Analysis of the lowest instrumented vertebra following anterior spinal fusion of thoracolumbar/lumbar adolescent idiopathic scoliosis: can we predict postoperative disc wedging?

Study Design: A retrospective radiographic study.
Objectives: To investigate which radiographic parameters correlate best to ultimate lowest instrumented vertebra (LIV) position and subjacent disc wedging following anterior spinal fusion (ASF) for thoracolumbar/lumbar (TL/L) adolescent idiopathic scoliosis (AIS).
Summary of Background Data: In an ASF of TL/L AIS, part of the operative goals are often to horizontalize and centralize the LIV, or potentially minimize subjacent disc wedging after surgery. To our knowledge, no study has investigated the specific radiographic parameters involved with obtaining these goals.
Methods: Sixty-one patients with TL/L AIS were treated with an instrumented ASF with a minimum 2-year follow-up. Preoperative and postoperative radiographs were examined measuring various radiographic parameters of the curve itself along with the LIV and subjacent disc. Specific correlation of these parameters to the coronal disc angle immediately below the LIV (disc angle), LIV translation, and global coronal balance (C7-CSVL distance) at 2 years postoperative was analyzed, respectively.
Results: The preoperative disc angle was 4.49 degrees +/- 5.48 and postoperative -5.85 degrees +/- 4.37. The change of the disc angle was significantly correlated to the LIV level relative to the preoperative lower end vertebra (LEV) (P < 0.006). Regressive analysis demonstrated the correlative parameters to the postoperative disc angle to be: preoperative upright disc angle; preoperative apex-LIV distance; and preoperative T12-LIV lordosis (P < 0.0001, r2 = 0.51). The correlative parameters to postoperative LIV translation were preoperative LIV translation and preoperative LIV rotation (P = 0.002, r2 = 0.2). The correlative parameter to postoperative C7-CSVL distance was only preoperative C7-CSVL distance (P < 0.0001, r2 = 0.3).
Conclusions: Postoperative subjacent disc wedging occurs most often when the preoperative subjacent disc is nearly parallel and when a shorter fusion excluding the LEV is performed. Preoperative LIV rotation significantly correlates to postoperative LIV translation. Surgeons should note these preoperative predictive factors to optimize radiographic results of the operative treatment of TL/L AIS.