In Vitro Evaluation of Stone Fragment Evacuation by Suction

Introduction

Kidney stones in the US population are increasingly more common, with a lifetime prevalence of ∼7.1% and 10.6% among women and men, respectively. ¹ Prevalence has steadily increased since the 1960s; the annual cost of management of urolithiasis in the United States is projected to be more than $2 billion. ^1,2 For less than 1-cm stones, shockwave lithotripsy and ureteroscopic fragmentation and retrieval are often employed; for stones in the 1 to 2 cm range, ureteroscopy yields higher stone-free rates and fewer procedures than shockwave lithotripsy. ^3,4

The gold standard outcome for ureteroscopy remains stone-free status, properly defined as complete elimination of all stone fragments based on computed tomography of the affected kidney. ⁵ Residual fragments serve as nidi for new stone growth, significantly increasing risk of retreatment, from 4% in patients with no residual fragments to 33% in those with fragments <2 mm over a median follow-up of 4.5 to 5.4 years. As this occurs regardless of whether the method of ureteroscopic lithotripsy was fragmentation or dusting, the mere presence of fragments at this size appears to correlate with stone recurrence. ^{6 –8} Of concern is that despite the most meticulous efforts in the most experienced hands, stone-free status is achieved in only 55% to 60% of flexible ureteroscopies performed for renal calculi less than 2 cm. ⁹ In particular, struvite stones show significant association with recurrence in the face of residual fragments. ^10,11

Standard procedures for evacuation of fragments are centered around the active retrieval of fragments using a variety of flexible stone baskets often in conjunction with a ureteral access sheath when the goal is fragmentation and basket extraction. ⁶ However, neither fragmentation/basketing nor dusting without a ureteral access sheath is effective at eliminating small stone fragments as 24% to 30% of patients have retained fragments ≤2 mm. ^12,13 Accordingly, we sought to assess the feasibility of suctioning ≤1-mm fragments through the 1.2-mm working channel of the flexible ureteroscope.

Materials and Methods

Phantom stones made from industrial plaster were hammered into small fragments that were then passed sequentially through 1- and 0.5-mm metal sieves into collection containers to create a group of stones in these two size ranges. This created one group of stones between 0.5 and ≤1 mm and one group of stones ≤0.5 mm. Both ≤1 and ≤0.5-mm groups were then divided into five trial samples; dry masses were obtained for each sample. A 10-mL Luer Lock syringe was connected to a fiber-optic Karl Storz Flex X2 ureteroscope (Karl Storz, Tuttlingen, Germany) with a 3.6F (1.2 mm) working channel.

Each stone trial group was individually mixed into a 400-mL beaker filled with 75 mL of normal saline on a stir plate at a continuous intermediate rate of spin. The 10- mL Luer Lock syringe was then used to suction stone fragments from the beaker. The deflection of the ureteroscope during the study was varied during the experiment to simulate real-time ureteroscopy. This was repeated five times for each trial sample for a total of 50 mL of suctioned solution, which was emptied into a labeled centrifuge tube. Time and number of extra flushings required to clear the working channel were recorded. After each 10-mL draw from the syringe, the apparatus was flushed with 10 mL of normal saline to dislodge any fragments remaining in the working channel of the ureteroscope or in the syringe. The remaining solution in the beaker, ∼25mL, was drained into a second centrifuge tube to measure the quantity of fragments remaining; care was taken to ensure that no visible fragments remained in the beaker and all remaining fragments were measured.

The centrifuge tubes containing both 50 mL of suctioned solution and 25 mL of remaining solution were centrifuged immediately at 1500 rpm for 5 minutes at 28°C. Supernatant saline was removed using a 20-mL syringe with an 18-gauge needle, and the remaining pellet of stone fragments was dried in an incubator at 33°C for 1 week.

Dried masses were then measured and recorded. The percentage of stone fragments suctioned in each group was determined by subtracting the remaining mass—those fragments that remained in the beaker following suction—from the starting dried mass of fragments in each trial sample. These values were then compared using Student's t-test, and a linear analysis comparing the dried mass before and after suction was conducted. The percentage of fragments stuck in the apparatus was determined by the unrecovered mass—those fragments that had been flushed out of the apparatus in between each 10-mL suction period—by subtracting the sum of the remaining mass and the mass of fragments that were suctioned from the starting mass.

Stone volume change in solution was also assessed. Stone fragments were suspended in 150 mL of normal saline for 5 minutes for fragments less than 0.5 mm and 10 minutes for fragments between 0.5 and 1.0 mm to correspond to average time suspended in saline during experimental trials. Stone fragments were mixed during this period. Fragments were passed through corresponding sieves to assess fragment size after suspension in fluid. Finally, the ureteroscope used in the experiment was assessed for any working port damage by assessing for any hindrance to the flow of irrigant through the working port; however, we did not perform a dissection of the ureteroscope itself to microscopically examine the working port for any abrasions from the aspirated stone fragments.

Results

The mean cumulative fragment masses for ≤0.5 and ≤1.0-mm stone groups at baseline were 0.807 g (SD = 0.002) and 0.806 g, respectively (SD = 0.01), corresponding approximately to a 0.90 to 0.94 cm diameter calcium oxalate stone, based on density and volume calculations. ¹⁴ After 50 mL of fluid and fragments was removed, a mean of 0.115 g of ≤0.5-mm fragments (standard deviation [SD] = 0.021) and 0.114 g of ≤1.0-mm fragments remained in the beaker (SD = 0.028). A mean of 0.693 g of ≤0.5 mm (SD = 0.019) and 0.692 g of ≤1.0-mm fragments was suctioned (SD = 0.031). A mean of 0.251 g of ≤0.5-mm fragments (SD = 0.020) and 0.154 g of ≤1.0-mm fragments was recovered from the syringe used after suction (SD = 0.069) (Table 1).

The mean percentage of stone fragments suctioned through the ureteroscope for both the ≤0.5 and ≤1.0-mm groups was 86% (p = 0.973) (Table 1). In addition, a linear analysis was conducted comparing suctioned stone mass and starting stone mass. When assessing a possible volume change of individual fragment size after suspension in the fluid, fragments were noted to clump together into larger groups; however, when the clumps were separated, the fragments passed through their respective sieves without increase or decrease in size.

During suctioning, the ureteroscope needed to be cleared with saline in addition to the times determined by the protocol as the small stone fragments blocked the working channel. This occurred 0 times when suctioning ≤0.5-mm fragments and 4 times per trial when suctioning ≤1.0-mm fragments (p = 0.02). Furthermore, 64% of suctioned fragments in the ≤0.5-mm group were trapped in either the working channel of the ureteroscope or within the Luer Lock syringe compared with 78% of suctioned fragments in the ≤1-mm group (p = 0.03) (Fig. 1). Finally, the mean time for completion of trials in the ≤0.5-mm group was 2 minutes 52 seconds (SD = 0.016) compared with 8 minutes 57 seconds (SD = 0.105) in the ≤1.0-mm group (p = 0.003). No noticeable damage occurred to the ureteroscope's working channel during the experimental study as there was maintenance of full deflection of the endoscope's tip, no alteration of optics, and ongoing satisfactory flow of irrigant.

OPEN IN VIEWER

FIG. 1.

Percent of overall stone mass comprising Steinstrasse that was then flushed from the endoscope. Percent of overall stone mass freely suctioned from the working channel.

Discussion

There is an unmet, currently underappreciated, and urgent need for an effective and consistent means to evacuate all stone fragments after laser lithotripsy, thereby rendering the patient truly stone free as corroborated by a noncontrast, low-dose CT scan of the affected kidney. Studies have shown that the presence of even small fragments leads to significant clinical repercussions, such as recurrent pain, obstruction, and the need for further treatment due to fragment growth. This is even more concerning among patients with infection-related calculi. ^7,10,11 Contemporary, flexible stone baskets, regardless of design, are incapable of removing all fragments that are ≤1 mm in diameter.

Stone evacuation by aspiration with a 10-mL Luer Lock syringe demonstrated significant efficacy in vitro, specifically for removing 86% of fragments ≤0.5 mm without causing frequent occlusion of the working channel. In contrast, while a similar percentage of fragments ≤1.0 mm could be removed using the Luer Lock syringe, the working port frequently became occluded, requiring the need to withdraw the ureteroscope and clear the channel, resulting in a nearly three-fold increase in the time needed to complete the evacuation protocol. As seen in our experiment, one potential cause of occlusion of the ureteroscope working channel may be the clumping of individual stone fragments into a larger fragment.

Our in vitro experience has mirrored our limited clinical practice. Indeed, suction aspiration in patients has proven to be too tedious to use as a routine procedure due to frequent plugging of the 1.2-mm working port with fragments. Clearly, what is needed is either a flexible ureteroscope with a larger working channel or the development of laser technology, such as perhaps the thulium fiber laser, capable of producing finer fragments.

While 86% of both fragments, ≤0.5 and ≤1.0 mm, could be effectively suctioned through the 1.2-mm working channel, the process was only efficient for the 0.5-mm group of fragments. The 1.0-mm trials were plagued by the need for repeated flushing of the working channel (4 per trial compared with 0 for the 0.5-mm samples) along with significantly more fragments becoming trapped within the endoscope (78% compared with 64%). Thus, aspiration through a standard working channel is efficient and effective for fragments ≤0.5 mm; however, increasing the size above that threshold rapidly makes it inefficient. Furthermore, while damage to ureteroscopes due to frequent shearing of the working channel by jagged stone edges is possible, no damage to the ureteroscope used in this study was observed. Deflection, optics, and flow of irrigant were unaffected throughout the study. Also of note, limiting potential damage is the fact that the suction activity occurred only at the end of the procedure when the stone was completely fragmented/dusted to the surgeon's satisfaction. In prior studies, the maximum pressure of vacuum/suction has been measured to be −435 Torr when using a 10-mL syringe. This is only 15% lower than the vacuum pressure that a 20-mL syringe would generate. ¹⁵ In these studies, it was found that using a larger syringe required more strength on the part of the operator. As such, we elected to utilize a 10-mL syringe for the study.

In the literature, myriad techniques to evacuate small stone fragments have been investigated. The first technique is the oldest and is based on creating a blood coagulum as in the prior reports of coagulum pyelolithotomy. ¹⁶ As recently reported, this involves drawing a 10-mL bolus of autologous venous blood from the patient and slowly injecting it through the ureteroscope into a fragment-containing area of the kidney and then waiting for the blood to clot around the fragments over the ensuing 7 to 10 minutes, followed by the use of a flexible basket to engage and remove the fragment-studded clot. ¹⁷ The second technique, which has only been used in a porcine study, involves a novel bioadhesive material that is injected and used to bind fragments together into a clot large enough to be entrapped and removed using a stone basket. ¹⁸ This technique showed similar efficacy to a coagulum, but appeared to be more efficient. ¹⁹ In contrast to these two approaches, endoscopic aspiration is far simpler and less costly; however, its effectiveness is markedly hampered due to the loss of time needed to clear ≥1.0-mm fragments from the working channel. In addition, fluid dynamics of aspiration has also been evaluated in in vitro studies evaluating the suction evacuation of blood clots causing a stroke. The success of aspiration appeared to be related to the proximity of the clot to the tip of the suction catheter and the applied force of suction. ²⁰ Although a thrombus has different characteristics than stone fragments, these factors may aid in future endoscopic device development. In this regard, it is of note that irrigation and suction platforms connected to the ureteral access sheath have recently been developed. ²¹ However, in one study, Huang et al. described the need to bring the flexible ureteroscope back into the access sheath repeatedly to remove small stone fragments. Although previously the concept of applying suction to the ureteroscope has been described in the literature, this has not been recommended due to concerns about the possible risk of damage to the ureteroscope. ^21,22 In addition, ClearPetra technology remains quite new and we are unaware of any studies detailing its overall effectiveness, specifically in the laboratory setting. However, there are two recently published studies on ClearPetra's use in vivo, which appear promising and lend credence to the concept of aspiration of small fragments. ^23,24

Limitations of this investigation are primarily due to the in vitro nature of the study. The renal pelvis is not a beaker, and stones are never reliably reduced into ≤1-mm fragments. Further investigation needs to be conducted with more correlative models such as cadaver models or in vivo porcine studies. In addition, the use of a plaster-based stone model resulted in clumping of the smaller ≤0.5-mm fragments; whether this would occur with different clinical uroliths remains to be tested (e.g., calcium oxalate monohydrate, calcium oxalate dehydrate, brushite, struvite, and cystine). Moreover, the sieves used were 1 and 0.5 mm. Therefore, there may have been some fragments in the 1-mm group and 0.5-mm group that may have been less than their respective sizes, potentially overestimating the percentage of stone fragments able to be suctioned during the experiment. Furthermore, an additional limitation of the study was that the ureteroscope was not broken down at the end of the experiment to microscopically evaluate any direct damage to the working channel. It is unknown at this time whether different stone types and shapes have an influence on stone suctioning capability; however, this is an area of interest for future studies. Moreover, although different suction forces may have different impacts on stone fragment suctioning capability, the limiting factor likely remains the small size of the working channel of the flexible ureteroscope. Future research studies are needed to evaluate suctioning efficiency utilizing different devices. Although stone weight may not be clinically relevant, it is a surrogate for stone size. Based on the literature density of calcium oxalate, for example, which is certainly different from other types of stones, a 0.8 g stone would correspond to a volume of 0.38 to 0.43 cm³. ¹⁴

In the overall scheme of things, the authors believe that two major developments in urolithiasis therapy need to be realized before stone-free rates can be increased. First, a new lithotripsy device needs to come on the scene, which truly has the ability to reduce all stones, regardless of composition, to dust (i.e., fragments that are 100 μm or smaller) rather than sand (i.e., fragments as large as 1 mm). In this regard, the recent advent of the thulium fiber laser appears promising. ²⁵ These dusting methods hold significant promise, especially as dusting with holmium lasers has shown comparable rates of complications, readmissions, and further procedures. ²⁶ However, as of yet, no longitudinal studies exist on the true, long-term stone-free rates of thulium fiber laser dusting procedures. Second, basketing of any large fragments needs to be complemented with the ability to sweep the kidney clear of all smaller fragments using either a newly designed basket with a tighter weave or a form of fragment aspiration. With regard to the latter, a larger, in-line, ureteroscope working channel is sorely needed. This same goal may also be accomplished by a second, parallel working channel. We are hopeful that with these advances, patients will one day be truly rendered stone free as documented by the complete absence of any stones on postoperative low-dose computed tomography. ²⁷ With regard to future studies, comparisons between different ureteroscopes can be made to assess the ability to suction small stone fragments through their working channels.

Conclusions

It is feasible to suction submillimeter stone fragments through the 1.2-mm working channel of the flexible ureteroscope using a 10-mL Luer Lock syringe. The limiting factor is creation of a Steinstrasse within the working channel necessitating frequent withdrawal of the endoscope and tedious clearing of the working channel, especially for fragments in the >0.5 to 1.0 mm range.