建立涵盖正常肩关节、反置肩关节置换术（reverse shoulder arthroplasty, RSA）及同心变径假体（concentric variable-diameter prosthesis, CVDP）的统一二维准静态力学模型，定量比较三者的力臂、关节接触反力及假体-骨界面应力，阐明RSA螺钉微动磨损的力学机制，并比较RSA螺钉固定与大面积压配固定的界面应力差异。

以假体球头几何中心作为坐标原点建立冠状平面直角坐标系，肩关节外展角度取值区间设定为0~90°。模型全部几何与力学参数均取自人体解剖实测结果：单侧上肢自重W=45 N，肢体重心距离旋转中心长度L_g=280 mm，三角肌止点沿肱骨轴线距离dd=100 mm，肱骨干半径R_shaft=15 mm，三角肌起点坐标A=（55, 45）mm。以力矩平衡方程组求解三角肌收缩力、关节接触反力及RSA螺钉剪切载荷，以悬臂挠曲理论量化螺钉界面微动幅度并结合磨损系数预估假体使用年限，经参数敏感性分析明确旋转中心偏移对螺钉剪切载荷的影响。

旋转中心位置对名义螺钉剪切力影响显著：内移的作用存在角度依赖性，小角度和大角度下不利，仅中间角度有利；下移在所有角度均有利，但受生理结构限制可调空间有限。优化后RSA外展60°时单钉剪切力降至28.1 N，螺钉-骨界面剪切应力τ_RSA≈2.81 MPa，微动位移δ ≈11.6 μm，磨损速率2.75× 10^-5 μm/cycle。磨损系数以临床随访数据（10年松动）标定后，模型在峰值角度的磨损寿命计算与标定基准自洽，提示螺钉剪切微动磨损机制在量级上与临床观测相容。CVDP采用无螺钉压配固定，界面面积A_fix≈400 mm²，其界面剪切应力峰值仅0.244 MPa，约为RSA的1/11。

RSA螺钉锚固设计存在生物力学缺陷：内移对名义螺钉剪切力的影响具有角度依赖性，在大小角度反而增大剪切力，难以借参数优化在全活动范围内改善假体寿命；下移虽全角度有利，可调空间却有限。CVDP恢复解剖旋转中心并采用大面积压配固定，在降低假体-骨界面应力方面具有生物力学优势。

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.