Michael Y Wang1. 1. Department of Neurological Surgery, Keck School of Medicine, University of Southern California, Los Angeles, California 90033, USA. myw@usc.edu
Abstract
OBJECTIVE: C2 screw fixation through the pars or pedicle involves a small but finite risk of vertebral artery injury. Recently, C2 crossing laminar screws were described as a method for rigid fixation of the axis. This technique, confined to the dorsal aspect of C2, presents minimal chance of injury to the neighboring neural or vascular structures. METHODS: Morphometric analysis was performed on 38 cadaveric spines obtained from the Keck School of Medicine at the University of Southern California. Critical measurements were determined for screw entry points, trajectories, and lengths for application of the technique, as described by Wright, using 3.5 and 4.0 mm screws. RESULTS: The average maximal screw length was 32 mm with a range of 27 to 37 mm. The minimal total laminar cross sectional area averaged 75 mm2, and the cancellous bone cross sectional area averaged 39 mm2. The minimal laminar cross sectional diameter was determined to be too small to accommodate a 3.5-mm diameter screw, assuming a 1 mm tolerance on each side, in 16 sides of 14 specimens, and to be too small to accommodate a 4.0 mm diameter screw, assuming a 1 millimeter tolerance, in 32 sides of 18 specimens. CONCLUSION: C2 laminar screw placement is straightforward and carries little risk of injury to the neural and vascular structures as long as the implants remain intraosseous. However, the highly variable thickness of the C2 lamina necessitates preoperative computed tomographic scanning to ensure that the patient's anatomy can accommodate screws entirely within the bone.
OBJECTIVE: C2 screw fixation through the pars or pedicle involves a small but finite risk of vertebral artery injury. Recently, C2 crossing laminar screws were described as a method for rigid fixation of the axis. This technique, confined to the dorsal aspect of C2, presents minimal chance of injury to the neighboring neural or vascular structures. METHODS: Morphometric analysis was performed on 38 cadaveric spines obtained from the Keck School of Medicine at the University of Southern California. Critical measurements were determined for screw entry points, trajectories, and lengths for application of the technique, as described by Wright, using 3.5 and 4.0 mm screws. RESULTS: The average maximal screw length was 32 mm with a range of 27 to 37 mm. The minimal total laminar cross sectional area averaged 75 mm2, and the cancellous bone cross sectional area averaged 39 mm2. The minimal laminar cross sectional diameter was determined to be too small to accommodate a 3.5-mm diameter screw, assuming a 1 mm tolerance on each side, in 16 sides of 14 specimens, and to be too small to accommodate a 4.0 mm diameter screw, assuming a 1 millimeter tolerance, in 32 sides of 18 specimens. CONCLUSION: C2 laminar screw placement is straightforward and carries little risk of injury to the neural and vascular structures as long as the implants remain intraosseous. However, the highly variable thickness of the C2 lamina necessitates preoperative computed tomographic scanning to ensure that the patient's anatomy can accommodate screws entirely within the bone.