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Subaxial sagittal alignment following atlantoaxial (A-A) posterior fusion was investigated retrospectively in patients with A-A subluxation.To evaluate the association between A-A fusion angle and postoperative subaxial sagittal alignment and to determine the optimal fusion angle for preservation of physiologic subaxial alignment.A-A posterior fusion has been used for patients with A-A instability and provided satisfactory clinical results. However, there are patients showing unexpected development of subaxial kyphosis after surgery. The reasons for subaxial kyphosis after A-A fusion remain unclear.Seventy-six patients with A-A subluxation who underwent several types of posterior A-A fusion were involved. There were 46 women and 30 men. The causes of A-A subluxation were rheumatoid arthritis in 47, trauma in 16, os odontoideum in 8, and unknown in 5. The methods of posterior fusion consisted of Magerl procedure with posterior wiring in 51, Brooks wiring in 18, and Halifax clamp in 7. Angles at C1–C2, C2–C7, and C1–C7 in the neural position were measured before surgery and at the final follow-up to find out any association between postoperative C2–C7 angle and the other radiologic parameters. The association between O-C1 range of motion and C2–C7 angle was also investigated.The mean angles of C1–C2, C2–C7, and C1–C7 before surgery were 18.4°, 14.5°, and 32.9°, respectively. Those at the final follow-up were 26.0°, 5.5°, and 31.5°, respectively. These results indicated that C1–C2 fixation in a hyperlordotic position led to a subaxial kyphosis after surgery. Statistics showed that there was a linear association between the C1–C2 lordotic fixation angle and the C2–C7 kyphotic angle.Surgical fixation of A-A joint in a hyperlordotic position will lead the lower cervical spine to a kyphotic sagittal alignment after surgery. To maintain the physiologic sagittal alignment of the subaxial cervical spine, C1–C2 should not be fixed in a hyperlordotic position.