Bio-Medical Materials and Engineering

A simple analytical model for damage evolution of bone fatigue is presented. A probabilistic method for characterizing the damage accumulation in terms of microcracks for bone fatigue was developed. The crack numerical density distributions were obtained from the Monte Carlo simulations with a Weibull distribution fit in this study. The results predicted from the present model are compared with existing experimental data and discussed. The quantitative relationship between stiffness loss, loading cycles and microdamage parameter developed in this study may be useful for fatigue life and failure stress predictions.

This paper describes a method of parametric optimisation to determine the optimal stiffness characteristics of cement, metal backing and UHMWPE (Ultra High Molecular Weight Polyethylene) materials, which minimises the probability of fatigue fracture of cement at all interfaces with the metal backing and the bone, while limiting the amount of bone resorbed. The parameters describing the elastic moduli of cement, metal backing and UHMWPE were considered as design variables. The method was applied to an axisymmetric finite element model of acetabular cup in combination with an optimisation procedure using the ANSYS program.

Young's moduli of about 0.63, 207 and 0.72 GPa are optimal materials for cement, metal backing (MB) and UHMWPE, respectively. These characteristics decreased fatigue notch factor K_f in cement by 8.2 and 10.6% and also decreased the maximum von Mises stress in cement by 21 and 27% at cement/bone and cement/metal backing interfaces, respectively. The optimal design reduces the probability of fatigue fracture of cement at all interfaces with the bone and the metal backing while limiting the amount of bone resorbed as a result of increasing von Mises stress and K_f in the central bone of the acetabulum by 34 and 30.6%, respectively.

Biodegradable microspheres were prepared with poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV, 85:15 by mole ratio of hydroxybutyrate to hydroxyvalerate) by an water‐in‐oil‐in‐water (W/O/W), oil‐in‐water (O/W) and oil‐in‐oil (O/O) solvent evaporation method for the sustained release of anti‐cancer drug, 5‐fluorouracil (5‐FU) with controlling the fabrication conditions. The shape of microspheres prepared was relatively rough due to highly crystalline property of PHBV and spherical. The efficiency of 5‐FU loading into the PHBV microsphere with O/O method was over 80% compared to that 7% for microspheres by O/W method and below 1% for microspheres by a conventional W/O/W method. However, the most desirable release pattern can be achieved from the O/W method due to the cosolvent effect. The effects of preparation conditions such as the type and amount of surfactant, initial amount of loaded drug, the temperature of solvent evaporation, and etc. on the morphology for W/O/W method were investigated. Possible mechanisms of the desirable sustained release pattern for O/W system have been proposed.

A multidirectional pin‐on‐plate reciprocating machine was used to compare the wear performance of UHMWPE sliding against cast cobalt chrome (CoCr) plates that were either untreated or coated with Amorphous Diamond Like Carbon (ADLC). The test conditions were based on a 1/5 scale model representative of in vivo motion at the tibial counterfaces of unconstrained mobile bearing knees [1]. The average ± STERR wear rates were 13.78 ± 1.06 mm³/Mcycles for the ADLC counterfaces and 0.504 ± 0.12 mm³/Mcycles for the control CoCr counterfaces. All of the pins run on the ADLC counterfaces exhibited the same patterns of blistering along the central axis, and severe abrasion elsewhere to the extent that all of the original machining marks were removed after just one week of testing. The average value of friction coefficient was 0.24 for the ADLC counterfaces and 0.073 for the control CoCr counterfaces. The factor of 3.5 increase was statistically significant at p<0.05. In the tribological evaluation of ADLC coatings for tibial trays in mobile bearing knees, this study shows that this specific Physical Vapour Deposition (PVD) ADLC showed significantly poorer frictional and wear performance than uncoated surfaces which was sufficient to negate any potential benefits of improved resistance to third body damage.

During its development, titanium was found to be incompatible with conventional dental porcelains due to weak bond strength brought about by titanium's high yet oxidative nature. In spite of the development of new low‐fusing porcelains designed for titanium application, previous studies have shown that sandblasting pre‐treatment prior to porcelain application led to weakening of the metal–ceramic bonding. The aim of this study is to search for an effective alternative to sandblasting for the surface treatment of the titanium substrate in the titanium–porcelain system. The research evaluated the bond strength of 165 samples of titanium–porcelain systems divided into 11 groups. A three‐point flexural bend test was conducted to measure the force required to fracture the porcelain on the titanium substrate. A correlation between the type of surface treatment and the bond strengths of each group was evaluated if it resulted to significant differences.

The study found significantly differences in the energy‐to‐break of titanium–porcelain systems treated with hydrochloric acid and sandblasting compared with the control group. The bonds strength achieved by the titanium–porcelain system when treated with hydrochloric acid is comparable to that of conventional metal–ceramic alloy system. Hydrochloric acid treatment of the titanium substrate is a promising alternative to sandblasting for the surface treatment of the titanium substrate in the titanium–porcelain system.

The titanium–porcelain system can be considered as a double‐layered structure, comprising of at least titanium substrate and porcelain body including bonding agent. Stress distribution pattern of such a double‐layered structure is not necessarily same as that of a single beam under the 3‐point bending testing mode. Previously tested porcelain‐fired commercially pure titanium samples (n=285) were re‐evaluated. All obtained data on bond strengths and bond toughness (energy‐to‐break) were re‐grouped in terms of c/t ratio, where c is the distance from the maximum compressive side of the beam to the calculated neutral axis and t is the thickness of titanium substrate plate thickness. It was found that (1) when the double‐layered structure falls in the c<t situation, both bond strength and bond toughness are in a lower zone, which is slightly lower than the cohesive tensile strength of the porcelain, (2) when the c>t situation (with relatively thick porcelain application) is established, both bond strength and toughness are in higher zone than the porcelain itself, and (3) there is a transition zone between the above.

Osteolysis and loosening of artificial joints caused by UHMWPE wear debris has prompted renewed interest in metal‐on‐metal (MOM) hip prostheses. This study investigated the wear and wear debris morphology generated by MOM prostheses in a physiological anatomical hip simulator for different carbon content cobalt chrome alloys.

The low carbon pairings demonstrated significantly higher “bedding in” and steady state wear rates than the mixed and high carbon pairings. The in vitro wear rates reported here were up to one or two orders of magnitude lower than the clinical wear rates for first‐generation MOM hip prostheses. Two methods for characterising the metal wear debris were developed, involving digestion, scanning electron microscopy and transmission electron microscopy. The metal wear particles characterised by the two methods were similar in size, 25–36 nm, and comparable to particles isolated from periprosthetic tissues from first and second‐generation MOM hip prostheses. Due to the small size of the metal particles, the number of particles generated per year for MOM prostheses in vitro was estimated to be up to 100 times higher than the number of polyethylene particles generated per year in vivo.

The volumetric wear rates were affected by the carbon content of the cobalt chrome alloy and the material combinations used. However, particle size and morphology was not affected by method of particle characterisation, the carbon content of the alloy or material combination.