Fatigue failure in the cement mantle of a simplified acetabular replacement model

Biomechanical performance of the cemented acetabular cup with combined effects of bone quality, implant material combinations and bodyweight

The objective of this study is to understand the combined effects of bone quality, implant materials and bodyweight on the biomechanical performance of cemented acetabular cup. Additionally, the performance of the cemented acetabular cup was evaluated for obesity cases or obese people. A total of 84 FE models (based on CT data) were developed based on combinations of three different cancellous bone material distributions to represent bone quality, four different implant material combinations and seven different bodyweights. The biomechanical performance of the acetabular cup was evaluated based on bone stress (both cortical and cancellous bone), cement mantle stress, micromotion and contact pressure between the acetabular cup and femoral head. Cortical bone stress, cancellous bone stress, cement stress, the contact pressure between implants and micromotion between implants are affected by different bone quality, implant material combinations and bodyweights. An increase in bodyweight would increase the cortical bone stress, cancellous bone stress, cement stress, contact pressure between implants and micromotion between implants. However, bodyweight affects the cortical and cancellous bone stress more (stiff rise of the bone stresses; nonlinear relation) as compared to other output parameters (mostly linear relation). Comparing cortical and cancellous bone stress, the stress versus bodyweight curve is much stiffer (stiff rise in the curve) for cortical bone than cancellous bone and that even further increases as bone quality decreases. Especially considering obesity cases or obese people (very high bodyweight), the performance of the cemented acetabular component is poor.

Graphical abstract

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Effect of liner offset and inclination on cement retention strength of metal-in-metal acetabular constructs: A biomechanical study

Cementing metallic liners into well-fixed acetabular shells facilitates utilizing dual-mobility cups in revision total hip arthroplasty without shell replacement. The current biomechanical study investigated the effect of increasing cemented liner (a) inclination; and (b) offset on the cement retention strength measured as the lever-out moment at cement failure. Eighteen metallic liner prototypes were cemented into cluster-hole acetabular shells at variable inclinations (0°, 10°, and 20°) and offsets (0 and 10 mm) relative to the enclosing acetabular shell (6 groups; n = 3 constructs per group). The constructs were connected to a material testing frame, and lever-out failure moments were tested through an established protocol. Failure occurred at the liner-cement interface (18/18). There was no correlation between liner inclination and the lever-out failure moment (r = -0.327, P = .185). Liner offset demonstrated a strong negative correlation to mean lever-out failure moments (r = -0.788, P < .001). There was no significant difference between mean lever-out failure moments at variable liner inclinations, regardless of offset (P = .358). Greater liner offset was associated with diminished mean lever-out failure moments (P < .001). Compared with neutral (0° inclination, 0 mm offset), the maximum inclination and offset group had the lowest mean lever-out failure moment (P = .011). Cemented metal-in-metal constructs are significantly affected by the liner positioning. While a correlation between liner inclination and cement retention strength could not be asserted, cement retention strength is significantly diminished by increased liner offset.

Non-anatomical placement adversely affects the functional performance of the meniscal implant: a finite element study

Non-anatomical placement may occur during the surgical implantation of the meniscal implant, and its influence on the resulting biomechanics of the knee joint has not been systematically studied. The purpose of this study was to evaluate the biomechanical effects of non-anatomical placement of the meniscal implant on the knee joint during a complete walking cycle. Three-dimensional finite element (FE) analyses of the knee joint were performed, based on the model developed from magnetic resonance images and the loading conditions derived from the gait pattern of a healthy male subject, for the following physiological conditions: (i) knee joint with intact native meniscus, (ii) medial meniscectomized knee joint, (iii) knee joint with anatomically placed meniscal implant, and (iv) knee joint with the meniscal implant placed in four different in vitro determined non-anatomical locations. While the native menisci were modeled using the nonlinear hyperelastic Holzapfel-Gasser-Ogden (HGO) constitutive model, the meniscal implant was modeled using the isotropic hyperelastic neo-Hookean model. Placement of the meniscal implant in the non-anatomical lateral-posterior and lateral-anterior locations significantly increased the peak contact pressure in the medial compartment. Placement of the meniscal implant in non-anatomical locations significantly altered the tibial rotational kinematics and increased the total force acting at the meniscal horns. Results suggest that placement of the meniscal implant in non-anatomical locations may restrain its ability to be chondroprotective and may initiate or accelerate cartilage degeneration. In conclusion, clinicians should endeavor to place the implant as closest as possible to the anatomical location to restore the normal knee biomechanics.

The implications of non-anatomical positioning of a meniscus prosthesis on predicted human knee joint biomechanics

Despite all the efforts to optimize the meniscus prosthesis system (geometry, material, and fixation type), the success of the prosthesis in clinical practice will depend on surgical factors such as intra-operative positioning of the prosthesis. In this study, the aim was therefore to assess the implications of positional changes of the medial meniscus prosthesis for knee biomechanics. A detailed validated finite element (FE) model of human intact and meniscal implanted knees was developed based on a series of in vitro experiments. Different non-anatomical prosthesis positions were applied in the FE model, and the biomechanical response during the gait stance phase compared with an anatomically positioned prosthesis, as well as meniscectomized and also the intact knee model. The results showed that an anatomical positioning of the medial meniscus prosthesis could better recover the intact knee biomechanics, while a non-anatomical positioning of the prosthesis to a limited extent alters the knee kinematics and articular contact pressure and increases the implantation failure risk. The outcomes indicate that a medial or anterior positioning of the meniscus prosthesis may be more forgiving than a posteriorly or laterally positioned prosthesis. The outcome of this study may provide a better insight into the possible consequences of meniscus prosthesis positioning errors for the patient and the prosthesis functionality.

Effects of interfacial crack and implant material on mixed-mode stress intensity factor and prediction of interface failure of cemented acetabular cup

This study deals with the effects of interfacial crack and implant material on mixed-mode stress intensity factor and prediction of interface failure of the cemented acetabular cup. A three dimensional (3D) finite element (FE) model of implanted pelvic bone was developed based on the computed tomography (CT) scan data. Combinations of four materials were considered for implant material. To understand the influence of interfacial crack at bone-cement and cement-implant interfaces on failure, 2D cracked models were developed based on the FE model and solved using the element-free Galerkin method (EFGM) by considering a rectangular section in the superior, inferior, anterior, and posterior locations. Interface failure was predicted in terms of mixed-mode stress intensity factor (SIF). The stress values obtained from FE analysis were transferred at the cut boundary of the rectangular section and considered as a mixed-mode loading condition to determine the SIF in the superior, inferior, anterior, and posterior locations at bone-cement and cement-implant interfaces using EFGM. Location wise, anterior seems to have more chances of failure because SIF in the anterior location was found to be higher than other locations. The bone-cement interface has more SIF and indicated more chances of failure than the cement-implant interface. Less SIF was found for the ceramic-ceramic material combination than other material combinations.

Study of an Additional Layer of Cement Mantle Hip Joints for Reducing Cracks

Failure of the cement mantle in total hip arthroplasty is not a simple phenomenon. Cracking, which can be caused by crack initiation and repeated loading, can cause loosening of the acetabular liner component. A previous study showed that addition of a metal layer between the liner and acetabular could reduce the stress at the contact surface of the cement mantle. This study elaborates on the performance of the additional layer. Several material properties of the layer were simulated using finite element analysis for maximum performance. A static contact analysis was used to simulate the stresses at the contact surface of the cement mantle. The results show that an additional layer of cobalt chrome produced the best performance.

Standardization of hemipelvis alignment for in vitro biomechanical testing

Although in vitro biomechanical tests are regularly performed, the definition of a suitable reference frame for hemipelvic specimens is still a challenge. The aims of the present study were to: (i) define a reference frame for the human hemipelvis suitable for in vitro applications, based on robust anatomical landmarks; (ii) identify the alignment of a hemipelvis based on the alignment of a whole pelvis (including right/left and male/female differences); (iii) identify the relative alignment of the proposed in vitro reference frame with respect to a reference frame commonly used in gait analysis; (iv) create an in vitro alignment procedure easy, robust and inexpensive; (v) quantify the intra-operator repeatability and inter-operator reproducibility of the procedure. A procedure to univocally identify the anatomical landmarks was created, exploiting the in vitro accessibility of the specimen's surface. Through the analysis on 53 CT scans (106 hemipelvises), the alignment of the hemipelvis based on the alignment of a whole pelvis was analyzed: differences between male/female and right/left hemipelvises were not statistically significant To overcome the uncertainty in the identification of the acetabular rim, a standard acetabular plane was defined. An alignment procedure was developed to implement such anatomical reference frame. The intra-operator repeatability and the inter-operator reproducibility were quantified with four operators, on male and female hemipelvises. The intra-operator repeatability was better than 1.5°. The inter-operator reproducibility was better than 2.0°. Alignment in the transverse plane was the most repeatable. The presented procedure to align hemipelvic specimens is sufficiently robust, standardized, and accessible. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1645-1652, 2018.

Assessment of failure of cemented polyethylene acetabular component due to bone remodeling: A finite element study

The aim of the study is to determine failure of the cemented polyethylene acetabular component, which might occur due to excessive bone resorption, cement-bone interface debonding and fatigue failure of the cement mantle. Three-dimensional finite element models of intact and implanted pelvic bone were developed and bone remodeling algorithm was implemented for present analysis. Soderberg fatigue failure diagram was used for fatigue assessment of the cement mantle. Hoffman failure criterion was considered for prediction of cement-bone interface debonding. Results indicate fatigue failure of the cement mantle and implant-bone interface debonding might not occur due to bone remodeling.

The effect of cup outer sizes on the contact mechanics and cement fixation of cemented total hip replacements

One important loosening mechanism of the cemented total hip arthroplasty is the mechanical overload at the bone-cement interface and consequent failure of the cement fixation. Clinical studies have revealed that the outer diameter of the acetabular component is a key factor in influencing aseptic loosening of the hip arthroplasty. The aim of the present study was to investigate the influence of the cup outer diameter on the contact mechanics and cement fixation of a cemented total hip replacement (THR) with different wear penetration depths and under different cup inclination angles using finite element (FE) method. A three-dimensional FE model was developed based on a typical Charnley hip prosthesis. Two acetabular cup designs with outer diameters of 40 and 43 mm were modelled and the effect of cup outer diameter, penetration depth and cup inclination angle on the contact mechanics and cement fixation stresses in the cemented THR were studied. The results showed that for all penetration depths and cup inclination angles considered, the contact mechanics in terms of peak von Mises stress in the acetabular cup and peak contact pressure at the bearing surface for the two cup designs were similar (within 5%). However, the peak von Mises stress, the peak maximum principal stress and peak shear stress in the cement mantle at the bone-cement interface for the 43 mm diameter cup design were predicted to be lower compared to those for the 40 mm diameter cup design. The differences were predicted to be 15-19%, 15-22% and 18-20% respectively for different cup penetration depths and inclination angles, which compares to the clinical difference of aseptic loosening incidence of about 20% between the two cup designs.

Numerical analysis of the crack growth path in the cement mantle of the reconstructed acetabulum

In this study, we use the finite element method to analyze the propagation's path of the crack in the orthopedic cement of the total hip replacement. In fact, a small python statement was incorporated with the Abaqus software to do in loop the following operations: extracting the crack propagation direction from the previous study using the maximal circumferential stresses criterion, drawing the new path, meshing and calculating again (stresses and fracture parameters). The loop is broken when the user's desired crack length is reached (number of propagations) or the value of the mode I stress intensity factor is negative. Results show that the crack propagation's path can be influenced by human body posture. The existing of a cavity in the vicinity of the crack can change its propagation path or can absolutely attract it enough to meet it. Crack can propagate in the outward direction (toward the acetabulum bone) and cannot propagate in the opposite direction, the mode I stress intensity factor increases with the crack length and that of mode II vanishes.

Fatigue in cemented acetabular replacements

The long-term stability of cemented total hip replacements critically depends on the lasting integrity of the bond between the cement and the bone, often referred to as fixation. In vitro assessment of fatigue behaviour of cemented acetabular, as opposed to femoral, replacements is of particular interest due to the more aggressive nature of late "loosening" found in acetabular replacements, reported to be three times that in femoral cases. Quantitative assessment of fatigue behaviour of cement fixation on acetabular side has been difficult due to the complexity of the pelvic bone geometry and the associated loading conditions.The purpose of this work was to develop a framework for in vitro assessment of fatigue integrity of cement fixation in acetabular replacements. To this end, a newly developed hip simulator was utilised, where the direction and the magnitude of the hip contact force (Bergmann et al., 2001) under typical physiological loading conditions including normal walking and stair climbing were simulated for the first time. Preliminary hip simulator experimental results seem to be consistent with those from constant amplitude fatigue tests, in that debonding at the bone-cement interface is identified as the main failure mechanism, although the numbers of cycles to failure are significantly reduced in samples tested in the hip simulator. Finite element analysis of implanted bone samples was carried out, where the effects of loading mode on the stress distribution in the cement mantle and at the bone-cement interface were evaluated. The effects of model geometry on the stress state and failure modes were also examined and discussed based on the results of the present and previously published work.