In vitro assessment of Function Graded (FG) artificial Hip joint stem in terms of bone/cement stresses: 3D Finite Element (FE) study
© Fouad et al.; licensee BioMed Central Ltd. 2013
Received: 6 November 2012
Accepted: 8 January 2013
Published: 16 January 2013
Stress shielding in the cemented hip prosthesis occurs due to the mismatching in the mechanical properties of metallic stem and bone. This mismatching in properties is considered as one of the main reasons for implant loosening. Therefore, a new stem material in orthopedic surgery is still required. In the present study, 3D finite element modeling is used for evaluating the artificial hip joint stem that is made of Function Graded (FG) material in terms of joint stress distributions and stem length.
3D finite element models of different stems made of two types of FG materials and traditional stems made of Cobalt Chromium alloy (CoCrMo) and Titanium alloy (Ti) were developed using the ANSYS Code. The effects on the total artificial hip joint stresses (Shear stress and Von Mises stresses at bone cement, Von Mises stresses at bone and stem) due to using the proposed FG materials stems were investigated. The effects on the total artificial hip joint system stresses due to using different stem lengths were investigated.
Using FG stem (with low stiffness at stem distal end and high stiffness at its proximal end) resulted in a significant reduction in shear stress at the bone cement/stem interface. Also, the Von Mises stresses at the bone cement and stem decrease significantly when using FG material instead of CoCrMo and Ti alloy. The stresses’ distribution along the bone cement length when using FG material was found to be more uniform along the whole bone cement compared with other stem materials. These more uniform stresses will help in the reduction of the artificial hip joint loosening rate and improve its short and long term performance.
FE results showed that using FG stem increases the resultant stresses at the femur bone (reduces stress shielding) compared to metallic stem. The results showed that the stem length has significant effects on the resultant shear and Von Mises stresses at bone, stem and bone cement for all types of stem materials.
It is well known that the fully understanding of stresses’ distribution in the hip joint is useful for both pre-operative planning and post operative rehabilitation. The short and long term behavior of the total hip joint replacement is dependent on obtaining optimal stresses’ distribution within the bone implant construct [1–3]. The structure, shape and material are the three main factors considered in the design of the hip prosthesis [4–7]. Initially, the artificial hip joint implants are used in orthopedic surgeries without prior pre-clinical testing that may lead to unsatisfactory clinical results. The pre-clinical testing helps in improving the clinical performance of the total hip joint replacement and limiting the possibility of joint revision operations .
The previous results indicated that if the design of hip joint stem resulted in high stresses in the fixation areas of the prosthesis, the joint fracture in short term or fatigue fracture in long term is quite likely to occur [9, 10]. Also, the presence of non uniform stresses’ distribution around the implanted stem will result in changes in bone density and shape . These non-uniform stresses’ distribution can cause the fixation to be altered as the bone density changes. For these reasons, many experimental and analytical studies have been implemented to optimize the stresses distribution in total hip joint replacement through the modification of stem design parameters [12–16]. These results indicated that the bone cement material, stem shape and material have great effects on the total hip joint short and long term performance. Chenm WP et al.  investigated the roles of bone cement in different fixation configurations of total hip arthroplasty initial stress shielding. Three different configurations of cement fixation: cement less, proximally-cemented and fully-cemented fixations were used in their study. The results revealed that, less stress shielding was found for femur with fully-cemented fixation. Takafumi U. et al.  studied the effects of bone cement type on the resulting stress shielding on the total hip joint replacement. Two types of bone cement; i.e. PMMA and Bioactive bone cement, were used. The results indicated that some bone resorption was observed in the use of bioactive bone cement 24 months after operation. However bone resorption was not observed 24 months after operation in the use of PMMA. Sabatini and Goswami  investigated the effect of stem cross section and material on the resultant Von Mises stresses on the stem designated locations. Stainless Steel (SS316L), Cobalt Chromium alloy (CrCoMo) and Titanium alloy (Ti 6Al 4 V) were used as stem materials. The results indicated that using high modulus stem created high stresses at the implant distal end while using low modulus stem created high stresses at the implant proximal end. These high stresses are known to affect the short and long term performance of the artificial hip joint.
As can be seen, the hip joint stem is usually made of SS, CrCoMo and Ti alloys. These materials have high modulus compared to the surrounding bone. This modulus mismatching is one of the main reasons for joint loosening due to the changes in the surrounding bone density (stress shielding). Therefore, the need for special stem material with suitable mechanical properties is essential for improving the performance of the total joint replacement. It is known that the Function Graded Material (FGM) can provide a reasonable compromise in terms of the properties of materials that would not be achieved otherwise. This is because the microstructure of the FGM is inhomogeneous and changing continuously in space. In comparison to conventional composite materials, FGMs exhibit a progressive change in composition, structure, and properties as a function of position within the material. With tailored design, the mechanical performance of FGMs can be superior to that of the conventional composite with a uniform composition. Wide varieties of available processes have been reported for FGM fabrication, such as plasma spraying, powder metallurgy, physical vapor deposition (PVD), and so on [19, 20].
Hedia HS et al. [21, 22] studied the effect of using 2D-FG Material cement less hip stem material in the reduction of bone resorption around cement less hip implants. The results indicated that the recommended FG stem reduced the stress shielding and reduced the maximum interface shear stress at the lateral and medial sides of the femur in comparison with Ti stem. Gong H. et al. identified the effects of using FG materials as cement less femoral stem on the functional adaptive behaviors of bone. The results showed that 2D-FG Material stem might produce more mechanical stimuli and more uniform interface shear stress compared with the stems made of other materials. Oshkour AA. et al.  developed 3D FE model of a FG femoral prosthesis. The model consisted of FG femoral prosthesis, bone cement, and femur. The results indicated that using FG stem resulted in uniform stress on the cement mantle layer and reduction of stress shielding in the joint.
Although FGMs have a wide range of applications from aviation structures to computer components and a promising potential in the biomedical usage, the use of FGMs is limited in the field of femoral prostheses. As such, the leading objective of this study was to develop new stem using FGMs in such a way that they reduce stress shielding on the hip prosthesis. This study intended to investigate the effects of using longitudinal FG material stem with low modulus at distal end and high modulus at proximal end in the improvement of the total hip joint replacement performance. In this work, the 3D FEM is used for investigating the modification on the hip joint stresses when using FG material as stem instead of traditional stems made of Ti and CoCrMo under static loading conditions. Also, the effects of stem length on the resultant stresses at the hip joint component will be considered.
Finite element model
Assigning materials properties to the FEM
E x is the Young’s modulus of the FG stem at any length, E d is the value of stem modulus at the distal end, E p is the value of stem modulus at the proximal end, L is the total length of the stem and X is the distance from the distal end towards the proximal end. It is evident that at X= 0, the value of E x equals E d and when X= L, the value of E x equals E p .
Mechanical properties of the materials used in the hip joint model
Co Cr Mo
Elastic Modulus GPa
220 to 50 GPa
110-to 10 GPa
Variation of FG stem modulus with stem length that used in the FE model
FG stem part No
The effects of mesh size and density on the predicted results are examined by increasing the number of elements until the predicted results become constant with increasing the mesh density. The contact interfaces between bone, bone cement and stem are represented in the finite element simulation as completely bonded surfaces. Also, the distal end of the femur is assumed to be completely fixed in all directions. The Finite Element Model loading conditions and constrains are shown in Figure 1.
The loading of the hip joint is based on the assumption that the patient body weight is 70Kg. The other initial conditions such as sex, age, activity etc. are neglected. This weight results in an equivalent hip joint load of 3KN which is applied as uniform pressure normal to the implant femur head  (Figure 1). This resultant load can be attributed to the fact that the typical gait human cycle generates forces in hip joint ranging from 4–6 times the body weight [14, 28].
Results and discussion
Effects of stem length on the joint stresses
Von Mises stresses at bone cement
Shear stresses at bone cement/stem interface
Von Mises stresses at bone
Effects of stem material on the joint stresses
Von Mises stresses at bone cement
From the above results, it can be concluded that the proposed FG stem results in significant reduction in the resultant Von Mises stresses at bone cement compared with Ti and CoCrMo stems. The significant decrease in bone cement stress due to using FG stems can be attributed to the fact that low stiffness FG material allows for more deformation and can absorb more strain energy compared to high stiffness ones. The resultant maximum Von Mises stresses induced in bone cement when using FG1 (14 MPa) is much lower than the yield strength of bone cement (PMMA), which is around 39 MPa . Similar trends are obtained by Senalp et al., where the resultant stresses on the artificial hip joint decrease when using Ti alloy stem instead of CoCrMo stems under static and dynamic loading conditions. Also, the results obtained by Sabatani et al.  indicate that the use of Ti6Al4V stem exhibits lower stresses than those obtained when using CoCrMo and SS stems. Finally, Rawal et al.  indicated that the use of stem material with lower modulus (E=117 GPa) would be more beneficial for artificial hip joint performance than the use of stiffer material such as steel (E=210 GPa).
Shear stresses at bone cement/stem interface
Von Mises stresses at bone
Stresses at stem
The results of Figure 15 also indicate that the maximum value of Von Mises stresses at stem decrease when using FG material stems instead of CoCrMo and Ti alloy stems at the same testing conditions. For example, for 135 mm stem, the stresses decrease from 88 MPa and 77 MPa to 43 MPa when FG1 stem is used instead of CoCrMo and Ti alloys stems respectively. For 220 mm stem, the stresses at the stem decrease by 57%,49 and 40% when FG1 stem is used instead of CoCrMo, Ti alloy and FG2 stems respectively. The decrease in stem stresses due to using low stiffness FG material can be attributed to the fact that a larger amount of load will be carried by the femur bone specially at the distal end (near fixation point). At this point, the FG stem has E=10 GPa, that is lower than the femur modulus (17 GPa). At case of Ti alloy or CoCrMo stems (that have very high modules compared to femur bone) larger amount of load will be carried by high stiffness stem resulting in a reduction of femur stresses and increases in stem stresses. For all kinds of stems, the maximum Von Mises stresses at stem take place at the distal end. There is no effect for stem length or material on the position of maximum Von Mises stresses at the stem or femur bone and also bone cement.
From the present FE results, it can be concluded that the maximum Von Mises stresses at the bone cement increase significantly with increasing the stem length while shear stresses decrease. For example, for CoCrMo stem, the maximum Von Mises stress at the bone cement increases by 50% when a stem of 190 mm length is used instead of 135 mm stem length. Similar trends have been also obtained for the Ti alloy, FG1 and FG2 stems with different lengths. The results also indicated that using FG stem (with low stiffness at stem distal end and high stiffness at its proximal end) resulted in a significant reduction in shear stress at the bone cement/stem interface and on the Von Mises stresses at the bone cement compared to CoCrMo and Ti alloy stems. The stresses’ distribution along the bone cement length when using FG material was found to be more uniform along the whole bone cement compared with other stem materials. Also, using FG stem increases the resultant stresses at the femur bone (reduces stress shielding) compared to metallic stems. Accordingly, the authors believe that using FG materials as a stem will be better for reducing the artificial joint stresses and stress shielding effects. This reduction will improve the artificial hip joint short and long term performance.
The Authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding the work through the research group project No. RGP-VPP-133”.
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