The CNN model15 based on deep-learning allowed us to evaluate luminal encrustation and stent volumes simultaneously with unparalleled accuracy. Once trained, the model can be applied to subsequent data sets without further tuning, and therefore reduces random error or bias imposed during the quantitative analyses. Moreover, to the best authors’ knowledge, this study is the first to measure both the encrustation volume and the stent volume. As such, the encrustation volume per stent unit was calculated, which is more representative than directly comparing the encrustation volumes, as was done in several previous studies13,14.
One observation in our study was that SHs were often the anchoring sites of encrustations even for stents with short indwelling times regardless of the patient group (Fig. 1). The initial deposits of encrustations near SHs might be explained by recent in vitro experiments such that SHs facilitate local urine flow stasis and promote particle accumulation in the neighbouring regions16–18. These initial deposits exacerbate the local encrustation process, causing severe stone burden near SHs11,19. The stony encrustation could compromise the stent tensile strength at the SHs, which deteriorate into fractures11,19.
Another observation was that that encrustation levels did not differ between patient groups in the first six weeks, but stone patients had a higher tendency to build up encrustations over prolonged indwelling time than transplant patients. We also observed the highest ERLs in the pigtails in both patient groups, suggesting higher risks of excessive encrustations per stent unit. Both pigtails exhibited high median in ERL and did not differ from each other, so the risk seemed equivalent in the renal and bladder ends. Since our results were based on encrustation volume per stent unit, the use of excessively long stents with superfluous pigtail materials should be avoided in both patient groups.
In practice, proximal stone burden has been associated with multiple surgical complications 20, and accurate determination of the stent section most susceptible to encrustations has been an active topic of discussion. Previous studies on stent encrustation patterns mainly focused on stone patients and no consensus was reached. While some suggested the renal pigtail as the most susceptible section followed by the bladder pigtail4,11,13, others reported that the two pigtails or the distal part of the stent were equally susceptible9, or that the distal part of stent12 was most encrusted. The recent study using µ-CT14 suggested the stent’s shaft body (the entire straight part) as the most susceptible region. However, no previous efforts attempted to derive encrustation volume per stent unit as an endpoint, which should be more meaningful than direct comparison of encrustation volumes. For one thing, the disputed conclusions can be attributed to the lack of quantitative tools to accurately measure the encrustation volumes. For another, the fact that there were no significant differences between certain comparisons, as demonstrated in Fig. 2, should also be acknowledged.
Other than stone patients, quantitative evaluation of stent encrustation patterns in transplant patients are still lacking to the best of the authors’ knowledge. Following kidney transplantation, stents are usually placed to prevent strictures or urine leakage. These prophylactic stents are usually much shorter than those in stone patients since the allograft ureter is kept short to ensure a good blood supply. Consequently, vesicoureteral reflux (VUR) in transplant patients often reaches up to the renal pelvis21. This retrograde flow might create a flushing effect, periodically removing the deposits on the luminal surface of the stent, whereas in stone patients the VUR would not reach up to renal pelvis as easily since the stents are often longer. This might explain the higher encrustation level in the proximal straight part in stone patients.
Moreover, stent implantation significantly impedes the peristalsis of the ureter22, and thus the urine transport from kidney to bladder is largely passive. The longer tube in stone patients creates larger resistance to the fluid, and local stasis of urine can be formed near the UPJ as the tract narrows, whereas the shorter tube in transplant patients could better facilitate urine flows through the stent, either from kidney to bladder or in presence of VUR. As such, both forward and retrograde urine flows influences the dynamics, co-creating the different encrustation patterns presented above.
In terms of limitation, the current study consists of stents collected from a single center with limited sample size, preventing further subgroup analysis. Moreover, the mechanism of urine flow dynamics in stented ureters remains unclear, which plays a pivotal role in stent encrustation23. Further studies in this regard could help elucidate the process of encrustations in stented native or allograft ureters.