Development of Novel Antibacterial Ti-Nb-Ga Alloys with Low Stiffness for Medical Implant Applications.

With the rising demand for medical implants and the dominance of implant-associated failures including infections, extensive research has been prompted into the development of novel biomaterials that can offer desirable characteristics. This study develops and evaluates new titanium-based alloys containing gallium additions with the aim of offering beneficial antibacterial properties while having a reduced stiffness level to minimise the effect of stress shielding when in contact with bone. The focus is on the microstructure, mechanical properties, antimicrobial activity, and cytocompatibility to inform the suitability of the designed alloys as biometals. Novel Ti-33Nb-xGa alloys (x = 3, 5 wt%) were produced via casting followed by homogenisation treatment, where all results were compared to the currently employed alloy Ti-6Al-4V. Optical microscopy, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) results depicted a single beta (ß) phase microstructure in both Ga-containing alloys, where Ti-33Nb-5Ga was also dominated by dendritic alpha (α) phase grains in a ß-phase matrix. EDS analysis indicated that the α-phase dendrites in Ti-33Nb-5Ga were enriched with titanium, while the ß-phase was richer in niobium and gallium elements. Mechanical properties were measured using nanoindentation and microhardness methods, where the Young's modulus for Ti-33Nb-3Ga and Ti-33Nb-5Ga was found to be 75.4 ± 2.4 and 67.2 ± 1.6 GPa, respectively, a significant reduction of 37% and 44% with respect to Ti-6Al-4V. This reduction helps address the disproportionate Young's modulus between titanium implant components and cortical bone. Importantly, both alloys successfully achieved superior antimicrobial properties against Gram-negative P. aeruginosa and Gram-positive S. aureus bacteria. Antibacterial efficacy was noted at up to 90 ± 5% for the 3 wt% alloy and 95 ± 3% for the 5 wt% alloy. These findings signify a substantial enhancement of the antimicrobial performance when compared to Ti-6Al-4V which exhibited very small rates (up to 6.3 ± 1.5%). No cytotoxicity was observed in hGF cell lines over 24 h. Cell morphology and cytoskeleton distribution appeared to depict typical morphology with a prominent nucleus, elongated fibroblastic spindle-shaped morphology, and F-actin filamentous stress fibres in a well-defined structure of parallel bundles along the cellular axis. The developed alloys in this work have shown very promising results and are suggested to be further examined towards the use of orthopaedic implant components.

Corroboration of coupled musculoskeletal model and finite element predictions with in vivo RSA migration of an uncemented acetabular component.

While finite element (FE) models have been used extensively in orthopedic studies, validation of their outcome metrics has been limited to comparison against ex vivo testing. The aim of this study was to validate FE model predictions of the initial cup mechanical environment against patient-matched in vivo measurements of acetabular cup migration using radiostereometric analysis (RSA). Tailored musculoskeletal and FE models were developed using a combination of three-dimensional (3D) motion capture data and clinical computerized tomography (CT) scans for a cohort of eight individuals who underwent primary total hip replacement and were prospectively enrolled in an RSA study. FE models were developed to calculate the mean modulus of cancellous bone, composite peak micromotion (CPM), composite peak strain (CPS) and percentage area of bone ingrowth. The RSA cup migration at 3 months was used to corroborate the FE output metrics. Qualitatively, all FE-predicted metrics followed a similar rank order as the in vivo RSA 3D migration data. The two cases with the lowest predicted CPM (<20 µm), lowest CPS (<0.0041), and high bone modulus (>917 MPa) were confirmed to have the lowest in vivo RSA 3D migration (<0.14 mm). The two cases with the largest predicted CPM (>80 µm), larger CPS (>0.0119) and lowest bone modulus (<472 MPa) were confirmed to have the largest in vivo RSA 3D migration (>0.78 mm). This study enabled the first corroboration between tailored musculoskeletal and FE model predictions with in vivo RSA cup migration. Investigation of additional patient-matched CT, gait, and RSA examinations may allow further development and validation of FE models.

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.

An Overview of the Stability and Fretting Corrosion of Microgrooved Necks in the Taper Junction of Hip Implants.

Fretting corrosion at the head-neck interface of modular hip implants, scientifically termed trunnionosis/taperosis, may cause regional inflammation, metallosis, and adverse local tissue reactions. The severity of such a deleterious process depends on various design parameters. In this review, the influence of surface topography (in some cases, called microgrooves/ridges) on the overall performance of the microgrooved head-neck junctions is investigated. The methodologies together with the assumptions and simplifications, as well as the findings from both the experimental observations (retrieval and in vitro) and the numerical approaches used in previous studies, are presented and discussed. The performance of the microgrooved junctions is compared to those with a smooth surface finish in two main categories: stability and integrity; wear, corrosion, and material loss. Existing contradictions and disagreements among the reported results are reported and discussed in order to present a comprehensive picture of the microgrooved junctions. The current research needs and possible future research directions on the microgrooved junctions are also identified and presented.

The Role of the Assembly Force in the Tribocorrosion Behaviour of Hip Implant Head-Neck Junctions: An Adaptive Finite Element Approach.

The cyclic loading, in the corrosive medium of the human body, results in tribocorrosion at the interface of the head-neck taper junction of hip implants. The resulting metal ions and wear debris adversely affect the local tissues. The force applied by surgeons to assemble the junction has proven to play a major role in the mechanics of the taper junction which, in turn, can influence the tribocorrosion damage. Recently, finite element method has been used to predict the material loss at the head-neck interface. However, in most finite element studies, the contribution of electrochemical corrosion has been ignored. Therefore, a detailed study to investigate the influence of the assembly force on the tribocorrosive behaviour of the head-neck junction, which considers both the mechanical and chemical material removal, is of paramount interest. In this study, a finite-element-based algorithm was used to investigate the effect of assembly force on the tribocorrosion damage at the junction interface, for over four million cycles of simulated level gait. The patterns of the material removal in the modelling results were compared with the damage patterns observed in a group of retrieved modular hip implants. The results of this study showed that for different cases, chemical wear was in the range of 25-50% of the total material loss, after four million cycles. A minimum assembly force (4 kN for the studied cases) was needed to maintain the interlock in the junction. The computational model was able to predict the damage pattern at the retrieved head-neck interface.

The mechanics of head-neck taper junctions: What do we know from finite element analysis?

Modular hip implants are widely used in hip arthroplasty because of the advantages they can offer such as flexibility in material combinations and geometrical adjustments. The mechanical environment of the modular junction in the body is quite challenging due to the complex and varying off-axial mechanical loads of physical activities applied to a tapered interface of two contacting materials (head and neck) assembled by an impact force intraoperatively. Experimental analogies to the in-vivo condition of the taper junction are complex, expensive and time-consuming to implement; hence, computational simulations have been a preferred approach taken by researchers for studying the mechanics of these modular junctions that can help us understand their failure mechanisms and improve their design and longevity after implantation. This paper provides a clearer insight into the mechanics of the head-neck taper junction through a careful review on the finite element studies of the junction and their findings. The effects of various factors on the mechanical outputs namely: stresses, micromotions, and contact situations are reviewed and discussed. Also, the simulation methodology of the studies in the literature is compared. Research opportunities for future are scrutinised through tabulating data and information that have been carefully retrieved form the reported findings.

A review on the finite element simulation of fretting wear and corrosion in the taper junction of hip replacement implants.

Taperosis/trunnionosis is a scientific term for describing tribocorrosion (fretting corrosion) at the head-neck taper junction of hip implants where two contacting surfaces are undergone oscillatory micromotions while being exposed to the body fluid. Detached ions and emitted debris, as a result of taperosis, migrate to the surrounding tissues and can cause inflammation, infection, and aseptic loosening with an ultimate possibility of implant failure. Improving the tribocorrosion performance of the head-neck junction in the light of minimising the surface damage and debris requires a better understanding of taperosis. Given its complexity associated with both the mechanical and electrochemical aspects, computational methods such as the finite element method have been recently employed for analysing fretting wear and corrosion in the taper junction. To date, there have been more efforts on the fretting wear simulation when compared with corrosion. This is because of the mechanical nature of fretting wear which is probably more straightforward for modelling. However, as a recent research advancement, corrosion has been a focus to be implemented in the finite element modelling of taper junctions. This paper aims to review finite element studies related to taperosis in the head-neck junction to provide a detailed understanding of the design parameters and their role in this failure mechanism. It also reviews and discusses the methodologies developed for simulating this complex process in the taper junction along with the simplifications, assumptions and findings reported in these studies. The current needs and future research opportunities and directions in this field are then identified and presented.

A finite element study on the mechanical response of the head-neck interface of hip implants under realistic forces and moments of daily activities: Part 2.

A finite element model was developed to investigate the effect of loading regimes caused by various daily activities on the mechanical behaviour of the head-neck taper junction in modular hip replacements. The activities included stair up, stair down, sit to stand, stand to sit, one leg standing and knee bending. To present the real mechanical environment of the junction, in addition to the force components, the frictional moments produced by the frictional sliding of the head and cup were applied to a CoCr/CoCr junction having a 12/14 taper with a proximal mismatch angle of 0.024°. This study revealed that stair up with the highest fretting work per unit of length (1.62 × 104J/m) was the most critical activity, while knee bending and stand to sit with 1.96 × 103J/m were the least critical activities. For all the activities, the superolateral region of the neck was identified as the most critical region in terms of having larger values of fretting work per unit of area. This study showed also that the relative micro-motions and contact stresses occurring at the head-neck interface for all the studied activities are mostly in the range of 0-38µm and 0-350MPa, respectively. These ranges may be accordingly employed for conducting relevant in-vitro tests to more realistically represent the mechanical environment of taper junctions with the same materials and geometry studied in this work.

A finite element study on the mechanical response of the head-neck interface of hip implants under realistic forces and moments of daily activities: Part 1, level walking.

This paper investigates the mechanical response of a modular head-neck interface of hip joint implants under realistic loads of level walking. The realistic loads of the walking activity consist of three dimensional gait forces and the associated frictional moments. These forces and moments were extracted for a 32mm metal-on-metal bearing couple. A previously reported geometry of a modular CoCr/CoCr head-neck interface with a proximal contact was used for this investigation. An explicit finite element analysis was performed to investigate the interface mechanical responses. To study the level of contribution and also the effect of superposition of the load components, three different scenarios of loading were studied: gait forces only, frictional moments only, and combined gait forces and frictional moments. Stress field, micro-motions, shear stresses and fretting work at the contacting nodes of the interface were analysed. Gait forces only were found to significantly influence the mechanical environment of the head-neck interface by temporarily extending the contacting area (8.43% of initially non-contacting surface nodes temporarily came into contact), and therefore changing the stress field and resultant micro-motions during the gait cycle. The frictional moments only did not cause considerable changes in the mechanical response of the interface (only 0.27% of the non-contacting surface nodes temporarily came into contact). However, when superposed with the gait forces, the mechanical response of the interface, particularly micro-motions and fretting work, changed compared to the forces only case. The normal contact stresses and micro-motions obtained from this realistic load-controlled study were typically in the range of 0-275MPa and 0-38µm, respectively. These ranges were found comparable to previous experimental displacement-controlled pin/cylinder-on-disk fretting corrosion studies.

An adaptive finite element simulation of fretting wear damage at the head-neck taper junction of total hip replacement: The role of taper angle mismatch.

An adaptive finite element simulation was developed to predict fretting wear in a head-neck taper junction of hip joint implant through a two dimensional (2D) model and based on the Archard wear equation. This model represents the most critical section of the head-neck junction which was identified from a 3D model of the junction subjected to one cycle of level gait loading. The 2D model was then used to investigate the effect of angular mismatch between the head and neck components on the material loss and fretting wear process over 4 million gait cycles of walking. Generally, junctions with distal angular mismatches showed a better resistance to fretting wear. The largest area loss in the neck after 4 million cycles of loading was 1.86E-02mm2 which was found in the junction with a proximal mismatch angle of 0.124°. While, the minimum lost area (4.30E-03mm2) was found in the junction with a distal angular mismatch of 0.024°. Contact stress, amplitude of sliding and contact length were found as the key parameters that can influence the amount of material loss and the process of fretting wear damage. These parameters vary over the fretting wear cycles and are highly dependent on the type and magnitude of the taper angle mismatch. This study also showed that lost area does not have a linear relationship with the mismatch angle of taper junctions.

Influence of geometry and materials on the axial and torsional strength of the head-neck taper junction in modular hip replacements: A finite element study.

The assembly force is important in establishing the mechanical environment at the head-neck taper junction of modular hip replacements. Previous experimental results of the assembled taper junctions with different material combinations (Co-28Cr-6Mo and Ti-6Al-4V) reported similar axial strengths (pull-off loads), but lower torsional strengths (twist-off moments) for the CoCr/CoCr junction. However, mechanics of the junction and the strength behaviour have not been understood yet. A three dimensional finite element model of an isolated femoral head-neck junction was developed to explore the assembly and disassembly procedures, particularly the axial and torsional strengths for different material combinations and geometries. Under the same assembly load, the contacting length between the CoCr head and titanium neck was greater than that of in CoCr/CoCr. The contact length in the titanium neck was more sensitive to the assembly force when compared to the CoCr neck. For instance, with increasing the assembly force from 1890 to 3700N, the contact length increased by 88% for CoCr/Ti and 59% for CoCr/CoCr junctions. The torsional strength of the junction was related to the lateral deformation of the neck material due to the applied moment. The angular mismatch existing between the head and neck components was found to play the main role in the torsional strength of the junction. The smaller mismatch angle the higher torsional strength. It is suggested to consider reducing the mismatch angle, particularly in CoCr/CoCr junctions, and ensure a sufficiently high assembly force is applied by impaction for this combination.

An Investigation on the Wear Resistance and Fatigue Behaviour of Ti-6Al-4V Notched Members Coated with Hydroxyapatite Coatings.

In this study, surface properties of Ti-6Al-4V alloy coated with hydroxyapatite coatings were investigated. Wear resistance and fatigue behaviour of samples with coating thicknesses of 10 and 50 µm as well as uncoated samples were examined. Wear experiments demonstrated that the friction factor of the uncoated titanium decreased from 0.31 to 0.06, through a fluctuating trend, after 50 cycles of wear tests. However, the friction factor of both the coated samples (10 and 50 µm) gradually decreased from 0.20 to 0.12 after 50 cycles. At the end of the 50th cycle, the penetration depth of the 10 and 50 µm coated samples were 7.69 and 6.06 µm, respectively. Fatigue tests showed that hydroxyapatite coatings could improve fatigue life of a notched Ti-6Al-4V member in both low and high cycle fatigue zones. It was understood, from fractography of the fracture surfaces, that the fatigue zone of the uncoated specimens was generally smaller in comparison with that of the coated specimens. No significant difference was observed between the fatigue life of coated specimens with 10 and 50 µm thicknesses.