Abstract: BACKGROUND: Due to the complex movement of the scapula, which is a six-degree-of-freedom activity in three-dimensional space, it is difficult to measure it accurately using traditional methods. The image and model matching technology based on dual-plane X-ray is a three-dimensional measurement method that has gradually developed and matured in recent years. Two high-speed cameras are used to project and shoot from orthogonal directions. Compared with a single perspective, this method has advantages in observation range and reduction of out-of-plane errors, and is suitable for the study of scapula kinematics.
OBJECTIVE: X-ray biplane and image-model registration technology were used to explore the differences in scapular kinematics between normal individuals and patients with rotator cuff tears, providing a basis for the treatment and rehabilitation of rotator cuff tear patients. 
METHODS: From April 2023 to January 2024, 10 patients with normal shoulders and 10 patients with rotator cuff tears who met the inclusion criteria were enrolled from Guanghua Hospital Affiliated to Shanghai University of Traditional Chinese Medicine. The subjects underwent a shoulder CT scan to create a 3D model and a local scapular coordinate system. X-ray biplane images were taken during shoulder abduction with two C-arm machines. The 2D and 3D images were registered to compare scapular kinematic differences between the two groups at 0°, 15°, 30°, 45°, 60°, 75°, and 90° of abduction, including scapular rotation angle and displacement distance. 
RESULTS AND CONCLUSION: (1) During shoulder abduction, the scapula of both groups showed upward rotation, but the upward rotation of the rotator cuff tear group was greater than that of the control group, and the difference was significant when the abduction was 30°-90° (P < 0.01). At the same time, the scapula internal rotation of both groups gradually increased, but the rotator cuff tear group was significantly greater than the control group when the abduction was 45°-90° (P < 0.01). In addition, the anteroposterior tilt of the scapula of the two groups was significantly different when the abduction was 15°-90° (P < 0.01). The scapula posterior tilt of the control group gradually increased during abduction, while the scapula of the rotator cuff tear group tilted forward except for a slight posterior tilt at 15°-30° abduction. (2) In terms of displacement, the upward displacement of the rotator cuff tear group was less than that of the control group during abduction, and the difference was significant at 15°-90° (P < 0.05), but there was no significant difference in lateral and anterior-posterior displacement between the two groups (P > 0.05). (3) Rotator cuff tear can cause scapular dyskinesis, characterized by increased upward rotation, internal rotation, and abnormal forward tilt during shoulder abduction. Identifying and addressing scapular dyskinesis is crucial for treating rotator cuff tear.

Key words: rotator cuff tear, scapular dyskinesis, scapular kinematics, shoulder abduction, biplane X-ray, image-model registration