Exoplanet detection. A terrestrial planet in a ~1-AU orbit around one member of a ~15-AU binary.

Using gravitational microlensing, we detected a cold terrestrial planet orbiting one member of a binary star system. The planet has low mass (twice Earth's) and lies projected at ~0.8 astronomical units (AU) from its host star, about the distance between Earth and the Sun. However, the planet's temperature is much lower, <60 Kelvin, because the host star is only 0.10 to 0.15 solar masses and therefore more than 400 times less luminous than the Sun. The host itself orbits a slightly more massive companion with projected separation of 10 to 15 AU. This detection is consistent with such systems being very common. Straightforward modification of current microlensing search strategies could increase sensitivity to planets in binary systems. With more detections, such binary-star planetary systems could constrain models of planet formation and evolution.

Discovery of a Jupiter/Saturn analog with gravitational microlensing.

Searches for extrasolar planets have uncovered an astonishing diversity of planetary systems, yet the frequency of solar system analogs remains unknown. The gravitational microlensing planet search method is potentially sensitive to multiple-planet systems containing analogs of all the solar system planets except Mercury. We report the detection of a multiple-planet system with microlensing. We identify two planets with masses of approximately 0.71 and approximately 0.27 times the mass of Jupiter and orbital separations of approximately 2.3 and approximately 4.6 astronomical units orbiting a primary star of mass approximately 0.50 solar mass at a distance of approximately 1.5 kiloparsecs. This system resembles a scaled version of our solar system in that the mass ratio, separation ratio, and equilibrium temperatures of the planets are similar to those of Jupiter and Saturn. These planets could not have been detected with other techniques; their discovery from only six confirmed microlensing planet detections suggests that solar system analogs may be common.

Fatigue-induced changes to the biaxial mechanical properties of glutaraldehyde-fixed porcine aortic valve leaflets.

Circumferential extension is a direct measure of the preservation of functional collagen crimp in the fibrosal layer of aortic valve leaflets. The aim of this study was to determine whether the elastic properties of zero-pressure, glutaraldehyde-fixed leaflets are changed by mechanical fatigue. Nine Medtronic Freestyle bioprostheses were subjected to 200x10(6) cycles of accelerated fatigue and then biaxially tested to quantify the elastic properties of the leaflets. At physiological load (60 Nm(-1)) the radial extensibility was approximately halved relative to controls (P<10(-4)); there were also lesser reductions in the circumferential extensions (P<.01). The pulsatile regurgitant volume showed no change relative to the control leaflets. The natural corrugations of the fibrosal layer were flattened by the fatigue cycling, but this was not related to an increase in the radial size of the leaflets. Valve competency was maintained.

Age-dependent changes in the radial stretch of human aortic valve leaflets determined by biaxial testing.

The maximum radial elastic extensibility (stretch) of human aortic valve leaflets was measured as a function of the donor age using biaxial testing techniques. The 36 leaflets tested were removed from 18 unimplanted cryopreserved aortic allograft valves retrieved from donors aged 15 to 58 years. The stretch was measured at a traction of 60 Nm-1 and analyzed as a function of the age of the donor. It was found that at 15 years, the radial stretch is about 80% but declines rapidly (linearized rate of 4% per year) to be 40% by the age of 25 years. The stretch stays approximately constant until the age of 40 years and then undergoes a steady decline at the rate of about 1% per year at least until the age of 58 years. These results demonstrate that stretch, which is important for the maintenance of adequate coaptation area, is not permanent but may be lost at a relatively rapid rate for reasons that are not yet understood. Stretch loss in the leaflets also must be a contributing factor to the development of aortic incompetence. The probability of postoperative stretch loss needs to be allowed for in sizing protocols for aortic allografts.

Biaxial mechanical properties of explanted aortic allograft leaflets.

The leaflets of 33 aortic allograft valves inserted using the freehand method were retrieved at reoperation, and the mechanical properties of the tissue were measured using biaxial testing methodology. Before insertion, the valves had been sterilized for 24 hours in either PSKA or CLPVA antibiotic solutions and then were either wet stored or cryopreserved. The cryopreserved valves were sterilized with CLPVA only, so there were three different treatment types in all. The controls were a set of unimplanted cryopreserved aortic valves. The radial stretch of the valve leaflets was measured and was analyzed as a function of the age of the donor in the case of the controls and the age of the donor plus the duration of implantation for the explanted valves. The biaxial tension test was used to determine the maximum radial stretch of each leaflet because this variable dictates the area of coaptation and, ultimately, valve competency. It was found that the radial stretch of the allograft leaflets after implantation declined faster with time than did the stretch of the native aortic valve leaflet controls. This greater rate of stretch reduction depended mostly on the degree of similarity between the stretch of the donor leaflets and that of the recipient at the time of implantation. No conclusion could be reached about the influence of the preimplantation storage method because of the different time intervals of implantation applicable to each case. After matching for annulus size, matching the ages of donor and recipient is a good graft-selection strategy, but it may be better to use a slightly older donor.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanical properties of porcine pulmonary valve leaflets: how do they differ from aortic leaflets?

This study measured the biaxial properties of pulmonary and aortic leaflets in extension in the fresh state and then in the same samples after fixation with glutaraldehyde. The results showed that when fresh, the valves had a similar response to load in the circumferential direction, but the pulmonary leaflets were more extensible in the radial direction. They were also less stiff. Fixation decreased the tissue extensibility and increased the stiffness of the pulmonary leaflets much less than in the aortic leaflets. This was interpreted to mean that the collagen content of the pulmonary leaflets is significantly less than that in the aortic leaflets. Reduced collagen content would be expected to enhance hemodynamic performance because of increased leaflet stretch and reduced stiffness. However, lower collagen levels may reduce implant durability.

A comparison between glutaraldehyde and diepoxide-fixed stentless porcine aortic valves: biochemical and mechanical characterization and resistance to mineralization.

As previously reported, we found that the fixation rate and thermal denaturation (shrink) temperature of the diepoxide-fixed tissue could be controlled by varying the concentration of the fixative as well as by adding alcohol and/or sodium chloride to the solution. In contrast to prior experience, however, we now found that the epoxide-fixed leaflets exhibited significantly higher resistance to mineralization compared to controls, but only when the tissue had not been exposed to phosphate ion.

What is the best bioprosthetic operation for the small aortic root?: allograft, autograft, porcine, pericardial? Stented or unstented?

Durability is assessed with regard to valve position, patient age, and the techniques of graft preparation for each of the tissue valves. Design affects both durability and the effective orifice area. It is assessed for each of the available devices, with particular emphasis on the stentless porcine valve. The effect that differences between this glutaraldehyde fixed device and the allograft valve may have on techniques of implantation is analyzed. On the basis of this information, an attempt is made to grade the currently available tissue valves with a satisfactory intermediate-term performance for use in the small aortic root.

A non-invasive technique to quantify in vitro bioprosthetic valvular coaptation.

The functional relationship between valvular leaflet stress and coaptive geometry may provide insight into a bioprosthetic heart valve's ability to maintain coaptation under a broad range of physiologic operating conditions. Previous investigators have only indirectly measured coaptation geometry. A non-invasive in vitro technique is demonstrated to directly quantify the coaptive geometry of bioprosthetic heart valves. A discussion is provided detailing the use of this information in a finite element model to determine the functional relationship between valvular leaflet stress and coaptation geometry. Video images of the stent post deflections and coaptive geometry for size 21 mm, 23 mm, and 25 mm Hancock Aortic Pericardial heart valves are obtained based on back light illumination of each valve's coaptive area at discrete back pressures. The resulting images are analyzed using image processing and motion analysis techniques. Results indicate that at back pressures greater than 50-75 mmHg the coaptive area remains constant. There is a decrease in coaptive depth at the valve's periphery, whereas the point of maximum coaptation maintains position approximately midway along the free margin. Good agreement is found between the experimentally determined and finite element model's predicted coaptation geometry.

Anatomy of aortic heart valve leaflets: the influence of glutaraldehyde fixation on function.

The basic function of the aortic valve is reviewed from an engineering perspective. Critical examination of the leaflet morphology can yield insights into how the leaflets function and transmit load to the aortic wall. From an understanding of both the structure and the function it is possible to estimate the impact of stent mounting and glutaraldehyde fixation. It is shown from simple engineering considerations that the major stresses are in the circumferential direction and that the radial components must be very small. Similarly, the largest strains are radial to facilitate the formation of a large coaptation area, while the circumferential strains are explained by the extension to the crimped collagen fibres. Glutaraldehyde fixation can greatly modify the mechanics of the leaflets but this can be minimized by fixation with no pressure differential across the closed valve. Zero-pressure-fixed leaflets are much softer and extensible than those from valves fixed under even low back-pressure (2-4 mmHg). This difference translates into a mode of valve function that more closely approximates that of the native aortic valve.

The stentless bioprosthesis: surgical challenges and implications for long-term durability.

An attempt is made to analyse the factors which are expected to influence clinical results following implantation of a stentless porcine bioprosthesis. Long experience with implantation of allograft aortic valves provides a meaningful basis for comparison with a glutaraldehyde-fixed device. Morphological differences between the two valves involve the aortic wall and muscle shelf, and differences in valve preparation include the strength and stiffness of the aorta and the extensibility of the valve leaflets. As a result of these differences, sizing of the porcine valve is expected to be more critical than the allograft valve and the porcine valve is also expected to be more obstructive. Methods for reducing the obstructive element include the use of a composite aortic valve, a porcine pulmonary valve, or a valve in which the aorta is glutaraldehyde-fixed under pressure while the leaflets are pressure-free. The techniques available for implantation, namely freehand insertion, total root replacement and mini-root replacement, are examined.

Computer modelling of bioprosthetic heart valves.

The underlying assumptions and principles of a computer-based model for tissue heart valves are described. The model is used to relate observations of leaflet morphology to the requirements of proper valve function. Stress is the fundamental mechanical factor that limits the longevity of bioprosthetic heart valves--the higher the stress levels in the leaflets, the shorter will be the time over which the leaflets can maintain satisfactory structural integrity. Direct measurement of stress is impractical with real heart valves, but the calculation of stress and the study of its interdependence on other key parameters of tissue valve design is a good alternative. The methodology of computer modelling of heart valves is discussed and the technique is illustrated with some examples. Models of pericardial valves, the aortic allograft and porcine bioprostheses are compared.

Identification of a failure mode of the antibiotic sterilized aortic allograft after 10 years: implications for their long-term survival.

Freehand aortic allograft valves that had been previously sterilized with antibiotics were recovered at reoperation for biomechanical testing. The elastic extensibility of two leaflets from each explanted allograft stored in modified Hank's solution at 4 degrees C were measured using biaxial testing techniques. It was found that the leaflets had lost a significant amount of the natural, preimplant tissue extensibility. Computer simulation was used to investigate the likely consequences of such a profound change in the biomechanical characteristics of the leaflets on whole valve function. We found that the valve becomes progressively more incompetent as leaflet extensibility is lost and the stress levels at the commissures rise sharply with reduced coaptation. The stress levels in the load-bearing leaflet center are increased in the ventricularis as load is transferred from the fibrosa. The measured change in the radial extensibility of the aortic allograft leaflet needs to be allowed for when sizing the graft at insertion. Inserting the largest possible graft will maximize the time to the onset of central incompetence and improve valvar longevity by lowering the stress during diastole.

On stress reduction in bioprosthetic heart valve leaflets by the use of a flexible stent.

It has been postulated that flexible stent posts can reduce tensile stress at the commissures of tissue heart valves by about 90% when compared with the same valve mounted on a rigid stent. We have used a detailed computer model to investigate the role of flexible stent posts in reducing stress in the leaflets of three types of bioprosthetic heart valves: the bovine pericardial and the high- and zero-pressure fixed porcine valves. The models use stress/strain data from biaxial experiments to characterize the tissue properties and are subjected to a back pressure of 120 mmHg. We found that strain was reduced linearly with stent post deflection and that this was a purely static process--it did not require the load to be applied impulsively. This finding was in close agreement with earlier experimental studies, which measured the same strain reduction whether the valve was loaded quasi-statically or at physiological rates. In addition, we found that for this mechanism to be effective the valve must have good coaptation at the center and the tissue should be stiff; in other cases, the advantages of strain reduction through the use of a flexible stent are considerably diminished.

An assessment of the mechanical properties of leaflets from four second-generation porcine bioprostheses with biaxial testing techniques.

The aim of this study was to determine whether second-generation porcine bioprostheses, glutaraldehyde fixed at pressures said to be less than 4 mm Hg, exhibit more natural leaflet material properties than earlier valves fixed at 80 to 100 mm Hg. Biaxial mechanical testing techniques were used to compare Carpentier-Edwards SAV, St. Jude Medical BioImplant, Hancock II, and Medtronic Intact bioprostheses (12 leaflets from four valves in each case) with fresh porcine aortic valves and high pressure-fixed Carpentier-Edwards 6625 bioprostheses (14 leaflets from five valves in each case). The circumferential extensibility of leaflets from Medtronic Intact bioprostheses and from fresh porcine aortic valves were not significantly different (p greater than 0.05), whereas leaflets from the other second-generation valves tested and from Carpentier-Edwards 6625 valves were highly inextensible in the circumferential direction. The radial material properties of leaflets from all bioprostheses differed from those of fresh porcine aortic valves, which were very extensible with a high pretransitional compliance. The radial extensibility and compliance of Hancock II, St. Jude Medical BioImplant, and Carpentier-Edwards 6625 leaflets were not significantly different (p greater than 0.05). In the radial direction, Carpentier-Edwards SAV and Medtronic Intact valve leaflets were substantially more extensible than Carpentier-Edwards 6625 leaflets (p less than 0.01), whereas Medtronic Intact leaflets were more compliant than all other bioprostheses. These data demonstrate (1) that second-generation porcine bioprosthetic valves do not necessarily exhibit more natural leaflet material properties than earlier high pressure-fixed xenografts and (2) that Medtronic Intact valve leaflets have material properties most closely approximating the fresh porcine aortic valve.

Graft detachment, a cause of incompetence in stent-mounted aortic valve allografts.

The detachment of allograft tissue from supporting stent posts has been a common mode of failure of stent-mounted aortic allografts. In an effort to reduce the localized stress loading on the tissue at the top of the stent posts, two changes were introduced to the previous fabrication protocol followed by Green Lane Hospital. Specifically, they were the use of a flexible acetyl-copolymer stent and covering of the peripheral attachment sutures with a bias strip of Dacron cloth. This study showed that these changes did not reduce the incidence of allograft failure caused by graft detachment. Unexpectedly, covering antibiotic-treated allograft tissue with a bias strip actually increased the likelihood of failure of the valve, by accelerating the biological degradation of the underlying graft aortic wall. The weakened aortic wall tissue was consequently less able to resist the high stress loadings at the top of the stent posts where detachment first occurred.