Interest of T1 parameters for sagittal alignment evaluation of adolescent idiopathic scoliosis patients

In the last few years several reports stressed the importance of sagittal alignment in adolescent idiopathic scoliosis (AIS) patients. It was recently reported that T1 slope, defined as the angle between the superior endplate of T1 and the horizontal, correlates strongly with overall sagittal parameters. The aim of this study was to assess the impact of T1 parameters (T1-slope and T1-tilt) on sagittal alignment of AIS hypokyphotic patients preoperatively and postoperatively.

Twenty-nine AIS patients with <20° preoperative hypokyphosis were included in a retrospective study. Surgery systematically comprised hybrid construct with screws below T11, sublaminar bands at thoracic level and a lamino-laminar claw on the upper instrumented vertebra. Preoperative, postoperative and 2-year follow-up radiological assessment included Cobb angle, T1 slope, T1 sagittal tilt, regional sagittal parameters and pelvic parameters.

In the series as a whole, coronal Cobb angle was significantly reduced postoperatively (58° vs. 17°; p < 0.001), thoracic kyphosis significantly improved (12.4° vs. 25.6°; p < 0.001) and cervical lordosis significantly restored (6.2° kyphosis vs. 4.1° lordosis; p < 0.001). There was a significant modification in T1-slope (10.2° vs. 18.2°; p < 0.001). Preoperatively, T1 slope was significantly correlated with T1 tilt (r = 0.427; p = 0.029). Postoperatively, T1 slope was significantly correlated with T1 tilt (r = 0.549; p = 0.002), thoracic kyphosis (r = 0.535, p = 0.005) and cervical lordosis (r = -0.444, p = 0.03). Restoration of cervical lordosis was significantly correlated to changes of T1-slope (r = -0.393, p = 0.032), which was significantly correlated to postoperative thoracic kyphosis.

According to these results, T1 seems to be of major interest in postoperative modifications of sagittal alignment. T1 slope and sagittal tilt are good indicators of postoperative changes for regional (cervical lordosis and thoracic kyphosis) and global parameters. We therefore consider these parameters as essential in the assessment of AIS patients. Further studies and correlation with clinical scores will, however, be necessary in order to confirm the present findings.