Reverse shoulder arthroplasty (RSA) shifts the glenohumeral rotation center medially and distally to lengthen the deltoid moment arm, which has become mainstream surgical treatment for patients with irreparable rotator cuff tear arthropathy. Nevertheless, long-term clinical follow-up records demonstrate a considerable revision rate caused by screw micromotion-induced wear, osteolysis and glenoid baseplate loosening, which originate from high interfacial shear stress generated by lever mechanics. Modified RSA implants with lateralized or distalized geometry can partially improve mechanical environments yet fail to eliminate screw shear damage throughout the full abduction range. A novel concentric variable-diameter prosthesis (CVDP) recovers the native anatomical rotation center and adopts humeral-side large-area press-fit fixation to avoid screw-related loosening complications. However, its biomechanical advantages have not been quantitatively verified under an integrated mechanical framework.

To establish a unified two-dimensional quasi-static rigid-body mechanical model encompassing the normal shoulder, RSA and CVDP, to quantitatively compare their moment arms, joint reaction forces, and implant-bone interface stresses, to elucidate the mechanical mechanism of screw micromotion wear in RSA, and to verify the biomechanical advantages of CVDP.

A coronal-plane coordinate system was built with the geometric sphere center as origin, and abduction angle θ ranged from 0-90°. All model parameters were derived from anatomical measurements: upper limb weight W=45 N, gravity arm Lg=280 mm, axial distance of deltoid insertion dd=100 mm, humeral shaft radius Rshaft=15 mm, and deltoid origin coordinate A= (55, 45) mm. Moment equilibrium equations were solved to calculate deltoid tension, joint reaction force and distributed shear load on RSA screws. A cantilever deflection model was adopted to quantify screw-bone micromotion, and service life was predicted via calibrated wear coefficient. Parametric sensitivity analysis was performed to assess how rotation center displacement alters screw shear load.

The COR position parameters (x_A, y_A) significantly affected screw shear force: medialization (increasing x_A) showed angle-dependent sensitivity—positive (unfavorable) at small and large abduction angles and negative (favorable) only at mid-ranges, whereas distalization (increasing y_A) showed negative sensitivity across all angles, indicating a universally favorable direction but with limited physiological adjustability. For the optimized RSA (x_A=70 mm, y_A=55 mm) , the single-screw shear force at 60° abduction decreased to 28.1 N, with screw-bone interface shear stress τ_RSA≈2.81 MPa, micromotion displacement δ≈11.6 μm, wear rate per cycle of 2.75×10^-5 μm/cycle (approximately 10.0 μm/year) , and the wear life calculation at the peak abduction angle (60°) is self-consistent with the calibration baseline (approximately 10 years) , indicating that the screw shear micromotion wear mechanism is quantitatively compatible with the 8-12 year radiolucent line progression window reported by Melis et al. CVDP, utilizing screwless press-fit fixation with an interface area Afix≈400 mm² (approximately 13 times the total screw area of RSA) , achieved a peak interface shear stress of only 0.244 MPa—approximately 1/11 of RSA.

The RSA screw anchorage design has inherent biomechanical deficiencies: COR medialization alters the direction of the joint reaction force, increasing the shear component acting on the fixation screws at small and large abduction angles, but its sensitivity to screw shear force is angle-dependent, being unfavorable at small and large abduction angles; distalization is universally favorable but constrained by anatomy. Consequently, parameter optimization alone cannot fundamentally resolve the mechanical conflict between "increasing deltoid moment arm" and "reducing screw shear force" in RSA. CVDP, by restoring the anatomical center of rotation and employing large-area press-fit fixation, is expected to reduce the risk of screw shear wear and demonstrates biomechanical advantages in reducing implant-bone interface stress.