Kenneth R. St. John

Evaluation of the wear performance of a polycarbonate-urethane acetabular component in a hip joint simulator and comparison with UHMWPE and cross-linked UHMWPE

Acetabular hip joint components manufactured from gamma-sterilized ultra high molecular weight polyethylene (UHMWPE), gamma cross-linked UHMWPE, or polycarbonate-urethane (PCU) polymers were evaluated in a hip joint simulator, using cobalt alloy femoral components, for at least 5 million cycles. The volume of material losses due to wear was calculated for each type of sample, based upon mass loss measurements, every 500,000 cycles. The loss of material for the conventional UHMWPE was much higher than for the cross-linked UHMWPE, showing about a 70% reduction in wear due to cross-linking. The material loss for the PCU samples appears to have been at least 24% lower than for the cross-linked UHMWPE. Based upon these results, the PCU material seems to have potential for use as an alternative bearing material to UHMWPE for total hip replacement surgeries.

Contributions of stress corrosion and cyclic fatigue to subcritical crack growth in a dental glass-ceramic

OBJECTIVE The objective of this study was to test the following hypotheses: (1) both cyclic degradation and stress-corrosion mechanisms result in subcritical crack growth (SCG) in a fluorapatite glass-ceramic (IPS e.max ZirPress, Ivoclar-Vivadent) and (2) there is an interactive effect of stress corrosion and cyclic fatigue to accelerate subcritical crack growth. METHODS Rectangular beam specimens were fabricated using the lost-wax process. Two groups of specimens (N=30/group) with polished (15μm) or air-abraded surface were tested under rapid monotonic loading. Additional polished specimens were subjected to cyclic loading at two frequencies, 2Hz (N=44) and 10Hz (N=36), and at various stress amplitudes. All tests were performed using a fully articulated four-point flexure fixture in deionized water at 37°C. The SCG parameters were determined using the ratio of inert strength Weibull modulus to lifetime Weibull modulus. A general log-linear model was fit to the fatigue lifetime data including time to failure, frequency, peak stress, and the product of frequency and logarithm of stress in ALTA PRO software. RESULTS SCG parameters determined were n=21.7 and A=4.99×10(-5) for 2Hz, and n=19.1 and A=7.39×10(-6) for 10Hz. After fitting the general log-linear model to cyclic fatigue data, the coefficients of the frequency term (α1), the stress term (α2), and the interaction term (α3) had estimates and 95% confidence intervals of α1=-3.16 (-15.1, 6.30), α2=-21.2 (-34.9, -9.73), and α3=0.820 (-1.59, 4.02). Only α2 was significantly different from zero. SIGNIFICANCE (1) Cyclic fatigue does not have a significant effect on SCG in the fluorapatite glass-ceramic evaluated and (2) there was no interactive effect between cyclic degradation and stress corrosion for this material.

Response of canine bone to a synthetic bone graft material

A model simulating a spiral diaphyseal fracture with butterfly fragments and bone loss was utilized to evaluate an hydroxyapatite/tricalcium phosphate, and collagen composite bone graft substitute in twelve dogs. The resultant grafted and contralateral control femora were tested in torsion at one year. This study examines the histological response to the graft material as well as crack propagation and fracture surface morphology using light microscopy and SEM. SEM and gross evaluation of the grafted bones revealed that 8/12 had fractured through bone outside the osteotomy site and all fractures included bone outside the graft site. No graft material was demonstrated at the points of initiation or termination of fracture for any of the bones. It was apparent that recorticalization had begun to occur at the graft site but the canal had not yet fully formed. The HA/TCP was seen to be tightly bound in tissue which had the appearance of new bone. Bone was found to be in direct apposition to the surface of the ceramic and within pores with no intervening soft tissue. Much of the new bone had remodeled into well organized Haversian systems with some patchy areas of woven bone and osteoid seen with polarized light illumination.

The Effect of Serum Protein Concentration on Wear Rates in a Hip Simulator

Ultra high molecular weight polyethylene cups, 22 mm in diameter, were aged for 5 years in the normal laboratory environment. Half of the samples had been processed by the standard radiation sterilization techniques, while the remainder had been cross-linked by a technique involving higher radiation doses and controlled temperature at the time of irradiation. The samples had been tested in a hip simulator for 5,000,000 cycles using a lubricant that had been diluted 1:1 with deionized water. Once that testing was completed, further testing was conducted using lubricant with greater and lesser serum protein concentrations, and the results compared with those that had already been recorded. Comparison of the wear rates within the study as well as to published data concerning the effect of serum concentration showed results that were consistent with assumed differences in lubrication ability at different concentrations. The results of other published studies were found to be inconsistent with each other and different from some of the results of this study. There is shown to be a need for carefully controlled and conducted studies to agree, if possible, on the importance of the serum concentration and the appropriate parameters to be used in testing, as well as variations that may be necessary with different bearing material characteristics.Ultra high molecular weight polyethylene cups, 22 mm in diameter, were aged for 5 years in the normal laboratory environment. Half of the samples had been processed by the standard radiation sterilization techniques, while the remainder had been cross-linked by a technique involving higher radiation doses and controlled temperature at the time of irradiation. The samples had been tested in a hip simulator for 5,000,000 cycles using a lubricant that had been diluted 1:1 with deionized water. Once that testing was completed, further testing was conducted using lubricant with greater and lesser serum protein concentrations, and the results compared with those that had already been recorded. Comparison of the wear rates within the study as well as to published data concerning the effect of serum concentration showed results that were consistent with assumed differences in lubrication ability at different concentrations. The results of other published studies were found to be inconsistent with each other and different from some of the results of this study. There is shown to be a need for carefully controlled and conducted studies to agree, if possible, on the importance of the serum concentration and the appropriate parameters to be used in testing, as well as variations that may be necessary with different bearing material characteristics.

Failure of Emperion modular femoral stem with implant analysis

Modularity in total hip arthroplasty provides multiple benefits to the surgeon in restoring the appropriate alignment and position to a previously damaged hip joint. The vast majority of modern implants incorporate modularity into their design with some implants having multiple modular interfaces. There is the potential for failure at modular junctions because of fretting and crevice corrosion in combination with mechanical loading. This case report details the failure of an Emperion (Smith and Nephew, Memphis, TN) femoral stem in a 67-year-old male patient 6 years after total hip replacement. Analysis of the implant revealed mechanically assisted crevice corrosion that likely accelerated fatigue crack initiation in the hip stem. The benefits of modularity come with the potential drawback of a combination of fretting and crevice corrosion at the modular junction, which may accelerate fatigue, crack initiation and ultimately reduce the hip longevity.

Fatigue loading and R-curve behavior of a dental glass-ceramic with multiple flaw distributions

OBJECTIVES To determine the effects of surface finish and mechanical loading on the rising toughness curve (R-curve) behavior of a fluorapatite glass-ceramic (IPS e.max ZirPress) and to determine a statistical model for fitting fatigue lifetime data with multiple flaw distributions. MATERIALS AND METHODS Rectangular beam specimens were fabricated by pressing. Two groups of specimens (n=30) with polished (15 μm) or air abraded surface were tested under rapid monotonic loading in oil. Additional polished specimens were subjected to cyclic loading at 2 Hz (n=44) and 10 Hz (n=36). All fatigue tests were performed using a fully articulated four-point flexure fixture in 37°C water. Fractography was used to determine the critical flaw size and estimate fracture toughness. To prove the presence of R-curve behavior, non-linear regression was used. Forward stepwise regression was performed to determine the effects on fracture toughness of different variables, such as initial flaw type, critical flaw size, critical flaw eccentricity, cycling frequency, peak load, and number of cycles. Fatigue lifetime data were fit to an exclusive flaw model. RESULTS There was an increase in fracture toughness values with increasing critical flaw size for both loading methods (rapid monotonic loading and fatigue). The values for the fracture toughness ranged from 0.75 to 1.1 MPam(1/2) reaching a plateau at different critical flaw sizes based on loading method. SIGNIFICANCE Cyclic loading had a significant effect on the R-curve behavior. The fatigue lifetime distribution was dependent on the flaw distribution, and it fit well to an exclusive flaw model.

The use of compliant layer prosthetic components in orthopedic joint repair and replacement: A review

The surgical repair or treatment of degenerative joint disease has traditionally involved the substitution of synthetic materials for one or both surfaces of the joint. Engineering thermoplastics, metals, and ceramics have either been widely accepted or experimentally evaluated for use as bearing surfaces in these prostheses. When engineering thermoplastics are used, the opposing surface is a metal or a ceramic, but metal-on-metal, metal-on-ceramic, and ceramic-on-ceramic have also been used or tested. Researchers have sought the opportunity to utilize materials with compressive mechanical properties more closely matching those of the natural articular cartilage. This review discusses the theory, testing, and application of elastomers for one bearing component of articular joint prostheses.

The use of compliant layer prosthetic components in orthopedic joint repair and replacement: A review: Compliant Layer Prosthetic Joints

The surgical repair or treatment of degenerative joint disease has traditionally involved the substitution of synthetic materials for one or both surfaces of the joint. Engineering thermoplastics, metals, and ceramics have either been widely accepted or experimentally evaluated for use as bearing surfaces in these prostheses. When engineering thermoplastics are used, the opposing surface is a metal or a ceramic, but metal-on-metal, metal-on-ceramic, and ceramic-on-ceramic have also been used or tested. Researchers have sought the opportunity to utilize materials with compressive mechanical properties more closely matching those of the natural articular cartilage. This review discusses the theory, testing, and application of elastomers for one bearing component of articular joint prostheses.