Comparison of Stress between Three Different Functionally Graded Hip Stem Implants Made of Different Titanium Alloys and Composite Materials

Abstract

This study aims to evaluate the mechanical behavior, by ways of the FEM, of three femoral stems made of a Ti-6Al-4V titanium alloy with transverse holes in the proximal zone and a stem made of a β-type titanium alloy with a stiffness varying from 65 GPa in the proximal zone to 110 GPa in the distal zone and the CFRP composite material. The purpose of the study was to evaluate the effect of stress shielding on an intact femoral bone. A three-dimensional model of the intact femur was created, and the three prostheses were inserted with perfect stem bone fit. Applying constraint conditions such as fixation in all directions of the distal part of the femur and the application of a static load simulating standing still during a gait cycle allowed the stresses of both the implants and the bone to be compared. Evaluating the stress shielding for the three proposed materials was possible by identifying the seven Gruen zones. We can see from the results obtained that the metal alloys produced observable stress shielding in all the Gruen zones. There was a difference for the β-type alloy which, as a result of its stiffness variation from the proximal to the distal zone, did not show any level of stress shielding in Gruen zones 1 and 2. The CFRP composite, in contrast, showed no stress shielding in all of the Gruen zones and is an excellent material for the fabrication of total hip replacements. Further in vitro and in vivo validation studies are needed to make the modeling more accurate and understand the biological effects of the use of the three materials.

1. Introduction

Total hip arthroplasty (THA) is a surgical procedure in which a diseased hip joint is replaced with an artificial femoral stem and acetabular cup. This surgery reduces pain and restores function to the joint [1,2,3]. Over the past century, THA has become one of the most successful surgical procedures for improving patients’ quality of life. With an annual growth rate of 25–30%, an estimated 400,000 THA procedures are performed in China alone each year [4]. However, one of the most common complications associated with this procedure is the implant becoming loose, possibly resulting in revision surgery [5,6,7]. After the stem is inserted into the femoral canal, new blood vessels develop around the implant. Reduced blood flow in this area can cause the resorption of cortical bone tissue, causing reduced bone density in the proximal cortical area [8]. The use of titanium and its alloys in the field of biomaterials has steadily increased over recent years since titanium alloys have properties like attractive mechanical behavior, good biocompatibility and corrosion resistance [9,10,11] that make them ideal for orthopedic implants. The most important mechanical requirements for orthopedic biomaterials are high mechanical strength to withstand human body loads and a controlled Young’s modulus to avoid bone absorption due to stress shielding. In fact, according to Wolff’s law, bone remodels its structure in response to mechanical stress [12]. If an implant is stiffer than the bone, this can lead to a loss of bone mass, which, in extreme cases, can lead to implant failure. This phenomenon, where high stiffness of the stem results in bone resorption, is known as ‘stress shielding’. However, bone resorption and stress shielding are related phenomena that are associated with changes in the mechanical load on the bone. In stress shielding, the physiological load on the bone is reduced, and as bone tissue is a dynamic tissue that adapts to mechanical loads, when it is not subject to adequate loads, as in stress shielding, osteoclasts are activated, leading to a reduction in bone mass and integrity [13,14,15]. As mentioned above, the stems currently available on the market are mainly made of dense metals, such as titanium alloys, cobalt–chrome alloys or 316L stainless steel, with elastic moduli between 110 and 230 GPa that is significantly higher than that of bone (0.3–22 GPa) [13,14]. As a result, the metal implant, which is much stiffer, sustains most of the load, while there is a reduction in stress in the surrounding bone. Maistrelli et al. [16] investigated the effect of the elastic modulus of the stem in total hip arthroplasty (THA) and found that the elastic modulus of the implant is related to the resorption of cortical bone tissue. Yang et al. [17] conducted a theoretical investigation of the effect of femoral implant regrowth using different materials, including porous titanium with different porosities. The results showed that bone loss around the implant is correlated with the elastic modulus of the implant: the higher the elastic modulus, the greater the bone loss. To ensure proper stress distribution and prevent bone resorption due to the stress-shielding effect, several solutions have been developed over time, particularly for the femoral part of hip replacements. These include alternative designs, coatings and biomaterials [18,19,20]. For example, a study by Bieger et al. [21] analyzed the primary stability and stress-shielding effect associated with a short-stem prosthesis and a standard-length prosthesis on nine pairs of cadaveric human femurs. The results showed that the short-stem prosthesis provided a more uniform load distribution in the proximal medial part with a stress change of approximately 47%. In addition, medial displacements were lower in the proximal section than in the distal section, suggesting greater stability for the short stem. However, the reduction in stem length may result in less implant stability, which also depends on the integrity of the soft tissues [22]. For example, Morscher et al. [23] developed an isoelastic stem as a low-elasticity solution to address the issue of stress shielding. Nevertheless, the results of the isoelastic stem proved disappointing due to the lack of primary fixation, which led to a high failure rate. Recently, β-type titanium alloys (also known as second-generation titanium alloys) have been developed to reduce the stress-shielding problem associated with implant materials with high Young’s moduli. These alloys have a Young’s modulus closer to that of bone. They offer better corrosion resistance and improved biocompatibility than the well-known Ti-6Al-4V alloy (Ti-90% Al-V-4%) [24]. Improved bone remodeling and reduced atrophy appeared with the use of beta-type titanium alloy implants when studying implants and intramedullary bars made of β-type titanium TNZT alloys and stainless steel in the tibia of fractured condyles. In a study by Chiba et al. [25], a new femoral stem made of an alloy of titanium, niobium (Ti-33.6% Nb-4% Sn), known as TNS alloy, also of the β-type, was developed. The TNS stem had a reduced Young’s modulus (40 GPa) and functional gradient properties, which meant that the proximal part of the stem had a higher magnetic strength while the distal part kept a lower Young’s modulus. The study, which included approximately 40 patients undergoing total hip arthroplasty, evaluated clinical and radiologic outcomes, including signs of loosening and stress shielding over a three-year period. The results showed good stem performance in TNS, with a stress-shielding incidence of 65% and no cases of loosening or fracture. In another study conducted by Lopes et al. [26], using a metastable beta titanium alloy and an appropriate combination of thermal treatments such as solution annealing, rapid cooling and aging, the authors obtained a variable elastic modulus for the stem ranging from 65 Gpa in the distal zone to 110 Gpa in the proximal zone. Today, the term composite materials is used to describe a class of innovative materials which include carbon fiber-reinforced materials (CFRPs). These fibers have biocompatible properties, which means they can interact with biological tissues without causing adverse reactions, and studies have shown that carbon fiber-reinforced composites can increase bone mass around the implant compared to traditional materials such as titanium. FEA studies conducted by Ceddia et al. [27,28,29] also found that the use of composite materials, particularly carbon fiber with stiffness properties close to those of bone, favored a greater distribution of stress from the proximal to the distal zones, with reduced stress shielding. In a finite element study by Delikanli et al. [30], using titanium alloys such as Ti-6Al-4V to fabricate the implants and take advantage of additive manufacturing technologies to create complex and porous geometries, a weight reduction of 15–17% was achieved compared to solid implants. This weight reduction allowed for reduced stem stiffness and improved bone loading. Hence, the use of porosity or holes on the surface of the implant provides benefits, particularly in terms of osseointegration, between the implant and the bone. Because holes increase the contact surface with the bone and improve cell adhesion, there is better interaction between the implant and the bone tissue, thus facilitating osseointegration. In addition, the holes improve the revascularization and stimulation of bone cells [31]. When placed inside the hole, the osteoblasts can be stimulated to form bone tissue inside the implant, which improves the long-term stability of the implant. Therefore, the aim of this study is to evaluate the mechanical behavior of three femoral stems made of a Ti-6Al-4V titanium alloy with transverse holes in the proximal zone, one stem made of a β-type titanium alloy with a stiffness varying from 65 GPa in the proximal zone to 110 GPa in the distal zone, and a third stem made of the composite material CFRP. Using the finite element method, it is possible to quantify the efficiency of the three solutions in terms of stress-shielding reduction.

2. Materials and Methods

2.1. Model Geometry

Femoral stems were designed and assembled using Autodesk Inventor CAD software, version 2024 (San Francisco, CA, USA). Figure 1 shows the three stems analyzed in this study: (a) a Ti-6Al-4V titanium alloy stem in which 0.3 mm transverse holes were drilled on the proximal part of the stem (Figure 1c); (b) a β-type Ti-30Nb-2Sn titanium alloy stem in which the proximal zone has a lower stiffness than the distal zone (Figure 1b); and (c) a comparison with a stem made entirely of CFRP composite material (Figure 1a).

The solid CAD model of the “Standard Muscular Femur” published by Viceconti et al. [32] was used in this study. This model is an intact femur which has been widely accepted by the scientific community and has become a reference for numerous studies using the finite element method (FEM). Figure 2 shows the 3D model of the intact femur with the prosthesis inserted. In the sectional view, the following three zones are shown: proximal, medial and distal.

2.2. Material Properties

Bone is composed of two main types of tissue: the cortical and the trabecular. Five different materials were selected in the finite element (FE) model: the mechanical properties of trabecular and cortical bone for the femur model, the mechanical properties of Ti-6Al-4V, Ti-30Nb-2Sn, and CFRP for the stems, and the Cr-Co alloy was used as the material for the head. In the three models analyzed, the femoral neck was modeled in the Ti-6Al-4V titanium alloy. The material specifications were found based on similar earlier studies [33,34,35]. Both cortical and trabecular bone were assumed to be homogeneous with orthotropic behavior. The mechanical properties of the materials are listed in

Table 1. Mechanical properties of the materials used in the study [33,34,35], (x, y, z) indicates the three directions in which the mechanical properties differ.

Material	Modulus of Elasticity	Shear Modulus (Gpa)	Poisson’s Ratio	Compressive Strength (Mpa)	Yield Strength (Mpa)	Density g/cm3
Cortical bone	Ex = 6979 (MPa) Ey = 18,132 (MPa) Ez = 6979 (MPa)	Gyz = 5.6 Gzx = 4.5 Gxy = 6.2	νyz = 0.25 νzx = 0.4 νxy = 0.25	195		2.02
Cancellous bone	Ex = 660 (MPa) Ey = 1740 (MPa) Ez = 660 (MPa)	Gyz = 0.211 Gzx = 0.165 Gxy = 0.260	νyz = 0.25 νzx = 0.4 νxy = 0.25	16		1.37
Ti-6Al-4V	Ex = Ey = Ez = 110 (GPa)		$\nu =$ 0.3	970	930	4.42
CFRP	Ex = 4 (GPa) Ey = 9.8 (GPa) Ez = 9.8 GPa)	Gyz = 3.5 Gzx = 3 Gxy = 3.5	νyz = 0.3 νzx = 0.3 νxy = 0.3
Ti–30Nb–2Sn	Proximal zone 65 GPa Distal zone 110 GPa		$\nu =$ 0.3	Proximal zone 900 Distal zone 500		5.72
Cr–Co (femoral head)	Ex = Ey = Ez = 200 GPa		$\nu =$ 0.33

.

2.3. Loading and Boundary Conditions

The loading condition was considered related to heel strike during the gait cycle, which includes loads applied to the femoral head [4.5 times body weight with force components: [(x, y, z) = (1492, 915, −2925) N] and abduction muscle force on the greater trochanter [3.45 times body weight with force components: [(x, y, z) = (−1342, −832, 2055) N], as shown in Figure 3. The lower part of the femur was constrained in all directions [36].

A fixed contact was considered between the femoral stem and the bone, simulating osseointegration and the absence of micromovement between the stem and the femur.

2.4. Finite Element Model

Ansys Workbench R2023 was used for simulation. The geometric models were imported from Inventor 2024 and the mesh was generated for all the parts considered in the study. A quadratic tetrahedral element type was selected for the static analysis [37,38]. It was noticed during convergence studies that the variation in results was significant for element sizes up to 5 mm. However, an average mesh size of 1.7 mm was chosen based on a finite element study [39]. This represents a perfect balance between the efficiency of the computation and the accuracy of the results. The element selected, SOLID187, is a three-dimensional, high-order quadratic solid element that is suitable for modeling irregular meshes. This element has 10 nodes, each with three degrees of freedom (x, y, and z directions) [40]. Finally, the FE model of the implanted femur is presented in Figure 4.

3. Results

A static analysis was performed using the von Mises stress analysis criterion to predict possible material failure points and evaluate how the different prosthesis configurations affected the stress distribution in the femur. Figure 5 shows a visualization of the stress results in the three implant configurations and the intact femur.

3.1. Stress Analysis on Implants

The results of stress in the implants provided critical information about their performance and ability to withstand mechanical loads. Figure 6 shows the results of stress distribution on the three analyzed femoral prostheses. Localized stress peaks in specific areas, such as the stem neck, are present in all configurations where the forces are concentrated.

Figure 6 shows the maximum stress value is reached for a rod made of Ti-6Al-4V, with a peak stress of 1234.24 MPa compared to 875.45 MPa for the CFRP and 1106.98 MPa for the titanium alloy Ti-30Nb-2Sn. We also see that the maximum stress is reached on the outer part of the femoral neck, where the stress state is generated because there is maximum load applied to the femoral head. This stress state is predominantly bending and extension. For the β-type titanium alloy (Ti-30Nb-2Sn), a maximum stress value of 297 MPa is reached, while for the CFRP composite, it is 308 MPa, and for the stem made of the titanium alloy (Ti-6Al-4V), it is 397 MPa. In the case of the CFRP stem, the stresses are lower both in the proximal zone (98 MPa) and in the distal zone (30 MPa) compared to the two titanium alloy versions. In contrast, for the β-type titanium alloy, the stresses are higher in the distal zone of the stem, 185 MPa, compared to 128 MPa for the Ti-6Al-4V titanium alloy and 30 MPa for the CFRP. This can be explained because heat treatment increased the stiffness in this area of the stem to 110 GPa compared to 65 GPa in the proximal area. The change in stiffness significantly changes the mechanical response of the material as well. Overall, the following conclusions can be obtained in which the CFRP stem shows a lower and more uniform stress distribution along its profile than the two metallic materials because of anisotropic behavior depending on the orientation of the carbon fibers and layers. The titanium alloy Ti-6Al-4V, on the other hand, has a higher total stress range (1234.24–4.21 MPa), being the material with higher stiffness than the other two. However, the execution of the transverse holes allowed a reduction of 42 MPa of stresses in the proximal zone.

3.2. Stiffness of the Stems

Figure 7 shows the load–displacement results for the three stems analyzed compared to that of an intact femur derived from the computational model. Stiffness was assessed by applying a horizontal load and evaluating the displacement along the direction of the application to the load on the model.

We can see how the stiffness of both stems deviates from that of the intact femur (blue line). The highest stiffness is given by the Ti-6Al-4V titanium alloy stem (57,161 N/mm) compared to 52,270 N/mm for the Ti-30Nb-2Sn titanium alloy and 52,026 N/mm for the CFRP. This helps to explain why the stiffness of metallic materials such as titanium alloys, which differs greatly from the stiffness values of bone, could create the problem of stress shielding. Composite materials, which have the characteristic of being designed according to the stiffness values to be achieved, allow a smaller discrepancy than the stiffness of bone.

3.3. Stress-Shielding Evaluation

The stress results obtained proved to be useful in assessing the effect of the implant on the surrounding bone tissue. An implant that produces more localized stress in the distal area and less in the proximal area can lead to the phenomenon of stress shielding and bone density loss as the bone near the greater trochanter is not stimulated to reabsorb. Conversely, an implant that distributes stress more evenly may promote bone growth on the implant surface and its complete biological integration. In this study, Gruen’s seven zones were used to analyze stress shielding, which are specific areas of the femur used in various studies [41] to evaluate load distribution and analyze bone strength in relation to orthopedic implants such as hip replacements (Figure 8).

Stress in the intact femur with the prosthesis in place was evaluated using the seven Gruen zones identified. This made it possible to evaluate the effect of stress shielding according to Equation (1) by expressing it in %:

$$Stress shielding factor\left(SSF\right)={\displaystyle \frac{{\sigma }_{intact}^{femur}-{\sigma }_{implant}^{femur}}{{\sigma }_{intact}^{femur}}}$$

(1)

When comparing von Mises stress values on the cortical surface for the three configurations, a significantly higher stress level was seen in the central part of the femur. This shows that the proximal zone might be an area susceptible to stress shielding (Figure 9).

The average stress values at the seven Gruen zones were divided and related to the stress values of the intact femur in the analysis of Figure 9. Using Equation (1) to calculate the shielding stress factor, the following results are obtained and are shown in Figure 10.

Figure 10 shows the stress-shielding factor (SSF) within each Gruen zone. A positive SSF value indicates a reduction in stress levels within the femur after prosthesis insertion, which could lead to bone loss. In all Gruen zones, the SSF value for the CFP composite prosthesis was significantly lower in all zones. There was a positive SSF value for the Ti-30Nb-2Sn prothesis in all Gruen zones except 1, 2 and 7. The same trend was seen for the Ti-6Al-4V titanium alloy. Given the results, we can conclude that the CFRP composite stem shows no clear signs of stress shielding in any of the Gruen zones. While the titanium alloy Ti-30Nb-2Sn has the absence of stress shielding in Gruen zones 1, 2 and 7 and from Gruen zones 3 to 6, there is an increase in stress shielding up to 60%. For the Ti-6Al-4V titanium alloy prosthesis, drilling transverse holes in the proximal stem zone eliminates stress shielding in Gruen zone 1. However, it has lower stress-shielding levels than Ti-6Al-4V. The results of this study may suggest design methods to limit problems such as stress shielding, especially in porous prosthesis, as highlighted above. The porosity distribution should be designed to promote uniform force distribution. In addition, the design should aim to promote osseointegration by encouraging bone growth around the prosthesis. It is possible to derive the stiffness parameters that a beta-type titanium prosthesis should have to avoid the stress-shielding phenomenon based on the mechanical properties of the bone. However, the use of composite materials has shown excellent results in reducing stress shielding and their optimization could improve long-term results.

4. Discussion

Long-term complications can occur, including an inflammatory response by the body to products from the implant material and stress shielding [42] after total hip arthroplasty. In the first case, there is an inflammatory response by the body to debris generated by the micro-movements between the implant and the surrounding tissue. Such inflammation can lead to pain, swelling, and the deterioration of osseous quality. On the other hand, stress shielding occurs because of the significant difference in elastic modulus between the implant material and the bone. Because the implant absorbs a significant part of loading that would normally be transferred to the bone, it may experience a reduction in mechanical stress, leading to the weakening of the surrounding bone which has not been stimulated enough to maintain its density and structural integrity. In recent years, many efforts have been made to find innovative solutions to mitigate these problems. Reducing the stiffness in the proximal femoral zone increases the stress on the bone with regard to the stress-shielding phenomenon, thus reducing bone resorption in the proximal femoral zone. Thanks to modern titanium alloys such as Ti5Al12.5Fe and Ti-6Al-7-Nb, also known as second-generation titanium alloys, it is possible to achieve higher stiffness under dynamic loading and a lower Young’s modulus. Another class of titanium alloys, β-Ti alloys, contain molybdenum, which stabilizes the β-phase of titanium at room temperature. The use of beta alloys, such as Ti-33.6Nb-4Sn, represents a significant step towards reducing stress shielding in femoral prostheses. Due to their excellent mechanical properties and ability to better adapt to the biomechanical characteristics of the bone, these alloys can improve implant stability and promote better bone integration. In 2014, Hanada et al. [43] developed a new stem using the β-Ti-33.6Nb-4Sn (TNS) alloy, which is characterized by a Young’s modulus that decreases from the proximal to the distal end. The strength and Young’s modulus of this alloy depend on the heat treatment temperature. A local treatment at 693 K for 5 h in a nitrogen atmosphere increased the strength of the prosthetic neck compared with the Ti-6Al-4V alloy. The TNS stem was evaluated in a clinical study by Chiba et al. [25] that followed 40 patients for 3 years and showed no radiologic evidence of stem loosening or failure. Although the study had some limitations, it confirmed that the TNS alloy is suitable for the manufacture of stems for total hip arthroplasty and showed a moderate stress-shielding effect in 65% of patients. Another study by Yamako et al. [44] showed that the TNS stem caused less bone resorption than the Ti-6Al-4V stem in almost all areas of the Gruen, with the bone loss occurring mainly in the proximal part of the femur. The differences in bone mineral density (BMD) between the two stems increase over time, with TNS showing a greater preservation of bone density. Although the stress-shielding phenomenon was not eliminated, the TNS stem showed beneficial effects on bone preservation and provided sufficient mechanical strength. In another study conducted by Lopes et al. [26], the design of a stem with decreasing stiffness from the proximal to distal zone helped to decrease the risk of stress shielding by improving the integration of the implant with the bone tissue. Today, the use of composite materials is revolutionary and allows the mechanical properties of the individual components to be combined in a single material. In the field of orthopedics, composite materials have been widely used to produce a variety of devices, including prostheses and bone fixation plates [45,46]. Most are made of CRRP, a material composed of carbon fibers immersed in an epoxy resin polymer material. These materials exhibit mechanical properties like those of human bone, making it a promising material for the fabrication of hip prostheses. Compared to other materials used for prostheses, CFRP allows for a more uniform transfer of loads from the implant to the bone, thus limiting the stress-shielding effect. The mechanical properties of CF/PEEK vary, depending on the orientation of the carbon fibers and their total proportion in the composite, with strength ranging from 70 to 1900 MPa and stiffness between 10 and 100 GPa. In addition, CFRP composites show excellent biocompatibility, environmental stability, and chemical resistance [47]. The ability to manufacture these materials with different configurations of carbon fibers allows the modulus along the stem to be adjusted, further helping to reduce the stress-shielding effect [47]. A study conducted by Carpenter et al. [48] compared a porous PEEK implant to a titanium implant and found that PEEK significantly increased load transfer to bone compared to titanium under compression, tensile, and shear conditions. This increase in load transfer averaged 83 percent, suggesting that differences in the intrinsic elastic modulus of the materials were more relevant than pore architecture. A finite element study conducted by Ceddia et al. [28] found that the use of both topological optimization and the use of composites in the realization of the femoral stem contributed to a reduction in stress shielding. Ceddia et al. [49] showed that the use of a CFRP plate, compared to the corresponding magnesium and titanium alloys, improved the stress distribution in the bone area and contributed to a rapid healing of the bone callus. The use of porosity allowed a reduction in the overall density and stiffness of the stem, thanks to modern additive manufacturing technologies. In a work by Arabnejad et al. [50], a high-strength femoral stem made entirely of porous titanium was developed. The microarchitecture of the implant was optimized using computational methods to mimic the properties of bone tissue, with the goal of minimizing the stress-shielding effect. Once the best density distribution was achieved, the stem was fabricated using an additive manufacturing technique called selective laser melting (SLM), which uses a high-power, high-density laser to melt and fuse metal powders. The porosity and pore size were set to 70% and 500 μm, respectively. The results of the finite element analysis (FEA) model showed that the total bone loss due to the stress-shielding effect for the traditional stem was 34%, concentrated in the medial proximal area (Gruen area 7). In contrast, for the porous implant, bone loss was limited to the same area with a value of 8%. The results presented in this study analyzed the main solutions to reduce stress shielding, including the use of composite materials such as CFRP, the use of β-Ti alloys, and the use of holes in the proximal area of a Ti-6Al-4V stem. Where the three materials mentioned above are concerned, the main zones in which stress shielding may occur based on the FEA results are the Gruen zones (4, 5, 6, and 7) when the calculated stress in the corresponding zones is lower than that calculated in the intact femur. In addition, the use of CFRP as a material to fabricate the prosthetic stem showed the total absence of stress shielding in all areas of Gruen, thus proving itself to be an excellent material and solution to solve the stress-shielding problem. Moreover, the use of carbon, and, in particular, carbon fibers, has clinically shown excellent biocompatibility, electrical conductivity, strength, light weight and the ability to stimulate bone growth, making it an excellent choice for medical applications [51]. It should be made clear that the results obtained are numerical and are subject to uncertainties due to some of the limitations made in this study. The main limitation is that only the single-legged stance phase of the gait cycle was simulated. This phase was chosen because it represents the period during which one leg supports the entire body weight, which is essential for stress analysis. Other phases of the gait cycle and other activities, such as climbing stairs or running, may influence the results of the loading. In addition, dynamic loading could lead to problems related to the fatigue strength of composite materials, such as CFRP. Further research will evaluate the influence of dynamic loading on phenomena such as fatigue strength and wear of the component. In addition, the accuracy of the results depends on how accurately the entire system is modeled. For example, the size of the elements should be appropriately selected through in vitro validation to improve the accuracy of the modeling. However, to perform the numerical analysis, the finite element method requires significant simplifications. The contact conditions between the bone and the implant should be carefully analyzed in in vitro studies so that future numerical studies can improve the accuracy of the simulation to ensure more accurate results.

5. Conclusions

This study analyzed the effect of stress shielding and the stress transmission of three total hip replacement implants made of three different materials: the β-Ti alloy, the CFRP composite, and the Ti-6Al-4V titanium alloy with transversal holes in the proximal zone. The results obtained in this study led to the conclusions that the metal alloys produced observable stress shielding in all the Gruen zones, with a difference for the β-type alloy, which, as a result of its variation in stiffness from the proximal to distal zone, did not show levels of stress shielding in Gruen zones 1 and 2. In contrast, the composite CFRP does not show any stress shielding in all of the Gruen zones and is therefore an excellent material for total hip arthroplasty. To better understand the performance of different materials under more realistic conditions, in vivo or pilot clinical studies are suggested to validate the results.