Encrustation on any urinary biomaterial results from ionic constituents in the urine crystallizing on its surface and becoming incorporated into the device biofilm. This biomaterial encrustation may be divided into two categories, as the mechanism of formation is slightly different.
The most common type of biomaterial encrustation is that associated with a UTI, specifically an infection with urease-producing bacteria. This mechanism of encrustation is most commonly seen with indwelling urethral catheters. Bacterial spread occurs from the urethral meatus and proceeds up the exterior or interior of the catheter into the bladder. Biomaterial colonization can occur even in the presence of a negative urine culture and often within the first 7 days from catheter insertion [10]. When the colonization is with a urease-producing organism, most frequently P. mirabilis, urea is cleaved to eventually form carbon dioxide, which raises the urinary pH and decreases both calcium phosphate and struvite solubility, leading to stone formation [11].
The second type of encrustation is that which is seen on ureteral stents. Internal stents are less commonly colonized with P. mirabilis and instead a non-urease-producing bacterium, such as E. coli, is more often found in a stent bacterial biofilm [12]. Biofilm on ureteral stents has been shown to promote encrustation by attracting and incorporating crystals on its surface. However, in the absence of significant pH changes associated with this non-urease colonization, stone deposition is driven by the same factors associated with nephrolithiasis, such as hypocitraturia, hypercalciuria, and hyperoxaluria, and stones are more often composed of calcium oxalate [11, 13].
It has been shown in this analysis that the rate of encrustation of Vesair intravesical balloons is minimal and tolerable. In no cases did the degree of encrustation lead to complications or difficulty removing the device when necessary. Additionally, subjects with the most severely encrusted balloons each had successful SUI outcomes despite the encrustation, and none were diagnosed with a UTI.
Mineralogical analysis found calcium oxalate on both balloons with surface granulation and the few with measurable stone deposition. Although the Vesair balloon is a chronic intravesical biomaterial, similar to a urethral catheter, the fact that there were no struvite or calcium phosphate stones suggests an alternate mechanism of stone deposition on this device. Lack of a direct external connection to allow rapid bacterial colonization may limit susceptibility of the Vesair balloon to this type of encrustation.
Calcium oxalate deposition on urinary biomaterials is much more commonly seen with indwelling ureteral stents. The incidence and rate of stone formation on ureteral stents, however, are much higher than what was found in the present study. Kawahara and colleagues analyzed 330 ureteral stents after removal and noted a nearly 50% rate of encrustation at a mean dwell time of only 52.8 days [14]. Severe encrustation, defined as inability to advance a stiff wire through the stent, was seen in 15.8% of stents removed. Dwell time was found to be a strong risk factor for stent encrustation, incrustation, and resistance to stent removal.
In addition to stent dwell time, other authors have suggested additional risk factors for ureteral stent encrustation. Sighinolfi and associates suggested different etiologies for stone deposition on the proximal versus distal coil of ureteral stents [13]. On the distal coil, a location more similar to that of the Vesair balloon, the presence of a UTI and advanced patient age contributed to stone formation. Advanced age may be a surrogate for an elevated post-void residual, which is a known risk factor for bladder stones and therefore would also lead to distal ureteral stent calcification.
Compared with indwelling ureteral stents, the rate of encrustation of the Vesair balloon in this study is remarkably low. This is despite the fact that the Vesair balloon is made of polyurethane, the same polymeric biomaterial used in most modern ureteral stents [15]. Additionally, Vesair balloons in this study were left indwelling for a longer duration (mean 182 days) than would be common for most ureteral stents. There are a variety of theoretical explanations for this difference. This study population excluded women who might be at high risk for stone formation, such as those with a history of nephrolithiasis and recurrent UTIs. Contrastingly, it is likely those exact risk factors that were the indication for stent placement in a majority of patients included in ureteral stent encrustation trials. Other than patient selection, however, the Vesair balloon has inherent properties that separate it from both ureteral stents and indwelling urethral catheters to reduce the risk of stone formation. The surface of the Vesair balloon is almost entirely smooth, with very few interstices that allow stone deposition to initiate. The balloon surface is elastic and can stretch and contract as a result of external forces placed upon it. It is also a freely mobile balloon, which floats around the bladder in continually different locations depending on patient positioning, urine volume, and activity. Compared with ureteral or urethral catheters, which are both anchored in place, the mobility and elasticity of the Vesair balloon likely limit the ability of surface deposition to begin and/or propagate.
There are several limitations to this study. While 97% of removed balloons were sent to the central laboratory, three balloons were not recovered. If any of these had significant stone deposition and had been captured it could significantly alter our results. Balloon removal is performed under direct vision as has previously been described [8, 9]. It is possible, however, that in the process of removing a balloon, encrustation that had been loosely adherent could have been detached and therefore would not be accounted for in the data analysis. Additionally, the surface characteristics of the balloon (as a result of the inherent variability of the manufacturing processes) and its delivery out of the catheter could contribute to the variability of the data measured. While all balloons passed manufacturing inspection for smooth surface characteristics, the surface characteristics of the balloon were not evaluated after removal in this analysis. Finally, bacterial contamination as a cause for the unusual bacteria noted in our DNA analysis of the balloon surface film cannot be ruled out. While care was taken to place each balloon into the collection container after removal, the sterility of these containers cannot be guaranteed and additional contamination may have occurred during balloon removal and transport. Limitations notwithstanding, the authors feel confident that the rate of encrustation on Vesair balloons in this cohort is comparatively low and does not appear to lead to adverse patient outcomes or device efficacy.