M.A. Pérez

Comparative analysis of bone remodelling models with respect to computerised tomography-based finite element models of bone

Subject-specific finite element models are an extensively used tool for the numerical analysis of the biomechanical behaviour of human bones. However, bone modelling is not an easy task due to the complex behaviour of bone tissue, involving non-homogeneous and anisotropic mechanical properties. Moreover, bone is a living tissue and therefore its microstructure and mechanical properties evolve with time in a known process called bone remodelling. This phenomenon has been widely studied, many being the numerical models that have been formulated to predict density distribution and its evolution in several bones. The aim of the present study is to assess the capability of a bone remodelling model to predict the bone density distribution of different types of human bone (femur, tibia and mandible) comparing the obtained results with the bone density estimated by means of computerised tomography. Good accuracy was observed for the bone remodelling predictions including the thickness of the cortical layer.

Stress transfer properties of different commercial dental implants: a finite element study

Dental implantology has high success rates, and a suitable estimation of how stresses are transferred to the surrounding bone sheds insight into the correct design of implant features. In this study, we estimate stress transfer properties of four commercial implants (GMI, Lifecore, Intri and Avinent) that differ significantly in macroscopic geometry. Detailed three-dimensional finite element models were adopted to analyse the behaviour of the bone-implant system depending on the geometry of the implant (two different diameters) and the bone–implant interface condition. Occlusal static forces were applied and their effects on the bone, implant and bone–implant interface were evaluated. Large diameters avoided overload-induced bone resorption. Higher stresses were obtained with a debonded bone–implant interface. Relative micromotions at the bone–implant interface were within the limits required to achieve a good osseointegration. We anticipate that the methodology proposed may be a useful tool for a quantitative and qualitative comparison between different commercial dental implants.

Computational modelling of bone cement polymerization: Temperature and residual stresses

The two major concerns associated with the use of bone cement are the generation of residual stresses and possible thermal necrosis of surrounding bone. An accurate modelling of these two factors could be a helpful tool to improve cemented hip designs. Therefore, a computational methodology based on previous published works is presented in this paper combining a kinetic and an energy balance equation. New assumptions are that both the elasticity modulus and the thermal expansion coefficient depend on the bone cement polymerization fraction. This model allows to estimate the thermal distribution in the cement which is later used to predict the stress-locking effect, and to also estimate the cement residual stresses. In order to validate the model, computational results are compared with experiments performed on an idealized cemented femoral implant. It will be shown that the use of the standard finite element approach cannot predict the exact temporal evolution of the temperature nor the residual stresses, underestimating and overestimating their value, respectively. However, this standard approach can estimate the peak and long-term values of temperature and residual stresses within acceptable limits of measured values. Therefore, this approach is adequate to evaluate residual stresses for the mechanical design of cemented implants. In conclusion, new numerical techniques should be proposed in order to achieve accurate simulations of the problem involved in cemented hip replacements.

Comparative Finite Element Analysis of the Debonding Process in Different Concepts of Cemented Hip Implants

Damage accumulation in the cement mantle and debonding of the bone–cement interface are basic events that contribute to the long-term failure of cemented hip reconstructions. In this work, a numerical study with these two process coupled is presented. Previously uniform bone–cement interface mechanical properties were only considered. In this work, a new approach assuming nonuniform and random bone–cement interface mechanical properties was applied to investigate its effect on cement degradation. This methodology was also applied to simulate and compare the degradation process of the cement and bone–cement interface in three different concepts of design: Exeter, Charnley, and ABG II stems. Nonuniform and random mechanical properties of the bone–cement interface implied a simulation closer to reality. The predicted results showed that the cement deterioration and bone–cement interface debonding is different for each implant depending on the stem geometry. Lower cement deterioration was obtained for the Charnley stem and lower bone–cement interface debonding was predicted for the Exeter stem, while the highest deterioration (cement and bone–cement interface) was produced for the ABG II stem.

Does Increased Bone–Cement Interface Strength have Negative Consequences for Bulk Cement Integrity? A Finite Element Study

Implant loosening is one of the most important modes of failure of cemented total hip replacement. It may be related to the cement strength, cement–prosthesis interface, cement–bone interface, surgical technique, or stem design. The main purpose of this study is to investigate the effect of bone–cement interface mechanical properties on cement degradation. The computational methodology proposed herein combines a previously developed bone–cement interface damage model and an accumulative damage model for bulk cement. This has been applied to a finite element model of an Exeter cemented hip implant. A higher strength of the bone–cement interface due to a higher amount of interdigitated bone results in faster cement deterioration. Over time, damage both at the bone–cement interface and in the cement mantle worsens. Also, a larger debonded area was predicted proximally, as observed in clinical practice. We conclude that the computational model proposed herein allows a realistic simulation of the bone–cement interface debonding and cement degradation, being a useful tool in the design of this kind of medical devices.

An interface finite element model can be used to predict healing outcome of bone fractures.

After fractures, bone can experience different potential outcomes: successful bone consolidation, non-union and bone failure. Although, there are a lot of factors that influence fracture healing, experimental studies have shown that the interfragmentary movement (IFM) is one of the main regulators for the course of bone healing. In this sense, computational models may help to improve the development of mechanical-based treatments for bone fracture healing. Hence, based on this fact, we propose a combined repair-failure mechanistic computational model to describe bone fracture healing. Despite being a simple model, it is able to correctly estimate the time course evolution of the IFM compared to in vivo measurements under different mechanical conditions. Therefore, this mathematical approach is especially suitable for modeling the healing response of bone to fractures treated with different mechanical fixators, simulating realistic clinical conditions. This model will be a useful tool to identify factors and define targets for patient specific therapeutics interventions.

Bone remodeling in the resurfaced femoral head: Effect of cement mantle thickness and interface characteristics

Metal-on-metal hip resurfacing prostheses were re-introduced during the last 10-15 years. These prostheses have the potential to better restore normal function with limited activity restriction, being an option for younger and more active patients. Resurfacing procedures have demonstrated high failure rates in national registers [1,2]. Multiple factors may affect early and long-term HR performance. The influence of femoral cement mantle thickness and different interface characteristics between the prosthesis components on the long-term performance of resurfacing prostheses is still unknown. In the present work, a model was used to predict bone remodeling with different mantle thicknesses and interface characteristics. A very thin cement mantle (0.25mm) increased bone resorption at the superior femoral head, while greater thickness (1 or 3mm) had a lesser effect. In all cases, bone apposition was predicted around the stem and at the stem tip. Bone formation and resorption were observed clinically in good agreement with the predictions calculated in simulations. Computed results showed that 1-mm cement mantle thickness combined with a bonded bone-cement interface and a debonded implant-cement interface was an appropriate configuration. Bone remodeling results and computed equivalent strains were correlated. In conclusion, we have been able to demonstrate the importance of choosing an adequate cement mantle thickness. Additionally, computational studies should consider realistic interface characteristics between the components in order to perform simulations closer to reality.

A finite element analysis of the vibration behaviour of a cementless hip system

An early diagnosis of aseptic loosening of a total hip replacement (THR) by plain radiography, scintigraphy or arthography has been shown to be less reliable than using a vibration technique. However, it has been suggested that it may be possible to distinguish between a secure and a loose prosthesis using a vibration technique. In fact, vibration analysis methods have been successfully used to assess dental implant stability, to monitor fracture healing and to measure bone mechanical properties. Several studies have combined the vibration technique with the finite element (FE) method in order to better understand the events involved in the experimental technique. In the present study, the main goal is to simulate the change in the resonance frequency during the osseointegration process of a cementless THR (Zweymüller). The FE method was used and a numerical modal analysis was conducted to obtain the natural frequencies and mode shapes under vibration. The effects were studied of different bone and stem material properties, and different contact conditions at the bone–implant interface. The results were in agreement with previous experimental and computational observations, and differences among the different cases studied were detected. As the osseointegration process at the bone–implant interface evolved, the resonance frequency values of the femur–prosthesis system also increased. In summary, vibration analysis combined with the FE method was able to detect different boundary conditions at the bone–implant interface in cases of both osseointegration and loosening.

Subject-specific musculoskeletal loading of the tibia: Computational load estimation

The systematic development of subject-specific computer models for the analysis of personalized treatments is currently a reality. In fact, many advances have recently been developed for creating virtual finite element-based models. These models accurately recreate subject-specific geometries and material properties from recent techniques based on quantitative image analysis. However, to determine the subject-specific forces, we need a full gait analysis, typically in combination with an inverse dynamics simulation study. In this work, we aim to determine the subject-specific forces from the computer tomography images used to evaluate bone density. In fact, we propose a methodology that combines these images with bone remodelling simulations and artificial neural networks. To test the capability of this novel technique, we quantify the personalized forces for five subject-specific tibias using our technique and a gait analysis. We compare both results, finding that similar vertical loads are estimated by both methods and that the dominant part of the load can be reliably computed. Therefore, we can conclude that the numerical-based technique proposed in this work has great potential for estimating the main forces that define the mechanical behaviour of subject-specific bone.

Biomechanical assessment and clinical analysis of different intramedullary nailing systems for oblique fractures

Abstract The aim of this study is to evaluate the fracture union or non-union for a specific patient that presented oblique fractures in tibia and fibula, using a mechanistic-based bone healing model. Normally, this kind of fractures can be treated through an intramedullary nail using two possible configurations that depends on the mechanical stabilisation: static and dynamic. Both cases are simulated under different fracture geometries in order to understand the effect of the mechanical stabilisation on the fracture healing outcome. The results of both simulations are in good agreement with previous clinical experience. From the results, it is demonstrated that the dynamization of the fracture improves healing in comparison with a static or rigid fixation of the fracture. This work shows the versatility and potential of a mechanistic-based bone healing model to predict the final outcome (union, non-union, delayed union) of realistic 3D fractures where even more than one bone is involved.