Abstract
These results are based on visual observation by two independent surgeons rather than quantitative analysis and therefore do not provide any objective indication of what severe vs moderate means. Furthermore, the authors based the degree of encrustation on the amount of encrusted area on the stent, which may be significantly affected by removal of the stent. Quantification of the thickness of encrustation would have given a better idea of the level of encrustation.
It is surprising to see that the authors placed a lot of emphasis on the discoloration of stents, which is a very common occurrence and can likely be attributed to urinary components or conditions (not present in all patients) interacting with the stent surface to result in a color change. The authors hypothesize that the color change and increased encrustation of discolored stents is because of the formation of iron sulfide, a product of a chemical reaction between hydrogen sulfide and iron, the former of which was hypothesized to be produced by uropathogens.
Although an interesting hypothesis, it is unlikely to be the cause of the level of encrustation described by the authors; the amount of iron in the urine is too low to cause extensive encrustation. Furthermore, uropathogens such as Escherichia coli and Klebsiella produce iron-scavenging proteins responsible for scavenging iron from the external environment and return it to the bacterium to be used in bacterial metabolism. As such, it is unlikely that in the presence of a bacterial infection, sufficient amounts of the limited iron in urine would remain to cause discoloration and encrustation. Considering this, as well as the lack of urinalysis data to verify the presence of uropathogens in these patients, the hypothesis presented is highly unlikely.
Furthermore, the authors state that stent discoloration is often associated with a higher encrustation rate compared with stents that retain their natural color. It is very likely that both stent discolouration and encrustation are associated with the deposition of certain urinary components that facilitate crystal attachment to the stent surface, resulting in changes to the physical appearance of the stent. As such, it would have been interesting to see urinalysis results for the patients with discolored stents to identify possible abnormalities. It is well established throughout the literature that the crystallization encrusting stents is typically the same composition as the patient's stone. It is therefore possible that discoloration changes the physical characteristics of the stent surface in such a way that encrustation is facilitated.
Furthermore, the type of encrustation may also have an effect on stent coloration and considering that no analysis of the encrusted material was performed, not much can be concluded from that aspect. The authors do present some interesting data, but we do take issue with how some of it is interpreted. At a panel of Japanese endourologists at the Société Internationale d'Urologie in Fukuoka, Japan, in September, 2012, all panelists had experienced the stent discoloration, but the consensus was that it was not clinically relevant. Further work will help us determine the clinical relevance.