In vivo and in vitro CT analysis of the occiput

Arguments concerning the best procedure for occipito-cervical fusion have r arely been based upon occipital bone thickness or only based on in vitro st udies. 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 Te n macerated human skulls were measured by means of CT and directly. Measure ments 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). Correla tion 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 fix ation using the 10% percentile value (6.68 mm for the maximum value of 11.8 7 mm) led to the conclusion that screws should only be inserted along the o ccipital 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 midl ine. A reduction of screw length to 7 mm (9 mm for the EOP) is proposed. Pr eoperative evaluation of the patient should be carried out by spiral CT wit h 1 mm slicing and sagittal reconstructions.