SA Orthopaedic Journal
On-line version ISSN 2309-8309
BUTEERA, A and LUKHELE, M. Anatomic study of the atlas for surgical planning of lateral mass screw fixation: is it safe in our population?. SA orthop. j. [online]. 2010, vol.9, n.4, pp. 38-44. ISSN 2309-8309.
INTRODUCTION: Atlantoaxial instability is not uncommon and its management can be a challenge. Several stabilisation techniques have been described. All have their challenges because of the paucity of some osseous elements, the neural and vascular structures found in that region. Lateral mass screw fixation is one of the techniques which has been described for selected cases. Like all the other techniques it is a challenging technique to use and calls for full familiarisation with the anatomy at that region. OBJECTIVES: The objective of the study was to assess if the atlas lateral mass screw can be safely accommodated in our population. The secondary aim was to establish if there were any differences in the size between the male and female or among the white and black population groups. METHODS AND MATERIALS: Samples of dry adult atlas vertebra from the Raymond Dart Collection of human skeletons at the Wits University Department of Anatomy were measured for several parameters including the height, width and length of the lateral mass using an electronic digital caliper. Atlas of persons below 18 years of age at the time of death and those who had anomalies or deformities were excluded from the study. Data was recorded in Microsoft Excel and was imported in SAS V9.1 (SAS Institute Inc., Cary, NC, USA) for analysis. RESULTS: One-hundred-and-fifty-nine atlas specimens were studied with almost equal numbers in both sexes and between black and white populations. The mean width of the lateral mass was 13.77 ± 1.23 mm, the height of lateral mass below the overhang was 4.51 ± 0.634 mm and the height of the lowest point of lateral mass was 11.94 ± 1.21 mm. The anteroposterior distance of the lateral mass was 17.64 ± 1.36 mm. The angle of inclination of posterior arch to lateral mass was 76.83 ± 5.12º. Of the 159 vertebrae, two vertebrae had lateral mass height below the overhang of 3.5 mm, and 37 vertebrae had height below overhang of 4 mm. There were no statistical significant differences between male and female and between the race groups. The reason for us to assess if there was any difference in the male and female and also in blacks and whites is that in our previous study on the size of the odontoid process we had found a significant difference between the groups. In that study we found that South African blacks had a much smaller odontoid process than South African whites. Interestingly we also found that South African blacks had a smaller odontoid process than African Americans. This indicated to us that there is more than genetic factors playing a role; environmental factors and nutritional factors could have influence. It was for that reason that we felt we could not take it for granted that these groups will have similar findings. This was the secondary aim of the study, not the main aim. CONCLUSION: The atlas mass morphology was found to be adequate to accommodate the lateral mass screw safely in our population. Only two of the 159 dry atlas bones examined had the potential of not accommodating a 3.5 mm screw risking violating the occiputo-atlas joints. In 37 of the specimens the height of the lateral mass was below 4 mm. For that reason it is advisable not to use 4 mm size screws in our population as there is a significant risk that the occiputo-atlas joint may be violated by it.