Combined wear and ageing of ceramic-on-ceramic bearings in total hip replacement under edge loading conditions

[Tribology in arthroplasty : Friction and wear, a key to a long lifetime]

This article is intended to highlight one of the key roles in endoprosthetic treatment with artificial implants and the extension of service life. Like every joint, artificial joints are subject to the physical laws of friction and wear-in short, tribology. Material pairings, surfaces and mechanisms of action in particular play a decisive role here. The special features and current findings relating to the three largest synovial joints (hip, knee and shoulder) will be discussed in detail and suggestions will be made for future developments. Continuous developments in the field of the tribology of artificial joints can massively improve care for patients. The revision figures and reasons already show the success of individual improvements in recent years.

A finite element study on the interactive effect between the damage of the cup-bone interface and the bone strain of hip implants under various fixation conditions

Interfacial damage has a high impact on the loosening of the acetabular cup. However, monitoring this damage induced by the variations in loading conditions, such as the angle, amplitude, and frequency in vivo, is challenging. In this study, we evaluated the risk of loosening of the acetabular cup due to interfacial damages induced by the deviation in loading conditions and amplitudes. A three-dimensional model of the acetabular cup component was developed, and the interfacial crack growth between the cup and the bone was modeled using a fracture mechanics approach, which simulated the extent of interfacial damage and associated cup displacement. The interfacial delamination mechanism changed with the increasing inclination angle, wherein a fixation angle of 60° exhibited the largest area of contact loss. The compressive strain of embedding the simulated bone at the remaining bonding area accumulated as the lost contact area widened. Such interfacial damages, namely, the growth of the lost contact area and accumulated compressive strain in the simulated bone, promoted both embedding and rotational displacement of the acetabular cup. In the worst case of a fixation angle of 60°, the total displacement of the acetabular cup exceeded the limit of the modified safe zone, suggesting a quantitative risk of dislocation of the acetabular cup induced by the accumulated interfacial damage. Furthermore, nonlinear regression analyses between the degree of displacement of the acetabular cup and the extent of the two types of interfacial damage demonstrated that the interactive effect of the fixation angle with the loading amplitude showed a significant effect on increasing cup displacement. These findings suggest that proper control of the fixation angle during operation is useful in preventing the loosening of the hip joint.

Damage analysis of retrieved BioloxⓇdelta components used in hard and soft bearings

This retrieval study included 43 Biolox delta explants (18 CoC, 25 CoP). Implants were examined macroscopically, whereby damage was evaluated using a semi quantitative scoring system. Confocal microscopy was used to examine wear related damage patterns of the articulating surfaces. Scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) was used to analyze wear marks on the implant surface and wear debris in periprosthetic tissue samples. Raman spectroscopy and X-ray diffraction (XRD) were used to quantify monoclinic zirconia fractions. On all components, in vivo wear resulted predominantly in different damage patterns caused by metal transfer. In CoC bearings stripe wear was additionally detected, and some implants underwent severe damage due to component breakage. The wear scores were higher for CoC components, with no differences between the scores for CoC heads and liners. Wear features caused comparable roughening on implants from CoC and CoP bearings. SEM studies demonstrated that most wear marks were caused by metal debris released from implant components. Grain pull-out was observed in stripe wear regions. Monoclinic phase shift was observed in a similar quantity on components from CoP and CoC bearings. The increase of monoclinic zirconia content around metal deposits was minimal and was more pronounced in areas of stripe wear. The results of this study indicate, that ZTA components in general undergo minimal wear in both, CoC and CoP bearings, however, it is more pronounced in the former. Metal deposits, as the most common wear feature, have no significant effect on monoclinic phase transition. STATEMENT OF SIGNIFICANCE: In this paper, we classify all damage patterns macroscopically according to an established scoring system and assess them regarding surface roughness (confocal microscopy) and monoclinic phase content (Raman spectroscopy) in order to derive the severity for patients. We compare hard-hard and hard-soft bearings and relate damage patterns with metal transfer based on SEM/EDS examinations. Furthermore, we work out correlations between patient-specific data, cause of revision and the physical condition of each individual sample Our cohort consists of 43 Biolox delta retrievals, a comparatively large quantity. In addition, we address current topics such as metal transfer and, based on the classification of damage patterns, provide incentives and/or meaningful focal points for further research.

Stability of fatigued and aged ZTA compared to 3Y-TZP and Al2O3 ceramic systems

To evaluate the effect of fatigue and aging on the crystalline content and reliability of a zirconia-toughened-alumina (ZTA) composite compared to its individual counterpart materials (3Y-TZP and Al₂O₃). Thirty-six disc-shaped specimens per group were obtained to comply with ISO 6872:2015. Crystalline content, microstructure and reliability of experimental groups were evaluated in four stages: 1) immediate; 2) aged; 3) fatigued; 4) aged + fatigue. Aging was performed in autoclave and Step-Stress-Accelerated-Life-Testing (SSALT) was performed using three stress profiles. Weibull statistics were used to determine Weibull parameters and life-expectancy. A significant increase in monoclinic phase in 3Y-TZP was observed after aging (19.31%), fatigue (17.88%) and aging + fatigue (55.81%), while ZTA evidenced minimal variation among all conditions (<5.69%). 3Y-TZP presented higher reliability than ZTA at 300 and 500 MPa, and ZTA outperformed Al₂O₃ at the same stress missions. None of the ceramics yielded acceptable reliability at 800 MPa. A higher characteristic strength was observed for 3Y-TZP, followed by ZTA and Al₂O₃. While after aging ZTA and Al₂O₃ remained stable, 3Y-TZP exhibited a significant increase in the characteristic stress. Aging did not affect the reliability of ZTA and Al₂O₃. 3Y-TZP demonstrated an increase in monoclinic content and characteristic strength after aging.

In vitro & in vivo investigation of the silicon nitride ceramic hip implant’s safety and effectiveness evaluation

Background

With regard to the ceramic hip joint implant, given the concerns in ceramic about the alumina brittleness and zirconia instability, is there any alternative material solution for the orthopedic implant? Beyond the metastable oxide ceramics, along the echelon of advanced technical ceramics, looking at the non-oxide ceramic, the silicon nitride could be an excellent candidate for the joint implant’s application. The purpose of this study is to investigate the safety, effectiveness and to demonstrate the potential of this silicon nitride hip implant.

Methods

According to the related ISO (International Organization for Standardization) standards, a series of in vitro (nine) & in vivo (five) tests, which had been accomplished for the aforementioned aim. Especially, the total hip replacement in pigs had been achieved, as per the authors’ knowledge, this is the first time to apply the THA (Total Hip Arthroplasty) in the big animal.

Results

Refer to the ISO 6474-2, in comparison with the current monopolized German product, this silicon nitride ceramic hip implant has high strength, high hardness, excellent fracture toughness, lower density, better wear resistance, good biocompatibility, inherent stability, corrosion resistance and bioactivity, bone integration capability.

Conclusions

This silicon nitride ceramic will be an admirable alternative solution with superior comprehensive property that can withstand the toughest conditions in the most demanding applications like in orthopedic and beyond.