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Ann Thorac Surg 2004;78:1060-1063
© 2004 The Society of Thoracic Surgeons
a Clinic of Cardiac Surgery, University Clinic of Schleswig-Holstein, Campus Lübeck, Lübeck, Germany
Accepted for publication September 22, 2003.
* Address reprint requests to Prof Dr Sievers, Clinic of Cardiac Surgery, University Clinic of Schleswig-Holstein, Campus Lübeck, Ratzeburger Allee 160, 23538 Lübeck, Germany
herzchir{at}medinf.mu-luebeck.de
Abstract
PURPOSE: Current replacement of the thoracic aorta performed with straight vascular prostheses may cause kinking, potentially affecting hemodynamics and promoting vortices and thrombus formation. A novel vascular prosthesis, resistant to pressure-related shape deformation, was designed to imitate the curved anatomy of the thoracic aorta.
DESCRIPTION: A woven velour prosthesis was trimmed with cross-sutures along a marked line, resulting in a curved-shaped anatomic form, and was compared with conventional straight and thermally fixed curved grafts. The vascular prostheses were fixed at both ends at various base distances (8, 10, 12, 14, and 16 cm) and pressurized. To imitate the neck vessels an abutment was fixed at the upper convexity of the grafts. Radius of curvature or depth of kinking was measured at different pressures (100, 125, and 150 mm Hg). Pressure gradients and flow profiles were further analyzed in an aortic arch glass model.
EVALUATION: When pressurized the straight and the thermally fixed protheses showed double kinking before and behind the abutment at all pressures and distances. Kinking depth increased with increasing pressure and less base distance. Transkinking pressure gradients increased with the degree of kinking. In a glass model flow profiles showed postkinking turbulences and vortex formation. The newly designed vascular prosthesis showed no kinking and remained form stable at all test conditions.
CONCLUSIONS: This novel curved vascular prosthesis for replacement of the thoracic aorta demonstrates form stability compared with conventional straight and thermally fixed vascular prostheses in an aortic arch model, with smaller pressure gradients and flow disturbances.
| Dr Sievers discloses that he has a financial relationship with Aesculap AG Tuttlingen, Germany.
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Until now, replacement of the curved aortic arch has been performed with straight grafts. However, these grafts are not form stable when pressurized. Thus, kinking may occur (Fig 1) with the potential risk of promoting turbulence, thrombus formation, unphysiologic flow pattern, development of pressure gradients, and nonuniform stress distribution at the anastomosis. Therefore, a novel prosthesis that keeps the curved design of the natural thoracic aorta stable after pressurizing was designed for replacement of the ascending aorta or aortic arch to overcome these potential problems.
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Three types of woven, collagen-impregnated vascular velour prostheses (Aesculap, Tuttlingen, Germany) of equal length (19 cm) and diameter (30 mm) were compared with each other. A straight prosthesis, a thermally fixed curved prosthesis (30 minutes at 160°C), and the novel, form-stable prosthesis were compared. The latter one showed a curved form, which was achieved by single, crossed stitches along a marked line (Premicron USP 3-0, Aesculap, patent pending). Grafts were fixed at each end on a horizontal plane at various base distances of 8, 10, 12, 14, and 16 cm to imitate the size variability of the curved aortic arch (Fig 2). A plate (length, 6 cm) was fixed at the upper convexity of each graft type to imitate the influence of the abutment of cranial vessels on graft curvature. Grafts were pressurized with increasing pressures (100, 125, and 150 mm Hg) by means of an adjustable water column. After pressurization at various base distances, a photograph of the prostheses with a centimeter scale was taken for further analysis. Changes in graft shapewere expressed as changes in radius of graft curvature or, if kinking occurs, as depth of kinking fold.
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Results
Straight (Fig 3A) and thermally fixed grafts (Fig 3B) showed double kinking before (ascending aorta) and after (descending aorta) the abutment, whereas the novel prosthesis preserved its form stability (Fig 3C).
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This study demonstrates thepressure-independent and radius-independent form stability of a newly designed vascular prosthesis for replacement of the ascending aorta or aortic arch compared with conventional vascular prostheses.
In the natural ascending aorta and aortic arch, intrinsic properties of the tissue architecture preserve the curved form of this vessel to guide the pulsatile flow in a helical fashion from the left ventricle to the descending aorta to almost a complete circle [1]. Optimal replacement of this structure should therefore imitate these anatomic conditions. As a standard method for the replacement of the thoracic aorta straight vascular grafts are used. They have to be trimmed in a certain manner to imitate the aortic arch. Even after these surgical maneuvers replacement of the ascending aorta or aortic arch could result in kinking of the prosthesis. Kinking, however, causes flow disturbances by creating turbulence and vortex formation. These irregularities of blood streaming may cause pressure gradients and serve as a source for thrombus formation. Furthermore, the trend to recoil to the straight original shape in relation to the amount of pressurizing could generate undue stress development at the side of the anastomosis during the cardiac cycle. So far new designs of vascular prostheses for replacement of the thoracic aorta predominantly address the imitation of the sinus of Valsalva to improve aortic valve function [2, 3], or the cranial branches of the aortic arch [4]. Thermally fixed vascular grafts have been designed to imitate the curved aortic arch, but they still have the risk of graft kinking, because systolic pressure overcomes forming forces introduced by thermal fixation. To improve form stability, a novelvascular prosthesis was designed, imitating the natural anatomy of the thoracic aorta even under pressure, and with a different radius of curvature imitating size variability of the aortic arch. This could be achieved by integrating a special suture technique, which fixes the graft at the concave curvature and consequently stabilizes the convex surface.
This prosthesis can also be trimmed for partial aortic arch replacement, whereby form stability is preserved. As this prosthesis is constructed of conventional vascular graft material, it can be used in all standard operations involving the ascending aorta or aortic arch. Our in vitro results confirm the theoretical considerations. We did not find any kinking and, thus, no flow turbulences in the novel prosthesis in contrast to straight and thermally fixed ones. These kinking folds cause pressure gradients that increase with systolic pressure and smaller radii of curvature. Especially if total arch replacement is performed, two kinking folds may occur (Figs 3A, 3B), potentially leading to clinically relevant pressure reduction in the descending thoracic aorta. In addition, turbulences were observed after the kinking folds (Fig 8), bearing the risk of thrombus formation. Whether this is of clinical significance needs further evaluation.
Theoretically, the form stability of the new vascular graft should be advantageous with respect to replacement of the thoracic aorta preserving the natural undisturbed curvature and, thus, the physiologic flow conditions. Further in vivo studies are required to investigate the intraoperative application of this prosthesis.
Disclosures and freedom of investigation
The novel vascular prosthesis was developed by the authors in collaboration with the Aesculap AG Tuttlingen, Germany. The authors have performed a free and independent evaluation of this new technology. Nevertheless, Dr Sievers partly owns patents on this prosthesis, licensed to Aesculap Tuttlingen, Germany; thus, there is a financial conflict of interest.
| DisclaimerThe Society of Thoracic Surgeons, the Southern Thoracic Surgical Association, and The Annals of Thoracic Surgery neither endorse nor discourage use of the new technology described in this article.
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References
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  A. Salemi and G. B. Pettersson Aortic arch reconstruction utilizing a simple reversed graft trimming technique Interactive CardioVascular and Thoracic Surgery, June 1, 2006; 5(3): 247 - 248. [Abstract] [Full Text] [PDF]
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