Biomechanical analysis of buckling alignment of the cervical spine - Predictive value for subaxial subluxation after occipitocervical fusion

Study Design. A biomechanical analysis of the buckling type of alignment on nonfused cervical segments was carried out in patients with occipitocervical fusion for atlantoaxial dislocation. Objectives. To examine whether biomechanical analysis is useful for preoperative prediction of subaxial subluxation after occipitocervical fusion. Summary of Background Data. Rheumatoid arthritis sometimes causes subaxial subluxation after occipitocervical fusion. At present, there are no widely accepted criteria for determining the appropriate extent of fusion to prevent subluxation. Methods. The subjects were 25 patients with rheumatoid atlantoaxial dislocation and 15 patients with nonrheumatoid atlantoaxial dislocation who underwent occipitocervical fusion. Preoperative and postoperative alignment of the cervical spine were analyzed biomechanically, using a specially developed computer program. Results. Five segments of nonfused cervical spine had subluxation after surgery in the rheumatoid group. For these segments, the preoperative value of buckling averaged 13.5 x 10(-4) and exceeded 10 x 10(-4) in all cases. For the segments that showed no subluxation after surgery, the preoperative value of buckling averaged 4.5 x 10(-4). Subluxation of the nonfused segments did not develop in the nonrheumatoid group. Conclusions. In patients with rheumatoid arthritis, segments that show abnormal buckling before surgery are likely to develop subluxation after occipitocervical fusion. Preoperative values of buckling over 10 x 10(-4) constitute a risk factor for subaxial subluxation after occipitocervical fusion.