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J Thorac Cardiovasc Surg 1994;108:1037-1042
© 1994 Mosby, Inc.
SURGERY FOR ACQUIRED HEART DISEASE |
Leeds, United Kingdom
Supported by Heart Research, Leeds, United Kingdom
Received for publication Nov. 19, 1993. Accepted for publication Mar. 16, 1994. Address for reprints: Professor John Fisher, Department of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom.
Abstract
The biomechanics and function of fresh porcine aortic roots and valves have been compared with those of glutaraldehyde-treated roots prepared in a conventional manner without dilation of the elastic aortic root and with glutaraldehyde-treated roots prepared with permanent predilation during fixation. The glutaraldehyde-treated aortic walls were significantly less extensible than fresh walls, with a mean dilation of only 6% compared with 45% for the fresh root at 120 mm Hg pressure. Permanent predilation of the aortic root during fixation allowed the total dilation of the fixed root to be increased to 19% at 120 mm Hg pressure. The effective orifice area of the fresh root and valve was significantly greater than those of the fixed roots and valves, with permanent predilation fixation producing a significantly greater orifice area than conventional fixation. The open-leaflet bending deformations were found to be lower in the valves fixed after permanent predilation than in the standard fixed valves. The glutaraldehyde-treated porcine root and valve does not reproduce the biomechanics and function of the fresh root, because of the reduced extensibility of the fixed aortic wall. Permanent predilation during fixation helps to partially overcome the shortcomings of the fixed root, producing better function than a standard fixed valve. (J THORACCARDIOVASCSURG1994;108:1037-42)
The use of free-sewn glutaraldehyde-treated porcine aortic roots and valves is generating considerable interest. 
1,2 The expectation is that they may more closely reproduce the function of the natural aortic valve and the homograft aortic root and valve. 3 The potential advantages of the free-sewn bioprostheses over frame-mounted porcine bioprostheses include a larger effective orifice area and lower pressure loss, improved leaflet function and reduced open-leaflet bending deformations, and an elastic support for the leaflets, which may reduce dynamic stresses during leaflet closure. However, no detailed biomechanical studies of the function of glutaraldehyde-treated porcine roots and valves have appeared in the literature and only qualitative statements about the benefits of these devices have been made. 4
In laboratory studies of fresh homograft aortic roots, low pressure drops and very low open-leaflet bending deformations have been found when the elastic aortic roots have been dilated at physiologic pressures of between 80 and 120 mm Hg. 5,6 The dilation of homograft aortic roots was between 33% and 44% at these physiologic pressures, and this large degree of dilation was found to be necessary to achieve good leaflet function. In a recent study, fresh porcine aortic roots dilated by 47% at a pressure of 120 mm Hg, and this dilation produced a triangular leaflet configuration in the open position with very low open-leaflet bending deformations. 7 In frame-mounted devices, partial dilation of the elastic aortic root within the constraint of the frame has helped to reduce open-leaflet bending deformations, 7 cited as a cause of rapid calcification inexisting commercial devices. 8 These studies have indicated that the dilation of the elastic aortic root is an important determinant of leaflet function. It is therefore useful to investigate the magnitude of dilation of the elastic aortic root and its effect on valve function in free-sewn porcine bioprosthetic roots and valves.
This paper compares the biomechanics and hydrodynamic function of fresh porcine aortic roots and valves, glutaraldehyde-treated roots and valves prepared in a conventional manner without dilation of the aortic root during fixation, and glutaraldehyde-treated roots and valves prepared with permanent predilation of the aorta during fixation.
MATERIALS AND METHODS
Porcine aortic roots and valves with an initial external diameter of between 18 and 22 mm were used in this study. The roots and valves were dissected from the hearts, cleaned, stored at 4° C, and either fixed in glutaraldehyde or used as fresh tissue within 36 hours of being harvested. Three groups of aortic roots were studied. In group A, 13 valves were used as fresh roots. In group B, 9 roots were fixed with glutaraldehyde with minimal or no dilation of the aortic root and zero pressure difference across the leaflets during fixation. This fixation was achieved by connecting both sides of the valve and the root to a manometer and by filling the system with 0.5% buffered glutaraldehyde to a hydrostatic pressure of 3 mm Hg. A hydrostatic pressure of 3 mm Hg was required to form the shape of the root during fixation. Valves were fixed under these conditions for 2
hours with glutaraldehyde, then removed from the fixation apparatus, and stored in the glutaraldehyde solution for final fixation. In group C, eight aortic roots were fixed with glutaraldehyde with permanent predilation of the aortic root, by applying an equal hydrostatic pressure of 60 mm Hg to both sides of the valve leaflets to dilate the elastic aortic root during fixation. This hydrostatic pressure was applied by connecting both sides of the manometer system to a pressure reservoir at 60 mm Hg. As with group B, the valves were fixed for 2 hours under the hydrostatic pressure and then subsequently placed in a container of glutaraldehyde. Each group of roots and valves then was subjected to a series of biomechanical and hydrodynamic tests.
The external diameters of the elastic aortic root and valves were measured in their undilated state immediately after fixation (2 hours) and subsequently after 48 hours of storage. The change in diameter or dilation of the aortic root was then determined as a function of increasing internal pressure. Measurements of the external diameter of the aortic root were taken 5 mm above the commissures with vernier calipers at three positions around the aorta as the pressure was increased in increments of 20 mm Hg. The extensibility of the aortic wall was also determined in uniaxial tests. Circumferential strips with a gauge length of 10 mm and a width of 3 mm were cut from the aortic wall 5 to 10 mm above the commissures and the stress/ extension ratio curve was determined in a uniaxial test. Fifteen fresh strips for group A, 6 fixed strips for group B, and 12 fixed strips for group C were tested, and the mean stress/extension ratio curve for each group was calculated.
Seven valves from each group were studied in a pulsatile flow simulator. 5 The roots were mounted in the aortic position as described previously and tested at flow rates of 60, 72, 80, and 100 beats/min with a stroke volume of between 60 and 80 ml. The mean pressure drop during forward flow (
DP, millimeters of mercury) and the root mean square forward flow (Q, milliliters per second) were measured, and the effective orifice area (EOA) 5 was calculated by this formula:
In addition, a video camera was used to determine the configuration of the open valve leaflets. Either a standard camera was used, in which case approximately two thirds of the valve orifice was seen because of the obstruction caused by the curved aortic wall, or, alternatively, an angioscopic camera was used, which allowed the complete orifice to be imaged. The open-leaflet bending deformations were determined from the still photograph of the fully open position in mid-systole. A bending deformation index of zero indicated no bending, whereas an index of one indicated maximum bending or folding back of the leaflet on itself. 7 Because of the quality of the images and the curvature of the aorta, it was not possible to determine deformation index on all the valves in each group. Finally, the dimensions of the leaflets were determined with respect to the aortic root diameter by dissecting the leaflets from the root and measuring the circumferential leaflet length between the commissures for all three leaflets. This circumferential leaflet length (S) was divided by the internal radius of the aortic root (Rc). The ratio S/Rc has previously been correlated with high open-leaflet bending. 7 The statistical analysis was carried out by comparisons of the mean values of the parameters measured for each group of valves by means of Student's t test.
RESULTS
The average external diameters of the each group of aortic roots after fixation in glutaraldehyde are shown in Table I. No significant difference in the mean aortic diameter was noted between group A (fresh) and group B (fixed without dilation). Diameters in group C (fixed with permanent predilation) were 12.5% larger after fixation than in their fresh state. The mean diameter of the permanently predilated roots in group C was significantly greater (p < 0.05) than the mean diameter of the fresh roots in group A. Fig. 1 shows a photograph of a fixed root from group B and a root from group C in their relaxed conditions. The root from group C was permanently predilated; hence the leaflets were more open and the valve orifice contained less redundant material than in group B. The ratio of the circumferential leaflet length (S) to the internal radius of the aortic root (Rc, measured above the commissures) was 2.94 for group B and 2.55 for Group C. The smaller value of S/Rc for the valves in Group C was due to the increase in Rc by permanent predilation of the root during fixation.
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Table II shows the bending deformation indexes for the three groups of valves. The lowest occurred in the fresh roots, group A, with significantly greater deformation in the standard fixed root, group B (p < 0.05). Group C specimens, fixed with permanent predilation, had a significantly lower leaflet deformation index than the standard fixed specimens of group B (p < 0.05), but deformation indexes were still higher than for the fresh valves. This difference was consistent with the amount of aortic root dilation described in Fig. 4.
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Previous studies with homograft roots and valves 5,6 have shown that the percentage dilation of the aortic root with respect to its relaxed state is an important determinant of valve leaflet function. Dilation of 33% at a diastolic pressure set of 80 mm Hg determined the physiologic boundary conditions and geometry for the valve leaflet. This study has shown that similar amounts of dilation of the aortic root in the free-sewn porcine bioprosthetic valve result in improved hydrodynamic performance. The fresh porcine aortic root showed similar extensibility and dilation to the homograft root with dilations of 30% and 45% at diastolic and systolic pressures. This produced a triangular open-leaflet configuration with extremely low bending deformation in the open leaflet. The glutaraldehyde-treated porcine aortic roots prepared in a standard method were much stiffer and produced only 6.1% dilation of the root at 120 mm Hg internal pressure. As a result, the leaflets were redundant, obstructive to flow, and had quite large bending deformations in the open position. The effective orifice area was only slightly higher and the bending deformations slightly smaller than those found in the frame-mounted valves. 6,7 This inadequate function was entirely attributed to the stiffening of the elastic aortic wall during fixation with glutaraldehyde, which restricted the dilation of the aortic root as the internal pressure was increased to physiologic levels. In this situation much of the potential advantage of the free-sewn bioprosthetic root over the frame-mounted valves was not realized.
The problem associated with reduced elasticity of the porcine bioprosthetic root was partially overcome by permanent predilation of the root during fixation with glutaraldehyde in group C. This predilation produced a total of 17% dilation at a diastolic pressure of 80 mm Hg, which resulted in a significantly higher effective orifice area and lower open-leaflet bending deformations than the standard fixed valve, group B. However, the roots fixed by permanent predilation did not reproduce fully the function of the fresh root and valve. In this study we chose to predilate the aortic root during fixation with 60 mm Hg internal pressure. This pressure was not an arbitrary choice, because pilot studies had also been carried out at 20 and 40 mm Hg internal pressure, and these showed less dilation on individual valves. Fixation at 60 mm Hg was selected because we believed further elastic deformation would occur as pressure was increased to diastolic pressures. In practice, the initial dilation at fixation relaxed somewhat from 19% back to 12.5% during storage. This dilation was then recovered as elastic deformation as internal pressure was applied. The results of this study are consistent with previous qualitative observations of free-sewn porcine bioprostheses with predilated roots. 7 In a previous design study, we have also emphasized the need to dilate the elastic aortic root before mounting in frame-mounted valves. In this study we recommend that dilation of at least 17% at the commissures be produced. This was similar to the predilation achieved in the group C valves in this study. It may well be that predilation of the elastic root at higher internal pressures will produce greater dilation of the fixed root and a further improvement in function may be achieved. In this study, we produced predilation of the root by applying an equal hydrostatic pressure to both sides of the valve leaflets during fixation. It is also possible to achieve predilation of the root by mechanically holding the elastic root in its appropriate dilated state and then applying a very low or zero pressure of fixative to the root and leaflets. This method has been used for the preparation of frame-mounted valves with the predilated roots. 7 Both methods can ensure zero pressure difference across the leaflets and a predetermined amount of dilation of the fixed aortic root, and both may be used for free-sewn devices.
Whole aortic roots were selected for this study to avoid variability associated with suturing free-sewn valves. The function of the free-sewn or miniroot-implanted valve within the host root will be dependent on the suture technique, the sizing protocol, and the subsequent compliance of the composite bioprosthesis/host tissue structure. Dilation of this composite structure is likely to be an important determinant of leaflet function in this configuration and will be the focus of a future study.
CONCLUSIONS
This study has shown that the extensibility and dilation of the fixed porcine root was significantly less than that of the fresh root. This reduced dilation increased the pressure drop and the level of open-leaflet bending deformations in the fixed root. Development of a new fixation method that produced permanent predilation of the glutaraldehyde-treated aortic root helped to increase the orifice area of the valve and reduced the open-leaflet bending deformation in the fixed root. However, the permanently predilated aortic root and valves showed biomechanical performance inferior to that of the fresh porcine roots and the previously studied fresh homograft roots.
References
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