## 4 Discussion
In clinical practice, deviations such as varus or valgus malalignment can occur, which may alter the stress distribution patterns around the femoral stem. Malalignment could shift the load-bearing regions, potentially exacerbating stress shielding in certain Gruen zones or increasing micromotion at the bone--implant interface. Although our FEM results demonstrated favorable biomechanical behavior under neutral conditions, future analyses incorporating surgical variability, such as different alignment scenarios, would be necessary to further validate the robustness of the modulus-graded Ti-Nb-Sn stem design. Fourth, the FEM analysis utilized a femoral morphology characterized by a champagne-flute-shaped canal, which is commonly associated with Dorr type A or B femora. This selection was based on its favorable cortical support and the widespread use of such morphologies in cementless total hip arthroplasty. However, different femoral canal shapes, such as stovepipe configurations observed in Dorr type C femora, may present distinct biomechanical environments. In particular, the reduced cortical thickness and wider canal in Dorr type C bones could affect the stress transfer mechanism and potentially exacerbate stress shielding. Therefore, future studies evaluating the modulus-graded Ti-Nb-Sn stem in a broader range of anatomical variations are warranted to ensure generalizability of the present findings. Fifth, the FEM analysis employed in this study was based on static loading conditions representing peak forces during walking. In clinical reality, implants are subjected to cyclic loading resulting from daily activities, such as walking, stair climbing, and rising from a chair. Repetitive loading could lead to cumulative micromotion at the bone--implant interface, potentially affecting long-term osseointegration and stress-shielding progression. Although our three-year clinical follow-up demonstrated favorable implant stability and minimal stress shielding, future studies incorporating cyclic loading simulations and long-term mechanical fatigue testing are warranted to comprehensively evaluate the durability of the modulus-graded Ti-Nb-Sn stem under repeated physiological stresses. Sixth, although the present study employed a FEM based on static boundary conditions and simplified muscle loading, it did not incorporate comprehensive musculoskeletal modeling. Advanced musculoskeletal simulations can better replicate physiological loading patterns by accounting for complex muscle forces, joint kinematics, and postural changes, thereby potentially improving the accuracy of implant-bone interaction assessments. Incorporating such approaches, as demonstrated in recent studies optimizing biomechanical strategies for injury prevention [48], may further enhance the predictive power of future biomechanical analyses for total hip arthroplasty. Seventh, although clinical validation was performed through radiographic assessment at one and three years postoperatively, the sample size was limited, and the follow-up duration was relatively short. Long-term follow-up studies are needed to confirm the durability of the stress-shielding suppression effect and the continued mechanical integrity of the modulus-graded Ti-Nb-Sn stem. Finally, this study focused on a single patient-specific femoral morphology--a champagne-flute-shaped canal in a female patient in her sixties. The generalizability of these findings to other morphotypes, age groups, or bone qualities (e.g., osteopororotic bone) has yet to be established.