Forgotten stents are a problem. Monga et al., 1995 found that 68% of stents forgotten more than 6 months were calcified and 10% were fragmented. Multiple urologic procedures were necessary to remove the stones. Long-term effects of these forgotten stents may lead to voiding dysfunction and renal insufficiency. Schlick, et al., 1998 are developing a biodegradable stent that will preclude the need for stent removal.
==== Encrustation ====
The urinary system presents a challenge because of its chemically unstable environment. Long-term biocompatibility and biodurability of devices have been problems due to the supersaturation of uromucoids and crystalloids at the interface between urine and the device. Encrustation of ureteral stents is a well-known problem, which can be treated easily if recognized early. However, severe encrustation leads to renal failure and is difficult to manage (Mohan-Pillai et al., 1999). All biomaterials currently used become encrusted to some extent when exposed to urine.
reported to fracture). Stent fragments can migrate to the bladder or renal pelvis with serious repercussions.
Surface science techniques were used to study three stent typesafter use in patients. The stent type, duration of insertion and age or sex ofthe patient did not correlate significantly with the amount of encrustation(Wollin et al., 1998). However, it has been suggested that factors whichaffect the amount of encrustation include the composition or the urine, thetype of invading and colonizing bacteria and the structure and surfaceproperties of the biomaterial used (Gorman 1995). A low surface energysurface seems to resist encrustation compared with a high surface energysurface (Denstedt et al., 1998). Many different types of stone can form in the urinary tract.Calcium oxalate, calcium phosphate, uric acid and cystine stones aremetabolic stones because they form as a result of metabolic dysfunction.They usually are excreted from the urinary tract. Struvite (magnesiumammonium phosphate) and hydroxyapatite (calcium phosphate) areassociated with infection (infection stones). These account for 1520% ofurinary calculi. ESWL is used to break up the larger infection stonesbecause they don't pass; recurrence of the problem occurs with incompleteremoval. Infection stones can manifest as poorly mineralized matrixstones, highly mineralized staghorn calculi or as bladder stones whichoften form in the presence of ureteral stents. Urea-splitting bacteriacolonize the surface and cause alkalinization of the urine, which lowers
the solubility of struvite and hydroxyapatite, and they deposit on the
surface. Bacterial biofilm associated with encrustation is a common