Are Flexible Metaphyseal Femoral Cones Stable and Effective? A Biomechanical Study on Hinged Total Knee Arthroplasty

doi:10.21203/rs.3.rs-1824209/v1

Background

Nowadays, cones currently available in the market are rigid and, unless they are custom-specific designed, are unable to correctly adapt to the shape of the patient’s bone. Therefore, flexible metaphyseal cones have been recently introduced to reduce potential bone trauma during implantation. Even if a preliminary clinical study on the use of such devices has shown promising results, no biomechanical study evaluates and quantifies their mechanical efficacy and safety.

Methods

Two commercial versions of flexible cones were analyzed in this study using finite element analysis, based on a previously validated model. Each cone geometry was modeled both as flexible and as rigid and implanted following surgical guidelines. Three activities were simulated in this study: surgical impaction, walking, and chair rise; the output was compared among configurations.

Results

During impaction, results show that the stress has been considerably reduced in flexible cones in comparison with rigid ones; the stress resulted also better distributed and more homogenous all over the cortical bone, with lower bone peaks. Considering the two different daily activities, the analysis didn’t show any remarkable differences between flexible and rigid cones both in terms of bone stress and implant micromotions.

Conclusions

Our findings demonstrate that metaphyseal flexible metal cones allow macro-deformation during impaction thanks to their flexibility, and therefore they are safer in comparison with rigid cones. However, for the daily tasks investigated results showed no major differences between rigid and flexible cones in terms of bone stress, implant stability, and micromotions. Therefore, the mechanical performances of the flexible cone can be considered similar to the rigid cone.

Cones

Revision TKA

Implant Stability

Micromotions

Biomechanics

When surgeons need to treat severe bone loss, off-the-shelf devices such as augments, porous metal cones or sleeves are possible solutions¹. Indeed, these techniques have shown optimal clinical and radiological results with relatively low complication rates^2,3. For instance, the use of metaphyseal cones is a suitable procedure to manage severe bone loss in revision total knee replacement ^4,5. Since the metaphysis of such patients could shows important defects, the bone is weak and might be easily damaged during the surgery ^6,7.

Nowadays, the cones available in the market are rigid and, unless they are custom-specific designed, are unable to correctly adapt to the shape of the patient’s bone ⁸. However, among the different designs available on the market, TrabecuLink® cones (Waldemar Link, Hamburg, Germany), that have been recently introduced, are characterized by a flexible design thanks to a posterior opening among two parallel flanges (Fig. 1); therefore, these devices, could, by improving the interaction between the cone and the bone, reduce bone stress and the related risk of bone fracture.

A preliminary clinical study on the use of such flexible devices has shown promising results ⁹; however, an in-vitro biomechanical study is required to properly evaluate and quantify their mechanical efficacy and safety during surgical implantation and their mechanical performance and stability during common daily activities.

Therefore, this biomechanical study focuses on two main objectives:

in the first part, the study aims to investigate the benefits, in terms of bone safety, achieved from the use of flexible cones, compared to rigid cones, during the surgical impaction;
in the second part, the study analyzes the mechanical performance, in terms of implant stability, of the flexible cone during two daily activities such as walking and chair raising, compared to rigid cones.

For these reasons, in this study, a Finite Element Analysis (FEA) model was developed in order to evaluate and compare the effects of the different cones rigidity options in a femur implanted with a rotating hinged TKA.

The finite element model developed for this study was based on a previously validated and published knee finite element model ^10–12 which includes the following features. This method is adopted as it allows to perform precise biomechanical analysis ¹³, providing the possibility to identify the effects of the different configurations on the same bone geometries (comparison that cannot be investigated in vivo nor in cadaveric bones) ^{11,12,14−18}.

Geometry

For the femoral bone, in agreement with previous biomechanical numerical and experimental studies ^{11,12,14−17}, a physiological three-dimensional femur was generated from computer tomography images of an artificial right femoral sawbone, size medium (3rd generation, Sawbones Europe AB, Limhamn, Sweden). Such femoral bone model includes cortical and cancellous bone.

For the implant devices, the following geometries, as CAD files, were received by the industrial manufacturer (WALDEMAR LINK GmbH & Co. KG, Hamburg, Germany):

Endo-Model Rotating Hinge, right, medium size;
2-Zones Symmetric Flexible TrabecuLink Femoral Cone, medium size, length 45 (Fig. 1.A);
3-Zones Asymmetric Flexible TrabecuLink Femoral Cone, right, medium size, length 45 (Fig. 1.B).

In comparison with the symmetric design of the 2-zone cone, the 3-zone cones are characterized by an additional asymmetric covering for the intercondylar box of the femoral knee arthroplasty implant to enhanced rotational stability (Fig. 1).

To properly evaluate the effect of flexibility, both cones were considered flexible (using the original geometry) and rigid. The rigidity of the cones was achieved by fixing the two metal posterior parallel flanges (see Fig. 2) to avoid any deformation of the cone. With this approach, in the analysis, both rigid and flexible cones are characterized by the same external and internal geometry, therefore the bone-cone interaction is the same, a part of the different flexibility of the two designs.

The surgical implantation of the hinged TKA was obtained by performing a femoral virtual cut according to the surgical guidelines provided by the industrial manufacturer.

Material Properties

Linear elastic models were chosen for all the materials involved in this study, in agreement with previous literature studies ^{10,19− 22}: this assumption allows a good approximation of all the mechanical properties, in order to gain a qualitative comparison among the different configurations. Cortical bone was modeled as transversely isotropic with a stiffer resistance along the anatomical axis of the femur ^{12,14,23− 25}. The cancellous bone was considered isotropic ^5,16,22. The material of the femoral components of the hinged TKA was a Cobalt–Chromium alloy (CoCr) ^14,26. The cones were produced, using additive manufacturing, in titanium alloy Ti–6Al–4V (Tilastan®) with different porosity in the region in contact with the bone (TrabecuLink®, see Fig. 1). The material properties for the cones were provided by the manufacturer. The material adopted for the cement (poly-methyl methacrylate, PMMA) was homogeneous and isotropic ^12,14,19. A full overview of the material models and properties used in the study is reported in Table 1.

Surgical Impaction: Configurations, Boundary conditions and Output

For this first part of the study, the following four different configurations were investigated:

2-zone Flexible cone;
2-zone Rigid cone;
3-zone Flexible cone;
3-zone Rigid one.

To simulate the surgical impaction, according to the surgical technique, a vertical compressive force of 2600 N was applied on the distal surface of the femoral cone, along the anatomical axis of the femur. The value of such force was experimentally evaluated in a preliminary study reproducing a real surgical impact test ⁶. For this analysis, in all the configurations considered, the proximal femur was considered fixed.

As the aim of this part of the study was to investigate the potential benefit in terms of bone safety induced by flexible cone, the magnitude, average, and distribution of the Von Mises stress in the bone during the impact were considered as the main output.

Daily Activities: Configurations, Boundary conditions and Output

In this phase, the hinged TKA and the 2-zone femoral Cone design were virtually implanted in the same bone used for the previous phase according to the surgical guidelines provided by the industrial manufacturer. In details, after the implantation of the femoral cone, press-fit with the bone, bone cement is applied to fill the cavity between the internal side of the cone and the prosthesis. However, the hinged implant itself, could be implanted with cemented and press-fit stems.

Therefore, the following four configurations were investigated:

2-zone Flexible Cone with Hinged TKA coupled with press-fit stem;
2-zone Rigid Cone with Hinged TKA coupled with press-fit stem;
2-zone Flexible Cone with Hinged TKA coupled with cemented stem;
2-zone Rigid Cone with Hinged TKA coupled with cemented stem.

For all the analysis a stem length of 120 mm was used for the model, as previous study ¹⁴ have demonstrated such length to be a compromised in terms of improving implant fixation and reducing femoral stress shielding in the bone; following the surgical technique and in agreement with previous study on proper femoral cementation ²⁷ a PMMA shell of 2mm thickness was used to fix the femoral component to the bone. For this study only the cone design 2-zones was used as it was considered the worst case scenario.

In agreement with previous study ^8,14, for the walking and chair rise daily activities, considered respectively at 0° and 90° of flexion, the maximum loads achieved were applied on the distal tibial component, along tibial axis. For this configuration the proximal femur was considered fixed.

In details, a force of 2100N, was considered for the walking motion, with a tibio-femoral flexion angle of 0°, while a force of 2600N, was considered for the chair raising, with a tibio-femoral flexion angle of 90°.

As the aim of this part of the study was to investigate the potential benefit in terms of implant stability and performance of flexible cone, the magnitude, average and distribution of the Von Mises stress in the bone, together with the implant micromotions, during the different motor tasks were considered as main output.

FE Analysis

In agreement with previous studies ^8,12,14,28, each finite element model analyzed in this study was developed and meshed by using tetragonal elements with an approximate element size of 2 mm. A refinement of the mesh, with an approximate element size of 0.5 mm, was performed in the contact region between bone and cone (for the analysis of surgical implantation) and between bone, cone, PMMA and TKA, for the analysis of daily activities. The proper size of the elements was chosen after a convergence test performed to verify that the mesh size adopted did not affect the requested outputs. Abaqus/Explicit version 2019 (Dassault Systèmes, Vélizy-Villacoublay, France) was used to develop the models and to perform the analyses of the impact test. Abaqus/Standard version 2019 (Dassault Systèmes, Vélizy-Villacoublay, France) was used to develop the models and to perform the analyses of the daily activities.

Impact Test

Figure 3 and Fig. 4 reported, for the four configuration analyzed, the Von Mises bone stress during the impact; results show that the magnitude of the Von Mises stress has been considerably reduced by the use of the flexible femoral cone in comparison with the rigid one. Moreover, the bone stress resulted also better distributed and more homogenous all over the cortical bone, with lower bone peaks both on the external surface (Fig. 3) that also on the internal one (Fig. 4).

Quantitatively, Fig. 5 reported the average of the Von Mises stress (in MPa) for the anterior cortical bone and the posterior cortical bone for all the configurations. Results show that, with the use of the 2-zones Flexible cone, the average stress is reduced of about 50% in the anterior region of the bone, and a reduction of 40% is observed in the posterior bone region. Also, the 3-zones Flexible cone shows a reduction of the bone stress of respectively 6% and 36% in the anterior and posterior bone region.

Daily Activities

Considering the two different daily activities, the analysis didn’t show any remarkable differences between Flexible and Rigid cones both in terms of bone stress (Fig. 6 and Fig. 7) and implant micromotions.

Overall, the stress distribution did not represent any risk of bone damaging, and the implant subsidence (relative displacement between cone and stem) of the flexible configurations never exceeded 5 micron difference from the respective rigid configurations.

Thus, the performances of the flexible cone are the same of the rigid cone when common activities are performed and, the presence the flexible cone guarantees the same implant stability of the rigid cone when common activities are performed.

The main aim of the present study is to evaluate the potential benefit on the use of flexible cone during surgical implantation and eventual implication of a flexible structure in patient during daily activities. Our findings demonstrate that the metaphyseal flexible metal cones allow during the impact, thanks to their flexibility, to deform and therefore to reduce the bone stress in comparison with the one obtained by rigid cones (not deformable) under the same boundary conditions. Therefore, for both 2-zone and 2-zone design, the flexibility reduced the relative risk of intraoperative fracture.

However, for the two the daily tasks investigated, both for cemented and press-fit configurations, results showed no major differences between rigid and flexible cones in terms of stress distribution and magnitude. In particular, both for cemented and press-fit hinged configurations, results showed no major differences between rigid and flexible cones in terms of implant stability and micromotions. Thus, the performances of the flexible cone are the same of the rigid cone when common activities are performed.

The results are compatible with the good results demonstrated clinically by different authors on the use of flexible ⁹ and rigid cone ^29–31. However, as reported by Bonazinga et al. ²⁹, in the case of rigid device, the incidence of intraoperative fractures due to cone was 2.91%, especially on the femoral bone (77%). The authors suggest, due to the bone fracture and, together with higher incidence of cones loosening on the femoral side, that particular attention should be dedicated during the preparation of the femoral metaphysis and that another possible clue may be the need for higher modularity and shape.

In terms of stability the value of micromotion at the bone implant interface identified by both cones during in-vivo activities are similar with the one determined by a recent computational study on metaphyseal tritanium cone ³² and with the one determined by Bori et al ¹⁴, on the same implant analyzed in the present study. Previous researchers have demonstrated and reported that bone ingrowth is achieved if interface micromotions are lower than 50 microns ^32–34 and as the micromotions measured in this study are far below 50 micron, for all designs, proper successful cone osseo-integration would be expected.

Undoubtedly, this study has some limitations. First of all, in terms of material properties implementation, heterogeneity of the cortical and cancellous bones (which could affect the stress results) were not implemented in our model, neither poor bone quality nor osteoporosis; furthermore, only one geometry was considered, as the geometries of the Sawbones femoral bone and no bone deformity was considered. However, this assumption but is commonly accepted in different literature studies ^5,13,14. In terms of configurations tested, then, only full extension and a flexion of 90 °were considered for analysis and only one prosthesis design (i.e. the Endo-Model Rotating Hinge) was implemented in this study, also in agreement with other biomechanical studies ^8,14.

The results of this study have therefore to be considered as a comparison of the different configurations in standardized bone geometries, and thus have mainly a comparative rather than an absolute value; further studies on eventual bone deformities or malpositioning issues should be per- formed in order to improve the current knowledge, and patient-specific models and simulations will however still represent the most reliable tool to predict clinical performances and/or evaluate the optimal options.

It is however to be reported that, despite the limitations listed above, the outcomes of the simulations found are in agreement with clinical results available in the literature, then supporting the relative discussions and conclusions.

In conclusion, the results of this study demonstrate that the metaphyseal flexible metal cones allow macro-deformation during surgical impaction thanks to their flexibility, and therefore they are safer in comparison with the one obtained by rigid cones (not deformable). However, for the daily tasks investigated, for both cemented and press-fit configurations, results showed no major differences between rigid and flexible cones in terms of stress distribution and magnitude, implant stability and micromotions. Therefore, the performances of the flexible cone can be considered similar to the rigid cone.

Competing interest

Research support from a company or supplier as a Principal Investigator: Link Orthopaedic. The funder had no role in the conceptualization, design, data collection, analysis, decision to publish, or preparation of the manuscript. All authors do not have any conflicts to declare.

Availability of materials and data

The data that support the findings of this study are available from the corresponding author, [BI], upon reasonable request.

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Table 1 Material model and properties used for this study. For the cortical bone, the direction E3 represents the axial direction.

Material	Material Model	Elastic Modulus (MPa)			Poisson’s ratio
Material	Material Model	E₁	E₂	E₃	v₁₂	v₂₃	v₃₁
Cortical bone	Transversely isotropic	11.500	11.500	17.000	0.58	0.31	0.31
Cancellous bone	Isotropic	2.130	-	-	0.31	-	-
Ti6Al4V	Isotropic	110.000	-	-	0.35	-	-
Porous Titanium	Isotropic	4.850	-	-	0.30	-	-
CoCrMo	Isotropic	240.000	-	-	0.30	-	-
UHMWPE	Isotropic	685	-	-	0.23	-	-
PMMA	Isotropic	3.000	-	-	0.30	-	-

Competing interest reported. Research support from a company or supplier as a Principal Investigator: Link Orthopaedic. The funder had no role in the conceptualization, design, data collection, analysis, decision to publish, or preparation of the manuscript. All authors do not have any conflicts to declare’

Are Flexible Metaphyseal Femoral Cones Stable and Effective? A Biomechanical Study on Hinged Total Knee Arthroplasty

Status:

Journal Publication

Version 1

Abstract

Background

Methods

Results

Conclusions

Figures

Introduction

Materials And Methods

Geometry

Material Properties

Surgical Impaction: Configurations, Boundary conditions and Output

Daily Activities: Configurations, Boundary conditions and Output

FE Analysis

Results

Impact Test

Daily Activities

Discussion

Conclusions

Declarations

References

Tables

Additional Declarations

Status:

Journal Publication

Version 1