Finite Elements in Analysis and Design 40 (2004) 2027–2047www.elsevier.com/locate/�nel

Finite element modeling of a generic stemless hip implantdesign in comparison with conventional hip implants

S.A. Asgaria, A.M.S. Hamoudaa;b;∗, S.B. Mansora, H. Singhc, E. Mahdia,R. Wirzaa, B. Prakashc

aInstitute of Advanced Technology, University Putra Malaysia, 43400 Serdang, Selangor, MalaysiabDepartment of Mechanical and Manufacturing Engineering, University Putra Malaysia, 43400 Serdang,

Selangor, MalaysiacOrthopedic and Traumatology Department, Hospital Seremban, 70300 Seremban, Malaysia

Received 30 March 2003; accepted 12 February 2004

Abstract

In this study, the �nite element modeling and comparison of the stress and strain analyses were carriedout for three di2erent structures that are intact bone, stemless implant and stemmed one. Currently proposedstemless design studied here is the generic concept of stemless implant. This generic stemless implant re-construction was numerically compared to the conventional stemmed implant and also to the intact bone ascontrol solution. Two loading conditions were applied to the most proximal part of the models, while themost distal part was �xed for all degrees of freedom. The models were divided into two regions and studiedalong two paths of medial and lateral aspect. The results of this study showed that the stemless implant hadless deviation from the control solution of the bone in all regions and in both loading conditions, comparingto the large deviation of the stemmed implant from the intact bone. However, it was shown that the �xationof this type of implant and its e2ect on sub-trochanter region must be carefully considered for designing the�nal product of any speci�c design of stemless implant.? 2004 Published by Elsevier B.V.

Keywords: Hip implant; Conventional implant; Stemless and �nite element analysis

1. Introduction

The famous Charnley technique for conventional hip implants has traditionally been very successfulfor older patients. Because the technique dramatically changes the normal stress and load patterns

∗ Corresponding author. Department of Mechanical and Manufacturing Engineering, University Putra Malaysia, 43400Serdang, Selangor, Malaysia. Tel.: +603-8946-6330; fax: +603-8656-7099.E-mail address: hamouda@itma.upm.edu.my (A.M.S. Hamouda).

0168-874X/$ - see front matter ? 2004 Published by Elsevier B.V.doi:10.1016/j.�nel.2004.02.003

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on the patient’s leg, however, it can cause problems in younger people, or those who are veryactive.

Pioneered in 1995 by Munting and Verhelpen [1], an alternate approach has more closely approx-imated normal loading conditions among patients in Europe. The design requires a much shorter rod,which is attached to the top of the thigh by several small bolts, inserted through the exterior side ofthe bone, the lateral cortex region. This design is �xed on the lateral side of the proximal femur bymeans of a bolt and employs �ns along the medial border to provide resistance to torsional motion.Their in vitro experiments show minimal micro-motion and the short-term clinical studies show lowinitial failure rates [2]. However, Munting design needs to insert the sti2 metal bolts through therelatively brittle lateral cortex.

More recently, Joshi et al. [3] designed a proximal �xation device that instead of bolts, usesa cabling system to anchor the prosthesis to the bone. Joshi et al. [3] propose that this cablingsystem helps to produce a more natural bending load over the cross-section of the femur by �xingthe trochanter to the implant. Their resulting design is patented to Advani et al. [4]. Advani designgreatly reduces the prosthetic rod, which seems to be responsible for much of the rigidity and alteredloading that occurs following hip replacement. In its place, they used a plate-like base to transferthe bending loads from the prosthesis to the bone. They suggested that further improvement couldbe achieved by using advanced composite materials to tailor the sti2ness of the implant.

Chieng et al. [5] proposed another recent design of stemless implant. The Chieng implant iscomposed of a conical-shaped mask; together with a central �xation screw, which further coupleswith the femoral modular head and acetabular cup to constitute the stemless total hip prosthesis(THP) [6]. They claim that the skirt of the conical-shaped mask anchored on the greater and lessertrochanter, causes no change in the stress distribution to the bone.

Although the stemless implant idea with metaphyseal anchoring is not a new idea, few systematicinvestigations were carried out on this type of implant. To the author’s knowledge, the only stemlessimplants that have been designed and studied so far are only the Munting implant, the Advaniimplants and the stemless implant designed and produced by Chieng et al. which is orientally inclinical practice in China but is without systematic follow-up study [7].

Meanwhile, several patents exist that propose di2erent approaches for proximal replacement thoughthere is no information on pre-clinical and clinical trails of these implants [8–10]. This shows the trendtoward revolutionary change from conventional implants to novel stemless implants. However, thestemless implants demand comprehensive pre-clinical and clinical testing before being widely applicable.

Today, �nite element analysis (FEA) is widely used to extend the boundaries of medical science.Bene�ts of FEA for total hip replacement (THR) are the ability to evaluate di2erent con�gurationsof the implant design on computer rather than by physical testing. Comparisons between the resultsobtained here and those obtained from experimental testing reveal a close correlation; hence, thismodeling approach has been proven to be viable. It is also possible to investigate several novel andunconventional implant ideas with numerical modeling. If FE models are to be used for pre-clinicaltesting purposes, they should be used on a comparative basis. A prerequisite for accurately predictingmechanical failure for di2erent implants is that the FE models accurately mimic the mechanical andphysical behavior of the hip reconstruction. This requires the validation of FE models with di2erentimplants relative to experimental measurements.

There are a number of papers describing the validation of FE models of hip reconstructionsrelative to bone surface strains [11–14] and in case of cemented implants, internal cement strains

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are measured to validate the FE models in few papers [15–17]. In all these validation studies, thefemurs were subjected to bending in the frontal plane.

The motive for this study is to analyze and evaluate a generic design of stemless hip implantin order to �nd the fundamental advantages or disadvantages of such implants, that can justifythe application of such implant as an alternative to the conventional ones. To achieve this end acomprehensive and validated numerical comparison was performed between generic stemless implant,conventional stemmed implant and intact bone.

2. Methodology

There are two approaches for the numerical modeling of femoral implant reconstruction. One is touse sample femoral bones and create the FE model based on CT scans. This demands experimentalanalyses of the same sample bone, as the analysis would be unique by its nature for every analysis.The other approach is to establish a standard basic geometrical model of the bone that would beapplicable for numerous analyses of the di2erent implant reconstructions. The geometrical reference,called standardized femur (SF) created by Viceconti [18] formed the basic geometry for FE analysesof the current study.

Based on common geometry, it is practical to compare results from di2erent FEM studies world-wide and besides, every FE models could be calibrated with data from experimental tests availablein the literature. The latter one is of great importance as it is not always possible in biomechanicsto do experimental tests for validating and verifying the numerical tests.

More information about the physical object from which the standardized femur model has beenderived is available from [18].

The IGES format of SF was imported to the commercial pre- and post-processor, Hypermeshsoftware. Using Hypermesh built-in ANSYS template, it was possible to export the model as anANSYS 3-D database [19,20]. The femur in this stage consisted of 3076 key points and 1350 lines.In several cases, there were non-continuous and non-smooth curves in the model that induced errorsin the output model; hence, a curve smoothing approach was implemented to avoid such incidenceand to allow mesh generators more Lexibility.

The dominating mathematical tool for describing curves and surfaces are splines or more speci�-cally non-uniform rational B-splines (NURBS). A function S(x) de�ned on a �nite interval [a; b] iscalled a spline function of degree k ¿ 0 and order (k+1) having key points with strictly increasingsequence xi where i = 0; 1; : : : ; n− 1, if the following two conditions are satis�ed:

1. On each key points interval [xi; xi+1], S(x) is given by a polynomial of degree k at most:

x∈ [xi; xi+1]; S(x)∈Pk; k = 0; : : : ; n− 2: (1)

2. The function S(x) and its derivatives up to order k − 1 are all continuous on [a; b]

S(x)∈Ck−1[a; b]: (2)

To give an aspect of the geometric properties of the splines, a theorem is presented [21]. The theoremis valid for cubic splines but it can be generalized for higher (odd) degree spline functions as well.Let f′′ be continuous in [a; b] and let a= x0¡x1¡ · · ·¡xn−1 = b. If s is the natural cubic spline

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Table 1Material properties used in this study for bone and implant

Material properties Ti–6Al–4V (TiAl) Bone

Ex′ (GPa) 117.24 17.9Ey′ (GPa) 117.24 18.8Ez′ (GPa) 117.24 22.8�x′y′ 0.3 0.26�x′z′ 0.3 0.31�y′z′ 0.3 0.37Gx′y′ 45.09 5.71Gx′z′ 45.09 6.58Gy′z′ 45.09 7.11

interpolating f at the key points ui for 06 i6 1 then∫ b

a[s′′(x)]2 dx6

∫ b

a[f′′(x)]2 dx; (3)

the curvature of a curve described by the equation y = f(x) is the quantity

|f′′(x)|(1 + [f′(x)2])−3=2: (4)

It is apparent that the natural spline produces the smoothest possible interpolating function and thevector space of functions satisfying Eqs. (1) and (2) will be denoted by Sk; t and its dimension isn+ k − 1.Once the curve smoothing was performed, the Boolean operations have been implemented in order

to only consider the proximal femur as a cortical shell bone. The models were meshed with 10 nodestetrahedral elements with the best possible mesh re�nement [22]. Two types of material, bone andimplant were assigned to the models. No contact element was considered for the interface of thebone and implant. The interface was formed by shared nodes between two types of material elementsof the bone and implant. The bone itself is an anisotropic material and the material properties ofcancellous bone are very much dependent on the amount of solid tissue material per unit volume, thepreferred direction of the sti2er trabeculae and the microstructural composition of the rods and plates.In this study, the bone modeled as an orthotropic material based on a recent study by Taylor et al.[23]. They produced a validated orthotropic �nite element model of a human femur by matchingthe predictions from FE modal analyses to the experimental natural frequencies, giving a maximumerror of 7.8% over four modes of vibration. Their predicted material properties, which are used inthis study, are listed in Table 1. For the implant, the material properties were assigned to mimic thebehavior of a metallic implant. It was modeled homogenous and isotropic with values correspondingto Ti–6Al–4V.

Intact bone (Fig. 1) was cut distally at a distance of about 260 mm from femoral head in orderto just simulate the proximal part of femur. Stemless implant reconstruction as it is shown in Fig.2 was modeled such that it evaluates the generic idea of using a stemless implant similar to ahemi-arthroplasty. Conventional implant was also modeled in a way that followed exact pattern ofmedullar channel as shown in Fig. 3. Dimensions shown in Figs. 1–3 are in millimeter. Two loading

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Fig. 1. Schematic view of intact bone model (dimensions in mm).

conditions of bending and torsion were applied separately to the models. In a one-legged stance, anaxial load case with magnitude of three times of body weight (BW = 826 N) applied to proximalend of femur. For torsion, the applied load on the femoral head was taken to represent terminalstance during horizontal walking [24]. Accordingly, the ratios of the three components of the forcewith respect to the antero-posterior force were 0.875, 0.625 for medio-lateral and antero-posteriorforces, respectively. Distal end of femur was �xed. The FE models were solved using ANSYSsolver running on a single Intel processor. The results of the comparison between the two types ofimplants were evaluated against a control solution for the intact bone, which is the ideal motherborn structure. Two paths were de�ned to study the desired location of the interface between thebone and the implant in stemless and stemmed model. Fig. 4 shows these two paths. Furthermore,to make it easier to study and compare the biomechanics of di2erent models, two regions namedR1 and R2 were identi�ed and are shown in Fig. 4. R1 was described, as region where the stemlessimplant had direct interface with the bone and R2 was region where the intramedullary part of thestemmed model had direct interface with the bone. The stemless implant had no interface with thebone in R2.

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Fig. 2. Schematic view of conventional implant reconstruction model (dimensions in mm).

3. Results and discussion

In Fig. 5, the average Von Mises stresses are shown for the intact bone, stemless and stemmedstructures.

For the intact bone, which was considered as control solution for evaluation of two other structures,it is depicted that the cortical shell assumes higher stresses than other regions up to 155:547 MPa.In stemless implant structure, the bone was carrying higher stresses than other regions while therewas some stress concentration reported in the neck of the femur as well. The cortical shell ofthe femur assumed higher stresses than inner side of the structure, up to156:337 MPa.In the stemless implant model, the stress contours showed similar distribution as intact bone,

transferring the loads to the cortical shell, but with higher stresses and a maximum displacement

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Fig. 3. Schematic view of stemless implant reconstruction model (dimensions in mm).

of about 17.39% higher than that of intact bone. There was higher stress concentration in the mostproximal region of the structure as well which is judged by the loads applied in this region.

In the stemmed implant reconstruction, the stem de�ned the stress pattern as it shielded the bonefrom the loads and carried the higher stresses on the surface over the implant and the bone interface.The maximum stress in the stem and bone interface region was up to 130:555 MPa.

However, it should be noted that the range of stress contours in Fig. 8 are di2erent for threemodels. These maximum and minimum values of color scheme are listed in Table 2.

3.1. Stress in medial and lateral aspect

The values of stresses were calculated for some speci�c locations along two paths of medial andlateral aspect of the femur and plotted against the distance from proximal head of femur towardthe distal end. Fig. 6 composes the average Von Mises stresses in lateral path for the conventionalstemmed implant, for the stemless implant and for the bone.

The stemmed implant showed stresses that were lower than that of the bone overall the lateral pathexcept for a region between range of 15–23 mm far from proximal head. As for stemless implant,starting from the most proximal point of the model (0 mm from the head), it had higher stressesby a factor of 1.7 than in the bone. It is shown that this trend is kept till 10 mm farther from head

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Fig. 4. Lateral and medial path and di2erent regions of study in lateral and medial aspect.

where the implant fell down the control solution of the bone and reported lower stresses till 23 mmdistance from the head. In R2 region, the maximum and minimum di2erences between the valuesof stress in stemless implant and the bone were not more than 0.683 and not less than 0:021 MPawith the maximum deviation of 1.5%.

For the medial aspect of the bone, Fig. 7 shows the values of stresses in the same bendingcondition for three structures. From this �gure, it was understood that again the di2erences betweenstress values in the stemless implant and the bone in region R2 is not signi�cant with the maximumdi2erence of 0:957 MPa and deviation of 10.4%. Meanwhile, due to geometrical parameter in theregion of femoral neck, the implant showed up and down behavior, as it assumed higher loads insome locations and less in others compared to the bone. The stemmed implant in the bending loadingcondition showed average Von Mises stresses over the medial aspect of femur that were below the

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Fig. 5. Stress pattern in three structure of intact bone, stemless and stemmed implants reconstructions.

Table 2The ranges of stress contours for three models

Stress (MPa) Intact bone Stemless implant model Stemmed implant model

Minimum 0.403 0.402 0.0875Maximum 465.834 458.205 391.49

values for control solution of the bone, except a part of region R1 from 9 to 23 mm proximaldistance. Similar to what was seen in lateral path, in this part of region R1, the stemmed implantshowed higher stresses while stemless implant fell below the bone at the same region. Stemmedimplant also had a Luctuating behavior at around 110 mm from the head. This region is stem’s tip,which changes the stress distribution in the bone. The stem tip had smaller radial circumference;hence, it had higher stress concentration in the tip of the stem, leading to bone loss and asepticloosening in conventional stemmed implant.

3.2. Strain in lateral and medial aspect

Fig. 8 shows the strain pattern in three structures of the bone model, the stemless implant modeland the stemmed implant model. The normal strain pattern in the intact bone was the control solutionto evaluate two other structure strains. In the stemless implant model in the R1 region, the neckof femur where the implant resides was not induced by any strains comparing to the intact bone.In the stemmed implant with higher strains overall the stem, the strain pattern was not so muchdi2erent from that of the intact bone. The same strain distribution can be seen in stem region forstemless implant model, but in great sub-trochanter region, there was a signi�cantly higher strainregion ranging from 440 to 1290 micro-strains. This region was the highest 15 mm part of R2

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0

5

10

15

20

25

0 10 20 30 40 50 60 70 80 90 100Proximal-distal distance (mm)

Von

Mis

es S

tres

s (M

Pa)

stemmed

stemless

bone(control)R1

R2

Fig. 6. Average Von Mises stresses over the lateral aspect of femur.

0

10

20

30

40

50

60

0 20 40 60 80 100 120 140Proximal-distal distance (mm)

Von

Mis

es S

tres

s (M

Pa)

.

stemmed

stemless

bone(control)

c

R1

R2

Fig. 7. Average Von Mises stresses over the medial aspect of femur.

region, which due to geometrical complications in the stemless implant model showed higher stressand strain concentration. Hence, it refers to the region, which is the weakest part in a stemlessreconstruction.

However, it should be noted that the range of strain contours in Fig. 8 are di2erent for threemodels. These maximum and minimum values are listed in Table 3.

In order to compare the exact value of strain in desired locations, same two paths of lateral andmedial were adopted to study the interface of bone with implant. Strain values as micro-strains wereplotted in Figs. 9 and 10 for lateral and medial aspects, respectively.For the intact bone, the strain along the same lateral path is higher than the stemless implant by

factor of 2.9 since it is shown to be 640.97 micro-strains. It was clearly shown that at the end of R1region from 32 mm proximal distance till 15 mm highest part of R2 region, there were higher strains

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Fig. 8. Strain pattern in three structure of intact bone, stemless and stemmed implants reconstructions.

Table 3The ranges of micro-strains contours for three models

Strain (micro-strains) Intact bone Stemless implant model Stemmed implant model

Minimum 33.1 15.3 12.2Maximum 4640 3840 1970

0

200

400

600

800

1000

1200

1400

1600

1800

0 10 20 30 40 50 60 70 80 90 100

Proximal-distal distance (mm)

Str

ain

(Mic

ro s

trai

n)

stemmed

stemless

bone(control)

R1

R2

Fig. 9. Average Von Mises strains over the lateral aspect of femur.

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0

500

1000

1500

2000

2500

0 20 40 60 80 100 120 140Proximal-distal distance (mm)

Str

ain

(Mic

ro s

trai

n) stemmed

stemless

bone(control)

R1R2

Fig. 10. Average Von Mises strains over the medial aspect of femur.

reported in stemless implant compared to the bone. This region is the weakest part in a stemlessreconstruction, which was also shown in Fig. 8.

Over the medial aspect, for the intact bone, the value of strain was again higher than stemlessimplant by the same factor of 2.9 in lateral path, as it was shown to be 700.08 micro-strains.

Similar to what was made known through Figs. 4 and 5, it is noticeable in Figs. 7 and 8 thatthe stemmed implant reported lower strains overall the both paths. This means that in the stemmedimplant the low value of stress in cortical shell induces less strain to the bone and implant interface,which is identi�ed by the medial and lateral paths. Meanwhile, for stemless implant the strain patternin the same location is similar to the bone model, unless some part of trochanter region in highestpart of R2 where the stemless implant had signi�cant increase in strain distribution. This region thatwas also identi�ed in Fig. 8 was in 22–26 mm distance from proximal head.

3.3. Loading condition

The results for stress and strain comparison in the models under torsion are presented in medialand lateral aspects of the models. Finally, the two loading conditions were compared by stress andstrain deviation of implant reconstruction to the control solution of the intact bone.

3.3.1. Torsion in lateral aspectFig. 11 shows the average Von Mises stress in lateral aspect of femur versus the distance from

the head of femur. Interestingly, in torsion condition the stemless implant model showed stressdistribution with the same trend of the intact bone along the lateral path, except in the femoral headregion (0–5 mm) and in the highest part of R2, below the sub-trochanter region (40–55 mm) whereits value deviated from that of the bone. On the other locations, the major di2erence between thestemless implant and the bone was not higher than 0:9254 MPa or deviation of 3.2% in R1 and3.2% in R2 regions.

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0

5

10

15

20

25

0 20 40 60 80 100Proximal-distal distance

Von

Mis

es s

tres

s (M

Pa)

stemmed

stemless

bone

R1R2

Fig. 11. Stress over the lateral path in torsion condition.

0

200

400

600

800

1000

1200

1400

1600

0 20 40 60 80 100Proximal-distal distance(mm)

Str

ain(

Mic

ro s

trai

n)

stemmed

stemless

bone

R1

R2

Fig. 12. Strain over the lateral path in torsion condition.

The twisting nature of this loading condition in lateral aspect of femur is well shown by oscillationsand sudden decrease and increase in stress distribution in the bone and the implants.

In stemmed implant model, though with smaller oscillations in stress concentration, the torsionalforces were transmitted by the implant along the interface of the bone and implant, causing thereduction of stress values all over the lateral path both in R1 and R2, except in the femoral neckof the model where there was higher stress concentration compared to the bone.

In order to investigate the e2ect of these stress concentrations in models, the strain distribution inlateral aspect was also studied and plotted in Fig. 12 for torsion loading condition.The strain changes in stemless implant were again following the same trend of its stress pattern

except in a small region of proximal head and also the highest part of R2, below the sub-trochanterregion. In other locations, the value of micro-strains had no signi�cant di2erence by maximum

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0

5

10

15

20

25

30

35

0 20 40 60 80 100 120 140Proximal-distal distance (mm)

Von

Mis

es s

tres

s (M

Pa)

stemmed

stemless

bone

R1R2

Fig. 13. Stress over the medial path in torsion condition.

deviation of 1.5% between stemless and bone models. The overall strains in lateral path, in thestemmed implant model, were lower than that of control solution of the bone as it was followingthe same trend of stress changes in this path. This was an indication that in stemmed implant model,the interface of the bone and implant had lower stresses causing strain reduction in this locationcompared to more stresses induced to the bone, which would result in more strain concentration inthe bone.

3.3.2. Torsion in medial aspectFig. 13 depicts the medial path in torsion, where the value of stress for the stemmed implant

model was 0:8166 MPa and it was 9:3118 MPa for the intact bone while it was the highest valuefor the stemless implant at 13:654 MPa. In R1, from 9 to 24 mm distance from head, the stemlessimplant showed stress values lower than the control solution of the bone, with minimum in 20 mmdistance from head where at the same location, the stemmed implant model reported high stressconcentration, which is still below the control solution. This could be justi�ed by considering thegeometrical design of two implants. That means, in this region the stemless implant is like a coverin outer space of femoral neck and in the stemmed reconstruction, the metallic implant is the innerpart, which is covered by the bone. Hence, in the interface of the bone and implant the stemlessimplant caused a region of stress reduction and the stemmed one showed higher stresses, which wereboth below the natural case of intact bone. The signi�cant di2erences of the stress between stemlessmodel and the bone in this region, which is the femoral neck of the models, was extended up to40 mm in medial aspect where afterwards in R2, the trend of stress alteration in both the stemlessimplant and the bone became one, with di2erences not more than 0.2% deviation.

Meanwhile, for the stemmed implant model, in medial aspect and toward the tip of stem from80 mm farther than the proximal head, there was an increase in value of stress compared to thebone. This could be justi�ed with the fact that in this loading condition most of torsional forceswere transmitted to the distal end of implant causing high stress concentration and twisting conditionin this region in the interface of the bone and the implant.

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0

200

400

600

800

1000

1200

1400

1600

1800

0 20 40 60 80 100 120 140Proximal-distal distance (mm)

Str

ain

(Mic

ro s

trai

n)m

stemmed

stemless

bone

R1 R2

Fig. 14. Strain over the medial path in torsion condition.

The strain distribution in medial aspect was studied and plotted in Fig. 14 for torsion loadingcondition.

For both implant models, in R1 region, which are femoral head, femoral neck and trochanter, thestrains reported in the stemless implant were less than that of the bone. However, in R2 region, thestemless implant showed the exact trend of strain distribution of the bone by di2erences not morethan 0.4%. For the stemmed implant model, in the interface of the implant with the bone, which ischaracterized by medial path of the models, the strains were lower all over the model in both R1and R2.

3.3.3. Torsion and bendingThe state of bending loading is compared with torsion loading condition; by deviation of stress

and strain values in implant reconstruction models. The stress and strain deviation in stemmed andstemless implant model was calculated with respect to the same values in control solution, naturalcase of intact bone.

Fig. 15 shows the stress deviation percent in stemmed and stemless implant models for both bend-ing and torsion conditions. It was clearly shown that the di2erence between bending and torsion instemless implant reconstruction was of slight importance especially in R2. Because in both loadingconditions, the stemless implant showed almost a perfect �t to control solution by very small devia-tions. Only in highest 15 mm part of R2, which is shown in magni�ed box in Fig. 15, the stemlessimplant deviated from the bone by stress alteration. This region is the weakest part of the stemlessreconstruction showed in Fig. 8.The stress deviation of the implanted reconstructions was comprehensive when the strain deviation

of the stemless and stemmed implant models with respect to the control solution were studied aswell. This is plotted in Fig. 16 showing the deviation of strain values in implant models in percent.Again it was clearly shown through this �gure that the models in bending and torsion cases had

no di2erent deviations in R2. In stemless implant reconstruction, the model in R1 had much higherdeviation from the bone in torsion compared to the same measure in bending. However, this modelagain showed a perfect �t to the intact bone in R2 region, both in bending and torsion cases. It

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-2

0

2

4

6

8

10

12

0 20 40 60 80 100 120 140

Proximal-distal distance

De

viat

ion

(%)

Stemmed(torsion)

Stemmed(bending)

Stemless(torsion)

Stemless(bending)

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

20 30 40 50 60 70 80

Fig. 15. Deviation of the average Von Mises stress of implants with respect to the bone (control solution).

-2

0

2

4

6

8

10

12

0 20 40 60 80 100 120 140

Proximal-distal distance

De

viat

ion

(%)

Stemmed(torsion)

Stemmed(bending)

Stemless(torsion)

Stemless(bending)

-0.1

0.05

0.2

0.35

0.5

20 30 40 50 60 70 80

Fig. 16. Deviation of the strain of implants with respect to the bone (control solution).

is clearly shown in the magni�ed box of Fig. 16 that in R2 region, the amount of deviation hasbecome very small after 15 mm highest part of R2. This region is the sub-trochanter area, whichwas also shown in Fig. 8. Again in this �gure it became clear that in R1, the strain distribution ofthe stemless implant reconstruction had the largest deviation from control solution of intact bone infemoral neck of the model. However, this deviation is less for the bending compared to the torsion.

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The stemmed implant showed very large deviation both in torsion and in bending in the mostproximal region of models in R1, which was depicted for stress and strain in Figs. 13 and 14,respectively. However, the stemmed model showed di2erent deviation in bending with torsion inboth R1 and R2. This is because of di2erent mechanisms of loading in two cases in which one ismulti-axial loading (torsion) and the other is only one axial load (bending).

3.4. Finite element analysis validation

The FE models of this research were based on SF model, which is widely used in several exper-imental and theoretical analyses. Hence, it was possible to compare and validate the results gainedhere with available experimental data in literature.

Several authors compared bone strains of FE models to experimental strains for the intact femur[13,25,26]. The level of agreement was generally usually low with R2 value ranged 0.59–0.98 withdi2erent models of intact femur. Stolk et al. [17] validated FE models of SF with experimentalmodels of composite femur and achieved the overall 10% agreement and R2 value of 0.992 withinthe bone and cement strains. They showed that high correlation between numerical and experimentalstrains could be obtained for both bone and cement strains, under both bending and torsional loadingconditions. However, their results could not be used as reference here as the current study is likethe non-cemented THA.

McNamara et al. [12] validated a mapped model of a reconstructed femur with a non-cementedstem. The loading condition and geometrical model of their study is the same of the current research;hence, it is possible to validate the FE results of this study based on that experimental results. Theycorrelated the FE results to experimental ones with the following equation:

y = 1109 + 1:05x; (5)

while the R2 value for their study was 0.96 showing that data-points belong to the same statisticaldistribution with the above-mentioned equation.

They studied three structures of intact femur, press-�t implant reconstruction and press-�t gluedimplant reconstruction models. The experimental results for intact femur were the basis for validationof the intact bone model in the current research. McNamara et al. [12] measured strain values inanterior, posterior, lateral and medial positions in di2erent levels. Three of the same positions wereidenti�ed in the FE models of the current study. Those were in R2 region starting from 50 mmdistance from proximal head in 10 mm intervals. The correlated experimental results were calculatedbased on Eq. (5). These values are shown in Table 4 with respect to experimental results ofMcNamara et al. [12].

The correlated experimental results of the current research and the literature’s experimental resultsare plotted for three positions in lateral and medial aspect in Fig. 17. In this �gure, PL1–PL3 referto three positions in lateral aspect and PM1–PM3 are positions in medial aspect of the femur. Theaverage deviation between these two quantities, which had the same nature, was calculated to be40.7% in medial and 22.7% in lateral positions.

However, an excellent �tting was found when the FE results of this research were compared withabove-mentioned experimental results. This is shown in Fig. 18 where the R2 value shown to be0.9945. That means the FE results of this study belongs to the same statistical distribution found

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Table 4Micro-strains of the current research and McNamara et al.

Proximal distal distance (mm) Micro-strain

FEA results of Correlated Literaturethe current research experimental results experimental results

of the current research (McNamara et al., 1997)

Lateral Medial Lateral Medial Lateral Medial

50 200 −1130 220.19 −1176.31 390 −95060 350 −830 377.69 −861.31 550 −65070 580 −1020 619.19 −1060.81 620 −800

-1500 -1000 -500 0 500 1000

PL1

PL2

PL3

PM1

PM2

PM3

Pos

ition

s

Micro strain

Current research

McNamara et al.

Fig. 17. Correlated experimental results of current research and literature experimental results in di2erent lateral and medialpositions.

R2 = 0.9945

-1200

-800

-400

0

400

800

-1500 -1000 -500 0 500 1000

FE results

Exp

erim

enta

l res

ults

Fig. 18. FE results of current study in comparison with literature experimental results.

S.A. Asgari et al. / Finite Elements in Analysis and Design 40 (2004) 2027–2047 2045

in experimental research. This is absolutely reliable since all the parameters such as BW, loadingcondition; geometrical model and boundary conditions in two studies are basically the same.

4. Discussion

The stemless implant was a perfect �t to the values of strain trend in the intact bone model in R2.In the same model, strain concentration had a sudden increase in strain values in region 22–26 mmfrom femoral head in R1, from about 200 to 1400 micro-strains. This region was in fact the femoralneck of the model where the stemless implant would be in direct contact with the bone, and theinterface of the implant and the bone had sudden changes in strain values in medial aspect either inbending condition or torsion. Hence, this region could become a source of problem post-operatively,where it might fail. This was also veri�ed by high deviation of stemless implant reconstruction fromthe intact bone in the femoral neck.

The other major problematic area for stemless implant concept was recognized to be the highestpart of R2, in sub-trochanteric area. In this region, a high strain concentration was seen in lateralside of model that lead to strain reduction in bone in surrounding areas. Once this happened, itcould result either in implant displacement or in bone atrophy and consequent bone loss in this area,which is formed by spongy cortical bone.

This is also consistent with results of [3], where they applied a cabling system to anchor theimplant in the same region. That might transfer some of strains in this region circumferentially. Inanother study of stemless type of implant, Munting and Verhelpen [1] suggest that the displacementbetween the greater trochanter and the upper part of the stemless implant are most likely to be lessin vivo. However, they reported fracture and large displacement in six experiments in the calcarregion. They associated those failed experiments to various position, lack of complete bone–implantcontact and incomplete cortical support of the inferior and medial surface of the implant. Hence,the high strain area shown in this study in cortical bone in sub-trochanter region could be a properjusti�cation for such failures.

Joshi et al. [3] compared two designs of stemless implant with strain values in di2erent levelsof same models. These two types of stemless implant were Munting and Verhelpen [1] designversus Advani et al. [4] design. They conclude that the Advani design consistently produces lessstress shielding than the traditional intramedullary design or Munting and Verhelpen design. This isveri�ed to be true in comparison to stemless generic type of implant versus intramedullary implantsby the current research. However, di2erent designs of stemless type of implant might have di2erentoutcomes as stated by Joshi et al. [3].

The other currently introduced stemless implant is Chieng et al. [5] design, which covers thefemoral neck by its skirt mask and �xes with metaphyseal anchoring by a central screw. This typeof implant is basically same as Munting and Verhelpen as the latter one also has central �xation bymetallic screws. While it has the advantage of bone stock preservation in femoral neck, if the bonegrowth and adaptive remodeling of the bone in femoral bone cannot provide enough �xations forthe implant, it might displace largely in great trochanter. However, Chieng et al. [5] design has twoside screws that �x the implant in greater and lesser trochanter. These screws might greatly improvethe �xation of the implant but they compromise the long-term service of the implant especially ingreater trochanter area. In this area the sti2 metallic screws of Chieng et al. design passes diagonally

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toward sub-trochanter region, which is shown to have high gradient of strain based on results of thecurrent study. The net e2ect would be a more complex redistribution of stress and strains in thisbrittle lateral area.

A large number of numerical and experimental in vivo and in vitro tests must be carried out inorder to comprehensively identify failure scenarios that might happen for a speci�c stemless implantdesign in service, especially in R1 region. This might lead to an ideal stemless implant that wouldbe able to preserves elasticity and vascularity of the proximal femur and acetabulum as well asthe bone stock preservation that would bring the advantage of easy revision surgery. The revisionsurgery would be applicable to any stemless implant reconstruction without further bone loss by anyconventional prosthesis. By its immediate �xation, it would be able to bear full body weight withoutmicro-motion and its long-term �xation would make it capable to withstand all loads supported bythe normal hip.

5. Conclusion

The stemless implant showed perfect �t to the control solution in R2 region except for the 14 mmhighest part of this region where the stemless implant showed strain reduction in the interface of thebone and the implant. This region was sub-trochanter and was concluded to practically be the weakpoint of this type of implant. Meanwhile, the stemless implant type had signi�cant changes in stressand strain distribution in R1 region. Great amount of care must be taken in femoral neck region whendesigning such an implant as this region showed to be a sensitive region to design variations. Hence,the speci�c stemless implant must be designed in a way that it could withstand the high gradientof stress and strain in this region. However, results with stemless implant shows that stress patterndoes not alter signi�cantly and if the loading mechanism of bone does not change, the probabilityof failing of implant service will be lessened. It is on the contrary with conventional implants thatgreatly deviate from loading mechanism of intact bone, and consequently lead to loosening. Thiscan justify further investigation for applicability of any speci�c stemless design based on generictype of stemless hip prostheses.

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