Influence of humeral liner constraint in reverse total shoulder arthroplasty on simulated range of motion in patients with various posture types – a modeling study

Background: The degree of constraint of the humeral liner is a key determinant of the stability of reverse total shoulder arthroplasty (rTSA). However, it remains largely unknown to what extent the degree of constraint affects range of motion (ROM). Purpose: To analyze the influence of various liner types on simulated ROM following virtual rTSA implantation in patients with different posture types. Methods: The CT scans of 48 consecutive patients scheduled for rTSA implantation were utilized to create patient-specific 3D bone models. For all cases, six shoulder surgeons independently virtually implanted a rTSA (lateralized 135° semi-inlay design) according to their respective preferences using an experimental surgical planning software. ROM was determined simulating posture type A (upright posture), B (intermediate), and C (kyphotic posture) utilizing a standard, constrained, and high mobility liner, resulting in a total of 2592 different rTSA configurations. Subgroup analyses were performed to determine the influence of the degree of constraint on simulated ROM across the three posture types. Results: Total ROM decreased with increasing liner constraint from a median of 611° [interquartile range 522° to 666°] with high mobility liners to 512° [IQR 441-570°] with standard liners to 411° [IQR 322-470°] with constrained liners (p < 0.01). For all posture types, the use of a high mobility liner led to a significant increase in ROM in all planes except abduction, except for flexion in posture type A. Constrained liners significantly decreased ROM in all planes except abduction in posture type A. For posture type A, the greatest increase in ROM due to more mobile liners was observed in extension (+17°; p <0.01) and internal rotation (+17°; p <0.01), while the greatest decrease (due to more constrained liners) was also seen in extension (-34°; p <0.01). In posture type B, the greatest increase in ROM occurred in adduction (+19°; p <0.01), whereas the greatest decrease was in high internal rotation (-28°; p <0.01). Similarly, for posture type C, the greatest increase in ROM was observed in adduction (+30°; p <0.01), while the greatest decrease was in high internal rotation (-21°; p <0.01). Conclusion: While in posture types B and C, high mobility liners improved particularly adduction and constrained liners decreased high internal rotation, in posture type A extension and internal rotation could be improved most prominently while extension was most prominently decreased by a constrained liner.