Reproducibility of thoracic kyphosis measurements in patients with adolescent idiopathic scoliosis

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RESEARCH

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Reproducibility of thoracic kyphosis measurements in patients with adolescent idiopathic scoliosis Søren Ohrt-Nissen1* , Jason Pui Yin Cheung2, Dennis Winge Hallager1, Martin Gehrchen1, Kenny Kwan2, Benny Dahl1, Kenneth M. C. Cheung2 and Dino Samartzis2*

Abstract Background: Current surgical treatment for adolescent idiopathic scoliosis (AIS) involves correction in both the coronal and sagittal plane, and thorough assessment of these parameters is essential for evaluation of surgical results. However, various definitions of thoracic kyphosis (TK) have been proposed, and the intra- and inter-rater reproducibility of these measures has not been determined. As such, the purpose of the current study was to determine the intra- and inter-rater reproducibility of several TK measurements used in the assessment of AIS. Methods: Twenty patients (90% females) surgically treated for AIS with alternate-level pedicle screw fixation were included in the study. Three raters independently evaluated pre- and postoperative standing lateral plain radiographs. For each radiograph, several definitions of TK were measured as well as L1–S1 and nonfixed lumbar lordosis. All variables were measured twice 14 days apart, and a mixed effects model was used to determine the repeatability coefficient (RC), which is a measure of the agreement between repeated measurements. Also, the intra- and inter-rater intra-class correlation coefficient (ICC) was determined as a measure of reliability. Results: Preoperative median Cobb angle was 58° (range 41°–86°), and median surgical curve correction was 68% (range 49–87%). Overall intra-rater RC was highest for T2–T12 and nonfixed TK (11°) and lowest for T4–T12 and T5–T12 (8°). Inter-rater RC was highest for T1–T12, T1-nonfixed, and nonfixed TK (13°) and lowest for T5–T12 (9°). Agreement varied substantially between pre- and postoperative radiographs. Inter-rater ICC was highest for T4–T12 (0.92; 95% CI 0. 88–0.95) and T5–T12 (0.92; 95% CI 0.88–0.95) and lowest for T1-nonfixed (0.80; 95% CI 0.72–0.88). Conclusions: Considerable variation for all TK measurements was noted. Intra- and inter-rater reproducibility was best for T4–T12 and T5–T12. Future studies should consider adopting a relevant minimum difference as a limit for true change in TK. Keywords: Adolescent idiopathic scoliosis, Thoracic, Kyphosis, Radiograph, Sagittal, Flexibility, Reproducibility, Reliability, Agreement, Intra-class correlation, Mixed effects model, Repeatability coefficient, Limits of agreement

Background Adolescent idiopathic scoliosis (AIS) is characterized by a lateral deviation of the spine in the coronal plane, vertebral rotation in the transverse plane, and often hypokyphosis in the sagittal plane [1, 2]. Current surgical treatment for AIS involves multisegmental pedicle screw * Correspondence: [email protected]; [email protected] 1 Spine Unit, Department of Orthopedic Surgery, Rigshospitalet, University of Copenhagen, Blegdamsvej 9, Copenhagen East 2100, Denmark 2 Department of Orthopedics and