Relation between alignments of upper and subaxial cervical spine: a radiological study

To reveal the relation between alignments of upper and subaxial cervical spine and deduce the optimal atlantoaxial fusion angle by a radiological study. 414 asymptomatic volunteers (213 males, 201 females) underwent cervical lateral radiographs in neutral position. The Oc-C2 angle, C1-C2 angle and C2-C7 angle were measured. Relations among these three angles and relations between angles and age were analyzed. The mean Oc-C2 angle was 16.3A degrees A A +/- A 7.0A degrees in females, significantly larger than 14.9A degrees A A +/- A 6.5A degrees in males. The mean C1-C2 angles were 28.2A degrees A A +/- A 4.0A degrees in females and 26.4A degrees A A +/- A 4.6A degrees in males, and C2-C7 angles were 12.7A degrees A A +/- A 6.6A degrees and 16.3A degrees A A +/- A 7.3A degrees, correspondingly. The mean C1-C2 angle in females was significantly larger than that in males, while C2-C7 angle smaller than that in males. The C2-C7 angle correlated significantly not only with C1-C2 angle but also with Oc-C2 angle. And correlation between C1-C2 angle and C2-C7 angle was stronger than that between Oc-C2 angle and C2-C7 angle. There were also significant positive correlations between C1-C2 and Oc-C2 angles. Oc-C2 angle, C1-C2 angle, and C2-C7 angle correlated significantly with age in both sexes. There were negative correlations between C1-C2 angle and C2-C7 angle as well as between Oc-C2 angle and C2-C7 angle, and the former correlation was stronger. C1-C2 fixation angle was the key to regulate postoperative subaxial alignment in atlantoaxial arthrodesis. The optimal atlantoaxial fusion angle may be between 25A degrees and 30A degrees.