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Eur J Cardiothorac Surg 2005;27:90-93
© 2005 Elsevier Science NL

In-vitro study on the relationship between progressive sinotubular junction dilatation and aortic regurgitation for several stentless aortic valve substitutes

Axel Nötzolda,1, Michael Scharfschwerdta, Lisa Thiedea, Michael Hüppeb, Hans-Hinrich Sieversa,*

a Department of Cardiac Surgery, University Hospital Schleswig-Holstein, Campus Lübeck, Germany
b Department of Anesthesiology, University Hospital Schleswig-Holstein, Campus Lübeck, Germany

Received 26 July 2004; received in revised form 28 August 2004; accepted 11 September 2004.

* Corresponding author. Address: Klinik für Herzchirurgie, Ratzeburger Allee 160, 23564 Lübeck, Germany. Tel.: +49 451 500 2108; fax: +49 451 500 2051. (E-mail: axel.noetzold{at}gmx.de).


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 
Objective: Stentless aortic valves are widely used due to their excellent hemodynamic properties. However, if the subcoronary implantation technique is used later dilatation of the sinotubular junction (STJ) can cause regurgitation. The aim of the study was to determine the dilatation tolerance of two commercially available stentless xenografts and fresh aortic and pulmonary roots against such dilatation. Methods: Four groups each comprising five specimens of fresh porcine aortic roots, pulmonary roots, Medtronic freestyle or Toronto SPV Xenografts were tested in a mock circulation using a special device for gradually increasing the diameter of the sinotubular junction. The smallest diameter Dr where regurgitation occurs was measured and correlated with the starting diameter Da and expressed as per cent values. Opening and closing patterns were obtained by a high speed camera and flow characteristics were determined. Results: The highest dilatation tolerance of STJ was found in the fresh porcine aortic roots (165%±10) followed by fresh pulmonary roots (146%±12), the Freestyle (143%±4) and the SPV (132%±5) bioprostheses. All differences were significant with P≤0.05 except that between the fresh pulmonary roots and the two commercial available bioprostheses. Conclusions: Our results indicate that aortic homografts provide higher resistance against regurgitation induced by dilatation of the STJ than an autograft or the stentless xenografts, Freestyle xenograft followed by the Toronto SPV. The use of the full-root technique should be considered if aortic dilatation seems to be likely.

Key Words: Stentless porcine valve • Sinutubular junction • Allograft • Autograft • Implantation technique


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 References
 
First aortic xenografts were implanted in a stentless fashion [1] prior to the introduction of stented bioprostheses which have been used for more than three decades with excellent results [2,3]. One shortcoming of stented bioprostheses is the fixation of the commissures which does not allow cyclic change of the commissural dimension as it normally occurs. This cyclic expansion of the commissural area serves reduction of stress on the leaflets. The principle of stentless implantation was clinically revived by Sievers et al. [4] who implanted stentless bioprostheses with different design features [5]. In the following years, the implantation of stentless bioprostheses was used more and more commonly. The advantage of stentless bioprostheses in comparison to stented bioprostheses is not only the distensibility at commissural level but also the smaller suture ring resulting in lower pressure gradients. However, the commissures of the stentless bioprostheses free-hand implanteted in subcoronary technique have to follow the dimensional changes of the aortic root not only in a cyclic mode but also concerning the naturally occuring increase of the aortic diameter [6]. This increase in dimension at commissural level pulls apart the commissures leading to reduction of coaptation area of the leaflets and finally regurgitation. The tolerance of the different stentless implants against regurgitation in relation to dilatation of the sinotubular junction (STJ) is still not clearly defined. David recommended not to use the Toronto SPV in case the STJ diameter exeeds the annular diameter by more than 3mm [7]. The pulmonary autograft is of special interest because a ‘low pressure graft’ is implanted in a high pressure environment which is the main characteristic of the Ross procedure [8]. The aim of this in vitro study was to investigate at which increase of STJ diameter regurgitation starts to occur.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 References
 
2.1. Experimental protocol
Four different types of stentless valve implants were used, firstly five fresh porcine native aortic roots (NAV) which were used at the latest within 2h after harvesting, five native pulmonary roots (NPV) of the same origin, also used within 2h after harvesting, five commercially available Medtronic stentless bioprostheses (Medtronic Freestyle®, Model 995, Medtronic, Minneapolis MN) (MF) and five also commercially available SJM bioprostheses (Toronto SPV®, Model SPA-101, St Jude Medical Inc, St Paul MN) (SPV). The diameter Da of the native aortic and pulmonary roots were measured with a caliper by pressure rising the roots to 80/25mmHg, respectively and was measured to be 26.0±0.26 and 25.98±0.21mm, respectively. The diameters of the prosthetic xenografts were labeled to be 25mm. The experimental setup consisted of a mock circulation with an integrated video camera system taking 500 pictures per second to visualise the opening and closing mechanisms of the implants and especially to detect the loss of coaptation, determining the onset of regurgitation. The STJ diameters were gradually increased using a special device in a pulsatile circuit.

2.2. Mock circulation
An open reservoir with its fluid level adjustable above valve position provides the atrial pressure. The pulsatile flow is generated by a short-stroke piston pump driven by waveform adapted cam plates at various frequencies and stroke volumes. To minimize negative pressure while filling the pump in diastole the input of the pump is built with two specially designed disc valves representing the mitral valve with little inertia during opening and nonleakage closure. An adjustable air compliance chamber at the pump outflow simulates ventricular impedance to avoid pressure and flow oscillations in systole. The composite graft was mounted in front of the pump, standing free between two holders in a fluid reservoir to keep the material moist. Above the aortic root section a box with an optical window at the upper side is mounted, permitting a view on the investigated valve.

The afterload system consists of three elements: a fluid column of variable height providing constant diastolic pressure, an adjustable air compliance chamber to provide the characteristic aortic compliance, and a nonlinear resistance element to simulate peripheral resistance. The pump rate was 60min–1 and the stroke volume were 60ml. The fluid consisted of physiological saline solution with a density of 1.05g/ml. A near normal pressure wave form with systolic pressures of 120mmHg and a diastolic pressure of 80mmHg was initiated.

2.3. Device for gradually increaseing sinotubular diameters
A special device (Fig. 1) was mounted at the area of the STJ and allowed a gradual increaseing in sizes of the sinotubular area equilaterally. The stentless grafts were implanted in a prosthetic tube. The prosthetic tube included a redundant patch material in the sinuses which allowed diameter increase at the STJ.



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  		Fig. 1. Schematic illustration of the Dacron® composite prosthesis including a device for gradually increasing the sinotubular junction diameter. If the hose clip is widened (circular arrows) the commissures are moving apart (outward arrows) while unfolding the extension patches. com, commissures; fep, folded extension patch; hc, hose clip; vl, valve leaflets.

 
2.4. Data collection and analysis
Valve motions were recorded with a Motionscope HR-1000 high-speed camera (Redlake Imaging Corp., Morgan Hill, CAL) at a rate of 500 frames per second, positioned right above the valve outflow. Video data were digitalized and analysed with a custom-designed motion evaluation software. Both video and data recording were started simultaneous and were synchronized by trigger signals of the high-speed camera. Time delay between measured data and video recording was less than 3ms. For every investigated valve each video sequence was carefully evaluated to assess the smallest STJ-diameter Dr at which a loss of central coaptation of the leaflets was observed. In every series the frame with the best coaptation was chosen. These diameters were compared to the inital values Da of the industrial prostheses, or respectively of the aortic and pulmonary roots. The Figs. 2 and 3 illustrate original pictures of a valve at a STJ diameter with first coaptation loss (Fig. 2) and under considerably higher distension of the STJ, causing severe regurgitation (Fig. 3). The pictures were obtained during diastole.



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  		Fig. 2. Onset of coaptation loss in the center of a SPV valve at D=31.5mm.

 


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  		Fig. 3. Advanced coaptation loss of the same valve at D=41.9mm.

 
Statistical analysis was performed with the statistical language R 1.8.1 [9]. Data were expressed as mean±SD. The minimum diameter of the STJ causing insufficiency was assessed by visually analysing the video records and was expressed as percentage of the initial diameter. Initially a Kruskal–Wallis rank sum test was performed and subsequently the differences between the groups were proven with the Wilcoxon rank sum test. Any difference was considered significant for P<0.05. Effect sizes were computed according to Cohen J. [10] and considered to be large if d>0.8 [10].


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 References
 
Native aortic valves exhibited the highest resistance against STJ-dilatation (165%±10). A significantly lower (P=0.03, d=1.75) but fairly high resistance can be demonstrated for the NPV-group, but however with a high variability (146%±12). The prosthetic specimens are tolerating dilatation to a significantly lesser extent than NAV (P=0.008 for both and dMF vs NAV=2.87 and dSPV vs NAV=4.15), the freestyle showing the better values (143%±4 vs 132%±5; P=0.004, d=2.28). The differences between SPV and NPV as well as between MF and NPV are not significant. A boxplot of the percentage of the Dr/Da-values is provided in Fig. 4. The absolute, relative and basic values are given in Table 1.



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  		Fig. 4. Boxplot of the percentage of the relative dilatation of the sinotubular junction which can be tolerated without regurgitation. Given are the P-values. NAV, fresh aortic valves; MF, Medtronic freestyle®; NPV, fresh pulmonary valves; SPV, Toronto SPV®.

 

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  		Table 1. Absolute and relative diameters of the sinotubular junction causing regurgitation and basic values
 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
References
 
In this study we provide evidence that the commercially available glutaraldehyde treated stentless xenografts have a reduced tolerance against of regurgitation with respect to dilatation of the STJ in comparison to native porcine aortic and pulmonary roots. In the field of bioprosthesis replacement of the aortic valve, stented bioprostheses were considered as the ‘Gold Standard’ for several decades. More or less fix commissures, however, induce abnormal stress on the leaflets which might have contributed to the limited durability of these implants. To overcome this problem, stentless bioprostheses were re-introduced 1985 by Sievers et al. [5,11] In the mean time, several commercially available stentless bioprostheses are on the market and have proven to be superior to stented bioprostheses with reference to pressure gradient and effective orifice area [12,13], as well as survival [14]. This is not only related to the more flexible commissures but also to the little amount of fabric tissue at the proximal suture line. Since the stentless valves are sutured to the native wall of the recipient aortic root, however, there exists a risk for the development of aortic regurgitation over time with increasing diameter at the STJ which occurs naturally during life to a greater or lesser extent [7,15–17]. If size increase exceeds certain limits, aortic regurgitation may occur after implantation of stentless implants, because the prosthesis commissures have to follow the dilatation of the root. This could implement a limitating factor for improved durability of stentless bioprostheses compared to stented bioprostheses. We found a significant difference of tolerance against aortic regurgitation with respect to dilatation of the STJ between native porcine aortic and pulmonary roots on the one hand and stentless bioprostheses on the other hand. Most likely this is related to the fixating procedure of the bioprostheses which stiffens and shrinks the leaflet material, preventing adaptive mechanisms and the natural tolerance against STJ dilatation. We found also a difference in the onset of regurgitation between the two commercially available bioprostheses from Medtronic® and SJM®. Whether the different fixation processes relate to this finding is speculative, but could have an important influence in this respect. The Medtronic® valve is fixated with zero pressure, the SJM® valve is fixated with low pressure. The different characteristics of these stentless bioprostheses might have an influence regarding late onset of regurgitation when undue dilatation of STJ occurs in the patient. These findings also emphasize the need for exact sizing of the STJ and valve during implantation of stentless bioprostheses. Undersizing of a stentless bioprosthesis could promote early occurence of regurgitation and should be avoided. On the other hand, oversizing could be a protective measure to get more tolerance against dilatation related regurgitation but it will also increase the stress on the leaflets due to bending and flexion. The lower tolerance of native pulmonary grafts against STJ dilatation compared to aortic grafts may be conditional upon the usage of a ‘low pressure graft’ in a high pressure environment which consumes part of the distensibility after being subjected to systemic diastolic pressure of 80mmHg [8]. Whether the lower resistance of pulmonary roots against coaptation loss under STJ distension is of clinical importance remains unclear even though the Yacoub group demonstrated functional and histological modes of adapation of pulmonary autografts to systemic pressure postoperatively [18].

One limitation of the study is that the experiments are performed under an unvariable diastolic pressure. It can be assumed that with a different diastolic load there may be also a change on distension tolerance for the different grafts. Furthermore, the results presented are valid only on the assumption that the tissue characteristics such as dimension, flexibility, movement during cyclic change of blood pressure and histological integrity of the valves do not change over time. For allogenic and xenogenic material it is likely that tissue alterations might reduce tolerance against dilatation related regurgitation. In addition, we did not use a non-Newtonian fluid. This may have an influence on hemodynamics and valve motion and possibly also on the amount of regurgitant volume.

In conclusion, there is a difference between tolerance towards aortic regurgitatiton (related to dilatation of the STJ) not only between native aortic and pulmonary porcine roots and stentless bioprostheses in systemic circulation, but also between different commercially available xenografts.


   Footnotes
 
{star} Presented at the joint 18th Annual Meeting of the European Association for Cardio-thoracic Surgery and the 12th Annual Meeting of the European Society of Thoracic Surgeons, Leipzig, Germany, September 12–15, 2004.

1 Present address: Segeberger Kliniken GmbH, Am Kurpark 1, Bad Segeberg, 23795, Germany. Back


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 References
 

  1. Binet JP, Carpentier A, Langlois J. Clinical use of heterografts for replacement of the aortic valve. J Thorac Cardiovasc Surg 1968;55(2):238-242.[Medline]
  2. Jamieson WR, Burr LH, Munro AI, Miyagishima RT. Carpentier–Edwards standard porcine bioprosthesis: a 21-year experience. Ann Thorac Surg 1998;66(6):S40-S43.
  3. David T, Ivanov J, Armstrong S, Feindel CM, Cohen G. Late results of heart valve replacement with the Hancock II bioprosthesis. J Thorac Cardiovasc Surg 2001;121(2):268-277.
  4. Sievers HH, Lange PE, Bernhard A. Implantation of a xenogeneic stentless aortic bioprosthesis. First experience. Thorac Cardiovasc Surg 1985;33(4):225-226.[Medline]
  5. Sievers HH, Mahmoodi M, Marquardt P, Nellessen U, Höfig M, Angell W, Bernhard A. Unstented and partial stented bioprostheses for aortic valve replacement-up to 6 years of follow-up. J Card Surg 1991;6(4):600-605.[Medline]
  6. Sievers HH, Onnasch DG, Lange PE, Bernhard A, Heintzen PH. Dimensions of the great arteries, semilunar valve roots, and right ventricular outflow tract during growth: normative angiocardiographic data. Pediatr Cardiol 1983;4(3):189-196.[CrossRef][Medline]
  7. David T. Aortic valve replacement with stentless porcine bioprostheses. J Card Surg 1998;13(5):344-351.[Medline]
  8. Sievers HH, Leyh R, Loose R, Guha M, Petry A, Bernhard A. Time course of dimension and function of the autologous pulmonary root in the aortic position. J Thorac Cardiovasc Surg 1993;105(5):775-780.[Abstract]
  9. R Development Core Team. R: A language and environment for statistical computing, Vienna, Austria; 2003, ISBN 3-900051-00-3..
  10. Cohen J. Statistical power analysis for behavioral sciences. Hillsdale: Erlbaum; 1988.
  11. Sievers HH, Lange PE, Bernhard A. Implantation of a xenogeneic stentless aortic bioprosthesis. First experience. Thorac Cardiovasc Surg 1985;33(4):225-226.
  12. del Rizzo DF, Goldman BS, Christakis GT, David TE. Hemodynamic benefits of the Toronto Stentless Valve. J Thorac Cardiovasc Surg 1996;112(6):1431-1445.[Abstract/Free Full Text]
  13. Melina G, Mitchell A, Amrani M, Khaghani A, Yacoub MH. Transvalvular velocities after full aortic root replacement: results from a prospective randomized trial between the homograft and the Medtronic Freestyle bioprosthesis. J Heart Valve Dis 2002;11(1):54-58.[Medline]
  14. del Rizzo DF, Freed D, Abdoh A, Doty D, Goldman BS, Jamieson WR, Westaby S. Midterm survival of stented versus stentless valves: does concomitant coronary artery bypass grafting impact survival?. Semin Thorac Cardiovasc Surg 2001;13(4 Suppl 1):148-155.[Medline]
  15. Shargall Y, Goldman B, Christakis G, David T. Analysis of explants and causes of mortality during long-term follow-up of the Toronto stentless porcine valve. Semin Thorac Cardiovasc Surg 2001;13(4 Suppl 1):106-112.[Medline]
  16. David T, Omran A, Ivanov J, Armstrong S, de Sa MP, Sonnenberg B, Webb G. Dilation of the pulmonary autograft after the Ross procedure. J Thorac Cardiovasc Surg 2000;119(2):210-220.[Abstract/Free Full Text]
  17. Keane MG, Wiegers SE, Plappert T, Pochettino A, Bavaria JE, Sutton MG. Bicuspid aortic valves are associated with aortic dilatation out of proportion to coexistent valvular lesions. Circulation 2000;102(19 Suppl 3):III35-III39.
  18. Carr-White GS, Afoke A, Birks EJ, Hughes S, O'Halloran A, Glennen S, Edwards S, Eastwood M, Yacoub MH. Aortic root characteristics of human pulmonary autografts. Circulation 2000;102(19 Suppl 3):III15-III21.



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This Article
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