• Tzu-Yung Chen, Kwen-Lung Lin, and Hwei-Hwang Ho.
• Chang-Gung Memorial Hospital, Department of Neurosurgery, Kweishan, Taoyuan, Taiwan. tychen@adm.cgmh.org.tw
• Spine. 2004 May 1; 29 (9): 1000-4; discussion 1005.

Study DesignA morphometric study of lateral mass from C1 to C2 and involving 42 patients with rheumatoid arthritis (RA).ObjectiveTo provide anatomic data on the lateral mass of the upper cervical spine and quantitatively assess structure feature of a C1-C2 lateral mass in RA and its association with adjacent structures.Summary Of Background DataNo anatomic study on C1-C2 lateral mass in Chinese RA patients exists, nor is there a study describing the risk of transarticular screws fixation in these patients.MethodsForty-two patients with RA were obtained for study of the bony structure of the C1-C2 lateral mass. Using reconstructed CT scan, the anatomic variability of bony structure to rheumatoid inflammatory change was assessed via C2 isthmus width and height measurement. The mediolateral diameter, superoinferior diameter, and sagittal length of the atlantoaxial lateral mass were also calculated. Additionally, the possible screw trajectory angles were assessed.ResultsForty-two patients displayed bony erosion of the C1-C2 mass. The dimension change of the C2 isthmus was weakly correlated with age and rheumatoid history. Furthermore, predominant destruction on either side of lateral mass is noted in 21.4% (n = 9) of patients. The mean shortest isthmus height of C2 is 4.69 +/- 1.66 mm, while its mean shortest width is 5.14 +/- 1.23 mm. Furthermore, the average distance between the anterior margin of C1 lateral mass and the same side posterior cortex of the C2 inferior facet is 36.53 +/- 3.94 mm. Meanwhile, the distance of coronal aspect of C1 lateral mass is 11.20 +/- 1.92 mm. The medial/lateral and caudo-cephalic inclinations of the isthmus with respect to the C2 inferior facet are 86.66 +/- 7.69 degrees and 40.82 +/- 7.29 degrees. Bilateral 3.5-mm screw placement could be safely achieved in only 30.9% (n = 13) of patients with chronic RA with upper cervical lesions.ConclusionsThe work provides detailed bony data on the rheumatoid C2 isthmus and C1 structure. Anatomic variation in either side or both sides of the C2 isthmus is severe during erosion in patients with RA. Unilateral C1-C2 transarticular screw, modification of screw diameter, or alternative techniques for C1-C2 arthrodesis should be considered in most Chinese rheumatoid cases.

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