ANATOMIC CONSIDERATIONS FOR PLATE-SCREW FIXATION OF THE CERVICAL-SPINE

A cadaveric study was done to investigate the variability of interfacet distances from C3 to C7, to learn pedicle morphology at C7-T2, and to determine potential dangers of plate-screw osteosynthesis technique of the cervical spine. Twenty-two cadavers were dissected from C2 to C7 to expose the lateral masses and nerve roots. The interfacet distances from the center of the lateral mass to the next lateral mass from C3 to C7 vary widely among different individuals, ranging from 9 mm to 16 (average, 13 mm). A new plate design is necessary to better accommodate the differences in interfacet distances among different patients and different levels. The second part of the study involved 11 dissected cadaveric specimens of the cervicothoracic junction. The pedicle entry point was 1 mm inferior to the midportion of the facet joint for C7, T1, and T2. The medial angulation averaged 34-degrees at C7, 31.8-degrees at T1, and 26.5-degrees at T2. The mediolateral and superoinferior outer pedicle diameters were 6.9 mm and 7.5 mm at C7, 8.5 mm and 9.5 mm at T1, and 7.5 mm and 10.7 mm at T2, respectively. The mediolateral inner diameter averaged 5.18 mm, 6.4 mm, 5.5 mm for C7, T1 and T2, respectively. The pedicle distances (from the entry point to the posterior vertebral body line) measured 9.1 mm, 9.9 mm, and 10.4 mm for respective levels. The margins for error are small when using pedicle screws at these levels. If a pedicle screw must be used at C7, T1, and T2, one must have precise knowledge of the entrance point, diameters, and 25-30-degrees medial direction. The third part of the experiment involved 18 fresh cadaveric specimens for the study of the anatomy of the nerve root, vertebral artery, and spinal cord in relation to drill trajectory and inserted screws. The vertebral artery and spinal cord were virtually free of danger with known techniques. The purpose of this experiment was then to find the trajectory that is safest for the nerve roots. The nerve root exits at the anterolateral portion of the superior facet, and the more medial angulation or the more cephalad angulation of the screw, the more likely it is to impinge on the nerve root. The ideal exit point of the drill was determined to be the juncture between the transverse process and the facet. The safest screw direction is determined to be 33-degrees lateral and 17-degrees cephalad starting 1 mm medial to the center of the lateral mass for C3-C6 in order to avoid both the nerve root and the facet joint. The depth of these screws ranged from 7 mm to 18 mm with an average of 10 mm, using this technique. The lateral mass is thin at the transitional C7 level, and lateral mass screw is dangerous in disrupting the nerve root or the facet joint.