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

The role of muscle forces and gait cycle discretization when assessing acetabular cup primary stability: A finite element study

The aim of this study was to investigate the influence of the muscle force contribution and loading cycle discretization on the predicted micromotion and interfacial bone strains in the implanted acetabulum. To this end, a patient specific finite element model of the hemipelvis was developed, based on the CT-scan and gait analysis results, collected as part of the authors' previous work. Outcomes of this study suggests that the acetabular cup micromotion and interfacial bone strains can be predicted just using the joint contact force. This helps to reduce the complexity of the finite element models by ignoring the contribution of muscle forces and the associated challenges of mapping these forces to the pelvis. However, the gait cycle needs to be adequately discretised to capture the micromotion at the bone-implant interface.

BACKGROUND AND OBJECTIVE

The Dalstra load case, which includes muscle forces, has been widely adopted in the literature for studying the mechanical environment in the intact and implanted acetabulum. To simplify the modelling approach, some researchers ignore the contribution of muscle forces. The Dalstra load case is also divided into eight separate load steps (five in the stance phase and three in the swing phase), however, it is unclear whether this adequately captures the micromotions, for a cementless acetabular cup, during a simulated activity. The aim of this study was to investigate the influence of the muscle force contribution and loading cycle discretization on the predicted micromotion and interfacial bone strains.

METHODS

In this work, a patient specific finite element model of the hemipelvis was developed, based on the CT-scan and gait analysis results, collected as part of the authors' previous work. Finite element simulations were performed using the joint contact and muscle forces derived from two sources. The first approach was used the load case proposed by Dalstra et al. The second approach used joint contact and muscle forces predicted by a musculoskeletal model. Additionally, the musculoskeletal load case was discretised into 50 equal load steps and the results compared with the equivalent Dalstra load steps.

RESULTS

The results showed that the contribution of the muscle forces resulted in minor differences in both the magnitude and distribution of the predicted acetabular micromotion (up to 4.01% in the mean acetabular micromotion) and interfacial bone strains (up to 10.34% in the mean interfacial bone strains). The degree of gait cycle discretisation had a significant influence on the acetabular micromotion with a difference of 20.89% in the mean acetabular micromotion.

CONCLUSION

Outcomes of this study suggests that the acetabular cup micromotion and interfacial bone strains can be predicted just using the joint contact force. This helps to reduce the complexity of the finite element models by ignoring the contribution of muscle forces and the associated challenges of mapping these forces to the pelvis. However, the gait cycle needs to be adequately discretised to capture the micromotion at the bone-implant interface.

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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Fracture Toughness of Acrylic PMMA Bone Cement: A Mini-Review

BACKGROUND

Acrylic PMMA bone cement is an essential component in cemented implants and formed the cement-bone and cement-implant interfaces. The information on the fracture parameters of PMMA bone cement would be decisive for all doctors, researchers, and orthopaedic surgeons.

PURPOSE

This review aims to indicate the parameters responsible for the variation in the fracture toughness of PMMA bone cement. This mini-review also points out some limitations of the earlier published research article, which can be added in the future analysis and can be helpful to get the more realistic data of the fracture parameters of PMMA bone cement.

CONCLUSION

Different mixing techniques, storage medium, temperature, loading conditions, frequency and environment, cement viscosity, type of specimen, and the ASTM standards (shape, size, and geometry), constituents, loading rate, and cement porosity were the critical parameters to affect the fracture toughness of PMMA bone cement. This study will also be helpful to increase the structural integrity of PMMA bone cement and the cemented implant.

Spatio-temporal evolution of hydroxyapatite crystal thickness at the bone-implant interface

A better understanding of bone nanostructure around the bone-implant interface is essential to improve longevity of clinical implants and decrease failure risks. This study investigates the spatio-temporal evolution of mineral crystal thickness and plate orientation in newly formed bone around the surface of a metallic implant. Standardized coin-shaped titanium implants designed with a bone chamber were inserted into rabbit tibiae for 7 and 13 weeks. Scanning measurements with micro-focused small-angle X-ray scattering (SAXS) were carried out on newly formed bone close to the implant and in control mature cortical bone. Mineral crystals were thinner close to the implant (1.8 ± 0.45 nm at 7 weeks and 2.4 ± 0.57 nm at 13 weeks) than in the control mature bone tissue (2.5 ± 0.21 nm at 7 weeks and 2.8 ± 0.35 nm at 13 weeks), with increasing thickness over healing time (+30 % in 6 weeks). These results are explained by younger bone close to the implant, which matures during osseointegration. Thinner mineral crystals parallel to the implant surface within the first 100 µm indicate that the implant affects the ultrastructure of neighbouring bone , potentially due to heterogeneous interfacial stresses, and suggest a longer maturation process of bone tissue and difficulty in binding to the metal. The bone growth kinetics within the bone chamber was derived from the spatio-temporal evolution of bone tissue's nanostructure, coupled with microtomographic imaging. The findings indicate that understanding mineral crystal thickness or plate orientation can improve our knowledge of osseointegration.