In vivo and in vitro CT analysis of the occiput.

Arguments concerning the best procedure for occipito-cervical fusion have rarely been based upon occipital bone thickness or only based on in vitro studies. To close this gap and to offer an outlook on preoperative evaluation of the patient, 28 patients were analysed in vivo by means of spiral CT. Ten macerated human skulls were measured by means of CT and directly. Measurements were taken according to a matrix of 66 points following a grid with 1 cm spacing based upon McRae's line. Maximum thickness in the patient group was met 4 cm above the reference plane in the median slice (11.87 mm; SD 3.41 mm) and 5 cm above it in the skull group (15.85 mm; SD 1.81 mm). Correlation between CT and direct measurements was good (91.79%). Intra-individual discrepancies from one side to the respective point on the other side are common (difference > 1 mm in 60%). Judging areas suitable for operative fixation using the 10% percentile value (6.68 mm for the maximum value of 11.87 mm) led to the conclusion that screws should only be inserted along the occipital crest in an area extending from 1.5 cm above the posterior margin of the foramen magnum to the external occipital protuberance (EOP). At the level of the EOP screws may also be inserted up to 1 cm lateral of the midline. A reduction of screw length to 7 mm (9 mm for the EOP) is proposed. Preoperative evaluation of the patient should be carried out by spiral CT with 1 mm slicing and sagittal reconstructions.