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Biomedical subjects

J B Medley

Publications and source records attributed to J B Medley.

13 recordsLinked to original sources
PubMed2006

Effect of microstructure on the dry sliding friction behavior of CoCrMo alloys used in metal-on-metal hip implants.

The microstructure and its effect on the friction behavior of a medical grade wrought cobalt-chromium-molybdenum (CoCrMo) alloy for surgical implants were studied in this work. In particular, the effects of compression and carbon (C) content on the above characteristics were analyzed. Increasing amounts of deformation resulted in a decrease in the number of annealing twins in the microstructures. In addition, there was an increase in the volume fraction of the hexagonal closed-packed (HCP) phase due to a strain-induced transformation (SIT) from the metastable face-centered cubic (FCC) phase. The high C (HC) alloy had a lower volume fraction of this SIT phase. Friction studies conducted on these alloys revealed a higher coefficient of friction for the HC alloy and no significant effect of SIT on the friction characteristics.

Chromium Alloys↗
PubMed2006

The effect of microstructure on the wear of cobalt-based alloys used in metal-on-metal hip implants.

The influence of microstructure on the wear of cobalt-based alloys used in metal-on-metal hip implants was investigated in a boundary lubrication regime designed to represent the conditions that occurred some of the time in vivo. These cobalt-chromium-molybdenum alloys were either wrought, with a total carbon content of 0.05 or 0.23 wt %, cast with a solution-annealing procedure or simply as-cast but not solution annealed. Bars of these different alloy grades were subjected to various heat treatments to develop different microstructures. The wear was evaluated in a linear-tracking reciprocating pin-on-plate apparatus with a 25 per cent bovine serum lubricant. The wear was found to be strongly affected by the dissolved carbon content of the alloys and mostly independent of grain size or the carbide characteristics. The increased carbon in solid solution caused reductions in volumetric wear because carbon helped to stabilize a face-centred cubic crystal structure, thus limiting the amount of strain-induced transformation to a hexagonal close-packed crystal structure. Based on the observed surface twining in and around the contact zone and the potentially detrimental effect of the hexagonal close-packed phase, it was postulated that the wear of cobalt-based alloys in the present study was controlled by a deformation mechanism.

Biocompatible Materials↗
PubMed2006

Wear particles from metal-on-metal total hip replacements: effects of implant design and implantation time.

Detailed characterization of wear particles is necessary to understand better the implant wear mechanisms and the periprosthetic tissue response. The purposes of the present study were to compare particle characteristics of current with older designs of metal-on-metal (MM) total hip replacements (THRs), and to determine the effect of implantation time on wear particle characteristics. Metal wear particles isolated from periprosthetic tissues from 19 patients with MM THRs of current and older designs and at different implantation times (very short, longer, and very long) were studied using transmission electron microscopy and energy dispersive X-ray analysis. The particles from the current design implants with implantation times of not more than 15 months (very short-term) were almost exclusively round to oval chromium oxide particles. In all other cases, although the predominance was still round to oval chromium oxide particles, greater proportions of cobalt-chromium-molybdenum (Co-Cr-Mo) particles, mainly needle-shaped, were detected. Very long-term THRs implanted for more than 20 years had the highest percentage of needle-shaped Co-Cr-Mo particles. Particle lengths were not markedly different between the different designs and implantation times except for the current design implants of not more than 15 months, which had a significantly smaller mean length of 39 nm. In conclusion, the implant design did not seem to have a significant influence on particle characteristics whereas the implantation time appeared to have the most effect on the particles. It should be noted that, because of the limited number of tissue retrievals available, some uncertainty remains regarding the generality of these findings.

Adult↗
PubMed2001

Effects of digestion protocols on the isolation and characterization of metal-metal wear particles. I. Analysis of particle size and shape.

Isolation of metal wear particles from hip simulator lubricants or tissues surrounding implants is a challenging problem because of small particle size, their tendency to agglomerate, and their potential for chemical degradation by digestion reagents. To provide realistic measurements of size, shape, and composition of metal wear particles, it is important to optimize particle isolation and minimize particle changes due to the effects of the reagents. In this study (Part I of II), transmission electron microscopy (TEM) was used to examine and compare the effects of different isolation protocols, using enzymes or alkaline solutions, on the size and shape of three different types of cobalt-based alloy particles produced from metal-metal bearings. The effect on particle composition was examined in a subsequent study (Part II). Large particles (<1200 nm) were generated by dry abrasion of CoCrMo alloy against itself and small particles (<300 nm) were generated by hip simulator testing of a metal-metal implant pair in the presence of either distilled-deionized water or a 95% bovine serum solution. The reagents changed particle size and to a lesser extent particle shape. For both large particles and small particles generated in water, the changes in size were more extensive after alkaline than after enzymatic protocols and increased with alkaline concentration and time in solution, up to twofold at 2 h and threefold at 48 h. However, when isolating particles from 95% serum, an initial protective effect of serum proteins and/or lipids was observed. Because of this protective effect, there was no significant difference in particle size and shape for both oval and needle-shaped particles after 2 h in 2N KOH and after enzymatic treatments. However, round particles were significantly smaller after 2 h in 2N KOH than after enzymatic treatments. Particle composition may also have been affected by the 2N KOH treatment, as suggested by a difference in particle contrast under TEM, an issue examined in detail in Part II.

Animals↗
PubMed1999

The Otto Aufranc Award. Wear and lubrication of metal-on-metal hip implants.

The implication of polyethylene wear particles as the dominant cause of periprosthetic osteolysis has created a resurgence of interest in metal-on-metal implants for total hip arthroplasty because of their potential for improved wear performance. Twenty-two cobalt chromium molybdenum metal-on-metal implants were custom-manufactured and tested in a hip simulator. Accelerated wear occurred within the first million cycles followed by a marked decrease in wear rate to low steady-state values. The volumetric wear at 3 million cycles was very small, ranging from 0.15 to 2.56 mm3 for all implants tested. Larger head-cup clearance and increased surface roughness were associated with increased wear. Independent effects on wear of material processing (wrought, cast) and carbon content were not identified. Implant wear decreased with increasing lambda ratio, a parameter used to relate lubricant film thickness to surface roughness, suggesting some degree of fluid film lubrication during testing. This study provided important insight into the design and engineering parameters that affect the wear behavior of metal-on-metal hip implants and indicated that high quality manufacturing can reproducibly lead to very low wear.

Analysis of Variance↗
PubMed1999

Temporomandibular Joint Arthroplasty Using Metal-on-Metal and Acrylic-on-Metal Configurations : Wear In Laboratory Tests and In Retrievals.

In the long-term performance ofthe temporomaridibular joint (TMJ) implant, wear must be considered. Thus retrieved and laboratory test implants were examined both optically and in a scanning electron microscope (SEM). In laboratory testing, the volumetric wear of metal-on-metal was about an order of magnitude less than that of acrylic-on-metal TMJ implants. This metal-on-metal wear was also about half of that reported in the literature for a laboratory test of polyethylene-on-metal TMJ implants, The retrieved TMJ implants showed some abrasive wear occurred during multi-directional articulation with smaller wear zones for the metal-on-metal compared to the acrylic-on-metal configuration. Further efforts to characterize and minimize wear were recommended as prudent in the continuing development of TMJ arthroplasty.

Journal Article↗
PubMed1997

Kinematics of the MATCO hip simulator and issues related to wear testing of metal-metal implants.

Metal-metal hip implants have been used clinically in Europe to reduce the risk of wear particle induced osteolysis. Joint simulator devices could provide useful information for design improvement of the modern generation of metal-metal hip implants. Early wear results for metal-metal hip specimens were obtained using a MATCO hip simulator. A detailed kinematic analysis was developed for the MATCO simulator and applied to two of the wear experiments to predict the starting surface motion, contact zone and lubricant film thickness. It was shown that points on cup surfaces were not subjected to a reciprocating interaction with the head during wear at the beginning of testing but as wear proceeded, it was suggested that, in some cases, reciprocating interaction did occur on the cup surface. Comparison between simulator and in vivo kinematics suggested a more realistic representation for cup than for head wear. In the simulator, the Hertzian contact zone moved in a circular path over the cup surface and changed in size in correspondence with the applied load. Elastohydrodynamic lubrication was considered to be possible in the simulator, with estimated fluid film thickness as great as 0.1 micron. However, such thick films were not likely to have occurred at the start of the two wear tests which were examined in detail, although some mixed film lubrication might have accounted for the relatively low wear of one of the specimens. The inclusion of kinematic details, contact mechanics and elastohydrodynamic lubrication analysis in simulator testing protocols and in design of metal metal hip implants was recommended.

Elasticity↗
PubMed1996

Comparison of alloys and designs in a hip simulator study of metal on metal implants.

Previous studies of metal on metal hip implants have shown that it is possible to obtain substantially lower volumetric wear than with metal on polyethylene pairings. To work toward design optimization, the gravimetric wear of serum lubricated, metal on metal implants was examined in a hip simulator apparatus. Seventeen implant specimens were fabricated from 3 alloys in various combinations of diameter and clearance and wear tested as many as 3 million cycles. Theory was developed to predict the thickness of elastohydrodynamic lubricant films, the linear wear, and wear zone geometry. To help interpret the data, implant diameter and clearance were combined into a single parameter called effective radius. Implants with larger effective radii (6-11 m) tended to experience lower wear, and theory suggested that this was a result of the correspondingly thicker elastohydrodynamic lubricant films providing an enhanced mixed film lubrication. As in most wear testing, the results were scattered but, overall, a low carbon, wrought alloy had lower wear than either a high carbon, cast alloy or a low carbon, commercial alloy. Because of the small number of specimens tested and possible variations in tribologic parameters such as calcium phosphate film formation and surface microgeometry, there was low statistical reliability of the results.

Alloys↗
PubMed1996

Engineering issues and wear performance of metal on metal hip implants.

A major concern in total hip arthroplasty is the generation of polyethylene wear particles at the articulating surfaces and resulting macrophage mediated periimplant osteolysis. There is renewed interest in metal on metal bearings as a solution to this problem in view of their potential for greatly improved wear performance. Using a commercially available hip simulator, the wear performance of metal on metal femoral head and acetabular cup combinations was evaluated and various parameters affecting metal on metal implant wear were identified. Nine implants custom manufactured from 2 medical grades of CoCrMo alloy (ASTM F1537-95 and F75-92) were tested within bovine serum as the lubricant to 3 million cycles (equivalent to approximately 3 years of service in vivo). The progressive wear of the components was determined by gravimetric methods at approximately every 300,000 cycles. The wear rates were characterized by an initial period of accelerated wear after which a lower steady state wear rate was observed for subsequent cycles. The presence of calcium phosphate films on the component surfaces, the microstructure of the lower carbon, wrought alloy, and increased effective radii (decreased diametral clearances) were identified as factors that may be favorable to improved wear performance. The extent of the effect on wear of each parameter, however, cannot be discerned at this point and necessitates a study in which parametric changes are more tightly controlled. The present study suggests that the use of metal on metal articulating surfaces may mitigate the problem of osteolysis by offering improved wear performance.

Hip Prosthesis↗
PubMed1990

Numerical [corrected] analysis of the load capacity of the human spine fitted with L-rod instrumentation.

L-rod procedures with sublaminar wiring have become widespread in orthopaedic practice, and serious mechanical problems involving fatigue failure, creep, and fixation have been reported. To address these problems, numerical [corrected] analysis was developed to provide estimates for the maximum bending stresses, approximately equal to maximum tensile stresses, in L-rods subject to various axial loads and postoperative Cobb angles. The superior load capacity of L-rods made from MP35N multiphase alloy compared with 316L stainless steel was demonstrated. Bending stresses in excess of the fatigue limit were predicted for eight patients with broken rods, suggesting that such failures might be prevented by prior analysis. This analytic approach provides a foundation for future studies of custom-designed L-rods and perhaps the successful use of L-rods without arthrodesis.

Alloys↗
PubMed1986

An experimental study of the buckling behavior of L-rod implants used in the surgical treatment of scoliosis.

A test facility was developed to study the buckling behavior of the scoliotic spine fitted with L-rod implants. The facility included a model, constructed from polymeric materials, that was similar in geometric detail and flexural stiffness to the ligamentous thoracolumbar spine with a severe scoliotic curvature. Nondestructive tests were performed in which the transverse deflection was recorded at various axial load levels. Southwell regression was used on the data to estimate the buckling loads. Attaching L-rods of 3.2-mm diameter did not increase the buckling load significantly, compared to the uninstrumented spine and thus were judged to be ineffective in halting the progression of a scoliotic curve. However, L-rods of 4.8-mm diameter increased the buckling load about four times and L-rods of 6.4 mm increased the buckling load about eight times.

Biomechanical Phenomena↗