## 1 Introduction  
Since the 1960s, total hip arthroplasty (THA) has proven to be successful in alleviating hip pain, enhancing mobility and quality of life, and enabling patients to resume regular activities. Over three million primary THAs are performed annually in Europe [1, 2], and different studies suggest that a further increase in demand is anticipated over the next 20 years [3, 4]. From an engineering point of view, THAs can be mainly classified as cemented or cementless, depending on the implant fixation modality. Cementless fixation, which relies on press fit and osseous integration, has gained popularity over the past decade, both for cups and stems [5, 6]. According to the latest report on primary THAs from the Register of the Orthopaedic Prosthetic Implants register, between 2017 and 2019, cementless cups and stems accounted for 99% and 96.7% of all operations, respectively, in Italy [7]. The latest National Joint Registry annual report also indicates a total number of primary uncemented hip replacements higher than that of cemented implants [8]. Despite being one of the most effective and commonly performed orthopaedic procedures, with a 10-year survival rate of over 90% shown in cementless implants [5], the number of implant revisions is continuously rising, mainly due to the growing number of primary surgeries. Given the significant medical and economic burden associated with the revision surgery, the significance of paying closer attention to THA failure and lowering the risk is self-evident. The THA failure patterns frequently observed include aseptic loosening (AL), infection, periprosthetic fracture, and instability. Aseptic loosening remains the leading cause of failure, as evident in most hip replacement registers. The reported values for the AL failure incidence range from 40% to 70% [9; 10; 11]. Aseptic loosening is described as the failure of the bone-implant fixation in the absence of infection [12]. Although AL can occur due to inadequate initial fixation (e.g., small-to-fit), it is primarily associated with mid- and long-term instability rather than a lack of primary stability. While substantial evidence has been observed regarding the mechanics of AL, it is difficult to attribute it to a single cause. Aseptic loosening is often accompanied by periprosthetic osteolysis, which refers to bone resorption around the implant as well as an inflammatory cellular response within the joint. Additionally, excessive micromotion at the bone-implant interface or the abrasion of polyethene particles from wear can also contribute to AL [12]. More practically speaking, AL is caused by various preoperative, intraoperative, and post-operative factors, including patients' bone quality, implant properties, surgical technique, and lifestyles. Among them, the implant designs play a crucial role in the procedure's success [13]. Clinical studies conducted long-term follow-up assessments to track the survival rate of THAs with various implant types, with some highlighting the AL rate of cementless femoral implants. In many cementless stem designs, a 10-year survivorship rate of over 96% has been observed (e.g., ABG I, Osteonics Cementless, R-B Interlok, Zweymuller-Alloclassic, Freeman Cementless) [14]. At 4.5 years, the estimated probability of revision for AL was 18.6% for the Bio-Fit stem (cobalt-chromium; straight stem) and 0.5% for the Corail stem (Titanium; straight stem) [15]. Another 5-year follow-up study conducted on 517 cementless THAs found an unusually high number of AL failure cases after using a modified stem with a shorter and shoulder-less double-taper [16]. The revision rate for the smaller Corail stem was four times higher than that for larger femoral diameters [17]. Different implant designs show a considerably different incidence of AL, which confirms that implant design is a major determinant. Therefore, studying the effect of various femoral implant types on failure risk is crucial for improving patient safety, reducing costs, and improving long-term outcomes.