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Ann Thorac Surg 2009;88:2061-2068. doi:10.1016/j.athoracsur.2009.06.060
© 2009 The Society of Thoracic Surgeons
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Reviews

Which Patients Benefit From Stentless Aortic Valve Replacement?

Helmut Gulbins, MD, PhD*, Hermann Reichenspurner, MD, PhD

Department of Cardiovascular Surgery, University Heart Center, University Hospital Eppendorf, Hamburg, Germany

* Address correspondence to Dr Gulbins, University Heart Center, University Hospital Eppendorf, Martinistr. 52, Hamburg, D-20246, Germany (Email: h.gulbins{at}uke.de).


   Abstract
Top
 Abstract
Introduction
 Material and Methods
 Comment
 References
 
This review article analyzes the literature to answer the question of whether stentless aortic bioprostheses possess proven advantages compared with stented bioprosthesis, and which patients might benefit from stentless valve implantation. For this purpose, the United States National Library of Medicine's PubMed and MEDLINE databases were searched for articles dealing with results of stentless aortic bioprostheses or studies comparing stented and stentless prostheses. Key word searches used were as follows: stentless aortic prostheses, stented aortic prosthesis, hemodynamic, hemodynamic performance, degeneration, durability, technique, and long-term follow-up. The analysis focused on stentless prostheses with a clinical experience for more than 5 years. Only a few randomized studies were found. Stentless prostheses were found to be advantageous in patients with severe impaired left ventricular function or a small aortic annulus (ie, evidence of grade II), but no specific advantages could be determined for the majority of patients. The durability results were mixed: the Toronto SPV (St. Jude Medical, Minneapolis, MN) showed an increase in degeneration after 10 years of follow-up, whereas the Freestyle porcine stentless prostheses (Medtronic, Minneapolis, MN) still showed excellent results after this period.


   Introduction
Top
 Abstract
 Introduction
Material and Methods
 Comment
 References
 
In history, aortic valve replacement was introduced by Barrat-Boyes [1] and Ross [2] using homografts for aortic valve replacement. These valve substitutes were the first biological prostheses used in clinical practice, and they were the first stentless valves, too. However, in this early period in cardiac surgery, the implantation technique was rather demanding; this technique and the restricted availability prevented the homografts from widespread use. Mainly due to the easier implantation and the unlimited availability, mechanical aortic valve replacement became an easier and reproducible therapeutic option [3]. Despite the progress in mechanical aortic valve design and function leading to improved hemodynamics, the main disadvantages of these devices remained the need for lifelong anti-coagulative therapy. To overcome the limited availability of homografts, and to use the easier implantation technique of the mechanical devices, xenogenic biological prostheses were developed. Either native porcine aortic valves or pericardial tissue were mounted on a stent resembling a tri-leaflet valve with a suture ring allowing comfortable implantation. After durability problems with the first generation of these valves, they were meanwhile accepted into clinical therapy for aortic valve replacement, especially in patients aged 65 years or older, with the risk for reoperation due to prosthetic degeneration at approximately 2%.

However, the stented valves do not present with an orifice area similar to that of a native valve, mainly due to the stent and the resulting turbulent flow through the prostheses. In addition, because the degeneration process usually started at the struts to which the cusps are attached, a stent-free design should overcome both issues of biological prostheses: (1) turbulent flow and (2) the limited durability.

Stentless substitutes were also fabricated either from pericardial tissue or by preparation of native porcine aortic roots. Durability was suspected to be superior to stent-mounted prostheses because of the lack of stress at the attachment to the stent. Although these valve substitutes can be regarded as being established in clinical routine aortic valve surgery today, there are still many more stent-mounted biological valves being implanted. In this review, we try to answer the question of whether the results reported in literature supports any recommendation to prefer stentless valves in specific patients, and which patients might benefit from a stentless valve.


   Material and Methods
Top
 Abstract
 Introduction
 Material and Methods
Comment
 References
 
Literature was searched using MEDLINE and the United States National Library of Medicine's PubMed databases for articles dealing with stentless bioprostheses, especially with direct comparisons between stented and unstented xenografts. Used keywords were as follows: stentless aortic prostheses, stented aortic prosthesis, hemodynamics, hemodynamic performance, degeneration, durability, technique, long-term follow-up. Case reports were excluded from analysis, and only valves with a clinical experience for more than 5 years were included.

Surgical Implantation
Whereas stent-mounted prostheses are fixed in the native aortic annulus by single or running sutures, the implantation of stentless prostheses is more demanding. There exist several different techniques of implantation, but two of them are used in most centers: (1) subcoronary fashion (Fig 1) or as (2) total root (Fig 2); the latter making reimplantation of the coronary arteries necessary. Other techniques, such as the mini-root, partial root, or others, were described but are rarely used presently.


Figure 1
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  		Fig 1. Schematic drawing of the subcoronary implantation technique for stentless aortic bioprosthesis. The valve is implanted into the native aortic root without touching the coronary arteries.

 

Figure 2
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  		Fig 2. Schematic drawing of the total root technique; the aortic root is replaced completely by the stentless prosthesis and the coronary arteries are re-implanted in the usual manner.

 
The subcoronary implantation technique has the advantage to avoid manipulations of the coronary arteries. The ascending aorta is opened the same way that is used for stented prostheses. The prosthesis is then sutured with its annulus to the aortic annulus as the baseline suture; this can be done by single sutures, but most centers prefer a running suture line. The commissures are then fixed to the wall of the aortic root by a second suture line restoring the physiological shape of the aortic valve. The disadvantages are the difficulties occurring in calcified aortic roots, the danger of valvular insufficiency by changing the shape due to torsions, and the fact that a healthy valve is implanted into a diseased aortic root.

The total root technique requires reimplantation of the coronary arteries, thus increasing the complexity of the procedure [4]. However, this technique results in excellent hemodynamics, especially in cases with a small aortic annulus; implantation of a total root avoids any patient-prosthesis mismatch.

Surgeons using both techniques usually decide on the local situation of the patient's anatomy, and how the stentless valve will be implanted (ie, in patients with small annular size, the total root technique is preferred, whereas in patients with a large aortic annulus, the valves are more often implanted in the subcoronary fashion).

Operative Outcome
The operative outcome of stentless and stented aortic valve prostheses does not differ in the literature. Randomized studies revealed similar perioperative mortality for stentless and stented aortic valves [5–11]. These results were also confirmed by several nonrandomized studies [12–18]. In these often retrospectively designed studies there was often a strong bias that existed toward younger patients in the stentless group. Stentless prostheses usually require a somewhat longer cross clamping time and subsequently time on extracorporeal circulation [7, 12, 17–20]. However, this did not impair the direct postoperative outcome. The postoperative complication rates of bleeding, myocardial infarctions, neurologic deficits, and heart block were similar in most studies with no advantages for either type of prostheses [12, 21, 22]. The learning curve seems to be more pronounced when using the total root technique resulting in elevated perioperative mortality in the early cases [19]. However, with increasing experience, the risk decreases to almost the same level reported for stented valves. Therefore, this technique is especially recommended for patients with high cardiac risk [22, 23].

There is a trend in the literature to favor stentless prostheses, especially in patients with aortic stenosis and moderate-to-severe depressed left ventricular (LV) function. One reason for improved perioperative outcome in these patients is the large effective orifice areas of stentless valves implanted, especially when the total root technique is used. This technique results in low transvalvular gradients and energy losses [24], a feature of great importance in patients with depressed LV function [22, 23, 25–27]; these advantages are still present when compared with stented pericardial prostheses of the newest generation [5]. In addition, the larger orifice areas help to avoid any patient-prosthesis mismatch [18, 27–29], an additional factor influencing perioperative outcome [18, 30, 31].

Long-Term Results
Regarding survival and hemodynamic situation, the long-term results do not differ in statistical significance when compared with stented prostheses [8, 13, 32–36]. The valve prostheses actually available for more than 10 years are the Toronto SPV (St. Jude Medical, Minneapolis, MN) and the Freestyle (Medtronic, Minneapolis, MN). The latter one was especially implanted with excellent 10-year follow-up results [33], and with extremely low rates of valvular degeneration during this 10-year period. However, degeneration is still a central issue in stentless valves; despite of the good 8-year results of the Toronto SPV (St. Jude Medical) valve [34], the results of the 12-year follow-up were very disappointing [26]. Longer follow-up must be awaited to compare these prostheses with the excellent results of some stented biological heart valve substitutes, such as the Carpentier-Edwards Perimount (Edwards Lifesciences, Irvine, CA) or Hancock II (Medtronic).

The hemodynamic performance, however, remains excellent. The stentless valves provide a rapid and effective reduction in LV hypertrophy with good clinical performance, even after 10 years. Several studies have reported an improved survival after implantation of a stentless aortic valve compared with stented valves [14, 36–40]. Most of these studies, however, were nonrandomized, and there seems to be a bias toward using stentless valves in younger patients; this surely affected survival in these patients, because most of the studies report an average patient age of 70 years in the stentless and 75 years in the stented valve groups. There also exist randomized trials [5, 7, 9, 11, 25, 36, 41, 42], as well as case-match studies [14] that support an improved survival of patients with stentless valves. An additional advantage for stentless valves was reported in patients with concomitant coronary artery disease requiring bypass grafting [43].

After valve replacement, thromboembolism is always an issue, although this is much more important for mechanical substitutes. The incidence of cerebrovascular events during follow-up after stentless valve implantation corresponds to the individual risk of the patient, regardless of his valvular heart disease [44], and this underlines the excellent biocompatibility meanwhile of biological valves.

Hemodynamic Performance
Early postoperative hemodynamic performance did not differ between stentless and stented prostheses, but the implanted valve sizes differed in some studies [20, 25, 45–50]. This mainly depended on the stented bioprostheses with which the stentless prostheses were compared. There was a trend toward larger sizes of stentless valves, especially when compared with stented porcine prostheses. Stentless pericardial valves seem to offer no advantage when compared with their stented pericardial counterparts [21]. The best hemodynamic results were reported for stentless valves implanted in total root fashion [51].

The excellent hemodynamic performance has always been one major argument for the use of stentless valves. In fact, there exists several studies reporting better [36, 37, 43, 46, 49], similar [25, 44, 52], or even worse hemodynamic results [21]. Totaro and colleagues [21] reported lower gradients (ie, mean peak gradients, 20 ± 6 vs 28 ± 12) and larger effective orifice areas (1.07 ± 0.4 cm2 vs 0.7 ± 0.3 cm2) in patients with stented valves (Perimount Magna; Edwards Lifesciences) compared with stentless substitutes (Edwards Prima Plus; Edwards Lifesciences), although larger stentless valves had been implanted. The study was prospectively randomized, but the randomization was done after judgment of the surgeon intraoperatively. No difference was found when the Prima Plus was compared with the Perimount pericardial valve [5, 12, 21, 44]. Compared with porcine stented valves, the hemodynamic results were better when the stentless prostheses were used [9], except for the Prima Plus (pericardial stentless) [52]. Cohen and colleagues [25] and Bevilacqua and colleagues [45] reported no significant differences in a nonrandomized study, but a faster and better recovery of LV function after stentless valve implantation. In part, these different results can be explained by the implantation technique. Subcoronary implantation of stentless valves usually does not allow for using larger valve sizes compared with their stented counterparts, because the baseline suture has to be placed within the aortic annulus. The largest effect arises from total root implantation, because the valve can be placed in a supravalvular fashion, allowing for upsizing compared with the original aortic annulus. Therefore, this technique is frequently used in cases with small aortic roots [30, 44, 52, 53] to avoid any prosthesis-patient mismatch [19]. The total root implantation also avoids torsions of the commissures or any other possibility of changes to the geometric shape, thus avoiding postoperative insufficiency or pressure gradients at all. Although this improved hemodynamic is paid with a little more demanding implantation procedure, the total root technique is the technique of choice in patients with small aortic roots.

Van Nooten and colleagues [54] reported clear advantages for the Toronto SPV (St. Jude Medical Inc, St. Paul, MN) compared with the Carpentier-Edwards supra-annular (Edwards Lifesciences), especially in patients with a smaller valve size implantation (ie, < 25 mm). However, there existed a remarkable bias in this nonrandomized study, because the experience of the surgeon and existing pathologies within the ascending aorta and aortic root mainly influenced the decision as to which type of valve was implanted. Therefore, it can be suggested that experienced surgeons in selected cases used the stentless valve, whereas more complex patients received stented ones. The study of Rao and colleagues [55] reported nearly the same results, but it was also biased the same way.

Dumesnil and colleagues [46] compared the stentless porcine Freestyle valve with the porcine stented Mosaic valve (Medtronic) and found significantly better hemodynamic results after stentless implantation. This randomized study is very interesting because both valves were treated in the same fashion. In a prospective, randomized trial, Cohen and colleagues [25] also found better hemodynamics after implantation of the Toronto SPV compared with the Carpentier-Edwards Perimount valve.

Effective Orifice Area and Transvalvular Gradients
Stentless valves, especially when implanted in total root technique, usually offer a nearly normal orifice area to the patient. Yun and colleagues [18] reported more frequent prosthesis and patient mismatch in patients with stented valves implanted. Therefore, during follow-up, the hemodynamic results of the stentless valves were significantly better.

Corresponding to the larger orifice areas, the transvalvular gradients described for stentless valves are lower compared with the stented valves. The orifice area (approximately 1.5 cm2) that is usually reported for stented valves resulted in trans-prosthetic gradients of 20 to 25 mm Hg. These gradients result from the stented design and the stent itself. Therefore, valves with a different design, such as the Mitroflow (Sorin Group Canada, Toronto, Ontario, Canada), present with lower gradient, thus not reaching the results of true stentless prostheses [56]. The difference is biggest when porcine stentless were compared with porcine stented valves [48].

Most of the studies report only gradients at rest. The change in gradients during exercise, however, is more interesting. When cardiac output increases, the transvalvular flow also increases and the raise in trans-prosthetic gradient is also an important measurement for adequate valvular function. These gradients under exercise are lower in patients with stentless prostheses, especially the raise in gradients, which is much lower compared with the stented valves.

Morsy and colleagues [50] compared stentless valves to mechanical prostheses using dobutamine stress echocardiography. They reported a significant rise in transvalvular gradients in the mechanical valve group, but not in the stentless group. Several studies reported superior hemodynamic under exercise of stentless valves compared with mechanical substitutes [50, 57–59]. Silberman and colleagues [57, 58] confirmed these findings and observed no difference in transvalvular gradients compared with native aortic valves of healthy controls. Chambers and colleagues [7] compared the Toronto SPV with the Carpentier-Edwards Perimount pericardial stented valve. Under exercise, the hemodynamic did not differ between the groups. In another study, Fries and colleagues [47] compared the Freestyle valve implanted as a total root with native aortic valves or the Carpentier-Edwards Perimount valvular prosthesis. They found no rise in transvalvular gradients under exercise for native and stentless valves, but transvalvular gradients increased significantly in patients with stented aortic valves. The validity of this study is increased by the fact that only 23-mm prostheses of each type were compared.

Left Ventricular Mass Reduction
Clinically, an improved hemodynamic performance should result in a faster left ventricular mass reduction, especially in patients with initial aortic stenosis and severe hypertrophy. Both stented and stentless prostheses were shown to lead to a significant reduction of LV mass and hypertrophy during postoperative follow-up [31, 60–68]. Most of the studies reported a faster decrease in LV mass in patients with stentless valves implanted. A meta-analysis by Kunadian and colleagues [35] also showed a faster LV mass reduction in patients with stentless valves. This effect seems to be more pronounced when LV function was preoperatively impaired. The endpoint, however, does not seems to differ between both types of aortic valve prostheses. When comparing the Prima Plus (pericardial stentless) with its stented counterpart in a small randomized study, no differences in LV mass reduction were seen [40]. Jasinski and colleagues [69] also found no differences in LV mass reduction between the Mosaic (stented) and Freestyle (stentless) valves, but this can be contributed to the small number of patients studies (n = 28).

Walther and colleagues [11] published a randomized, prospective trial reporting a significantly faster LV mass regression after implantation of stentless valves compared with stented ones. The authors especially emphasize the possibility of oversizing when using stentless valves. This allows for the implantation of larger valves compared with stented substitutes. This aspect was also reported by Williams and colleagues [42].

Tamim and colleagues [65] reported a very fast LV mass reduction after stentless valve implantation (Toronto SPV), but found an increase in LV mass starting at 5 years postoperatively. This could be an early marker of the onset of degeneration, because a significant degeneration rate was reported at 10-years follow-up for this valve [34].

Degeneration and Durability
Degeneration is an important issue in biological valve prostheses, because the limited durability represents the main disadvantage of these valve substitutes. The actually xenogenic stentless aortic valves were introduced in clinical routine in the early 1900s; therefore, only a few of the available prostheses present with sufficient long-term results. The 10-year results of the first valve types used, however, support the theory, that these valves are at least not any worse compared with their stented counterparts. The hypothesis that durability is improved by avoiding the tissue stress caused by the artificial struts can not be proven yet. In fact, the mean age of patients who get a biological valve implanted makes it difficult to get sufficient patient numbers to be at risk for valve degeneration at 15 or more years of follow-up. The freedom from structural degeneration is actually given to be at 95% after 10 years for the porcine Freestyle stentless prosthesis [33], which is rather similar compared with the stented valves. The Toronto SPV performed worse after 10 years compared with these results [34]. For this prosthesis, the 8-year results were excellent and the increase in structural failure occurred later, but only within a 2-year interval of further follow-up. The further experience will show whether these results are maintained after 15 or more years.

For the first stentless valves, the homografts long-term follow-ups are reported by several groups. The results are very good when the younger mean age of the patients is taken into account [70]. However, recent data reported faster calcifications in homografts compared with stentless valves [71], indicating possible better durability for the xenografts. But there exists a lack of information about the durability of actually available xenogenic prostheses in younger patients, because the majority of these valves are implanted in patients older than 60 years. Therefore, xenogenic and allogenic valves can not be compared concerning durability, because the patient collectives available are too different.

Biological prostheses, especially homografts are often believed to be the substitute of choice in cases with endocarditis. In fact, reports in the literature with excellent results using mechanical heart valve prostheses also exist [72], and the direct comparison of allogenic and mechanical valve replacement showed no advantage for the homografts [73]. The susceptibility of prosthetic infections also does not differ; at least, no randomized, controlled studies exist that deal with this problem. Therefore, the recommendation for biological prostheses in endocarditis has a low level of evidence (grade IV).


   Comment
Top
 Abstract
 Introduction
 Material and Methods
 Comment
References
 
When introduced into clinical practice, stentless prostheses were believed to be revolutionary valve substitutes, increasing both hemodynamic performance and durability. More than 15 years later, these hopes were fulfilled only in parts. Stented biological prostheses are still in use, and the number of stented valves implanted still exceeds the number of stentless valves. In 2008, approximately 11.8% of all aortic valve prostheses implanted in Europe were stentless substitutes (European Cardiovascular Monitor [BIBA Research, London, United Kingdom], see: http://www.bibaresearch.com/services/bibaresearch---services/european-cardiovascular-monitor). One reason for the rather low number of stentless valves implanted is surely the more demanding technique of implantation required for stentless prostheses. The subcoronary technique always implies the danger of changing the geometrical shape of the implanted valve, thus leading to either elevated transvalvular gradients or valvular insufficiency. In addition, a healthy valve is implanted into a diseased aortic root. These suggestions, however, did not affect outcome [74]. The total root technique offers the best hemodynamic results with the lowest risk of postoperative valve dysfunction. The need for reimplantation of the coronary arteries, and the possible requirement of interponating a vascular prosthesis in cases with a mismatch between the patients aortic diameter and the porcine aortic root, does not automatically mean prolonged ischemic and bypass times and an increased risk to the patient.

When comparing the results of stented and stentless valves, the different expertise of the surgeons usually can not be disclosed from analysis. The longer ischemic and bypass times are an issue, especially in high-risk patients. Although experienced centers reported excellent results with stentless valves, even in these cases, and also in octogenarians, most surgeons tend to implant a stented valve in these patients to keep the procedure quick, safe, and simple. The need for stentless valves with easier implantation techniques exist, and there are new prostheses introduced into the market such as the Solo stentless aortic valve (Sorin, Italy) with excellent initial results [75]. Subcoronary implantation is reduced to one suture line facilitating implantation; however, the long-term results have to be waited on. This leads to an important issue in biological valve replacement (ie, durability).

The durability of the stentless valves has yet to be determined. Actually, the 10-year results with sufficient patient numbers included show that the performance of these substitutes are at least not worse compared with stented valves. An improved durability, as assumed in the beginning of the stentless era, can not be proven yet. The reoperations for degenerated stentless valves, however, are technically very demanding and exhibit a rather high mortality, although the number of patients reported is actually very low. An interesting finding was reported by Tamim and colleagues [65]; they found a larger reduction in LV mass during the first 3 years of follow-up after stentless valve implantation. However, they reported a slow increase in LV mass after 5 years, possibly an indicator for beginning degeneration. In their study, they used the Toronto SPV stentless valve, with initial excellent hemodynamic results and a reduced durability compared with the pericardial stented valve, which was reported by David and colleagues [34].

So the excellent hemodynamic performance with low transvalvular gradients remains the strongest argument to use stentless valves. This results in a survival benefit even during long-term follow-up in patients with impaired LV function at the time of surgery. This effect is clearly contributable to the excellent hemodynamic performance, thus keeping the transvalvular energy loss low, allowing the functionally depressed left ventricle to remodel. However, when comparing valvular prostheses with regard to the LV mass reduction, the initial situation plays an important role [37]. This explains the necessity of studies enrolling larger numbers of patients or very sophisticated imaging techniques to detect differences, because in most studies the initial state was not uniformly defined.

In patients with good LV function, this effect is less pronounced and seems not to affect clinical outcome. Because the mean age of the patients with a biological prosthesis implanted is approximately 75 years, the number of patients who frequently stress themselves with sports activity requiring more than twice the normal cardiac output is low. However, in patients with good LV function, the improved hemodynamic performance plays an important role only when increasing heart-minute volume; then the transvalvular gradients rise to significantly higher levels when a stented valve is implanted compared with a stentless substitute. Therefore, stentless valves are assumed to be beneficial in more active patients, although there is no true evidence to this yet. In patients with a small aortic root the excellent hemodynamic results are of great advantage, because stentless valves help to avoid surgical enlargements of the native aortic annulus. A 19-mm stentless valve, implanted with the total root technique, offers a hemodynamic performance comparable with that of a 23-mm or 25-mm stented prosthesis.

In conclusion, the results of the various studies and their level of evidence are given in Table 1. There seems to be an advantage for stentless prostheses with a small aortic root or impaired LV function; in these cases, the excellent hemodynamic result improves perioperative outcome and even long-term results. The long-term durability can not be defined yet, because the number of patients at risk 15 years after stentless valve implantation is too low. These results, however, will strongly influence the use of stentless prostheses in the future. The actual literature, however, does not allow for a general recommendation to prefer stentless prostheses in all patients.


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  		Table 1 Different Statements and the Number of Studies and Patients Supporting Them a
 


   References
Top
 Abstract
 Introduction
 Material and Methods
 Comment
 References
 

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