Advanced material modelling in numerical simulation of primary acetabular press-fit cup stability

Modeling the debonding process of osseointegrated implants due to coupled adhesion and friction

Cementless implants have become widely used for total hip replacement surgery. The long-term stability of these implants is achieved by bone growing around and into the rough surface of the implant, a process called osseointegration. However, debonding of the bone-implant interface can still occur due to aseptic implant loosening and insufficient osseointegration, which may have dramatic consequences. The aim of this work is to describe a new 3D finite element frictional contact formulation for the debonding of partially osseointegrated implants. The contact model is based on a modified Coulomb friction law by Immel et al. (2020), that takes into account the tangential debonding of the bone-implant interface. This model is extended in the direction normal to the bone-implant interface by considering a cohesive zone model, to account for adhesion phenomena in the normal direction and for adhesive friction of partially bonded interfaces. The model is applied to simulate the debonding of an acetabular cup implant. The influence of partial osseointegration and adhesive effects on the long-term stability of the implant is assessed. The influence of different patient- and implant-specific parameters such as the friction coefficient [Formula: see text], the trabecular Young's modulus [Formula: see text], and the interference fit [Formula: see text] is also analyzed, in order to determine the optimal stability for different configurations. Furthermore, this work provides guidelines for future experimental and computational studies that are necessary for further parameter calibration.

Determinants of the primary stability of cementless acetabular cup implants: A 3D finite element study

Primary stability of cementless implants is crucial for the surgical success and long-term stability. However, primary stability is difficult to quantify in vivo and the biomechanical phenomena occurring during the press-fit insertion of an acetabular cup (AC) implant are still poorly understood. The aim of this study is to investigate the influence of the cortical and trabecular bone Young's moduli E_c and E_t, the interference fit IF and the sliding friction coefficient of the bone-implant interface μ on the primary stability of an AC implant. For each parameter combination, the insertion of the AC implant into the hip cavity and consequent pull-out are simulated with a 3D finite element model of a human hemi-pelvis. The primary stability is assessed by determining the polar gap and the maximum pull-out force. The polar gap increases along with all considered parameters. The pull-out force shows a continuous increase with E_c and E_t and a non-linear variation as a function of μ and IF is obtained. For μ > 0.6 and IF > 1.4 mm the primary stability decreases, and a combination of smaller μ and IF lead to a better fixation. Based on the patient's bone stiffness, optimal combinations of μ and IF can be identified. The results are in good qualitative agreement with previous studies and provide a better understanding of the determinants of the AC implant primary stability. They suggest a guideline for the optimal choice of implant surface roughness and IF based on the patient's bone quality.

Comparison of Test Setups for the Experimental Evaluation of the Primary Fixation Stability of Acetabular Cups

Sufficient primary fixation stability is the basis for the osseointegration of cementless acetabular cups. Several test methods have been established for determining the tilting moment of acetabular press-fit cups, which is a measure for their primary fixation stability. The central aim of this experimental study was to show the differences between the commonly used lever-out test method (Method 1) and the edge-load test method (Method 2) in which the cup insert is axially loaded (1 kN) during the tilting process with respect to the parameters, tilting moment, and interface stiffness. Therefore, using a biomechanical cup block model, a press-fit cup design with a macro-structured surface was pushed into three cavity types (intact, moderate superior defect, and two-point-pinching cavity) made of 15 pcf and 30 pcf polyurethane foam blocks (n = 3 per cavity and foam density combination), respectively. Subsequently, the acetabular cup was disassembled from the three artificial bone cavities using the lever-out and the edge-load test method. Tilting moments determined with Method 1 ranged from 2.72 ± 0.29 Nm to 49.08 ± 1.50 Nm, and with Method 2, they ranged from 41.40 ± 1.05 Nm to 112.86 ± 5.29 Nm. In Method 2, larger areas of abrasion were observed in the artificial bone cavity compared to Method 1. This indicates increased shear forces at the implant–bone interface in the former method. In conclusion, Method 1 simulates the technique used by orthopedic surgeons to assess the correct fit of the trial cup, while Method 2 simulates the tilting of the cup in the acetabular bone cavity under in situ loading with the hip resultant force.

Fixation Stability of Uncemented Acetabular Cups With Respect to Different Bone Defect Sizes

BACKGROUND

In total hip arthroplasty, acetabular press-fit cups require a proper bone stock for sufficient primary implant fixation. The presence of acetabular bone defects compromises the primary fixation stability of acetabular press-fit cups. The aim of the present study is to determine the fixation stability of a cementless acetabular cup regarding standardized bone defects in an experimental setup.

METHODS

An acetabular defect model was developed and transferred to a biomechanical cup-block model. The lack of superior cup coverage was divided into 4 stages of superior rim loss (33%, 50%, 67%, and 83%) in the anterior-posterior direction and into 4 stages of mediolateral wall absence (11%, 22%, 33%, and 50%). This resulted in 11 different defect cavities, which were compared to the intact cavity in push-in and lever-out tests of one press-fit cup design (56 mm outer diameter). Thereby, push-in force, lever-out moment, lever-out angle, and interface stiffness were determined.

RESULTS

The determined lever-out moments range from 15.53 ± 1.38 Nm (intact cavity) to 1.37 ± 0.54 Nm (83%/50% defect). Smaller defects (33%/11%, 33%/22%, and 50%/11%) reduce the lever-out moments by an average of 33.9% ± 2.8%.

CONCLUSION

The lack of mediolateral acetabular coverage of 50% was assessed as critical for cementless cup fixation, whereby the contact zone between implant and bone in the defect is lost. A lack of 20% to 30% mediolateral coverage appears to be acceptable for press-fit cup fixation in the presence of primary stability. A defect of 50%/50% was identified as the threshold for using additional fixation methods.

Influence of Synthetic Bone Substitutes on the Anchorage Behavior of Open-Porous Acetabular Cup

BACKGROUND

The development in implants such as acetabular cups using additive manufacturing techniques is playing an increasingly important role in the healthcare industry.

METHOD

This study compared the primary stability of four selectively laser-melted press-fit cups (Ti6Al4V) with open-porous, load-bearing structural elements on the surface. The aim was to assess whether the material of the artificial bone stock affects the primary stability of the acetabular cup. The surface structures consist of repeated open-porous, load-bearing elements orthogonal to the acetabular surface. Experimental pull-out and lever-out tests were performed on exact-fit and press-fit cups to evaluate the primary stability of the cups in different synthetic bone substitutes. The acetabular components were placed in three different commercially available synthetic materials (ROHACELL-IGF 110, SikaBlock M330, Sawbones Solid Rigid). Results & conclusions: Within the scope of the study, it was possible to show the differences in fixation strength between the tested acetabular cups depending on their design, the structural elements used, and the different bone substitute material. In addition, functional correlations could be found which provide a qualitative reference to the material density of the bone stock and the press-fit volume of the acetabular cups.

Calibration of crushable foam plasticity models for synthetic bone material for use in finite element analysis of acetabular cup deformation and primary stability

Polyurethane (PU) foam is a material often used in biomechanical experiments and demands for the definition of crushable foam plasticity (CFP) in numerical simulations of the primary stability and deformation of implants, to describe the crushing behaviour appropriately. Material data of PU foams with five different densities (10-40 pounds per cubic foot were ascertained experimentally in uniaxial compression test and used to calibrate CFP models for finite element modelling. Additionally, experimental and numerical deformation, push-out and lever-out tests of press-fit acetabular cups were carried out to assess the influence of the chosen material definition (linear elastic and CFP) on the numerical results. Comparison of the experimentally and numerically determined force-displacement curves of the uniaxial compression test showed a mean deviation of less than 3%. In primary stability testing, the deviation between the experimental and numerical results was in a range of 0%-27% for CFP modelling and 64%-341% for the linear elastic model. The material definition selected, highly influenced the numerical results in the current study. The use of a CFP model is recommended for further numerical simulations, when a deformation of the foam beyond the yield strength is likely to occur.

Experimental Characterization of the Primary Stability of Acetabular Press-Fit Cups with Open-Porous Load-Bearing Structures on the Surface Layer

Background: Nowadays, hip cups are being used in a wide range of design versions and in an increasing number of units. Their development is progressing steadily. In contrast to conventional methods of manufacturing acetabular cups, additive methods play an increasingly central role in the development progress. Method: A series of eight modified cups were developed on the basis of a standard press-fit cup with a pole flattening and in a reduced version. The surface structures consist of repetitive open-pore load-bearing textural elements aligned right-angled to the cup surface. We used three different types of unit cells (twisted, combined and combined open structures) for constructing of the surface structure. All cups were manufactured using selective laser melting (SLM) of titanium powder (Ti6Al4V). To evaluate the primary stability of the press fit cups in the artificial bone cavity, pull-out and lever-out tests were conducted. All tests were carried out under exact fit conditions. The closed-cell polyurethane (PU) foam, which was used as an artificial bone cavity, was characterized mechanically in order to preempt any potential impact on the test results. Results and conclusions: The pull-out forces as well as the lever moments of the examined cups differ significantly depending on the elementary cells used. The best results in pull-out forces and lever-out moments are shown by the press-fit cups with a combined structure. The results for the assessment of primary stability are related to the geometry used (unit cell), the dimensions of the unit cell, and the volume and porosity responsible for the press fit. Corresponding functional relationships could be identified. The findings show that the implementation of reduced cups in a press-fit design makes sense as part of the development work.

A novel approach to determine primary stability of acetabular press-fit cups

Today hip cups are used in a large variety of design variants and in increasing numbers of units. Their development is steadily progressing. In addition to conventional manufacturing methods for hip cups, additive methods, in particular, play an increasingly important role as development progresses. The present paper describes a modified cup model developed based on a commercially available press-fit cup (Allofit 54/JJ). The press-fit cup was designed in two variants and manufactured using selective laser melting (SLM). Variant 1 (Ti) was modeled on the Allofit cup using an adapted process technology. Variant 2 (Ti-S) was provided with a porous load bearing structure on its surface. In addition to the typical (complete) geometry, both variants were also manufactured and tested in a reduced shape where only the press-fit area was formed. To assess the primary stability of the press-fit cups in the artificial bone cavity, pull-out and lever-out tests were carried out. Exact fit conditions and two-millimeter press-fit were investigated. The closed-cell PU foam used as an artificial bone cavity was mechanically characterized to exclude any influence on the results of the investigation. The pull-out forces of the Ti-variant (complete-526 N, reduced-468 N) and the Ti-S variant (complete-548 N, reduced-526 N) as well as the lever-out moments of the Ti-variant (complete-10 Nm, reduced-9.8 Nm) and the Ti-S variant (complete-9 Nm, reduced-7.9 N) show no significant differences in the results between complete and reduced cups. The results show that the use of reduced cups in a press-fit design is possible within the scope of development work.

Assessment of the biomechanical stability of a dental implant with quantitative ultrasound: A three-dimensional finite element study

Dental implant stability is an important determinant of the surgical success. Quantitative ultrasound (QUS) techniques can be used to assess such properties using the implant acting as a waveguide. However, the interaction between an ultrasonic wave and the implant remains poorly understood. The aim of this study is to investigate the sensitivity of the ultrasonic response to the quality and quantity of bone tissue in contact with the implant surface. The 10 MHz ultrasonic response of an implant used in clinical practice was simulated using an axisymmetric three-dimensional finite element model, which was validated experimentally. The amplitude of the echographic response of the implant increases when the depth of a liquid layer located at the implant interface increases. The results show the sensitivity of the QUS technique to the amount of bone in contact with the implant. The quality of bone tissue around the implant is varied by modifying the bone biomechanical properties by 20%. The amplitude of the implant echographic response decreases when bone quality increases, which corresponds to bone healing. In all cases, the amplitude of the implant response decreased when the dental implant stability increased, which is consistent with the experimental results.

Finite element simulation of ultrasonic wave propagation in a dental implant for biomechanical stability assessment

Dental implant stability, which is an important parameter for the surgical outcome, can now be assessed using quantitative ultrasound. However, the acoustical propagation in dental implants remains poorly understood. The objective of this numerical study was to understand the propagation phenomena of ultrasonic waves in cylindrically shaped prototype dental implants and to investigate the sensitivity of the ultrasonic response to the surrounding bone quantity and quality. The 10-MHz ultrasonic response of the implant was calculated using an axisymetric 3D finite element model, which was validated by comparison with results obtained experimentally and using a 2D finite difference numerical model. The results show that the implant ultrasonic response changes significantly when a liquid layer is located at the implant interface compared to the case of an interface fully bounded with bone tissue. A dedicated model based on experimental measurements was developed in order to account for the evolution of the bone biomechanical properties at the implant interface. The effect of a gradient of material properties on the implant ultrasonic response is determined. Based on the reproducibility of the measurement, the results indicate that the device should be sensitive to the effects of a healing duration of less than one week. In all cases, the amplitude of the implant response is shown to decrease when the dental implant primary and secondary stability increase, which is consistent with the experimental results. This study paves the way for the development of a quantitative ultrasound method to evaluate dental implant stability.

Assessment of in vitro dental implant primary stability using an ultrasonic method

Dental implants are used for oral rehabilitation. However, there remain risks of failure that depend on the implant stability. The objective of this study is to investigate whether quantitative ultrasound technique can be used to assess the amount of bone in contact with dental implants. Ten implants are first inserted in the bone samples. The 10 MHz ultrasonic response of each implant is measured using a dedicated device and an indicator I is derived based on the amplitude of the signal. Then, the implant is unscrewed by 2 π radians and the measurement is realized again. A statistical analysis of variance was carried out and revealed a significant effect of the amount of bone in contact with the implant on the values of I (p value < 10⁻⁵). The results indicates the feasibility of quantitative ultrasound techniques to assess implant primary stability in vitro.

Ultrasonic evaluation of dental implant osseointegration

Dental implants are widely used for oral rehabilitation. However, there remain risks of failure which are difficult to anticipate and depend on the implant osseointegration. The objective of this in vivo study is to determine the variation of the echographic ultrasonic response of a dental implant to bone healing around the implant interface. Twenty one dental implants were inserted in the femur of seven New Zealand white rabbits. Two animals were sacrificed after a healing duration of two weeks, three animals after six weeks and six animals after eleven weeks. The 10 MHz ultrasonic response of the implant was measured just after the implantation using a dedicated device positioned at the emerging surface of each dental implant. The measurements were realized again before the sacrifice with the same device. An indicator I˜ was derived based on the amplitude of the rf signal obtained for each configuration. The bone-Implant Contact (BIC) ratio was determined by histological analyses. The average value of the relative variation of the indicator I˜ obtained after initial surgery and after the corresponding healing period varies between 7% and 40%. A Kruskal-Wallis test (p<0.01) revealed a significant decrease of the value of the indicator I˜ as function of healing time. The indicator I˜ was significantly correlated (R(2)=0.45) with the BIC ratio. The results show that the ultrasonic response of a dental implant varies significantly as a function of healing time, which paves the way for the development of a new quantitative ultrasound (QUS) method in oral implantology.