The A-Palp: A Digitized Manual Palpation Method for Sagittal Spine Assessment—A Study of Reliability Over Time and Between Operators

Background/Objectives: The A-Palp enables a calibrated anatomical systems technique (CAST) approach. Previous studies have demonstrated repeatability and concurrent validity for selected spinal curvature angles in patients with scoliosis. However, the inter-operator reproducibility, temporal repeatability, and reliability of sagittal spinal curvature measurements and spinopelvic parameters remain to be established. Methods: Eighteen healthy adults without spinal pathology were assessed. Two operators sampled sagittal spinal profiles with the A-Palp in a 14-camera optoelectronic setup, applying reflective markers and palpating spinous processes. One operator repeated measurements after seven days. Marker data were processed in MATLAB (R2019b) to smooth trajectories, fit curvature arcs, and compute extracorporeal kyphosis, lordosis, and pelvic parameters. Reliability and repeatability were evaluated using Bland & Altman analysis, intraclass correlations (ICCs), standard error of measurement (SEM), mean detectable change (MDC95), root-mean-squared errors (RMSEs), and Statistical Parametric Mapping (SPM). Results: Reliability and repeatability were strong. For global spinal angles, ICCs exceeded 0.90 across operators and sessions. The tangent method yielded low SEM (1–2°) and MDC95 (3–6°) values, whereas the circle-fit/trigonometric methods showed larger errors. Most spinopelvic angles had moderate-to-excellent ICCs (0.65–0.98) with SEM/MDC95 values ≈2.1–4.5°/5.9–12.4°. Ground reaction force-referenced distances showed good ICCs and small intra-operator error (SEM: 3.8–4.8 mm; MDC95: 10.7–13.4 mm) but wider inter-session thresholds (SEM: 10.3–11.6 mm; MDC95: 28.6–32.8 mm). Bland & Altman biases were ~0, with narrower limits for the tangent (≈±5°) than circle-fit/trigonometric (≈±8–12°) methods. Curve tracking was consistent (RMSE: 2.7–3.7 mm, <5% amplitude), and SPM detected no point-wise differences. Conclusion: The A-Palp method demonstrated high reliability and repeatability for extracorporeal sagittal spinal and sacro-spinal evaluation. Variability was low across operators and sessions, supporting its use as a robust, non-invasive clinical and research tool.