Dynamic digital radiography is a valid and reliable alternative to 3 Dimensional registration for scapulohumeral rhythm analysis

Background: Scapulohumeral rhythm (SHR) is a key metric for evaluating shoulder joint function. Current established methods of SHR analysis involve 3-dimensional (3D) to 2-dimensional (2D) registration, which requires significantly greater resources to generate dynamic positioning data of the humerus and scapula. Dynamic digital radiography (DDR) captures pulsed low-dose radiographs in the plane of the scapular elevation to create a cine loop for analysis. This study aimed to establish the most reliable measurement technique by identifying bony landmarks that minimize measurement error and validate manual SHR measurements on DDR against an established 3D to 2D registration technique. Methods: DDRs performed on 22 reverse shoulder arthroplasty shoulders at least 6 months post-operatively were obtained. Manual measurements, including glenohumeral and scapulothoracic angles, were performed on DDRs by 2 authors every 10° of motion from rest to 120° of shoulder elevation. The scapulothoracic angles were measured using either the lateral or medial border of the scapula or scapula spine. Manual measurements were used to calculate SHR across the total range of motion and between various intervals. Results: SHR values were also calculated from a 3D to 2D registration model. Interclass correlations were statistically significant for glenohumeral angles and scapulothoracic angles using the lateral border – 0.989 (P < .001) and 0.955 (P < .001), respectively. Intraclass correlations were statistically significant for the glenohumeral angles (0.999 [P < .001]) and all scapulothoracic angles using the lateral border, medial border, and scapular spine – 0.999 (P < .001), 0.963 (P < .001), and 0.989 (P < .001), respectively. Paired t-tests revealed no significant differences in SHR between manual and 3D to 2D registration measurements across all intervals of shoulder elevation. Discussion: SHR measurements on DDR images are a simple and valid technique that can be readily incorporated into clinical workflows, where advanced 3D modeling may be impractical due to resource constraints. DDR provides a novel imaging modality to analyze in vivo shoulder biomechanics with comparable accuracy and validity to that of resource-restricted and time-consuming 3D imaging modalities.