Application of an interface failure model to predict fatigue crack growth in an implanted metallic femoral stem.

A novel computational modelling technique has been developed for the prediction of crack growth in load bearing orthopaedic alloys subjected to fatigue loading. Elastic-plastic fracture mechanics has been used to define a three-dimensional fracture model, which explicitly models the opening, sliding and tearing process. This model consists of 3D nonlinear spring elements implemented in conjunction with a brittle material failure function, which is defined by the fracture energy for each nonlinear spring element. Thus, the fracture energy criterion is implicit in the brittle material failure function to search for crack initiation and crack development automatically. A degradation function is employed to reduce interfacial fracture properties corresponding to the number of cycles; thus fatigue lifetime can be predicted. Unlike other failure modelling methods, this model predicts the failure load, crack path and residual stiffness directly without assuming any pre-flaw condition. As an example, fatigue of a cobalt based alloy (CoCrMo) femoral stem is simulated. Experimental fatigue data was obtained from four point bending tests. The finite element model simulated a fully embedded implant with a constant point load. Comparison between the model and mechanical test results showed good agreement in fatigue crack growth rate.

Simulation of a knee joint replacement during a gait cycle using explicit finite element analysis.

The stress distribution within the polyethylene insert of a total knee joint replacement is dependent on the kinematics, which in turn are dependent on the design of the articulating surfaces, the relative position of the components and the tension of the surrounding soft tissues. Implicit finite element analysis techniques have been used previously to examine the polyethylene stresses. However, these have essentially been static analyses and hence ignored the influence of the kinematics. The aim of this work was to use an explicit finite element approach to simulate both the kinematics and the internal stresses within a single analysis. A simulation of a total knee joint replacement subjected to a single gait cycle within a knee wear simulator was performed and the results were compared with experimental data.The predicted kinematics were in close agreement with the experimental data. Various solution-dependent parameters were found to have little influence on the predicted kinematics. The predicted stresses were found to be dependent on the mesh density. This study has shown that an explicit finite element approach is capable of predicting the kinematics and the stresses within a single analysis at relatively low computational cost.

A computational model for the prediction of total knee replacement kinematics in the sagittal plane.

A computational model has been developed using a current generation computer-aided engineering (CAE) package to predict total knee replacement (TKR) kinematic in the sagittal plane. The model includes friction and soft tissue restraint varying according to the flexion angle. The model was validated by comparing the outcomes of anterior-posterior (A-P) laxity tests of two contemporary knee replacements against data obtained from a knee simulating machine. It was also validated against predictions from a computer model reported in the literature. Results show good agreement in terms of A-P displacements. Further tests were performed to determined the influence of the soft tissue restraints varying with flexion angle. This work represents the first attempt to use a sophisticated commercial CAE package to predict TKR motions and the advantages of the modelling procedure chosen are discussed.

Effect of mechanical surface pretreatment on metal ion release.

The degree of metal ion dissolution from Ti-6Al-4V alloy hip replacement stems subjected to various mechanical and chemical surface pretreatments was analysed in vitro. High-dissolution rates were observed for nitric acid passivated samples that had been mechanically surface treated to increase the implant surface area. Significantly lower ion release levels were observed for mechanically treated samples which had been aged in de-ionised water. The application of an hydroxyapatite coating decreased the metal ion release from the nitric acid passivated samples (compared to the uncoated sample) and increased the metal ion dissolution from the aged samples. The dissolution behaviour of the samples is explained in terms of the diffusion processes occurring at the stem/solution interface and the morphological and chemical characteristics of the surface treated stems.

Reliability theory for load bearing biomedical implants.

At present, load-bearing implants are designed on a deterministic basis in which the structural strength and applied loading are given fixed values, and global safety factors are applied to (i) cover any uncertainties in these quantities, and (ii) to design against failure of the component. This approach will become increasingly inappropriate as younger and more active patient demands become more exacting and as devices become more complex. The present work describes a preliminary investigation in which a scientific and probabilistic technique is applied to assess the structural integrity of the knee tibial tray. It is envisaged that by applying such a technique to other load bearing biomedical devices, reliability theory may aid in future lifing procedures and materials/design optimisation.

In vivo biocompatibility and mechanical study of novel bone-bioactive materials for prosthetic implantation.

Two epoxy materials with or without adhesively bonded hydroxyapatite (HA) coatings were studied for their biocompatibility and mechanical pushout strength using in vivo implantation in the rabbit lower femur for a duration of 10 days to 6 months. Both were two-part epoxies cured at room temperature for 24 h, with material 1 (Ampreg 26; SP Systems Limited, Cowes, UK) postcured at 110 degrees C (Tg approximately 80 degrees C) and Material 2 (CG5052; Ciba Geigy Limited, Cambridge, UK) at 125 degrees C (Tg approximately 120 degrees C). Implantation in dead rabbit bone was performed to provide mechanical baseline levels. Polymethylmethacrylate (PMMA) and conventionally HA-coated titanium alloy (Ti-6Al-4V) were used as control materials. In the biological study, different fluorescent dyes were used to label newly formed bone. After 6 weeks of implantation, results from mechanical pushout tests showed that the interfacial shear strength (ISS) values were significantly higher than for dead bones with each of the different implants (p < .01-.001). HA-coated material 2 showed a significantly higher ISS value than the uncoated material (p < .05) after 6 weeks' implantation. However, the ISS value for the uncoated material 2 was significantly higher than for PMMA controls (p < .05). No significant differences in the ISS values were shown between HA-coated materials 1 and 2 and Ti-6Al-4V on in vivo implantation for 6 weeks. Failure points of the pushout test from the three HA-coated materials were defined by scanning electron microscopy. Specimens implanted with both HA-coated epoxies were fractured within the HA-coatings or the bone, while with HA-coated Ti-6Al-4V cracked between the coating and metal implant. The percentage of bone in contact with the implant surface was obtained by image analysis which showed that there were no significant differences between different materials after short time implantation (up to 6 week). Long-term implantation of the HA-coated material 2 showed that the percentage of bone contact had increased from 52.8+/-1.1% (6 week) to 80.0+/-0.3% (3 months) (p < .01) and remained at 81.0+/-0.8% (6 months). Measurements of bone mineralization rate (BMR) showed that after 3 weeks of implantation, there were no significant differences between PMMA and uncoated materials 1 and 2. After 6 weeks, the BMRs in animals implanted with either HA-coated material 1 or 2 were significantly higher than with HA-coated Ti-6Al-4V (p < .05-.0001 in both cases), but with HA-coated material 2 was lower than with this material uncoated (p < .05-.001). No significant differences were found between the two HA-coated epoxy materials. In addition, there were always lower BMRs during the third week of implantation than other periods regardless of biomaterial implanted. The study indicated that the adhesively bonded HA-coated novel epoxy materials were superior to conventional plasma-sprayed Ti-6Al-4V implants with respect to both BMR and bone integration with the implant surfaces. Adhesively bonded HA-coated epoxy materials had similar ISS values to HA-coated Ti-6Al-4V, but the former failed within the bone and coating, while the latter showed splitting between coating and metal.

Analysis of push-out test data based on interfacial fracture energy.

Push-out testing is frequently used to assess the interfacial shear strength developed at a bone-biomaterial interface during in vivo experiments. The aim of the present research was to assess the in vivo performance of a novel substrate/coating combination and to introduce a more rigorous fracture mechanics analysis of the push-out test data. An adhesively bonded hydroxyapatite (HA), and a Ti-6Al-4V alloy plasma sprayed with HA, were implanted in female New Zealand white rabbits for up to 6 months in duration. After death, push-out tests were carried out and the shear strength was calculated in the conventional way, together with microscopical examination of crack paths. A finite element model was drawn up representing four potential failure mechanisms. The measured "failure shear strengths" in conventional analysis were approximately equal for the two coatings. However, JC at failure calculated from the model was 210 J m(-2) at the novel adhesively bonded HA/bone interface and 5 J m(-2) at a conventional titanium/plasma-sprayed HA interface. The conventional shear strength approach is strongly test dependent, and we believe that the fracture energy approach represents a more rigorous analysis of the real failure criterion in the implant/host tissue structure.

Composite technology in load-bearing orthopaedic implants.

Composite materials have been widely promoted as possible orthopaedic biomaterials but to date have found few successful commercial applications, due to the many challenging problems presented by their design, fabrication and testing. The range of possible composite biomaterials is reviewed, together with the possible methods of fabrication and the limitations that these place on the design of composite components. The use of composite materials allows many new design possibilities, but this freedom of design requires a clearer understanding of the objectives and constraints on the design process. The testing of composite components also presents many challenging problems, which are not adequately addressed by existing standards developed for testing conventional monolithic materials. The interaction of composite materials with the body is more complex than that of the component materials, and the prediction of their long-term mechanical performance also presents many intractable difficulties. However, despite these challenges composite materials are likely to prove invaluable in the future development of orthopaedics.

Development of fatigue lifetime predictive test methods for hip implants: part I. Test methodology.

The fatigue failure of hip joint prostheses will be expected to assume more importance in second generation implants aimed at younger, more active patients. Furthermore, new designs and material combinations including coatings (e.g. hydroxyapatite) may introduce fatigue problems that as yet have not been considered. The current research makes an initial attempt to develop accelerated fatigue testing procedures to enhance the methodology of hip implant lifetime prediction. Tests conducted on a 'model' four point bendbar testpiece (mill-annealed Ti-6A1-4V) highlighted that the accelerated test must be conducted in a physiological solution such as Ringer's at 37 degrees C. The introduction of superimposed block overloads (50 cycles) to signify stair ascent/descent or fast walking and single overloads to signify sit/stand movements or stumbling were found to reduce fatigue life by > 50%. The findings of this fatigue study were combined with biomechanics studies to construct a variable amplitude 'in-service' load spectrum for testing hip implants. Using ambulatory trial data, a simple load sequence was designed containing 4 single (sit/stand movements) and 3 block (stair ascent/descent) overloads that repeated ten times gave one days activity; single overloads repeated every 110 base cycles (normal walking) and block overloads 80, 110 (morning/evening) and 250 (daytime) base cycles.

Surface modification of titanium alloy implants.

Hip replacement stems manufactured from Ti6Al4V titanium alloy were surface treated in one of four ways and tested for dissolution resistance in bovine serum. Those stems treated thermally were found to have significantly lower metal ion release compared with those receiving standard commercial treatments. The improved dissolution behaviour is associated with a change in the surface oxide structure from mixed titanium oxides to a more stable rutile structure.

Wear induced by motion between bone and titanium or cobalt-chrome alloys.

We studied the wear generated by motion between polished and shot-blasted titanium-alloy (Ti-6Al-4V) or cobalt-chrome alloy (Co-Cr) surfaces and cortical bone in vitro. Semicircular sections of human proximal femoral cortex were reamed to fit metal cylinders of each alloy. The cylinders were then fitted in the bone, loaded and rotated in physiological saline. Ti-alloy resulted in more wear both of the bone and of the metal than did Co-Cr alloy. Metal wear was reduced and bone wear was increased by shot-blasting, a procedure which introduces surface residual stresses and roughens the metal surface. We conclude that when there is gross motion between a metal implant and bone, Ti-alloy is likely to generate more wear debris than Co-Cr alloy. The least wear both of bone and of metal was produced by polished Co-Cr.

Effect of surface treatment on the dissolution of titanium-based implant materials.

Titanium and its alloys are widely used in load-bearing implants as a result of their excellent mechanical properties and corrosion resistance, but there is concern over the release of metal ions from the prosthesis. Our research investigated the influence of the surface oxide on the dissolution of the substrate material in saline solution, using a combination of atomic absorption spectroscopy, ellipsometry and transmission electron microscopy techniques. It is demonstrated that a substantial reduction in the release of metal ions may be achieved by ageing the surface oxide in boiling distilled water or by thermal oxidation; this is discussed in terms of the structure of the oxide film.

Application of PVD TiN coating to Co-Cr-Mo based surgical implants.

The requirements for successful joint arthroplasty are particularly exacting; a balanced combination of mechanical properties together with good biocompatibility are essential. Co-Cr based alloys have been used for many years on account of their relative inertness, good load bearing properties and excellent wear resistance. There is, however, concern that a slow accumulation of metal ions such as cobalt and chromium can lead to adverse clinical reactions; modern cementless fixation techniques may exacerbate this problem. In an attempt to reduce the release of potentially harmful metal ions from Co-Cr-Mo based surgical implants, a thin coating of TiN has been applied via Physical Vapour Deposition (PVD). In vitro corrosion performance has been investigated using electrochemical techniques, and also by atomic absorption analysis. The release of cobalt and chromium ions is shown to be reduced by the presence of the TiN coating, and these results are discussed in terms of the electrochemistry and microstructure of the coating and substrate.