Improve the performance of coated cemented hip stem through the advanced composite materials.

Design of hip joint implant using functionally graded material (FGM) (advanced composite material) has been used before through few researches. It gives great results regarding the stress distribution along the implant and bone interfaces. However, coating of orthopaedic implants has been widely investigated through many researches. The effect of using advanced composite stem material, which mean by functionally graded stem material, in the total hip replacement coated with the most common coated materials has not been studied yet. Therefore, this study investigates the effect of utilizing these two concepts together; FGM and coating, in designing new stem material. It is concluded that the optimal FGM cemented stem is consisting from titanium at the upper stem layers graded to collagen at a lower stem layers. This optimal graded stem coated with hydroxyapatite found to reduce stress shielding by 57% compared to homogenous titanium stem coated with hydroxyapatite. However, the optimal functionally graded stem coated with collagen reduced the stress shielding by 51% compared to homogenous titanium stem coated with collagen.

A new design of cemented stem using functionally graded materials (FGM).

One of the most frequent complications of total hip replacement (THR) is aseptic loosening of femoral component which is primarily due to changes of post-operative stress distribution pattern with respect to intact femur. Stress shielding of the femur is known to be a principal factor in aseptic loosening of hip replacements. Many designers show that a stiff stem shields the surrounding bone from mechanical loading causing stress shielding. Others show that reducing stem stiffness promotes higher proximal interface shear stress which increases the risk of proximal interface failure. Therefore, the task of this investigation is to solve these conflicting problems appeared in the cemented total hip replacement. The finite element method and optimization technique are used in order to find the optimal stem material which gives the optimal available stress distribution between the proximal medial femoral bone and the cement mantle interfaces. The stem is designed using the concept of functionally graded material (FGM) instead of using the conventional most common used stem material. The results showed that there are four feasible solutions from the optimization runs. The best of these designs is to use a cemented stem graded from titanium at the upper stem layer to collagen at the lower stem layer. This new cemented stem design completely eliminates the stress shielding problem at the proximal medial femoral region. The stress shielding using the cemented functionally graded stem is reduced by 98% compared to titanium stem.

Effect of coating thickness and its material on the stress distribution for dental implants.

Dental implants have been increasingly used to recover the masticatory function of lost teeth. It has been well known that the success of a dental implant is heavily dependent on initial stability and long-term osseointegration due to optimal stress distribution in the surrounding bones by the concept implant surface coating. Hydroxyapatite (HAP), as a coating material, has been widely used in dentistry due to its biocompatibility. Some investigations show a benefit of coating dental implants with HAP, and others concluded that HAP coating reduces the long-term implant survival. Therefore, the aim of this investigation is to design a new functionally graded dental implant coating, as well as studying the effect of coating thickness on the maximum von Mises stresses in bone adjacent to the coating layer. The gradation of the elastic modulus is changed along the longitudinal direction. Stress analysis using a finite element method showed that using a coating thickness of 150 microm, functionally graded from titanium at the apex to the collagen at the root, will successfully reduce the maximum von Mises stress in bone by 19% and 17% compared to collagen and HAP coating respectively.

Improved design of cementless hip stems using two-dimensional functionally graded materials.

Increasingly, it is acknowledged that bone resorption around cementless hip implants may cause future problems. The solution is frequently sought in reducing implant stiffness. However, this confronts the designer with a true design conflict: how to reduce the stiffness without excessively loading the proximal bone/prosthesis interface? The aim of this work is to improve the design of cementless hip stem material, using two-dimensional (2D) functionally graded material (FGM) concept in order to solve the above problems. Two models were used in this analysis, using three materials with different elastic moduli, E(1), E(2), and E(3). In model I, the elastic moduli E(1) and E(2) gradually change along the upper stem surface, while E(3) is maintained constant along all the lower surface of the stem. However, in model II, the elastic moduli E(1) and E(2) gradually change along the lower stem surface, while E(3) is maintained constant all along the upper stem surface. It is found that the recommended model is model I, which has three distinct materials of hydroxyapatite, Bioglass, and collagen. The recommended design of 2D FGM is expected to reduce the stress shielding by 91% and 12%, respectively, compared with titanium stem and model II of FGM. It is found that this new design reduces the maximum interface shear stress at the lateral and medial sides of the femur by about 50%, compared with titanium stem. Furthermore, the maximum interface shear stress is reduced by about 17% and 11% at the lateral and medial sides of the femur, respectively, compared with that of model II of FGM.

Design optimization of cementless metal-backed cup prostheses using the concept of functionally graded material.

Metal backing has been widely used in acetabular cup design. A stiff backing for a polyethylene liner was initially believed to be mechanically favourable. Yet, recent studies of the load transfer around acetabular cups have shown that a stiff backing causes two problems. It generates higher stress peaks around the acetabular rim than those caused by full polyethylene cups and reduces the stresses transferred to the dome of the acetabulum causing stress shielding. The aim of this study is to overcome these two problems by improving the design of cementless metal-backed acetabular cups using the two-dimensional functionally graded material (FGM) concept through finite-element analysis and optimization techniques. It is found that the optimal 2D FGM model must have three bioactive materials of hydroxyapatite, Bioglass and collagen. This optimal material reduces the stress shielding at the dome of the acetabulum by 40% and 37% compared with stainless steel and titanium metal backing shells, respectively. In addition, using the 2D FGM model reduces the maximum interface shear stress in the bone by 31% compared to the titanium metal backing shell.

Design of functionally graded dental implant in the presence of cancellous bone.

In a previous work by the author [Hedia HS, Mahmoud NA. Biomed Mater Eng 2004;14(2):133--143], a functionally graded material (FGM) dental implant was designed without cancellous bone in the model. In this investigation, the effect of the presence of cancellous bone as a thin layer around the dental implant was investigated. It is well known that the main inorganic component of natural bone is hydroxyapatite (HAP) and that the main organic component is collagen (Col). HAP implants are not bioabsorbable, and because induction of bone into and around the artificially made HAP is not always satisfactory, loosening or breakage of HAP implants might occur after implantation in the clinical application. The development of a new material that is bioabsorbable and that has osteo-conductive activity is needed. Therefore, the aim of the current investigation was to design an implant, in the presence of cancellous bone as a thin layer around it, from FGM. In this study, a novel biomaterial, Col/HAP, as a FGM, was developed using the finite element and optimization techniques that are available in the ANSYS package. These materials have a self-organized character similar to that of natural bone. The investigations have shown that the maximum stress in the cortical bone and cancellous bone for the Col/HAP functionally graded implant has been reduced by about 40% and 19%, respectively, compared with currently used titanium dental implants.

Comparison of one-dimensional and two-dimensional functionally graded materials for the backing shell of the cemented acetabular cup.

Among the factors that have been suggested as contributing to the failure of a total joint replacement are stress shielding and the subsequent bone resorption. Recent studies have shown that when a backing shell made from a Ti alloy is used, high stresses are generated in the cement at the edges of the cup, and low stresses are generated at the dome of the bone in the acetabulum; thus, the bone at the dome suffers stress shielding and the cement edge suffers high stresses. The aim of this study was to investigate the effect of using a functionally graded material (FGM), instead of Ti alloy, for the backing shell (BS) on the stress distribution in the BS-cement-bone system. Finite-element and optimization techniques were used to obtain the optimal distribution of materials in the tangential direction only of the backing (1D FGM) as well as in the tangential and radial directions of the backing (2D FGM). It was found that the stress distribution in the BS-cement-bone system was about the same, regardless of whether the BS was fabricated from a 1D or 2D FGM. The stress-shielding factor in the bone at the dome of the acetabulum and the maximum von Mises stress in cement at the cement interfaces for 1D and 2D FGM were reduced by about 51%, 69%, and 50%, respectively, compared to the case when the shell was fabricated from a Ti alloy. The optimal elastic modulus of the 1D FGM was obtained with the materials graded from HA at the dome of the acetabulum to a Ti alloy at the rim of the shell. The optimal elastic modulus of the 2D FGM was obtained with the materials graded from Ti alloy at the right edge of the rim, to Bioglass 45S5 at the left edge of the rim, and to HA at the dome of the shell.

Effect of cancellous bone on the functionally graded dental implant concept.

In a previous work by the author [H.S. Hedia and M. Nemat-Alla, Design optimization of functionally graded dental implant, submitted to be published in the J. Bio-Medical Materials and Engineering], a functionally graded material dental implant was designed without cansellous bone in the model. In this investigation the effect of presence cancellous bone as a thin layer around the dental implant was investigated. It is well known that the main inorganic component of natural bone is hydroxyapatite (HAP) and that the main organic component is collagen (Col). Hydroxyapatite HAP implants are not bioabsorbable, and because induction of bone into and around the artificially made HAP is not always satisfactory, loosening or breakage of HAP implants may occur after implantation in the clinical application. The development of a new material which is bioabsorbable and which has osteoconductive activity is needed. Therefore, the aim of the current investigation is to design an implant, in the presence of cancellous bone as a thin layer around it, from functionally graded material. In this study, a novel biomaterial, collagen/hydroxyapatite (Col/HAP) as a functionally graded material (FGM), was developed using the finite element and optimization techniques which are available in the ANSYS package. These materials have a self-organized character similar to that of natural bone. The investigations have shown that the maximum stress in the cortical bone and cancellous bone for the Col/HAP functionally graded implant has been reduced by about 40% and 19% respectively compared to currently used titanium dental implants.

Design optimization of functionally graded dental implant.

The continuous increase of man's life span, and the growing confidence in using artificial materials inside the human body necessities introducing more effective prosthesis and implant materials. However, no artificial implant has biomechanical properties equivalent to the original tissue. Recently, titanium and bioceramic materials, such as hydroxyapatite are extensively used as fabrication materials for dental implant due to their high compatibility with hard tissue and living bone. Titanium has reasonable stiffness and strength while hydroxyapatite has low stiffness, low strength and high ability to reach full integration with living bone. In order to obtain good dental implantation of the biomaterial; full integration of the implant with living bone should be satisfied. Minimum stresses in the implant and the bone must be achieved to increase the life of the implant and prevent bone resorption. Therefore, the aim of the current investigation is to design an implant made from functionally graded material (FGM) to achieve the above advantages. The finite element method and optimization technique are used to reach the required implant design. The optimal materials of the FGM dental implant are found to be hydroxyapatite/titanium. The investigations have shown that the maximum stress in the bone for the hydroxyapatite/titanium FGM implant has been reduced by about 22% and 28% compared to currently used titanium and stainless steel dental implants, respectively.

Stress and strain distribution behavior in the bone due to the effect of cancellous bone, dental implant material and the bone height.

The stress and strain distribution in the bone surrounding a dental implant have been analyzed using the finite element and optimization techniques. The effect of removing cancellous bone completely or not was investigated. Two models were used, the first model without cancellous bone and the second with it. The elastic modulus of the implant material and the length of the implant neck or the height of bone surrounding the implant were used as design variables in the two models. In the first model a higher level of stress in the cortical bone surrounding the neck of the implant was found. While in the second model, it was found surrounding the tip of the implant. The result indicates that the stress concentration factor in the bone of the first model is reduced by 30% compared to the initial design. However, when the implant was surrounded by sleeve of cancellous bone (second model) the stress concentration is reduced by 16% for cortical bone and 15% for cancellous bone. This reduction help to reduce fatigue failure and bone resorption.

Parameteric optimisation of materials for acetabular cup.

This paper describes a method of parametric optimisation to determine the optimal stiffness characteristics of cement, metal backing and UHMWPE (Ultra High Molecular Weight Polyethylene) materials, which minimises the probability of fatigue fracture of cement at all interfaces with the metal backing and the bone, while limiting the amount of bone resorbed. The parameters describing the elastic moduli of cement, metal backing and UHMWPE were considered as design variables. The method was applied to an axisymmetric finite element model of acetabular cup in combination with an optimisation procedure using the ANSYS program. Young's moduli of about 0.63, 207 and 0.72 GPa are optimal materials for cement, metal backing (MB) and UHMWPE, respectively. These characteristics decreased fatigue notch factor Kf in cement by 8.2 and 10.6% and also decreased the maximum von Mises stress in cement by 21 and 27% at cement/bone and cement/metal backing interfaces, respectively. The optimal design reduces the probability of fatigue fracture of cement at all interfaces with the bone and the metal backing while limiting the amount of bone resorbed as a result of increasing von Mises stress and Kf in the central bone of the acetabulum by 34 and 30.6%, respectively.

Stiffness optimisation of cement and stem materials in total hip replacement.

It is acknowledged that bone resorption and fatigue fracture of cement in total hip replacement may cause feature problems. The solution is frequently sought associated with the stiffness of cement and stem. The purpose of this paper is firstly to describe the effect of changes in modulus of elasticity of the cement material for the implanted prosthesis on the fatigue notch factor (Kf). The paper further describes a method of numerical optimisation to determine the optimal stiffness characteristics of cement and stem materials, which minimises the probability of fatigue fracture of cement at all interfaces with the stem and the bone, while limiting the amount of bone resorbed. The parameters describing the elastic moduli of cement and stem were considered as design variables. The method was applied to an equivalent 2D finite element model of femoral hip replacement in combination with an optimisation procedure using the ANSYS program. The results of the first study suggest that lower modulus of elasticity of cement material decreases Kf in the cement at all interfaces and proximal bone while higher values increase Kf. For the second aim, Young's moduli of about 0.6 and 22 GPa are optimal for cement and stem materials, respectively. These characteristics decreased the probability of fatigue fracture of cement at all interfaces with the stem and the bone as a result of decreasing Kf in cement at all interfaces, while limiting the amount of bone resorbed as a result of increasing Kf in the proximal bone.

Shape optimisation to maximise crack initiation time.

Most of machine parts, artificial joints and their components used for human body contain fillets, which connects two different diameters or widths. These fillets may cause a failure for these components. So, in this paper an optimisation method is presented which calculates and predicts optimal shape of the longitudinal profile for transition length of a tension bar, which connects two different diameters, using the ANSYS program. This method is applied to maximise the crack initiation time by minimising the fatigue notch factor Kf. A specified program has been developed to calculate the fatigue notch factor using Ansys Parametric Design Language (APDL) which is associated to the ANSYS Package. It was observed that the maximisation of crack initiation time with higher value of the statistical parameter, k, is also a minimisation of stress concentration factor, kt, at the same time.A tension bar with four different transition length, t (t/d=0.333, 0.4167, 0.5 and 0.583) is considered. The crack initiation time of the optimal shape for all cases was increased by 17%, 12%, 9% and 7%, respectively. The stress concentration factor was reduced by 20%, 17%, 15% and 12.5%, respectively.

Shape optimization of metal backing for cemented acetabular cup.

Stresses are generated in implant materials and bone, and at their interfaces. These stresses may affect the structural properties of the implant/bone system, or bring it to failure at some time in the postoperative period. Due to these stresses, acetabular cup loosening becomes an important problem for long term survival of total hip arthroplasty. It was found that metal backing would tend to reduce stresses in the underlying acrylic cement and bone. Yet, recent studies of load transfer around acetabular cups have shown that metal backing generates higher stress peaks in cement at the cup edges, while generates lower stress peaks in bone at the central part of acetabulum (dome), thus the bone at the dome becomes more stress shielded. In this study a numerical shape optimization procedure in combination with an axisymmetric finite element model was used in order to optimize the shape of a stainless steel metal backing shell. The design was to minimize fatigue notch factor in cement along cement/bone and cement/metal backing interfaces in order to prevent failure of cement mantel and loosening of acetabular components, at the same time increasing fatigue notch factor in bone at the center of acetabulum to prevent stress shielding. The results of this study indicate that cemented acetabular cup designs can be improved by using metal backing shells of non-uniform thickness, thick at the dome and thin at edges. Fatigue notch factor in cement was reduced by 2.3% at cement/metal backing interface and increased by 1.3% in the central bone of acetabulum. Von Mises stresses in the cement edge were reduced by 17.8% and 19.3% along cement/bone and cement/metal backing interfaces, respectively. Thus the optimal design will reduce the possibility of fatigue fracture of cement and decrease the stress shielding effect and the likely incidence of bone resorption, whereby extend the expected life of the prostheses.

The effect of elastic modulus of the backing material on the fatigue notch factor and stress.

In cemented acetabular cup design it is acknowledged that bone resorption and fatigue fracture of cement may cause the most common problems after total hip replacement. Previous studies have optimized the shape of metal backing (MB) shell used in cemented acetabular components in order to minimize the fatigue notch factor (Kf) in cement, whilst at the same time maximizing Kf in bone at the central part of acetabulum to prevent stress shielding and subsequent bone resorption [1]. The optimal shape was found to be thin at the edges and thick at the dome. The present study describes the effect of changing the elastic modulus of the backing material on Kf and stresses as predicted by the initial shape of the backing shell of (3 mm) thick, and the optimized backing shape of non-uniform thickness in order to find the optimal material for the backing shell. It is recommended to use a backing shell material with elastic modulus equals 70 GPa (which can be readily attained using a fiber reinforced polymer composite). It is shown that such a material will decrease the fatigue notch factor and the stresses in cement at cup edges, at the same time it will increase the stresses and the fatigue notch factor in bone at the central part of acetabulum. Thereby, reducing the possibility of fatigue fracture of cement, whilst at the same time decreasing the stress shielding effect and the resulting bone resorption. The effect of lower bone resorption and lower probability of fatigue fracture of the cement will also reduce the incidence of loosening and premature revision operations.

Material optimisation of the femoral component of a hip prosthesis based on the fatigue notch fatigue approach.

Previous studies have optimised the shape of a cemented stainless steel stem in order to minimise the fatigue notch factor Kf in the cement whilst at the same time maximising Kt in the proximal medial bone to prevent bone resorption [1]. The present study firstly describes the effect of changes in the modulus of elasticity of the stem material for both the original Charnley stem and the optimised shape on Kf as predicted by a 2D finite element (FE) model of the implanted prosthesis. The paper further describes a method for parametric optimisation to determine the best material properties of a layered composite femoral stem consisting of a core material (stainless steel) and an outer layer of a different material, the elastic modulus of which is used as a design variable. The overall objective of the optimisation was to maximise Kf in the proximal bone whilst at the same time constraining Kf at all cement interfaces to be no greater than its initial value. The results of the first study suggest that Young's moduli of about 145 and 210 GPa are optimal for the monolithic Charnley and optimised stems, respectively. A composite prosthesis with a layer of modulus 31 GPa added to the optimised stainless steel stem in the proximal region only was found to significantly increase the stresses in the proximal bone and reduce Kf in the cement whilst retaining the advantages of an outer stem profile very similar to that of the original Charnley prosthesis.

A method for shape optimization of a hip prosthesis to maximize the fatigue life of the cement.

The long term success of total joint replacement can be limited by fatigue failure of the acrylic cement and the resulting disruption of the bone-cement interface. The incidence of such problems may be diminished by reduction of the fatigue notch factor in the cement, so that stress concentrations are avoided and the fatigue crack initiation time maximized. This study describes a method for numerical shape optimization whereby the finite element method is used to determine an optimal shape for the femoral stem of a hip prosthesis in order to minimize the fatigue notch factor in the cement layer and at interfaces with the bone and stem. A two-dimensional model of the proximal end of a femur fitted with a total hip prosthesis was used which was equivalent to a simplified three-dimensional axisymmetric model. Software was developed to calculate the fatigue notch factor in the cement along the cement/stem and cement/bone interfaces and in the proximal bone. The fatigue notch factor in the cement at the cement/stem interface was then minimized using the ANSYS finite element program while constraining the fatigue notch factor at the cement/bone interface at or below its initial level and maintaining levels of stress in the proximal bone to prevent stress shielding. The results were compared with those from other optimization studies.

Effect of FE idealisation, load conditions and interface assumptions on the stress distribution and fatigue notch factor in the human femur with an endoprosthesis.

The load transferred through the hip joint is one of the major forces occurring in the human body. After the replacement of this joint in THR arthroplasty, the load is transferred through the implant to the femoral bone. Loosening of the fixation of the implant and the fatigue failure of prosthetic stems create problems for both patient and surgeon. Both problems can be reduced by the use of Finite Element (FE) analysis to predict stresses and fatigue lifes but the results are sensitive to assumptions regarding the loading conditions and the idealisation of the components. Consequently the stress distributions and resulting fatigue notch factors in the human femur with an endoprosthesis have been determined for different assumptions regarding the form of the idealisation, the load conditions, and the interface conditions. The FE results show that a realistic loading condition without a tension banding force always produces the highest fatigue notch factor and von Mises stresses. An equivalent 2D plane stress model obtained by varying the thickness is likely to give more realistic stresses because it predicts more realistic strains than other 2D approximations. The full bonded interface is a satisfactory approximation for the real interface conditions because it predicts stress distributions of the correct form without excessive stress concentration.

Shape optimisation of a Charnley prosthesis based on the fatigue notch factor.

The femoral component of the artificial hip joint implanted in a patient's femur is subjected to a complex set of forces exerted due to normal life activities. It is thought that high values of stress in the cement in a cemented prosthesis can lead to fractures of the cement mantle and loosening of the stem. The incidence of such problems may be diminished by reduction of the fatigue notch factor in the cement, such that stress concentrations are avoided and the crack initiation time maximised. This study describes a method for numerical shape optimisation to determine an optimal shape for the femoral stem of a hip prosthesis in order to minimise the fatigue notch factor in the cement layer at the interfaces with the bone and stem whilst at the same time maintaining or increasing the stress levels in the proximal medial femoral bone to help prevent stress shielding. The method when used to optimise the shape of a stainless steel Charnley stem was found to be extremely efficient and effective. The resulting optimal shape was heavily waisted in the proximal region below the neck but distally was quite similar to the original design. The fatigue notch factors in the cement were reduced by 16% and 19% for medial and lateral cement/stem interfaces, respectively, and by 8% and 2% for the corresponding cement/bone interfaces. The fatigue notch factor in the proximal medial bone was increased by 57% which indicates that the general stress level in this region is markedly increased. Thus the optimised design should increase the fatigue life of the cement and at the same time reduce stress shielding in the proximal bone. Both of these effects may help prevent loosening of the femoral component and hence reduce the need for early revision operations.