Is There an Ideal Biliary Metal Stent?

In this issue, a prospective uncontrolled experience with a permanent self-expanding metal stent (SEMS) is reported [1]. The stent utilized for this study has been referred to as the “Diamond” stent due to the appearance of its mesh pattern (Boston Scientific Corporation, Natick, Massachusetts). This stent, in comparison with the more frequently reported Wallstent (Boston Scientific) is more flexible, has less radial expansile force, is constructed of nitinol (nickel-titanium alloy) as opposed to a stainless steel alloy (cobalt-chromium-nickel-iron-based), has continuous (as opposed to individually exposed) wire ends, and best yet, is no longer available. The authors of this study were clearly pleased to report a median stent patency of 477 days, representing an all-time record for a prospective study with a SEMS in the biliary tree!

Overall this stent did well, in the sense that it provided the majority of its recipients with uncomplicated palliative biliary drainage. The issue at hand for this editorial opinion is not the Diamond stent, but rather permanent SEMS as a categorical group of biliary prostheses. On a personal note, a decade of experience using all versions of SEMS in the biliary tree will influence my comments regarding the possibility of an ideal biliary metal stent. After I was confronted with the invitation to write this editorial, I allowed some time to elapse (too much for an anxious editor with a deadline) for reflection and to allow an overall impression of the SEMS to surface. My impression is that you have to “love’em” and you have to “hate’em” which is something that might surface more often in a country-and-western ballad than an endoscopic editorial. My answer to the question “Is there an ideal biliary metal stent?” is therefore, “No.” Why so? Let’s find out by examining what it is about the Diamond stent that is less than ideal and then offer what my opined “ideal” features are for a SEMS, features which would allow our theoretical country-and-western crooner to just “love’em,” “plain and simple”.

The less than ideal feature which the stent in this study shares along with other SEMS is the 30 % shortening inherent in the deployment design. The ideal stent would not shorten after deployment. This would eliminate guesswork in centering the stent within a stricture thus allowing a stent to be user-friendly for the infrequent user. This is important. It would also eliminate the difficulty in selecting a precise length, e. g. for those patients with a mid common bile duct stricture. The nonshortening ideal SEMS would allow the endoscopist several placement options, such as to deploy the stent to remain entirely within the common bile or an off-center placement allowing extension of the stent into the duodenum. Shortening from a suboptimally placed SEMS would require at some point a more costly, second stent to be coaxially deployed. On the other hand, if too long a stent is selected, as overcompensation for shortening, this may extend into the bifurcation and result in what we have over the years referred to as “stent prison,” the isolation of the intra- and extrahepatic biliary tree from anything but convenient future access options. Finally, foreshortening makes it difficult to palliate hilar strictures and can result in the need for additional stents at considerable expense.

The stents used in this study were available in 40, 60, and 80 mm lengths, similarly to the Diamond stent’s more frequently used competitor. Why fixed lengths? Just another reason to “hate’em.” There is always the stricture that just doesn’t quite conform to these predetermined sizes. For the inexperienced clinician dealing with a stricture in the extrahepatic biliary tree, a mistake in length selection may also result in the need for coaxial stacking of additional stents, or else in a single stent which is too long for the stricture, creating a proximal “stent prison” as described above when it crosses the bifurcation unilaterally. The ideal SEMS, as one of my colleagues has suggested, could be of a standard length and just trimmed to size before deployment with a wire cutter. You have to “love” this feature.

Let’s discuss expense. Any SEMS, all of which list at a price of approximately $1000 USD each, is much too expensive regardless of the anticipated patency period. Realistically, it is the upfront expense of the stent which drives the desire for significantly longer patency than plastic stents and, for the fiscally conscious, limits the use of multiple stents, either coaxially or bilaterally. Reimbursement for a single SEMS, much less multiple SEMS, is problematic. The ideal SEMS would be priced downward from their outlying position to within range of the upper end accessories currently available.

Stent patency for this study was well beyond the expected 200 - 250 days more typically reported, which is difficult to believe, despite the argument the authors have deliberately given to support their calculation of the patency period. Previous experiences with the Diamond stent have more often reported patency rates statistically inferior to those of the Wallstent, with a single study directly comparing the Diamond stent to the Wallstent resulting in similar patencies [2] [3] [4] [5] . The ideal permanent SEMS would have a patency of at least 1 year. The ideal SEMS would also retard all tissue ingrowth, malignant as well as benign. Covered SEMS have not satisfactorily (or statistically) resolved this problem. They have in fact, created problems with migration and cholecystitis. The ideal stent would be either pharmacologically active, with a slow-release of tissue growth inhibitory substances, or externally activated to induce necrosis of ingrowing tissue. For the permanent SEMS, this would expand applicability to benign strictures.

The Diamond stent is not removable. To be honest, none of the currently or formerly available SEMS are or were removable, except by either the gifted or dogged; in either case the experience has been typically not far short of an endoscopic nightmare for all involved. The ideal SEMS should be removable as needed, despite the length of time that the stent may have been in place. A satisfactory alternative to this “ideal” criterion would be a reliable bioabsorbable stent. which may ultimately prove to be the ideal self-expanding stent [6].

In summary, there does not exist an ideal SEMS, but those currently available are workable and their use is very much indicated for the palliation of malignancy in those patients with an anticipated survival beyond 3 months. The ideal permanent SEMS should be inexpensive, easily deployed (especially bilaterally), have sufficient radial force yet flexibility, should not shorten, should be removable, should retard tissue ingrowth, and it should be possible to trim it to any desired length. For the record, I “love’em” more than I “hate’em”.

References

• 1 Ferlitsch A, Oesterreicher C, Dumonceau J M, et al. Diamond-stents for palliation of malignant bile duct obstruction: a prospective, multicenter evaluation.  Endoscopy. 2001;  33 645-650
• 2 Dumonceau J M, Cremer A, Auroux J, et al. A comparison of Ultraflex Diamond stents and Wallstents for palliation of distal malignant biliary strictures.  Am J Gastroenterol. 2000;  95 670-676
• 3 Rajiman I, Amin V, Siddique I, et al. The use of the Diamond stent (DS) in the treatment of malignant bile duct stricture.  Gastroenterol Endosc. 1999;  49 AB235
• 4 Seecoomar L F, Cohen S A, Kasmin F E, et al. Preliminary experience with the Ultraflex Diamond stent for the management of malignant biliary obstruction.  Gastrointest Endosc. 1999;  4 AB236
• 5 Siqueira E, Martin J A, Vargas J S, et al. Prospective evaluation of a new metal stent for treating malignant biliary obstruction.  Gastrointest Endosc. 1999;  4 AB236
• 6 Freeman M L. Bioabsorbable stents for gastrointestinal endoscopy.  Tech Gastrointest Endosc. 2001;  3 120-125