## 4 Discussion  
This study demonstrated that a FEM with a validated Ti-Nb-Sn [PERSON]'s modulus gradient provides a highly accurate simulation of in vivo mechanical behavior. Importantly, the proximal micromotion values suggested favorable conditions for osseointegration and long-term stability. Additionally, in the three-year clinical follow-up of Ti-Nb-Sn stem implantation, stress shielding was markedly suppressed. According to [PERSON]'s classification, no cases exceeding Grade 3 stress shielding were reported. This supports the biomechanical findings from FEM simulations and underscores the clinical relevance of [PERSON]'s modulus gradient control. FEM studies of hip arthroplasty indicated that the prosthesis design materially influences stress shielding and load transfer [34; 35; 36; 37]. The reviewed papers employed diverse \begin{table}
\begin{tabular}{c c c} \hline \hline
**[PERSON]’s Classification** & **1-Year Follow-Up** & **3-Year Follow-Up** \\ \hline  & n\({}_{\text{r}}\) (ratio) & n\({}_{\text{r}}\) (ratio) \\ \hline None & 19, (47.5\%) & 14, (35\%) \\ Grade 1 & 17, (42.5\%) & 10, (25\%) \\ Grade 2 & 4, (10\%) & 16, (40\%) \\ Grade 3 & 0, (0\%) & 0, (0\%) \\ Grade 4 & 0, (0\%) & 0, (0\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Incidence of stress shielding with the Ti-Nb-Sn stem. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Gruen Zone** & **Zone 1** & **Zone 2** & **Zone 3** & **Zone 4** & **Zone 5** & **Zone 6** & **Zone 7** \\ \hline Stress shielding & 9, (23\%) & 0, (0\%) & 0, (0\%) & 0, (0\%) & 0, (0\%) & 1, (2.5\%) & 21 (53\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Frequency of stress shielding by Gruen zone. Figure 8: Three-year follow-up radiographs of the Ti-Nb-Sn hip implant. (**A**) Immediately post-surgery, (**B**) one year post-surgery, and (**C**) three years post-surgery. (**D**) Schematic illustration of the Gruen zones (1–7) used for the radiographic evaluation of femoral stress shielding. No apparent stress shielding was observed. Reprinted with permission from [22]. FEM approaches--including micro-level, CT-based, and patient-specific 3D models--to assess both proximal and distal bone responses [34; 38; 39]. For example, an optimized design featuring dorsal ridges and perforations achieved a 31.46% reduction in proximal stress shielding, while one comparative study noted that a short stem (Metha) retained 20% of native proximal stress, versus 6% for a standard stem [35; 40]. Other investigations documented proximal strain reductions of 40-50% and a ribbed anatomic design with a 71.6% stress-shielding ratio in the posterior quadrant [38; 39]. Distal load transfer also varied by design [34; 38]. Increased mid-to-dist stresses were associated with some configurations, with one paper reporting distal densification and another observing improved metaphyseal loading linked to higher bone mineral density [40]. Material properties and geometric features--from variable stem rigidity to surface modifications--appeared to govern these biomechanical outcomes [36; 38]. Overall, the studies substantiated that modifications in prosthesis design and material parameters are associated with quantifiable differences in stress distribution and bone remodeling after total hip arthroplasty [35; 40]. This study provided critical insights from a FEM of a stem fabricated from Ti-Nb-Sn alloy, which enabled the formation of a Young's modulus gradient through localized heat treatment. By incorporating a controlled gradient in Young's modulus, the FEM accurately simulated the mechanical environment of the implanted stem, including micromotion and stress distribution. Moreover, the study is of high clinical relevance, as the FEM-derived predictions were validated through mid-term postoperative radiographic evaluations, demonstrating a strong correlation between computational analysis and actual clinical outcomes.