Structural simulation of Ti–Ha–CaCO 3 biocomposites using ﬁnite element analysis (FEA) for biomechanical stability of hip implant

The structural integrity of new biocomposite implants is critical in ensuring the success of biomedical implants under physiological loading conditions. Studying the stress distribution, deformation, and potential failure modes under different loading scenarios is complex, expensive, and time-consuming, as it involves repeated surgery on clinical assessment. The present study aims to investigate the biomechanical stability of hip implants made of a Ti–Ha–CaCO 3 biocomposite using ﬁnite element analysis. The Ti–Ha–CaCO 3 biocomposite was modeled and simulated using Solidworks. The model mesh was generated to represent the implant’s geometry accurately, and normal human activities (standing and jumping) were considered the boundary conditions with the lower part of the femur ﬁxed. The model was subjected to static loading following ISO 7206-4 with an equivalent load of 2300 N according to ASTM F2996-13 standard. The Ti–Ha–CaCO 3 biocomposite demonstrated outstanding biomechanical stability under loading circumstances. The maximum von Mises stress (354.7 MPa) observed with the GSB-femur model in the implant was below the yield strength of the titanium implant, indicating that the implant can withstand applied loads without experiencing permanent deformation. However, 74.11 MPa was obtained as acceptable von Mises stress using GSB intramedullary rods for bone ﬁxation. The most stable implant is DSB, with the lowest displacement value of 2.68 mm. Low equivalent strains were achieved for all the implants, as the highest strain (0.012) was obtained in the simulation of the stem DSB-femur model. Low-stress signals (SS) were obtained for the implant-femur models, indicating they are suitable for replacing bone for that loading. The DSB (7.19) is the most suitable among the studied stem-femur models, and GSB (0.87) remains the suitable intramedullary rod-femur model with the lowest SS.


Introduction
Finite element analysis (FEA) is critical in determining how the bone-implant system behaves under compressive and tensile loads during pre-clinical testing of novel devices for articular cartilage, joints, and bone structures.In order to ascertain stress, strain, and displacement in the implant and bone tissue, finite element analysis (FEA) is a valuable tool in the research of load-bearing implants.Applications of this technology have been extended to biomechanics and biomechanical engineering simulations of various phenomena, particularly in complicated systems that are not accessible through experimentation.For example, Brekelmans et al. [1], applied a new approach to a basic two-dimensional model of the human femur and presented a new technique for analyzing the mechanical behaviour of skeletal components.These findings proved that complicated human anatomy, such as that of the femur, can be studied effectively using finite element analysis.Kumar et al. [2] modelled a three-dimensional virtual femur bone using Solid Edge V19.The bone was analysed for hip contact stresses/forces during normal walking, standing, running and jumping activities.The stress analyses were carried out using ANSYS 14.0, a Finite Element code to investigate the behaviour of the femur bone during these activities.The stress results obtained are compared with the previous studies and were found to be optimized for the material required to join/repair the fractured bone.Ebrahimi et al. [3] predicted overall stiffness and peak bone stress in the same femur after injury, repair, and healing concerning its intact condition.Experiments were conducted on a synthetic femur with strain gages and subjected to 1500 N of axial force.Finite element (FE) models were validated against experiments and then re-analyzed using a clinicallevel force of 3000 N. At 3000 N, FE bone stresses yielded peaks of 75.7 MPa at the load application point (Stage 1), 29.0 MPa near the hip implant tip (Stage 2), 126.3 MPa at the distal portion of the plate (Stage 3), and 69.3 MPa at the proximal portion of the plate (Stage 4), showing that Stage 3 was most vulnerable to re-injury compared to Stage 1.The structural analysis of natural femur bone was recently compared with AZ31 (magnesium alloy) and CP Ti (Commercially Pure Titanium Alloy) to predict the suitable alternative material using the finite element method.It was found that AZ31 is the best-suited material for bone implants, and its weight is approximately the same as that of natural bone [4].Razaei et al. [5] executed a comparative finite element analysis to evaluate the load transfer between the carbon/PEEK composite prosthesis, stainless steel/titanium, and the femur bone.Compared to the stainless steel/titanium prosthesis, the strain, von Mises stress, and primary stress were much lower.Taqriban et al. [6] evaluated the safety aspects and stresses on the hip joint implant utilizing four different AISI 316L material types using software for analyzing structures.Based on the static structural analysis, the Undip hip joint implant design obtained the total deformation, equivalent elastic strain, equivalent von Mises stress, and the safety factor.
The structural integrity of the 3D femoral model under forces ranging from 2.5 to 6.3 kN was investigated using a finite element analysis (FEA).The findings revealed that the force magnitudes operating on the implant are significant and can potentially alter the implant's stress field and cause instability, leading to malfunctioning implants [7].Al-sanea et al. [8] utilized SolidWorks to design a 2D/3D CAD model of a prosthetic hip joint.Hip joint components were designed using biocompatible materials like Alumina ceramic (Al 2 O 3 ) and Ti6Al4V alloy.Furthermore, following the application of static loads of 250N, 350N, and 450N, respectively, the design was examined using computer-aided engineering (CAE) software to determine its endurance.Static testing confirmed the model's stability.Roatesi [9] studied the rehabilitation of implant prosthetics through functional loadings' behaviour with FEA using SolidWorks software.FEA analysis helps prepare the design to indicate the suitable materials applied under different load circumstances to lower the clinical practice's material consumption or failure.Hassan et al. [10] utilized ANSYS 17 to examine the structural characteristics of functionally graded (FG) implants as a new implant from several titanium alloys (Ti35Nb7Zr5Ta and Ti12Mo6Zr2Fe) layered with hydroxyapatite (Ha) developed to avoid tissue responses to metals (vanadium and aluminium).The results revealed increasing von Mises stress and strain energy in the cancellous and cortical bones with FG femoral stem.Mahmoudi et al. [11] reduced the destructive stresses in functionally graded materials (FGMs) with the class of inhomogeneous materials using Abaqus/CAE 6.13 software.Alumina crowns produced the largest von Mises stress at the crown margin (77.7 MPa), reducing the fracture likelihood.The maximal von Mises stresses of the crown margin created by FGM crowns were lower than those generated by homogeneous crowns (70.8 vs. 46.3MPa).
Ibrahim et al. [12] reported that the mechanical property of biocomposite experimentally improves the longevity and performance of hip implants with low modulus value closer to the host bone and ultimately can improve the state of patients' lives undergoing hip replacement surgery.Therefore, the structural behaviour of the new biocomposite implants may be studied before a clinical trial using FEA and simulation tools in designing and evaluating biomedical implants.This study examines the structural stability of Ti-Ha-CaCO 3 Biocomposite implants using Finite Element Analysis (FEA) for replacement hip bone.

Structural analysis of biocomposite samples using finite element analysis
Two models were designed: a stem, cylindrical intramedullary rod implant, and a generic implant type on the femur, and they were assembled with a fractured femoral bone model.This study's geometrical analysis used the Zimmer hip joint prosthetic design [13] and cylindrical intramedullary implant design, as shown in Figs. 1 and 2, respectively.The head and the stem are the two components of this design.The standard femur dimension was obtained in millimetres (mm) from O'Connor et al. [14], as shown in Fig. 3.The 3D implant model was developed using a commercially available computer-aided design tool (SolidWorks 2017, Dassault Systems, Waltham, MA, USA) to investigate the behaviour of cementless hip implants developed from titanium, hydroxylapatite (Ha), and CaCO 3 powder using powder metallurgy technique.Case studies of two femoral fractures were considered with the implant replacement.It comprises a layer of spongy or hard cancellous bone in the  intramedullary area and a layer of cortical bone in the exteriors.The assembled model for the implants with the femur bone is shown in Fig. 4, which was modelled using the same software to prepare the mesh, define contact, and establish the boundary conditions.

Material properties
The Ti-Ha-CaCO 3 biocomposites used in this study are dense structure biocomposite (DSB) and graded Structured Biocomposite (GSB), and their required properties, including Young Modulus (E), Poisson's ratio (r) and density (p) were experimentally obtained as shown in Table 1 as obtained from Ibrahim et al. [11] findings.The cortical bone properties in Table 1 were obtained from Anderson and Madigan [15].

Loading and boundary conditions
The natural boundary conditions of the femur are complex and involve various forces and constraints acting at the proximal and distal ends and along the shaft.These conditions are essential for understanding the biomechanical behaviour of the femur under physiological loads.They are crucial for designing biomedical implants, conducting finite element analysis, and ensuring the stability and functionality of orthopedic treatments.Everyday human activities such as standing and jumping that reflect real-world conditions were considered boundary conditions, and equivalent human weight was taken as the applied static load on the femur head, as established by He et al. [16].The implant head and stem were bonded using a process called freeze contact, in which the secondary node is frozen with respect to the master surface.Additionally, freeze contact was used to secure the implant stem to the femur bone, allowing the transfer of compressive stresses.The femur's lower truncated end was set with fixed support, and static structural load was applied to the femur head for the static structural analysis, following the ISO 7206-4 standard.Loading of 2,300 N was applied to the stem head in compliance with ASTM F2996-13.The load is applied in a perpendicular direction on the femur head bottom section.

Generation of mesh
Utilizing SOLIDWORKS/Solidmesh simulation was selected to generate free mesh based on 10-node blended curvature-based mesh.For quality mesh to be achieved, the convergence of the simulation was conducted by varying mesh size from 7 to 1 mm with obtained max Von-Mises stresses increasing until stable max Von-Mises stresses were achieved at a mesh size of 3-1 mm, which is in agreement refined mesh of Aghili et al. [17].Therefore, the 3 mm mesh size was considered the optimum mesh for the simulation, and the details of the meshes for all models are given in Table 2.

Structural analysis of biocomposite using FEA
For the simulation convergence, the model's mesh information is given in Table 2.The information includes the node, element, and mesh size obtained remained almost the same for both assemblies of bone and the two design implant materials, which can be attributed to the similar input properties like Poisson's ratio and Young Modulus utilized to simulate the static structure of biocomposites as reported by Taqriban et al. [6].Figures 5 and 6 show the mesh model for the two models considered in this study.
The SOLIDWORKS simulation for Static Structural analysis shows the displacement (deformation), equivalent strain, and von Mises stress in Fig. 5 a and b for the assembled stem and intramedullary rod implant with fractured femur model.The study models were designed to represent the best artificial femoral joint and intramedullary rod fixation, and the stress distribution was determined utilizing various implant materials.Figures 6,7,8,9,10,11,12,13,14,15,16,and 17 show the simulation results, including maximum, von Mises stress, displacement, and equivalent strains.
It was confirmed that the model complied with the requirements when the maximum von Mises bone stress was seen at the distal end, where the model was fixed, and there was zero movement of the same surface.The highest implant Von Mises stress for DSB biomaterial in the solid neck portion was 354.3 MPa.The maximal implant stress's value and position are consistent with the findings of earlier research by Hassan et al. [10].Observations reveal that the von Mises emphasis for the DSB biocomposites was 323.7 and 103.4 MPa for stem and intramedullary rod implants, respectively.The lowest equivalent strain and displacement for the intramedullary rod were obtained from DSB 15.23 mm and 0.003073, respectively.Similarly, the equivalent strains and displacements were obtained for the stem DSB (0.012 and 2.68 mm) and GSB (0.0092 and 5.542 mm) implant, respectively.It can be observed that the results of the highest von Mises stress, equivalent strain, and displacement from the assembly of stem-femur were obtained as 354.3 MPa from GSB, 0.012 from DSB, and 6.128 mm from PSB, respectively.
The max.Von Mises stresses for the biocomposites obtained in Figs. 6, 7, 8, and 9 for assembled stem and intramedullary rod with femur revealed substantially lower value to the yield stresses of common Ti-6Al-4V (851 MPa) implant [18].Von Misses stresses are more than the yield strength of 316 L steel but smaller than the yield strength of Ti-6Al-4V and Co-Cr alloy in the findings of static numerical analysis, even at the maximum postulated load magnitudes.However, the stress, strain, and displacement distribution occur mainly at the implant and femur bone interface.This agrees with Taqriban et al. [6] results with the 234 MPa, 0.00121, and 0.154 mm for stainless steel, respectively.
To ensure that new models can provide mechanical resistance to the physiological load, preclinical experiments must be conducted to determine the optimal geometry and biomaterials for implant replacement.It was discovered that the different femur designs used in the studies exhibited lower stress values on the stem than in some previously published papers.Additionally, Li et al. [14] noted that material parameter differences prevent numerical results from being compared between finite element studies.
It is found that with a relatively low strain of 5% obtained, the von Mises stresses are much higher than the material's initial yield strength.Thus, Milone et al. [19] obtained the maximum mises stress of 359.4 for Ti6Al4V porous and 385.1 MPa for solid Ti6Al4V implants.The improved Fig. 7 von-Mises stress result of simulated assembly of DSB stem and femur implant was examined using load standards from ISO7206-4 and ISO7206-6, and a maximum stress of 575 MPa was achieved [16].
Bittredge et al. [20].reported that Lattice 5 has an MVM stress of 389.6 MPa, while Lattice 2 and 3 have 37.91 and 37.81 MPa, respectively, despite being porous implants with lower stiffness and yield strength.However, high porosity caused low yield strength, as obtained for lattices 2 and 3.It may result in premature failure because a high pore in the implant structure reduces the load-bearing application ability [21].
Colic et al. [7] obtained Max.von Mises stress for slow working on a flat floor, climbing upstairs, climbing downstairs, and tripping are 256.3,361.2, 312.6, and 535.5 MPa with a maximum load of 2490, 3143, 6358, and 3417 N, respectively.This was the first study where the stress distribution of the composite hip stem was evaluated and compared directly with that of a standard metal hip joint.It has been reported that to prevent plastic deformation and improve functional stability, an implant needs to have a high yield strength [20].Human cortical bones have a compressive yield strength that ranges from 100 to 130 MPa, according to Morgan et al. [22].Therefore, the results showed that the studied biocomposite implants are suitable for replacing bone atrophy.
However, it has been reported that an increase in von Mises stress, strain, and deformation is precisely correlated with the static load, which depends on the nature of femur routine activities.Consequently, there will be a rise in the risk ratio of hip joint damage [8].This result validates the necessity of testing hip implant designs with varying loading scenarios [23].The applied load distribution between the stem and the femur is shown with the von Mises stresses experienced by the hip stems implanted femurs.The point load, walking load, and climbing load in solid stems have higher stress values of 324 MPa, 124 MPa, and 196 MPa, respectively.
However, to minimize the stress-shielding effects and keep the interfacial stresses below user-defined maximum stresses, the highest stress levels that intact and implanted femurs can endure are used to calculate the stress shielding signal (SSS).The SSS expression has been reported based on the hypothesis of bone remodelling, which depends on the strain energy density.However, Alkhatib et al. [23] established that the strain energy density is replaced with the max von Mises femur stress to quantify the SSS due to constant density (constant modulus) and low strain values.The SSS was calculated with expression in Eq. 1; where MSimp and MSref are the corresponding maximal von Mises stresses for the undamaged and implanted femurs.
The maximum stress of healthy femur bone was reported to occur at the femoral neck in the range of 17.2-39.5MPa [24], and this was confirmed with the numerical simulation conducted by Dash et al. ( 2013) obtained as 28.859 MPa, This phenomenon demonstrates how the unique geometry of the human femur bone serves as a crucial zone for biomechanical failure, as the stress distribution was on the stem neck.While utilizing the intramedullary rod for replacement, the critical zone was noticed on the femur shaft where the implant occupied.The stress shielding signal for the stem-femur model was 7.19 and 7.97 for the DSB and GSB, respectively.The SS for the intramedullary rodfemur model was obtained as 1.62 and 0.89 for the DSB and GSB, respectively.It has been established that higher SSS indicates a high-stress shielding effect on the implantbone structures [25].The standing load results obtained for the maximum SSS showed positive signal values for studied models, showing that the healthy femur stresses are minimized compared to those experienced with implanted femur [23].It can be observed that the stem-femur model exhibits high-stress shielding signals compared to the intramedullary rod-femur model, which indicates the DSB (7.19) is the most suitable among the studied stem-femur models and GSB (0.89) remain the suitable intramedullary rod-femur models with lowest stress shielding signals.The maximum displacements at both implant-femur assemblies' stem and distal part indicate that more shearing stress occurs in the structure as it experiences maximum displacement at the point of load application.These results agree with previous literature [26].
The maximum von stress, equivalent strain, and intramedullary rod and stem displacement values were less than those reported in earlier publications [7].This could result from the various biomaterials or femoral stem designs employed, as reported by Li et al. [26].The distributions of strain and stress on the intramedullary rods and femoral stems with various biocomposite designs show that the biocomposites can produce stress concentrations on the femoral neck and distal stem and reduce stress on the stem body.Therefore, to validate the results quantitatively, the magnitude of the VMS of all the developed biocomposite implants in this study is compared with the results obtained for implants by Colic et al. [7].

Validation of the simulation models
The biocomposite implant was validated with the maximum stress experienced by the installed metallic and other implant materials, as shown in Fig. 18.In comparing the obtained result of maximum von Mises stress (354.7 MPa) for the GSB-femur model and the DSB implant (323.7 MPa), The maximum stress of intact femur bone has been established to 75.7 MPa [3], and 582 MPa were obtained from FEA for 316 L stainless steel and 216 MPa for CF/PA-12 hybrid implant [27].The stress values of the biocomposites were lower than the results of some previously utilized metallic implants.However, the biocomposite's maximum stresses are higher than the hybrid implant and intact femur maximum stress.This may be due to the different designs of the femoral stem used.It has been established that it is impossible to compare numerical results between different finite element studies owing to differences in anatomy, meshing, loading conditions, and material parameters.The distributions of stress on the femoral stem with different cross-sections indicate that some cross-sections can reduce stress on the stem body yet also generate stress concentrations on the corner of the femoral neck and distal stem [23], making the results more controversial.
However, fracture repair along the femur shaft has been carried out over the years with metallic plates and screws, which usually resulted in carrying a large portion of the load by the metal implants compared to the bone, which still leads to stress shielding.The present study utilized intramedullary rod fixation with obtained maximum von Mises stresses of 74.11 MPa for the GSB intramedullary rod fracture repair and 103.4 MPa for the DSB implant, as shown in Fig. 19.The result was validated with the obtained 462.6 MPa peak stress carried by the hip implant after insertion for femur fracture repair along the shaft with plate and screw (Ebrahimi  Better agreement with previous results proves the accuracy and validity of the finite element model for the present study.This highlights the importance of appropriate load application on the model.Nevertheless, all results show that the maximum stress occurs at the inferior root of the femoral neck in the epiphysis and the middle of the femoral shaft, as concluded by Aghili et al. [24].

Conclusion
The studied model and simulation results effectively showed the behaviour of the DSB and GSB implants under loading as established by ASTM F2996-13 guidelines for standing and jumping boundary conditions.The max. von Mises stress (354.7 MPa) for the GSB-femur model and the lowest values for the DSB implant (323.7 MPa) were obtained.Meanwhile, the displacement with the most stable implant is DSB with the lowest displacement value of 2.68 mm.However, 74.11 MPa was obtained as acceptable Von Mises stress using GSB intramedullary rods for bone fixation.The displacement value of DSB shows the most stable implant with 15.23 mm.The equivalent strains for all the implants showed low strains as the highest strain (0.012) was obtained in the simulation of the assembly of the stem DSB-femur model.Low SS were obtained for the implant-femur models, indicating they are suitable for replacing bone for that loading.The DSB (7.19) is the most suitable among the studied stem-femur models, and GSB (0.87) remains the suitable intramedullary rod-femur model with the lowest SSS.The findings from this study suggest that the Ti-Ha-CaCO 3 biocomposite is a promising material for use in hip implants, and further investigations are warranted to evaluate the clinical efficacy of the implant in vivo.
Funding Open access funding provided by University of Johannesburg.

Fig. 4
Fig. 4 Assembled femur and stem joint (a) and intramedullary Rod (b) Femoral Implants

Fig. 5
Fig. 5 Mesh model for assembly a stem-implants with femur with loading; b Stem-intramedullary rod with loading

Fig. 8
Fig. 8 von Mises stress result of assembly of GSB intramedullary rod and femur

Fig. 15
Fig. 15 Equivalent strain result of simulated assembly of DSB stem and femur

Fig. 16
Fig. 16 equivalent strain result of assembly of GSB intramedullary rod and femur

Fig. 17 Fig. 18
Fig. 17 Equivalent strain result of assembly of DSB intramedullary rod and femur

Fig. 19
Fig. 19 Comparison of intact femur, metallic plat, and screw and studied implant materials for femur fracture repair

Table 1
Required properties of femur, dense titanium, and biocompos-

Table 2
The mesh information of both assembly and femoral stem and intramedullary biomaterials