The goals of internal fixation of the spine are to (1) correct deformity, (2) improve the fusion rate, (3) protect neural elements, and (4) reduce the rehabilitation time. These rationales are also applied to the cervical spine. Various operative
techniques of internal fixations for cervical spine are introduced. But now we have some question about the appropriate diameter and length of the screws and the critical range of the safe zone for screwing.
For the exact preopertive planning, such as selection of the appropriate size of implant and direction of the screwing, we measured in 100 cases of normal adults and 10 cases human cadaveric dry bones and obtained following results;
1. The mid sagittal and coronal diameter of the isthums of odontoid in male and female is 10.14±1.69mm by 9.99±0.41mm and 9.20±0.52mm by 9.27±0.49mm respectively.
2. Mean length of screwing for odontoid in male and female is 46.38±1.99mm and 43.02±2.34mm.
3. Anteroposterior diameter of the vertebral body of all cases are over 14mm.
4. Radiologic length for posterior screwing into the C2 in male and female is 38.67±1.91mm and 35.20±1.15mm. And the depth of the route for screwing is 9.27±0.70mm, 8.68±0.65mm in male and female respectively. But direct measurement of the
isthmic
portion of the cadaveric dry bone is 5.59±1.09mm.
5. By the method of Magerl, the appropriate the safe maximal length of the screw from C3 to C7 are 12.6mm to 16.3mm (see in text).
|