia.
