| 1. |
- Clarke, IC, et al.
(författare)
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Ultra-low wear rates for rigid-on-rigid bearings in total hip replacements
- 2000
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Ingår i: Proceedings of the Institution of Mechanical Engineers. Part H, Journal of engineering in medicine. - : SAGE Publications. - 0954-4119 .- 2041-3033. ; 214:H44, s. 331-347
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Tidskriftsartikel (refereegranskat)abstract
- With the increased clinical interest in metal-on-metal and ceramic-on-ceramic total-hip replacements (THRs), the objective of this hip simulator study was to identify the relative wear ranking of three bearing systems, namely CoCr-polyethylene (M-PE), CoCr-CoCr (M-M) and ceramic-on-ceramic (C-C). Volumetric wear rates were used as the method of comparison. The seven THR groupings included one M-PE study, two M-M studies and four C-C studies. Special emphasis was given to defining the ‘run-in’ phase of accelerated wear that rigid-on-rigid bearings generally exhibit. The hypothesis was that characterization of the run-in and steady state wear phases would clarify not only the tribological performance in vitro but also help correlate these in vitro wear rates with the ‘average’ wear rates measured on retrieved implants. The implant systems were studied on multichannel hip simulators using the Paul gait cycle and bovine serum as the lubricant. With 28 mm CoCr heads, the PE (2.5 Mrad/N2) wear rates averaged 13 mm3/106 cycles duration. This was considered a low value compared with the clinical model of 74 mm3/year (for 28 mm heads). Our later studies established that this low laboratory value was a consequence of the serum parameters then in use. The mating CoCr heads (with PE cups) wore at the steady state rate of 0.028 mm3/106 cycles. The concurrently run Metasul™ M-M THRs wore at the steady state rate of 0.119 mm3/106 cycles with high-protein serum. In the second Metasul M-M study with low-protein serum, the THR run-in rate was 2.681 mm3/106 cycles and steady state was 0.977 mm3/106 cycles. At 10 years, these data would predict a 70-fold reduction in M-M wear debris compared with the clinical PE wear model. All M-M implants exhibited biphasic wear trends, with the transition point at 0.5 × 106 cycles between run-in and steady state phases, the latter averaging a 3-fold decrease in wear rate. White surface coatings on implants (coming from the serum solution) were a confounding factor but did not obscure the two orders of magnitude wear performance improvement for CoCr over PE cups. The liners in the alumina head-alumina cup combination wore at the steady state rate of 0.004 mm3/106 cycles over 14 × 106 cycles duration (high-protein serum). The zirconia head-alumina cup THR combination wore at 0.174 and 0.014 mm3/106 cycles for run-in and steady state rates respectively (low-protein serum). The zirconia head and cup THR combination wore slightly higher initially with 0.342 and 0.013 mm3/106 cycles for run-in and steady state rates respectively. Other wear studies have generally predicted catastrophic wear for such zirconia-ceramic combinations. It was noted that the zirconia wear trends were frequently masked by the effects of tenacious white surface coatings. It was possible that these coatings protected the zirconia surfaces somewhat in this simulator study. The experimental ceramic Crystaloy THR had the highest ceramic run-in wear at 0.681 mm3/106 cycles and typical 0.016 mm3/106 cycles for steady state. Since these implants represented the first Crystaloy THR sets made, it was likely that the surface conditions of this high-strength ceramic could be improved in the future. Overall, the ceramic THRs demonstrated three orders of magnitude wear performance improvement over PE cups. With zirconia implants, while the cup wear was sometimes measurable, head wear was seldom discernible, Therefore, we have to be cautious in interpreting such zirconia wear data. Identifying the run=in and steady state wear rates was a valuable step in processing the ceramic wear data and assessing its reliability. Thus, The M-M and C-C THRs have demonstrated two to three orders of reduction in volumetric wear in the laboratory compared with the PE wear standard, which helps to explain the excellent wear performance and minimal osteolysis seen with such implants at retrieval operations.
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| 2. |
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| 3. |
- Lauge-Pedersen, Henrik, et al.
(författare)
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Arch-shaped versus flat arthrodesis of the ankle joint: strength measurements using synthetic cancellous bone
- 2002
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Ingår i: Proceedings of the Institution of Mechanical Engineers. Part H: Journal of Engineering in Medicine. - : SAGE Publications. - 0954-4119 .- 2041-3033. ; 216:H1, s. 43-49
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Tidskriftsartikel (refereegranskat)abstract
- The aim Of this Study was to see if preservation of the arch shape of the ankle at arthrodesis contributes to stability. The ankle joint was simulated by paired blocks of a synthetic material corresponding to rheumatoid cancellous bone with low stiffness and strength. Flat end constructs with and without subchondral bone were compared with arch-shape constructs with and without subchondral bone. The pairs were fixed with two screws simulating an arthrodesis. These constructs were then tested to failure in four-point bending and torque. In four-point bending the subchondral bone increased the strength, regardless of shape. Stiffness was higher in the arch-shaped specimens but was not influenced b the subchondral bone. In torque, both arch-shape and subchondral bone increase the strength. Stiffness was increased by arch-shape but not subchondral bone. The results imply that the arch-shape and subchondral bone should be preserved when performing an ankle arthrodesis, especially in weak rheumatoid bone.
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| 4. |
- Thompson, Mark, et al.
(författare)
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Effects of acetabular resurfacing component material and fixation on the strain distribution in the pelvis
- 2002
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Ingår i: Proceedings of the Institution of Mechanical Engineers. Part H: Journal of Engineering in Medicine. - : SAGE Publications. - 0954-4119 .- 2041-3033. ; 216:H4, s. 237-245
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Tidskriftsartikel (refereegranskat)abstract
- A 3D finite element (FE) model of an implanted pelvis was developed as part of a project investigating an all-polymer hip resurfacing design. The model was used to compare this novel design with a metal-on-metal design in current use and a metal-on-polymer design typical of early resurfacing implants. The model included forces representing the actions of 22 muscles as well as variable cancellous bone stiffness and variable cortical shell thickness. The hip joint reaction force was applied via contact modelled between the femoral and acetabular components of the resurfacing prosthesis. Five load cases representing time points through the gait cycle were analysed. The effect of varying fixation conditions was also investigated. The highest cancellous bone strain levels were found at mid-stance, not heel-strike. Remote from the acetabulum there was little effect of prosthesis material and fixation upon the von Mises stresses and maximum principal strains. Implant material appeared to have little effect upon cancellous bone strain failure with both bonded and unbonded bone-implant interfaces. The unbonded implants increased stresses in the subchondral bone at the centre of the acetabulum and increased cancellous bone loading, resembling behaviour obtained previously for the intact acetabulum.
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| 5. |
- Thompson, M.S., et al.
(författare)
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A comparison of structural and mechanical properties in cancellous bone from the femoral head and acetabulum
- 2004
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Ingår i: Proceedings of the Institution of mechanical engineers. Part H, journal of engineering in medicine. - : SAGE Publications. - 0954-4119 .- 2041-3033. ; 218:6, s. 425-429
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Tidskriftsartikel (refereegranskat)abstract
- Mechanical interlock obtained by penetration of bone cement into cancellous bone is critical to the success of cemented total hip replacement (THR). Although acetabular component loosening is an important mode of THR failure, the properties of acetabular cancellous bone relevant to cement penetration are not well characterized. Bone biopsies (9 mm diameter, 10 mm long) were taken from the articular surfaces of the acetabulum and femoral head during total hip replacement. After mechanical and chemical defatting the two groups of bone specimens were characterized using flow measurement, mechanical testing and finally serial sectioning and three-dimensional computer reconstruction. The mean permeabilities of the acetabular group (1.064 × 10-10 m2) and femoral group (1.155 × 10-10 m2) were calculated from the flow measurements, which used saline solution and a static pressure of 9.8 kPa. The mean Young's modulus, measured non-destructively, was 47.4 MPa for the femoral group and 116.4 MPa for the acetabular group. Three-dimensional computer reconstruction of the specimens showed no significant differences in connectivity and porosity between the groups. Results obtained using femoral head cancellous bone to investigate bone cement penetration and fixation are directly relevant to fixation in the acetabulum. © IMechE 2004.
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