Investigations on the galvanic corrosion of multialloy total hip prostheses

Total hip prostheses combining cobalt and titanium alloys in direct physical contact are currently being used. The purpose of this paper was to investigate the possibility of an accelerated corrosion occurring due to the coupling of these alloys. Electrochemical corrosion studies based on mixed potential and protection potential theories were used to study the in vitro effects of coupling the titanium and cobalt alloys. Verification of these studies was made by direct coupling experiments. Macroscopic examination of' surface features of four retrieved total hip prostheses ranging from 0-6 years implantation were conducted. The electrochemical studies predicted coupled corrosion potentials of -0.22 V and low coupled corrosion rates of 0.02 microA/cm2. Direct coupling experiments verified these results. Cobalt-titanium interfaces on the implants were macroscopically examined, and no instances of extensive corrosion were found. Overall, the in vitro corrosion studies and the examination of retrieved prostheses predicted no exaggerated in vivo corrosion due to the coupling of these cobalt and titanium alloys.     

Observations at the articular surface of hip prostheses: an analytical electron microscopy study on wear and corrosion.

We used scanning electron microscopy in combination with X-ray microanalysis to evaluate Co-, Cr-, and Mo-based human femoral hip prostheses. In total, 23 retrieved implants and four new implants were included in this study. Scanning electron microscopy of the polished surface of all arthroplasties showed, in addition to the polishing marks, small round and angular holes or pits. Other types of surface irregularities were interpreted as wear or corrosion of the metal compound. In all cases studied, corrosion propagated from holes at the surface of the polished prosthesis heads, in some cases also along phase boundaries. X-ray microanalysis of the intact prosthetic surface showed a relative composition of the elements Co, Cr, and Mo, which was in agreement with the manufacturer's information (63:33:4%). However, X-ray microanalysis spot analysis of the surface holes showed deviation in the relative composition of the elements Co, Cr, and Mo and also the presence of Si, Ti, and Al. Furthermore, Ti and Al could be traced back at an artificially made fracture plane of a new prosthesis. Therefore, Ti and Al have to be present during the manufacturing process. These impurities in the metal prosthesis alloy may create a galvanic element with the Co, Cr, Mo alloy of the implant. If this is the case, such a galvanic element in combination with the electrolyte environment formed by body fluids, can induce galvanic corrosion with release of metal particles.     

An elastomeric material for facial prostheses: synthesis, experimental and numerical testing aspects

Current materials used for maxillofacial prostheses are far from ideal and there is a need for new improved materials which better simulate the tissues they are replacing. This study was based on a mixed experimental/analytical/numerical approach. A new polymeric material was developed to provide a better alternative to the materials currently used in maxillofacial prosthetics. A series of experimental tensile tests were performed in order to characterise the tensile properties of the material. A Mooney-Rivlin type hyperelastic formulation was chosen to describe the constitutive behaviour of the polymer which operates at the finite strain regime. The material parameters (two) of the constitutive law were identified with the experimental data. The Mooney-Rivlin material was found to be suitable to represent accurately the mechanical behaviour of the polymer up to 50% strain as shown by the excellent agreement between analytical and experimental results. An FE model reproducing all the characteristics of the experimental tensile tests was built and a series of three FE analyses were conducted and has proven the proper finite element implementation of the material model. This preliminary study will serve as a basis to introduce more complex features such as viscoelasticity and wrinkling of the soft polymeric structure in order to optimise the performances of the final prosthetic material.     

Textile arterial prostheses: is water permeability equivalent to porosity?

Porosity and water permeability are two distinct terms that describe different characteristics of vascular prostheses. The porosity is a measure of the void fraction within the prosthesis wall and is believed to give a rough prediction of the capacity of the graft to anchor newly formed surrounding tissue after implantation, whereas the water permeability indicates the rate at which water can flow through the prosthesis wall and, when measured under physiological pressure conditions, provides the surgeon with information about the need for preclotting prior to implantation. The literature has not always clearly distinguished between these two terms, and some authors in fact have suggested that they both refer to the same property of a prosthesis. In an attempt to clarify the issue, porosity and water permeability measurements were made on 34 commercial vascular prostheses having different textile constructions. Linear regression analysis demonstrated that these two characteristics are only weakly related (r = 0.59). It is therefore recommended that the current draft standards for such devices reference both properties: porosity and water permeability.     

Luminal surface fabrication for cardiovascular prostheses

Methods to produce highly ordered, specific surface morphologies on poly-tetrafluoroethylene (PTFE Teflon) surfaces were developed. These methods included the use of photolithographic techniques for pattern definition and directed argon ion beam sputter etching to produce the desired surface morphology. Use of these techniques resulted in the formation of regular arrays of sharply defined hexagonal pillars with smooth, vertical walls with heights of up to 80 microns. Pillar height-to-width ratios ranged up to 5.2-1. Surface hole depths of up to approximately 80 microns were also obtained. These surface morphologies could have an important application in medicine for improving the patency of cardiovascular prostheses. This would be accomplished by creating a luminal surface in the implant which promotes the development of a healthy neointima lining.     

Sensory feedback for powered limb prostheses

Conclusions The results of these tests confirm that a prosthesis is easier to control with sensory feedback and that the proposed technique is acceptable to patients as well as to professionals working in this field. Work in this Institute is now directed toward development of the feedback system to a form suitable for inclusion in a prosthesis. This requires miniaturisation, minimisation of power drain, and, hopefully, provision for use of common electrodes for stimulation and control.     

Neurophysiological approach to the design of visual prostheses: a theoretical discussion

This paper deals with the design of systems of sensory substitution of vision in case of blindness. Using the rules of man-machine interactions, one proposes the following optimal design for sensory substitution systems. A model of the visual system might process visual information similarly to its natural counterpart in order to deliver a signal comparable to the one reached in polysensory nervous structures. Then this artificial signal might reach these associative structures through a substitutive sensory system. To achieve this aim, one might model this substitutive system, reverse the information flow, and then trust the natural substitutive system with the stimuli generated by this inverse model. As a result, this artificial system consists of a model of vision connected to an inverse model of the substitutive sensory system. It would first process visual information up to a complex level. Then it would translate this complex signal into coded substitutive stimuli. These stimuli, after processing by the sensory substitutive system, would evoke a vision-like perception.     

Control of upper-limb prostheses: a case for neuroelectric control.

A discussion is presented on the control aspects of upper-limb prostheses, with emphasis on the areas of necessary improvements in current designs. Arguments are presented to indicate that it should be possible to obtain a substantial improvement in prostheses control by properly training the amputee, improving the dynamics response of the prostheses, and improving the quality of the forward-path control signal. Augmentation of feedback information, although useful, may not be essential. The limitations of the myoelectric (muscle) signal as a forward-path control signal, especially for multiple degrees-of-freedom prostheses, is discussed. Most of the limitations of the myoelectric signal can be overcome if the neuroelectric (nerve) signal is used as a forward-path control signal. Results of a series of experiments which demonstrate the feasibility of constructing an electrode capable of being implanted around severed nerves and of detecting neuroelectric signals for prolonged periods of time are presented. A possible scheme for employing neuroelectric control is also presented.     

Pneumocystis carinii: oxygen uptake, antioxidant enzymes, and susceptibility to oxygen-mediated damage.

The ability of Pneumocystis carinii obtained by alveolar lavage of rats with glucocorticoid-induced pneumocystosis to utilize molecular oxygen, the concentrations of selected antioxidant enzymes, and the susceptibility of P. carinii to in vitro killing by oxygen radical-generating systems have been evaluated. As expected of an organism which has been found to convert radiolabeled glucose to CO2, the parasite utilizes molecular oxygen. No evidence for pathways of oxygen utilization other than the cytochrome pathway was found; cyanide virtually abolished oxygen consumption. Although readily detectable levels of the antioxidant enzymes superoxide dismutase, catalase, and glutathione peroxidase were present in the P. carinii preparations, only superoxide dismutase was present at levels that suggested that the activity was indeed a property of the parasite. Almost certainly, P. carinii does not possess effective concentrations of catalase. In addition, it was found that P. carinii is susceptible to the lethal actions of hydrogen peroxide and superoxide, but the parasite seems to be resistant to the effects of a hydroxyl radical-generating system.     

Surface myoelectric signal classification for prostheses control

This paper represents an ongoing investigation for surface myoelectric signal segmentation and classification. The classical moving average technique augmented with principal components analysis and time-frency analysis were used for segmentation. Multiresolution wavelet analysis was adopted as an effective feature extraction technique while artificial neural networks were used for classification. Results of classifying four elbow and wrist movement signals recorded from biceps and triceps gave 5.1% classification error when two channels were used.     

Cochlear implant prostheses: Strategies and progress

Progress in development of elements of higher-generation cochlear implant devices is briefly summarized. Initial tests with early model devices indicate that these devices will gain wide application, as aids capable of effecting significant levels of speech recognition in the profoundly (totally) deaf. This project has been supported by NIH Contract NO1-NS-7-2367, NIH Grant NS-11804, Hearing Research, Inc., and The Coleman Fund.     

Materials for internal prostheses: the present position and possible future developments

In this paper we consider solid phase forming as a method of manufacturing acetabular cups. Static and cyclic creep tests have been carried out on Charnley cups and indicate that creep, rather than wear, plays the major role in the penetration of the socket by the femoral ball, at least in the first few years. We have investigated a new type of cross-linked polyethylene and found it to be extremely good from both a wear and creep viewpoint, particularly under high loads and at high sliding velocities.     

Biocompatibility of Delrin 150: a creep-resistant polymer for total joint prostheses

A thermoplastic polymer, Delrin 150 (polyoxymethylene homopolymer), with creep resistance ten times that of ultrahigh-molecular-weight polyethylene, is used as a material for total joint prostheses. A study was made of the local and systemic host response to this polymer when implanted in three different mammalian species. 316 LC stainless steel was used as a control. The materials were implanted into muscle and bone as solid cylinders. A total of 446 samples were implanted into 74 animals. The duration of implantation ranged from 2 weeks to 2 years. A semi-quantitative evaluation of local tissue reaction was performed. For each implant, 16 histological criteria were graded for severity of host tissue reaction. The liver, spleen, kidneys, pancreas, and lungs from each animal were also studied for evidence of systemic toxicity. The polymer implants exhibited a mild tissue reaction with the same characteristics as the control. Local tumor formation, bone osteolysis, and surrounding muscle necrosis were not seen. No pathological changes compatible with systemic toxicity by Delrin 150 were observed in the study of the organs. Delrin 150 in solid form did not exhibit local or systemic toxicity and is therefore biocompatible by this study. Powder implantation studies should be performed to simulate tissue response to wear particles.     

Pathway to carcinogenesis: the role of bacteria

A bacterium is involved in the etiology and physiopathology of the malignant cell. It is not any bacterium, but one with specific characteristics suggestive of a plant-type metabolic and respiratory physiology. Utilizing mice growing sarcomatous lesions as an experimental model, we have been able to recover, in 100% of culture procedures, the Staphylococcus aureus coagulase positive micro-organism, provided that a modified media, as described in a later paragraph, is utilized to sustain these microbes while in transfer from in vivo to in in vitro measures. In human cancer tissue studied, the Ascomycete fungus as well as the Staphylococcus could be isolated (1). Bacteria, fungi, or viruses, isolated by the culturing of cancerous tissue or by their recognition through microscopic study, in itself do not constitute satisfactory reasons to conclude that such microbes are the causative factors in cancer etiology. There must be a definite pathway of bacterial activity to justify such a conclusion. Our experimental studies have defined such a pathway. Of extreme importance is the fact that the Staphylococcus, as a separate entity, does not and cannot produce a malignant cell. Our studies indicate that the malignant cell can only occur in a warm-blooded living animal or human, and it is dependent upon three factors. The bacterium as the first factor must possess characteristics of a plant micro-organism. Specifically, they must have a reducing factor capable of creating an oxygen free millieu in an enclosed area, must be able to convert during duress to a unicellular micro-coccus, and must remain viable within a sac or cell despite the absence of their outside cell wall.(ABSTRACT TRUNCATED AT 250 WORDS)