## 1 Introduction  
Osteoarthritis and osteoporosis can affect any joint in the body, but they are more common in the hip and knee joints and can be secondary to trauma, avascular necrosis, some metabolic diseases, infection, or alterations of joint morphology, as in hip dysplasia [1] ([PERSON], 2012). Many adults and elderly people with severe osteoporosis end up with bone fractures, especially hip fractures, due to low-energy falls [2]. There is no curative treatment for hip joints and in severe cases total hip arthroplasty (THA) is the only choice. Accidents are another factor in the loss of functionality at the hip joint due to fractures, which are also treated by THA. According to epidemiological projections, by 2050, the total number of THAs globally will reach 6.26 million [3]. In the United States, it was estimated that around 498,000 hip replacement procedures will have been performed in 2020, the number increasing to 652,000 by 2030 [4]. For the year 2011, there were a total of 1.63 million hip replacement surgeries carried out in the Organization for Economic Co-operation and Development (OECD) countries and in 2017 the average was 2.4 million. To treat this huge number of patients, femoral stems made of metallic biomaterials, such as titanium alloy, cobalt-chrome alloy, and stainless steel, are conventionally used in THAs. The stiffness of these materials is 4-12-fold greater than the stiffness of the bone [5, 6, 7, 8]. The high stiffness of the dense stems in comparison to the femur bone causes several complications, such as aseptic loosening and stress shielding, which may lead to poor bone growth, stem failure, and revision of surgery [9, 8]. The revision rate of THAsperformed in the United States is about 6% after 5 years, 12% after 10 years, and 18% between 10 and 20 years after the initial surgery [10; 11; 12]. The risk of revision surgeries is higher for young adults (35%) due to active lifestyles [13; 14]. To treat the arthritis/trauma initially, the femoral stem was made of stainless steel and its alloys; 316 L, especially, was used extensively. However, the usage of stainless steel has decreased tremendously, primarily due to corrosion issues [15]. Stainless steel stems are replaced by titanium and cobalt chrome alloys, but more pain has been reported with the use of cobalt-chrome alloy stems compared to titanium-based stems [16]. This is because the stiffness of the cobalt-chrome alloy is almost twice that of titanium alloy, which promotes stress shielding, a phenomenon that causes loosening of the femoral stem. Accordingly, in this study, a titanium alloy (Ti-6 Al-4V) was selected for the stem material because of its lightweight, biocompatibility, good strength, corrosion resistance, and flexibility [17; 18]. However, the stiffness of bulk Ti-6 Al-4V alloy (114 GPa) is still 5-10-fold that of cortical bone (10-18 GPa), which may cause stress shielding. Stress shielding is where the stiffer femoral stem takes more body weight, with insufficient load sharing with the femur bone. As a result, the density of the bone close to the implant starts to decrease [11; 12; 19; 20]. The consequence of this is aseptic loosening of the stem [21; 22; 23], which is one of the major factors for revision surgeries for THA [11; 12]. Due to the inherent limitations on conventional manufacturing of femoral stems, such as casting and machining, it is not possible to produce femoral stems with porous cellular architectures to reduce the stiffness and give the unique shape for stability. It is also costly and time-consuming to produce specifically customized implants for patients using these traditional manufacturing processes. Introducing cellular porous structures with different porosities is used to tailor the stiffness of the stem for mimicking bone properties [5; 6; 7; 8; 9]. Additive manufacturing (AM) technology allows the manufacture of such cellular geometrical structures and provides flexibility to allow control of the shapes and geometries as needed for each particular patient.