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It has been suggested that the mechanical durability of glutaraldehyde-treated porcine aortic valve (PAV) bioprostheses (BPs) may be increased if collagen crimp approximating that observed in native valvular tissue is maintained during preimplantation processing. Among the factors affecting PAV fixation, the magnitude of the pressure applied to the aortic valve during fixation influences the extent of collagen crimp. Polarized light microscopy is useful for both the qualitative assessment of collagen morphology and the quantitative measurement of collagen crimp in BPs. The purpose of this study was to determine the feasibility of using this optical method to detect quantitative differences in PAV collagen crimp following zero-, low-, and high-pressure fixation. The results of this study demonstrate the following: 1) coupling polarized light microscopy and morphometric measurements provides a quantitative method for the evaluation of the relationship between fixation pressure and collagen crimp in PAVs; 2) a significant association between increasing fixation pressure and absence of collagen crimp was observed in cords and the mid-cusp region; 3) collagen crimp length in the cords is affected by fixation pressure; 4) the alteration of collagen crimp by low- and high-pressure fixation does not uniformly affect all regions of the cusp; and, 5) collagen crimp was consistently present in all cusp regions in zero-pressure fixed PAVs.

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A new morphologic method is described for the simultaneous quantitation of porcine aortic valve collagen crimp length and the assessment of biomechanical properties. This method utilizes the simultaneous real-time video recording of collagen crimp morphology and acquisition of crimp length data through the combination of polarized light microscopy and morphometry. We felt that the development of this method was warranted, due to the fundamental role played by collagen in porcine aortic valve performance. The development of this method involved the design and fabrication of a uniaxial microtensile stage, suitable for mounting on a standard microscope stage. The validation of our test method was accomplished by a comparison of untreated and glutaraldehydetreated porcine aortic valve leaflet tissue, because the biomechanical and morphologic characteristics of the native and fixed aortic valve have been extensively studied. The method described in this communication enables the collection of morphologic and biomechanical data from a single tissue specimen, eliminating the need for independent studies of multiple specimens. Furthermore, this method obviates the need for making assumptions, which may be difficult to verify, concerning the homogeneity of different test specimens with respect to their morphology and corresponding mechanical response to different experimental conditions.

This study describes an application of the life table method for examining the risk of death from exposure to low-level ionizing radiation. The method provides estimates of the expectation of premature death and the resultant life shortening. Applications to occupational exposure situations are used to demonstrate the technique. Doses ranging from 0.2 to 5.0 rem/ yr and over age spans from 10 yr to working lifetimes are investigated. A technique for comparing radiation related mortality with nonradiation related risk of death from on-the-job accidents is introduced. Results show that workers receiving whole body doses equal 5 rem/yr for sustained periods of time incur significant risk of premature death from radiation induced cancer. The risk is estimated, in some cases, to be as high as, or higher than, the average risk of accidental death in high risk industries, depending upon factors such as length of exposure, age at time of exposure, and the radiation risk model used.