## 4 Discussion
This integration of material science, computational modeling, and clinical validation underscores the potential of modulus-graded implants in optimizing stress distribution and minimizing stress shielding in vivo. Previous studies evaluating conventional Ti-6 Al-4V stems have reported considerable stress shielding, with 35-50% of cases exhibiting Engh's classification Grade 3 or higher within the first 2-3 years postoperatively [4; 37]. Additionally, micromotion at the stem-bone interface for Ti-6 Al-4V implants has often been reported in the range of 50-150 um under physiological loading conditions, occasionally exceeding thresholds associated with compromised osseointegration [32]. In contrast, the Ti-Nb-Sn stem with a continuous Young's modulus gradient demonstrated substantially reduced stress shielding, with no cases exceeding Grade 2 at three years, and achieved micromotion values well below 40 um in the critical proximal press-fit region. These favorable outcomes highlighted the clinical advantages of using a modulus-graded Ti-Nb-Sn alloy stem to enhance load transfer, preserve proximal bone stock, and promote stable long-term fixation in cementless total hip arthroplasty. \(\beta\)-type titanium alloys for orthopedic implants are primarily based on Ti-Nb-Zr-Ta systems, with occasional additions of elements, such as Sn, Si, and Sc. These compositions have been shown to offer lower Young's moduli compared to the conventional Ti-6 Al-4V alloy [41; 42; 43; 44]. Reported modulus values ranged from 37 to 85 GPa (versus roughly 110-120 GPa for Ti-6 Al-4V) [42; 45]. For example, one study of a (Ti-35 Nb-7 Zr-5 Ta)\({}_{98}\)Si\({}_{2}\) alloy reported a Young's modulus of 37 GPa with a yield strength of 1296 MPa and an ultimate strength of 3263 MPa, while another evaluating a Ti24 Nb38 Zr2 Mo0.1 Sc alloy noted a modulus of 65.4 GPa, yield strength of 780 MPa, and ultimate strength of 809 MPa [45; 46]. Previous studies indicated that stabilizing the beta phase with niobium and tantalum led to mechanical properties suited for load-bearing applications and that fine-tuning compositions via elements such as Zr, Sn, and Si enhanced strength without sacrificing a low modulus [42; 43; 44]. In vitro and animal studies documented favorable biological responses--increased cell attachment, proliferation, osteogenic differentiation,and osseointegration--and noted corrosion resistance that was comparable or superior to Ti-6 Al-4V [45; 46; 47]. Processing methods, such as cold rolling, laser deposition, and spark plasma sintering, further tailored the alloys' microstructures and properties [46]. Although other promising \(\beta\)-type titanium alloys have been reported, TiNbSn alloy remains the only one that has been clinically applied in orthopedic devices, such as joint prostheses. Several limitations of this study should be acknowledged. First, although the FEM analysis was based on a validated [PERSON]'s modulus gradient derived from experimental data and incorporated patient-specific bone properties, the simulations assumed ideal implantation conditions. In clinical practice, however, variations in stem positioning--including varus or valgus alignment--may affect the stress distribution and compromise the intended stress-shielding suppression effect. It remains unclear whether the biomechanical advantages observed in this study would be preserved under such suboptimal alignment conditions. Second, while the micromotion thresholds used to evaluate implant stability (150 \(\upmu\)m for loosening and 40 \(\upmu\)m for osseointegration) are widely accepted in the literature, they represent general guidelines and may not fully account for patient-specific biological responses. Additionally, the micromotion analysis did not account for dynamic loading cycles over time, which may influence long-term bone--implant interactions and the progression of stress shielding. Third, the FEM analyses in this study were conducted under the assumption of ideal neutral stem alignment.