Interact CardioVasc Thorac Surg 2009;9:672-676. doi:10.1510/icvts.2009.211102
© 2009 European Association of Cardio-Thoracic Surgery
ESCVS article - Experimental |
Cutting precision in a novel aortic valve resection tool. Research in progress ,
Daniel Wendta,*,
Sebastian Stühlea,
Hermann Wendta,
Matthias Thielmanna,
Brigitte Kipfmüllerb,
Florian Haucka,c,
Bernd Vogelc and
Heinz Jakoba
a Department of Thoracic and Cardiovascular Surgery, West-German Heart Center Essen, University Hospital Essen, Hufelandstraße 55, 45122 Essen, Germany
b Endosmart GmbH, Stutensee, Germany
c Department of Physical Engineering, Fachhochschule Gelsenkirchen, Gelsenkirchen, Germany
Received 4 May 2009;
received in revised form 24 June 2009;
accepted 29 June 2009
 Presented at the 58th International Congress of the European Society for CardioVascular Surgery, Warsaw, Poland, April 30–May 2, 2009. 
This study was supported by the ‘Arbeitsgemeinschaft industrieller Forschung' (AIF) (Grant PROINNO KF0373601PK6).
*Corresponding author. Tel.: +49-201-723-84912; fax: +49-201-723-5451.
E-mail address: daniel.wendt{at}uk-essen.de (D. Wendt).
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Abstract
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Background: We recently demonstrated the first in-vitro cutting results of a minimal-invasive aortic valve resection tool. The current study was designed to assess the cutting accuracy of this new device improved by the implementation of a linear motor-based propulsion unit. Methods: Native aortic valves of isolated swine hearts (valve diameter 17.8±0.9 mm, mean±S.D.) were artificially stenosed and calcified (n=7). Subsequently, valves were resected by the use of a new aortic valve resection tool. The cutting process was performed by fitting the instrument with foldable Nitinol cutting blades (diameter 15 mm) and two software-operated linear motors combined with separated manual rotation. Aortic valve area was measured pre- and postprocedure by software-guided binary area calculation. Aortic valve residue has been determined and the grade of accuracy has been assessed via calculating the average midpoint of the neoannulus. Furthermore, radial deviation of concentricity was calculated and cutting time was measured. Results: Aortic valve resection was successful in all cases and nearly all leaflets (2.5±0.4) with a weight of 0.22±0.12 g were cut. Aortic valve area increased significantly (0.3±0.1 cm2 vs. 1.1±0.2 cm2, P<0.001) with a mean cutting time of 49.7±15.0 s. Mean lateral leaflet rim within the annulus was 3.2±3.2 mm. Cutting precision revealed a median deviation of the cutting ring from the desired position of 1.3±0.6 mm (y-axis) and 1.4±0.5 mm (x-axis). Median center deviation of the cutting ring was 2.6±0.8 mm. Conclusions: The present study clearly confirmed ability of an accelerated cutting of stenotic aortic valve by the aortic valve resection tool. Nearly all leaflets were cut and a small rim was left within the annulus, hence providing an ideal ‘landing zone' for the new prosthesis. Nevertheless, the aortic valve resection tool should be enhanced by adding a centering mechanism, thus achieving a more precise cutting process in order to avoid secondary damage.
Key Words: Aortic stenosis; Aortic valve resection; Resection tool; Sutureless valve implantation; Cutting performance; Minimal-invasive
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1. Introduction
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Degenerative and calcified aortic stenosis (AS) is the most frequent form of adult heart valve disease with an increased prevalence in the elderly and the prognosis is poor once symptoms occur [1]. At present surgical aortic valve replacement (AVR) by the use of extracorporeal circulation is the golden standard which is commonly performed with optimal results [2]. However, valve excision and thorough debridement of the annulus may be rather time consuming especially in the case of severe calcification followed by encircling the annulus with sutures for valve fixation. Therefore, owing to an aging and multimorbid patient population, contemporary patients will benefit from reduced ischemic and procedure time [3]. Thus, the accelerated resection of the stenotic native aortic valve would be favorable prior to the implantation of the new prosthetic aortic valve. For this purpose, the West German Heart Center Essen in collaboration with Endosmart® GmbH and the Department of Physical Engineering, Fachhochschule Gelsenkirchen, developed an innovative prototype of a novel aortic valve resection tool based on foldable nickel-titanium (NiTi) blades (European and US patent pending) which offers the possibility of an accelerated valve resection [4, 5].
The aim of the present study was, therefore, to determine the cutting accuracy and precision of this new aortic valve resection tool in an in-vitro study.
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2. Material and methods
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2.1. Study design
Resection of artificially stenosed aortic valves was performed in ex-vivo porcine hearts (n=7). All studies were performed with the prototype of our minimal-invasive aortic valve resection tool equipped with rotating and foldable Nitinol cutting edges (further details will be listed below). In contrast to our previous studies an implemented linear motor drive system enabled precise translation.
During each experiment the following data were obtained: - Aortic valve diameter (mm)
- Aortic valve area, pre- and post-resection (cm2)
- Resecting time (s)
- Number of resected leaflets
- Maximum aortic valve residue per valve leaflet (mm)
- Distance from aortic valve rim to the aortic wall at nine defined measuring points (mm)
Aortic valve diameter, aortic valve residue and the distance from the valve rim to the aortic wall was measured by an analog slide gauge with a resolution of 0.05 mm (Promat RN 11, Max Schön AG, Lübeck, Germany). Aortic valve area was calculated by image processing and analysis software (ImageJ, U.S. National Institutes of Health, Bethesda, Maryland, USA). Calculation was based on a digitally recorded photography of the aortic valve (including a metric reference scale) prior to and after the resection process. The relevant section was enlarged and the aortic valve area was digitally inked (Adobe Photoshop Elements, Adobe Systems, San Jose, California, USA). Additional shading and filling of the relevant parts improved contrast. Afterwards, a binarization was performed and by the use of the reference scale, the aortic valve area was automatically calculated. The process of aortic valve area calculation is illustrated in Fig. 1.
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Fig. 1. Artificially ‘stenosed' porcine heart (a), aortic valve after digital processing (b) and calculation of aortic valve area before resection (c).
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2.2. Cutting precision
Cutting precision was defined as the deviation of the midpoint of the neo-annulus (center deviation) and the deviation of the whole neo-annulus from the calculated central position. After the cutting process, the rim dimensions were obtained at nine defined and fixed measuring points (Fig. 2). Afterwards, the Cartesian coordinates of these measuring points and the designated positions, depending on determined aortic valve and cutting ring diameter (15 mm) were calculated. In order to calculate the cutting accuracy, the midpoint of the cutting ring (15 mm) was calculated and was compared for each experiment with the new created midpoint after the resection procedure. The Pythagorean theorem calculated the Cartesian coordinates of the created midpoint after the resection. By the use of a linear system of equations, in each case, three arbitrary real positions (out of the nine measuring points) computed the midpoint of the neo-annulus. The arithmetic mean of 15 calculated midpoints defined the created midpoint of the neo-annulus. The distance between the midpoint of the neo annulus and the desired midpoint defined the median center deviation and was additionally calculated in x- and y-direction. Furthermore, the median deviation of the cutting ring from the desired position was calculated. This deviation was calculated by subtracting the real positions of the annulus from the desired positions within the Cartesian coordinates graph and was calculated for x- and y-deviation.
2.3. Aortic valve resection tool
Aortic valve resection was performed by using the aortic valve resection tool as previously described by our group [5]. The prototype of this innovative, easy-handling and short-time-procedure device is illustrated in Fig. 3. Due to anatomical reasons (in contrast to our previous studies) the current experiments were performed with 15 mm diameter cutting edges. Opposed to our preliminary tests, the resection tool was equipped with a linear motor drive system. This technical innovation offers on the one hand the decoupling of the two motions and on the other hand, the translation could be easily changed by digitally adjusting form feed of both linear motors.
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Fig. 3. Aortic valve resection tool. A, apex of the instrument including cutting edge; B, acrylic instrument shaft; C, body including blade unfolding mechanism; 1, lever inducing blade release; 2, lever inducing blade expansion; D, gear and linear motor housing; E, handle including ratchet drill; F, linear motors combined and G, embedded linear drive system.
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2.4. Linear motors and motion controller
The implementation of two synchronal linear motors (LM 1247 ‘Quickshaft', Dr Fritz Faulhaber GmbH & Co KG, Schönaich, Germany) decoupled feed motion and manual rotation during the cutting procedure. The linear motors allowed precise heading with automated linear form feed (0.175 mm/step) by a computer-controlled, customized sequence program. Maximum and nominal force of each motor was 9.3 N and 3.1 N, respectively. A ring connected both motors with the manual driven rotation part of the instrument, decoupling automated heading from manual rotation (Fig. 3 F/G). Motion control units operated (Motion Controller MCLM 3003/06 C, Dr Fritz Faulhaber GmbH & Co KG, Schönaich, Germany) and the Motion Manager© software package offered user-defined heading of both linear motors (version 3.0, Dr Fritz Faulhaber GmbH & Co KG, Schönaich, Germany).
2.5. Test specimens: swine hearts
Seven porcine hearts, one day after explantation (stored at 5 °C), taken from five-month-old swine were prepared for aortic valve resection. After removing of blood residue, the aorta was cut one centimeter above the aortic valve. The left ventricle was opened 40 mm below the aortic valve enabling unlimited view during the cutting process.
The aortic valve was artificially stenosed by fusing the three unrestricted leaflets (Prolene 6-0, Ethicon, Norderstedt, Germany) and additionally, the aortic valve was freezed by a cooling spray prior to the resection process (75 Super, Kontakt Chemie, CRC Industries Deutschland GmbH, Iffezheim, Germany). Both methods simulate aortic valve ‘calcification'. A ‘stenosed' valve is illustrated in Fig. 1a.
2.6. Statistics
Continuous and normally distributed data were reported as mean±standard deviation (S.D.). A Student t-test was used for paired data testing. A P<0.05 was considered to indicate statistical significance. All statistical analyses were performed using the Sigma-Stat® software package, version 2.0 (Systat Software Inc, San Jose, CA, USA).
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3. Results
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3.1. Test setup
Seven artificial stenosed porcine aortic valves with a diameter of 17.8±0.91 mm were used for the resection procedures. All valves were cut and no signs of complications occurred during the resection. A total of 2.5±0.4 leaflets were resected, leaving a flat and smooth surface within the annulus.
Neither the resection tool nor the linear motor drive system showed any signs of malfunction during the experiments. A resection result is illustrated in Fig. 4. Results are listed in Table 1.
3.2. Resection time
Resection time was 49.7±15.0 s (range 26–64 s). Results are listed in Table 1.
3.3. Cutting precision
The mean lateral leaflet rim after resection was 3.2±3.2 mm. Aortic valve area increased from 0.29±0.1 cm2 to 1.1±0.2 cm2 (P<0.001). The mean center deviation of the cutting ring was 2.6±0.8 mm (x-axis: 2.1±0.9 mm; y-axis: 1.2±1.2 mm). The median deviation of the whole neo-annulus from the desired position was 1.4±0.5 mm (x-axis) and 1.3±0.6 mm (y-axis), respectively. Results are listed in Table 1. An example of Cartesian coordinates based cutting precision calculation is illustrated in Fig. 5.
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Fig. 5. Cartesian coordinates calculation. Desired position of the 15 mm cutting edge (square), measured position (circle), measuring points (diamond) and deviation from midpoint (arrow).
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4. Discussion
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The major findings of the present in-vitro study are that (1) the cutting precision of our aortic valve resection tool was acceptable and (2) the feed motion could be performed by the use of an integrated linear motor drive system.
Currently, AVR is the golden standard treatment option in patients presenting with symptomatic aortic valve stenosis as there is no effective medical therapy [6, 7]. Furthermore, AVR should be performed promptly after the onset of symptoms since morbidity and mortality will increase once symptoms occur [2, 8]. During the last years, patients referred for surgery present gradual increases in age and overall risk profile. However, age is not a contraindication to surgical AVR and actual mortality in patients undergoing isolated AVR has been reported to be 1.0% for patients <60 years, 1.3% for patients <70 years and below 3.5% for those <80 years [9]. Despite these excellent results in conventional AVR, new minimal invasive treatment options, like partial sternotomy or sutureless valve implantation in the open approach or transcatheter aortic valve implantation techniques in the beating heart situation, aim to reduce operative trauma and myocardial ischemia [3, 10–14]. Nevertheless, additional improvements of the current techniques are necessary to maintain and improve these excellent results, even with the actual trend of growing age in the Western industrial population resulting in an increase of comorbidities. The present work aimed to obtain the cutting precision of a new minimal-invasive aortic valve resection tool. This current prototype was developed to decrease the required time for surgical aortic valve resection. Conventional, isolated AVR will result in aortic cross-clamp times around 55–60 min (stented prosthesis) and around 75–80 min by the use of stentless valve prostheses [15, 16]. During this time, probably 10 min are needed for valve excision and thorough debridement of the calcified aortic annulus and another 15–20 min will elapse for placing the sutures and subsequent valve deployment. Both are essential steps in surgical AVR. Valve excision and prosthesis implantation represent the major time periods during conventional AVR. Therefore, an accelerated valve resection, even when combined with an accelerated valve implantation will result in a decreased aortic cross-clamp time and myocardial ischemia. In a previous in-vitro study, we confirmed the feasibility of an accelerated aortic valve resection within 60 s by the use of our new aortic valve resection tool [5]. The present study confirmed these findings in ex-vivo porcine hearts, resulting in a mean resection time of 49.7±15.0 s.
Aortic cross-clamp times were reported to vary between 18 and 56 min by the use of sutureless aortic valve prostheses [3, 14, 17], but further improvements and developments in sutureless valve techniques as well as growing experience using these concepts will definitively reduce ischemic times.
Moreover, determining the valve diameter prior to initiation of extracorporeal circulation represents one of the major advantages of the present philosophy. The aortic annulus can be measured by TEE guidance prior to extracorporeal circulation and a corresponding cutting blade can be predetermined and fixed within the instrument's tip. Additionally, an appropriate sutureless or balloon-expandable aortic valve prosthesis can be rinsed and prepared in advance, thus decreasing ischemic time as well. Not only the implantation of biological, sutureless aortic valve prostheses will be possible, but also the implantation of a mechanical, sutureless aortic valve will be feasible [18].
A small circular remnant of 1–2 mm within the aortic annulus should be left, which acts as the ‘neo-annulus'. This ‘neo-annulus' plays an essential role as a kind of ‘landing zone' for the implantation of the new prosthesis. In the present study, the mean lateral leaflet rim was 3.2±3.2 mm allowing to anchor the new prosthesis. The new prosthesis should be equipped with a ‘c-shaped' annulus, covering the ‘neo-annulus' from below and above, preventing paravalvular leakage by this method.
In this entire context, the cutting precision of this new aortic valve resection device is of utmost importance. The precise alignment, fixation and stabilization before and during the whole cutting process must be guaranteed and evaluated since secondary injury of the aortic wall, the ventricular septum or the mitral valve leaflets may occur. Within the present study a median deviation of the cutting ring from the desired position was moderate (1.4±0.5 mm on the y-axis and 1.3±0.6 mm on the x-axis). But keeping in mind, that the new created ideal ‘neo-annulus' should only be 1–2 mm in width, this accuracy seems to be imprecise, particularly when considering the median center deviation of 2.6±0.8 mm. In the present study, the imprecise centring may be due to porcine anatomical reason as the non-coronary cusp of the trileaflet aortic valve appeared to be partly fused with the porcine ventricular septum. Moreover, this may be a potential reason for incomplete cutting of all three leaflets (2.5±0.4). It remains unclear if the present in-vitro results were comparable to the human in-vivo situation, as we used artificially calcified porcine aortic valves, which may not mimic native valve pathology. Future tests should be performed in a model on real native aortic valves (e.g. worn out biological root prosthesis or human cadaver experiments). Furthermore, different intensities of calcification, especially asymmetric calcifications should be tested. Nevertheless, additional centring mechanisms must be implemented in the current instrument to achieve an optimal position in order to prevent secondary damage.
Furthermore, the particle generation during the cutting process is crucial. During valve resection, created debris must be removed by additional safety features like the incorporation of filter systems or integration of a suction device. In this open heart situation using extracorporeal circulation, flushing of the outflow tract and the whole situs after resection is essential. As well, the secure protection of the coronary ostia during the resection process has to guaranteed [5].
In conclusion, the present concept enables an accelerated AVR in combination with a self-expanding or balloon-expandable sutureless aortic valve prosthesis, reducing aortic cross-clamp times to maybe <10 min. Nevertheless, further improvements, like centring mechanisms are necessary in order to prevent secondary damage. Anyway, this concept may result in accelerated conventional AVR, making, therefore, classic surgery again more attractive.
4.1. Limitations
Within the present study, aortic valve resection was based on porcine swine hearts prepared for aortic valve resection. Therefore, this model does not simulate ‘real-life' human conditions due to different valve anatomy, as the native non-coronary porcine cusp is fused with the septum of the ventricle. Moreover, our design of valve calcification by suturing and freezing the native porcine aortic valve cusps is still lacking. Future studies should be performed in more realistic models, like human cadaver experiments before considering the first clinical application.
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References
|
|---|
- Kahlert P, Khandanpour S, Sack S, Erbel R. Perkutaner Aortenklappenersatz – Eine neue Alternative zur Operation. Journal für Kardiologie 2008;15:124–131.
- Bonow RO, Carabello BA, Chatterjee K, de Leon AC Jr, Faxon DP, Freed MD, Gaasch WH, Lytle BW, Nishimura RA, O'Gara PT, O'Rourke RA, Otto CM, Shah PM, Shanewise JS. ACC/AHA 2006 Guidelines for the Management of Patients with Valvular Heart Disease: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Writing Committee to Revise the 1998 Guidelines for the Management of Patients With Valvular Heart Disease): developed in collaboration with the Society of Cardiovascular Anesthesiologists: Endorsed by the Society for Cardiovascular Angiography and Interventions and the Society of Thoracic Surgeons. Circulation 2006;114:e84–e231.[Free Full Text]
- Wendt D, Thielmann M, Buck T, Janosi RA, Bossert T, Pizanis N, Kamler M, Jakob H. First clinical experience and 1-year follow-up with the sutureless 3F-Enable aortic valve prosthesis. Eur J Cardiothorac Surg 2008;33:542–547.[Abstract/Free Full Text]
- Wendt D, Stühle S, Kawa E, Thielmann M, Kipfmüller B, Wendt H, Hauck F, Vogel B, Fischer H, Jakob H. NiTinol-based cutting edges for endovascular heart valve resection: first in-vitro cutting results. Minim Invasive Ther Allied Technol 2009;18:1–7.[Medline]
- Wendt D, Müller W, Hauck F, Thielmann M, Wendt H, Kipfmüller B, Vogel B, Jakob H. In vitro results of a new minimally invasive aortic valve resecting tool. Eur J Cardiothorac Surg 2009;35:622–627.[Abstract/Free Full Text]
- Kvidal P, Bergstrom R, Horte LG, Stahle E. Observed and relative survival after aortic valve replacement. J Am Coll Cardiol 2000;35:747–756.[Abstract/Free Full Text]
- Lund O. Preoperative risk evaluation and stratification of long-term survival after valve replacement for aortic stenosis. Reasons for earlier operative intervention. Circulation 1990;82:124–139.[Abstract/Free Full Text]
- Bonow RO, Rosing DR, McIntosh CL, Jones M, Maron BJ, Lan KK, Lakatos E, Bacharach SL, Green MV, Epstein SE. The natural history of asymptomatic patients with aortic regurgitation and normal left ventricular function. Circulation 1983;68:509–517.[Free Full Text]
- Brown JM, O'Brien SM, Wu C, Sikora JA, Griffith BP, Gammie JS. Isolated aortic valve replacement in North America comprising 108,687 patients in 10 years: changes in risks, valve types, and outcomes in the Society of Thoracic Surgeons National Database. J Thorac Cardiovasc Surg 2009;137:82–90.[Abstract/Free Full Text]
- Webb JG, Pasupati S, Humphries K, Thompson C, Altwegg L, Moss R, Sinhal A, Carere RG, Munt B, Ricci D, Ye J, Cheung A, Lichtenstein SV. Percutaneous transarterial aortic valve replacement in selected high-risk patients with aortic stenosis. Circulation 2007;116:755–763.[Abstract/Free Full Text]
- Walther T, Simon P, Dewey T, Wimmer-Greinecker G, Falk V, Kasimir MT, Doss M, Borger MA, Schuler G, Glogar D, Fehske W, Wolner E, Mohr FW, Mack M. Transapical minimally invasive aortic valve implantation: multicenter experience. Circulation 2007;116:I-240.[Abstract/Free Full Text]
- Thielmann M, Wendt D, Kahlert P, Eggebrecht H, Kottenberg-Assenmacher E, Janosi RA, Buck T, Peters J, Erbel R, Jakob H, Sack S. Transcatheter off-pump aortic valve implantation in patients with very high risk for conventional aortic valve replacement. Circulation 2008;118:S944–S945; (Abstract).
- Tabata M, Umakanthan R, Cohn LH, Bolman RM III, Shekar PS, Chen FY, Couper GS, Aranki SF. Early and late outcomes of 1000 minimally invasive aortic valve operations. Eur J Cardiothorac Surg 2008;33:537–541.[Abstract/Free Full Text]
- Shrestha M, Khaladj N, Bara C, Hoeffler K, Hagl C, Haverich A. A staged approach towards interventional aortic valve implantation with a sutureless valve: initial human implants. Thorac Cardiovasc Surg 2008;56:398–400.[CrossRef][Medline]
- Walther T, Falk V, Langebartels G, Kruger M, Bernhardt U, Diegeler A, Gummert J, Autschbach R, Mohr FW. Prospectively randomized evaluation of stentless versus conventional biological aortic valves: impact on early regression of left ventricular hypertrophy. Circulation 1999;100:II-6.
- Deleuze PH, Fromes Y, Khoury W, Maribas P, Lemaire S, Bical OM. Eight-year results of freestyle stentless bioprosthesis in the aortic position: a single-center study of 500 patients. J Heart Valve Dis 2006;15:247–252.[Medline]
- Magovern GJ, Kent EM, Cromie HW, Cushing WB, Scott S. Sutureless aortic and mitral prosthetic valves. Clinical results and operative technique on sixty patients. J Thorac Cardiovasc Surg 1964;48:346–361.[Medline]
- Berreklouw E, Vogel B, Fischer H, Weinberg F, van Rijk-Zwikker GL, Van Nooten GJ, Cohn LH. Fast sutureless implantation of mechanical aortic valve prostheses using Nitinol attachment rings: feasibility in acute pig experiments. J Thorac Cardiovasc Surg 2007;134:1508–1512.[Abstract/Free Full Text]
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Abstract
1. Introduction
