## 1 Introduction  
Stress shielding after total hip arthroplasty (THA) shows clear radiographic features, while only a few studies detail related symptoms [1, 2, 3]. [PERSON] et al. reported that patients display a lower walking score and reduced ambulation when stress shielding is present [4]. [PERSON] et al. noted that high-degree stress shielding accompanies a lower Harris Hip Score [5]. In contrast, most studies focus on radiographic patterns and implant factors without detailing clinical complaints [1, 2]. Thus, the available evidence supports that reduced walking ability and thigh pain are the primary symptoms linked to stress shielding after THA [4; 5]. Stress shielding caused by stiffness mismatch between femoral stems and surrounding bone remains a major concern in cementless THA [4; 6; 7; 8]. Moreover, the extent and clinical impact of stress shielding have been reported to differ depending on the proximal femoral morphology. Specifically, patients with Dorr type C femora, characterized by a wide canal and thin cortical bone, are at greater risk of developing high-degree stress shielding due to reduced mechanical support around the implant [9; 10; 11]. Severe stress shielding has been associated with a higher incidence of implant loosening, periprosthetic fractures, and compromised long-term fixation [4; 12]. These complications are thought to arise from the altered stress conduction patterns caused by the mismatch between the prosthesis stiffness and the surrounding bone, which triggers bone resorption in unloaded regions [7]. Therefore, minimizing the modulus mismatch between the implant and the host bone is critical not only for preventing radiographic changes but also for preserving mechanical stability and reducing clinical complications over time. Conventional Ti-6 Al-4V alloy stems, despite their excellent mechanical strength, exhibit Young's modulus values far exceeding that of cortical bone, often leading to periprosthetic bone resorption [13]. With a Young's modulus of 110 GPa, Ti-6 Al-4V exhibits high stiffness, which causes stress to be transferred away from the medial proximal femur. As a result, bone mineral density can decrease by as much as 45.8% over a ten-year period [14; 15]. To address this, titanium-based alloys with a lower and gradable Young's modulus, such as Ti-33.6 Nb-4 Sn (Ti-Nb-Sn), have been proposed by [PERSON] et al. [16]. They developed a novel \(\beta\)-type titanium alloy, Ti-Nb-Sn, characterized by an exceptionally low Young's modulus of approximately 40 GPa. In addition to its low stiffness, a key advantage of the Ti-Nb-Sn alloy lies in its tunable mechanical properties through heat treatment. The localized heat treatment of the Ti-Nb-Sn alloy stem was designed to induce a gradual increase in Young's modulus along the longitudinal axis. By applying controlled heating, the proximal region of the stem achieved a higher modulus to maintain mechanical strength and resist neck fracture, while the distal portion retained a lower modulus to promote physiological load transfer to the surrounding bone and reduce stress shielding. This approach enabled functional optimization without altering the alloy composition. Utilizing this feature, the researchers engineered a new cementless femoral stem exhibiting a functional gradient in both Young's modulus and mechanical strength. Previous studies have confirmed the alloy's safety and biocompatibility [17; 18; 19; 20]. Unlike conventional Ti-6 Al-4V alloy stems, which exhibit a high and uniform Young's modulus (\(\sim\)110 GPa) and are associated with significant stress shielding, the Ti-Nb-Sn stem utilizes a novel approach to stiffness control [16; 21]. Traditional functionally graded materials achieve modulus variation through compositional changes, which can introduce heterogeneity in biocompatibility and mechanical reliability.