The assignment provides a detailed analysis of femur models and boundary conditions, comparing the effects of mid-shaft fixation, distal condyle fixation, and joint constraint on strain and deflection. It also discusses the implications for physiological and biomechanical modeling.
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3rd-3rdCombined Musculoskeletal and Finite Element Modelling ….
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3. Review of musculoskeletal and finite element analysis of the proximal femur For the purpose of relief in hip-pain, the most successful procedure in terms of surgery accounts to be Total hip arthroplasty. It also proves to be helpful in the restoring-function with respect to joint and is further contributes in improving life quality (Bachmeier et al. 2001; Karrholm et al. 2008). Despite the factor of enhancement in population, a very large amount of patients are generally represented by even a small percentage of arthroplasty-surgeries that have failed. From a total of 71,400 hip replacement which occurred in the year 2008, nearly 6,600surgerieswentthroughrevision.Thishoweveroccurredintheengland& Wales(National Joint Registry 2009). This obtained data is higher than the one recorded in the year 2005 wherein from a total of 62,000 hip-replacements namely 5,800 revision were observed (National Joint Registry 2005) . With reference to this data, the amount of people in need of arthroplasty surgery is expected to increase (Birrell et al. 1999). There has beena significant increase in the weight (National Joint Registry 2009) of patients as an outcome of which their demographics have also changed. They are gradually becoming more young (Karrholm et al. 2008) thereby tending to be rather active. There are namely three categories wherein hip implants could be analysed. They are clinical-trials in case of patients, modelling in computational style and in-vitro lab tests. For the purpose of comparing three diversified implants, fixation methods, levels of activities contextualising patients and even certain types of surgical procedures, gait analysis & reviews that assess arthroplasty throughout the entire lifetime of patient can be utilised under clinical- studies which are non-invasive. On the contrary, invasive processes like radiostereometric analysis (RSA) wherein beads of tantalum are implanted as internal-markers around joint to be replaced, can be of use for assessing patients as well as their implants. In order to obtain details about the information regarding implant displacement , RSA can be referred. However, there is an associated limitation as well. Only a selected set of patients would be taken in consideration since the processes is of invasive nature & additionally expensive as well. Due to the studying of patients within in-vivo, very dependable outcomes are obtained by the aid of clinical study. Despite this, the flexibility of this clinical study is still in question due to the involvement of ample confounding-variables that have potential. Adding more detail, in every study, only one variable would be taken in consideration for investigation. In order to provide significant
outcomes of research based on statistics, large amount of patients are required as an additional requirement. For the purpose of finding out overall performance by a prosthesis, procedure or hospital, the most suitable measure accounts to be revision of surgery. However, on the contrary, in order to ascertain the entire performance of the hip that has been replaced, this proves to be rather crude procedure. It has been found out that the life quality as well as pain- levels would be measured by the aid of clinical studies (Karrholm et al. 2008). Despite this, the process of assessing them is considered as very difficult. Whenever a person begins to walk, the positioning of their legs get recorded by the aid of gait analysis. By the utilisation of force-plate, the reaction of their foot upon ground gets recorded (Section 3.2). In the duration of entire gate-cycle, joint angles could be calculated by using this recorded data. Furthermore, to predict joint and muscle contact-forces, musculoskeletal model can be taken into account (Section 3.3). For the purpose of comparing diversified hip replacement that have occurred in total, utilisation of joint & muscle force could be done. Despite this, another disadvantage is there that only a limited amount of individuals can be assessed during the monitoring procedure of study. Ample of patients can be considered for investigation for hip replacementsince it is not an invasive study.In order for informing experimental & computational analysation in hip replacements, a diversified range of forces determined by musculoskeletal modelling in clinical study can be utilised. The investigation of stability in case of an implant & wear upon bearing-surface are assisted through laboratory experiments. A rather greater level of flexibility is aided through this type of analysation in comparison to clinical study. The reason behind this is that implants are available along with ample conditions for loading. These conditions of loading that are going to be used in other tests need to be obtained from other study. Adding more to this, the phenomenon of testing tends to be very expensive as well as slower in comparison to computational-modelling. However, ample of design range, criteria for loading& scenarios that have to be modelled is assisted by very flexible and quick in silica analyses (Section 3.2). In this also, only limited data can be utilised and hence their validity is directly proportional to load & input-geometry. Since vast of scenarios can be investigated by aid of this modelling process, it is considered as good approach to examine trends. This will further assist for investigation of the most appropriate and relevant situation. Afterwards these could be taken into account for comparison with other experimental or clinical study. This is mainly for evaluation of modelling process's robust nature.
3.1.Musculoskeletal analysis Forthepurposeofactualoccurringforcesaroundthehip,atechniquecalled instrumented– hip prostheses is usually taken in consideration (Section 2.5). However, there is a very less amount of patients who might have undergone through total hip-arthroplasty. During the process of gate, angle as well as movement at the joint could be assisted from gate analysis (Section 2.4.1).For the purpose of predicting movements at joints, gait analysis can be utilised by the musculoskeletal analysis of dynamic nature. The already predicted internal level forces can be used as an prediction element within forward dynamic musculoskeletal analysis. Despite this, even torques can be utilised instead of gait analysis for validating the formulated assumptions in movement generation. Despite this, there is noaccuracy in the process of determining the force-production though either methods or data.Difficulty is involved in this procedure with respect to accuracy which further adds up errors into the analysis. Contextualising upon inverse dynamic musculoskeletal models, in the process of gait analysis , role of kinetics & kinematics is there. This is done within the motion equation for determination of net-force as well as torque which act around the joint (Erdemir et al. 2007). For the purpose of prediction in forces regarding joint contact & muscle, optimisation is highly required as observed from the outcomes through inverse dynamic analysis. 3.1.1.Inverse dynamics By utilising anthropometric data with respect to every modelled segment of limb, external force and kinematic-data, joint moments as well as forces are calculated through inverse dynamics (Robertson et al. 2004). Length, mass-centre, mass as well as property of inertia is usually included within the anthropometric data in context to every limb. Scaling of all of these is usually done from cadaver measurement towards the obtained data from gait analysis. This generally takes in consideration height and weight in relation to body of subject. For the purpose of measuring velocity, position & acceleration in case of individual-limb, kinematic data is obtained via aid of markers mounted on kin within the procedure of gait analysis (Section 2.4.1). In order to carry the inverse dynamic analysis of an individual's lower limb, forces from ground reaction are also taken in consideration. For calculating torque as well as net-joint forces, the obtained equations of motion are utilised. There is an inclusion of 3 markers attached to each of the limb-segment for capturing the orientationaswellasposition.Addingmoretothis,thereareonlysixdegreeof freedom(DOF) attached to each of modelled segment. With the addition of constraints upon the joints, there is further reduction observed.
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A few coordinates measured via marker are neglected by standardized kinematic-analysis within inverse dynamics for the purpose of reduction in this system that is over-determinant. For calculating of joint movement from the measured data of markers, approach based upon optimisation can be taken into account when applying towards musculoskeletal-model within the constraints of joints as stated by Andersenet ai.(2009). As observed from the findings, trajectories in relation to measurable markers were followed by the optimised-marker. In comparison to markers that have been modelled as per standardized approach, the optimised markerprovidedmorecloserperspectivewithreferencetotrajectories.Incontextto acceleration of markers, the root-mean-square that is RMS observed an reduction of nearly 60 % by aid of approach based upon optimisation. 3.1.2.Predicted muscle and joint contact forces Through the aid of establishing balance among the external force acting upon each of the segments in limbs, forces of muscles as well as joint-contact can be easily calculated. On the contrary, in comparison to dynamic equilibrium equations, this tends to have more inclusion of muscles. Henceforth, in the process of relating muscular forces with accelerations of limb- segment, involved system of equations tends to be indeterminate. The prediction of individual muscular forces can be done via aid of two methods. These methods are either by reduction in muscle numbers within models (Paul 1966) or via aid of utilising optimisation techniques (Seireg and Arvikar 1975; Johnston et al. 1979; Brand et al. 1 986; Brand et al. 1 994; Glitsch and Baumann 1 997; Stansfield et al. 2003; Lenaerts et al. 2008).Assumptions regarding manner as per which muscles are recruited by body for calculation of muscle-force, tend to be included within the optimisation approach. Within the literature, there has been inclusion of some of the suggested criteria for optimisation. The literature has been carried on the criteria for minimisation which is attained by either muscle-stress (Johnston et al. 1 979; Brand et al. 1 986; Brand et al. 1 994; Glitsch and Baumann 1 997; Stansfield et al. 2003; Lewis et al. 2007) oreventhroughmuscle-force(SeiregandArvikar1973;SeiregandArvikar1975; Crowninshield et al. 1978; Patriarco et al. 1981; Glitsch and Baumann 1997; Rasmussen et al. 2001). PCSA is the responsible entity for its normalisation. Among most of these studies, optimisation is responsible for minimising the individual-level defined criterion's sum-total (Crowinshield et al. 1978; Johnston et al. 1979; Patriarco et al. 1981; Lenaerts et al. 2008). Despite this, ample of studies have observed a rise in terms of order which is further of power 2 or 3. This can be more clearly understood by an example : the studies have observed a rise in
power raised to either cubed or squared muscle-force (Brand et al. 1 986; Glitsch and Baumann 1 997; Hoek van Dijke et al. 1 999). Van Bolhuis and Gielen (van Bolhuis and Gielen 1999) carried out the study in which comparison was done of different techniques that are used for optimization. various models of optimization were investigated by them by making comparison of the results of patterns of electromyography (EMG) pertaining to the investigation of the arm that was carried out through an isometric experiment. It was inferred from their research that there was no similarity and association between the models investigated and the activation patterns derived from EMG data. but, it was found that there was no fit between the minimization of either metabolic consumption pf energy or sum of forces. The best match that was found for the experimental data was that of the second order. This comprise of minimizing the sum of squares of muscle activation, forces, stress. For the purpose of validating their musculoskeletal model, measured hip-contact forces were utilised by Brandet ai.(1994), Stansfieldet ai.(2003) and Helleret ai.(2001).An instrumented hip was utilised for comparison of hip-contact force with the separate gait analysis which was recordedasmeasured byBrand et al.This was basically obtained from the same patient as a result of which correlation of very good quality was able to be reported. The muscle recruitment was utilised by them that further minimised muscle stress sum tht was cubed. In comparison to the measured force upon heel-strike & peaks of toe-off, hip-contact forces were predicted to be higher by approximately 0.5BW. Despite this, it is quite difficult to assessthecomparison'svalidity.Thereasonisduetothenon-simultaneouship-force measurement as well as motion-capture. Adding more, there were also some recorded variation in between the gait cycles. The data in context to gait analysis which was recorded in simultaneous way along-with measured hip-contact forces by use of instrumented hip-implant (Bergmann et al. 1993; 2001) was used by Stansfieldet al.and Helleret al.From both the studies, a very good quality of comparison was determined among the predicted & measured hip-contact forces. In order to minimise sum of muscle-forces, muscle recruitment was utilised by Heller et al. On the contrary before the reduction of forces in context to joint & muscle forces, maximised muscle-stresswas minimised by Stansfield et al. The hip-contact force is generally better in stance in comparison to swing-phase as predicted by Heller et al. Although slight overestimation was done in case of the predicted force. Nearly a deviation of 33% has
been recorded from measured-force. However, contextualising upon duration of normal- walking nearly 2-23 % average difference was recorded. In addition to this, contextualising upon diversified activities from various walking-speeds & from standing to sitting, 14-18 % difference was recorded as well as stated by Stansfield et al. Near about 6-21 % variation was observed in difference among peak within predicted & measured forces at the time of walking normally. However, in comparison to toe off difference was rather low at heel-strike. When compared with the predicted forces of musculoskeletal model at time of early-swing & late- stance, measured forces tended to be higher. Also, in comparison to measured forces, pattern in relation to predicted-force was rather smooth. Despite the presence of difference among predicted & measured forces, from the observations obtained through studies, hip-contact forces can reasonably be predicted by musculoskeletal models. Although, slight variation could be observed in muscle-activity because of the activated muscles through used diversified criterion of recruitment.However, this I not likely to affect hip-contact force that has been obtained as resultant. For the purpose of validating various musculoskeletal models,utilisation of EMG has been done. Muscle contraction intensity of first order was used by Hoek van Dijke et al. (1999). Muscle force of first order was used by Patriarco et al. (1981). A combination of force along-with joint-moments was further utilised by Seireg and Arvikar (1975). From all of the these, a very good correlation was observed in between their model's output and reading of EMG. The use of diversified criterion leads to the fact that towards the optimisation-criteria models don't prove to sensitive enough during the comparison among muscle's activity. Similar to this, as determined by Brand et al. (1986) forces of output were rather sensitive towards individual muscle's PCSA in comparison to criteria for analysis. Using of optimisation technique for calculation of muscle-activity does not proves to be accurate enough to give a detailed description about activation of real muscle. From the observations it can also be stated that by use of EMG, force is produced that is counter towards joint-movement as a whole, bycertain active-muscles (Glitsch and Baumann 1997) . The muscles in this category are classified as antagonistic-muscles. The activity of antagonist muscle was imposed by Hoek et al. (1999) within the model formulated by them. Although, it only had minor-effect upon non-antagonist muscle's activity. 2005)Muscle modelling Usingtherecruitmentprocedureofreducingthemuscleforceisrelativelyavery straightforward process but it does not considers about the various sizes of the muscles. The
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muscle stress after the whole criteria of the recruitment normalizes the muscle with the help of the physical size measured by their PCSA(Section 2.3). Using the current force of the muscle that generates the capacity and evaluates the activity of the muscle too, acts as an alternate method of normalizing the muscle force. Various muscle models have also been investigated by some groups which helps in calculating the potential force of the muscle in a particular situation. Their comparison have also been done on the basis of how they relate to the observed behavior (Hill 1926; Hill 1938; Hill 1950; Hill 1 953; Lloyd and Besier 2003; Thaller and Wagner 2004; Scovil and Ronsky 2006).According toHill (1926; 1938; 1950; 1953), for creating a model for the prediction of force in a single muscle, a series of experiments have been done on frog and toad muscles. The velocity of the contraction in the muscle and the maximum force available are directly or indirectly related (Section 2.3.3). According to Delpet ai.(1995), a musculoskeletal model which is specifically used for the prediction of the effects including the changes in location of the hip centre of rotation which is done by reshaping the joint angles. A hill-type muscle model including the activation patterns are being documented by EMG for calculating the muscle force in their musculoskeletal model. Before the prediction of the muscle force through the caliberation of the force velocity and the force length curves to the EMG activity, the length and velocity of the muscle tendon unit has been calculated. This model requires activation patterns of the muscle for each modelled scenario. It is very useful in the comparison of the muscle generating capacity of the different scenarios (Delp and Maloney 1993).With the help of EMG data and an optimised strategy , hybrid models are created for the calculation of the force in the remaining muscles (Cholewicki and Mcgill 1994). A hybrid musculoskeletal model has been created for gait and it uses the EMG data from the surface electrodes. After that calculation of the activity and strength of the deeper muscles is done using the inverse dynamics. There researches have resulted successful till now, although it is quite difficult to obtain all the specified data from the same research. Hybrid models are those which require the relevant data from the same patient. Peak force generated by a muscle is during the peak isometric contraction, when the muscle has its optimal fiber length. According to Wickiewiczet al.(1983), the optimal muscle length and other related parameters in the lower extremity permits the overall evaluation of the tendon length. Muscles differs in their sizes and it influences the overall strength of the muscle. Most commonly used in musculoskeletal are the hill muscle models which helps in predicting the muscle strengths (Zajac et al. 2002). A huge number of muscle models are
generated from Hill’s original equations and the ones used in the physiological cross sectional area of the muscles(Brand et al. 1986; Klein Horsman et al. 2007). Presently, no method is there for the determination of PCSA of all the muscles in a living subject. Some elements of scaling must take place to cover the parameters reported to the individual patients in the literature. 2006)Anatomical modelling Mathematically, musculoskeletal models determines the lien of action of the muscle by two discrete points. With respect to the joint axis, the moment arm of the muscle is directly concerned with the joint angle and the origin-insertion length. So, it acts as animportant aspect that the accuracy of the body model that is constituting the muscles with the attached points and the anthropometric data of the body’s segment is considered (An et al. 1984).A muscle has a specific area of attachment which has splitted a few larger muscles into several sub units to permit the different lines of action and various muscle activities which are to be modelled. The requirement of the segregated units and the quantity which is essential to provide an appropriate representation of the human anatomy ( Van der Helm and Veenbaas 1991). It included the modelling of the muscles at the shoulder with almost 200 units and found that the sub units required are dependent on the number of degrees of freedom as influenced by the muscles. Because of the small attachment sites and the muscles with fibers that runs parallel with the line of action of the muscle, the muscles are need to be divided into different sub units in the lower extremity than the shoulder. Various researches have said that the muscle attachment points have divided the muscles into various sub parts. It should be appropriate to the size of the attachment and the line of action (Dostal and Andrews 1981; Klein Horsman et al. 2007). 3.1.2. Predicted forces Predictions have been made by the musculoskeletal models regarding the broad variety of peak forces pertaining to normal gait from approximately 3-7BW (Table 3). The predicted values of the forces were found to be higher as compared to those that were obtained in vivo (chapter 2.5.1). In some of the musculoskeletal models, the participants used were healthy (Paul 1966; Crowinshield et al. 1978; Johnston et al. 1979; Glitsch and Baumann 1997). These participants could have possessed forces that were higher ascompared those of patients with hip
replacement. Generally, the forces in these models were higher in comparison to the models of gait pertaining to THA patients. But, antagonistic muscles which carry the potential of bringing increase in the forces of joint contact, are often ignored by the musculoskeletal models. There are various researches in which the predicted forces were compared with the forces that were measured experimentally so that the musculoskeletal models can be validated. As per the discusseion in chapter 3, the predicated hip forces were very well compared with the measured forces by Brand et al. (1994), Heller et al. (2001) and Stansfield et al. (2003). 3.1.2.Finite element analysis Analysis of Hip prostheses have been done with the range of various computational models. Different implant types have been compared by various static tests and they often represent the peak load on the hip during gait (Rohlmann et al. 1983; Huiskes 1990). As generated through the CT scans, these models act more specific and patient specific (Schileo et al. 2008). Various implant coatings have also been developed and they are used to investigate their effects and also of altered bone laoding on the biological responses (Bitsakos et al.2005). Representation of only one model takes place but the hip arthroplasty is being performed over a huge range of the population and that specific persons which is likely to receivethe hip replacement.Many probabilistic models tried to liberate the issues different patients face. It may be differing the bone geometry or the likely positions for the whole implant (Kayabasi and Ekici 2008). The finite models as becoming more complicated in their geometry, there are wider differences too that are applied in the models, their boundary conditions and the external parameters. A range of conditions are used by the computational studies of hip implants for more complex scenarios which also includes several muscle forces to the hip contact force.The magnitude of the forces differs between various studies. Although, most of them only represent an average patient at the time of peak in hip loading in the normal gait. While analyzing the computations,the boundary conditions act quite changeable. Many finite element studies constrained by the potential scenarios that are available in the vitro. There are various external parameters as stress, strain, bone density, deflection, micro motion etc., that are used in the finite element analyses. A model generally produces a large amount of data, because of which these external parameters are minimizing to a single system response. It can be maximum, minimum or the mean value as well. The overall range of the external parameters makes the
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comparisons more complicated between the researches and the clinical results. To find the most useful parameter as a predictor of a clinically relevant outcome can be a complex procedure. 3.2.1.Computational studies of cementless hip implants AsCementedimplantsareanalyzed,thecementlessimplantdesign'sarealso implemented in the same way but there are some additions like investigation about the in growth of bone in the implant surface. Various researches use cement less implants for the internal investigation of their effect on the surrounding bone, also for predicting the outcome of various situations. Unlike the cemented hip arthroplasties, the expected fixation of the implant to the bone not occurs the direct way. Therefore, the internal stability of the cementless implant can be evaluated through modelling the bone and implanting immediately after the surgery which is done before the bone in growth. An alternate to the secondary stability of the implant is evaluated by the assumptions about the full osseointegration of the femur and implant. The implant is assumed to have a frictional coefficient in the primary stability. It is to be done between the implant and bone to present either the rough coated surface or a smooth one where the representation of the ideal osseointegrated is done by a fully bonded model.Post operation, it would not occur for several weeks and must be dependent on the primary stability. In Table 4, some of these finite element studies are being identified along with the summary of the implant type, boundary conditions, loading conditions and the external parameters reported along with the major findings of the study. Table 4: Using finite element analysis the studies performed… page 57 Various researches are there for the comparison of the fixation type, to differentiate the differences between the cemented and the cementless implants. It also includes the examine the effects of incomplete, complete or no coating on the cementless stems (Tensi et al. 1989; Huiskes 1990; Taylor et al. 1995). According to Tensiet ai.(1989), compressive stress in the distal-lateral region in a completely coated stem is calculated during the one legged stance and the greater stresses with level walking loads although they use only abductor and hip forces. It was also founded that there came an increase in the lateral stress along with a coated stem when compared to a fully coated stem. According to (Huiskeset al.,1990), although the stress
was minimized on the proximal side of the implant, the stress pattern present at the bone interface was identical in both the cemented and cementless cases. However, much hip load was being transferred by the distal portion of the implant that is compared with the cemented model. Interface stresses were found and evaluated as a resultant of the interface normal stress along with the shear stress. It must be of more than 10MPa at its distal end of only fully coated cementless stem. According to (Huiskeset ai.1990), a titanium stem when compared to the CoCrMo alloy, resulted in a less rigid titanium which transferred more hip load to the proximal femur. However, it minimized the interface stress at the distal interface. Less stress was calculated significantly with a partially coated stem when being compared to a fully coated stem at the distal interface. According to Huiskeset al.(1990), a press fit stem is being also produced and made the interface stresses higher than the founded in either cemented or HA coated stem models. It was also found that the fixation of the implant used only a press fit, either a smooth implant surface or a rigid onethat generates comparatively greater peak principal stresses and strains. This all is done in the cancellous bone than either the cemented or HA coated fixation models. In the cancellous bone, the peak principle stress was evaluated as approx -4MPa and also in both cemented and HA coated models. In the press fit models, both the ridged and the smooth coated stems for the peak principle stress was evaluated approximately as -13MPa when compared to -2MPa in the intact femur model. An identical pattern was founded in the peak principal strains where the calculation of peak tensile strain of approximately 3000^6 in the cemented while as about 10000^6 in the HA coated models and of the smooth press fit model although the ridged model already had a peak tensile strength which is evaluated approximately about 5000^6. According to (Huiskeset al.1990), the effects of bone in growth which seems identical to the researches that compared stem coating quantity. In this, the models assume the full bone ingrowth in teh coated scenarios. According to (Keavenyet al.), the ideal bone ingrowth of the proximal loading is minimised when comapared with the no bone ingroeth. The relative motion between the implant and the bone was minimised in the bone ingrowth cases compared to the non ingrowth.In contradiction to thatBiegleret ai.(1995) found that there is a slight difference between the relative motion of the smooth and porous coated surfaces.
Between different cementless implant designs, comparisons have also been made (Ando et al. 1999; Folgado et al. 2009; Hu et al. 2009). New material types have also been investigated according to various new researches irrespective of the impant designs (Huiskes 1990; Cheal et al. 1992; Rotem 1994). According to (Andoet al.1999), lower von Mises and relativemotionhavebeenfoundattheboneimplantinterfaceinanimplantthatis anatomicallybasedwhencomparedwithwithconventionallydesignedstems.The anatomically based stem has generated a large area of contact within the bone cortex and the implant. It has been attributed to the minimisation in the interface stress when compared with the other implant models. It allows the transferring of the load fromthe stem to the femur in the proximal and the distal areas. According to Huet al.(2009), the stresses within the bones covering the implant designs along with the sharp corners were larger than the ones having the more rounded stem. It also predicted that the higher stresses may enlarge the likelihood of the fractures in the bone. In contradiction to that Vicecontiet al.(2006) suggested that the finned implant had a larger primary stability, evaluated by a lower micromotion of the stem than a stem due to the larger contact area between the implant and the bone. The interface stresses are said to be affected by the stiffness of the stem as a much rigid stem transferring the load to the distal regions. It is not like a flexible stem that transfers the load to the proximal regions and helps in minimizing the stress in those distal regions (Huiskes, 1990). According toHuiskeset al.(1990), a change in the stem material from CoCrMo to the titanium is being evaluated and would be minimised by more than 20% of the medial distal interface.According to Chealet al.(1992) and Rotemet al.(1994), a minimisation in the prosthesis stiffness is found in lower interface stresses.According to (Folgadoet al.,2009), evaluated a lesser bone mass in the proximal femur. It si being mainly done on the medial side by comparing CoCr stems with the titanium stems. In the analysis of the finite elements, two scenarios are usually modelled. In primary stability, the stem ahs been recently implanted and the totally bonded scenarios while assuming that the osseointegration has reached till the equilibrium (Table 4). According to Tayloret ai. (1995) and Tensiet ai.(1989), the HA coated models were assumed to be bonded perfectly to the bone which helps in simulating an ingrown situation. As per many researches, use of friction coefficient at the bone implant interface is done to model the stem before the ingrowth
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of the bone has taken place (Park et al. 2009; Pettersen et al. 2009; Sakai et al. 2010). The calculation of micromotion is often carried out at the interface because the bone growth at the surface can be impacted by it significantly. It can also affect the stability of the implant (Jasty et al. 1991). According to the various researches conducted, it has been stated that micromotion is inversely proportional to the surface of the stem in frictional contact which represents the ingrown area (Keaveny and Bartel 1993; Keaveny and Bartel 1995). According to the study conducted by Vicecontiet al.micromotion is impacted by the thickness of the layer of soft tissue, modelling of which was done with a low frictional coefficient. It was also found that osteointegration on the surface of the implant could be prevented by a layer which was greater than 500^m.The micromotion at the interface is also affected by the contact area that lies between the bone and the stem (1999; 2006; 2008). Andoet al.(1999) found that with ananatomicallyshapedstem,therewasalowerrelativemotionincomparisonto conventionally shaped stems due to better fit between the bone and stem. Vicecontiet al. (2006) found that the contact region that lies between the stem and the bone significantly impacted peak micromotion and Parket al.(2008) also explored that micromotion was reduced if the gaps at the bone implant interface are reduced. This happens due to an increase in the contact region. However, according to the study carried out by Parket al.(2009) at low ratios, micromotion is affected by the bone-implant contact ratio. But, a contact greater than 40 per cent does not affect micromotion significantly.Several research have been studied related to bone growth and tissue differentiation beyond the analyze of independent static and the change in tissue type basedcan be considered by making use of adaptive models on the loading of respective elements. Folgadoet al.(2009) in an adaptive model make use ofbone strain to bring out the comparison between the amount of coating on a stem and the material type. To symbolize coated surfaces on hip stems a frictional coefficient of 0.6 was utilized and they discovered that less amount of bone resorption are produced by uncoated stems in comparison to partial and full coated stem models. Some past study have discovered thatthe lifespan of uncoated stem is likely to beshorter due to high stresses at the interface (Huiskes 1 990; Taylor et al. 1 995) and Folgadoet ai.also discovered that uncoated stream have higher strain at the interface however to calculate bone resorption an algorithm is used which uses the high strain to remodel thebone density. They studied that the effect of coating for cylindrical and the final bone mass was samein all the coating types whereas with a tapered stem. wasless when the stem was tapered .
An affect was produced in the interfacial strain and Speirset al.(2007) due to position of stem that showed an anteverted or medialised stem would produce higher strain across the length of the femur in comparison withproperly positioned stem. In particular, they discoveredthat the medialised stem producesa higher strain energy density during stair climbingtin comparison with reference stem position. However these differences that are discovered are tiny in comparison to change in strain from an intact femur to an implanted situation. Reggianiet al.(2008) also analyzedimplant position impacts by examining a planned stem position with the surgically achieved position. By examining these two models only a slight difference was found although, at peak, discovered with the help of achieved position there is an increase of12% in the von Mises stresses. A wide range of scenarios to be modeledby making use of Probabilistic studies (Viceconti et al.2006; Park et al. 2009; Dopico-Gonzalez et al. 2010). By varying the bone density, patient’s body weight, bone size and the quantity of bone-implant contact area Vicecontiet al.(2006)analyzed1000models of an implanted cementless stem .They discovered that due to total variationthat was applied to the model and the average micromotion 206±159^m under stair climbing loads the peak micromotion was affected. The bone size, body weight and region of implant contact were found to significantly affect The peak shear stress and peak micromotion were affected by thevariation in cortical bone density , bone size, body weight and region of implant contact . However, although the report was varied with the body weight and it was then usedto vary the loads on the femur, other report have shown that body weight cannot be considered alone for inter-patient variation (Bergmann et al. 2001; Taylor and Walker 2001). By making use of probabilistic model Dopico-Gonzalezet ai.(2010) discovered thatpeak micromotion areaffected by bone and implant geometry at the bone-implant interface .The bone-implant interface area are affected by implant geometry and as a outcome ithas been discoveredto changethe micromotion (Park et al. 2009). Parket al.(2009) found so that with an increase in contact ratio between the implant and bone using a statistical model the bone-implant micromotion can getv reduced. 2007)The applied forces limitations in finite element studies Despite the enhancementin modeling techniques, the previous finite element studied shows the consideration of the loads acting across the hip joint have changeda little.Hip
contact force are used by most models, which uses multiple of body weight, that can be measuredin vivodata or calculated from a musculoskeletal model. Many models only consider an ‘abductor’ muscle force in addition to the hip contact force (Verdonschot et al. 1993; Keaveny and Bartel 1995; Kayabasi and Erzincanli 3.2.2., Alsoa wider range of muscle forces are also includedin few models. (Stolk et al. 2001; Bitsakos et al. 2005; Jonkers et al. 2008; Afsharpoya et al. 2009). However, the number of papers that are used are quite smallto provide these forces and proper investigation have not been conducted on the potential range of joint and muscle force . Utilization of finite element models for loading conditions are either scaled from measured data or from calculated forces. To measure the hip contact force in the body various vivo studies have been done using an instrumented hip replacement (Chapter 2.5) and the studies conducted by Bergmannet al.(1993; 2001) aremost popularly used for measuring forces. Muscle forces are not measured in the body and therefore if the inclusion of muscle forcesfrom computational model can betaken from either an analytical model such as Paul (1966; 1966) or a musculoskeletalmodel, the most popular areusedby Brandet a.(1982; 1986; 1994), Crowninshieldet a.(1978; 1980), Dudaet a.(1996) Helleret a.(2001) or Patriarcoet a.(1981) (Table 4). Through recent evaluation of cementless implants many studies (Wong et al. 2005; Speirs et al. 2007; Behrens et al. 2008; Hu et al. 2009; Park et al. 2009) have made use of the musculoskeletal forces predicted by Helleret a.(2001; 2005). Thestatic finite element models and the applied hip contact forces range from 2.3BW can be used for peak joint contact forces for walking(Stolk et al. 2001) to 4.64BW (Cheal et al. 1992). Stumbling loads can also beutilized or the model is loadedin the femur toinduce fracture(Lotz et al. 1991; El'Sheikh et al. 2003) and variousloads are utilizedin the models related to computationalto permit them in comparison with theexperimental analysis (Pedersen et al. 1 997; Abdul-Kadir et al. 2008; Afsharpoya et al. 2009).Using static models there are also some studies which include various instances in the gait cycle such as heel strike, toe off and mid-stance (Bitsakos et al. 2005) During the gait cycle. heel strike is generally used as the peak in hip contact force. An abductor forceare included by various FE models and this can range from approximately 1 BW (Wong et al. 2005) to approximately 3.5BW (Keaveny and Bartel 1995) for gait models at the peak hip contact force. Theabductor muscle force can further be
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divided into the glutei muscles (Watanabe et al. 2000; El'Sheikh et al. 2003). Some extra muscle forces are added to some models and the most popular forces include the iliotibial tract, iliopsoas and vastus muscles (Cheal et al. 1992; Taylor et al. 1996; Simoes et al. 2000; Stolk et al. 2001; Bitsakos et al. 2005; Shih et al. 2008; Afsharpoya et al. 2009). To investigate the effect of adding muscle forces to FE models these muscle forces have been included in FE studies (Duda et al. 1 998; Stolk et al. 2001; Bitsakos et al. 2005; Speirs et al. 2007) or to give a more specific pattern of sprain in the femur. Various report have discovered that with the inclusion of a more physiological selection of muscle forces the deformation of the femur have to be more physiological in FE models. (Cheal et al. 1992; Polgar et al. 2003; Speirs et al. 2007) in comparison onapplying a hip contact force or hip and abductor forces. Speirset ai.(2007) study the muscle forces effects on FE models of the femur. By making use of three muscle configurations along with the forces taken from a musculoskeletal model containing 95 muscle units published by Helleret al. (2001). All the models mainly have hip contact force and in addition to that the first model contained only an abductor muscle ,the abductor, adductor and vasti muscles are contained in second and in third all themuscles forces are connected to the femur as well as the knee and patella forces from the musculoskeletal model. In the simplifiedandin the complex loading scenario, the muscle forces have the same magnitude . The abductor force isa collection of the glutei muscles .The simplified load cases are not necessarily reflected that areused in other studies, specificallythose which makeuse offorces predicted with only a restricted number of muscle groups. Speirset al.discoveredthat with thecomplex muscle loading inclusion , the deformation of the femur and by inferring tothe strain in the femur is more physiological. Polgaret al.(2003) also discovered thatthe strain and displacement measured in an intact femur from unrealistic values calculated with simplified loading can be reduced by the inclusion of complex muscle loading. Muscle and hip contact forces were used from Duda et al.(1 998) and the loads were consideredat 1 0% of the gait cycle, selected because these were the peak in abductor and adductor muscle force and the hip contact force which was less than 1BW.A single time step in the gait cycle is considered ,particularly the effect of the muscle forces on the femur models at the peak in hip contact forcecan be represented by tiny chip contact force relative to the muscle force . However, whenthe whole gait cycle is investigated it is essential to include a more complex loading regime. Polgaret al.(2003) also discoveredthat the muscle forcedistribution affects the femoral strain. The muscle forces
appliedaremodelledasconcentratedforcesatthemuscles’attachmentcentroidsand comparison is done over the insertion area where the muscle forces are distributed uniformly. The principal strain for peak tensile isreduced from approximately 9000^6 to approximately 1000^6and the internal compressive andwith the distributed model. principal strains for tensilesalso decreasesTayloret al.(1995) discovered an enchancement to the abductor, iliotibial tract and iliopsoas forces to a hip contact force only reduced the peak minimum principal stress slightly in the cancellous bone in an HA coated implant model although in the intact femur modelit reduced the peak minimum principal stress to -0.75MPa which is less than a half the stress calculated by the hip contact force alone. The influence of loading conditions was also investigated by Bitsakoset ai.(2005) and to calculate bone loss in the femur after a hip arthroplasty made use of an adaptive models. From a musculoskeletal model (Brand et al. 1994; Duda et al. 1996) their study also used a set of muscle forces that were taken and reduced the number of muscles included in the FE study without altering the magnitude of the remaining forces. They discovered that by using models a smaller quantity of bone loss surrounding the femur can be calculated with the more realistic set of muscles and this can be compared well with clinical data although they commented that using the algorithm to calculate the bone remodelling over-estimates bone loss. In an intact femur the effect on the strain pattern was also investigated in a finite element model by Dudaet al.(1998) with respect to the inclusion of muscle forces. Muscle forces were used as they were predicted from a musculoskeletal model published by Dudaet al.(1996) by utilizing data from Brandet al.(1982; 1986).With an inclusion of all the femur attached muscle-forces from musculoskeletal model, maximum resulted principal strain within femur was below 2000qe . This was low when compared with the simple load cases where up to 3000qe was reached by principal strain. Pattern observed in case of femur varied less in context with complex muscle-loading. From the study it was estimated that a very well comparison was there among the calculated principal-strain within their model along with mesurements of in vivo. These were observed to be near about 850qe over anteromedial side of tibia midshaft (Lanyon et al. 1975). The stress & strain in their Femodel of cemented hip-implant had a major impact of included abductor force as stated by Stolket al.(2001). On the contrary,deflection of femoral-
head, implant stress, cement-mantle, surface-strain & density of strain energy had a minor effect through added illotibial-tract, adductors & vasti-muscles. However, surface-strain within the area close to implant's tip was reduced by the added muscle-forces during gait-cycle of 10%. In similarity with stated scenario by Duda et al. (1998), 19 muscles along with calculated hip-contact force by Brand et al (1982; 1986; 1994) and Duda et al (1996). A conclusion was made that hip-contact and abductor-force have a requirement in adequately reproducingin vivo loading in context to cemented THA reconstruction . This was based upon caused minor- changes which was result of adding complex-loading. From this study, an observation could be made that added muscle had very small effect but still femur strain was affected. Adding more details, stem tip's strain-energy density was affected in particular. Certain levels of variations were also found within micromotionat interface of bone- implant atc the time of comparison in between diversified loads from different activities as stated by Pancantiet al.(2003) and Biegleret al.(1995). In comparison to normal-walking, highermicromotionwasobservedincaseofstair-climbingfrombothstudies.Also, suggestions were given by Bieglar et al that high torque-activities should be avoided by patients for example climbing stairs till occurrence of bone ingrowth. Pancantiet al.(2003) and Biegleret al.(1995) determined that higher strain was generatedwithin the bone through climbing stairs among both intact & implant femurs. The calculated muscle forces foo selected patient is of utter importance in context with resulting FE model. Through the predictions of Jonkerset al.(2008) musculoskeletal model of Lenaertset al.(2008) was used to determine joint contct & muscle forces. This was done for 2 subjects. Along with an inclusion of glutei adductors, iliotibial tract, 19 muscles & hip-contact forces were applied to FE model specific subject in context to their femur implanted. With the body-weight of patient, normalisation of the forces was done. Afterwards in subsequent manner these were applied to other FE model. In comparison with bone geometry, altering of patient forces had a rather strong affect upon tress within femur. The occurring changes in properties of bone material have been affecting micromotion as well as bone-strain despite the less importance of bone geometry (Wong et al. 2005). At the time while consideration of micromotion at bone-implant interface, higher inter-patient variation was determined. This might possibly contribute towards the loading effect upon femoral-stresses as found by Pancantiet ai.(2003). In the duration of upstairs-walking scenarios near about 75-107qm variation observed among patients within peak-micromotion. This range was lower near about 56-75qm when patients were modelled to walk normally. Along with diversified implant &
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positions of hip-centreforces around hip varied (Bartel and Johnston 1 969; Johnston et al. 1 979; Iglic et al. 1 993; Bicanic et al. 2009; Erceg 2009). Despite the changes within modelling geometry, forces are intended to remain as per the assumptions of parametric & probabilistic studies(Dopico-Gonzalez et al. 2010). For full utilisation of parametric analyses concerning hip-arthroplasty, effects upon hip-forces by geometrical changes need to incorporated within studies. But there is an additional requirement of understanding in detal concerning the effects of changing hip-centre or even the positioning of implant along with forces trange to which the hip might be subjected. 3.2.3.Boundary conditions For the purpose of providing equilibrium within model of FE models,either a nodes plane gets constrained or femur's distal end gets fixed up (Table 4). A greater level of displacement occurred regarding femoral head through distal end fixation in comparison to the one fixed at midshaft as shown by Polgaret al.(2003). Adding more, no changes were found within the peak tensile principal-strain and also peak compressive principal strain observed a reduction. Despite this, physiological boundary-condition led to the production of model having more level of physiological deflection concerning the patterns of bone & strain as shown by Speirset al.(2007).The authors made comparison between three different boundary conditions. The first condition was femur fixed at mid shaft, the second was femur fixed at nodes on distal condyles while the third condition was a ‘joint’ constraint. The joint constraint condition was characterized by fixing of knee centre node in all DOF, fixing the hip node in two DOF while fixing the lateral condyle node in one DOF. The femoral head was found to be deflected by 19mm in the model that comprised of constrained distal condyles. In the anterior- posterior direction, there was a particularly large deflection. In the model where there was a joint constraint, the deflection of femoral head was found to be only 2mm. also, a lower surface strain was found throughout the length of the femur as compared to either of the two models. In the mid shaft model, there was a higher deflection in comparison to the joint model. But surface strain was almost same in both the models being slightly higher in the lateral and medial mid- shaft. There was similarity in the deflection that was calculated at the modelled
femoral head comprising of joint constraint and the deflection that was measured in the study carried outin vivoon single legged stance (Taylor et al. 1996). The maximum deflection that was found was approximately 3 mm inferiorly and 4 mm medially. Phillips et al.(2009) generated a model associated with loading and boundary constraints of femur which was more complex. They modelled muscles as well as ligaments and considered them as the spring elements. An acetabular was also modelled by them. This applied a static load of %BW. The calculation of the force present in each muscle unit was done by making use of a force displacement relationship. There were two models that made use of a force displacement relationship that was either cubic or linear. This was for all the muscles that possess a lower and upper boundary for activation of muscle. Principal strains that were present on the surface of femur were indicative of presence of tensile strain on lateral side and compressive strain on medial side. This was at peak value between 2000-2500^6. A substantially lower strain was found to be present on the anterior and posterior surfaces. In the non- linear model, an overall deflection of less than 2mm and only 1.6 mm was found in the femoral head.