Porous coated surfaces for fixation of total hip replacement are a current trend in clinical orthopedics. Such devices are designed to be fixed by ingrowth of bony tissue, although in the absence of FDA approval for biologic fixation, fixation with PMMA cement is recommended by the implant manufacturers. In order to characterize the mechanical properties of the micro-interlocked stem-cement interface, we tested both porous coated and smooth femoral components in cement mantles of consistent overall geometry. Under conditions of increasing load the smooth stems demonstrated stepwise irreversible subsidence into the mantle. Axial and circumferential strains measured in the cement containment vessels with the smooth stems showed that stepwise increases in tensile hoop strain occurred concomitantly with the stepwise incidents of stem subsidence. When subjected to the same loading conditions, the porous coated stems did not undergo stepwise incidents of subsidence, and hoop strain generation was reduced. In addition, a twofold increase in the failure load of the stem-cement interface was measured with the porous coated stems. Fatigue loading for 10(7) loading cycles did not result in gross failure of either the micro-interlocked or smooth interfaces. However, the data showed that during fatigue loading, stepwise subsidence of the smooth stems again occurred. The final subsidence magnitude of the smooth stem-cement interface at 10(7) loading cycles was six times greater than the value associated with the porous coated stem. Thus the porous coating of femoral stems was shown to dramatically improve the load carrying capability and fatigue characteristics of the stem-cement interface.
To aid in understanding loosening following cemented total hip arthroplasty, we conducted a cadaver study of the proximal femur with implanted cobalt-chromium and titanium femoral components of recent design, loaded through the head of the prosthesis. Stem subsidence and strain in the proximal femur were measured. After proximal support of the implant collar was removed, we found that cobalt-chromium implants had a greater tendency to subside than titanium implants. Subsidence of a femoral component within the cement mantle caused an increase in tensile hoop strain measured in the proximal cortex. When implants were loaded until failure of the cortex, a direct relationship between increase in subsidence and increase in cortical hoop strain was demonstrated. Our data show that implants that resist subsidence into the cement mantle tend to decrease hoop strains in the proximal femoral cortex.
orman Daniels' proposal to distribute health care on the basis of fair equality of opportunity, is, in this writer's opinion, unworkable. His concepts of species-typical activity and normal opportunity range are unclear; so is the relationship between them. His view that justice accords disease a better claim on the health dollar than other causes of death, pain, and disability, commits him unknowingly to indefensible positions on particular sorts of health care, such as the care of the aging and of pregnant women. Daniels' concept of opportunity is so inclusive, his notion of balancing opportunities so vague, that his theory loses systematic power. I offer a different account from Daniels' concerning why health care needs are objective and of special importance. I also argue for a voucher system which levels out class inequalities and which finances current medical practices more or less uncritically, but allows for change through a diversity of insurance plans available to consumers. This system is just, and more practical than rating health care needs by impact on opportunity.
The canine larynx was studied to obtain qualitative and quantitative data accurately defining the destructive effects of the carbon dioxide laser on epithelial tissue by administering doses of varied energy to the dorsal surface of the vocal cords. Resultant lesions subsequently were analyzed by light, transmission, and scanning microscopy. Three major morphologic patterns of vocal cord lesions were observed: 1. cup shape, 2. doughnut shape and, 3. complex star shape. The patterns produced in the laryngeal tissue reflect the operative mode (energy distribution within the focused spot of energy) of the laser at the time of surgery. The mode resulting in the doughnut shaped pattern (energy distribution around the periphery of the spot) produced deepest lesions, with minimal thermal conduction laterally, whereas the mode resulting in the cup‐shaped pattern (energy distribution uniform throughout the spot) produced at comparable doses shallower lesions with larger diameter. Too few star‐shaped patterns were observed to permit meaningful morphometric analysis.
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