Timing of Changes in Three-Dimensional Spinal Parameters After Selective Thoracic Fusion in Lenke 1 Adolescent Idiopathic Scoliosis: Two-Year Follow-up

Abstract

Study design: Retrospective analysis of the prospectively collected data.

Objective: To investigate the relationship between the axial rotation of the unfused lumbar spine and the parameters of the instrumented thoracic spine at varying time points after selective thoracic fusion (STF) in Lenke 1B and 1C adolescent idiopathic scoliosis (AIS).

Summary of background data: The impact of STF on the spontaneous lumbar curve correction in AIS has been studied mainly in the frontal planes. The relationship between the spontaneous transverse plane correction of the lumbar spine and the parameters of the fused thoracic spine is not well documented.

Methods: Twenty-one Lenke 1B and 1C patients who had received STF with minimum two years' follow-up were selected. Thoracic and lumbar Cobb angles, kyphosis, lordosis, and thoracic and lumbar apical vertebrae rotations were measured at preoperative, first-erect, six-month, one-year, and two-year follow-ups. The association between the lumbar apical vertebral rotation and other thoracic and lumbar variables at different time points were determined using regression analysis. The variables significantly predicting the lumbar axial rotation correction at two years were determined from the preceding follow-up visits.

Results: Kyphosis, thoracic Cobb, thoracic apical vertebral rotation, and lumbar Cobb were significantly different between the preoperative and all the postoperative follow-ups (p < .05). At the two-year follow-up, a decrease in thoracic rotation and lumbar Cobb and a higher residual thoracic Cobb were associated with an improved spontaneous lumbar rotation (R² = 0.41, p < .05). Lumbar rotation at two years was predicted from thoracic derotation and lumbar Cobb at first erect (R² = 0.30, p < .05).

Conclusion: Spontaneous lumbar curve rotation correction correlated to the fused and unfused spinal parameters in the three anatomic planes. The relationship between thoracic and lumbar rotation persist up to two years after STF. Thoracic derotation is an important factor determining the lumbar rotation correction at two years after STF.

Keywords: 3D spinal parameters; Adolescent idiopathic scoliosis; Axial plane parameters; Predictive model; Selective thoracic fusion.