The determination of biomechanical force systems of implanted femurs to obtain adequate strain measurements has been neglected in many published studies. Due to geometric alterations induced by surgery and those inherent to the design of the prosthesis, the loading system changes because the lever arms are modified. This paper discusses the determination of adequate loading of the implanted femur based on the intact femur-loading configuration. Four reconstructions with Lubinus SPII, Charnley Roundback, Müller Straight and Stanmore prostheses were used in the study. Pseudophysiologic and nonphysiologic implanted system forces were generated and assessed with finite element analysis. Using an equilibrium system of forces composed by the Fx (medially direction) component of the hip contact force and the bending moments Mx (median plane) and My (coronal plane) allowed adequate, pseudo-physiological loading of the implanted femur. We suggest that at least the bending moment at the coronal plane must be restored in the implanted femur-loading configuration.

## Introduction

Finite element analysis (FEA) is a powerful tool and even though the use of it has been criticized because of the lack of validation, it is the only way forward to explore “possible” solutions, even if quantitative results cannot be obtained due to the missing of biological information (1,2). Besides other merits, finite element models can be used to distinguish and predict the performance of implants.

FE analyses depend on simulation parameters like tissue geometry replication, material properties, boundary conditions (loading and fixation), and finite element selection. Musculoskeletal tissues have irregular geometry and so finite element modeling is increasingly carried out using digitized images generated from computer tomography scanning (1). Bone materials are normally assumed to be isotropic and homogeneous medias, whereas it is known that they are highly anisotropic and inhomogeneous, in particular cancellous bone (see, e.g., (3)). Time-dependent mechanical properties of tissues are also seen as critical to the improvement of biomechanical models (1,4). Loading and fixation conditions are relevant input data that can strongly influence results (5,6). Loads applied to finite element models have been strongly simplified and many papers report all sorts of loading configurations, namely, in total hip and knee replacements (see e.g., (7,8)) and seem to be a strong limitation on the quantitative accuracy of finite element results. Some researchers have focused on the development of improved geometric precision, whereas others have focused on improved representations of material behavior (1).

There are other factors intrinsically related to the FEA itself. The finite element mesh is a key factor for an efficient analysis and much research has been done on meshing and element performance (see, e.g., (9,10,11,12,13,14,15,16,17,18,19,20,21,22,23)). Finite element models must be sufficiently refined to accurately represent the geometry and mechanical behavior of the bone structure (17,19). The results of these models are mesh sensitive and convergence tests must be made to test the model accuracy (16). Convergence tests can be done comparing nodal displacements and/or total strain energy (18,19), or stresses and strains (19).

The biomechanics of the hip is an extremely complex system and the correct knowledge of the functioning of muscles, ligaments, and the hip contact force (HCF) is unknown. Many authors have studied this problem using numerical and experimental models (8,24,25,26,27). Simulation of physiological loading of the hip is of considerable importance to improve prostheses design, bone remodeling simulations and mechanical testing of implants (27).

Other aspect of loading of the implanted femur is related to the change of forces (magnitude and direction) of the hip when a replacement is performed. The HCF and muscle forces are modified due to geometric alterations, as for example, changes of the position of the head or of the great trochanter provoked by surgery. The prosthesis does not exactly restore the original head center and lever arms are different. Although the effect of a femoral head displacement is important from a surgical point of view (28), this variable must be controlled when numerical or experimental studies are performed and since the head location cannot be restored exactly, it is necessary to analyze how it can be misleading in assessing the performance of different designs (28,29,30).

The intact femur undergoes a system of loads and moments that cannot be transposed in simulations of the implanted femur. We remind that the femur is statically undetermined due to the joint, ligaments, and muscle forces. There is no perfect solution when one tries to replicate the three forces and three moments as in the intact femur with identical femora implanted with different stems. Following Cristofolini and Viceconti (5), there are two main options: one that applies the same force magnitude for the hip joint and the abducting force (same forces) and a different moment will be applied due to the changed lever arms; and one that applies the same bending moment to the femur and different force values are required. In this case, the magnitude, direction, and position of the resultant vector force must stay constant with respect to the femoral diaphysis (5).

There seems to be no agreement in the literature on the preferred solution (31). However, Cristofolini and Viceconti (5) refer that to compensate for the unavoidable geometry changes, the implanted femur should be loaded in such a way as to apply the same bending moment, rather than the same forces as in the intact femur. If this is not done, errors can be expected in the strain measurement that possibly overshadow existing differences between implants, or give the impression that the difference exists when, in fact, the variations observed depend merely on the loading setup (5). In several cases in the literature, large strain differences are found below the stem tip in the implanted femur in comparison to the intact femur (32,33,34).

A detailed numerical study was performed to determine how load configurations of the implanted femur can undermine the reliability of strain measurements. Based on the intact femur loading configurations of walking during gait, implanted femora with Lubinus SPII, Charnley Roundback, Müller Straight, and Stanmore loading configurations were analyzed. A set of plausible combinations of forces with bending and torsion moments was simulated and analyzed. The difference of strain in all aspects of the intact and the implanted femur was compared to select the suitable loading configuration(s) for each of the prostheses assessed in this study.

## Materials and Methods

The assessment of different types of cemented hip arthroplasties was made through finite element analysis. Three-dimensional (3D) computer aided design (CAD) models representing cemented hip joint reconstructions with Charnley Roundback, Lubinus SPII, Stanmore, and Müller Straight (Fig. 1) prostheses were used. For this purpose, in vitro femoral replacements were performed on synthetic composite femurs (third generation, left, mod. 3306, Pacific Research Labs, Vashon Island, WA) by a surgeon that followed strictly the surgical protocol of each one of the prostheses. The replacements provoked offsets ranging from $28.25mm$ (Charnley Roundback) to $35.48mm$ (Stanmore). The large left composite femur (mod. 3306, Pacific Research Labs, Vashon Island, WA) was used as reference geometry for the finite element analysis (35). It is a 3D solid model made available in public domain derived from computer tomography (CT)-scan dataset of the composite femur. The CAD models of the prostheses were obtained by reverse engineering. The geometry of the cement mantle and position of the stem in the femur were determined from CT scans of the reconstructions.

Figure 1
Figure 1
Close modal

The numerical simulations were preformed with $Hyperworks®$ 5.1 (Altair Engineering, Inc., Troy, MI) finite element analysis software, using pre- and post-processor $HYPERMESH®$ 5.1 and solver $HYPERSTRUCT®$ 5.1, respectively. Four-node linear tetrahedral elements were selected to generate the numerical models. In Table 1 the number of nodes and elements for the implanted femurs are identified. Convergence tests of the finite element meshes were previously performed and the models refinement was sufficient enough to obtain accurate stress and strain predictions. The experimental models were validated successfully by strain gauge measurements.

Table 1

Nodes $∣$ elements of the FEA models

 Model Lubinus SPII Charnley Roundback Müller Straight Stanmore Cortical bone $38002∣167051$ $38087∣166023$ $41328∣181625$ $36291∣157684$ Cancellous bone $16452∣68996$ $16887∣74334$ $19145∣82413$ $19070∣80483$ Cement mantle $26476∣105621$ $23281∣96612$ $26562∣109375$ $25147∣105868$ Prosthesis $19024∣82821$ $12599∣53933$ $16831∣73279$ $14072∣60572$
 Model Lubinus SPII Charnley Roundback Müller Straight Stanmore Cortical bone $38002∣167051$ $38087∣166023$ $41328∣181625$ $36291∣157684$ Cancellous bone $16452∣68996$ $16887∣74334$ $19145∣82413$ $19070∣80483$ Cement mantle $26476∣105621$ $23281∣96612$ $26562∣109375$ $25147∣105868$ Prosthesis $19024∣82821$ $12599∣53933$ $16831∣73279$ $14072∣60572$

The cortical and cancellous bone replicating materials, femoral component and cement mantle were assumed to be isotropic and linearly elastic. The elastic properties of all materials of the reconstructions are presented in Table 2. The loading configuration used in the study represents the one around the hip during the most strenuous phase of the walking cycle (Table 3) (36). The loading comprises the hip contact force (HCF), the glutei, the tensor fasciae latae, and the vastus lateralis (Fig. 2) and was proposed by Bergmann et al. (37,38) and Heller et al. (39) for mechanical testing of hip replacement reconstructions (36).

Table 2

Mechanical properties of the finite element models

 Part of model Material type Elastic modulus (GPa) Poisson’s ratio Cortical bone Glass-fiber reinforced epoxy 14.2 0.28 Cancellous bone Polyurethane foam 0.280 0.3 Cement Polymethylmethacrylate 3.0 0.3 Stem $CoCr$ alloy 210 0.3
 Part of model Material type Elastic modulus (GPa) Poisson’s ratio Cortical bone Glass-fiber reinforced epoxy 14.2 0.28 Cancellous bone Polyurethane foam 0.280 0.3 Cement Polymethylmethacrylate 3.0 0.3 Stem $CoCr$ alloy 210 0.3
Table 3

Representation of the hip joint and muscle force magnitudes during walking

 Vector forces (N) $X$ medially $Y$ anteriorly $Z$ proximally Hip force contact (HCF) $−405$ $−246$ $−1719$ Abductors (glutei) 435 32 649 Tensor fasciae latae (proximal part) 54 87 99 Tensor fasciae latae (distal part) $−4$ $−5,3$ $−143$ Vastus Lateralis $−7$ 139 $−697$
 Vector forces (N) $X$ medially $Y$ anteriorly $Z$ proximally Hip force contact (HCF) $−405$ $−246$ $−1719$ Abductors (glutei) 435 32 649 Tensor fasciae latae (proximal part) 54 87 99 Tensor fasciae latae (distal part) $−4$ $−5,3$ $−143$ Vastus Lateralis $−7$ 139 $−697$
Figure 2
Figure 2
Close modal

One possible way to determine the implanted femur loading system is to compare the strain measured below the femoral stem tip before and after implantation. Theoretically, these values must be identical because they belong to an area far from the influence of the implant (5). To determine the changed biomechanical forces of the implanted femur, the moments and forces transmitted through the intact femur must be determined. The moments and forces, at a region of the cortex of the intact femur, far enough from the tip of the prosthesis and from the region of the fixed condyles can be compared with those of the implanted femur. To generate tentative loading configurations for the different hip replacements, intact and implanted femur strains were compared in a region of the diaphysis of the femur out of influence of the stem. To obtain the implanted HC and abductors forces, two situations were analyzed:

• Equilibrium of forces and moments considering the implanted HC vector force (direction and magnitude) variable;

• Equilibrium of forces and moments considering the implanted HC and abductors vector forces variable, but the direction of the abductor force was kept unchanged.

The magnitudes and directions of the distal and proximal tensor fasciae latae and the vastus lateralis were kept unchanged.

Considering the equilibrium forces for the intact femur, the following equations can be setup (Figs. 2,3):
$∣∑Fxint_femur=ABDx+HCFx∑Fyint_femur=ABDy+HCFy∑Fzint_femur=ABDz+HCFz∑Mxint_femur=HCFya−HCFze+ABDyd∑Myint_femur=−HCFxa−HCFzb−ABDxd+ABDzc∑Mzint_femur=HCFyb−ABDyc∣$
1
If the abductor force direction is considered unchanged:
$∣ABDy=αABDzABDx=βABDz∣$
2
where $k1$ and $k2$ are constants. For the abductors direction force considered, $k1=−1.490$ and $k2=−0.0741$. Figure 3 shows the geometric dimensions (Table 4) used to derive the force systems of the reconstructed femurs. Dimensions $c$ and $d$ (distances from the glutei insertion point) were kept constant. The system of Eqs. 1 allows 35 possible combinations, where 27 have solutions and 8 have indeterminate solutions. All solutions were analyzed, but only the first nine load cases (Table 5) are discussed in this paper that includes physiological and non-physiological load configuration systems.
Table 4

Geometric dimensions of intact and implanted femurs

 a b c d e Intact femur 230 44 25 205 0 Charnley Roundback 227.16 28.25 25 205 0.50 Lubinus SPII 223.55 33.37 25 205 $−1.51$ Stanmore 230.48 35.48 25 205 0.61 Muller Straight 221.83 32.79 25 205 $−4.71$
 a b c d e Intact femur 230 44 25 205 0 Charnley Roundback 227.16 28.25 25 205 0.50 Lubinus SPII 223.55 33.37 25 205 $−1.51$ Stanmore 230.48 35.48 25 205 0.61 Muller Straight 221.83 32.79 25 205 $−4.71$
Table 5

Moments and forces considered in the tentative load cases studied

 Fx Fy Fz Mx My Mz $Case_0$ Intact hip force system $Case_1$1 X X X $Case_2$1 X X X $Case_3$1 X X X $Case_4$1 X X X $Case_5$ X X X X $Case_6$ X X X X $Case_7$ X X X X $Case_8$ X X X X $Case_9$ X X X X $Case_10$ X X X $Case_11$ X X X $Case_12$ X X X $Case_13$ X X X $Case_14$ X X X $Case_15$ X X X $Case_16$ X X X $Case_17$ X X X $Case_18$ X X X $Case_19$ X X X X $Case_20$ X X X X $Case_21$ X X X X $Case_22$ X X X X $Case_23$ X X X X $Case_24$ X X X X $Case_25$ X X X X $Case_26$ X X X X $Case_27$ X X X X
 Fx Fy Fz Mx My Mz $Case_0$ Intact hip force system $Case_1$1 X X X $Case_2$1 X X X $Case_3$1 X X X $Case_4$1 X X X $Case_5$ X X X X $Case_6$ X X X X $Case_7$ X X X X $Case_8$ X X X X $Case_9$ X X X X $Case_10$ X X X $Case_11$ X X X $Case_12$ X X X $Case_13$ X X X $Case_14$ X X X $Case_15$ X X X $Case_16$ X X X $Case_17$ X X X $Case_18$ X X X $Case_19$ X X X X $Case_20$ X X X X $Case_21$ X X X X $Case_22$ X X X X $Case_23$ X X X X $Case_24$ X X X X $Case_25$ X X X X $Case_26$ X X X X $Case_27$ X X X X

Abductors force magnitude and direction unchanged.

Figure 3
Figure 3
Close modal

For load $Case_1$, load $Case_2$, load $Case_3$, and load $Case_4$ the HCF was allowed to change in magnitude and direction and the other muscle forces were kept unchangeable. For the other cases studied, the HCF and the abductors force was allowed to change in magnitude, maintaining the direction of the abductors force unchangeable. Table 6 contains the HCF and abductor forces derived using Eqs. 1,2 for the four hip reconstructions analyzed and for the load cases selected for discussion. These load cases were simulated and strains compared with those obtained for the intact femur. Table 7 contains all the other (18) possible load configurations.

Table 6

HC and abductors force components used in the simulations of reconstructed femurs (first nine load cases of Table 5)

 Lubinus SPII Charnley Roundback Müller Straight Stanmore Force (N) Med. Ant. Prox. Med. Ant. Prox. Med. Ant. Prox. Med. Ant. Prox. HFC $−405$ $−246$ $−1719$ $−405$ $−246$ $−1719$ $−405$ $−246$ $−1719$ $−405$ $−246$ $−1719$ $Case_0$ ABD 435 32 649 435 32 649 435 32 649 435 32 649 HFC $−291$ $−383$ $−1719$ 137 $−349$ $−1719$ 117 $−328$ $−1719$ 142 $−355$ $−1719$ $Case_1$ ABD 435 32 649 435 32 649 435 32 649 435 32 649 HFC $−405$ $−383$ $−2637$ $−405$ $−349$ $−5347$ $−405$ $−328$ $−5108$ $−405$ $−355$ $−5420$ $Case_2$ ABD 435 32 649 435 32 649 435 32 649 435 32 649 HFC $−291$ $−253$ $−1719$ $−335$ $−265$ $−1719$ $−341$ $−250$ $−1719$ $−333$ $−292$ $−1719$ $Case_3$ ABD 435 32 649 435 32 649 435 32 649 435 32 649 HFC $−405$ $−255$ $−2637$ $−405$ $−268$ $−2188$ $−405$ $−251$ $−2137$ $−405$ $−302$ $−2206$ $Case_4$ ABD 435 32 649 435 32 649 435 32 649 435 32 649 HFC $−858$ $−280$ $−2394$ $−634$ $−263$ $−2060$ $−634$ $−263$ $−2061$ $−635$ $−263$ $−2062$ $Case_5$ ABD 888 66 1324 664 49 990 664 49 991 665 49 992 HFC $−1492$ $−320$ $−3150$ $−1289$ 284 $−2808$ $−918$ $−282$ $−2391$ $−1901$ $−263$ $−3500$ $Case_6$ ABD 1394 103 2080 1166 87 1738 886 66 1321 1630 121 2430 HFC $−1461$ $−324$ $−2071$ 2851 $−4$ $−4016$ $−972$ $−288$ $−1949$ $−5362$ $−614$ 887 $Case_7$ ABD 1491 111 2223 $−2821$ $−209$ $−4207$ 1002 74 1494 5392 400 8042 HFC $−2465$ $−375$ $−4309$ 29 $−225$ $−1304$ $−1558$ $−316$ $−3136$ 633 $−192$ $−622$ $Case_8$ ABD 2172 161 3239 157 12 234 1385 103 2066 $−301$ $−22$ $−449$ HFC $−1365$ $−320$ $−3150$ $−1136$ $−284$ $−2808$ $−856$ $−282$ $−2391$ $−1600$ $−263$ $−3500$ $Case_9$ ABD 1394 103 2080 1166 87 1738 886 66 1321 1630 121 2430
 Lubinus SPII Charnley Roundback Müller Straight Stanmore Force (N) Med. Ant. Prox. Med. Ant. Prox. Med. Ant. Prox. Med. Ant. Prox. HFC $−405$ $−246$ $−1719$ $−405$ $−246$ $−1719$ $−405$ $−246$ $−1719$ $−405$ $−246$ $−1719$ $Case_0$ ABD 435 32 649 435 32 649 435 32 649 435 32 649 HFC $−291$ $−383$ $−1719$ 137 $−349$ $−1719$ 117 $−328$ $−1719$ 142 $−355$ $−1719$ $Case_1$ ABD 435 32 649 435 32 649 435 32 649 435 32 649 HFC $−405$ $−383$ $−2637$ $−405$ $−349$ $−5347$ $−405$ $−328$ $−5108$ $−405$ $−355$ $−5420$ $Case_2$ ABD 435 32 649 435 32 649 435 32 649 435 32 649 HFC $−291$ $−253$ $−1719$ $−335$ $−265$ $−1719$ $−341$ $−250$ $−1719$ $−333$ $−292$ $−1719$ $Case_3$ ABD 435 32 649 435 32 649 435 32 649 435 32 649 HFC $−405$ $−255$ $−2637$ $−405$ $−268$ $−2188$ $−405$ $−251$ $−2137$ $−405$ $−302$ $−2206$ $Case_4$ ABD 435 32 649 435 32 649 435 32 649 435 32 649 HFC $−858$ $−280$ $−2394$ $−634$ $−263$ $−2060$ $−634$ $−263$ $−2061$ $−635$ $−263$ $−2062$ $Case_5$ ABD 888 66 1324 664 49 990 664 49 991 665 49 992 HFC $−1492$ $−320$ $−3150$ $−1289$ 284 $−2808$ $−918$ $−282$ $−2391$ $−1901$ $−263$ $−3500$ $Case_6$ ABD 1394 103 2080 1166 87 1738 886 66 1321 1630 121 2430 HFC $−1461$ $−324$ $−2071$ 2851 $−4$ $−4016$ $−972$ $−288$ $−1949$ $−5362$ $−614$ 887 $Case_7$ ABD 1491 111 2223 $−2821$ $−209$ $−4207$ 1002 74 1494 5392 400 8042 HFC $−2465$ $−375$ $−4309$ 29 $−225$ $−1304$ $−1558$ $−316$ $−3136$ 633 $−192$ $−622$ $Case_8$ ABD 2172 161 3239 157 12 234 1385 103 2066 $−301$ $−22$ $−449$ HFC $−1365$ $−320$ $−3150$ $−1136$ $−284$ $−2808$ $−856$ $−282$ $−2391$ $−1600$ $−263$ $−3500$ $Case_9$ ABD 1394 103 2080 1166 87 1738 886 66 1321 1630 121 2430
Table 7

The other HC and abductors force components of reconstructed femurs

 Lubinus SPII Charnley Roundback Müller Straight Stanmore Force (N) Med. Ant. Prox. Med. Ant. Prox. Med. Ant. Prox. Med. Ant. Prox. HFC $−405$ 246 1719 $−405$ 246 1719 $−405$ 246 1719 $−405$ 246 1719 $Case_10$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−405$ 246 $−1397$ $−405$ 246 1051 $−405$ 246 194 $−405$ 246 427 $Case_11$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−405$ 246 148972 $−405$ 246 $−48360$ $−405$ 246 124312 $−405$ 246 $−15356$ $Case_12$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−405$ 253 1719 $−405$ 241 1719 $−405$ 250 1719 $−405$ 219 1719 $Case_13$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−405$ 383 1719 $−405$ 324 1719 $−405$ 305 1719 $−405$ 330 1719 $Case_14$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−81$ 246 1719 $−67$ 246 1719 $−81$ 246 1719 $−42$ 246 1719 $Case_15$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−405$ 383 60913 $−405$ 324 $−10551$ $−405$ 305 22514 $−405$ 330 $−3534$ $Case_16$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−74$ 246 $−1397$ $−71$ 246 1051 $−77$ 246 194 $−70$ 246 427 $Case_17$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−211$ 383 60913 $−150$ 324 $−10551$ $−136$ 305 22514 $−154$ 330 $−3534$ $Case_18$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−2142$ 375 4309 $−127$ 225 1304 $−1355$ 316 3136 331 191 622 $Case_19$ ABD 2172 $−161$ $−3239$ 157 $−12$ $−234$ 1385 $−103$ $−2066$ $−301$ 22 449 HFC $−1386$ 319 3183 $−1009$ 291 2620 $−872$ 281 2416 $−1049$ 294 2679 $Case_20$ ABD 1416 $−105$ $−2112$ 1039 $−77$ $−1550$ 902 $−67$ $−1346$ 1079 $−80$ $−1609$ HFC $−4889$ 578 13336 $−4115$ 521 $−2328$ $−5934$ 656 17316 $−3830$ 500 $−481$ $Case_21$ ABD 4919 $−365$ $−7336$ 4145 $−307$ $−6182$ 5964 $−442$ $−8895$ 3860 $−286$ $−5757$ HFC $−1386$ 319 1827 $−1009$ 291 500 $−872$ 281 1641 $−1049$ 294 256 $Case_22$ ABD 1416 $−105$ $−2112$ 1039 $−77$ $−1550$ 902 $−67$ $−1346$ 1079 $−80$ $−1609$ HFC $−1386$ 319 119285 $−1009$ 291 $−40863$ $−872$ 281 115266 $−1049$ 294 $−12561$ $Case_23$ ABD 1416 $−105$ $−2112$ 1039 $−77$ $−1550$ 902 $−67$ $−1346$ 1079 $−80$ $−1609$ HFC $−5044$ 578 8638 $−4993$ 507 8561 $−6287$ 661 10491 $−6393$ 439 10650 $Case_24$ ABD 5074 $−376$ $−7568$ 5023 $−372$ $−7491$ 6317 $−468$ $−9421$ 6423 $−476$ $−9580$ HFC $−1131$ 322 3183 $−716$ 276 2620 $−574$ 283 2416 $−725$ 242 2679 $Case_25$ ABD 1416 $−105$ $−2112$ 1039 $−77$ $−1550$ 902 $−67$ $−1346$ 1079 $−80$ $−1609$ HFC 2535 576 237895 6024 681 $−128126$ 3618 515 202172 7241 762 $−48559$ $Case_26$ ABD $−2505$ 186 3735 $−5994$ 444 8939 $−3588$ 266 5352 $−7211$ 535 10754 HFC $−1128$ 319 1827 $−730$ 291 500 $−572$ 281 1641 $−111$ 294 256 $Case_27$ ABD 1416 $−105$ $−2112$ 1039 $−77$ $−1550$ 902 $−67$ $−1346$ 1079 $−80$ $−1609$
 Lubinus SPII Charnley Roundback Müller Straight Stanmore Force (N) Med. Ant. Prox. Med. Ant. Prox. Med. Ant. Prox. Med. Ant. Prox. HFC $−405$ 246 1719 $−405$ 246 1719 $−405$ 246 1719 $−405$ 246 1719 $Case_10$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−405$ 246 $−1397$ $−405$ 246 1051 $−405$ 246 194 $−405$ 246 427 $Case_11$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−405$ 246 148972 $−405$ 246 $−48360$ $−405$ 246 124312 $−405$ 246 $−15356$ $Case_12$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−405$ 253 1719 $−405$ 241 1719 $−405$ 250 1719 $−405$ 219 1719 $Case_13$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−405$ 383 1719 $−405$ 324 1719 $−405$ 305 1719 $−405$ 330 1719 $Case_14$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−81$ 246 1719 $−67$ 246 1719 $−81$ 246 1719 $−42$ 246 1719 $Case_15$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−405$ 383 60913 $−405$ 324 $−10551$ $−405$ 305 22514 $−405$ 330 $−3534$ $Case_16$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−74$ 246 $−1397$ $−71$ 246 1051 $−77$ 246 194 $−70$ 246 427 $Case_17$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−211$ 383 60913 $−150$ 324 $−10551$ $−136$ 305 22514 $−154$ 330 $−3534$ $Case_18$ ABD 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ 435 $−32$ $−649$ HFC $−2142$ 375 4309 $−127$ 225 1304 $−1355$ 316 3136 331 191 622 $Case_19$ ABD 2172 $−161$ $−3239$ 157 $−12$ $−234$ 1385 $−103$ $−2066$ $−301$ 22 449 HFC $−1386$ 319 3183 $−1009$ 291 2620 $−872$ 281 2416 $−1049$ 294 2679 $Case_20$ ABD 1416 $−105$ $−2112$ 1039 $−77$ $−1550$ 902 $−67$ $−1346$ 1079 $−80$ $−1609$ HFC $−4889$ 578 13336 $−4115$ 521 $−2328$ $−5934$ 656 17316 $−3830$ 500 $−481$ $Case_21$ ABD 4919 $−365$ $−7336$ 4145 $−307$ $−6182$ 5964 $−442$ $−8895$ 3860 $−286$ $−5757$ HFC $−1386$ 319 1827 $−1009$ 291 500 $−872$ 281 1641 $−1049$ 294 256 $Case_22$ ABD 1416 $−105$ $−2112$ 1039 $−77$ $−1550$ 902 $−67$ $−1346$ 1079 $−80$ $−1609$ HFC $−1386$ 319 119285 $−1009$ 291 $−40863$ $−872$ 281 115266 $−1049$ 294 $−12561$ $Case_23$ ABD 1416 $−105$ $−2112$ 1039 $−77$ $−1550$ 902 $−67$ $−1346$ 1079 $−80$ $−1609$ HFC $−5044$ 578 8638 $−4993$ 507 8561 $−6287$ 661 10491 $−6393$ 439 10650 $Case_24$ ABD 5074 $−376$ $−7568$ 5023 $−372$ $−7491$ 6317 $−468$ $−9421$ 6423 $−476$ $−9580$ HFC $−1131$ 322 3183 $−716$ 276 2620 $−574$ 283 2416 $−725$ 242 2679 $Case_25$ ABD 1416 $−105$ $−2112$ 1039 $−77$ $−1550$ 902 $−67$ $−1346$ 1079 $−80$ $−1609$ HFC 2535 576 237895 6024 681 $−128126$ 3618 515 202172 7241 762 $−48559$ $Case_26$ ABD $−2505$ 186 3735 $−5994$ 444 8939 $−3588$ 266 5352 $−7211$ 535 10754 HFC $−1128$ 319 1827 $−730$ 291 500 $−572$ 281 1641 $−111$ 294 256 $Case_27$ ABD 1416 $−105$ $−2112$ 1039 $−77$ $−1550$ 902 $−67$ $−1346$ 1079 $−80$ $−1609$
An equivalent strain value that takes into account the strain values at the medial, lateral, anterior, and posterior aspects of the femur was obtained using the following equation:
$Δε=(ϵMint−ϵMimp)2+(ϵLint−ϵLimp)2+(ϵAint−ϵAimp)2+(ϵPint−ϵPimp)24$
3
being eM, eL, eA, and eP the strain at the medial, lateral, anterior, and posterior aspects of the intact (int) and implanted (imp) femur. This root-mean-squared strain value approximates in excess the difference between the intact and implanted strains.

The intact femur load system was used for the implanted configurations and is referred as load $Case_0$. The other load cases were simulated to illustrate the relevance of the vector components of the HCF on the bending (all load configurations) and torsional moments ($Case_1$, $Case_2$, $Case_6$, and $Case_9$). Some of the load cases were simulated to show that although the force system is in equilibrium, they can provoke non-physiological load configurations (e.g., $Case_7$ for the Charnley Roundback and Stanmore prostheses and $Case_8$ for all designs). The strains were compared in the $x$, $y$, and $z$ directions. The most suitable loading configuration of the implanted femur will be the one that minimizes the difference in strains (intact femur strain minus implanted femur strain).

## Results and Discussion

Considering the HC and abductors forces derived from Eqs. 1,2 and although the system forces is in equilibrium, loading configurations for which the sense of the force is opposite to the physiological one can be obtained (for $Case_7$ and Stanmore prosthesis we can observe that the sense of the HCF is toward proximal, whereas physiologically it is toward distal) and the intensity is significantly different than what is observed in vivo (e.g., $Case_2,Case_6$, and $Case_9$ provoke very high intensity forces and $Case_7$ and $Case_8$ provoke very low intensity forces). Other load cases show considerable high or low force magnitudes that are unlikely to occur in vivo. Interesting to note that independently on the prosthesis design, the $z$ component of the HCF is considerable higher for all implanted configurations. The $z$ component of the HCF of the implanted reconstruction for load $Case_4$, relatively to the intact femur, is higher in an excess of 918, 469, 418, and $487N$ for the Lubinus SPII, Charnley Roundback, Müller Straight, and Stanmore prostheses, respectively.

For the load cases presented in Table 6, the strain values in the $x$, $y$, and $z$ directions were picked at a distance of $20mm$ down from the tip of the longest stem (Lubinus SPII). This region of the femur is out of the influence of the stem, which was confirmed numerically with finite element models and with uniaxial strain gauges glued to composite replica femurs. The absolute difference $(Dez=∣ezintact−ezimplanted∣)$ between strains generated by the intact and implanted femurs are presented in Table 8. Table 9 presents the mean-squared strain difference for the tentative solutions analyzed.

Table 8

Absolute difference between strains generated by the intact and implanted femurs (all tentative load configurations)

 Strain $(με)$ Lubinus SPII Chamley Müller Straight Stanmore Med Post Ant Lat Med Post Ant Lat Med Post Ant Lat Med Post Ant Lat $εx$ 161 62 52 116 92 37 39 75 102 44 23 49 129 6 1 70 $Case_0$ $εy$ 153 6 45 126 89 44 28 77 82 44 22 54 109 2 3 82 $εz$ 524 210 168 389 301 138 107 242 315 149 98 180 405 19 4 247 $εx$ 65 115 115 24 444 85 123 455 427 1111 142 667 387 115 176 481 $Case_1$ $εy$ 45 115 105 27 469 68 108 455 483 47 98 490 443 120 151 451 $εz$ 192 378 359 92 1528 263 381 1551 1498 185 342 1641 1374 393 164 1573 $εx$ 24 180 122 29 633 406 207 423 600 314 196 503 538 524 332 445 $Case_2$ $εy$ 1 176 115 34 681 378 200 438 683 300 158 503 626 518 309 423 $εz$ 51 585 1513 90 2180 1317 1812 3717 2105 1037 531 1655 1905 1686 1060 1455 $εx$ 45 56 30 1 22 44 37 2 36 41 10 20 54 42 279 10 $Case_3$ $εy$ 30 52 32 8 18 51 29 5 12 41 14 14 33 38 19 5 $εz$ 127 184 115 9 64 162 106 2 89 134 66 49 149 139 73 19 $ϵx$ 3 12 20 4 3 6 29 6 14 7 1 20 32 1 11 8 $Case_4$ $εy$ 15 11 19 14 9 15 20 7 13 8 6 15 8 2 9 7 $εz$ 14 4 75 8 20 39 77 7 14 24 38 51 74 5 38 10 $εx$ 32 75 45 14 17 38 33 3 27 48 15 29 54 1 16 11 $Case_5$ $εy$ 16 70 47 8 10 46 25 4 2 48 21 25 26 4 14 1 $εz$ 84 245 167 58 44 146 93 23 59 159 87 82 144 2 44 23 $εx$ 20 81 50 28 2 60 52 23 21 48 14 37 21 55 29 45 $Case_6$ $εy$ 2 77 52 24 8 69 44 29 7 47 20 36 17 50 35 57 $εz$ 40 267 185 107 8 223 159 96 34 157 84 110 25 187 118 163 $εx$ 72 174 65 36 48 99 27 41 $Case_7$ $εy$ 60 163 71 35 24 97 34 39 $εz$ 218 564 245 139 129 325 130 124 $εx$ 0 101 66 48 38 27 29 27 3 56 19 55 $Case_8$ $εy$ 20 95 68 48 36 35 20 33 28 53 26 60 $εz$ 29 333 241 183 120 106 77 93 26 182 104 177 $εx$ 112 85 33 160 151 63 37 181 45 49 5 105 274 64 4 360 $Case_9$ $εy$ 136 79 44 158 165 73 36 185 75.5 49 17 101 326 56 14 359 $εz$ 408 271 154 556 529 232 122 626 187 161 68 334 986 405 42 1204
 Strain $(με)$ Lubinus SPII Chamley Müller Straight Stanmore Med Post Ant Lat Med Post Ant Lat Med Post Ant Lat Med Post Ant Lat $εx$ 161 62 52 116 92 37 39 75 102 44 23 49 129 6 1 70 $Case_0$ $εy$ 153 6 45 126 89 44 28 77 82 44 22 54 109 2 3 82 $εz$ 524 210 168 389 301 138 107 242 315 149 98 180 405 19 4 247 $εx$ 65 115 115 24 444 85 123 455 427 1111 142 667 387 115 176 481 $Case_1$ $εy$ 45 115 105 27 469 68 108 455 483 47 98 490 443 120 151 451 $εz$ 192 378 359 92 1528 263 381 1551 1498 185 342 1641 1374 393 164 1573 $εx$ 24 180 122 29 633 406 207 423 600 314 196 503 538 524 332 445 $Case_2$ $εy$ 1 176 115 34 681 378 200 438 683 300 158 503 626 518 309 423 $εz$ 51 585 1513 90 2180 1317 1812 3717 2105 1037 531 1655 1905 1686 1060 1455 $εx$ 45 56 30 1 22 44 37 2 36 41 10 20 54 42 279 10 $Case_3$ $εy$ 30 52 32 8 18 51 29 5 12 41 14 14 33 38 19 5 $εz$ 127 184 115 9 64 162 106 2 89 134 66 49 149 139 73 19 $ϵx$ 3 12 20 4 3 6 29 6 14 7 1 20 32 1 11 8 $Case_4$ $εy$ 15 11 19 14 9 15 20 7 13 8 6 15 8 2 9 7 $εz$ 14 4 75 8 20 39 77 7 14 24 38 51 74 5 38 10 $εx$ 32 75 45 14 17 38 33 3 27 48 15 29 54 1 16 11 $Case_5$ $εy$ 16 70 47 8 10 46 25 4 2 48 21 25 26 4 14 1 $εz$ 84 245 167 58 44 146 93 23 59 159 87 82 144 2 44 23 $εx$ 20 81 50 28 2 60 52 23 21 48 14 37 21 55 29 45 $Case_6$ $εy$ 2 77 52 24 8 69 44 29 7 47 20 36 17 50 35 57 $εz$ 40 267 185 107 8 223 159 96 34 157 84 110 25 187 118 163 $εx$ 72 174 65 36 48 99 27 41 $Case_7$ $εy$ 60 163 71 35 24 97 34 39 $εz$ 218 564 245 139 129 325 130 124 $εx$ 0 101 66 48 38 27 29 27 3 56 19 55 $Case_8$ $εy$ 20 95 68 48 36 35 20 33 28 53 26 60 $εz$ 29 333 241 183 120 106 77 93 26 182 104 177 $εx$ 112 85 33 160 151 63 37 181 45 49 5 105 274 64 4 360 $Case_9$ $εy$ 136 79 44 158 165 73 36 185 75.5 49 17 101 326 56 14 359 $εz$ 408 271 154 556 529 232 122 626 187 161 68 334 986 405 42 1204
Table 9

Root-mean-squared strain difference (all tentative load configurations)

 Load Case Lubinus SPII Charnley Round. Müller Straight Stanmore $Case_0$ 279 171 189 159 $Case_1$ 226 886 981 993 $Case_2$ 499 1630 1254 1174 $Case_3$ 99 82 123 71 $Case_4$ 31 36 33 28 $Case_5$ 123 71 60 82 $Case_6$ 135 115 110 85 $Case_7$ 265 1001 1411 156 $Case_8$ 177 81 1221 111 $Case_9$ 300 342 637 170
 Load Case Lubinus SPII Charnley Round. Müller Straight Stanmore $Case_0$ 279 171 189 159 $Case_1$ 226 886 981 993 $Case_2$ 499 1630 1254 1174 $Case_3$ 99 82 123 71 $Case_4$ 31 36 33 28 $Case_5$ 123 71 60 82 $Case_6$ 135 115 110 85 $Case_7$ 265 1001 1411 156 $Case_8$ 177 81 1221 111 $Case_9$ 300 342 637 170

Ideally, the most adequate implanted femur-loading configuration should provoke the same strain magnitudes as the ones provoked by the intact femur. However, this is not possible, unless extra forces are added to the loading system. Therefore, the suitable loading configuration is the one that minimizes the differences in strains in all aspects of the femur. Due to the higher magnitudes of strains in the axial direction of the femur ($z$ direction) at the lateral and medial aspects of the femur, the $εz$ component of the strain seems to be an adequate parameter to select the suitable loading configuration for each of the hip replacements assessed.

Figure 4a shows the strain $(εz)$ distribution at the medial aspect of the femur considering all the reconstructions loaded with the loading configuration of the intact femur $(Case_0)$, which does not take into account the correction of the load system. Figure 4b illustrates identical results but with the loading configuration corrected $(Case_4)$. The comparison of these two figures show that it can be misleading in assessing the performance of different designs if the implanted loading system is not derived adequately. Figure 4b also shows the strain shielding effect, which does not depend on the stem geometry since very similar strain magnitudes were observed for all designs.

Figure 4
Figure 4
Close modal

For the hip replacements simulated with loading configuration of the intact femur simulated, the Lubinus SPII provoked the highest strain differences in all aspects of the femur; the Charnley Roundback and Müller Straight provoked very similar differences and the Stanmore provoked differences of strains higher at the proximal portion of the femur. The results presented in Fig. 4 clearly show that it is not correct to use the same loading configuration for the intact and implanted femurs. Apparently, and observing Fig. 4a we are forced to conclude that the Lubinus SPII prosthesis provokes higher strain shielding, when in fact this does not really occur. Figure 4b shows that strain shielding is very similar for all prostheses if the loading configuration is corrected. Since the head location of the prosthesis cannot be restored, the lever arms change and also the force system and if this difference is not considered, errors can be expected in the strain measurements as evidenced in Fig. 4a. It is necessary to determine how the consequences of this unavoidable source of error can be minimized (5). Many authors have realized numerical and experimental studies using the same force system for the intact and implanted femurs (40,41,42,43,44,45,46,47,48).

Load $Case_4$, that considers the force Fx and moments Mx and My (with the abductor force direction kept unchanged), was of all loading configurations simulated the one that provoked the lowest strain deviations either using the absolute strain difference (Table 7) or the root-mean-squared strain (Table 8) parameter. Other loading configurations, like load $Case_1$ and load $Case_2$, provoked significant differences in strains and seem to be nonsuitable implantable load configurations. It is interesting to note that the load cases that provoke the lowest strain differences all include the bending moment My, reinforcing that this bending moment at the coronal plane must be restored in simulations of implanted hip reconstructions. This moment plays a key role because it is obtained using the highest vector force magnitudes and lever arms.

Cristofolini and Viceconti (5) suggest that to compensate for the unavoidable geometry changes, the implanted femur should be loaded in such a way as to apply the same bending moment as in the intact femur. In fact, the strain deviations for load cases that included the two bending moments ($Case_3$, $Case_4$, $Case_6$, $Case_7$, and $Case_8$) were significantly lower. Load $Case_5$ that includes all forces (Fx, Fy, and Fz) and the My moment allowed relatively lower strain deviations, partially due to the influence of this bending moment (My). Overall, the load cases considering the torsional moment (Mz) ($Case_1$, $Case_2$, and $Case_9$) provoked very high strain deviations and it is therefore questionable the relevance of replicating this moment in the implanted femur system force, although it plays an important key role in the fixation of prostheses. It must be said that the load transfer mechanism between the intact and implanted femur is inherently different and the bending moments are reflected more pronouncedly in the strain pattern, while the torsional moment is more deleterious at the bone-prosthesis interface.

Figure 5 shows the highest strain error committed if the load system of the intact femur is used in implanted femur simulations. Depending on the type of geometry, some designs provoke higher errors than others. The highest errors were observed at the medial aspect of the femur and for the Lubinus SPII stem. Errors were smaller at the anterior and posterior aspects of the femur. We can also observe that the Lubinus SP II stem provoked the highest error differences which seem too be related to its anatomical geometry. All other stems provoked similar strain differences.

Figure 5
Figure 5
Close modal

The study showed that it is important to derive correctly the implanted femur-loading configuration, especially if different designs are being compared. For a certain design, including bending in the coronal plane can be sufficient; for others probably not. It is necessary to understand which the most critical load component is and try to restore the constant conditions for that one, having in mind that for others the deviations are less relevant. The adequate determination of the load configuration for implanted femurs has not been realized in many studies published (40,41,42,43,44,45,46,47,48).

## Concluding Remarks

The study performed aimed to derive adequate loading configurations for implanted femurs with different hip femoral components. If so is not done, errors can be expected in the strain distributions that possibly hide differences between different femoral designs. For the designs analyzed, adequate implanted system forces were generated using for the equilibrium system the Fx (medially direction) of the HCF and the bending moments (Mx and My) provoked by the HCF and abductors forces. We suggest that at least the bending moment at the coronal plane must be restored in the implanted femur-loading configuration.

## Acknowledgment

The authors gratefully acknowledge Fundação para a Ciência e a Tecnologia do Ministério da Ciência e do Ensino Superior for funding António Ramos with Grant No. SFRH/BD/63217/2002. Part of the work was supported by Project No. POCTI/EME/38367/2001 and No. POCTI/EME/44644/2002.

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