Biomedical subjects

A Unsworth

Publications and source records attributed to A Unsworth.

At least 19 recordsLinked to original sources

PubMed2001

A five-station hip joint simulator.

A five-station hip joint wear simulator was designed and built which featured simplified motion and loading. An elliptical wear path was produced using approximately sinusoidal motion in the flexion/extension and internal/external rotation axes and the dynamic loading approximated to a square wave. Five 28 mm diameter zirconia femoral heads articulated against ultra-high molecular weight polyethylene acetabular cups in 25 per cent bovine serum for 5 x 10(6) cycles. Gravimetric wear measurement was used with moisture absorption compensation using a dynamically loaded soak control. With motion of physiological magnitude, the mean acetabular cup wear rate was 52.2 mm3/10(6) cycles which is comparable with a number of clinical studies.

Hip Prosthesis↗

PubMed2000

A tribological study of UHMWPE acetabular cups and polyurethane compliant layer acetabular cups.

A novel design of polyurethane compliant layer acetabular cup has been developed through a series of friction, creep and wear tests. Friction tests were initially conducted on ABG standard form, polyurethane acetabular cups and an ABG standard form, UHMWPE acetabular cup for comparison. The polyurethane cups showed lower friction than the UHMWPE cup with maximum friction factors between 0. 008 and 0.02 compared with 0.035 for the UHMWPE cup. This indicated that, in the polyurethane cups, more of the load across the joint was carried by the fluid entrapped in the joint space rather than with asperity contact, compared with the UHMWPE cup. The inherent compliance of the polyurethane is used to promote elasto-hydrodynamic lubrication. However, this compliance raised concerns over excessive creep, which may in turn adversely affect tribological performance. Therefore, creep tests were undertaken on the ABG standard form, polyurethane acetabular cups followed by further friction tests. Small amounts of creep occurred in the polyurethane cups at ambient temperature, which reduced the friction slightly (maximum friction factors of 0.009) due to increased conformity between the head and the cup. However, at 37 degrees C, greater creep occurred causing pinching of the femoral head by the acetabular cup resulting in lubricant starvation and higher friction (maximum friction factors of 0.035). The design of the polyurethane cups was subsequently modified to incorporate a flared rim to eliminate the possibility of fluid starvation through pinching. Creep in polyurethane acetabular cups is also affected by the method of fixation of the cups, due to the conformity with and the stiffness of the cup backing. Hence, a one-million-cycle wear test was performed on five ABG flared form, polyurethane acetabular cups on the Mk. I Durham Hip Joint Wear Simulator to evaluate the best method of fixation for the polyurethane cups. The smallest amount of penetration, due to creep and wear, was found with cement fixation (0.30 mm penetration with cement fixation, 0.44 mm with polyethylene holder mounting, and 0.52 mm with metal shell mounting). A 4. 25-million-cycle wear test was then conducted on a further five ABG flared form, polyurethane acetabular cups with cement fixation. Five ABG standard form, UHMWPE acetabular cups were also wear-tested to 5. 0-million cycles. The mean and standard error of the wear rate for the polyurethane cups were 14.1 +/- 4.3 mg/10(6) (12.0 +/- 3.6 mm(3)/10(6)), cycles compared with 44.8 +/- 3.4 mg/10(6) (48.2 +/- 3. 7 mm(3)/10(6)), cycles for the UHMWPE cups. This study showed that the novel polyurethane-compliant layer acetabular cup with cement fixation was tribologically superior to the ABG standard form UHMWPE design currently being used clinically.

Biocompatible Materials↗

PubMed2000

Variation in the wear rate during the life of a total hip arthroplasty: a simulator and retrieval study.

The limitation of wear is fundamental to the optimization of total hip arthroplasty longevity. The maintenance of the supersmooth femoral head surface is considered to be paramount in maximizing prosthesis life expectancy. Ex vivo studies have failed to substantiate a relationship between roughness and the clinical wear factor, however. A hip simulator wear study was undertaken to investigate this contradiction. Three explanted femoral heads were articulated for 5 million cycles against new acetabular liners. The simulator wear rate was 5 times the ex vivo value. This difference can be explained only if the explant head roughness was not that which existed for most of the joint's life. The relationship between surface roughness and wear deduced for simulator testing is substantially different from that of unidirectional wear screening methods. The multiphasic nature of wear in cementless joints has been illustrated: a wear-in period, followed by a steady-state phase, until a head-roughening event causes a rapid wear period.

Arthroplasty, Replacement, Hip↗

PubMed2000

A frictional study of total hip joint replacements.

Polymeric wear debris produced by articulation of the femoral head against the ultra-high-molecular-weight polyethylene socket of a total hip replacement has been implicated as the main cause of osteolysis and subsequent failure of these implants. Potential solutions to this problem are to employ hard bearing surface combinations such as metal-on-metal or ceramic-on-ceramic prostheses. The aim of this study was to investigate the difference in lubrication modes and friction of a range of material combinations using synthetic and biological fluids as the lubricants. The experimental results were compared with theoretical predictions of film thicknesses and lubrication modes. A strong correlation was observed between experiment and theory when employing carboxy methyl cellulose (CMC) fluids as the lubricant. Under these conditions the ceramic-on-ceramic joints showed full fluid film lubrication while the metal-on-metal, metal-on-plastic, diamond-like carbon-coated stainless steel (DLC)-on-plastic and ceramic-on-plastic prostheses operated under a mixed lubrication regime. With bovine serum as the lubricant in the all ceramic joints, however, the full fluid film lubrication was inhibited due to adsorbed proteins. In the metal-on-metal joints this adsorbed protein layer acted to reduce the friction while in the ceramic coupling the friction was increased. The use of bovine serum as the lubricant also significantly increased the friction in both the metal-on-plastic and ceramic-on-plastic joints. The friction produced by the DLC-on-plastic joints depended on the quality of the coating. Those joints with a less consistent coating and therefore a higher surface roughness gave significantly higher friction than the smoother, more consistently coated heads.

Adsorption↗

PubMed2000

Comparison of friction and lubrication of different hip prostheses.

It is well documented that an important cause of osteolysis and subsequent loosening of replacement hip joints is polyethylene wear debris. To avoid this, interest has been renewed in metal-on-metal and ceramic-on-ceramic prostheses. Various workers have assessed the lubrication modes of different joints by measuring the friction at the bearing surfaces, using different lubricants. Measurements of friction factors of a series of hip prostheses were undertaken using carboxymethyl cellulose (CMC) fluids, silicone fluids, synovial fluid and different concentrations of bovine serum as the lubricant. The experimental results were compared with theoretical predictions of film thicknesses and lubrication modes. A strong correlation was observed between experiment and theory when employing CMC fluids or silicone fluids as the lubricant. Mixed lubrication was found to occur in the metal-on-metal (CoCrMo/CoCrMo) joints with all lubricants at a viscosity within the physiological range. This was also the case for the metal-on-plastic (CoCrMo/ultra-high molecular weight polyethylene) joints. The ceramic-on-ceramic (Al2O3/Al2O3) joints, however, exhibited full fluid film lubrication with the synthetic lubricants but mixed lubrication with the biological lubricants. Employing a biological fluid as the lubricant affected the friction to varying degrees when compared with the synthetic lubricants. In the case of the ceramic-on-ceramic joints it acted to increase the friction factor tenfold; however, for the metal-on-metal joints, biological fluids gave slightly lower friction than the synthetic lubricants did. This suggests that, when measuring friction and wear of artificial joints, a standard lubricant should be used.

Adsorption↗

PubMed2000

Design of a surface replacement prosthesis for the proximal interphalangeal joint.

A surface replacement finger joint prosthesis was designed specifically for the proximal interphalangeal joint (PIPJ). The two-piece design consisted of a bi-condylar proximal phalangeal head and a conforming bi-concave middle phalangeal base. The bearing surfaces were designed as close to the original anatomy of the PIPJs as possible, using detailed information obtained from a previous anatomical study of 83 PIPJs by the present authors. Four sizes of prosthesis were designed with maximum head diameters of 7, 8, 9 and 10 mm. Fixation of the joint prosthesis was achieved by an interference fit between the stems of semicircular cross-section and the phalangeal bone shafts. The main considerations for the stem designs were the offset from the centre of rotation, angle of inclination, length, and cross-sectional shape and size. It is proposed that the two components will be made from cross-linked polyethylene (XLPE) because it can be injection moulded to produce the complex shapes of the joint prosthesis. In addition, XLPE against itself has shown comparable wear rates with stainless steel against ultra-high molecular weight polyethylene from previous work by Joyce et al.

Biocompatible Materials↗

PubMed2000

Long-term results for Kinemax and Kinematic knee bearings on a six-station knee wear simulator.

A long-term wear test was performed on Kinemax and Kinematic (Howmedica Inc.) knee bearings on the Durham six-station knee wear simulator. The bearings were subjected to flexion/extension of 65-0 degrees, anterior-posterior translation of between 4.5 and 8.5 mm and a maximum axial load of 3 kN. Passive abduction/adduction and internal/external rotation were also permitted, however, two of the stations had a linkage system which produced +/- 5 degrees active internal/external rotation. The bearings were tested at 37 degrees C in a 30 per cent bovine serum solution and the test was run to 5.6 x 10(6) cycles. The bearings from stations 2 and 3, and stations 4 and 5 were swapped during the test to investigate the effects of interstation variability. The average wear rate and standard error was 3.00 +/- 0.98 mg/10(6) cycles (range 1.33-4.72 mg/10(6) cycles) for the Kinemax bearings and 3.78 +/- 1.04 mg/10(6) cycles (range 1.87-4.89 mg/10(6) cycles) for the Kinematic bearings. There were no significant differences in wear rates between the different bearing designs, the addition of active internal/external rotation or a change of stations. However, the wear tracks were different for the two types of bearings and with active internal/external rotation. The wear rates and factors were generally lower than previously published in vitro wear results; however, this may have been due to a difference in the axial loads and lubricants used. The appearance of the wear tracks with active internal/external rotation was comparable with those seen on explanted knee bearings.

Bias↗

PubMed2000

The design of a finger wear simulator and preliminary results.

A dual-cycle finger wear simulator has been designed, manufactured and commissioned. The simulator interspersed dynamic flexion-extension motion under light load with a heavier static 'pinch' load to a test prosthesis immersed in a lubricant heated to 37 degrees C. A validation test was undertaken on a size 2 Swanson prosthesis, leading to prosthesis failure in less than 1 million cycles. A second test was carried out on a Durham metacarpophalangeal prosthesis. After 4.8 million cycles a total wear factor for the joint of 0.60 x 10(-6) mm3/N m was calculated, with no cracks or damage visible. Both test results compare well with earlier tests undertaken on the Stokoe finger wear simulator.

Artificial Limbs↗

PubMed2000

Simplified motion and loading compared to physiological motion and loading in a hip joint simulator.

Two wear tests were conducted using the Durham Hip Joint Wear Simulator to investigate the effects of simplified motion and loading on ultra-high molecular weight polyethylene (UHMWPE) acetabular cup wear rates. Bovine serum was used as a lubricant and a gravimetric technique was used to measure wear. The first wear test duration was 7.1 x 10(6) cycles and investigated the effect of simplified loading. This was achieved by using full physiological motion and loading for the first 5 x 10(6) cycles of the test, then physiological motion with simplified loading for the final 2.1 x 10(6) cycles of the wear test. The UHMWPE acetabular cup wear rates using full physiological motion and loading were 32.2 and 51.7 mm3/10(6) cycles against zirconia and CoCrMo femoral heads respectively. Using simplified loading the cup wear rates were 30.1 and 49.2 mm3/10(6) cycles against zirconia and CoCrMo respectively which was not significantly different from wear rates with physiological loading. The effect of simplified motion was investigated in a second wear test of 5.0 x 10(6) cycles duration. Physiological loading was applied across the prosthesis with physiological motion in the flexion/extension plane only. Mean wear of the acetabular component dropped to 0.197 mm3/10(6) cycles. The surfaces of all the acetabular cups were subject to gross examination, optical microscopy and scanning electron microscopy. No notable difference was observed between the cups subjected to physiological motion and loading and those subjected to simplified loading. The cups worn with a single plane of motion had a much smaller worn area and a notable difference in surface features to the other cups. Simplifed loading is therefore an acceptable simplification in simulator testing but simplifying motion to the flexion/extension plane axis only is unacceptable.

Acetabulum↗

PubMed2000

A multi-directional wear screening device and preliminary results of UHMWPE articulating against stainless steel.

This paper describes the design of a pin on plate rig which has been modified to give multi-directional motion to the test pins, resulting in elliptical and quasi-elliptical wear paths. Such paths are closer to those seen in vivo by a femoral head articulating against an acetabular cup. The description of the rig is augmented by the results of a test of ultra high molecular weight polyethylene pins rubbing against stainless steel plates. with dilute bovine serum at 37 degrees C being used as the lubricant. With standard reciprocating motion, a mean wear factor of 0.085 x 10(-6) mm3/N m was found, but with multi-directional motion, the mean wear factor increased to 1.10 x 10(-6) mm3/N m, which is very similar to in vivo wear factors.

Animals↗

PubMed1999

The influence of femoral head surface roughness on the wear of ultrahigh molecular weight polyethylene sockets in cementless total hip replacement.

A theoretical relationship was recently proposed relating the wear behavior of polymetric bearing materials articulating against hard counterfaces.(1) This model attempts to predict the influence of surface roughness on wear. Laboratory-based studies have been used to establish the validity of these relationships, but their application to the clinical situation has not been investigated fully. Forty-two retrieved PCA hip joints have been assessed. The total wear volume was calculated from the penetration measured using the shadowgraph method, and roughness of the articulating surfaces was recorded using noncontacting profilometry. The roughness of the explanted femoral heads was observed to increase (median S(a) - 10. 35 nm worn region, 3.05 nm peripheral region), while that of the acetabular liner fell dramatically (median S(a) - 41 nm worn region, 212 nm unworn region). No evidence of a relationship between the topography of the worn regions of the femoral head and that of the acetabular liner could be found. Similarly, the strength of the association between the surface roughness and the clinical wear factor was considerably poorer than that achieved in laboratory experiments. A number of reasons for this observation are proposed. Most deleterious was considered to be the inability of the roughness parameters to describe the damaging features of the surface adequately. Uncertainty as to when the surface of the component degrades during its life serves to introduce further doubt as to the application of the wear models in the clinical environment. In conclusion, this study fails to provide clinical evidence to substantiate the relationship between surface finish and wear rate. The adoption of standardized measurement parameters and techniques would facilitate the direct comparison of joint types and the selection of the most advantageous materials.

Arthroplasty, Replacement, Hip↗

PubMed1999

Evaluation of a hip joint simulator.

To evaluate the functioning of the Durham hip joint wear simulator, the wear rates of ultra high molecular weight polyethylene (UHMWPE) and polytetrafluoroethylene (PTFE) acetabular cups articulating against 22 mm diameter cobalt-chromium-molybdenum (CoCrMo) femoral heads were studied. A wear test was conducted in a lubricant of distilled water at 37 degrees C for a duration of 4.8 million cycles. The average penetration rate for the CoCrMo femoral heads against UHMWPE acetabular cups was 0.03 mm/10(6) cycles, while penetration rate for PTFE cups was some twenty times greater. These results are of a similar order of magnitude to other simulator studies in distilled water and are in a similar ratio to clinical data.

Acetabulum↗

PubMed1999

A comparison between gravimetric and volumetric techniques of wear measurement of UHMWPE acetabular cups against zirconia and cobalt-chromium-molybdenum femoral heads in a hip simulator.

Five cobalt-chromium-molybdenum (CoCrMo) and five zirconia femoral head components have been wear tested against 28 mm diameter ultra high molecular weight polyethylene (UHMWPE) acetabular cups for 5 million cycles in the Durham hip joint wear simulator using bovine serum as a lubricant. Wear measurements used gravimetric and volumetric techniques and no statistically significant difference was found between the measurement methods. The wear rates of the acetabular cups against both femoral heads are presented for both measurement methods. The UHMWPE acetabular cups showed a statistically significant higher linear wear rate for the first 2 million cycles than the lower linear wear rate from 2 million cycles to the end of the test, against both femoral head materials. Over the full duration of the wear test, the wear rates of acetabular cups articulating against zirconia femoral heads were lower than against CoCrMo femoral heads. The wear rates up to 2 million cycles and from 2 to 5 million cycles for both femoral head materials were consistent with other studies.

Acetabulum↗

PubMed1998

Prevalence of impingement in explanted Charnley acetabular components.

: Impingement of the femoral neck against the rim of the socket bore has often been cited as one of the contributory factors in the acetabular loosening process. However, there has been little research on its clinical prevalence or on the effects of both linear wear and the diameter of the femoral neck. With this aim in mind, 74 Charnley prostheses were examined after being removed at revision surgery and the sockets interrogated for evidence of impingement. The probability of impingement was assessed using logistic regression analysis. A strong positive association was observed between penetration depth and impingement (chi2 = 12.8; P = 0.0004) regardless of differences in the femoral neck diameter. Further, the introduction of femoral components which comprised a reduced diameter neck had a positive effect, in that the 50% probability of impingement occurred at approximately 2 mm of penetration. For those components with standard necks, the 50% probability of impingement occurred at zero mm of penetration. If impingement is a problem then a reduced diameter neck would appear to be a solution in cutting rates of long-term loosening. However, whether or not this reduction in rim damage, and therefore impingement, is clinically significant in terms of loosening can only be fully assessed from long-term survival analysis and comparison with autopsy retrieved specimens.

Acetabulum↗

PubMed1998

Differences in the rates of penetration determined from radiographic and shadowgraphic measurements of acetabular sockets.

A number of previous studies have observed a marked difference in the penetration depths recorded between direct and radiographic measurement, although no assessment of how these discrepancies affect the rates of penetration has been undertaken. In this report, the penetration depth of 95 sockets was determined from both prerevision radiographs and casts of the retrieved sockets using the uniradiographic and shadowgraphic techniques, respectively. It was observed that the mean discrepancy between the penetration rates derived from the two measurement methods was 0.046 (SE 0.017) mm yr(-1), which was significantly different from zero at P = .007. It was concluded that the penetration rates derived from this method of radiographic assessment seriously underestimated the true value as measured by a direct method by approximately 20%. As a consequence, the discrepancy in penetration rates between those sockets that are loose at revision surgery (direct measurement) and the general population of acetabular components (radiographic measurement) may not be as large as previously thought.

Acetabulum↗

PubMed1998

Wear in retrieved acetabular components: effect of femoral head radius and patient parameters.

Forty-seven explanted Porous Coated Anatomic (PCA, Howmedica, Rutherford, NJ) cementless acetabular components were acquired at revision surgery. All the components articulated against CoCrMo femoral heads of 32-mm diameter. The penetration depth and angle were measured using the shadowgraph technique. The wear volume was then calculated using Kabo's formula. Using weighted linear regression analysis, the mean penetration rate and mean volumetric wear rate were calculated to be 0.23 (SE, 0.03) mm3/y and 96 (SE, 13) mm3/y, respectively. The creep component was not found to be significantly different from zero. The clinical wear factor, k(clinical), for this cohort was also calculated using linear regression analysis but with the assumption that creep was zero. The value found, k(clinical) = 1.93 (SE, 0.29) x 10(-6) mm3/N-m, was similar to those in previous studies involving cemented joints with a 22-mm femoral head diameter. The similar k(clinical) values of these substantially different joint types suggest that the high volumetric wear rate for the PCA joint can be attributed entirely to its larger head size and the younger, more active, patient profile. Fixation technique and metal backing seem not to influence the rate of wear.

Acetabulum↗

PubMed1998

Discrepancy between penetration depths derived from radiographic and direct measurement of acetabular components.

The most common technique for assessing penetration due to wear in acetabular components is with the aid of the most recent serial radiograph. This approach, which is often termed the uni-radiographic method, has been shown to underestimate the more reliable value of the penetration depth deduced from direct measurement of explanted sockets. In this article the causes of the discrepancies between the two data sets are explored. Ninety-six sockets were available from revision surgery for which both the penetration depth and angle could be measured using the shadowgraphic technique in both the coronal and wear planes. Further, the penetration depth for each of the sockets was also assessed from pre-revision X-rays. A significant discrepancy was observed between the penetration depths measured in the wear plane of the replica delta Pw and that measured from the radiograph, delta PX-ray. The discrepancy was greatest for loose sockets as opposed to those that were still fixed at revision surgery. Using the corresponding data from the shadowgraph measurements, it was possible to deduce that the errors have arisen from the radiographic measurement of wear in the coronal plane and the formula used in calculating delta PX-ray. If these errors (which cannot be calculated from the X-ray data alone) were taken into consideration, then the systematic bias between radiographic and shadowgraphic measurement was greatly reduced. The largest portion of the discrepancy was accounted for by wear occurring out of the plane of the radiograph, and this, in general, coincides with the coronal plane. Overall, these results indicate that the accurate measurement of wear from serial radiographs is not possible and that improved performance in terms of accuracy can only be achieved when a three-dimensional system is used. An alternative method for deducing the radiographic penetration depth is proposed which, theoretically, negates the error arising from the inaccuracy of the formula.

Acetabulum↗

PubMed1998

The association between rates of wear in retrieved acetabular components and the radius of the femoral head.

Current evidence suggests that loosening of the acetabular socket is related to the volume of ultra-high molecular weight polyethylene wear debris generated at the articulating surfaces, through a process of bone resorption. Therefore it is important that the rate of volumetric wear is minimized in an attempt to extend the useful life of the procedure. Laboratory evidence indicates that a reduction in sliding distance would be beneficial in achieving this target and may be attained by a reduction in femoral head radius. To investigate the relationship between femoral head size and the rates of both volumetric wear and penetration, 200 acetabular components were retrieved at the time of revision surgery. The joints had femoral heads ranging in size from 11.1 to 19.8 mm radius. For those sockets that were loose at revision surgery, a significant correlation was observed, between the rate of volumetric wear and the radius of the femoral head. For this cohort an increase in radius of 1 mm resulted in an increased rate of volumetric wear of 5.1 (SE 1.4) mm3/yr. However, the explained variance in the regression was low and exemplifies the multifactorial nature of the wear process. In particular, it is anticipated that the activity of the patient will have a significant effect on the rate at which the debris is produced. No significant correlation was observed between the rate of linear wear and femoral head radius. These results would indicate a benefit in using head sizes of a smaller radius, which generate debris at a reduced rate, and therefore require more time to accumulate large volumes of wear products.

Aged↗