OBJECTIVETo determine the location of the vertebral artery foramens from C(3) to C(6) and their relationship to the point 1 mm medial to the center of the lateral mass and to identify the value of oblique radiograph for cervical lateral mass screw trajectory by a cadaveric study.

METHODS(1) Twenty-eight cervical specimens (C(3)-C(7)) of human cadavers aged from 28 to 79 years were analysed. The transverse radiographs of C(3)-C(6) vertebrae were taken and the angle between the parasagittal plane and the line connecting the point of the lateral mass with the lateral limit of the transverse process foramen of C(3)-C(6) were measured. (2) The K-wires were drilled into lateral mass of C(3)-C(6) starting 1 mm medial to the center of the lateral mass and exiting by the juncture between the transverse process and the facet in ten specimens. Four wire placements under direct visualization, including placement of the wire tip staying the ventral cortex and 2, 4, 6 mm over-penetration of the ventral cortex of lateral mass, were performed separately on each specimen. After each placement, radiographs were taken on 45 degrees oblique left and 45 degrees oblique right views. Each intervertebral foramen on the oblique radiographs was divided into two parts: superior and inferior parts. The former is the true intervertebral foramen, while the latter is the intertransverse foramen on the gross specimen. The number of wire tips in each part was quantified for each placement. All results on the radiographs were compared with those on the gross anatomy.

RESULTS(1) The angles between the parasagittal plane and the line connecting the posterior starting point of the lateral mass with the lateral limit of the transverse foramen (C(3)-C(6)) were lateral to the sagittal plane, ranging from 5 degrees to 12 degrees. Among the vertebrae, there were no statistically significant difference (P > 0.05). (2) 15% of the wires without over-penetration and 41.3% with 2 mm over-penetration were found in the inferior parts of the intervertebral foramen in oblique views, while the wires were not noted in the intervertebral foramen by gross anatomy. with 4 mm over-penetration of the ventral cortex, 35% and 65% of wires were noted in the superior and inferior parts of the intervertebral foramen respectively, while only 28.8% of wires were found in the inferior part approximating the nerve roots in gross specimens. With 6 mm over-penetration, the number in the intervertebral foramen were 63.8% superiorly and 36.2% inferiorly on the oblique radiographs while all the tips were at the inferior part (intertransverse foramens) in gross specimens. The tip of wire crossed the line connecting the posterior borders of the intervertebral foramens in oblique radiographs when it penetrated the ventral cortex of lateral mass 4 mm or more.

CONCLUSIONS(1) There is no risk of damaging the vertebral artery if a screw is directed more than 15 degrees lateral to the sagittal plane at C(3 approximately 6) starting 1 mm medial to the center of the lateral mass. (2) Ideal screw tip position on oblique radiograph may not cross the line connecting the posterior borders of the intervertebral foramen on radiograph. If the screw tip is noted in the superior part of intervertebral foramen on the oblique radiograph, the screw may be identified as dangerous.