Does humeral head size predict the lateralization required to preserve near-anatomic posterosuperior rotator cuff length in reverse shoulder arthroplasty?

Background: Reverse total shoulder arthroplasty (rTSA) lacks an anatomy-based framework for individualized implant planning. We hypothesized that native humeral head size (HHS) could serve as a patient-specific anatomical reference to guide lateralization in rTSA. The purpose of this study was to determine whether HHS predicts the amount of baseplate lateralization required to preserve near-anatomic posterosuperior rotator cuff muscle length, and whether muscle lengths can be reliably estimated in deformed joints using a statistical shape model (SSM). Methods: In this computational study, 83 pre-operative computed tomography scans were analyzed using a validated SSM to estimate rotator cuff muscle-tendon lengths. Five rTSA implantation strategies were modeled while controlling implant geometry, ranging from a medialized Grammont-type configuration to lateralized Frankle-type and hybrid strategies. Best-fit humeral head size was correlated with patient-specific lateralization quantified by overlap-based graft thickness and with posterosuperior cuff muscle length. Results: Best-fit humeral head size demonstrated an excellent correlation with graft thickness required for near-anatomic overlap (r = 0.93) and strong correlations with posterosuperior cuff muscle length, particularly the infraspinatus (r = 0.88), as well as the subscapularis (r = 0.76) and teres minor (r = 0.62). Among implantation strategies, the Frankle-type configuration most closely reproduced near-anatomic graft thickness and posterosuperior cuff muscle length. Implantation strategy produced large to very large differences in baseline muscle-tendon length across configurations (Kendall W ≈ 0.9-0.97). The SSM reliably reconstructed muscle-tendon geometry across a broad spectrum of degenerative pathology. Conclusion: HHS predicts the lateralization required to preserve near-anatomic posterosuperior cuff muscle length in rTSA. A Frankle-type lateralization strategy most closely reproduced the overlap-based anatomic reference, while alternative strategies produced systematic deviations in muscle length. These findings support HHS as an anatomy-based, patient-specific guide for lateralization planning. This framework is geometric and conceptual in nature and requires future clinical validation.