Mechanical stress as cause of aortic valve disease. Presentation of a new aortic root prosthesis

We present an overview of studies on the aortic valve and propose that mechanical stress is a main causative factor in the degenerative valvular disease. In the normal aortic valve, the leaflets have a smooth surface, free of wrinkles and creases, throughout the opening process. This smooth leaflet surface during motion is achieved by the "pull and release" movement of the commissures, which occurs because of the compliance of the aortic root. When the aortic root is stiffened, either by artificial means or by the loss of elasticity due to aging, the leaflet dynamics change significantly. The leaflets develop a significant number of creases and wrinkles during the opening process. In the bileaflet valve, the leaflets develop similar creasing and wrinkling during the opening process. This happens mainly due to the less-than-ideal design of the bileaflet valve and in spite of the compliant aortic root. When the aortic valve is spared using a noncompliant tube graft, a similar phenomenon of leaflet creasing occurs. Because the creasing produces high stresses from bending and buckling, it is damaging to the leaflet tissue and can lead to degenerative and calcific valvular disease. Based on these observations a new aortic root prosthesis with compliant sinuses has been designed for the valve sparing operation.     

The mechanism of opening of the aortic valve.

The mechanism of opening of the aortic valve was investigated in dogs by attaching radiopaque markers to the commissures and the leaflets. Analysis of abnormal cardiac cycles demonstrated that, when the ventricular pressure first equalled the aortic pressure, the intercomissural distances increased 9 percent, and the valve opened with a stellate orifice without forward flow and without a rise in aortic pressure. Further opening of the aortic valve was dependent on forward flow over a narrow range. A new mechanism of aortic valve opening is proposed. This mechanism results in minimal flexion stresses on the leaflets and is important for the longevity of the normal aortic valve. It can occur only if the leaflets arise from an expansile aortic root.     

Mechanical Stress as Cause of Aortic Valve Disease Presentation of a New Aortic Root Prosthesis

We present an overview of studies on the aortic valve and propose that mechanical stress is a main causative factor in the degenerative valvular disease. In the normal aortic valve, the leaflets have a smooth surface, free of wrinkles and creases, throughout the opening process. This smooth leaflet surface during motion is achieved by the “pull and release” movement of the commissures, which occurs because of the compliance of the aortic root. When the aortic root is stiffened, either by artificial means or by the loss of elasticity due to aging, the leaflet dynamics change significantly. The leaflets develop a significant number of creases and wrinkles during the opening process. In the bileaflet valve, the leaflets develop similar creasing and wrinkling during the opening process. This happens mainly due to the less-than-ideal design of the bileaflet valve and in spite of the compliant aortic root. When the aortic valve is spared using a noncompliant tube graft, a similar phenomenon of leaflet creasing occurs. Because the creasing produces high stresses from bending and buckling, it is damaging to the leaflet tissue and can lead to degenerative and calcific valvular disease. Based on these observations a new aortic root prosthesis with compliant sinuses has been designed for the valve sparing operation.     

Aortic root dilatation may alter the dimensions of the valve leaflets

Objective: Valve-sparing surgery can be used in patients with dilated aortic roots and aortic insufficiency (AI) but has not become a common practice, in part because the spared valve may be incompetent. Our goal was to study how the dimensions of the aortic root and leaflets have changed in such patients. Methods: Fourteen patients with dilated aortic root and AI were examined by transesophageal echocardiography. The annulus diameter, sinotubular junction (STJ) diameter, sinus height, leaflet free-edge length, and leaflet height were measured. Correlations among these dimensions and with the AI grades were explored. Measurements were also made in 19 normal human aortic valves from silicone molds. Results: There was no evident change in the average diameter of the annulus between the normal valves and those in the dilated aortic roots. The STJ diameter was obviously increased in the dilated aortic roots; the aortic sinuses also appeared to be taller and the leaflets larger than normal. The leaflet free-edge length, the leaflet height, and the sinus height were found to increase with the dilated STJ diameter. The degree of AI was not found to correlate well with any of the dimensions measured. Conclusions: The dimensions of the leaflets may change parallel to aortic root dilatation with AI. Therefore, during valve sparing, it may be necessary to correct both the dilatation of the root and the leaflet free-edge length to achieve a competent valve.     

The hydrodynamic function and leaflet dynamics of aortic and pulmonary roots and valves: an in vitro study.

Fresh homograft aortic and pulmonary roots were tested in a pulsatile flow simulator to assess their hydrodynamic function and leaflet opening characteristics. Simultaneous flow and pressure measurements were obtained for a range of cardiac outputs. The effective orifice area and regurgitant volumes were calculated. The mean pressure difference across the pulmonary roots was obtained under both left and right side pressures. A video recording of valve leaflet function was also obtained for each valve. A comparison was made with four porcine bioprosthetic heart valves (21 and 23 mm). The mean pressure difference with respect to flow for the 21 mm Hancock II and Intact porcine bioprostheses was significantly higher than that for all aortic roots tested. The 24 mm aortic root showed significantly lower pressure drop compared to all porcine valves tested. The mean pressure difference across each pulmonary root at pulmonary pressures was significantly greater than at systemic pressures. At systemic pressures the fully open leaflets had a triangular orifice with low leaflet open bending strains at the commissures. At the lower internal pressures, with reduced dilation of the root, higher bending strains were noted. These were not as severe as seen in porcine valves.     

The in vitro hydrodynamic characteristics of the porcine pulmonary valve and root with regard to the ross procedure

OBJECTIVE: The hydrodynamic parameters and leaflet motion of the porcine pulmonary root and valve and the performance of the pulmonary autograft implanted in subcoronary position or as a free-standing root were investigated at systemic and pulmonary pressures in vitro. METHODS: Ten fresh pulmonary and aortic roots (anulus diameter, 20-25 mm) were tested in a pulsatile flow simulator. Five free-sewn pulmonary valves were implanted in aortic roots in the subcoronary position, and 5 pulmonary roots were implanted as free-standing roots. The external diameter of the roots was measured at the sinotubular junction in a pressure range of 0 to 120 mm Hg. The transvalvular gradient and regurgitation were measured, and the effective orifice area was calculated. The leaflet motion was recorded on video tape. RESULTS: The fresh pulmonary roots were more compliant than their aortic counterparts (33% +/- 3. 0% vs 7% +/- 1.5% with dilatation at 0-30 mm Hg and 46% +/- 8.4% vs 35% +/- 7.8% with dilatation at 0-120 mm Hg). The pulmonary roots had a lower pressure drop at systemic than at pulmonary pressures. The pressure drops of the pulmonary roots were also lower than those of the aortic roots in the systemic pressure range. The leaflet opening of the pulmonary valve was triangular, with low bending deformation at all pressures. Implanting the free-sewn pulmonary valve in the subcoronary position or the pulmonary root as a free-standing root did not affect the hydrodynamic parameters and leaflet motion adversely. CONCLUSION: The pulmonary valve and root could easily withstand aortic pressures in vitro. A biphasic dilatation curve ensures that higher pressures did not overdilate the pulmonary root. Moreover, valve performance was better at systemic pressures.     

Transpecies heart valve transplant: advanced studies of a bioengineered xeno-autograft

Background. Tissue engineering approaches utilizing biomechanically suitable cell-conductive matrixes should extend xenograft heart valve performance, durability, and growth potential to an extent presently attained only by the pulmonary autograft. To test this hypothesis, we developed an acellular, unfixed porcine aortic valve-based construct. The performance of this valve has been evaluated in vitro under simulated aortic conditions, as a pulmonary valve replacement in sheep, and in aortic and pulmonary valve replacement in humans.

Methods. SynerGraft porcine heart valves (CryoLife Inc, Kennesaw, GA) were constructed from porcine noncoronary aortic valve cusp units consisting of aorta, noncoronary aortic leaflet, and attached anterior mitral leaflet (AML). After treatment to remove all histologically demonstrable leaflet cells and substantially reduce porcine cell-related immunoreactivity, three valve cusps were matched and sewn to form a symmetrical root utilizing the AML remnants as the inflow conduit. SynerGraft valves were evaluated by in vitro hydrodynamics, and by in vivo implants in the right ventricular outflow tract of weanling sheep for up to 336 days. Cryopreserved allograft valves served as control valves in both in vitro and in vivo evaluations. Valves were also implanted as aortic valve replacements in humans.

Results. In vitro pulsatile flow testing of the SynerGraft porcine valves demonstrated excellent valve function with large effective orifice areas and low gradients equivalent to a normal human aortic valve. Implants in sheep right ventricular outflow tracts showed stable leaflets with up to 80% of matrix recellularization with host fibroblasts and/or myofibroblasts, and with no leaflet calcification over 150 days, and minimal deposition at 336 days. Echocardiography studies showed normal hemodynamic performance during the implantation period. The human implants have proven functional for over 9 months.

Conclusions. A unique heart valve construct has been engineered to achieve the equivalent of an autograft. Short-term durability of these novel implants demonstrates for the first time the possibility of an engineered autograft.     

In vitro investigation of aortic valve annuloplasty using prosthetic ring devices

Objectives: The remodeling of the dilatated valve annulus with a prosthetic ring for the repair of valve insufficiency is a well-established concept in mitral valve surgery, and may also be suitable for aortic valve reconstruction. In this study, two models of prosthetic aortic annuloplasty devices were investigated. Methods: Fresh porcine aortic roots (n = 16) were investigated in a pulsatile flow simulator after patch dilatation of the annulus and subsequent reconstruction using both an external and an internal prosthetic ring. For each configuration, leakage was determined by ultrasonic flow measurements and leaflet co-aptation by transesophageal echocardiography. In addition, valves’ motions were recorded by high-speed video. Results: By the use of the prosthetic annuloplasty rings, leakage volumes decreased significantly compared with the dilatated root, more pronounced with the intra-annular ring. Similarly, the co-aptation height of the leaflets increased. Pressure gradients were not significantly influenced by the ring application, but leaflet motion patterns changed from the usual trapezoid to a more rectangular opening characteristic, visible at both echocardiographic and high-speed video analysis. Conclusions: The reconstruction of a dilatated aortic valve annulus using external and internal ring devices is feasible and effective for reduction of regurgitation at which the internal ring provides a greater potential to decrease valve insufficiency.     

Effect of cryopreservation techniques on aortic valve glycosaminoglycans

This study was designed to evaluate the effect of cryopreservation on the glycosaminoglycan (GAG) content of the aortic allografts. Twenty-one porcine aortic valves were obtained. Five aortic roots were immediately analyzed without cryopreservation, eight were cryopreserved in closed leaflet position, and eight in open leaflet position. The groups were compared in terms of GAG concentration and subclass proportion in three different zones including the aortic root wall, the commissures, and the leaflets. GAG content at the commissures was significantly lower in the closed leaflet group than in the other groups (P = 0.001). The electrophoretic analysis did not show any significant difference in the zonal distribution of GAG classes between groups. Quantitative analysis in various aortic valve zones suggests that cryopreservation can alter the GAG content. Cryopreservation of the aortic valve in an open leaflet position can preserve the matrix more efficiently and might prolong the durability of the aortic allograft.     

Obstruction of St Jude Medical valves in the aortic position: significance of a combination of cineradiography and echocardiography

BACKGROUND: Obstruction of the St Jude Medical valve (St Jude Medical, Inc, St Paul, Minn) is a rare but serious complication. METHODS: Cineradiographic and echocardiographic evaluations of aortic St Jude Medical valves were simultaneously performed on 54 patients, with no signs of prosthetic valve dysfunction late after surgery. RESULTS: Although closing angles of the leaflets corresponded closely with the manufacturer data, restricted opening of the leaflets (opening angle >/= 20 degrees ) was found in 16 (group D) of the 54 patients by means of cineradiography. The opening angles were equal to or less than 14 degrees in the other 23 patients (group N) and between 15 degrees and 19 degrees in the remaining 15 (group M). Doppler-derived transprosthetic pressure gradients were significantly higher (P =.03) and the velocity index was significantly lower (P =.003) in group D than in group N. However, no significant differences were found in those values between group N and group M. Replacement of the aortic St Jude Medical valves was performed in 5 of the 16 patients, and the remaining 11 have been followed up because of relatively low pressure gradients. The cause of restricted leaflet movement was pannus formation without thrombosis in 4 patients and valve thrombosis with pannus formation in one. CONCLUSIONS: Reduced valve orifice area and restricted opening of the leaflets resulting from excess growth of pannus probably led to obstruction of the aortic St Jude Medical valves. A combination of cineradiography and echocardiography makes it possible to provide an accurate and detailed diagnosis of obstruction of the valve.     

Opening and closing characteristics of the aortic valve after valve-sparing procedures using a new aortic root conduit

BACKGROUND: The durability of aortic valve-sparing procedures is negatively affected by increased leaflet stress in the absence of normally shaped sinuses of Valsalva. We compared valve motion after remodeling procedures using a standard conduit and a specifically designed aortic root conduit. METHODS: Echocardiographic studies of the aortic valve dynamics were performed in 14 patients after remodeling of the aortic root (7 standard conduits, group A; 7 new conduits, group B) and in 7 controls (group C). Opening and closing leaflet velocities and percent of slow closing leaflet displacement were measured. Root distensibility and the pressure strain of the elastic modulus were measured at all root levels. RESULTS: Root distensibility and the pressure strain of the elastic modulus were different in group A and B only at the sinuses (p < 0.001). Opening and closing leaflet velocities were not different among groups. Slow closing leaflet displacement was markedly more evident in group B patients (24.2%+/-1.9% versus 2.5%+/-1.9% in group A, p < 0.001) and similar to controls (22.1%+/-7.9%). CONCLUSIONS: The new conduit guarantees dynamic features of the aortic valve leaflets superior to those obtained with standard conduits and more similar to normal subjects.     

Preservation of aortic leaflets during rapid deployment valve implantation in patients with aortic insufficiency

Development of a new surgical technique for aortic valve replacement with the use of rapid deployment/sutureless valve: a leaflet preservation technique applying imbrication methods to pliable aortic leaflets. We aim to decrease the incidence of paravalvular leak by preserving aortic leaflets in patients with aortic insufficiency and large aortic annulus.     

Mechanisms of aortic valve incompetence: finite element modeling of aortic root dilatation

Background. Idiopathic root dilatation often results in dysfunction of an otherwise normal aortic valve. To examine the effect of root dilatation on leaflet stress, strain, and coaptation, we utilized a finite element model.

Methods. The normal model incorporated the geometry, tissue thickness, stiffness, and collagen fiber alignment of normal human roots and valves. We evaluated four dilatation models in which diameters of the aortic root were dilated by 5%, 15%, 30%, and 50%. Regional stress and strain were evaluated and leaflet coaptation percent was calculated under diastolic pressure.

Results. Root dilatation significantly increased regional leaflet stress and strain beyond that found in the normal model. Stress increases ranged from 57% to 399% and strain increases ranged from 39% to 189% in the 50% dilatation model. Leaflet stress and strain were disproportionately high at the attachment edge and coaptation area. Leaflet coaptation was decreased by 18% in the 50% root dilatation model.

Conclusions. Idiopathic root dilatation significantly increases leaflet stress and strain and reduces coaptation in an otherwise normal aortic valve. These alterations may affect valve-sparing aortic root replacement procedures.     

Dynamic analysis of the aortic valve using a finite element model

BACKGROUND: The major aim of this study was to examine the leaflet/aortic root interaction during the cardiac cycle, including the stresses developed during the interaction. METHODS: Dynamic finite element analysis was used along with a geometrically accurate model of the aortic valve and the sinuses. Shell elements along with proper contact conditions were also used in the model. Pressure patterns during the cardiac cycle were given as an input, and a linear elastic model was assumed for the material. RESULTS: We found that aortic root dilation starts before the opening of the leaflet and is substantial by the time leaflet opens. Dilation of the root alone helps in opening the leaflet to about 20%. The equivalent stress pattern shows an instantaneous increase in stress at the coaptation surface during closure. Stresses increase as the point of attachment is approached from the free surface. CONCLUSIONS: The complex interplay of the geometry of the valve system can be effectively analyzed using a sophisticated dynamic finite element model. Results not previously brought out by the earlier static analysis shed new light on the root/valve interaction.     

Aortic valve sparing operations: an update

Background. Aortic valve sparing operations in patients with ascending aorta and/or aortic root aneurysms have been performed for a decade in our institution. Initially only patients with normal aortic valve leaflets had these operations, but more recently we utilized them in patients with prolapse of a single leaflet and in those with a bicuspid aortic valve. This article is an update on the clinical results of these operations.

Methods. From May 1988 to December 1997, 126 patients with ascending aorta and/or aortic root aneurysms and aortic insufficiency underwent replacement of the ascending aorta with reconstruction of the aortic root and preservation of the native aortic valve. There were 85 men and 41 women, with a mean age of 54 years (range, 14 to 84). Thirty-two patients had the Marfan syndrome; 17 patients had acute and 10 had chronic type A aortic dissection; 23 had a transverse arch aneurysm; 26 had coronary artery disease, and 8 had mitral regurgitation. The aortic valve sparing operation consisted of simple adjustment of the sinotubular junction in 33 patients, adjustment of the sinotubular junction and replacement of one or more aortic sinuses in 60, and reimplantation of the aortic valve in a tubular Dacron (C.R. Bard, Haverhill, PA) graft in 33. Fifteen patients also had repair of aortic leaflet prolapse. Only 4 patients had a bicuspid aortic valve.

Results. There were 3 operative deaths due to cardiac failure. Patients were followed from 2 to 117 months, with a mean of 31. There were 11 late deaths: 7 cardiovascular and 4 from unrelated causes. The actuarial survival was 72 ± 8% at 7 years. Two patients required aortic valve replacement; the freedom from aortic valve replacement was 97 ± 2% at 7 years. Doppler echocardiography revealed absent, trivial or mild aortic insufficiency in most patients; only 9 patients had moderate aortic insufficiency.

Conclusions. Aortic valve sparing operations are feasible in most patients with ascending aorta and/or aortic root aneurysms who have normal or near normal aortic leaflets. The functional results of the repaired aortic valve are excellent, and the repair appears to be durable.     

Mechanisms of aortic valve incompetence: finite-element modeling of Marfan syndrome.

OBJECTIVES: Progressive aortic root dilatation and an increased aortic root elastic modulus have been documented in persons with Marfan syndrome. To examine the effect of aortic root dilatation and increased elastic modulus on leaflet stress, strain, and coaptation, we used a finite-element model. METHODS: The normal model incorporated the geometry, tissue thickness, and anisotropic elastic moduli of normal human roots and valves. Four Marfan models were evaluated, in which the diameter of the aortic root was dilated by 5%, 15%, 30%, and 50%. Aortic root elastic modulus in the 4 Marfan models was doubled. Under diastolic pressure, regional stresses and strains were evaluated, and the percentage of leaflet coaptation was calculated. RESULTS: Root dilatation and stiffening significantly increased regional leaflet stress and strain compared with normal levels. Stress increases ranged from 80% to 360% and strain increases ranged from 60% to 200% in the 50% dilated Marfan model. Leaflet stresses and strains were disproportionately high at the attachment edge and coaptation area. Leaflet coaptation was decreased by approximately 20% in the 50% root dilatation model. CONCLUSIONS: Increasing root dilatation and root elastic modulus to simulate Marfan syndrome significantly increases leaflet stress and strain and reduces coaptation in an otherwise normal aortic valve. These alterations may influence the decision to use valve-sparing aortic root replacement procedures in patients with Marfan syndrome.     

In vitro study of percutaneous aortic valve replacement: selection of a tissue valve

BACKGROUND: Selection of the best tissue valve is an essential step before percutaneous aortic valve replacement (PAVR) becomes a clinical reality. The aim of this study was to evaluate in vitro three different tissue valves mounted within the same endovascular stent. METHODS: Thirty stented valves (10 aortic porcine, 10 pulmonary porcine, and 10 pericardial tubular) were sutured within a 32-mm long by 23-mm diameter cobalt-nickel stent. The porcine valves were trimmed down close to the cusps. All valves were delivered with a percutaneous valvuloplasty catheter and placed orthotopically in a latex root that was cast from a sheep's aorta. The roots were tested in a pulse duplicator at a rate of 60 beats per minute and 3.5 liters per minute. The transvalvular gradient, maximum valve orifice area, and presence of central and paravalvular leaks were recorded echocardiographically. RESULTS: Within the limitations of implantation in a synthetic, noncalcified annulus, the pericardial valve performed best in terms of orifice area, transvalvular gradients, and tissue bulk; but four of the ten valves showed a central leak due to the type of stent used. CONCLUSION: The ideal valve for PAVR should collapse with minimal bulk to avoid coronary obstruction and central and paravalvular leaks. The tubular pericardial valve showed the lowest pressure gradients and was the most compressible, but was more open to manufacturing errors.     

Aortic valve replacement with stentless porcine aortic bioprosthesis

Twenty-nine patients were entered in a clinical trial on aortic valve replacement with a stentless glutaraldehyde-fixed porcine aortic valve. This bioprosthesis is secured to the aortic root by the same technique used for aortic valve replacement with aortic valve homografts. The functional results obtained from this operation have been most satisfactory. To assess the hemodynamic benefit of eliminating the stent of a porcine aortic valve, we matched 22 patients with a stentless porcine bioprosthesis for age, sex, body surface area, valve lesion, and bioprosthesis size to 22 patients who had aortic valve replacement with a Hancock II bioprosthesis. Mean and peak systolic gradients across the aortic bioprosthesis and effective aortic valve areas were obtained by Doppler studies. Gradients across the stentless bioprosthesis were significantly lower than gradients across the Hancock II valve for every bioprosthesis size. Effective aortic valve areas of the stentless bioprosthesis were significantly larger than the valve areas of the Hancock II valve. Our data demonstrate that the hemodynamic characteristics of a glutaraldehyde-fixed porcine aortic bioprosthesis are greatly improved when the aortic root is used as a stent for the valve. This technique of implantation is expected to enhance the durability of the bioprosthesis, because the aortic root may dampen the mechanical stress to which the leaflets are subjected during the cardiac cycle.     

Modified patch repair for bicuspid aortic valve with thickened raphe

We report a successful repair of bicuspid aortic valve having thickened raphe using pericardial patch. After excising the thickened portion, the patch was sewn to the remaining leaflet and root. To note, the height the patch was sewn to the root was lowered to the equivalent level of the corresponding portion of the other cusp, to create symmetrical bicuspid configuration. Postoperative echocardiography revealed trivial aortic regurgitation with improved leaflet motion and transvalvular flow.     

Kangaroo versus porcine aortic valve tissue--valve geometry morphology, tensile strength and calcification potential

OBJECTIVE: Valve related factors and patient related factors are responsible for calcification of valvular bioprostheses. Recent studies showed different donor and recipient species have different influences on the total calcification rate of bioprostheses. This study was performed to evaluate and compare Kangaroo aortic valve leaflets with porcine aortic valve leaflets. Experimental design. Prospective study. Setting. Cardio-thoracic experimental research of a university department. MATERIALS AND METHODS: Glutaraldehyde-fixed Kangaroo and porcine valve leaflets were evaluated in vitro according to valve geometry (internal diameter and leaflet thickness), morphology (light and electron microscopy) and tensile strength. In vivo evaluation consisted of implantation in a rat model for 8 weeks, Von Kossa stain for calcium and atomic absorption spectrophotometry for total extractable calcium content. RESULTS: Kangaroo valves indicated a smaller internal valve diameter as well as a thinner valve leaflet (p<0.01, ANOVA) at corresponding body weight, less proteoglycan spicules in the fibrosa, increased elasticity (p<0.05) and low calcification potential (p<0.01, confidence interval 95%). CONCLUSIONS: Kangaroo aortic valve leaflets have different valvular qualities compared to porcine valve tissue. Kangaroo valve leaflets are significantly superior to porcine valve leaflets as far as calcification is concerned. These results are encouraging and suggest further in vivo evaluation in a larger animal model before clinical application can be considered.