Exploratory Concurrent Associations between K-Kyphometer and Flexicurve-Derived Kyphosis Measures (Kyphosis Index and Kyphosis Angle): preliminary study

Background: Quantifying thoracic kyphosis is essential in musculoskeletal and rehabilitation practice. Although radiographic Cobb angle is commonly treated as a reference standard, repeated imaging is limited by cost and radiation exposure. Therefore, clinicians frequently use non-radiographic tools such as inclinometers and flexicurve rulers, which have shown acceptable reliability and variable levels of validity in different populations. The objective of this study was examine Pearson correlations between a patented K-Kyphometer (Utility Patent No. 23366) and two flexicurve-derived measures—Kyphosis Index (KI) and Kyphosis Angle (K angle). Methods: In this cross-sectional correlation study (N=28), thoracic kyphosis was assessed using the K-Kyphometer and the flexicurve ruler. From the flexicurve tracing, KI and Kangle were computed using standard geometric derivations from the same tracing. The Pearson correlations (two-tailed) between measures obtained from the K-Kyphometer with KI and KAngle were analyzed using statistical significance value (alpha) of 0.05. Results: K-Kyphometer showed negligible correlations with KI (r=0.045, p=0.818) and Kangle (r=0.043, p=0.826). In contrast, KI and Kangle demonstrated a perfect/near-perfect linear relationship (r=1.000, p< 0.001), consistent with both indices being derived from the same flexicurve geometry. Conclusion: K-Kyphometer measurements were not linearly associated with flexicurve-derived kyphosis metrics. KI and Kangle were essentially mathematically linked as flexicurve-based descriptors of thoracic curvature. However, clinical interchangeability with reference standards (e.g., radiographic Cobb or inclinometer methods) should be established using agreement analyses and external validity testing. Keyword: Thoracic kyphosis, Kyphosis Index, Kyphosis Angle, K-Kyphometer