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D Dowson

Publications and source records attributed to D Dowson.

At least 19 recordsLinked to original sources
PubMed1999

A multi-station hip joint simulator study of the performance of 22 mm diameter zirconia-ultra-high molecular weight polyethylene total replacement hip joints.

The commissioning of a new form of 10-station hip joint simulator is described and the results of a study of the performance of zirconia-ultra-high molecular weight polyethylene (UHMWPE) total replacement hip joints in the familiar Charnley head size of 7/8 inch (22.225 mm) diameter are presented. The head size is referred to as 22 mm for brevity and consistency throughout the paper. The simulator provided very consistent and repeatable results and the new machine, together with the methods of investigation adopted, offer an excellent facility for the further evaluation of existing and new prostheses. The findings are compared with the outcome of previous laboratory simulator and clinical studies of ceramic-polyethylene implants of similar diameter. It was found that a relatively rapid penetration of the head into the cup was followed by a very low, steady, long-term penetration rate after about two million loading cycles. The mean long-term volumetric penetration rate was 6.28 mm3/10(6) loading cycles. When the linear penetration rates were assessed by direct measurement on a coordinate measuring machine, or deduced from the tunnelling expression, the resulting values were very similar and small at 0.019 and 0.016 mm/10(6) loading cycles respectively. It is generally assumed that one million loading cycles is equivalent to about one year of service in the body and if this equivalence is accepted, these penetration rates compare very favourably with a clinical evaluation of alumina heads of the same diameter, which yielded a mean long-term penetration rate of 0.022 mm/year.

Aluminum Oxide↗
PubMed1999

Development of a ten-station, multi-axis hip joint simulator.

Joint simulators are now used extensively to evaluate the performance of materials and designs of total replacement hip and knee joints. In this Technical Note a new ten-station hip joint simulator with biaxial rotational articulation synchronized to a physiological loading cycle is described. The current simulator manufactured by ProSim Limited (Manchester, UK) is a development of a first generation machine designed and built in-house at DePuy International Limited (Leeds, UK). The use of this new form of ten-station hip simulator to evaluate the performance of 22 mm zirconia femoral heads and ultra-high molecular weight polyethylene acetabular cups over some 7 million loading cycles is described elsewhere [1].

Biomechanical Phenomena↗
PubMed1998

A comparative tribological study of the wear of composite cushion cups in a physiological hip joint simulator.

A composite cushion acetabular cup for a total hip replacement has been designed and developed jointly by Leeds University and DePuy International. In order to assess the long-term performance of this novel design, two sets of simulator tests of more than 4 million cycles duration have been carried out with the cushion bearings using the Leeds PA hip joint simulator with bovine serum as the lubricant. The results of these simulator tests were compared to the results from a previously reported study that used 32 mm ultrahigh molecular weight polyethylene (UHMWPE) acetabular cups. Under a physiological walking cycle simulation, with continuous cyclic motion and loading, the composite cushion cups produced negligible wear compared to a volumetric wear rate of 32 mm3 per million cycles for the conventional UHMWPE acetabular cups. This study has demonstrated for the first time the beneficial effects of fluid film lubrication in reducing wear in composite cushion acetabular cups.

Animals↗
PubMed1998

Prediction of transient lubricating film thickness in knee prostheses with compliant layers.

The transient lubricating film thickness in knee prostheses using compliant layers has been predicted under simulated walking conditions based upon the elastohydrodynamic lubrication theory. Qualitative agreement has been found between the present theoretical predictions and the experimental measurements using an electric resistance technique reported earlier. It has been shown that the contact geometry plays an important role in the generation of fluid film lubrication in knee prostheses using compliant layers. The maximum lubricating film thickness is predicted for the maximized contact area of a transverse conjunction where the semi-minor contact radius lies in the direction of entraining. The additional advantage of the transverse contact conjunction is that the possibility of lubricant starvation due to small stroke length can be minimized. All these factors, together with the kinematic requirements in the natural knee joint, should be taken into consideration when designing artificial knee joint replacements.

Biocompatible Materials↗
PubMed1997

Analysis of fluid film lubrication in artificial hip joint replacements with surfaces of high elastic modulus.

Lubrication mechanisms and contact mechanics have been analysed for total hip joint replacements made from hard bearing surfaces such as metal-on-metal and ceramic-on-ceramic. A similar analysis for ultra-high molecular weight polyethylene (UHMWPE) against a hard bearing surface has also been carried out and used as a reference. The most important factor influencing the predicted lubrication film thickness has been found to be the radial clearance between the ball and the socket. Full fluid film lubrication may be achieved in these hard/hard bearings provided that the surface finish of the bearing surface and the radial clearance are chosen correctly and maintained. Furthermore, there is a close relation between the predicted contact half width and the predicted lubrication film thickness. Therefore, it is important to analyse the contact mechanics in artificial hip joint replacements. Practical considerations of manufacturing these bearing surfaces have also been discussed.

Biocompatible Materials↗
PubMed1997

The wear of ultra-high molecular weight polyethylene sliding on metallic and ceramic counterfaces representative of current femoral surfaces in joint replacement.

A number of studies have investigated the influence of surface roughness on the wear of ultra-high molecular weight polyethylene (UHMWPE) in total joint replacement. The results of these studies have shown that the wear factor is proportional to the counterface roughness raised to a power greater than one. In this laboratory study, the effect of surface finish of several biomaterials on the wear of UHMWPE was studied. The study was conducted using reciprocating pin-on-plate wear tests with bovine serum as a lubricant. The biomaterials investigated as the counterface material included stainless steel, cast cobalt chrome (CoCr), CoCr (ASTM F799), alumina ceramic and zirconia ceramic. The counterface topographies of the wear plates were produced using techniques representative of current manufacturing methods. The surface roughness of the wear plates was varied in the range Ra = 0.005-0.04 micron; this was representative of femoral heads and femoral knee components currently used clinically. Metals and ceramics with a similar surface roughness produced a similar wear rate of UHMWPE. For the limited range of smooth counterfaces used in this study only a moderate correlation was found between the surface roughness and the wear factors. For a change in counterface roughness Ra of 0.005 to 0.04 micron, the wear factor increased from 7.4 +/- 1.6 to 16.5 +/- 2.4 x 10(-9) mm3/N m (mean +/- standard error). This variation in counterface roughness had much less effect in wear than previously reported for rougher counterfaces. For an extended range of counterface roughness, a stronger correlation was found using an exponential function for the regression fit. The exponential function shows the benefits of decreased wear with decreased surface roughness. Although the wear rate decreased less rapidly with decreased counterface roughness for Ra values below 0.05 micron, there were significant advantages to be gained from improved femoral head roughness to below 0.01 micron Ra.

Aluminum Oxide↗
PubMed1996

An investigation into the origins of time-dependent variation in penetration rates with Charnley acetabular cups--wear, creep or degradation?

The total penetration of femoral heads into acetabular cups is achieved by a combination of long-term wear and early non-recoverable deformation or creep. The former is important in determining the total rate of production of polyethylene wear debris, recently implicated in the development of osteolysis and loosening, while the latter contributes to the overall penetration and the possibility of neck impingement in some designs of implants. Attention is drawn to the need to evaluate and to separate out these two physical processes in the assessment of clinical penetration rates. This is particularly important with more recent designs and combinations of materials, which are capable of operating with much reduced wear rates. Measurements of the penetrations of metallic femoral heads into polyethylene acetabular cups in 87 explanted Charnley hip arthroplasties from 85 patients are reported and assessed. The vast majority of the acetabular cups had been sterilized by gamma irradiation and the established shadowgraph technique was used to determine the penetrations. The average implantation time was 8.75 years, with a range from 0.2-18.6 years and the average age was 55 years, covering the span 19-73 years. The influence of sterilization procedure, implantation time and patient age are considered. Assessments of the relative magnitudes of wear and non-recoverable deformation are made by different statistical techniques and compared with previously reported data from clinical and laboratory studies.

Adult↗
PubMed1996

Prospective clinical and joint simulator studies of a new total hip arthroplasty using alumina ceramic heads and cross-linked polyethylene cups.

We report the findings from independent prospective clinical and laboratory-based joint-simulator studies of the performance of ceramic femoral heads of 22.225 mm diameter in cross-linked polyethylene (XLP) acetabular cups. We found remarkable qualitative and quantitative agreement between the clinical and simulator results for the wear characteristics with time, and confirmed that ceramic femoral heads penetrate the XLP cups at only about half the rate of otherwise comparable metal heads. In the clinical study, 19 hips in 17 patients were followed for an average of 77 months. In the hip-joint simulator a similar prosthesis was tested for 7.3 million cycles. Both clinical and simulator results showed relatively high rates of penetration over the first 18 months or 1.5 million cycles, followed by a very much lower wear thereafter. Once an initial bedding-in of 0.2 mm to 0.4 mm had taken place the subsequent rates of penetration were very small. The initial clinical wear during bedding-in averaged 0.29 mm/year; subsequent progression was an order of magnitude lower at about 0.022 mm/year, lower than the 0.07 mm/year in metal-to-UHMWP Charnley LFAs. Our results show the excellent tribological features of alumina-ceramic-to-XLP implants, and also confirm the value of well-designed joint simulators for the evaluation of total joint replacements.

Adult↗
PubMed1995

Contact pressure prediction in total knee joint replacements. Part 1: General elasticity solution for elliptical layered contacts.

A general elasticity contact theory has been developed to predict the contact area and the contact pressure in total knee joint replacements with elliptical contacts where the thickness of ultra high molecular weight polyethylene (UHMWPE) is similar or less than the contact half width. The interfacial boundary condition between the UHMWPE component and the underlying metal substrate has been considered to be either perfectly bonded or perfectly unbonded in the model. Poisson's ratio for UHMWPE has been assumed to be 0.3 or 0.4. The effect of the thickness of the UHMWPE layer on the contact area and the contact pressure has been examined. The predictions of the maximum contact pressure and the contact area have been presented in non-dimensional forms and can readily be applied for typical design configurations of current total knee joint replacements. Furthermore, the present results can readily be applied to design considerations for total knee joint replacements to reduce contact stresses within the UHMWPE component.

Elasticity↗
PubMed1995

Contact pressure prediction in total knee joint replacements. Part 2: Application to the design of total knee joint replacements.

The general elasticity contact theory for elliptical geometry developed in Part 1 (1) has been applied to the design of current total knee joint replacements. A two-step curve-fitting technique using cubic spline interpolation routines has been adopted to represent the full elasticity solutions. The curve fit results of the maximum contact pressure have been compared with the full elasticity solution for a specified elliptical geometry and different polyethylene thicknesses and good agreement has been demonstrated. The computing time required by the curve-fitting technique is very small compared with the full elasticity solution and therefore can readily be applied to the design of knee joint replacements. Furthermore, reasonable agreement has also been found for the contact area for a typical knee joint design between the present theoretical prediction and the experimental measurement using pressure-sensitive film. Predictions of the maximum contact pressure have been made for an existing knee joint replacement in order to illustrate the present analysis in the design cycle. It has been shown that the effect of the thickness of ultra high molecular weight polyethylene is relatively small on contact stress predictions provided a sufficiently large value is chosen. On the other hand, the effect of conformity has a much greater influence on the contact stress distribution, particularly in the direction of the smaller principal radius.

Elasticity↗
PubMed1995

Cushion form bearings for total knee joint replacement. Part 1: Design,friction and lubrication.

Cushion knee prostheses have been designed and constructed that produce approximately equal initial contact areas and theoretical film thicknesses compared with a conventional UHMWPE (ultra-high molecular weight polyethylene) joint. These compliant bearings had a flat tibial component which imposed fewer biomechanical constraints and allowed a greater range of movement. Friction experiments have been carried out on a pendulum simulator apparatus. The results showed that the cushion knee joints operated just within the mixed lubrication regime, but that they benefited from a substantial measure of fluid film lubrication. Microelastohydrodynamic lubrication was effective in preserving low friction and thin but effective lubricating films.

Biomechanical Phenomena↗
PubMed1995

Cushion form bearings for total knee joint replacement. Part 2: Wear and durability.

Cushion knee prostheses have been designed and constructed to produce larger initial contact areas and thicker theoretical film thicknesses than a conventional UHMWPE (ultra-high molecular weight polyethylene) joint. The compliant bearing had a flat tibial component which imposed fewer biomechanical constraints and allowed greater range of movement. Wear tests were performed in a knee joint simulator and creep tests were carried out in a servo-hydraulic apparatus. Various failure modes of cushion joints that require further study were identified. However, the results showed that adequate durability was achieved from a 20 MPa polyurethane material in joint simulating tests carried out over 0.5, 1.0 and 5.0 million cycles. Most importantly, during these tests, no detectable wear debris was generated. It is believed that this is the first time that the full potential of cushion bearings has been demonstrated in a joint simulator over periods corresponding to about five years of service in vivo.

Friction↗
PubMed1994

A parametric analysis of the contact stress in ultra-high molecular weight polyethylene acetabular cups.

It is well known that the wear factor for ultra-high molecular weight polyethylene (UHMWPE) sliding on metallic or ceramic counterfaces is largely independent of contact stress for modest loading conditions and sliding distances. However, it is now recognized that under more severe stress levels and with sliding distances comparable to those encountered in current replacement synovial joints, subsurface fatigue contributes to the volume of wear debris. Since the fatigue process is influenced by surface stress levels it is becoming increasingly important to limit the contact stress through design in order to minimize the volume of UHMWPE wear debris in implants. The contact pressure in UHMWPE acetabular cups has been predicted using both the simple elasticity analysis and the finite element method. It has been shown that the radial clearance between the femoral head and the socket is the dominant parameter in determining the contact stress. Thus, the radial clearance should be controlled so the contact half width is close to the femoral head radius (a total included angle of contact of 120 degrees) to minimize the contact pressure. There is little benefit to be gained by increasing the contact half width greater than the femoral head radius. This is consistent with the geometrical constraint of the anatomical position and the direction of loading. It has been shown that the radius of the femoral head has the most significant effect on the maximum contact pressure for these closely conforming contacts where the contact half width is close to the femoral head radius. The effect of the elastic modulus and the thickness of UHMWPE is relatively small under these contact conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetabulum↗
PubMed1994

A tribological study of a series of retrieved accord knee explants.

A tribological study of a series of 27 retrieved Accord meniscal bearing knee joints has been carried out. The roughness of the articulating surfaces of the meniscal and the femoral components was investigated, and the penetration into the polyethylene meniscal component was determined. There was generally little damage to the highly polished metallic femoral components. A general polishing and smoothing of the ultra high molecular weight polyethylene meniscal components was observed, although there was some subsequent deterioration of the polyethylene surfaces in certain cases. The penetration rate of the femoral components into the menisci was low, and was found to be similar to that reported for other meniscal knee joints.

Adult↗
PubMed1994

Comparative study of the wear of UHMWPE with zirconia ceramic and stainless steel femoral heads in artificial hip joints.

The wear of ultra high molecular weight polyethylene (UHMWPE) when sliding against zirconia ceramic and stainless steel counterfaces has been compared in a pin-on-plate reciprocator and in a hip joint simulator. A lower wear factor was found for the UHMWPE when sliding on the zirconia ceramic counterfaces in the pin-on-plate tests. In the hip joint simulator test, the acetabular cups articulating on zirconia heads showed consistently lower volume changes than the cups articulating on stainless steel heads. The higher volume changes found with the stainless steel heads were associated with an increased roughness of the femoral heads during the tests. This roughening was caused by the adherence of a rough polymer transfer film.

Biocompatible Materials↗
PubMed1993

Friction and lubrication in cushion form bearings for artificial hip joints.

Two hip joint prostheses were designed and constructed to be elastohydrodynamically equivalent producing approximately equal initial contact areas and theoretical film thicknesses. One was made from conventional UHMWPE (ultra-high molecular weight polyethylene) and the other was a cushion component which had a low modulus layer introduced into the joint space. Friction measurements were carried out on a pendulum simulator apparatus and the two joints were compared. In addition the experimental results were compared with theoretical values of friction predicted from elastohydrodynamic lubrication theory. Values for the friction factor at peak load and peak velocity in the cushion cup (0.003-0.009) were much lower than in the UHMWPE cup (0.017-0.042). The low friction values in the cushion cup are consistent with fluid film lubrication in the contact with the thin lubricating film being preserved by microelastohydrodynamic action.

Friction↗
PubMed1992

The effect of porosity of articular cartilage on the lubrication of a normal human hip joint.

The effect of porosity of articular cartilage on the lubrication of a normal human hip joint has been studied. The poroelasticity equation of articular cartilage and the modified Reynolds equation for the synovial fluid lubricant have been successfully solved under squeeze-film motion and for the conditions experienced in a normal human hip joint. It has been shown that porosity of the articular cartilage depletes the lubricant film thickness, rather than increasing it, particularly when the lubricant film thickness becomes small. Furthermore, it has been shown that articular cartilage can be treated as a single-phase incompressible elastic material in the lubrication modelling under physiological walking conditions.

Cartilage, Articular↗