Micromechanics and mathematical modeling: an inside look at bioprosthetic valve function

In vitro testing of bioprostheses: influence of mechanical stresses and lipids on calcification

BACKGROUND

Structural valve deterioration of bioprostheses is mainly caused by the progressive development of calcification. Mechanical stresses or lipid deposits in porcine aortic leaflets have been proposed as major factors contributing to the calcification process.

METHODS

A new test protocol consisting of nondestructive holographic interferometry, which allows a quantitative deformation analysis of heart valves, and accelerated dynamic in vitro calcification was used. The rapid calcification fluid contained a final combined calcium and phosphorus concentration of 130 (mg/dL)2 in barbital buffer solution. The calcification of 32 bioprostheses donated by different manufacturers (SJM Bioimplant, Biocor standard, Biocor No-React, Carpentier-Edwards SAV, Bravo, pericardial prototype) was assessed after up to 25 x 10(6) cycles by microradiography and the areas of calcification were compared with the holographic interferograms. The distribution of lipid droplets of four porcine prostheses were visualized by Sudan III stain before the calcification process.

RESULTS

Most of the tested bioprostheses had areas presenting with stress concentrations, and the dynamic in vitro testing resulted in leaflet calcification corresponding to the holographic irregularities. A strong correlation between calcification and stress distribution or lipid accumulation was found (r = 0.72; r = 0.81, respectively). After 19 x 10(6) cycles, the Carpentier-Edwards SAV and the pericardial valves had significantly less calcification than other prostheses tested (p = 0.003), but the variation among individual prostheses from the same manufacturer was even more pronounced.

CONCLUSIONS

Mechanical stresses or lipid accumulation seems to play an important role in the calcification process of bioprostheses. Quality control of bioprosthetic valves using holographic interferometry has the potential to predict calcification before implantation.

Implantation of the Toronto SPV stentless porcine bioprosthesis in dilated ascending aorta

In the presence of severe dilatation of the ascending aorta the implantation of a Toronto SPV stentless bioprosthesis is compromised by the risk of postoperative central regurgitation. A modification of the implantation technique is described that restores the normal shape of the ascending aorta and thereby avoids the risk of dysfunction of the prosthesis.

Dynamic glutaraldehyde fixation of a porcine aortic valve xenograft. I. Effect of fixation conditions on the final tissue viscoelastic properties

Sixty porcine aortic valves were fixed under dynamic conditions at specific durations, pressures and vibration rates in a 0.5% glutaraldehyde phosphate buffer (pH 7.4, 0.2 M). Tensile relaxation tests were performed at low through high extension rates (0.3, 3 and 30 mm s-1) and tissue denaturation temperatures were determined by the hydrothermal isometric tension method. Conventional statically fixed valves and fresh valves were used as controls. No differences between dynamic and static treatment were observed at pulsation rates above those expected in the physiological range (i.e. above 1.2 Hz) or at higher pressures such as 30 mmHg. However, differences in both stress relaxation rates and denaturation temperatures were delineated in milder fixation conditions, i.e. at low pressures (< 4 mmHg) and low vibration rates similar to that of the normal heart beat (approximately 1.2 Hz). In these conditions the relaxation rate of the dynamically fixed tissue (-7.4 +/- 0.7% of stress remaining per log(s)) was similar to that of the fresh tissue (-6.7 +/- 1.2% log(s-1)) and significantly higher than the statically treated tissue (-3.9 +/- 1.7% log(s-1)). The rates of stress relaxation appeared to be strain rate dependent in both radial and circumferential directions when the tissues were strained at physiological rates during testing (> approximately 15000% min-1). Dynamically treated valves showed higher denaturation temperatures (mean +/- SD) (89.4 +/- 0.5 degree C) compared with the statically fixed (82.7 +/- 1.4 degrees C) or untreated (fresh) valves (65.5 +/- 0.8 degree C). The results suggest a higher degree of internal cross-linking owing possibly to enhanced penetration of the glutaraldehyde reagent and a greater accessability of reactive cross-linking sites on the collagen molecules. Better stress relaxation rates are likely associated with an increase in potential shearing between adjacent collagen fibres thus preserving the natural stress-reducing mechanism of the fresh, untreated valves. The dynamically treated valves therefore possess characteristics that may enable them to better resist long-term mechanical fatigue and in vivo degradation.

Comparison of low-pressure versus standard-pressure fixation Carpentier-Edwards bioprosthesis

Intermediate-phase clinical results of 51 low-pressure (LP) and 234 standard-pressure (SP) fixation porcine Carpentier-Edwards (CE) valves implanted between 1977 and 1991 were compared for valve-related events. Group similarities included New York Heart Association functional class, ejection fraction, and sex. Patients with SP valves were younger (mean age, 58 versus 68 years; p = 0.0001). There were 20 in-hospital deaths (8.6%) in the SP valve group and 5 (9.8%) in the LP valve group (p = 0.79). Follow-up was 99%, with a mean of 104 months in the SP valve group versus 55 months in the SP valve group (p = 0.0001). The actuarial survival rate was 48.2% and 22.3% at 10 and 15 years, respectively, in the SP valve group and 34.1% at 10 years in the LP valve group (p = 0.42). Freedom from events at 5, 10, and 15 years in the SP valve group and at 5 years in the LP valve group was as follows: for late valve-related events, 86.3%, 51.4% and 20.2%, respectively, in the SP valve group versus 85% in the LP valve group (p = 0.44); for valve-related death, 96.4%, 93.6%, and 87.3% in the SP valve group versus 100% in the LP valve group (p = 0.20); for structural valve failure, 96%, 68%, and 35% in the SP valve group versus 100% in the LP valve group (p = 0.09); and for reoperation, 95%, 61%, and 30% in the SP valve group versus 92% in the LP valve group (p = 0.82). In conclusion, this study revealed no significant statistical difference between LP and SP valves. In the LP valve group, structural valve failure/valve-related death was not observed, perhaps indicating a more favorable result. Absolute verification of this trend awaits long-term follow-up.

Pathology of substitute heart valves: new concepts and developments

All types of contemporary cardiac valve substitutes suffer deficiencies and complications that limit their success. Mechanical and bioprosthetic valves are intrinsically obstructive, especially in small sizes. Mechanical valves are associated with thromboembolic problems; the chronic anticoagulation used in virtually all mechanical valve recipients causes hemorrhage in some. Calcification limits the success of porcine and pericardial bioprostheses, allograft valves, and the yet experimental trileaflet polymeric prostheses. The predominant mechanism of calcification in porcine, pericardial, and allograft valves is cell mediated, being nucleated at the membranes and in organelles of the transplanted cells. In polymeric leaflet valves, calcification is both extrinsic (in adherent thrombus) and intrinsic (subsurface and acellular in the solid elastomer). Nevertheless, except for a few notable exceptions, contemporary mechanical valves are durable. Other important potential complications of prosthetic and bioprosthetic valves include paravalvular leak, endocarditis, or extrinsic interference with function. Moreover, aortic valvular allografts undergo progressive noncalcific degeneration, tearing, sagging, and/or retraction. Studies of retrieved long-term cryopreserved allograft explants demonstrate severe degeneration, with distortion of normal architectural detail, loss of endothelial and deep connective tissue cells, and variable inflammatory cellularity. Thus, they are morphologically nonviable valves, whose structural basis for function seems primarily related to the largely preserved collagen, and they are unlikely to have the capacity to grow, remodel, or exhibit active metabolic functions. Since calcification intrinsic to the cusps is the major pathologic process necessitating bioprosthetic valve reoperations, efforts to prevent formation of mineral deposits are active.(ABSTRACT TRUNCATED AT 250 WORDS)

Numerical simulation of leaflet flexure in bioprosthetic valves mounted on rigid and expansile stents

Recent studies suggest that flexural stresses induced during the opening phase may be responsible for much of the mechanical failures of bioprosthetic heart valves. Sharp leaflet bending is promoted by the mounting of valves on rigid stents that do not mimic the systolic expansion of the natural aortic root. We, therefore, hypothesized that flexural stresses could be significantly reduced by incorporating a flexible or expansile supporting stent into the valve design. Using our own non-linear finite element code (INDAP) and the pre- and post-processor modules of a commercial finite element package (PATRAN), we simulated the opening and closing behaviour a trileaflet bovine pericardial valve. The leaflets of this valve were assumed to be of uniform thickness, with a non-linear elastic behaviour adapted from experimentally obtained bending stiffness data. Our simulations have shown that during maximal systolic valve opening, sharp curvatures are induced in the leaflets near their commissural attachment to the supporting stent. These areas of sharp flexure experience compressive stresses of similar magnitude to the tensile stresses induced in the leaflets during valve closure. By incorporating a stent with posts that pivot about their base, such that a 10% expansion at the commissures is realized, we were able to reduce the compressive commissural stressing from 250 to 150 kPa. This was a reduction of 40%. Conversely, a simple pliable stent with stent posts that deflect inward and outward under load did not achieve a significant reduction of compressive stresses. This numerical analysis, therefore, supports the theory that (i) high flexural and compressive stresses exist at sites of sharp leaflet bending and may promote bioprosthetic valve failure, and (ii) that proper design of the supporting stent can significantly reduce such flexural stresses.