Feasibility and accuracy of low-angle, non-orthogonal biplane fluoroscopic 2D-3D registration for dynamic analysis of the thumb carpometacarpal joint.

Background Accurate three-dimensional dynamic assessment of the thumb carpometacarpal (CMC) joint is important for understanding the pathomechanics of CMC osteoarthritis and evaluating treatment outcomes. Intensity-based biplane 2D-3D registration is a promising approach for dynamic bone motion analysis; however, conventional orthogonal biplane configurations are often difficult to implement for small-joint imaging in clinical settings. Methods We validated the accuracy of intensity-based biplane 2D-3D registration for thumb CMC joint kinematic analysis using a phantom model under a low-angle, non-orthogonal biplane fluoroscopic configuration (53°). Registration-derived bone poses were compared with radiostereometric analysis (RSA)-derived ground truth across 252 biplane fluoroscopic image pairs during palmar and radial abduction tasks. Translational and rotational errors were calculated for the thumb proximal phalanx, first metacarpal, trapezium, and second metacarpal. Results The method achieved submillimeter mean overall component-wise translational errors for all evaluated bones (range: 0.11-0.31 mm). Mean overall component-wise rotational errors were below 3° for all bones (range: 0.44-2.64°). Larger axis-specific rotational root mean square errors were observed for the trapezium and thumb proximal phalanx, reflecting bone- and axis-dependent error patterns. Conclusion Intensity-based biplane 2D-3D registration can provide sufficient accuracy for dynamic analysis of the thumb CMC joint using a low-angle, non-orthogonal biplane configuration. This approach may serve as a clinically feasible platform for assessing small-joint kinematics in settings where conventional orthogonal biplane systems are difficult to implement, supporting future in vivo studies of dynamic thumb CMC joint motion.