Teres minor circle and subscapularis lengthening patterns during forward elevation in lateralized reverse total...

Area of Science:

• Orthopedic surgery
• Biomechanics
• Musculoskeletal research

Background:

• The role of subscapularis (SSC) repair after lateralized reverse total shoulder arthroplasty (rTSA) is debated.
• The teres minor (TM) is crucial for external rotation in rTSA, especially with infraspinatus (ISP) insufficiency.
• TM length-tension behavior during shoulder elevation in rTSA is not well understood.

Purpose of the Study:

• To characterize the length-tension behavior of the TM and other rotator cuff muscles after simulated lateralized rTSA.
• To analyze the impact of different glenoid baseplate positions and neck-shaft angles (NSA) on muscle function.

Main Methods:

• Generated 3D shoulder models from 15 patient CT scans.
• Simulated four lateralized rTSA configurations with varying glenoid baseplate positions and NSA (135°, 145°, 155°).
• Computed muscle-tendon length changes for TM, SSC, ISP, and deltoid during simulated forward elevation (0°–120°).

Main Results:

• The TM demonstrated a consistent biphasic length-tension pattern (shortening then lengthening) across all tested rTSA configurations.
• The TM exhibited a significantly earlier transition point (circle angle) from shortening to lengthening compared to ISP, SSC, and deltoid.
• Subscapularis (SSC) consistently lengthened, while ISP, TM, and deltoid showed biphasic patterns with varying prevalence.

Conclusions:

• The TM's biphasic length-tension pattern provides a biomechanical basis for understanding its role in external rotation after rTSA.
• Findings suggest potential vulnerability of SSC repair due to consistent lengthening during elevation.
• The study highlights the TM's significant contribution to shoulder function in lateralized rTSA, particularly in elevated positions.