Harms C1-C2 instrumentation technique: anatomo-surgical guide.

Study Design. Anatomic study. Objective. To measure C1 and C2 critical areas related to the screws trajectory, according to Harms technique, in Latin specimens. To investigate vertebral's artery course in cadavers. Summary of Background Data. To our knowledge there are no studies addressing vertebral surface measurements for screw placement, according to Harms C1-C2 instrumentation technique, nor cadaveric measurements of the trajectory of the vertebral artery in Latin specimens. Methods. C1 and C2 specimens were measured. C1 measurements: height, width, anteroposterior diameter (intraosseus screw length) and convergence in the axial plane of the lateral mass; length from the posterior border of the posterior C1 arch to the anterior cortex of the articular mass (total screw length). C2 measurements: width, height, convergence and sagittal inclination of the pars interarticularis. Direction of the trajectory of the vertebral artery in the suboccipital region in fresh cadavers. Results. C1: left mass width 14.20 mm, right: 14.32 mm; left intraosseus screw length: 17.17 mm, right 16.9 mm; left total length of the screw: 27.14 mm, right: 26.72 mm; left mass height: 10.22 mm, right: 10.29 mm. Right mass convergence: 24.68 degrees, left: 22.44 degrees. C2: width: left 8.75 mm, right: 8.53 mm; height: left 10 mm, right 9.81 mm; convergence: left 42.15 degrees, right: 38.98 degrees; sagittal inclination: left 35.50 degrees, right 33.07 degrees. Vertebral artery's medial border is between 13 and 22 mm from the middle line of C1 posterior arch. Conclusion. Convergence and inclination of the pars are slightly greater than the suggested by Harms. Individual and/or racial variations must be considered. There is enough space for safe placement of a 3.5 mm screw in the lateral masses of C1 and through the pars of C2. Dissecting the superior face of the posterior arch of C1 laterally more than 10 mm from the posterior tubercule could injure the vertebral artery.