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Eur J Cardiothorac Surg 2006;29:150-155
© 2006 Elsevier Science NL

Forces at single point attached commissures (SPAC) in pericardial aortic valve prosthesis

Wolfgang A. Goetz * , Khee Hiang Lim, Romain Ibled, Nil Grousson, Sidney Le Hung Salgues, Joon Hock Yeo

Nanyang Technological University, 50 Nanyang Avenue, School of Mechanical and Aerospace Engineering, Division of Thermal and Fluids Engineering, Singapore 639798, Singapore

Received 17 October 2005; received in revised form 21 November 2005; accepted 2 December 2005.

* Corresponding author. Tel.: +65 90281501; fax: +65 90281501. (Email: wogoe{at}web.de).


   Abstract
Top
 Abstract
1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Limitations of the...
 6. Summary
 Appendix A
 References
 
Objective: New pericardial aortic bioprostheses (3F Therapeutics and temporarily stented autologous pericardial valve prosthesis) were developed recently. These valves are designed with commissures connected to the aortic wall at only three single points (single point attached commissures (SPAC)). The aim of this study was to investigate the forces acting on SPAC during varying pressure load. Methods: Aortic roots with diameters 19, 25, and 29 mm were made using silicone polymer. A bovine pericardial SPAC aortic valve prosthesis was constructed using a 3D-mold and was implanted in the silicone aortic root. The base of the valve was sutured onto the aortic annulus with 4-0 polypropylene running suture and each commissure was sutured to a miniaturized force transducer with only one 3-0 polypropylene U-stitch. Three silicon aortic roots of each size were pressurized up to 200 mmHg and forces on SPAC were measured. Results: All valves remained competent at a pressure of 200 mmHg. Recordings showed a linear correlation between applied pressure and forces measured at SPAC. At a pressure of 80 mmHg (equivalent to diastolic pressure), the forces were 0.44 ± 0.22 N, 1.15 ± 0.18 N, and 2.00 ± 0.35 N in annular diameters 19 mm, 25 mm, and 29 mm, respectively. It was observed, that the main forces were acting along the axial direction and not along the radial direction. Conclusions: Forces on "single point attached commissures" in pericardial aortic valves were measured. These forces were acting mainly in axial direction and not in radial direction. This knowledge is important for the implantation technique of SPAC pericardial aortic valves.

Key Words: Single point attached commissure • SPAC • Aortic valve prosthesis • Commissure • Force


   1. Introduction
Top
 Abstract
 1. Introduction
2. Materials and methods
 3. Results
 4. Discussion
 5. Limitations of the...
 6. Summary
 Appendix A
 References
 
The main reason for stented biological aortic valve prostheses to be implanted is to avoid the need for anticoagulation. Stents, however, have been shown to increase stresses on the valve leaflets contributing to leaflet calcification and structural failure [1]. The rigid stents are by its nature stenotic and result in non-physiologic blood flow pattern [2,3]. Stentless xenografts are used to overcome these problems, nevertheless, stentless xenografts are supported by a Dacron cloth as well as the aortic wall, causing obstruction and contribute to calcification [4]. Homografts and stentless bioprosthesis are hemodynamically superior [5] but technically more difficult to implant. The pulmonary autograft, in spite of its claim to be a permanent solution, has not reached universal acceptance because of the limited availability of homografts and the serious surgical complications secondary to its technical complexity [6].

To further reduce the stent material and the obstructive component of bioprosthesis, new aortic valve prosthesis made of pericardium has been developed.

The 3F aortic valve (3F Therapeutics, Lake Forest, CA, USA) [7,8] consists of a tubular structure assembled from three equal sections of equine pericardium treated with glutaraldehyde. A polyester ring reinforces the base of the valve. Three tabs have been placed at commissures that allow fixation of the commissures to the sinotubular junction of the aortic root at three single points.

The "temporarily stented autologous pericardial valve prosthesis" [9] is made of a fan-shaped flat strip of autologous pericardium treated for a short time with glutaraldehyde. The lateral sides of the pericardial strip are sutured together with a running 4-0 polypropylene suture converting the fan-shaped strip into a truncated cone. After suturing the base of the valve to the aortic annulus, a temporary annular stent is removed leaving the valve completely flexible without any foreign material. The commissures are connected to the aortic wall at the level of sinotubular junction, each with only one single 4-0 polypropylene U-stitch.

Both valves described are designed as tubular structure with the base of the valve sutured to the aortic annulus in a circular line and the commissures connected to the aortic wall at the level of sinotubular junction at three single points (single point attached commissures (SPAC)).

A common concern regarding the force distribution and durability of the single point attached commissures led us to investigate forces acting on SPAC in an "in vitro" setting. In this experiment, we used the molded valve design invented by Duran et al. [10] that was further developed to a SPAC pericardial aortic bioprosthesis.


   2. Materials and methods
Top
 Abstract
 1. Introduction
 2. Materials and methods
3. Results
 4. Discussion
 5. Limitations of the...
 6. Summary
 Appendix A
 References
 
2.1 Aortic root design
Aortic root was constructed using silicone polymer (VTV 750 silicon rubber from MCP-HEK Tooling GmbH, 41564 Kaarst, Germany) with a wall thickness of 3 mm. Silicon characteristics: clear transparent; tensile strength, 6.5 MPa; elongation at break, 350%; tear strength, 17 MPa; and curing time, 120 min at 70 °C. The dimension of the roots with diameters 19 mm, 25 mm, and 29 mm were taken from publications from Swanson and Clark [11] and Thubrikar [12]. The aortic root included three sinuses, a clover-shaped aortic annulus and a sinotubular junction having the same diameter as the annulus.

2.2 Valve design
For valve construction, fresh bovine pericardium was purchased from a slaughterhouse. Only pericardium with a thickness of 0.20–0.25 mm was utilized. The thickness of pericardium was measured with Digital Micrometer (Fowler; Electronic Micrometer; Fred V. Fowler Co. Inc., Newton, MA, USA). The valve prosthesis was made from a molded piece of pericardium as described by Duran [13]. The pericardium was placed between two molds with three consecutive bulges of appropriate size corresponding to the aortic annulus and the three aortic leaflets. The pericardium was then treated for 10 min in buffered glutaraldehyde (0.625%) at room temperature [13]. The shorter sides of the trapezoid were sewn together with a 5-0 polypropylene suture creating a valve prosthesis with three molded leaflets (Fig. 1 ).


Figure 1
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  		Fig. 1. Molded pericardial aortic valve.

 
2.3 Force transducer
At the level of the sinotubular junction a custom-made force transducer was designed and incorporated into the silicone wall of the aortic root at the three equidistant commissural points (Fig. 2 ). These force transducers were constructed of a V-shaped spring steel (22 mm x 4 mm x 0.2 mm; Z257; Hasco Industries Inc., MI, USA). On the free end of the spring metal there was a hole to connect the SPAC with the force transducer using 3-0 polypropylene suture. On the free arm of the spring metal two miniature strain gauges (FLE-1-11, length 1 mm, resistance 120 ± 0.3 {Omega}, factor 2.14, Tokyo Sokki Kenkyujo Co. Ltd., Gunma, Japan) were glued to the inner and outer surface (M-Bond 200 adhesive and 200 Catalyst-C; Measurements Group Inc., Raleigh, NC, USA) and coated with "M-coat A" air drying polyurethane coating (Measurements Group Inc.). The two opposite strain gauges were formed to a Wheatstone half-bridge circuit and connected to a control unit (Potable data logger DTS 303, Tokyo Sokki Kenkyujo Co. Ltd., Gunma, Japan). The transducers were individually calibrated in a range of 0–5 N.


Figure 2
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  		Fig. 2. Aortic root with force transducers and SPAC valve implanted.

 
2.4 Implantation of the valve
The base of the valve prosthesis was sutured to the aortic annulus with one running 4-0 polypropylene suture following the clover shape of the aortic annulus.

A 3-0 polypropylene suture was sewn and tied to each pericardial commissure by performing an axial U-stitch with a height of 3 mm. One arm of this suture was passed outside the aortic root through the silicone wall at each commissure and tied to the miniaturized force transducer (Fig. 2). Incorporating a felt in the wall and applying silicone oil to the suture reduced the friction of the suture, when passing through the silicone wall.

2.5 Data acquisition pressure
The aortic root was mounted on an apparatus supplying varying water pressure and including an endoscope for video recording. The silicone root was pressurized from 0 mmHg to 200 mmHg. Pressure readings were taken using Millar pressure transducer control unit (TCB 600) and Mikro-Tip® Pressure Transducer Catheters SPC 330A (Millar Instruments Inc., Houston, TX, USA) at 5 mm above commissural level.

2.6 Finite element analysis
A computational model was developed to determine the direction of the forces acting at SPAC. The developed model contained the aortic wall with sinuses and leaflets. Material properties of the wall and leaflets were 6 MPa and 0.6 MPa, respectively. The Poisson ratio was 0.45 to simulate incompressible behavior of the tissues. Since the model developed was assumed symmetrical, therefore, only one-sixth of the model was used for simulation under static pressure of 80 mmHg. Details of the model were previously described by Lim et al. [14].

Finite element analyses of the valves were carried out using ANSYS 5.7 (ANSYS Inc., Canonsburg, PA, USA) running on an SGI Origin 2000 platform server operating at the Center for Advanced Numerical Engineering Simulation (CANES) at Nanyang Technological University, Singapore.

2.7 Tensile test
Fresh porcine aortic root and bovine pericardium were purchased from a public slaughterhouse, kept moist at 4 °C and tested within 24 h. Before testing, all soft tissues were carefully removed from the outer surface of the specimens, without damaging the surface of the aortic root or pericardium. Thickness of the aortic wall was measured to be between 2.5 mm and 3.5 mm. Pericardial strips with 2 cm width by 5 cm length and thickness of 0.20–0.25 mm were treated with buffered glutaraldehyde in the same way as the valves were constructed (0.625% glutaraldehyde for 10 min) [13]. The pericardial strips were sutured to a porcine aortic root at the sinotubular junction with one axial U-stitch (width 3 mm) using 4-0 polypropylene suture. No reinforcement, like felt or pericardial pledgets, were used on both sides of the suture. Tensile strength was tested in nine specimens until cut-off point for reversal elasticity (maximum tensile strength) and breaking point in axial and radial direction. Additionally, nine tensile tests were performed with two strips of pericardium sutured together with one axial U-stitch (width 3 mm) using a 4-0 polypropylene suture and nine tensile tests with a strip of pericardium only. Maximum tensile strength of 20 standardized strip of pericardium was tested to acquire the Young modulus. The stepwise tensile tests were performed as described before [15], using Instron 5566 tensile tester including PC control unit (Instron Ltd., High Wycombe, Bucks, UK). The tests were conducted at a constant stretch rate of 10 mm min–1 and results were recorded graphically.

2.8 Statistical analysis methods
Force measurements were recorded in three silicone roots of sizes 19 mm, 25 mm, and 29 mm at each of the three commissures in six separate experiments at seven pressure steps (0 mmHg, 20 mmHg, and 40 mmHg and every subsequent step at intervals of 40 mmHg until 200 mmHg), finally giving 378 data points for each root size.

Data are given as mean ± standard deviation. Data were tested with linear regression analysis (Pearson's r) using SPSS 11.0.1. for Windows (SPSS Inc., 233 S. Wacker Drive, Chicago, IL 60606, USA).


   3. Results
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
4. Discussion
 5. Limitations of the...
 6. Summary
 Appendix A
 References
 
Video recording of the pressurized aortic root showed an immediate competent valve. Pressurizing of the aortic root up to 200 mmHg revealed that the 3-0 polypropylene sutures were primarily pulled in the axial direction towards the base of the aortic valve and not in radial direction (Fig. 3 ). All valves remained competent up to a pressure of 200 mmHg. There was no tear or rupture at the commissures in all valves tested.


Figure 3
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  		Fig. 3. Aortic root pressurized with 200 mmHg. The suture at SPAC is pulled in axial direction to the aortic annulus.

 
Forces measured at the commissures showed a linear correlation in each valve size between pressures applied and forces acting on the commissures with an excellent level of significance for the coefficients (19 mm, R 2 = 0.98; 25 mm, R 2 = 0.99; and 29 mm, R 2 = 0.99) (Table 1 , Fig. 4 ). At a pressure of 80 mmHg (equivalent to normal diastolic pressure), the forces acting on the commissures of 19 mm, 25 mm, and 29 mm valves were 0.44 ± 0.22 N, 1.15 ± 0.18 N, and 2.00 ± 0.35 N, respectively. Maximum forces measured on the commissures of 19 mm, 25 mm, and 29 mm valves at a pressure of 200 mmHg were 1.33 ± 0.33 N, 2.82 ± 039 N, and 4.27 ± 0.55 N, respectively.


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  		Table 1. Forces in Newton (N) at SPAC in aortic roots (19 mm, 25 mm, and 29 mm in size) at varying pressures (20–200 mmHg)
 

Figure 4
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  		Fig. 4. Forces acting at SPAC in aortic roots with diameters 19 mm, 25 mm, and 29 mm.

 
Finite element study confirmed that the main forces at SPAC are acting in axial direction. The main force vectors at SPAC are directed at an angle of 20.59° with respect to the axial axis (Fig. 5 ).


Figure 5
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  		Fig. 5. Finite element study of SPAC showing force vectors on pericardial leaflet.

 
Tensile test of the connection between aortic root and pericardial valve revealed a maximum tensile strength (limit beyond elastic deformation will change to plastic deformation where material will be permanently deformed) in axial direction of 4.73 ± 0.88 N before tearing of the aortic root along the circumferential collagen fibers [16]. Tensile test in radial direction revealed a maximum tensile strength of 18.6 ± 1.79 N before tearing of the aorta in seven specimens and 22.12 ± 1.80 N before breaking of the 4-0 polypropylene suture in two specimens. Tensile test of two pericardial strips sutured together with a 4-0 polypropylene U-stitch revealed a maximum tensile strength of 18.20 ± 0.90 N before breaking of the 4-0 polypropylene suture. There was no obvious tearing of the pericardium in all specimens. Glutaraldehyde-treated bovine pericardium strips only, had a maximum tensile strength of 100.84 ± 26.60 N. Standardized strips of bovine pericardium treated for 10 min with glutaraldehyde were found to have a Young modulus of 19.17 ± 3.67 MPa.


   4. Discussion
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
5. Limitations of the...
 6. Summary
 Appendix A
 References
 
New aortic bioprostheses "3F Therapeutics aortic valve prosthesis" and the "temporarily stented autologous pericardial valve prosthesis" were developed. The base of these valves is sutured to the aortic annulus in a circular line and the commissures are connected to the aortic wall at the level of sinotubular junction at three SPAC. The unique properties of these valves are their easy and fast implantation as well as the low transvalvular gradient because of the lack of any stent or foreign material at the outflow portion [7–9]. We experienced that these valves are extraordinary forgiving in terms of asymmetric implant. The valves remain competent over a wide range of different diameters at the sinotubular junction, with the commissures implanted unevenly at different heights or at three non-equidistant points. A common concern regarding the force distribution of the SPAC led us to investigate SPAC in this study.

We observed that the forces at SPAC acted mainly in the axial direction towards the aortic annulus (Fig. 3). Finite element study confirmed that the main forces acting at SPAC have an angle of 20.59° to the aortic axis (Fig. 5). The SPAC is barely exposed to forces in the radial direction as long as the free edges of the leaflets are longer than the radius at the sinotubular junction, the valve coaptation is complete and there exist no central leakage. Material properties of pericardium and aortic root were studied. Tensile strength analysis revealed that the maximum tensile strength of the aortic root at the connection of SPAC was four times higher in radial direction (18.6 ± 1.79 N) than in axial direction (4.73 ± 0.88 N). This discrepancy can be explained by the circumferential orientation of collagen fibers in the aortic wall [16].

Maximum tensile strength of the glutaraldehyde-treated pericardial strips sutured together with a 4-0 polypropylene suture (18.20 ± 0.90 N) is more than four times higher than the maximum tensile strength of the aortic wall in the axial direction (4.73 ± 0.88 N). Therefore, glutaraldehyde-treated pericardium is the strongest and the aortic wall is the weakest component of the SPAC.

The force acting on a SPAC in a 29 mm root in axial direction at normal diastolic pressure load (2.00 ± 0.35 N at 80 mmHg) is half of the maximum tensile strength of the aortic wall (4.73 ± 0.88 N) in axial direction. At a pressure of 200 mmHg, the forces at SPAC (4.27 ± 0.55 N) are equal to the maximum tensile strength of the aortic wall (4.73 ± 0.88 N) and with higher pressure it was expected that the aortic wall will tear along the circumferential collagen fibers.

Although the diastolic pressures in human will unlikely exceed the critical values, it is recommended that all knots will be tied over pericardial pledgets placed outside the aorta, particularly when the aortic wall is thin and affected by post-stenotic dilatation.

Nevertheless, studies in large animal models [9] and clinical implants [7,8] have proven mid-term durability of the SPAC valve. The acquired knowledge about force distribution and material properties is important for implantation technique of SPAC pericardial aortic valve. Further studies including fatigue tests and extensive finite element analysis are required to gain more knowledge on SPAC valves.


   5. Limitations of the study
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Limitations of the...
6. Summary
 Appendix A
 References
 
We present an "in vitro" study, which is not directly applicable to an "in vivo" situation in human. The valves were implanted in a self-made silicon aortic root according to average dimensions published before [11,12] which is different from natural asymmetric morphologic structures and material properties. The valves implanted were equally constructed using a standardized mold design. Therefore, this study cannot address disparities between different SPAC valve designs. The study was done in a static setting because the main forces on SPAC are occurring during diastole and cannot represent a dynamic situation. The friction of the suture, when passing through the silicone wall, was reduced by incorporating a felt in the wall and applying silicone oil to the suture. Linearity of the results showed that the friction was negligible. As this study is only an acute experience, the results do not reflect the long-term durability of SPAC valves. Pig aortic roots were used, as human aortic roots were not available. The maximum tensile strength parameters are widely used to compare properties of different materials. However, they can be misleading and of limited value in biomaterials in which a permanent deformation is nearly always equivalent to functional failure. Permanent deformations produce distortion in the valve mechanics that can render the structure susceptible to further deformations, leading to valve failure. Nevertheless, the results of this study give an insight into the direction and magnitude of forces acting at SPAC as well as material properties of SPAC and aortic root.


   6. Summary
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Limitations of the...
 6. Summary
Appendix A
 References
 
Forces on "single point attached commissures" in pericardial aortic valves were measured, showing a linear correlation between pressures and resulting forces. These forces are acting mainly in axial direction and not in radial direction. Tensile tests of SPAC revealed that the maximum tensile strength of the aorta in axial direction is four times higher than in radial direction. The aortic wall is the weakest component of the SPAC and reinforcement with pledgets is recommended.


   Appendix A
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Limitations of the...
 6. Summary
 Appendix A
References
 
Conference discussion

Dr A. Wahba (Trondheim, Norway): Could you explain a little bit to us, since you didn’t have time to tell us more about your work, what does that mean in practice, and would you say that valves that have a longer attachment not only in the root but higher up, that they have to be preserved, or are the forces quite small in relation to other valves? Could you elaborate a little bit on that?

Dr Goetz : Finally our question was: what are the consequences of all these findings for this kind of valve? We performed tensile tests on SPAC at the pericardial valve and at the aortic root and found that the pericardial leaflets can easily withstand these forces measured at SPAC valves, whereas the aortic root can be torn. One consequence for us is that we recommend a reinforcement of the sutures at SPAC with pledgets outside of the aortic root.

Dr R. De Simone (Heidelberg, Germany): We have done similar observations where we found a downward movement of the annulus. Do you think you can apply this kind of model to experiments in vivo?

Dr Goetz : We thought about it, but it is very difficult to implant this miniaturized force transducer in an in vivo model. And we wanted to know the maximum force at SPAC, which will appear only during diastole. We did an in vitro experiment, placing this aortic root in a pulsatile valve tester and we could see exactly the same magnitude of forces during diastole.

Dr De Simone : I am suggesting that perhaps you can use a Doppler, or tissue Doppler to get the same kind of information, if you know the angle.

Dr Goetz : As far as I know, tissue Doppler is not sensitive enough to identify these small changes and to reflect the forces acting on a pericardial leaflet.

Dr R. Gallo (Khamis Mushayt, Saudi Arabia): I have been working with Dr Duran before, and it is a little bit different than the first aortic valve that he developed. My comment is in the pulse duplicator that you are using to measure the pressure exactly, you don’t have the escape of the coronaries.

Dr Goetz : No, we did not. We had a pressurized root in a static experiment. We did not care about the coronary flow.

Dr Gallo : But to measure the exact pressure, you need the coronaries as an escape.

Dr Goetz : In a pulsatile experiment including the coronary flow or in an in vivo experiment we would have the coronary flow, which will change the situation. But our question was how durable is SPAC, when will it tear, what is the magnitude of the forces acting, and what is the direction of the forces. To answer this questions we thought we can ignore the coronary flow and we need no do a pulsatile in vitro test or to perform an in vivo experiment.

Dr Gallo : But the forces will change when you have the escape of both coronaries?

Dr Goetz : I assume that the magnitude of forces might be smaller.

Dr C. Mestres (Barcelona, Spain): You referred to how important this knowledge would be with regards to an implantation technique. So what do you suggest to us? Will it be an influence on the coaptation area of the leaflets, or what?

Dr Goetz : We measured a maximum force of 4.27 N on a 29 mm diameter valve. If you do a tensile test on an aortic root with one stitch at SPAC, you will experience that the aortic root will tear in a circumferential way. If we would pressurize a living aortic root with 200 mmHg during diastole, what we don’t do, this kind of valve will tear the aortic root. That's why we recommend that there should be a reinforcement at the outside of the aortic root.

Dr L. von Segesser (Lausanne, Switzerland): I think this knowledge has a major impact on the design of valved stents, because if you know you don’t have a force in the horizontal axis, then you don’t have to have a big expansion force in this area. So that is important knowledge.

Dr Goetz : And this is what happens at the valves from 3F, which are this kind of valves. At SPAC they will experience mainly forces in axial and not in radial direction.


   Acknowledgments
 
We appreciate the technical assistance by Associate Professor Chiam Kerm Sin Nanyang Technological University, Division of Manufacturing and Tissue Engineering Laboratory in performing Tensile Strength Analysis. The study was funded by Biomedical Research Council, Singapore.


   Footnotes
 
{star} Presented at the joint 19th Annual Meeting of the European Association for Cardio-thoracic Surgery and the 13th Annual Meeting of the European Society of Thoracic Surgeons, Barcelona, Spain, September 25–28, 2005.


   References
Top
 Abstract
 1. Introduction
 2. Materials and methods
 3. Results
 4. Discussion
 5. Limitations of the...
 6. Summary
 Appendix A
 References
 

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