Then, thanks to Mr.Shaikh Karimulla, my best friend who received me in this IIT Guwahati and helped me a lot. Abhigyan Prasad, Mr.G.Senthil Raja, Dr.William Johnbosco, Mr.Antony, Mr.K.Rajesh, Mr.S.Manikandan,Mr.
Introduction
A metallic device (compression screw and side plate) holds the fractured bone in place while allowing the femoral head to move normally in the hip socket.
Total Hip Replacement
Correct operative technique for fixation
- Cementless Fixation
The cement layer also acts as an intermediate bumper between the very stiff metal in the total hip prosthesis and the weak skeleton. The components of the total hip replacement are pushed directly into the space made by reaming the skeleton and held there by the elastic force generated in the bone tissue.
Choice of Bio-Compatible Materials
The risk of pieces of bone marrow substance being pushed into the circulation during hard hammering of the total hip without cement in place. The risk of a fracture of the skeleton during surgery, when the surgeon blows the total bone too strongly into a small bone bed.
Satisfactory Design of Prosthesis
The force transferred to the bone depends on the stiffness ratio of the bone to the implant. Load transfer must occur through shear stress at the bone-implant interface.
Motivation of the Present Work
This shear stress repeats many cycles per day and determines the longevity of the bone-implant interface. Another important aspect of THR is wear of the acetabular cup due to constant friction between the cup and the spherical head.
Organisation of the Report
LITERATURE REVIEW
Introduction
Phillips [13] used a 2D FE model of the acetabulum consisting of four layers viz. plastic, cement, graft and bone using plane strain elements. Predicted performance of the new design was compared with that of conventional single modulus prosthesis.
Materials selection for hip prosthesis
To overcome the low stiffness of the matrix, carbon fiber reinforcement was proposed. 43] conducted a comparative study of the flow, plastic flow and fracture behavior of two implantable grades of UHMWPE (GUR 1120 versus 4150 HP).
Experimental analysis
- Experimental stress analysis
- Experimental fatigue analysis
- Experimental wear analysis
The wear mechanisms, including the size and shape of the wear particles, were in good agreement with those seen in clinical studies. 80] conducted a study of the influence of dimensional and microstructural parameters on the wear behavior of metal-metal hip implants of a Co-Cr cast alloy using laboratory simulation.
Finite element analysis of hip prosthesis
- Finite element analysis of femoral stem
- Finite element analysis of acetabular cup
95] produced a biomechanical study of resurfacing hip arthroplasty using finite element analysis of the femoral component. The stress levels in the femoral component of the total hip replacement were calculated using finite element analysis (FEA).
Fatigue analysis of hip prosthesis
Harrigan [99] carried out a study on the analysis of the stresses at the cement-metal interface using a large-scale linear finite element analysis. 30] described a new method for shape optimization of a hip prosthesis to maximize the fatigue life of the cement.
Contact and wear analysis
Laurian and Tudor [120] analyzed the influence of clearance in the pressure distribution in the total hip prostheses. Sfantos and Aliabadi [130] analyzed an application of the boundary element method for the simulation of wear in total hip replacement.
Objectives of the present work
This chapter provides a brief description of finite element modeling of a hip prosthesis to estimate the fatigue life of the prosthesis and the wear rate of the acetabular cup. In the second model, a conical stem with a spherical head and cementum layer together with cancellous and cortical bone was considered.
Finite element analysis of the prosthesis
- Characteristics of tetrahedral element
- Characteristics of contact elements
- Stress analysis of model 1
- Stress analysis of model 2
Ti6Al4V, CoCr alloy and UHMWPE in terms of stress shielding, a very simple model (Fig. 3.4) of the prosthesis was initially considered. The material properties of the cortical bone, the cancellous bone, the cement layer and the prosthesis are shown in Table 3.3.
Summary
It can be noted that maximum stress is developed in stepped stems as compared to conical and straight stems. Because insertion of the stem into the bone would be easier to perform using a conical stem than with a straight stem, conical stems were taken into account in the further analysis. It is also seen from Table 3.5 that the ratio of induced stress to yield stress is very high for UHMWPE and therefore it could not be used as stem material. It was decided to study only Ti6Al4V and CoCr materials as prosthetic materials. .
FATIGUE ANALYSIS OF HIP PROSTHESIS USING EQUIVALENT STATIC LOADING
Introduction
Residual Strength degradation model
- Algorithm for determination of K, b, c
- Validation of the algorithm
- Determination of fatigue parameter c, b, K for Ti6Al4V
- Determination of fatigue parameter c, b, K for CoCr alloy
- Determination of fatigue parameter c, b, K for UHMWPE
Since it is known from experimental data in Table 4.1 that the ultimate strength of graphite/epoxy ranges from 60 to 85 ksi [65], a random set of data has been generated assuming that the number of samples m is 14. These are given in Table 4.2. Based on the experimental data for the fatigue test (Yang and Liu) [65] from Table 4.3, the S-N curve was plotted for 25 samples as shown in Figure 4.1. Similar to equation (4.2) for static data, here too the first three central moments were calculated using the data in Table 4.2 using equations (4.5).
Constant values of c, b, K obtained for three different biomaterials, i.e. Ti6-Al4-V, CoCr alloy and UHMWPE using simulated data are listed in Table 4.11.
Evaluation of fatigue life
- Fatigue life of CoCr alloy prosthesis
The von Mises stresses thus obtained from the FE analysis were used in a residual strength degradation model to calculate the fatigue life of the prosthesis corresponding to different activities. Table 4.16 shows that with prosthesis model 1, the endurance life of the prosthesis for patient 1 is minimal, which corresponds to fast walking, which is. Figures 4.12 to 4.17 show the fatigue life curves for the CoCr prosthesis for three different patients in different activities.
It can be observed that in model 1 of the prosthesis, the fatigue life of the prosthesis for patient 1 is minimum corresponding to fast walking, which is cycles.
FATIGUE ANALYSIS OF HIP PROSTHESIS USING DYNAMIC LOADING
Introduction
Finite Element Analysis
Figures 5.8 to 5.13 show the fatigue life curves for three different patients corresponding to nine different activities for model 1 and model 2. It can be seen from the fatigue life curves as well as from table 5.10 that for model 1 the minimum life of fatigue corresponds to fast walking in patient 1, slow walking in patient 2, and standing 2-1-2 legs in patient 3. However, for model 2, the minimum fatigue life corresponds to standing 2-1 -2 for patient 1, slow walk for patient 2 and 2-1-2 stance for patient 3.
The Von Mises stresses obtained for the CoCr alloy prosthesis under dynamic loading for three patients with nine different activities are obtained using 3D FE analysis and are given in Table 5.12.
Comparison of performances of Ti6Al4V and Co Cr prosthesis
Summary
CONTACT AND WEAR ANALYSIS OF HIP PROSTHESIS
Introduction
Contact stress analysis and wear volume calculation using FEM
- FE modeling of acetabular cup and spherical head
The contact stress on the junction is the yield stress of the softer material p, the load on the junction. It applies to all junctions, and the total wear rate V is the sum of all v. This expression assumes that each junction forms a wear fragment, but in fact only a fraction K of the volume is removed, therefore the wear volume rate is given by
In equation (6.5), V is the wear volume away from the softer material, A is the contact area and σ is the normal contact stress.
Results and Discussions
- Wear of acetabular cup with Ti6Al4V head
- Wear Analysis with CoCr head with UHMWPE acetabular Cup
Tables 6.5 to 6.7 show the wear volume of the acetabular cup for four different patients with body weights of 860N, 980N, 800N and 702N respectively for different roughnesses of the acetabular cup ranging from 0.6 µm to 0.8 µm. Figures 6.7 to 6.9 show the variation in wear volume during 9 different activities for 4 different patients and the roughness of the acetabular cup ranging from 0.6 µm to 0.8 µm. In the case of a CoCr head, the contact stress, sliding distance and load-bearing volume for four different patients are shown below for nine different activities.
Tables 6.8 through 6.11 show the contact stress, sliding distance, and wear volume for four different patients and nine different CoCr head activities.
Development of an empirical relation for determination of wear volume
In this section, an attempt has been made to develop an empirical relationship that can be used to calculate the wear volume without actually performing the FE analysis. 73], the modified Archard's wear volume equation for softer material in terms of contact stress, sliding distance can be given by. In this section, an empirical relation for the wear volume in terms of body weight, head radius and roughness was developed through the maximum contact stress and sliding distance obtained by FE analysis for nine different activities. From fig 6.13 it can be observed that the wear volume is almost the same in both the cases and the maximum error will be around 5% for static analysis.
Consequently, the developed empirical relation can be used to find the wear volume for different patients knowing their weight and daily activity.
Summary
In this work, empirical relationships were proposed by which wear rate can be determined as a function of body weight, femoral head radius and roughness that can be used for the assessment of wear at the interface of acetabular cup and the femoral head in an artificial hip prosthesis. These relationships are developed for nine different daily activities viz. climbing down stairs, climbing stairs, fast walking, slow walking, normal walking, standing on 2-1-2 legs, standing up, sitting and kneeling postures. From the present study, it was also observed that the contact stress and wear volume is maximum in the downward pedaling activity and is minimum in the case of knee flexion activity.
General Conclusions
Specific conclusions
Although much previous work has been reported on estimating fatigue life using equivalent static loading, the current study found that equivalent static loading predicts a service life that is almost ten times higher compared to that obtained using actual dynamic load. It has been observed that Ti6Al4V as a prosthetic material leads to increased wear of the acetabular cup made of UHMWPE. The wear rate of the acetabular cup increases as the head radius and surface roughness increase.
Scope for future work
Bragdon, 2006, RSA Wear measurements with and without markers in total hip arthroplasty, Journal of Biomechanics. An elastoplastic finite element model for polyethylene wear in total hip arthroplasty, Journal of Biomechanics. Rullkoetter, 2005, Finite element simulation of early creep and wear in total hip arthroplasty, Journal of Biomechanics.
Callaghan, 1996, A sliding - distance - coupled finite element formulation for polyethylene bearing in total hip arthroplasty, Journal of Biomechanics.
APPENDIX
