Can We Predict Postoperative Disc Wedging Below Anterior Instrumented Thoracolumbar/Lumbar Adolescent Idiopathic Scoliosis Fusions?

Introduction: In anterior spinal fusion (ASF) for thoracolumbar/ lumbar (TL/L) adolescent idiopathic scoliosis (AIS), the goals are to obtain and maintain optimal lowest instrumented vertebra (LIV) position and to avoid disc wedging immediately below the LIV postoperatively.

Purpose: To investigate which preoperative x-ray parameters correlate best to ultimate LIV position and the disc angle immediately below the LIV following ASF for TL/L AIS.

Methods: 61 patients with TL/L AIS (Lenke type 5C or 6C) were treated with an instrumented ASF with a min. 2-year follow-up (range 2-6 years). The LIV was L2 (n=5), L3 (n=46), L4 (n=10). An average of 5.15 levels were fused (range 3 to 7). LIV equaled the lower end vertebra (LEV) (LIV=LEV) in 47 patients, one above LEV (LEV-1) in 10 patients, and one below LEV (LEV+1) in 4 patients. Three postoperative parameters were chosen to best represent coronal balance: disc angle immediately below the LIV;LIV-center sacral vertical line (CSVL) distance (regional balance), and C7-CSVL distance (global balance). Then the correlation of each radiographic parameter to these three was evaluated statistically.

Results: The mean preoperative TL/L Cobb was 49.48º +/- 10.20 and postoperative 17.30º +/- 8.89 (65% correction). The change of the absolute value of the disc angle immediately below the LIV over 2-year follow-up was significantly different between LIV=LEV and LIV=LEV-1 (p<0 .006). Stepwise multiple linear regression selected four correlative parameters to the postoperative disc angle: preoperative upright angle, supine apex-LIV distance, and T12-LIV lordosis (p<0.0001, r2="0.3)." Additionally, apex-CSVL distance (p="0.002," LIV number of fused vertebrae were found correlate it significantly as well. The LIV-CSVL rotation parameter C7-CSVL was DISCUSSION: Postoperative global regional balance in coronal plane difficult predict preoperatively. However, approximately 50% angle could be explained by parameters. Especially when a parallel above LEV is excluded from fusion, wedging will occur very high incidence. natural history observed unknown, significance this occurrence uncertain. these radiographic that should noted.

Conclusions: Postoperative disc wedging occurs most often when the preoperative disc angle in the upright and in supine position is small, preoperative T12-LIV lordosis is small, and the LIV is one level short of, or equals the LEV. The advantage of a shorter fusion is preservation of more mobile segments, however the long-term morbidity of disc wedging in the "saved" disc is unknown.