Biomedical subjects

A Unsworth

Publications and source records attributed to A Unsworth.

At least 19 recordsLinked to original sources

PubMed2006

Polyurethane unicondylar knee prostheses: simulator wear tests and lubrication studies.

Many materials are used as artificial joint bearing surfaces; these include conventional stainless steel or CoCrMo-on-ultra-high molecular weight polyethylene (UHMWPE), CoCrMo on itself and alumina-on-alumina. However, these joints have a limited lifespan resulting in failure of the prosthesis and the need for revision surgery. A number of materials have been introduced recently in an attempt to overcome these problems. Polycarbonate urethane (PU) is a compliant material that can be used as an artificial joint bearing surface which has been developed to mimic the natural synovial joint more accurately by promoting fluid film lubrication. Tribological tests were performed on CoCrMo-on-PU unicondylar knee prostheses to assess their performance in vitro. The wear produced by these components was considerably lower than that found for conventional joints. They also exhibited low friction and operated close to full-fluid film lubrication with viscosities of lubricant similar to those found in patients with arthritis. These tests gave encouraging results for the tribological performance of this material couple for use as an alternative bearing combination.

Biocompatible Materials↗

PubMed2006

Compliant layer acetabular cups: friction testing of a range of materials and designs for a new generation of prosthesis that mimics the natural joint.

Total joint replacements (TJRs) have a limited lifetime, but the introduction of components that exhibit good lubricating properties with low friction and low wear could extend the life of TJRs. A novel acetabular cup design using polyurethane (PU) as a compliant layer (to mimic the natural joint) has been developed. This study describes a series of friction tests that have been used to select the most appropriate material, optimize the design parameters, and fine-tune the manufacturing processes of these joints. To determine accurately the mode of lubrication under which these joints operate, a synthetic lubricant was used in all these tests. Friction tests were carried out to assess the lubrication of four PU bearing materials. Corethane 80A was the preferred material and was subjected to subsequent testing. Friction tests conducted on acetabular cups, manufactured using Corethane 80A articulating against standard, commercially available femoral heads, demonstrated friction factors approaching those for full-fluid-film lubrication with only approximately 1 per cent asperity contact. As the joint produces these low friction factors within less than half a walking cycle after prolonged periods of loading, start-up friction was not considered to be a critical factor. Cups performed well across the full range of femoral head sizes, but a number of samples manufactured with reduced radial clearances performed with higher than expected friction. This was caused by the femoral head being gripped around the equator by the low clearance cup. To avoid this, the cup design was modified by increasing the flare at the rim. In addition to this the radial clearance was increased. As the material is incompressible, a radial clearance of 0.08 mm was too small for a cup diameter of 32 mm. A clearance of between 0.10 and 0.25 mm produced a performance approaching full-fluid-film lubrication. This series of tests acted as a step towards the optimization of the design of these joints, which has now led to an in vivo ovine model.

Acetabulum↗

PubMed2006

The effects of proteins on the friction and lubrication of artificial joints.

The tribological testing of artificial hip and knee joints in the laboratory has been ongoing for several decades. This work has been carried out in an attempt to simulate the loading and motion conditions applied in vivo and, therefore, the potential for the success of the joint. However, several different lubricants have been used in these tests. The work documented in this paper compares results obtained using different lubricants and makes suggestions for future work. Hip joints and knee joints of different material combinations were tested in a friction simulator to determine their friction and lubrication properties. Both carboxymethyl cellulose (CMC) fluids and bovine serum (with CMC fluids added) were used as the lubricants. These were prepared to various viscosities to produce the Stribeck plots. Human synovial fluid, of just one viscosity, was used as the lubricant with some of the joints to give a true comparison with physiological lubricants. The results showed that, in most cases, the lubricant had a significant effect on the friction developed between the joint surfaces. This is thought to be due to the proteins that are present within the bovine serum adsorbing to the bearing surfaces, creating 'solid-like' films which rub together, protecting the surfaces from solid-to-solid contact. This would be beneficial in terms of wear but can either increase or decrease the friction between the contacting surfaces. It is important to simulate the conditions in vivo as closely as possible when testing these joints to try to obtain a better comparison between the joints and to simulate more accurately the way that these joints will operate in the body. In an attempt to simulate synovial fluid, bovine serum seems to be the most popular lubricant used at present. It would be beneficial, however, to develop a new synthetic lubricant that more closely matches synovial fluid. This would allow us to predict more accurately how these joints would operate long-term in vivo.

Body Fluids↗

PubMed2006

The tribology of metal-on-metal total hip replacements.

Total hip surgery is an effective way of alleviating the pain and discomfort caused by diseased or damaged joints. However, in the majority of cases, these joints have a finite life. The main reason for failure is osteolysis (bone resorption). It is well documented that an important cause of osteolysis, and therefore the subsequent loosening and failure of conventional metal- or ceramic-on-ultra-high molecular weight polyethylene joints, is the body's immunological response to the polyethylene wear particles. To avoid this, interest has been renewed in metal-on-metal joints. The intention of this paper is to review the studies that have taken place within different laboratories to determine the tribological performance of new-generation metal-on-metal total hip replacements. These types of joint offer a potential solution to enhance the longevity of prosthetic hip systems; however, problems may arise owing to the effects of metal ion release, which are, as yet, not fully understood.

Biocompatible Materials↗

PubMed2006

The effect of 'running-in' on the tribology and surface morphology of metal-on-metal Birmingham hip resurfacing device in simulator studies.

It is well documented that hard bearing combinations show a running-in phenomenon in vitro and there is also some evidence of this from retrieval studies. In order to investigate this phenomenon, five Birmingham hip resurfacing devices were tested in a hip wear simulator. One of these (joint 1) was also tested in a friction simulator before, during, and after the wear test and surface analysis was conducted throughout portions of the testing. The wear showed the classical running in with the wear rate falling from 1.84 mm3 per 10(6) cycles for the first 10(6) cycles of testing to 0.24 mm3 per 10(6) cycles over the final 2 x 10(6) cycles of testing. The friction tests suggested boundary lubrication initially, but at 1 x 10(6) cycles a mixed lubrication regime was evident. By 2 x 10(6) cycles the classical Stribeck curve had formed, indicating a considerable contribution from the fluid film at higher viscosities. This continued to be evident at both 3 x 10(6) and 5 x 10(6) cycles. The surface study complements these findings.

Biocompatible Materials↗

PubMed2006

Comparative in vitro wear testing of PEEK and UHMWPE capped metacarpophalangeal prostheses.

Six metacarpophalangeal prostheses were each wear tested to five million cycles. Each prosthesis consisted of a metacarpal component with an approximately hemispherical shell on a titanium body, articulating against a titanium phalangeal component. Four prostheses had a shell made from ultra-high molecular weight polyethylene (UHMWPE) and two had a shell made from polyether ether ketone (PEEK). The tests were undertaken using a finger wear simulator. Despite pre-soaking and the use of control components, lubricant uptake by the metacarpal components was significant. Gravimetrically, the UHMWPE test components showed a greater weight gain than the UHMWPE control components. Therefore there was no apparent wear of any of the UHMWPE test metacarpal components. The original concentric machining marks of the UHMWPE components could still be seen after five million cycles of testing. For the metacarpal components with PEEK shells, gravimetric wear could be measured. Gravimetrically, all of the titanium phalangeal components showed little or no wear. Light scratches in the direction of sliding appeared on the articulating faces of all metacarpal and phalangeal test components, indicating slight abrasive wear.

Benzophenones↗

PubMed2005

NeuFlex metacarpophalangeal prostheses tested in vitro.

This paper describes the testing of three single-piece silicone NeuFlex metacarpophalangeal prostheses in a finger function simulator and describes the resulting modes of prosthesis failure. In all cases, failure was due to imminent fracture of the prosthesis across the pivot of the central hinge section. This result is in contrast with previous in-vitro and in-vivo experience with single-piece silicone Swanson and Sutter metacarpophalangeal prostheses, which both tend to fracture at the junction of the distal stem and the hinge. In comparison with earlier in-vitro simulator tests of the Sutter metacarpophalangeal prosthesis, the NeuFlex prostheses showed a greater longevity before fracture. To date, no other reports of fracture of the NeuFlex metacarpophalangeal prosthesis have been reported, either in vitro or in vivo.

Equipment Failure Analysis↗

PubMed2005

Design aspects of compliant, soft layer bearings for an experimental hip prosthesis.

Currently, an artificial hip joint can be expected to last, on average, in excess of 15 years with failure due, in the majority of cases, to late aseptic loosening of the acetabular component. A realistic alternative to the problem of wear in conventional joints is the introduction of bearing surfaces that exhibit low wear and operate in the full fluid-film lubrication regime. Contact analyses and friction tests were performed on compliant layer joints (metal-on-polyurethane) and the design of a prototype ovine arthroplasty model was investigated. When optimized, these components have been shown to achieve full fluid-film lubrication.

Animals↗

PubMed2004

The effect of bone cement particles on the friction of polyethylene and polyurethane knee bearings.

Compliant layer knee joints have been considered for use in an attempt to increase the serviceable life of artificial joints. If designed correctly, these joints should operate within the full-fluid film lubrication regime. However, adverse tribological conditions, such as the presence of bone and bone cement particles, may breach the fluid film and cause surface wear. The frictional behaviour of both polyurethane (PU) and conventional polyethylene (PE) tibial components against a metallic femoral component was therefore assessed when bone cement particles were introduced into the lubricant. The bone cement particles caused a large increase in the frictional torque of both the PE and PU bearings; however, the friction produced by the PU bearings was still considerably lower than that produced by the PE bearings. The volume of bone cement particles between each of the bearings and the resultant frictional torque both decreased over time. This occurred more quickly with the PE bearings but greater damage was caused to the surface of the PE bearings than the PU components.

Biocompatible Materials↗

PubMed2004

Is the wear factor in total joint replacements dependent on the nominal contact stress in ultra-high molecular weight polyethylene contacts?

The exact dependence of wear factor on contact stress, load and apparent contact area is much disputed in the literature. This study attempts to solve this dispute. Pin-on-plate studies of ultra high molecular weight polyethylene against stainless steel were conducted under different combinations of load (33-250 N), nominal stress (0.56-12.73 MPa) and face diameter, as well as two tests where both stress and load were kept constant, while the diameter was changed. For these tests the centre of the pin face was bored out to create four different average pin diameters with similar face areas. Diameter and load were found to have no significant effect on the wear factor, while the wear factor decreased with increasing contact stress according to the relation K = 2 x 10(-6) sigma(-0.84).

Compressive Strength↗

PubMed2003

[Tribologic investigation of a metacarpophalangeal prosthesis].

It is the goal of this paper to introduce modern tribological investigations into the development of a new finger prosthesis, with the particular aim of optimising the material component.A new, unconstrained metacarpophalangeal prosthesis restoring the anatomy of the joint has been developed in two versions (UHMWPE-metal and PEEK-metal). In order to determine the version having better wear behaviour, these two versions were tested on a Joyce finger simulator. As the UHMWPE-metal version showed a lower amount of wear in vitro, this version was chosen to conduct a clinical study. This investigation showed the importance of conducting an in vitro wear test before any clinical studies. Such tests allow the optimisation of the wear behaviour of the tested metacarpophalangeal prosthesis and therefore permit a minimisation of the possible risks to the patients.

Equipment Failure Analysis↗

PubMed2003

A comparison of ex vivo and in vitro Sutter metacarpophalangeal prostheses.

Forty-one Sutter metacarpophalangeal prostheses were implanted into 11 hands of nine patients. Twelve of these prostheses were revised from three patients after a mean period of 42 months. Of the 12 prostheses, 11 showed fracture at the junction of the distal stem and the central hinge region. Two Sutter metacarpophalangeal prostheses were tested on a single station finger simulator and both failed due to fracture at the junction of the hinge and the distal stem.

Arthritis, Rheumatoid↗

PubMed2003

Relative movements between Kinemax Plus tibial inserts and the tibial base-plates.

Tests were performed on six large Kinemax Plus knee bearings (snap-fit design) to evaluate the amount of movement between 10- and 15-mm-thick tibial inserts and the tibial base plates. The knee bearings were tested up to 1 x 10(6) cycles on the Durham six-station knee wear simulator which subjected the bearings to similar motion and loading profiles that would be experienced by the natural knee during walking. Although passive internal/external (I/E) rotation was allowed, no active I/E rotation was applied. The movement of the tibial inserts was measured with dial gauges (accuracy +/-0.01 mm) before and after the bearings were tested on the simulator, when unloaded, and throughout the tests while the bearings were being dynamically loaded in the simulator. Movement occurred between the tibial insert and the tibial base plate after initial assembly due to the snap-fit mechanism used to locate the tibial insert within the tibial base plate. However this decreased appreciably when the bearings were loaded in the simulator. The amount of movement did not change with time when the bearings were continuously loaded in the simulator. However, after each test the amount of movement of the tibial inserts, when unloaded, was only 65 per cent (anterior-posterior) and 46 per cent (medial-lateral) of the values before the test. This was thought to be due to creep of the ultra-high molecular weight polyethylene (UHMWPE) inserts. The movement between the tibial insert and tibial base plate in situ is likely to be much less than that observed by a surgeon at the time of assembly due to loading of the knee bearing in the body. However, the amount of movement when the tibial inserts are loaded may still be great enough to produce a second interface where wear of the tibial insert may take place.

Coated Materials, Biocompatible↗

PubMed2002

Design and validation of a surrogate humerus for biomechanical testing.

At present biomechanical testing of fracture plating strategies is conducted using animal or cadaveric whole bone models. This may introduce experimental error into these studies. This communication summarises the design and validation of a novel bone and fibre-reinforced plastic construct conceived to minimise intra-experimental error. A tubular surrogate humerus was produced with dimension and strength matched to that of the human humerus. Bone inserts placed into the wall of the tube allow for the fixation of the plates with bone screws. Three-point bending tests of the flexural rigidity of the surrogate humerus (EI=100.1 (SD 6.0)Nm(2)) showed it to be comparable to the human humerus. Further, pull-out tests of the screws showed that the bone slots adequately mimicked the whole bone scenario. This testing construct will be used for a comparative study of humeral plating techniques.

Animals↗

PubMed2002

A literature review of "failures" of the Swanson finger prosthesis in the metacarpophalangeal joint.

This paper reviews the use of the Swanson finger prosthesis, concentrating on clinical results from the metacarpophalangeal (MCP) joint and modes of prosthetic "failure". While "failure" is generally associated with fracture, it is recognised that fracture does not always necessitate replacement of the Swanson prosthesis. Fracture tends to occur at the junction of the distal stem and hinge of the prosthesis. Initial improvements in ulnar deviation and range of motion (ROM) tend to be gradually reduced over the duration of implantation, and there is little evidence to suggest any long-term improvement in hand strength. Bone erosion and silicone synovitis have been seen but at a much lesser incidence than in other joints implanted with silicone spacers. An evaluation of retrieved Swanson prostheses, tied in with patient history and hand measurements might provide additional information to improve the design of the Swanson prosthesis itself and of other finger prostheses.

Arthritis↗

PubMed2002

A test procedure for artificial finger joints.

This paper highlights the lack of an agreed testing standard for artificial finger joints. It reviews the anatomy, pathology and biomechanics of finger joints as well as the various designs of finger prostheses and the machines used to test them. While pre-implantation testing should be fundamental, increasing regulation of the biomedical engineering industry will further demand testing of prostheses to pre-agreed standards. Standards relating to the testing of other artificial joints are reviewed before possible parameters for testing finger prostheses are offered.

Equipment Failure Analysis↗

PubMed2001

Heel pad stiffness in runners with plantar heel pain.

OBJECTIVE: To evaluate significant differences in heel pad stiffness within a cohort of runners with diagnosed plantar heel pain and to explore the clinical importance of maximum heel pad stiffness values. DESIGN: A cross-sectional design was used to quantify the heel pad stiffness of 166 runners with 33 diagnosed with plantar heel pain. BACKGROUND: Palpation is still widely used to evaluate heel pad stiffness subjectively in everyday clinical practice. However, there is limited quantifiable data pertaining to heel pad stiffness measurements in runners and those with heel pain. METHODS: A portable hand-held device measured force applied by a metal probe, and its displacement into the plantar surface of the heel pad. Non-linear modelling allowed curve coefficients b0 and b1 to be evaluated and was described by an exponential function using a non-linear regression equation. Exploratory analysis was used to describe a single-point approximation for clinical use. RESULTS: An independent t-test demonstrated a statistically significant difference between the curve coefficient b1 (p<0.05). No significant difference was found for coefficient b0 between the plantar heel pain group and the non-plantar heel pain group (p>0.05). Exploratory analysis demonstrated maximum mean stiffness of 3.22 N/mm for the non-plantar heel pain group and 2.87 N/mm for the plantar heel pain-group, an 11% mean difference. CONCLUSION: The results suggested that heel pad stiffness may be associated with plantar heel pain subjects. RELEVANCE: Heel pad stiffness measurements may give a better insight into the mechanical properties of the heel pad in subjects with plantar heel pain.

Adolescent↗

PubMed2001

Pin-on-plate studies on the effect of rotation on the wear of metal-on-metal samples.

An important cause of osteolysis and subsequent loosening of replacement hip joints is the body's biological response to polyethylene wear debris. Interest has thus been renewed in hard bearing surfaces such as metal-on-metal implants. Tests were performed on a pin-on-plate machine to determine the effects of pin rotation on the wear of two different compositions of cobalt chrome molybdenum (CoCrMo) against itself (high carbon and low carbon). With reciprocating motion only, the low carbon material gave an order of magnitude higher wear than the high carbon material. The overall wear (that for both the pin and the plate) was significantly reduced with added rotation for the low carbon material but remained approximately the same for the high carbon material. However, the wear of the low carbon material was not reduced below that of the high carbon material which remained the best material in terms of wear.

Journal Article↗