Exploratory analysis of shoulder motion strategies for achieving functional hand behind the back in controls and...

After reverse shoulder arthroplasty (RSA), most patients recover a satisfactory anterior forward elevation and external rotation elbow at side. However, the recovery of functional hand behind the back (fHBB), essential for daily life activities, remains unpredictable. This study aims to describe the complex mechanisms enabling fHBB and to explore their association with patient anatomy.

A retrospective comparative study was conducted on patients who underwent RSA and a control group, all of whom had upper limb motion capture analysis. Baseline demographic and anatomical characteristics were collected for each subject. In the control group, humeral torsion, glenoid version, and scapular type were assessed through computer tomography (CT) imaging. For both the RSA and control groups, maximal glenohumeral range of motion (ROM)-flexion/extension, adduction/abduction, and internal/external rotations-were quantified using an optical motion capture system and 3D reconstructions. Movements used to reach the highest spinal level possible were then classified into distinct strategies using clustering. In controls, these movement strategies were correlated with anatomical parameters (humeral torsion, glenoid version, and scapular type). RSA patients were then descriptively projected onto the control clusters to explore similarities. Within the RSA cohort, ROM were compared between patients who succeeded or failed to achieve fHBB (hand above L5).

Among the controls who reached HBB type 3 (above T12) (n = 45 shoulders), an extension-dominant cluster (n = 25, extension 24° ± 9°, abduction 2° ± 5°, IR 27° ± 8°) and an internal rotation dominant group (n = 20, extension 17° ± 7°, abduction 5° ± 11°, IR 53° ± 10°) were identified. No statistically significant differences in humeral torsion (p = 0. 534), glenoid version (p = 0. 229), or scapular orientation (p = 0. 825) were found between the two clusters. In the RSA group, six out of ten reached fHBB. The successful and unsuccessful groups did not differ in terms of extension (17° ± 11° vs. 20° ± 12°, p = 0.761) or abduction (15° ± 12° vs. 8° ± 2°, p = 0.257). Taken together, these exploratory findings point to IR and extension as candidate contributors to fHBB that merit confirmation in larger cohorts.

This exploratory study identified two strategies for achieving HBB above T12: extension-dominant and internal rotation-dominant, emphasizing the complex, multiplanar kinematics required to reach the back. Patients with RSA who regained fHBB (above L5) showed a tendency for increased IR but did not show significant differences from the unsuccessful patient suggesting that other factors are also involved.