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Ann Thorac Surg 2000;70:785-791
© 2000 The Society of Thoracic Surgeons

Original articles: cardiovascular

Survival advantage of stentless aortic bioprostheses

Stephen Westaby, FRCSa, Matthew Horton, FRACSa, Xu Yu Jin, MD, PhDa, Takahiro Katsumata, MD, PhDa, Omar Ahmed, MDa, Satoshi Saito, MD, PhDa, Hui-Hua Li, MDa, Gary L. Grunkemeier, PhDa

a Oxford Heart Centre, John Radcliffe Hospital, Oxford, United Kingdom

Address reprint requests to Dr Westaby, Department of Cardiac Surgery, Oxford Heart Center, John Radcliffe Hospital, Headington, Oxford OX3 9DU, United Kingdom
e-mail: swestaby{at}ahf.org.uk

Presented at the Thirty-sixth Annual meeting of The Society of Thoracic Surgeons, Ft. Lauderdale, FL, Jan 31–Feb 2, 2000.


   Abstract
Top
 Abstract
Introduction
 Patients and methods
 Results
 Comment
 References
 
Background. Bioprostheses (BPs) are used to avoid anticoagulation after aortic valve replacement (AVR) in patients over 65 years of age. Stentless BPs offer established hemodynamic benefits. We sought to determine whether these advantages translate into improved survival.

Methods. Between 1993 and 1997, follow-up data (for Food and Drug Administration submission) were collected prospectively for 160 consecutive, unselected hospital survivors who received the Freestyle valve (FS). Equivalent data were collected for 247 Carpentier-Edwards (CE) porcine xenograft patients. Detailed comparative statistical analysis was used to compare events and survival between the groups. Follow-up was 100% complete for the FS (5.2 years maximum; mean 3.2 ± 1.0 years) group and 98% (7.2 years maximum; mean 3.8 ± 2.0 years) for CE.

Results. The groups were well matched in age (FS, 73 ± 6 years; CE, 74 ± 6 years), gender (FS, 58% male; CE, 62% male), ventricular function, and number of patients requiring coronary grafts (FS, 41%; CE, 37%). Actuarial survival at 5 years was 84% for FS versus 69% for CE (p = 0.023 Kaplan Meier, p = 0.009 Cox). Annual mortality rates were 3.6% for FS versus 7.1% for CE (p = 0.001). Thromboembolic rate was 0.8% per year for FS and 2.4% for CE (p = 0.024) without a difference in cardiac rhythm. Incidence of nonstructural dysfunction (paravalvular leak) was 0.2% for FS versus 1.3% for CE (p = 0.020).

Conclusions. By 5 years, the stentless valve patients had improved survival and reduced adverse events. Though differences in durability are yet to be proved, our findings support the use of stentless bioprostheses in this age group.


   Introduction
Top
 Abstract
 Introduction
Patients and methods
 Results
 Comment
 References
 
The bioprosthetic valve stent was adopted to simplify xenograft aortic valve replacement when prolonged cardiopulmonary bypass was an important risk factor and myocardial protection was poorly developed. This apart, the stent provides no further advantage. In contrast, freehand-sewn aortic homografts convey improved survival and fewer valve-related events, though limits of supply restrict their use to younger patients [1]. Recently, stentless aortic xenografts, first used in the 1960s, were reintroduced primarily to benefit the elderly patient with aortic stenosis [2]. The stentless design removes the obstructive element in the aortic root and theoretically reduces stress on the xenograft valve cusps when suspended from a rigid frame [3]. In effect, stentless xenografts are the porcine equivalent of the aortic homograft with virtually identical flow characteristics [4]. Unlike stented bioprostheses, the transvalvular gradients remain very low on exercise. Very low transvalvular gradients allow rapid resolution of left ventricular hypertrophy, and by eliminating a rigid ring in the left ventricular outflow tract, their effective orifice area increases with time [5, 6]. Experience also shows that the modicum of increased surgical effort has not added to hospital mortality and possibly the reverse [5, 7].

Recently, David and associates [8] reported that stentless bioprostheses improve survival in elderly patients, probably as a consequence of their hemodynamic superiority. Further studies are required to substantiate this important information. Ideally, the question: "Do stentless valves convey survival benefit?" should be addressed by a prospective randomized trial. In practice, there are already ethical and logistic problems with this approach. After a decade of experience, the hemodynamic advantage to the patient of a stentless valve is well recognized [3–5]. Second, the few current randomized trials have resorted to patient selection in the stentless group depending on operative findings, which might increase surgical risk [9, 10]. Size discrepancy between the annulus and sinotubular junction, calcification of the aortic sinuses, and adverse anatomy of the coronary ostia in relation to the valve annulus or commissures have excluded some high-risk patients from the stentless cohort.

Accordingly, we elected to study the outcome in 160 consecutive stentless valve patients from one center and compare these with 247 stented bioprosthesis patients, operated in the same time frame and identical facilities.


   Patients and methods
Top
 Abstract
 Introduction
 Patients and methods
Results
 Comment
 References
 
The study aim was to determine differences in outcome between hospital survivors with stented and stentless bioprostheses. We investigated late postoperative morbid events and longevity in concurrent series of patients undergoing aortic valve replacement with the Freestyle stentless xenograft (FSV; Medtronic Inc, Minneapolis, MN) from July 1993 to June 1997, and the Carpentier-Edwards (CE) SAV bioprosthesis model 2650 (Baxter Health Care Inc, Irvine, CA) from January 1992 to February 1998. Both valves were selected for patients 65 years of age or older and for a small number of younger patients in whom anticoagulation with warfarin was contraindicated or refused.

The 160 Freestyle valve patients were studied prospectively and continuously as part of the Medtronic IDE submission to the Food and Drug Administration. The valves were implanted by the modified subcoronary method, using separate inflow and outflow suture lines [5]. This was a consecutive series of bioprosthesis implants by one surgeon (S.W.), during which all patients older than 65 years received the FSV, irrespective of adverse anatomical (including aortic dissection) or patient-related risk factors. No patient over the age of 65 years received a mechanical valve during the study. The 247 CE valves were inserted by two other surgeons. All patients were operated using the same myocardial protection technique (cold antegrade St. Thomas’ cardioplegia). All patients received identical pre- and postoperative care by the same cardiologists, anesthetists, and surgical residents. Patient data for each valve type are summarized in Table 1. CE patients were anticoagulated with warfarin to achieve an International Normalized Ratio between 2.5 and 3.0 for 3 months or longer if in atrial fibrillation. After the first 20 FSV implants, formal anticoagulation was discontinued and aspirin 75 mg daily prescribed in preference.


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  		Table 1. Patient Demographics

 
Follow-up for both valves was by scheduled clinic visits, then before data submission, by direct telephone contact with both the general practitioner and the patients themselves. Retrospective accumulation of data for the CE patients was by the same valve research group. For deceased patients, a copy of the death certificate was obtained and relevant information sought from relatives or the general practitioner. All valve-related events and causes of cardiac and noncardiac deaths were recorded according to The Society of Thoracic Surgeons Guidelines for Reporting Morbidity and Mortality after cardiac valvular operations [11]. In view of the established relationship between prosthesis type and alterations in left ventricular hypertrophy and dynamics, we included readmission for medical treatment of heart failure as a valve-related event [5, 6]. Maximum, mean, and total follow-up times were, respectively, 5.2 years, 3.2 ± 1 years, and 506 patient-years for the FSV patients and 7.2 years, 3.8 ± 1 years, and 937 patient-years for the CE patients. Follow-up was 100% complete for the FSV patients and 98% complete for the CE patients.

Statistical methods
Data evaluation and statistical analysis were undertaken independently by Drs Grunkemeier and Li at the Medical Data Research Center, Providence Health System (Portland, OR). All analyses were performed using the SPSS software package, version 9.0 (SPSS, Inc, Chicago, IL). Late events (> 30 days) were expressed as percent per year (%/yr). Definitions were in agreement with STS guidelines for the reporting of cardiac valvular morbid events [11]. Linearized rates were compared using the likelihood ratio method [12]. Actuarial estimates were constructed with the Kaplan-Meier method and comparison among curves was carried out with the log/rank test. Multivariate analysis was performed with the Cox’s proportional hazard method to assess the relationship of various risk factors with survival. Variables entered into the model included: age at operation, gender, valve type, preoperative New York Heart Association (NYHA) class, preoperative ejection fraction, presence of renal failure, number of coronary grafts, presence of aortic stenosis, presence of aortic insufficiency, preoperative angina, preoperative myocardial infarction, and valve size. A p value less than 0.05 was considered statistically significant.


   Results
Top
 Abstract
 Introduction
 Patients and methods
 Results
Comment
 References
 
Analysis of preoperative data showed the FSV patients to be sicker (NYHA class III/IV, FSV 73%, CE 46%; p < 0.01) with a greater incidence of renal impairment (Creatinine > 140 mg/dL; FSV 36%, CE 18%; p < 0.01) (Table 1). This may reflect the cardiologists’ referral practice, because in every other respect the patients were closely matched. Sizes of prostheses implanted (reflecting annulus size) were also comparable (Table 2). There was no significant difference in operative mortality between the two groups (FS 8.0%, 14 operative deaths/174 total patients; CE 12.1%, 34 operative deaths/281 total patients; p = 0.171). Late deaths are summarized in Table 3 and valve-related complications in Table 4. Annual mortality for the CE patients was double that of the FSV. Cardiac deaths were four times as frequent in the CE group. Nonstructural dysfunction (paravalvular leak) and systemic thromboembolism were all more frequent in the CE patients (Table 4). Although the incidences of reoperation and readmission for medical management of left ventricular failure were greater in the CE patients, the difference did not achieve statistical significance.


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  		Table 2. Sizes of Prosthesis Implanted

 

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  		Table 3. Late Deaths

 

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  		Table 4. Late Valve-Related Complications

 
At 5 years, there was a significant difference in survival in favor of the FSV patients (FSV 84%, CE 69%; p = 0.023 Kaplan Meier, p = 0.009 Cox) (Fig 1). In the Cox regression model, predictors of late mortality were use of the CE valve (p = 0.009), poor ejection fraction (p = 0.035), higher number of bypass grafts in patients with coronary artery disease (p = 0.003), and preoperative renal failure (p = 0.006).



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  		Fig 1. Actuarial survival curve for Freestyle (FS) and Carpentier Edwards (CE) valves.

 

   Comment
Top
 Abstract
 Introduction
 Patients and methods
 Results
 Comment
References
 
Limitations of the study
This was not a prospective randomized trial, but a comparative study, which describes the outcome in concurrent groups of hospital survivors who received a stented or stentless porcine xenograft at the same center. However, in contrast to the small number of randomized trials, our patients were not selected as favorable candidates for the stentless valve [9, 10]. Paradoxically, sicker patients were referred to the surgeon who used stentless valves and could have biased survival against this prosthesis. However, different surgeons implanted the different type of valves, and those followed prospectively in a clinical trial may do better than those seen routinely. The fact that the FSV was an investigational device did not influence patient selection in the UK, because only isolated aortic valve replacement patients (with or without coronary artery bypass graft) were included in both study groups. All patients from the FSV learning curve were included in this study, which again could have biased survival against the stentless cohort. The outcome in hospital survivors was compared in order to rule out the influence of different surgeons’ hospital mortality. In fact, there was no difference in hospital mortality, which might have influenced overall survival curves had hospital deaths been included in the late survival analysis. The hospital mortalities for both groups are higher than contemporary reported valve replacement series in the US [8]. This perhaps reflects the advanced deterioration in many UK patients who are subject to waiting times of 12 to 18 months after acceptance for surgery (waiting list mortality 5% to 8%). In fact, several patients received cardiac massage on the way to the operating room.

Although the prospective follow-up of FSV patients was continuous and 100% complete, the retrospective data collection for CE patients may have underestimated nonfatal valve-related events.

Lastly, in order to eliminate bias in data analysis, we invited an experienced independent group to handle the raw data.

Difference in survival
Our findings suggest that late clinical outcome after aortic valve replacement in patients over 65 years of age is significantly improved by the use of a stentless (versus stented) bioprosthesis. The stented valve manifests a fourfold incidence of cardiac death and a double incidence of noncardiac death over the stentless valve. The unexplained difference in noncardiac deaths is similar to that reported in the case-matched study of David and colleagues [8]. Moreover, without a difference in primary tissue failure, the stentless valve had a much lower incidence of valve-related events than the stented valve.

This report is the third to document a clear survival difference in favor of stentless bioprostheses. David and associates’ case control study compared the Toronto SPV (stentless valve) and the Hancock II (stented valve) bioprostheses [8]. At 8 years after operation, the actuarial survival rate was 91% in the SPV group and 69% in the Hancock II group. Freedom from cardiac-related death was 95 ± 4% for the Toronto SPV and 81 ± 8% for the Hancock II (p = 0.01). Remarkably, the freedom from any valve-related complication was 81 ± 5% for the Toronto SPV versus 50 ± 10% for the Hancock II (p = 0.008).

Del Rizzo and associates, in conjunction with the author and other FSV investigators, compared the clinical results of two comprehensive multicenter trials (for Investigational Device Exemption submission) of the Hancock II stented and Freestyle stentless bioprosthesis, implanted by the subcoronary method [13]. The study included a significant number of patients under the age of 60 years, and all patients were followed for at least 5 years. The overall 5-year actuarial survival rate was 86% for Freestyle versus 77% for Hancock II patients (p < 0.05). Late mortality occurred in 8.8% of Freestyle patients compared with 16.1% of Hancock II patients (p = 0.0074). Analysis by Cox’s proportional hazard model showed the interaction of age with valve type to have an important effect on outcome. Remarkably, for patients less than 60 years of age at the time of operation, aortic replacement with the Hancock II valve was associated with a virtually fivefold risk of death, compared with patients of the same age who received the FSV (hazard ratio = 4.97, p = 0.0004). The survival advantage decreased by 50% with each decade thereafter (valve/age interaction hazard ratio = 0.50, p = 0.0027). The probability of dying within 5 years of surgery was 50% greater in patients who received the stented valve (hazard ratio = 1.50, p = 0.0442).

Actuarial survival curves for stentless valves in these studies are similar to those for aortic homografts, subject to modern preservation techniques [1]. These show almost no decline in survival in the first 10 years and compare favorably with contemporaneously published series of stented bioprostheses and mechanical valves for which 3-year survival is consistently less than 90% (range 82% to 86%) [14–16]. Furthermore, the mean age of patients in these series is younger than our stentless valve patients and fewer underwent concomitant coronary artery surgery. In the current study, our 3-year survival for patients over 65 years of age with a stentless valve was 92%, despite the fact that 41% required concomitant coronary bypass.

Reasons for improved survival
The stentless porcine xenograft resembles and provides flow characteristics equivalent to an aortic homograft and similar to the normal human aortic valve [6]. Both inflow and outflow suture knots are excluded from the blood path, and by eliminating a rigid sewing ring in the annulus, the dynamic nature of the aortic root is preserved. The nonturbulent flow, with very low transvalvular gradients, mitigates against thromboembolism or prosthetic valve endocarditis, and formal anticoagulation is unnecessary. This reduced propensity for valve-related events is now confirmed by clinical experience [8, 13].

The survival benefits of stentless valves may also be related to early regression of left ventricular hypertrophy and normalization of left ventricular function [17, 18]. The Framingham Study showed left ventricular hypertrophy to be an important predictor of cardiac mortality [19]. Experience shows regression of left ventricular hypertrophy after valve replacement with most types of prosthesis [20]. This follows a decrease in transvalvular flow velocity and pressure gradient, which lowers energy expenditure during systolic ejection. However, comparative studies show that for mechanical valves and stented bioprostheses, the reduction in left ventricular mass (LVM) never reaches baseline levels, and prosthetic effective orifice area (EOA) remains constant [21, 22]. The pressure in the left ventricle exceeds that in the aorta throughout the entire cardiac cycle, because of the rigid structure in the left ventricular outflow tract [23]. The transvalvular gradients are substantial on exercise and do not decrease over time. Consequently, the genetic stimulus for left ventricular remodeling never disappears.

In contrast, aortic homografts and stentless xenografts allow rapid and complete resolution of left ventricular mass (LVM) and a progressive increase in EOA as a function of improved haemodynamics [18, 19].

Using repeated-measures analysis after stentless valve replacement, Del Rizzo and colleagues showed a statistically discernable relationship between LVM regression and the changes in transvalvular velocity and EOA over time [24]. There was a strong correlation between the change in transvalvular velocity and a change in mean gradient as a function of time. The findings suggest that resolution of hypertrophy depends on the residual mean gradient and EOA.

Dumesnil and Yoganathan identified prosthesis patient mismatch as a major contributor to high residual gradients after aortic valve replacement [25]. They showed an inverse exponential relationship between indexed EOA and transvalvular gradients. The rapid part of the ascent curve occurs at an indexed EOA less than or equal to 0.8 cm2/m2. With lower EOAs, transvalvular gradients are substantially enhanced during exercise. In contrast, at EOA values 1 cm2/m2 or greater (on the flat part of the exponential curve), an increase in cardiac output results in only small increases in transvalvular gradient. We have also shown that left ventricular mass index (LVMI) 3 years after aortic valve replacement is significantly greater in patients with an indexed EOA less than 0.8 cm2/m2 [26].

Yun and colleagues specifically addressed the problem of prosthesis patient mismatch in a hemodynamic comparison of the Freestyle and Mosaic stented (Medtronic Inc, Minneapolis, MN) valves [27]. Comparing implanted prostheses of the same external diameter, all FSVs had larger EOAs and lower mean gradients than their stented counterparts at 1 year. When EOA was indexed to body surface area (EOAI), the mean EOAI for FSV sizes 21 mm or more was always higher than 0.85 cm2/m2. Even size 19 FSVs had an EOAI of 0.82 cm2/m2. In contrast, for the Mosaic valve, only patients with sizes 25 and 27 mm consistently attained a mean EOAI greater than 0.85 cm2/m2. For Freestyle and Mosaic patients, the incidence of prosthesis patient mismatch (EOAI < 0.85 cm2/m2) at 1 year was 19% versus 64% respectively (p <= 0.001).

In a study of factors that affect resolution of hypertrophy after stentless valve replacement, we stratified LVMI% at 3 years according to EOA [26]. For patients with an EOAI of greater than 0.8 cm2/m2, the 3-year LVMI was reduced to 78.7 ± 23.4% of early postoperative LVMI. In sharp contrast, for patients with EOAI less than 0.8 cm2/m2, the 3-year LVMI remained at 95.5 ± 26.5% of base line. Consequently, an important reduction in LVMI was restricted to those with an index EOA greater than 0.8 cm2/m2, a situation found only in larger sized stented bioprostheses.

Besides the reduction in left ventricular outflow obstruction, we have found LVM regression to depend also on patient-related factors including prior myocardial infarction, chronic atrial fibrillation, systemic hypertension, and carotid artery disease [23, 26]. Resolution of hypertrophy is also dependent on base-line LVM, consistent with the fact that long-standing valve disease can lead to irreversible changes within the myocardium.

This study strongly supports the premise that prosthetic valve design influences long-term outcome after aortic valve replacement. Stentless bioprostheses resemble human valves and, as such, produce fewer valve-related complications. Survival benefit may also result from their excellent hemodynamics and early resolution of left ventricular hypertrophy. The survival data strongly support the use of stentless bioprostheses for the elderly patient and suggest extended application into younger age groups. With improved tissue preservation, the stentless bioprosthesis now provides an alternative to the homograft in acquired aortic valve disease.


   References
Top
 Abstract
 Introduction
 Patients and methods
 Results
 Comment
 References
 

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R. O. Bonow, B. A. Carabello, K. Chatterjee, A. C. de Leon Jr, D. P. Faxon, M. D. Freed, W. H. Gaasch, B. W. Lytle, R. A. Nishimura, P. T. O'Gara, et al.
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
J. Am. Coll. Cardiol., August 1, 2006; 48(3): e1 - e148.
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 Asian Cardiovasc. Thorac. Ann.Home page
S. Beholz, S. Dushe, and W. Konertz
Continuous Suture Technique for Freedom Stentless Valve: Reduced Crossclamp Time
Asian Cardiovasc Thorac Ann, April 1, 2006; 14(2): 128 - 133.
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P. Totaro, N. Degno, A. Zaidi, A. Youhana, and V. Argano
Carpentier-Edwards PERIMOUNT Magna bioprosthesis: A stented valve with stentless performance?
J. Thorac. Cardiovasc. Surg., December 1, 2005; 130(6): 1668 - 1674.
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G. Polvani, F. Barili, L. Dainese, M. Muratori, M. Porqueddu, A. Sala, and P. Biglioli
Long-Term Results After Aortic Valve Replacement With the Bravo 400 Stentless Xenograft
Ann. Thorac. Surg., August 1, 2005; 80(2): 495 - 501.
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I Florath, A Albert, U Rosendahl, T Alexander, I C Ennker, and J Ennker
Mid term outcome and quality of life after aortic valve replacement in elderly people: mechanical versus stentless biological valves
Heart, August 1, 2005; 91(8): 1023 - 1029.
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 Asian Cardiovasc. Thorac. Ann.Home page
M. Tamim, T. Bove, Y. Van Belleghem, K. Francois, Y. Taeymans, and G. J Van Nooten
Stentless vs. Stented Aortic Valve Replacement: Left Ventricular Mass Regression
Asian Cardiovasc Thorac Ann, June 1, 2005; 13(2): 112 - 118.
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J. N. Clark, M. F. Ogle, P. Ashworth, R. W. Bianco, and R. J. Levy
Prevention of Calcification of Bioprosthetic Heart Valve Cusp and Aortic Wall With Ethanol and Aluminum Chloride
Ann. Thorac. Surg., March 1, 2005; 79(3): 897 - 904.
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J P A Puvimanasinghe, J J M Takkenberg, M B Edwards, M J C Eijkemans, E W Steyerberg, L A van Herwerden, K M Taylor, G L Grunkemeier, J D F Habbema, and A J J C Bogers
Comparison of outcomes after aortic valve replacement with a mechanical valve or a bioprosthesis using microsimulation
Heart, October 1, 2004; 90(10): 1172 - 1178.
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 Ann. Thorac. Surg.Home page
T. G. Gleason, T. E. David, J. S. Coselli, J. W. Hammon Jr, and J. E. Bavaria
St. Jude Medical Toronto biologic aortic root prosthesis: Early FDA phase II IDE study results
Ann. Thorac. Surg., September 1, 2004; 78(3): 786 - 793.
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A. John
Risk in Elderly Patients After Stentless versus Stented Aortic Valve Surgery
Asian Cardiovasc Thorac Ann, March 1, 2004; 12(1): 92 - 92.
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D. S. Bach
Choice of prosthetic heart valves:update for the next generation
J. Am. Coll. Cardiol., November 19, 2003; 42(10): 1717 - 1719.
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S. H. Rahimtoola
The next generation of prosthetic heart valves needs a proven track record of patient outcomes at >=15 to 20 years
J. Am. Coll. Cardiol., November 19, 2003; 42(10): 1720 - 1721.
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 Ann. Thorac. Surg.Home page
J. C. Halstead and S. S. Tsui
Randomized trial of stentless versus stented bioprostheses for aortic valve replacement
Ann. Thorac. Surg., October 1, 2003; 76(4): 1338 - 1339.
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J. Ennker, U. Rosendahl, I. C. Ennker, S. Bauer, and I. Florath
Risk in Elderly Patients After Stentless Versus Stented Aortic Valve Surgery
Asian Cardiovasc Thorac Ann, March 1, 2003; 11(1): 37 - 41.
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T. Fukui, S. Suehiro, T. Shibata, K. Hattori, H. Hirai, and T. Aoyama
Aortic root replacement with Freestyle stentless valve for complex aortic root infection
J. Thorac. Cardiovasc. Surg., January 1, 2003; 125(1): 200 - 203.
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D. B. Doty and J. R. Doty
Stentless Aortic Valve Replacement: Bioprostheses
Card. Surg. Adult, January 1, 2003; 2(2003): 889 - 898.
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G. Dellgren, M.J. Eriksson, L.A. Brodin, and K. Radegran
Eleven years' experience with the Biocor stentless aortic bioprosthesis: clinical and hemodynamic follow-up with long-term relative survival rate
Eur. J. Cardiothorac. Surg., December 1, 2002; 22(6): 912 - 921.
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 Ann. Thorac. Surg.Home page
G. B. Luciani, G. Casali, S. Auriemma, F. Santini, and A. Mazzucco
Survival after stentless and stented xenograft aortic valve replacement: a concurrent, controlled trial
Ann. Thorac. Surg., November 1, 2002; 74(5): 1443 - 1449.
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 Ann. Thorac. Surg.Home page
A. R. Akar, A. Szafranek, C. Alexiou, R. Janas, M. J. Jasinski, J. Swanevelder, and A. W. Sosnowski
Use of stentless xenografts in the aortic position: determinants of early and late outcome
Ann. Thorac. Surg., November 1, 2002; 74(5): 1450 - 1457.
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X. Y. Jin and J. R. Pepper
Do stentless valves make a difference?
Eur. J. Cardiothorac. Surg., July 1, 2002; 22(1): 95 - 100.
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N. D. Kon, R. D. Riley, S. M. Adair, D. W. Kitzman, and A. R. Cordell
Eight-year results of aortic root replacement with the freestyle stentless porcine aortic root bioprosthesis
Ann. Thorac. Surg., June 1, 2002; 73(6): 1817 - 1821.
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G. Dellgren, C. M. Feindel, J. Bos, J. Ivanov, and T. E. David
Aortic valve replacement with the Toronto SPV: long-term clinical and hemodynamic results
Eur. J. Cardiothorac. Surg., April 1, 2002; 21(4): 698 - 702.
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T. E. David, J. Ivanov, M. J. Eriksson, J. Bos, C. M. Feindel, and H. Rakowski
Dilation of the sinotubular junction causes aortic insufficiency after aortic valve replacement with the Toronto SPV bioprosthesis
J. Thorac. Cardiovasc. Surg., November 1, 2001; 122(5): 929 - 934.
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