Reliability Analysis for Manual Radiographic Measures of Rotatory Subluxation or Lateral Listhesis in Adult Scoliosis

Study Design. Retrospective observational study. Objective. To define the inter-and intraobserver reliability of 3 measures of rotatory subluxation (RS) in adult scoliosis (AS). Summary of Background Data. RS is a hallmark of AS. To accurately track this measure, one must know its reliability. Reliability testing has not been performed. Methods. PA 36 '' films of 29 AS patients were collected from one surgeon's practice. Three observers on 2 separate occasions measured all levels with >= 3-mm RS (60 levels, 360 measurements) on the convexity of the involved segment using 3 different techniques-midbody (MB), endplate (EP), and centroid (C). These data were then analyzed to determine the intraclass correlation coefficient (ICC) for inter-and intraobserver reliability. Results. The thoracolumbar/lumbar curve (average 58 degrees) was the major curve for the majority (62%) of patients. RS at L3/4 was most common (35%). The overall inter-and intraobserver reliability was good-excellent for all methods, but the centroid method consistently had the highest ICC. ICC correlated with observer experience. Moderate-severe arthritic change (present in 55%) and poor image quality (52%) decreased ICC, but it still remained good-excellent for each measure. The reproducibility coefficient for each measure was 4 mm for MB and 2.8 mm for C and EP. Conclusion. MB, EP, and C are reliable techniques to measure RS even in elderly arthritic spines, but the methods inherently produce different values for a given level. The centroid method is most reliable and least influenced by experience. The EP method is easy to perform and very reliable. Spine surgeons should pick their preferred method and apply it consistently. Changes >= 3-mm suggest RS progression. RS may be a useful measure in addition to Cobb angle in AS. Having defined measurement reliability, the role of RS progression in surgical indications and patient outcomes can be evaluated.