Dynamic modelling of prosthetic chorded mitral valves using the immersed boundary method.

Current artificial heart valves either have limited lifespan or require the recipient to be on permanent anticoagulation therapy. In this paper, effort is made to assess a newly developed bileaflet valve prosthesis made of synthetic flexible leaflet materials, whose geometry and material properties are based on those of the native mitral valve, with a view to providing superior options for mitral valve replacement. Computational analysis is employed to evaluate the geometric and material design of the valve, by investigation of its mechanical behaviour and unsteady flow characteristics. The immersed boundary (IB) method is used for the dynamic modelling of the large deformation of the valve leaflets and the fluid-structure interactions. The IB simulation is first validated for the aortic prosthesis subjected to a hydrostatic loading. The predicted displacement fields by IB are compared with those obtained using ANSYS, as well as with experimental measurements. Good quantitative agreement is obtained. Moreover, known failure regions of aortic prostheses are identified. The dynamic behaviour of the valve designs is then simulated under four physiological pulsatile flows. Experimental pressure gradients for opening and closure of the valves are in good agreement with IB predictions for all flow rates for both aortic and mitral designs. Importantly, the simulations predicted improved physiological haemodynamics for the novel mitral design. Limitation of the current IB model is also discussed. We conclude that the IB model can be developed to be an extremely effective dynamic simulation tool to aid prosthesis design.

Doppler microembolic signals in patients with two different types of bileaflet valves.

OBJECTIVES: This study was performed to evaluate the prevalence and counts of Doppler microembolic signals in patients with St Jude Medical valves (St Jude Medical, Inc, St Paul, Minn) and patients with ATS valves (ATS Medical, Inc, Minneapolis, Minn) and their relation to clinical parameters. METHODS: A total of 179 outpatients of the department of cardiothoracic surgery were examined. They included 98 men and 81 women, aged 61 +/- 11 years, with ATS (n = 91) or St Jude Medical (n = 88) valves in the aortic (n = 110), mitral (n = 39), or both positions (n = 30). Neurologic examination was followed by transcranial Doppler monitoring for microembolic signals. Monitoring was performed bilaterally over the middle cerebral arteries for 1 hour per session. RESULTS: Microembolic signal counts and prevalence were significantly higher in patients with St Jude Medical as compared with ATS valves. Valve type and presence of diabetes mellitus were the only predictors of microembolic signal prevalence on multivariate analysis. No influence of microembolic signals on cerebral embolic complications was established. Additionally, patients with a postoperative history of cerebral embolic complications did not have a higher number of microembolic signals than remaining patients. Interobserver variability was satisfactory. CONCLUSIONS: Patients with St Jude Medical valves were shown to have significantly higher microembolic signal counts than patients with ATS valves. However, our results suggest that microembolic signal counts cannot be used to predict cerebral embolic complications. Their relation to neuropsychologic deficits remains to be evaluated.

Calcification and fatigue failure in a polyurethane heart value.

The prosthetic heart valves were fabricated from a polyurethane containing a 4,4'-diphenylmethane diisocyanate hard segment, chain-extended with butanediol and with a polyether soft segment. The rate of calcification of these polyurethane heart valves was much slower in a dynamic in vitro test system than similar bioprosthetic heart valves. The calcified deposits were located exclusively at regions of material failure. Fourier transform infrared (FTIR) spectroscopy indicated the involvement of the polyether soft segments of the polymer directly in the calcification process. Calcification of polymer fractions also suggested that small molecular weight extractable components are accelerating factors in the calcification process.

In vitro blood compatibility of surface-modified polyurethanes.

Polyurethanes have proven durable materials for the manufacture of flexible trileaflet heart valves, during in vitro tests. The response of two polyurethanes of differing primary structure to parameters of blood compatibility has now been investigated, using an in vitro test cell. Platelet (beta-thromboglobulin) release, complement (C3a) activation, the activation of free plasma and surface-bound factor XII were studied using fresh, human blood (no anticoagulant) or citrated plasma in control and surface-modified polyurethane. Surface modifications were designed to affect material thrombogenicity and included covalent attachment of heparin, taurine, a platelet membrane glycoprotein fragment, polyethylene oxide (PEO), 3-aminopropyltriethoxysilane, and glucose or glucosamine. Unmodified control polyurethanes caused platelet release and complement activation. High molecular weight (2000 D) polyethylene oxide reduced platelet release slightly but only glucose attachment to the surface produced a significant reduction in platelet activation. All modifications reduced C3 activation compared with controls, but the greatest reduction was achieved with polyethylene oxide attachment or glycosylation. Most surface modifications were more activating of factor XII, both in plasma and on the material surfaces, than the control polyurethanes. Heparin and high molecular weight PEO produced the greatest activation of factor XII in the free plasma form, but low molecular weight PEO and glucosamine produced the greatest activation of surface-bound factor XIIa. The least activating surfaces, affecting both free plasma and surface-bound factor XIIa, were those treated with platelet membrane glycoprotein fragment and glucose. PEO surfaces performed relatively well, compared with controls and most surface modifications. The best overall surface, however, was the glucose-modified surface which was least activating considering all parameters of blood compatibility.

Chemical modification of bovine pericardium and its effect on calcification in the rat subdermal model.

Specific modification of functional groups in collagen has been used to investigate their influence on calcification and thermal stability of bovine pericardium. Pretreatment of pericardium with iron (III) citrate reduced calcification in the rat subcutaneous implant model, as did acyl azide activation of carboxyl and amide groups. Chondroitin sulphate had no significant effect, while cyanamide treatment was mainly effective in combination with iron (III) citrate. Glutaraldehyde pretreatment restricted reaction with other modifying agents, but, as a post-fixation treatment, improved the thermal stability of other agents. Glutaraldehyde post-fixation had no significant relationship to the calcification rate.

New polyurethane heart valve prosthesis: design, manufacture and evaluation.

In light of the thrombogenicity of mechanical valves and the limited durability of bioprosthetic valves, alternative designs and materials are being considered for prosthetic heart valves. A new tri-leaflet valve, made entirely from polyurethane, has been developed. The valve comprises three thin polyurethane leaflets (approximately 100 microns thick) suspended from the inside of a flexible polyurethane frame. The closed leaflet geometry is elliptical in the radial direction and hyperbolic in the circumferential direction. Valve leaflets are formed and integrated with their support frame in a single dip coating operation. The dipping process consistently gives rise to tolerably uniform leaflet thickness distributions. In hydrodynamic tests, the polyurethane valve exhibits pressure gradients similar to those for a bioprosthetic valve (St Jude Bioimplant), and levels of regurgitation and leakage are considerably less than those for either a bi-leaflet mechanical valve (St Jude Medical) or the bioprosthetic valve. Six out of six consecutively manufactured polyurethane valves have exceeded the equivalent of 10 years function without failure in accelerated fatigue tests. The only failure to date occurred after the equivalent of approximately 12 years cycling, and three valves have reached 527 million cycles (approximately 13 years equivalent). The simplicity of valve manufacture, combined with promising results from in vitro testing, indicate that further evaluation is warranted.

Dynamic in vitro calcification of porcine aortic valves.

Previous work in our laboratory has demonstrated a simple, dynamic in vitro calcification method for studying bovine pericardial heart valves. The calcification produced closely resembled that found in clinical explant valves. The current study extends this technique to the porcine aortic bioprosthesis. Five Carpentier-Edwards porcine aortic bioprostheses were calcified in vitro in a modified wear tester. All valves calcified to a similar degree as bovine pericardial valves. Calcification predominated on the ribbed tissue structures near the commissures on the outflow surfaces. The same calcification pattern was seen in clinical explant valves. A number of anti-calcification modifications of porcine aortic valves were also investigated. These had all previously inhibited calcification of bovine pericardium in a rat subdermal implant model but had failed to reduce calcification in whole pericardial valves calcified in vitro under dynamic conditions. The modified porcine valves produced similar results: no modification achieved reduction of calcification on exposure to the functional valve calcification model. The dynamic in vitro calcification test has been shown to be useful for the study of both main types of bioprostheses, bovine pericardial and porcine aortic valves, and for the assessment of alterations to these biomaterials.

Confocal laser scanning microscopy of calcified bioprosthetic heart valves.

Calcification of bioprosthetic heart valves is a major factor limiting their long-term function. Current methods of microscopic examination of calcific deposits require dehydration and processing of the leaflet material, e.g. wax embedding, sectioning, gold coating. Confocal laser scanning microscopy is a new technique which allows serial optical sectioning of thick biological specimens in their normal hydrated state. The current study has examined bovine pericardium and porcine aortic valve materials calcified in vitro under static and dynamic conditions. A series of clinical explants of bovine pericardial and porcine aortic valve types has also been examined. The calcium-specific stain, Alizarin Red S, has been used as a fluorescent marker for calcium deposits. Fluorescent images, generated by Argon ion laser light at 488nm, have been obtained at varying depths into samples. These have been reconstructed to demonstrate the relationship between calcium deposits and autofluorescent collagen fibers. The patterns of calcification were similar in both in vitro and explant valve material. The calcification was of three main patterns: calcium depositing longitudinally on collagen fiber bundles; calcium forming a banded pattern perpendicular to the fiber direction and large, dense, focal deposits obliterating the underlying collagen structure. Involvement of collagen fibers in the calcification pattern was a consistent finding in all sample types. The method is a useful addition to the tools available for study of calcification processes. The potential exists for three-dimensional reconstruction of leaflet architecture and its inter-relationship with calcium deposition, in a normally hydrated state.

In vitro calcification of bioprosthetic heart valves: report of a novel method and review of the biochemical factors involved.

The lifetime of bioprosthetic heart valves is limited by primary tissue failure and calcification of the valve leaflets. There are indications that synthetic elastomeric materials may also be subject to this problem. The mechanism of calcification is not known, but it is of interest that calcification can be induced in tissue even in the absence of cellular mechanisms, outside the body. Many hypotheses relate to inhibitory or promotory factors rather than primary instigators of calcification and none has led to a satisfactory solution of the problem. The study of calcification in replacement valves generally utilises in vivo test methods i.e. complex biologic systems. This creates difficulty in defining the primary factors involved. The use of in vitro test methods, including a novel fatigue tester method, has been reviewed. Various test media have been used, including simple salt solutions (allowing definition and controlled modification of the calcification medium) and bovine plasma. Comparison of static and dynamic in vitro methods with the rat subcutaneous implant model indicated a lower degree of calcification in vitro: the calcification achieved was, however, significantly greater than similar material not subject to calcification processes. Dynamic in vitro tests produced greater calcification than static in vitro tests. Porcine aortic valve material, in static tests, behaved similarly to bovine pericardium. In vitro calcification testing has a useful role to play in the economic screening of new materials or modifications of existing materials prior to in vivo testing. It may also aid the definition of the mechanism of calcification and hence the development of solutions to the problem.

In vitro function and durability of a polyurethane heart valve: material considerations.

BACKGROUND AND AIMS OF THE STUDY: Flexible trileaflet polyurethane heart valves can be designed which have good hydrodynamic function in vitro. The choice of polyurethane for fabrication of such valves is not simple; several similar polyurethanes are available, but relatively minor differences in their structure and composition may have profound effects on long term fatigue behavior with little effect on their short term mechanical properties. METHODS: The relative functional characteristics of flexible trileaflet polyurethane valves made from two different polyetherurethanes were compared before and after long term fatigue testing. The polyetherurethanes had similar gross mechanical properties, differing in that one was chain-extended with butanediol (PEU) and the other with ethylene diamine (PEUE). Six valves of each type, with similar leaflet thickness distributions, were studied. RESULTS: Hydrodynamic function of both valve types was similar to a similarly sized porcine aortic valve. Mean pressure drop across the open valve was higher in PEU valves than in PEUE valves, although PEUE valves had greater energy losses during closure and when closed. Reverse flow decreased with time in the fatigue tester. In long term fatigue tests, all six PEU valves failed by 307 million cycles, with failure primarily by development of holes at the coaptation region of the leaflets associated with localized calcification. Three PEUE valves exceeded 800 million cycles without failure and all PEUE valves exceeded 450 million cycles. CONCLUSIONS: A combination of good phase separation of polyurethane soft and hard segments with good rubbery characteristics can explain the better results achieved in the PEUE valves compared with similar PEU valves. These results suggest the general type of polyurethane structure suitable for heart valve fabrication and have implications for development of novel polyurethanes for this application.

Platelet aggregation and high-intensity transient signals (HITS) in a sheep model of mitral valve replacement.

BACKGROUND AND AIMS OF THE STUDY: The composition of microemboli detected as high-intensity transient signals (HITS) by Doppler ultrasound in patients with prosthetic heart valves is still debated. Here, platelet aggregation and HITS were investigated in a sheep model. METHODS: Insonation of the carotid artery was performed in 20 sheep with either a mechanical or a biological mitral valve prosthesis in place. The effect of ICI 170809, a 5HT2a antagonist, on the frequency of HITS and on platelet aggregates, counted in arterial blood smears per nine high-power fields, was assessed at three and six months after valve implantation. The mitral transvalvular gradient was measured by transthoracic echocardiography at three and six months. RESULTS: Data are expressed as median and interquartile range. At three months, there were 36 (20-114) HITS/h in the mechanical group, and 0 (0-15) HITS/h in the biological group. At six months, there were 21 (0-82) and 0 (0-2) HITS/h, respectively. The occurrence of HITS was unaffected by either ICI 170809, or by duration of implant in either group. Platelet aggregate counts were higher with the mechanical than with the biological valve at three months, but not at six months. ICI 170809 reduced platelet aggregate counts in both valve types; the reduction was not significant in the bioprosthetic valve group. The pressure gradient across the bioprosthesis increased during the study from 2 (2-3) mmHg to 7.5 (6-10) mmHg, but was unchanged in the mechanical valve. CONCLUSIONS: (i) It was confirmed that the frequency of HITS is higher with the mechanical prosthesis than the bioprosthesis; (ii) circulating platelet aggregates in the bioprosthetic valve group tended to increase as structural valve deterioration occurred; (iii) the frequency of HITS was not influenced by either an increase or a decrease in circulating platelet aggregates; and (iv) HITS detected in patients with prosthetic valves are unlikely to be due to circulating platelet aggregates.

Porcine versus pericardial bioprostheses: eleven-year follow up of a prospective randomized trial.

BACKGROUND AND AIM OF THE STUDY: There is renewed interest in the pericardial heart valve as an alternative to the porcine bioprosthesis. The long-term results of a randomized trial comparing a second-generation pericardial valve against a well-tested porcine bioprosthesis are presented. Seven-year follow up has been reported previously. Production of the Bioflo pericardial prosthesis used in this trial was discontinued due to fears related to bovine spongiform encephalopathy. METHODS: Between February 1987 and March 1990, 170 patients undergoing aortic (AVR) or mitral (MVR) valve replacement were assigned randomly to receive either the Bioflo pericardial bioprosthesis or the Carpentier-Edwards (CE) supra-annular porcine bioprosthesis. Eighty-five patients received 93 Bioflo valves (46 AVR, 31 MVR, eight AVR+MVR), and the remaining 85 received 99 CE valves (48 AVR, 23 MVR, 14 AVR+MVR). Mean patient age was 61.0 years (range: 38-77 years) for the Bioflo group and 62.1 years (range: 41-77 years) for the CE group. Current follow up is 100% complete and totals 1,391 patient-years; mean +/- SD follow up was 8.2 +/- 3.4 years (maximum 12.2 years). RESULTS: The operative mortality rate was 4.12%. There were 70 patients still at risk at 11 years (31 Bioflo, 39 CE); of these, 91.4% were in NYHA classes I/II. No significant difference in survival or valve-related complications was seen between the groups. Mean (+/- SEM) survival at 11 years was 41.4 +/- 6.8% in the Bioflo group and 55.3 +/- 6.8% in the CE group (p = 0.15). There were 16 valve-related deaths (nine in the Bioflo group, seven in the CE group). At 11 years, freedom from valve-related mortality was 89.5 +/- 3.9% for the Bioflo group and 91.0 +/- 3.5% for the CE group (p = 0.4). Valve position had no impact on survival. At 11 years, freedom from structural valve deterioration was 83.9 +/- 5.4% and 87.5 +/- 4.2% in the Bioflo and CE groups, respectively (p = 0.9). CONCLUSION: Over the 11-year period of follow up, clinical performance of the Bioflo pericardial valve was comparable with that of the Carpentier-Edwards supra-annular porcine bioprosthesis. No difference was apparent between the two valve types when implanted in either the aortic or the mitral position.

Polyurethane: material for the next generation of heart valve prostheses?

OBJECTIVES: The prospects for a durable, athrombogenic, synthetic, flexible leaflet heart valve are enhanced by the recent availability of novel, biostable polyurethanes. As a forerunner to evaluation of such biostable valves, a prototype trileaflet polyurethane valve (utilising conventional material of known in vitro behaviour) was compared with mechanical and bioprosthetic valves for assessment of in vivo function, durability, thromboembolic potential and calcification. METHODS: Polyurethane (PU), ATS bileaflet mechanical, and Carpentier-Edwards porcine (CE) valves were implanted in the mitral position of growing sheep. Counting of high-intensity transient signals (HITS) in the carotid arteries, echocardiographic assessment of valve function, and examination of blood smears for platelet aggregates were undertaken during the 6-month anticoagulant-free survival period. Valve structure and hydrodynamic performance were assessed following elective sacrifice. RESULTS: Twenty-eight animals survived surgery (ten ATS; ten CE; eight PU). At 6 months the mechanical valve group (n=9) showed highest numbers of HITS (mean 40/h, P=0.01 cf. porcine valves), and platelet aggregates (mean 62.22/standard field), but no thromboembolism, and no structural or functional change. The bioprosthetic group (n=6) showed low HITS (1/h) and fewer aggregates (41.67, P=1.00, not significant), calcification and severe pannus overgrowth with progressive stenosis. The PU valves (n=8) showed a small degree of fibrin attachment to leaflet surfaces, no pannus overgrowth, little change in haemodynamic performance, low levels of HITS (5/h) and platelet aggregates (17.50, P<0.01 cf. mechanical valves, P=0.23 cf. porcine valves), and no evidence of thromboembolism. CONCLUSIONS: In the absence of valve-related death and morbidity, and retention of good haemodynamic function, the PU valve was superior to the bioprosthesis; lower HITS and aggregate counts in the PU valve imply lower thrombogenicity compared with the mechanical valve. A biostable polyurethane valve could offer clinical advantage with the promise of improved durability (cf. bioprostheses) and low thrombogenicity (cf. mechanical valves).

Randomised, prospective evaluation of a new pericardial heart valve: outcome after seven years.

Between 2 February 1987 and 20 March 1990, 170 patients were randomly allocated to receive a new pericardial heart valve (the Bioflo) or the Carpentier-Edwards supra-annular porcine bioprosthesis. Eighty-five patients (mean age 61 years, range 38-77) received 93 Bioflo valves, 46 having aortic valve replacement (AVR), 31 mitral valve replacement (MVR) and 8 aortic and mitral valve replacement (A+MVR); 85 patients (mean age 62.1 years, range 41-77) received 99 Carpentier-Edwards porcine valves (48 AVR, 23 MVR and 14 A+MVR). Cumulative follow-up totals 926 patient-years (mean 5.45 +/- 1.93, median 6.03, maximum 7.47 years). The overall operative mortality rate for the Bioflo group was 2.4%, and 5.9% for the Carpentier-Edwards group. At 7 years, there was no statistically significant difference in survival or in any prosthesis-related complication between the pericardial and porcine valve recipients overall, or when the data were analysed according to valve implant position. The actuarial survival rate at 7 years for the Bioflo group was 80.1 +/- 5.1% and 72.3 +/- 5.9% for the Carpentier-Edwards group. Freedom from structural valve deterioration (SVD) at 7 years for Bioflo patients was 98.5 +/- 1.5% and for Carpentier-Edwards patients 91.2 +/- 4.1%. No patient in the Bioflo group has required reoperation for SVD. The randomised prospective trial has proven practical and effective and has shown the pericardial valve to perform at least as well as the porcine valve up to 7 years when all of the standard outcome measures of valve performance are assessed.

Surface modification of polyurethane heart valves: effects on fatigue life and calcification.

Polyurethane heart valves can be functionally durable with minimal calcification, in vitro. In vivo, these characteristics will depend on the resistance of the polyurethane to thrombogenesis and biodegradation. Surface modification may improve the polyurethane in these respects, but may adversely affect calcification and durability. This study investigates the effects of surface modifications of two polyurethane heart valves (PEU and PEUE) on their in vitro fatigue and calcification behaviour. Modifications included heparin, taurine, 3-aminopropyltriethoxysilane and polyethylene oxide (PEO). Neither hydrodynamic function nor leaflet thickness distribution was significantly altered by surface modification. PEO-modification was detrimental to valve fatigue durability and calcification. Heparin, taurine or aminosilane modifications of PEU valves increased durability. Aminosilane modification of PEUE valves increased durability compared with PEO modification. Appropriate surface modification may be useful to improve blood compatibility of implantable polyurethanes, and may also be advantageous as regards fatigue durability of flexing materials in longterm applications.

Laser profiling of bovine pericardial heart valves.

Laser profiling techniques have been used to examine the 2-dimensional and 3-dimensional patterns of leaflet motion in functioning bovine pericardial heart valves (1 normal valve and 1 fatigued/calcified). In the normal valve the general patterns of opening and closing were similar for all leaflets; however, localised variations such as areas of high curvature, retarded motion and high speed motion were identified. In the fatigued/calcified valve significant differences from the normal leaflet motion were observed e.g. increased crimping, gross leaflet lag and irregular deformation. The laser profiling technique was able to reveal changes in the functional dynamics of pericardial valve leaflets not otherwise detectable by conventional hydrodynamic measurements of valve performance.

Polyurethane heart valve durability: effects of leaflet thickness and material.

A flexible trileaflet polyurethane valve has been made by dip-moulding leaflets directly onto an injection-moulded frame. The durability of this value is, in part, determined by the thickness of its leaflets. Leaflet thickness is also a major determinant of hydrodynamic function. This study examines valves (n = 31) with leaflets made of a polyetherurethane (PEU, n = 22) or a polyetherurethaneurea (PEUE, n = 9), of varying thickness distributions. The valves were subjected to accelerated fatigue test at 37 degrees C and failure monitored. Leaflet thicknesses ranged from 60 to 200 microns. PEU leaflet thickness bore no relationship to durability, which was less than 400 million cycles. PEUE valves, in contrast, exceeded 800 million cycles. Durability in PEUE valves was directly related to leaflet thickness (r = 0.93, p < 0.001), with good durability achieved with median leaflet thicknesses of approximately 150 microns. Thus polyurethane valves can be made with good hydrodynamic properties and with sufficient durability to consider potential clinical use.

Calcification modelling in artificial heart valves.

This study has examined a range of methods of studying the calcification process in bovine pericardial and polyurethane biomaterials. The calcification methods include static and dynamic, in vitro and in vivo tests. The analytical methods include measurement of depletion rates of calcium and phosphate from in vitro calcifying solutions, analysis of tissue contents of calcium, histological staining of tissue sections for calcium, X-ray elemental analysis, by scanning electron microscopy, of calcium and phosphorus distributions over valve leaflets calcified in vitro under dynamic conditions. Bovine pericardium, in all test settings, calcified to a much greater degree than polyurethane biomaterials. Polyurethane extracts calcified to a greater degree than bulk polyurethanes. The test protocol used allows progress through increasingly demanding calcification tests, with the possibility of eliminating unsuitable materials with tests of limited complexity and expense.

Hydrodynamic function of a biostable polyurethane flexible heart valve after six months in sheep.

Survival to six months for sheep with a non-biostable polyurethane mitral heart valve prosthesis has been reported previously, however, with surface degradation and accumulation of calcified fibrin/thrombus that impaired leaflet motion and compromised hydrodynamic function. Newly available biostable polyurethanes may overcome this problem. Six adult sheep with biostable polyurethane trileaflet heart valve prostheses of documented hydrodynamic performance, implanted in the mitral position, were allowed to survive for 6 months. Explanted valves were photographed, resubmitted to hydrodynamic function testing, and studied by light and electron microscopy. Explanted valves were structurally intact and differed little in appearance from their preimplant state. Hydrodynamic testing showed no deterioration in pressure gradient or energy losses compared with pre-implant values. Biostable polyurethanes demonstrated improved blood compatibility leaving leaflets flexible and valve function unimpaired. Biostable polyurethanes may thus improve prospects for prolonged function of synthetic heart valve prostheses.

Effect of bending rigidity in a dynamic model of a polyurethane prosthetic mitral valve.

We investigate the behaviour of a dynamic fluid-structure interaction model of a chorded polyurethane mitral valve prosthesis, focusing on the effects on valve dynamics of including descriptions of the bending stiffnesses of the valve leaflets and artificial chordae tendineae. Each of the chordae is attached at one end to the valve annulus and at the other to one of two chordal attachment points. These attachment points correspond to the positions where the chords of the real prosthesis would attach to the left-ventricular wall, although in the present study, these attachment points are kept fixed in space to facilitate comparison between our simulations and earlier results obtained from an experimental test rig. In our simulations, a time-dependent pressure difference derived from experimental measurements drives flow through the model valve during diastole and provides a realistic pressure load during systole. In previous modelling studies of this valve prosthesis, the valve presents an unrealistically large orifice at beginning of diastole and does not close completely at the end of diastole. We show that including a description of the chordal bending stiffness enables the model valve to close properly at the end of the diastolic phase of the cardiac cycle. Valve over-opening is eliminated only by incorporating a description of the bending stiffnesses of the valve leaflets into the model. Thus, bending stiffness plays a significant role in the dynamic behaviour of the polyurethane mitral valve prosthesis.