Original Article

Proc IMechE Part H:J Engineering in Medicine1–9� IMechE 2019Article reuse guidelines:sagepub.com/journals-permissionsDOI: 10.1177/0954411919856138journals.sagepub.com/home/pih

Stability of femoral neck fracturefixation: A finite element analysis

Shabnam Samsami1,2, Peter Augat1,3 and Gholamreza Rouhi4

AbstractFemoral neck fractures represent a relatively uncommon injury in the non-elderly population often resulting from high-energy trauma. Clinical outcome in these patients can be improved by optimizing surgical procedures and selectingappropriate fixation methods. The aim of this study was to develop a numerical fracture model to investigate the influ-ence of critical mechanical factors on the stability of fixation methods for femoral neck fractures. The mechanical stabi-lity of fracture fixation was assessed through employing finite element models and simulating progressive consolidationof the fracture for a vertical femoral neck fracture (i.e. Pauwels type III in which the angle between the fracture line andthe horizontal plane is greater than 70�). Mechanical performance was compared among three different fixation methods(cannulated screws, dynamic hip screw with de-rotational screw, and proximal femoral locking plate). Axial femoral headdisplacement varied from 2.3 mm for cannulated screws to 1.12 mm for proximal femoral locking plate, although dynamichip screw with de-rotational screw indicated a value of 0.94 mm. Considering a consolidated fracture and full weight-bearing load case, average displacements of fracture fragments were obtained of about 1.5, 3 and 70 mm for dynamic hipscrew with de-rotational screw, proximal femoral locking plate and cannulated screws methods, respectively. In terms ofinterfragmentary movements at the fracture site, outcomes of this study demonstrated that, in agreement with our pre-vious experimental research, the dynamic hip screw with de-rotational screw implant is a more effective choice than can-nulated screws and proximal femoral locking plate techniques for vertical femoral neck fractures in young patients. Thus,one may conclude that the use of dynamic hip screw with de-rotational screw, particularly during the early stages ofbone healing, could provide suitable mechanical environments that facilitate direct bone formation and shorter healingtimes.

KeywordsVertical femoral neck fracture, stability, interfragmentary movement, finite element analysis, bone fracture healing, non-homogeneous bone distribution

Date received: 22 November 2018; accepted: 14 May 2019

Introduction

Femoral neck fractures include injuries that involve thearea between the head of the femur and the intertro-chanteric line.1 These fractures are not very commonamong physiologically young adults (under the age of65 years) and account for 3% of all hip fractures.2,3

Their incidence has been reported to be as low as 5 in10,000 populations per year.4 Despite the low preva-lence of this injury in a younger population, providingappropriate fracture managements is important due tothe inherent higher activity level in youth and severecomplications associated with their fractures.5 Inyounger patients, femoral neck fractures occur in high-energy traumas such as road traffic accidents or sportinjuries, with a common pattern of a vertical fracturein which shearing stress is dominant.6

The cornerstone of treating these fractures is ana-tomic reduction and stable internal fixation of thefemoral neck in an attempt to salvage the femoralhead.3 Complications such as vascular necrosis of thefemoral head (38%) which could result in post-

1Institute for Biomechanics, Trauma Center Murnau, Murnau, Germany2Faculty of Medicine, Ludwig Maximilian University of Munich (LMU),

Munich, Germany3Institute for Biomechanics, Paracelsus Private Medical University,

Salzburg, Austria4Faculty of Biomedical Engineering, Amirkabir University of Technology,

Tehran, Iran

Corresponding author:

Gholamreza Rouhi, Faculty of Biomedical Engineering, Amirkabir

University of Technology, Tehran 15875-4413, Iran.

Email: grouhi@aut.ac.ir

traumatic osteoarthritis and non-union (10%) are notuncommon.7 Clinical outcomes after femoral neck frac-ture can be improved with good pre-operative plan-ning, prompt and optimized surgical procedures andapplication of stable fixation techniques.1

Union of femoral neck fractures should be achievedvia primary bone healing, which necessitates absolutestability at the fracture site.8,9 Compared to other frac-tures which heal through periosteal callus formation,the nature of this intracapsular fracture makes it morevulnerable to non-union. Synovial fluid prevents bloodclot formation, which contributes to secondary bonehealing. In addition, the intracapsular part of thefemoral neck has no periosteal layer to participate inthe bone healing process.8,9 Finally, direct bone healingrequires rigid stabilization to suppress the formation ofa callus in either cancellous or cortical bone.10 Hence,the goal of internal implantation of an intracapsularfemoral neck fracture is to provide stable fixation withcompression of the fracture fragments.11

Finite element (FE) models can be used to evaluatethe mechanical performance of fracture fixation. UsingFE models as opposed to conducting experiments hasthe advantage of assessing new implants before manu-facturing.12 It also enables the evaluation of stressesand strains generated within the bone, or at interfacesof bone and implant components which are difficult, ifnot impossible, to measure experimentally.13 Thus, uti-lizing the FE method may provide some predictionsabout the clinical performance of implants.14 The levelof confidence in FE predictions critically depends onthe accuracy of the models, which is determined by geo-metrical accuracy, applied material properties (such aslinear elastic, elastic-plastic, viscoelastic, visco-plasticand plastic), as well as type and number of elements.15

Typically, FE models determine the initial stability offracture fixation by simulating the fracture as an emptygap. However, as the fracture begins to consolidate, thefracture stability progressively increases. It has beenpreviously shown that this consolidation influences con-struct stability and may thus affect the relative mechani-cal performance among fracture fixation implants.16

Hence, a valid computerized fracture model using afinite element method (FEM) might allow the investi-gation of the influence of critical mechanical factors onthe stability of femoral neck fracture fixations.17 Thisstudy aimed to investigate the mechanical stabilityobtained in femoral neck fracture fixation during thebone healing process for three different implants: can-nulated screws (CSs), dynamic hip screw with de-rotational screw (DHS + DS) and proximal femorallocking plate (PFLP), using FEM. In order to comparethe mechanical stability during different phases of thehealing process, the progress of fracture consolidationwas simulated by progressively increasing the frictioncoefficients between fracture fragments. The aforemen-tioned fixation methods were selected for this investiga-tion based on previous studies in which common

fixation techniques for vertical femoral neck fractureswere compared.18–23

Materials and methods

First, the FE model of intact proximal femur was con-structed and validated by considering non-homogeneous bone distribution. Validation of thismodel aimed to demonstrate that the current non-homogeneous FE model could provide promisingresults. In this process, the FE model of the intactproximal femur without implants was used to compareFEM outcomes with our experimental data24 and thoseof previous studies.25,26

Then, this validated model was utilized to prepareFE models of vertical femoral neck fractures fixed withthree different implants (CSs, DHS + DS, and PFLP).Interfragmentary movements, that is, relative displace-ments of the femoral head with respect to the femoralshaft which can be calculated using equation (3) eluci-dated in the penultimate paragraph of this section, wereanalyzed as a measure of mechanical fixation perfor-mance of the implants. Finally, the effect of fractureconsolidation on the mechanical performance of frac-ture fixation was assessed by progressively increasingfriction between the fracture fragments. Increasing thefriction coefficient at the fracture site is representativeof ossification and progression of the bone healing pro-cess at the fracture site.

To extract a three-dimensional (3D) geometricmodel of the proximal femur, computed tomographic(CT) images of a healthy 65 year-old male (image reso-lution=512*512 pixels, pixel size=0.33mm, slicethickness=1.25mm, slice increment=1.25mm) wereused to create point clouds of the right femur usingMimics software (V.10.01). CT data of this patient wereevaluated by a senior surgeon in terms of osteoporosisand other diseases which can affect bone mineral den-sity and was found to be representative of a clinicallyhealthy and young subject. Then, CAD models of theproximal femur were obtained by changing the point-cloud model to a solid structure in Catia software(V5.R21). In FE models, a femoral neck fracture ofPauwels type III (i.e. when the angle between the frac-ture line and the horizontal plane is greater than 70�27)was created by defining the slice plane in the transcervi-cal region. The cutting plane extended from the super-ior femoral neck to the basicervical region with theorientation of 70� to the horizontal plane (Figure 1(b)).Geometric models of the implants (CSs, DHS + DSand PFLP) were additionally prepared by use of coordi-nate measuring machine (CMM) data and Solidworks2011. All implants used in this study were made byPooyandegan Pezeshki Pardis (3P) Company (Iran). Allscrew threads were replaced by a smooth surfacein which the size corresponds to the mean thread dia-meter.24,28–31 CAD models of the implants were assem-bled with the geometrical models of the femoral neck

2 Proc IMechE Part H: J Engineering in Medicine 00(0)

fractures in appropriate positions. Figure 1 summarizesdifferent steps taken to prepare the FE models for threedifferent fixation methods.

The CS model includes three 7.3mm CSs (threadlength: 32mm) inserted into the femoral head in aninverted triangle configuration, parallel to the femoralneck axis. In the DHS + DS model, a 135� three-holeDHS plate was positioned with the central screwdirected into the middle of the femoral head. Three 4.5-mm cortical screws were used to fix the side plate to the

femoral shaft. A superior neck 7.3-mm cannulated can-cellous de-rotational lag screw was inserted parallel tothe central screw. For the PFLP model, a fixed-anglePFLP was secured with two locking screws in thefemoral head: one 7.3-mm cannulated conical screw at95� to the plate shaft and one 5.0-mm cannulated coni-cal screw at 110� to the plate shaft. The side plate wasfixed to the proximal femur using four 4.5-mm non-locking screws. For all fixation methods, the tips of allscrews were positioned 5mm from subchondral bone.

The main material constituent of all three implants,stainless steel, was modeled as a homogeneous, isotro-pic, and linear elastic material, with an elastic modulusof E=200GPa and Poisson’s ratio of y =0.3.8,32,33 Inall models, the proximal femur was modeled as an elas-tic, isotropic, and non-homogeneous material. In com-parison with our previous study,8 the novelty of thiswork was to take into account and simulate a non-homogeneous bone mass distribution in the proximalfemur FE model in order to provide a more realisticassessment of the mechanical behavior of the bone-implant constructs during the bone healing process. Inorder to consider the non-homogeneity of bone, thePython (3.3.4) scripting tool in ABAQUS (V.6.14,Simulia, Dassault Systemes, Waltham, MA, USA) wasemployed to assign a specific Young’s modulus to eachelement. In order to calculate each element’s modulusof elasticity, the Hounsfield Unit (HU) was extractedfrom CT images and calculated at the centroid of eachelement. A mineral density calibration phantom(HEAD CT Calibration Phantom, Mindways, Austin,TX, USA) was used to determine the coefficients ofequation (1) and thus the relation between CT dataHUs and bone density. Then, the apparent density (r)and Young’s modulus (E) of each element were com-puted using equations (1) and (2), respectively34,35

r =1:141 � 10�3HU+1:184 � 10�1 g=cm3 ð1Þ

E MPað Þ= 2014r2:5

1763r3:2

�for r41:2 g=cm3

for r ø 1:2 g=cm3

�ð2Þ

Furthermore, a value of 0.3 was assigned as thePoisson’s ratio of each bone element.36,37 The distribu-tion of Young’s modulus is illustrated in Figure 2,which is a cut-view along the axial axis of femur.

It is worth mentioning that in the field of CT, theHU is a dimensionless unit proportional to the degreeof x-ray attenuation. This value is allocated to eachpixel in terms of tissue density and is shown in the formof a shade of grayscale. The CT Hounsfield scale is cali-brated such that the HU value for water is 0 HU andfor air is 21000 HU at all tube energies used.38

Different friction coefficients between the fractureplanes were used to mimic different phases of the bonehealing process. Thus, the space between fracture frag-ments of the femur was modeled in all FE models usingthe ‘‘HARD’’ type of normal contact with various fric-tion coefficients (f=0.1, 0.3, 0.5, 0.7 and 0.9). Thesame method was used to model contact between the

Figure 1. Various steps taken for finite element modeling ofeach fixation method: (a) preparing CAD models of the proximalfemur and the implants, (b) assembling CAD models of implantsand femur to generate three CAD models of femoral neckfractures fixed with three different implants (highlighted circlesshow the fracture site which is a 70� fracture line passing fromthe superior femoral neck to the basicervical region) and (c)mesh generation as a pre-processing step in FE analysis of themodels fixed with CSs, DHS + DS and PFLP fixation methods.

Samsami et al. 3

DHS plate and bone (f=0.3) in the DHS + DSmodel.28,29 In all models, coupling of bone with allscrew types (cannulated, cortex, conical and DHS) wasmodeled using a tied contact method in order to mimicbonded contact of the threaded screws within thebone.28–31 Moreover, the tied contact approach wasused to model coupling of conical, cortex and DHSscrews with their plates.28 There was no interactionbetween bone surface and the implant in the PFLPfixation method because the PFLP implant includes alocking plate, as opposed to a conventional compres-sion plate.

This FE analysis simulated one-legged stance withpartial or full weight bearing corresponding to 700 or1400N loading, respectively. The distal ends of all prox-imal femur models were fully fixed. A distributed cou-pling was used to apply external forces by which singleforces acting in a control reference node were equallydistributed to the bone tissue at contact points betweenthe femoral head and acetabulum.28 During the courseof FE analyses, the models were loaded by horizontaland vertical hip contact force components correspond-ing to partial or full weight bearing in the one leg stanceposition (Figure 3). In partial weight-bearing loads,axial displacement of the control node was evaluatedfor different fixation methods

The bone-implant structures were imported toAbaqus (V.6.14, Simulia, Dassault Systemes, Waltham,MA, USA) where 10-node tetrahedral elements wereused to discretize all parts of differently fixated femoralneck fracture models. The results were converged tothe parameter of interest, that is, axial femoral headdisplacement, with 97,000–114,000 elements dependingon the fixation methods.

To evaluate interfragmentary movement in the cur-rent FE analysis, equation (3) was used to calculate theaverage value of relative motion of the fracture frag-ments, CDave

CDave=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffifficopen2ave+ cslip12ave+ cslip22ave

qð3Þ

where copenave, cslip1ave and cslip2ave (Figure 4) repre-sent the average normal separation of the fracture frag-ments and average femoral head sliding relative to thefemoral shaft in tangential directions, respectively.8,25

Moreover, each interfragmentary displacement compo-nent (axial, anterior–posterior shear or superior–inferior shear) was compared among the implants toprovide more detailed information about their mechani-cal behavior during the bone healing process (Figure 4).

For validation purposes, the FE model of the intactproximal femur without implants was utilized to com-pare results of this study with our experimental data24

and those of previous studies.25,26 Similar to FE modelsof fixed fractures, the distal end of the proximal femurmodel was fully fixed, and a control reference nodewith a distributed coupling was used to apply horizon-tal and vertical components of the hip contact forcecorresponding to the partial weight bearing in one-legged stance (i.e. a 680N vertical force and a 190Nhorizontal force, or in other words, Fx=190N, Fy=0and Fz=680N).39 The control reference node was theclosest node to the center of loading detected in themechanical test. The axial displacement of thiscontrol node was compared with our experimentalmeasurements24 and those of previous investiga-tions.25,26 In our previous experimental assessment,24

all samples were positioned at 25� of adduction. Usinga quasi-acetabulum fixture, samples were verticallyloaded at a rate of 1mm/min displacement up to 700Ncorresponding to partial weight bearing (Figure 5).

Figure 3. Boundary conditions in the FE model of the femoralneck fracture fixed with DHS + DS method as an illustration forthe applied boundary conditions in all FEM models: (a) loadingand boundary conditions (the highlighted coordinate systemshows the selected directions for x, y and z axes) and (b)distributed coupling to simulate acetabulum contact forces.

Figure 2. A cut-view along longitudinal axis of femur showingYoung’s modulus distribution.

4 Proc IMechE Part H: J Engineering in Medicine 00(0)

This situation simulated the loading condition in whichforce components were Fx=190N, Fy=0, andFz=680N. The red arrow in Figure 5 shows the down-ward displacement of the femoral head.

Results

Axial femoral head displacement of the intact modelwith partial weight bearing was found to be 0.45mm,consistent with the findings in our previous experimen-tal study.24 Results for this parameter, which was calcu-lated under partial weight bearing, can be found inTable 1 for each fixation method. CS fixation techniqueshowed the highest axial displacement for the femoralhead, as expected, followed by PFLP and DHS + DS(Table 1).

By increasing contact friction at the fracture site,relative displacement of the fracture fragmentsdecreased by about 30% for all loading conditions andfixation techniques (Figure 6). For the DHS + DSand the PFLP implant, the resultant relative displace-ments of fracture fragments were almost negligible(\ 7mm). In contrast, CS fixation resulted in a morethan 30 times higher fracture displacement value(Figure 6).

Under full weight bearing, the CSs implant showedthe highest opening of fracture fragments compared to -angle-stable implants, DHS + DS and PFLP(Figure 7(a)). Also, CS technique demonstrated thehighest shear displacement compared to two other fixa-tion methods. Moreover, superior–inferior shear dis-placement of fracture fragments for the CSs implantwas almost two times that of anterior–posterior displa-cement (Figure 7(b) and (c)).

Discussion

The aim of this study was to use a non-homogeneousFE model of the femoral neck fracture to assess threedifferent fixation methods (CSs, DHS + DS andPFLP) during the bone healing process in terms ofinterfragmentary movements, which is representative ofmechanical stability as an initial factor for fractureconsolidation.

The mentioned fixation methods were selected forthis investigation with previous biomechanical studiesin mind. The CS is one of the traditional fixation

Figure 4. Local coordinate system considered at fracture planewhich shows the direction of shear (cslip1 cslip2) and axial(copen) displacements.

Figure 5. Test setup, red arrow shows downward femoral headdisplacement creating a resulting force vector with angulation of25� with respect to the femoral shaft. The highlighted coordinatesystem shows the selected directions for x, y and z axes.24

Table 1. Axial femoral head displacement for different finite element models, under partial weight bearing load condition.

Finite element models Axial femoral head displacement (mm)

Intact femur model 0.45Femural neck fracture fixed with CSs 2.3Femural neck fracture fixed with DHS + DS 0.94Femural neck fracture fixed with PFLP 1.12

DHS + DS: dynamic hip screw with de-rotational screw; PFLP: proximal femoral locking plate.

Samsami et al. 5

methods for femoral neck fractures. According to pre-vious research that compared different CS configura-tions, three CSs in an inverse triangle arrangementprovides high mechanical stability for vertical femoralneck fractures.18,19 The DHS as a fixed-angle plateis another implant commonly used to managefemoral neck fractures. An additional CS is usuallyinserted parallel to the main DHS screw to providesupplemental rotational resistance for vertical femoralneck fractures.20,21 Locking plates, which allow formultiple points of angled fixation in the femoral headto resist the shear forces dominant in Pauwels IIIfemoral neck fractures, have also been recently investi-gated for fixation of this type of fracture.22,23

In our FE model of femoral neck fracture fixation,DHS + DS fixation provided the most stable mechani-cal conditions during the bone healing process. TheDHS + DS method was approximately three timesmore stable than PFLP. In contrast to the other fixa-tion techniques, CS fixation resulted in the least stablefixation and allowed much higher fracture displace-ments, in particular for shear displacements. Thus, ourfindings confirm the clinical experience that the CSsmethod is mechanically unsuitable for the fixation ofvertically unstable fractures of the femoral neck.40

In order to provide a validated numerical modelto compare the performance of different implants,downward femoral head displacement of an intact

model, that is, an FE model of the proximal femurwithout an implant, was compared with experimentaldata collected on cadaveric specimens in our previousinvestigation.24 Considering inhomogeneous bone massdistribution, error of numerical results (against experi-mental data)24 declined from 73% to 25%, when com-pared with our previous homogeneous FE model.8 Inaddition, previous experimental and FEM studiesdemonstrated 0.5–0.75mm and 0.63mm of verticalfemoral head displacement, respectively; thus, the cur-rent obtained result for this parameter (0.45mm) iswithin the range of other investigations.25,26

Primary bone healing requires absolute stability andanatomical alignment of the fracture fragments. Givenan optimal vascular supply, bone healing mainlydepends on the interfragmentary movement, which isdetermined by the applied load and stability of the fixa-tion method.40 Shear forces are dominant in verticalfemoral neck fractures,41 and shear motion at the frac-ture site is a detrimental factor to diaphyseal and meta-physeal healing processes.42,43 Therefore, a stablefixation method for this fracture should provide astrong resistance to shear motion. Results of this studyshowed that there was a lack of overall shear resistancefor CS fixation technique, which could not reducesuperior–inferior shearing displacement at the fracturesite during bone healing (Figure 7(b)). It is known thatanterior–posterior sliding at the fracture site, as a sign

Figure 6. Average displacement at the fracture site for three fixation methods in different frictional contact and loading conditions:(a) DHS + DS, (b) PFLP and (c) CSs.

6 Proc IMechE Part H: J Engineering in Medicine 00(0)

of relative rotation of fracture fragments, and fracturefragment separation are damaging factors for bonehealing.25 Our findings demonstrated that CS, with thehighest relative rotation and separation of fracturefragments, is unable to provide appropriate mechanicalfixation for bone ossification (Figure 7). For the CSmethod of fixation, superior-inferior sliding at the frac-ture site was two times that of anterior-posterior direc-tion, which indicates the domination of rotationalinterfragmentary displacement as an undesirablemechanical factor for bone union (Figure 7(b, c)).However, the DHS + DS technique showed the low-est shear movement and fragment separation, as wellas almost negligible rotation at the fracture site; there-fore, DHS + DS may be able to result in bone unionand reduce bone healing time (Figure 7). In a clinicalstudy on high shear-angle femoral neck fractures, itwas found that fixed-angle implants improved frac-ture stability and decreased rates of non-union andosteonecrosis in comparison with the CS method.40

Hence, as it was expected, angle-stable implants(DHS + DS and PFLP) demonstrated higher shearresistance compared to CSs (Figure 7). However, theDHS + DS method illustrated higher shear resis-tance and almost negligible fracture separation whencompared to PFLP which includes only two lockingscrews (Figure 7).

Tetrahedral elements were used in this study becauseemploying hexahedral elements was quite difficult dueto the complexity of the FE model. Tetrahedral 10-node elements (C3D10) were automatically generatedusing the ABAQUS software. The second-order shapefunctions of this element type ensured a mesh that rep-resented the bone’s boundary surfaces appropriately. Itshould be noted that compared to hexahedral meshingmethods, tetrahedral meshes are easier to generate butincrease run times; initial sensitivity analyses are alsorequired to optimize solving time for mesh-independentresults. On the other hand, generating hexahedralmeshes, which are more accurate than tetrahedral onesand reduce computational time of the FE models,requires more effort in which specific numericalapproaches such as multi-block meshing can beemployed.44–46 Considering the advantages and disad-vantages of the aforementioned meshing methods,second-order tetrahedral meshing method was optedfor use in this study. However, for more complex geo-metries, the increased computational costs may belargely outweighed by the advantages of an automaticmeshing approach.

There are some limitations in this work, whichshould be kept in mind for future research. First, inorder to reduce the computational complexity of theFE analysis, bone-screw couplings were assumed to be a

Figure 7. Maximum values of interfragmentary displacement components in various values of friction coefficients for three fixationmethods under full weight bearing: (a) axial displacement, (b) superior–inferior shear displacement and (c) anterior–posterior sheardisplacement.

Samsami et al. 7

tied contact instead of a frictional contact, which isthought to be more realistic. Second, for a more accurateevaluation of the fixation methods using FE analysis,cyclic loading should also be applied in order to morerealistically mimic normal physiological loading. Third,although the applied forces are physiologically represen-tative of one-legged stance, much higher forces can beexperienced by the hip during situations such as stair-climbing or stumbling.47 However, it would still be possi-ble for different fixation methods to be compared andanalyzed under these loading conditions using the pre-sented FE models. Finally, physiologic force componentsacting across the hip joint, such as muscle forces thatplay a crucial role in the stability of fixation techniques,were neglected in this research for the sake of simplicity.

This study aimed to assess the mechanical stabilityof different fixation methods for vertical femoral neckfractures by comparing them in terms of interfragmen-tary movement during bone healing. Nonetheless, inregard to the side effects of each fixation method onstress shielding, and consequently on the rate of boneremodeling, future research needs to be done.

Conclusion

Particularly for the early healing phase, a stable con-struct such as the DHS + DS will provide the mostsuitable mechanical environment in comparison withthe PFLP and CS methods. Hence, this fixation methodis most likely to result in favorable healing outcomesand shorter fracture repair times, which is in agreementwith our previous investigations.8,24

Acknowledgements

The authors thank Biomedical Engineering Faculty ofAmirkabir University of Technology, and Institute forBiomechanics, Trauma Center Murnau.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interestwith respect to the research, authorship and/or publica-tion of this article.

Funding

The author(s) received no financial support for theresearch, authorship and/or publication of this article.

ORCID iD

Gholamreza Rouhi https://orcid.org/0000-0001-5592-4970

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