Postoperative Segmental Motion of the Unfused Spine Distal to the Fusion in 100 Patients With Adolescent Idiopathic Scoliosis

Study Design. A cross-sectional study. Objective. The purpose of this study was to assess intervertebral segmental and cumulative motion in the distal unfused segments of the spine in patients with adolescent idiopathic scoliosis after instrumentation as a function of the lowest instrumented level. Summary of Background Data. The implications of hyper- or hypomobility in the unfused segments of the spine after instrumentation are poorly understood. There is little research on changes in functional movement capabilities of the spine after thoracolumbar spinal fusion. Methods. Patients were prospectively offered inclusion into this institutional review board-approved cross-sectional study at their routine 2-, 3-, 4-, or 5-year postoperative visits at 1 of the 5 participating centers. Motion was assessed by standardized radiographs acquired in maximum right, left and forwarding bending positions. The intervertebral angles were measured via digital radiographic measuring software at each level from T12 to S1. The relationship of the vertebral segmental motion for each interspace to the lowest instrumented vertebrae was evaluated with an analysis of variance. The relationship between the cumulative preserved motion and each domain of the Scoliosis Research Society questionnaire were evaluated using a Pearson correlation coefficient. Results. The data for 100 patients are included. The lowest instrumented vertebrae ranged from T10 to L4. In lateral bending, an association was detected between the lowest fused vertebral level and the degree of motion at the distal unfused segments. With a more distal instrumented vertebrae, there was significantly greater L2-L3, L3-L4, and L4-L5 segment motion (P = 0.002, 0.009, and 0.001, respectively). A similar trend was noticed at L5-S1 level. In addition, the summed motion from L3 to S1 also increased with a more distal fusion (P = 0.001). Similar results were not found in forward bending. None of the domains of the Scoliosis Research Society questionnaire correlated with the preserved L3-S1 motion. Conclusion. In a group of postoperative patients with adolescent idiopathic scoliosis, evaluation of the distal unfused intervertebral motion showed that preservation of vertebral motion segments allowed greater distribution of functional motion across more levels. With each distal fusion level, motion was significantly increased at the L2-L3, L3-L4, and L4-L5 segmental levels in lateral bending. The relationship between the increased motion and subsequent disc degeneration with a more distal fusion is unknown, but suspected.