Impact of lowest instrumented vertebra tilt and rotation on uninstrumented lumbar curve and L4 tilt in thoracic adolescent idiopathic scoliosis

Akira Iwata MD, PhD 1 , Hideki Sudo MD, PhD 1 , Kuniyoshi Abumi MD, PhD 2 , Manabu Ito MD, PhD 3 , Katsuhisa Yamada MD, PhD 1 and Norimasa Iwasaki MD, PhD 1
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  • 1 Department of Orthopaedic Surgery, Hokkaido University Hospital;
  • 2 Spine Center, Sapporo Orthopaedic Hospital; and
  • 3 Department of Spine and Spinal Cord Disorders, Hokkaido Medical Center, Sapporo, Hokkaido, Japan
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OBJECTIVE

Controversy exists regarding the effects of lowest instrumented vertebra (LIV) tilt and rotation on uninstrumented lumbar segments in adolescent idiopathic scoliosis (AIS) surgery. Because the intraoperative LIV tilt from the inferior endplate of the LIV to the superior sacral endplate is not stable after surgery, the authors measured the LIV angle of the instrumented thoracic spine as the LIV angle of the construct. This study aimed to evaluate the effects of the LIV angle of the construct and the effects of LIV rotation on the postoperative uninstrumented lumbar curve and L4 tilt in patients with thoracic AIS.

METHODS

A retrospective correlation and multivariate analysis of a prospectively collected, consecutive, nonrandomized series of patients at a single institution was undertaken. Eighty consecutive patients with Lenke type 1 or type 2 AIS treated with posterior correction and fusion were included. Preoperative and 2-year postoperative radiographic measurements were the outcome measures for this study. Outcome variables were postoperative uninstrumented lumbar segments (LIV tilt, LIV translation, uninstrumented lumbar curve, thoracolumbar/lumbar [TL/L] apical vertebral translation [AVT], and L4 tilt). The LIV angle of the construct was measured from the orthogonal line drawn from the upper instrumented vertebra to the LIV. Multiple stepwise linear regression analysis was conducted between outcome variables and patient demographics/radiographic measurements. There were no study-specific biases related to conflicts of interest.

RESULTS

Predictor variables for postoperative uninstrumented lumbar curve were the postoperative LIV angle of the construct, number of uninstrumented lumbar segments, and flexibility of TL/L curve. Specifically, a lower postoperative uninstrumented lumbar curve was predicted by a lower absolute value of the postoperative LIV angle of the construct (p < 0.0001). Predictor variables for postoperative L4 tilt were postoperative LIV rotation, preoperative L4 tilt, and preoperative uninstrumented lumbar curve. Specifically, a lower postoperative L4 tilt was predicted by a lower absolute value of postoperative LIV rotation (p < 0.0001).

CONCLUSIONS

The LIV angle of the construct significantly affected the LIV tilt, uninstrumented lumbar curve, and TL/L AVT. LIV rotation significantly affected the LIV translation and L4 tilt.

ABBREVIATIONS AIS = adolescent idiopathic scoliosis; AVT = apical vertebral translation; CSVL = central sacral vertical line; LIV = lowest instrumented vertebra; SRS-22 = Scoliosis Research Society-22 questionnaire; TL/L = thoracolumbar/lumbar; UEV = upper end vertebra.

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Contributor Notes

Correspondence Hideki Sudo: Hokkaido University, Hokkaido, Japan. hidekisudo@yahoo.co.jp.

INCLUDE WHEN CITING Published online June 12, 2020; DOI: 10.3171/2020.4.SPINE191297.

Disclosures The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

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