Feasibility of reconstructing the glenohumeral center of rotation with a single camera setup

Background: An accurate estimation of the glenohumeral joint center of rotation (CoR) is important during alignment of braces and exoskeletons, as a misalignment will introduce undesired forces on the human body. The aim of this research was to develop a new method to estimate the glenohumeral CoR and register the location to the body using a single camera and two printed markers.
Methods: During shoulder anteflexion, the arm roughly describes an arc in the sagittal plane, with the glenohumeral joint in the center. Two binary square-fiducial ArUco markers were secured to the upper arm and the scapula, their position and orientation were obtained, and a sphere was fitted to the coordinates of the arm marker. The sphere center position was then registered on the skin. The accuracy was assessed with a test bench with a known rotational center. The repeatability was assessed in vivo with five healthy participants.
Results: The mean absolute offset between the true CoR of the test bench and the fitted sphere centers across multiple trials was 2.7 mm at a velocity of 30 degrees/s, and 2.5 mm at 60 degrees/s. The root mean squared distance from the estimated sphere centers after each trial to the mean sphere center across all trials per participant was 5.1 mm on average for the novice examiner and 5.2 mm for the expert examiner.
Conclusions: The proposed method is able to accurately and precisely estimate the glenohumeral CoR.