Finite element analysis of mechanical behavior of SMP hip joint implanted in femur bone.
|Abstract:||Uncemented THR can present component fixation problem. The loosening of the prosthesis is one of the main complications leading to revision of THR surgery. Bending in prosthesis stem is a notable feature. The geometric shape of prosthesis has a major role on the stress transfer in supporting structures. The objective of the present study is to analyse the mechanical satibility of steel SMP prosthesis and compare it with its equivalent implant using finite element method.The effect of joint force due to body weight and motion on the interface of prosthesis and bone are evaluated using F.E.M. Finite element model of healthy cadeveric femur bone implanted with SMP hip joint has been developed. SMP is Sancheti Modular Prosthesis which is the hip joint prosthesis developed as indegenised product. 3D finite element model is validated with experimental measurements. The mechanical properties and elastic constants of cortical femur bone are evaluated from cadveric femur donors of Indian origin. Displacement and sinking of shaft is observed experimentally. The stresses and displacements of interface boundary of prosthesis and bone are analysed to observe their effect on stresses and displacements at the interfaces and within the cortical bone. The stress level in bone at inters face of prosthesis and bone are compared for SMP and imported prosthesis. This analysis provided an insight/comparison of the mechanical effect of an implanted SMP which was developed as indeginised product over available imported prosthesis of its kind. Results emphasized the role of newly developed SMP, which will be manufactured in India. It is expected to be available at lesser price than imported prosthesis. Larger number of patients in India is expected to be benefited due to development of this hip joint. Reduced surgery cost will facilitate many patients to go in for THR which is otherwise unaffordable.|
Finite element method
Smart materials (Mechanical properties)
Smart materials (Research)
Implants, Artificial (Mechanical properties)
Implants, Artificial (Usage)
Implants, Artificial (Research)
Prosthesis (Mechanical properties)
|Publication:||Name: Trends in Biomaterials and Artificial Organs Publisher: Society for Biomaterials and Artificial Organs Audience: Academic Format: Magazine/Journal Subject: Health Copyright: COPYRIGHT 2009 Society for Biomaterials and Artificial Organs ISSN: 0971-1198|
|Issue:||Date: May-August, 2009 Source Volume: 23 Source Issue: 1|
|Topic:||Event Code: 310 Science & research|
|Product:||SIC Code: 3842 Surgical appliances and supplies|
|Geographic:||Geographic Scope: India Geographic Code: 9INDI India|
The present study analyses the stability of the SMP prosthesis design during initial stages of THR surgery. SMP is Sancheti Modular Prosthesis which is the hip joint developed at SIOR (Sancheti Institute of Orthopedic Research) as indegenised Total Hip Replacement (THR) prosthesis. The effect of joint force on the bone-prosthesis composite due to body weight and different postures during different activities is evaluated using F.E.M. Loosening of hip joint is one of the major causes of failure of THR arthoplasty. Clinical and radiographic observations show that loosening generally occurs at the bone-cement interface. Fundamentally loosening which is characterized by bone resorption and interfacial micromotion depends upon the state of underlying bone, its quality ,the implant design ,the implantation technique as well as the condition of the surrounding muscular system. The hip joint reactive forces are found by various researchers to range from W to 4.9W where W is body weight depending upon the activity and bone frame of a subject. The joint loading on hip stem creates bending in hip stem. This flexure initiates loosening of joint. Consequently the durability of prosthesis is one important step in THR arthoplasty
The hip joint analysis is sensitive to joint force acting on it. It is necessary to know loading condition on hip joint. Foucher & Debra (1) has evaluated hip joint contact force and rotary torque during walking and stair climbing in THR patients. They establishes the hypothesis that peak contact forces and rotary torque are higher during stair climbing than during walking. The peak contact force is reported as 2.7 to 4.0 times body weight for walking and 2.7 to 3.7 for stair climbing. Rotatory torque ranged from 1.9 to 4.0% body weight*meters(BW*m) during walking and 3.5 to 6.0% BW during stair climbing. Edmund (2) advocates role of finite element models in orthopedics and recommends approaches to orthopedic problems should be analytical approach combined with experimental. Clingir et.al. (3) found out that while using FEM, the results show 0.5% difference between 3D anatomic and axisymmetric models, and 7%differance between 3D anatomic and 2D axisymmetric FE models, and recommends use of 3D FE models. Huiskies (4,5) also highlights significant difference in von mises stress values between 2D axisymmetric and 3D anatomic models. Crowninshield (6) gives guidelines for prosthesis design ,material of prosthesis design and the effect of collar on stability of prosthesis. He remarks Titanium femoral components have more flexible stem than steel. He also reports that titanium produces more stress in proximal region than steel stem; and is responsible for loosening of prosthesis. Weinans & Huskies (7) have developed finite element programme that describes of interface debonding, dependent upon interface stress criteria, and soft tissue interface interposition dependent on relative interface motions. Sauwen et al (8) uses straight forward mechanical model to obtain algebraic formulas to estimate primary stability of uncemented hip prosthesis. The resistance parameters derived from this model can be used to estimate the effect of stem geometry on the primary stability due to the actual load imposed upon the prosthesis. The derived formula is not validated using FEM or clinical observations. Huiskies (4) gives, two dimensional FEM analysis of uncemented prosthesis and concludes that load transfer mechanism in intramedullary hip prosthesis and stresses generated at the bone interface are highly susceptible to details of stem design. Tarr & Clark (9) has presented study of loosening of prosthesis using radiography, and has observed radiolucent lines of x-ray to detect interface problem. Barrington & Johnssson (10) in their study has discussed complications that occurred with fracture of the femur associated with hip replacement. Lee & Ling (11) have studied in brief stress related loosening. They have stated about no firm conclusion with regards to true place of uncemented implants being drawn till then. The study has pointed out that removing or revising these devices may present difficulties. They have also iterated that development of hip pro must have reasonable objectives. Neither surgeon nor engineer will ever make an artificial hip-joint which will last 30 years and at some point of time during this period enable the patient to play football.
Materials and Methods
Finite element analysis
Finite element analysis software ANSYS 10.0 is used for carrying out analysis. In this study static structural analysis is carried out using SOLID BRICK-PLANE 45 elements for 3D analysis. It is a 8 Node hexahedral element with lagarangian shape function. This element provides three translational degrees of freedom for each node. The material model allows to completely define orthotropic behavior of material by inputting appropriate elastic constants in three orthogonal directions X-Y-Z. As the study is for initial stability of prosthesis after THR surgery, the stem and bone are assumed to be fully bonded.
The joint force 4.9 times the body weight (maximum force during step climbing) is applied as a distributed force on the prosthesis head. The weight of subject is considered as 85 kg. For 3D analysis, all degrees of freedom at distal end of prosthesis are constrained. The translational degrees of freedom in saggetial plane are constrained as prosthesis shaft is free to rotate in frontal plane of the hip socket.. The 3D models of bone- prosthesis composite are made in ANSYS using average bone geometry. SMP with variation in geometrical designs and a Imported prosthesis are the cases studied .These geometrical designs are based on variation in stem geometry and provision of proximal collar and are indicated as SMP, SMP with collar, Slender stem SMP, Imported prosthesis.
Experimental investigation is done in two parts, Determination of material properties of femur bone, Investigation of stability of hip stem in intramedulary canal of cadeveric femur bone, using simulated motion of walking.
During first part, testing of cortical tissue from femur bone was carried out to know elastic properties of bone material. Standards of materials testing code like ASTM provide a source of mechanical testing techniques. Elastic properties of cortical bone are evaluated form test data and are listed in Table 1.
In second part of testing, bone prosthesis composite specimens are made. Bones used for the experiment are in the good condition. The implant is inserted in bone using operative techniques. Implant used to make specimen are SMP made of M.S. and imported prosthesis made of titanium alloy along with UHMWPE and stainless steel sleeve femoral cup. The sample is gripped into electrically operated hip simulator. The samples were loaded for 36000 walking cycles with frequency approximately one cycle per second. It accounts for walking motion for upto three months. The stability of prosthesis was checked after the test by reading the shift in radiolucent lines of bone -prosthesis interface, in X-Rays before and after the test.
[FIGURE 1 OMITTED]
The mechanical testing of cortical femur bone revels that cortical bone possesses orthotropic elastic properties. The ultimate tensile strength of cortical femur bone in the longitudinal direction is observed to be 135 MPa, the ultimate compressive strength was 205 MPa, and the shear strength was observed to be 67 MPa. The tensile strength of the femur in the transverse direction was obtained as only 53 MPa, as compared to 135 MPa in the longitudinal direction. The cyclic load tests performed on bone- prosthesis composite gives set of displacement observations measured by X-Ray radiograph at medial interface of bone and prosthesis interface. Displacement observations for the boneprosthesis interface using X-Ray was a difficult task especially in proximal region of SMP. This region showed larger shift in radiolucent line as shown in fig1. In some samples this zone of bone showed bending compression failure. It may be indication higher stress in proximal region. In sample with imported implant, radiolucent line was clearly observed for its shift. This may be attributed to better geometrical design that dissipates strain without localized stress concentrations in proximal zone. It appears that even after testing the specimen the bond in proximal region was better for longer duration. The consistent displacement readings indicate towards better behavior of bond before failure. The SMP sample without a collar and bulky proximal section produces more von mises stress in the adjoining bone. The slender stem in SMP reduces this stress to a very small extent. While collar provided to SMP shows considerable reduction in von mises stress as shown in fig2.
[FIGURE 2 OMITTED]
The medial region shows reduced interface shift. It also indicates pivotal region, which undergoes normal tension and compression in contact surface. But the consistency in results indicates that the interface was able to sustain this repetitive, reversal of stress.
Distal region results were also observed to be consistent and indicate that interface was good enough to sustain repeated load, Surrounding bone material was strong enough to take up the load that was transferred to bone from prosthesis. The simulated walking results are indication of better contact of bone and prosthesis in distal region.
Understanding the behavior of proximal medial and distal zone from displacement photographs before and after the sample was subjected to simulated walking cycle of particular duration, It may be said that medial and distal region effectively take part in load transfer from prosthesis to bone through interface under repeated cycles of loads. This region is observed to be sensitive to prosthesis design. While proximal region appears to be less elastic and prone to debonding of interface.
To evaluate the performance of the bone-composite prosthesis and evaluate the performance of the SMP geometry vis-a-vis imported prosthesis FEM results are compared. The Table 2 compares maximum stresses in bone due to joint force. It is found that bone orthotropy plays important role in resisting compressive loads and shear stresses in bone, this has impact on design of artificial joint since assumption of isotrophy underestimates the flexural stresses in bone. Providing slender shaft can be anatomically advantageous due to lesser bone loss and lesser bone resorption but design parameters do not allow for this shape and needs further revision in geometry.
Collar can reduce interface stresses as it provides larger seating on proximal bone during load transfer. SMP shows better resistance to flexural stress than imported prosthesis. It shows uniform increase in resistance along the length of prosthesis.
Suggested shape of the collared SMP model shows better performance as compared to imported prosthesis. The FEM results however show that orthotropy of bone material doesn't contribute in sustaining flexural stresses, and isotropic model ignores weakness of material in orthogonal direction. Material orthotrophy contributes in resisting shear stress induced in bone tissue. Isotropic model shows 10.25% larger flexural stresses than orthotropic flexural stresses. The collared SMP implant gives near about 62% stress shielding in proximal zone. Established the material properties of human bone under Indian environment enabled to test and evaluate the behavior of hip joint prosthesis. 3D analysis results show better agreement with experimental observation for displacement. Comparison of results of 3D FEM results obtained by Tarr & Clark (9) and axisymmeteric model validate the 3D FEM analysis showing better agreement than 2D analysis. SMP shows better resistance to flexural stress than imported prosthesis. It shows uniform increase in resistance along the length of prosthesis. Flexural stress in bone at interface of imported prosthesis shows large variation, this may be attributed to larger variation in radius of curvature of medial shaft for imported prosthesis. This nature of result suggests need for slender shaft.
Design of stainless steel SMP has lesser bone stress as compared to Titanium alloy imported prosthesis. Hence it should have better stability after surgery and expected to give more duration of service life as compared to other prosthesis. In the transient analysis joint force during gait cycle (one cycle of walking motion)is less than the peak load, therefore the stresses obtained are less than those obtained in the static analysis. Therefore transient analysis may not be necessary to predict the behavior of prosthesis when subjected to gait cycle.
There are various methods adopted by researchers for measuring micro motion, like ultra sound method, strain gauges placed on the surface of femur bone. Ultra sound method is based on apparent density of the medium through which the sound waves travel. Bone having not really consistent density, gives results with accuracy range that may vary with quite a large percentage and cannot read the micromotion changes with expected accuracy. One of the researcher who measured micro motion using strain gauges, could record strains on the surface of bone. So with this method it was not possible to read bone prosthesis interface strain. Here method adopted for micromotion measurement is from X-ray radiograph. The experimental results are plotted in the form of graph in fig 2. The comparative graphs of displacement on lateral side and medial side are plotted. For comparison the analytical results by FEM are also plotted along with experimental results. Some of the observations from this comparison are as follows. Lateral displacement of lateral side of prosthesis shows following things. The displacement in medial zone appears to be in well agreement for all samples.
The displacement of imported prosthesis is more than that of SMP in proximal zone. In medial zone imported prosthesis appears to displace less as compared to SMP.
More or less all sample have same nature of response in medial zone.
Proximal cancellious bone appears to show greater deformation while subjected to cyclic load. FEM results of lateral displacement of SMP and Imported prosthesis closely follow pattern of displacement observed experimentally. From lateral as well as medial side displacement results, the zone of larger micromotion is evident in proximal region. This can be called as stress shielding effect. This indicates of design improvement needed in proximal zone of prosthesis. The maximum Von mises stress in SMP-bone composite is 2.750 Mpa, which is 6.9% of the failure criteria for tensile failure stress. The Von mises stresses in SMP has shown a reduction up to 80%. Bending stress is about 80% in proximal and distal zones and up to 12% in medial zone. These reductions in stress levels in bone by providing a collar would result in reduction in post surgical pain. The prosthesis functions better as the maximum stress levels reached is about 1/12th of failure stress of bone thus stability of prosthesis in body post surgery is better.
Received 21 November 2008; Accepted 15 June 2009; published online 25 June 2009
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M.S. Kulkarni , S.R. Sathe , K.C. Sharma , K.H. Sancheti 
[1,2] Government College of Engineering, Pune, Maharashtra, INDIA 411004
 University of Pune, Maharashtra, INDIA 411016
 Sancheti Institute of Orthopedic Research,
Table 1: Elastic constants of Cortical Bone Modulus of Modulus of Direction Elasticity Poisons Rigidity N\mm2 ratio N\mm2 X 11737 0.22(y-z) 4010.2 Y 8755 0.20(X-z) 3647.9 Z 7896 0.22(X-y) 3262.81 Table 2: Comparison of stem design with respect to FEM results Max. Bending Model Description stress at Max. Shear prozimal stress zone (MPa) (MPa) 1 SMP 4.7 0.6385 2 SMP with collar 0.638 1.17 3 SMP with slender stem 8.132 0.9062 4 SMP with slender stem 6.636 2.048 and collar 5 Imported Prosthesis 3.872 2.44 6 Tarr and Clark 5.01 -- Max. Von mises stress (MPa) 1 2.57 2 2.58 3 17.22 4 17.488 5 16.33 6 --
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