Key points

Introduction

Percutaneous nephrolithotomy (PCNL) was first described in 1976, just a few years before the introduction of shockwave lithotripsy (SWL). The strong initial interest in PCNL, however, was subsequently quelled by the explosion of SWL as the first noninvasive treatment of kidney and ureteral stones. Although early on SWL was used almost indiscriminately for the management of upper tract calculi, the limitations of the technique for large and complex stones became evident over time, and PCNL became firmly established in the therapeutic armamentarium of nephrolithiasis. In recent years, however, as the indications for ureteroscopic management of upper tract stones have expanded, ureteroscopy (URS) has, in some cases, supplanted SWL and PCNL for the treatment of some stones. Nonetheless, there have been efforts underway to reduce the morbidity and increase the efficiency and effectiveness of PCNL, making it more competitive with SWL and URS for the first-line management of upper tract stones. The contemporary advances in surgical technique, instrumentation, and perioperative care that continue to refine PCNL are reviewed here.

Introduction

Percutaneous nephrolithotomy (PCNL) was first described in 1976, just a few years before the introduction of shockwave lithotripsy (SWL). The strong initial interest in PCNL, however, was subsequently quelled by the explosion of SWL as the first noninvasive treatment of kidney and ureteral stones. Although early on SWL was used almost indiscriminately for the management of upper tract calculi, the limitations of the technique for large and complex stones became evident over time, and PCNL became firmly established in the therapeutic armamentarium of nephrolithiasis. In recent years, however, as the indications for ureteroscopic management of upper tract stones have expanded, ureteroscopy (URS) has, in some cases, supplanted SWL and PCNL for the treatment of some stones. Nonetheless, there have been efforts underway to reduce the morbidity and increase the efficiency and effectiveness of PCNL, making it more competitive with SWL and URS for the first-line management of upper tract stones. The contemporary advances in surgical technique, instrumentation, and perioperative care that continue to refine PCNL are reviewed here.

Indications

One of the most important factors in selecting the optimal surgical modality for the patient with nephrolithiasis is stone size because size has been shown to strongly influence stone-free rate, need for secondary procedures, and complication rate for some treatment modalities. Historically, PCNL has been the treatment of choice for the management of large and/or complex stones. Indeed, the American Urologic Association (AUA) Guidelines for the Management of Staghorn Calculi states that “percutaneous nephrolithotomy should be the first treatment used for most patients” with stones. According to their meta-analysis of published clinical trials evaluating outcomes for surgical management of staghorn calculi, the stone-free rate for PCNL was 78% versus 71% for open surgery, 66% for combined PCNL and SWL, and 54% for SWL monotherapy. When comparing the total number of procedures required to successfully treat the stone and to manage complications, PCNL required 1.9, combination therapy 3.3, SWL 3.6, and open surgery 1.4 procedures. Likewise, the European Association of Urology Guideline on Urolithiasis (updated Feb 2012) recommends PCNL for the treatment of all stones greater than or equal to 2 cm and lower pole stones greater than or equal to 1.5 cm.

With improvements in the safety and efficacy of PCNL as a result of advances in instrumentation and a growing experience with the technique, some investigators have argued that the indications for PCNL should be broadened to include smaller stones throughout the kidney and specifically to those in the lower pole calyces. Deem and colleagues randomized 32 subjects with moderate sized (1–2 cm, median 1.2 cm) upper or middle calyceal or renal pelvis stones to PCNL or SWL and evaluated them at 3 months with nonenhanced CT. PCNL stone-free rate was superior to SWL (85% vs 33%, respectively) and none of the PCNL patients required a secondary procedure, whereas 77% of the SWL subjects required at least one other procedure and 17% required more than one. Quality of life, as assessed by the short form (SF)-8 quality of life survey, also favored PCNL for both mental and physical domains.

Stone location is also an important determinant of stone-free rate for some treatment modalities. Lower pole location has been shown to be associated with poor stone-free rates for SWL, likely because of limited clearance of fragments from the dependent lower pole calyces. On the other hand, PCNL stone-free rate is independent of stone burden. The Lower Pole I Study Group conducted a prospective, multicenter randomized clinical trial (RCT) comparing SWL and PCNL for symptomatic, greater than 1.0 cm, lower pole stones and found that stone-free rates overall were threefold higher for PCNL compared with SWL (95% vs 37%, respectively, P <.001). When stone-free rates were stratified by stone size (less than 1 cm, 1–2 cm, and greater than 2 cm), PCNL stone-free rates were relatively uniform at 100%, 93%, and 86%, respectively, whereas SWL stone-free rates were inversely related to stone size (63%, 23% and 14%, respectively).

PCNL has increasingly been used as an alternative to URS and SWL for large, proximal ureteral stones as well. Sun and colleagues randomized subjects with greater than 1 cm proximal ureteral stones to PCNL or URS and found that PCNL with antegrade URS achieved a higher stone-free rate than retrograde ureterolithotripsy according to imaging obtained at discharge (95% vs 79.5%, P = .027) and 1 month postprocedure (100% vs 86%, P = .026). Several other series have corroborated these results.

In addition to stone size and location, other factors, including stone composition, patient factors, and renal anatomy, can influence the success of specific treatment modalities. SWL success is influenced by stone composition, with harder stones, such as cystine, calcium oxalate monohydrate, and brushite, being relatively shockwave-resistant; therefore, patients with these stone compositions are best treated endoscopically. Although one study suggested poorer outcomes for PCNL in subjects with increasing calcium phosphate content of their stones, this finding was subsequently disputed by another group and, in general, PCNL outcomes have not been definitively shown to be influenced by stone composition. Although predicting stone composition based on preoperative imaging has met with limited success, investigators have correlated CT attenuation coefficient (Hounsfield units [HU]) as a surrogate for stone composition with SWL success and found an inverse relationship between the two. Perks and colleagues, and others, have shown that harder stones, particularly those with HU greater than 900, are less likely to be successfully treated with SWL and may, therefore, be more amenable to PCNL or URS. Skin-to-stone-distance (SSD), another CT-derived parameter, has been shown to correlate with SWL success and can be used to identify patients in whom endoscopic management, PCNL or URS, is advisable. Perks and colleagues showed that an SSD greater than 9 cm is associated with diminished SWL success (79% for subjects with SSD <9 cm vs 57% for subjects with SSD >9 cm). On the other hand, PCNL is not affected by body mass index with respect to stone-free rate, complication rate, or cost.

Positioning

PCNL has historically been performed with the patient in a prone position. Retrograde placement of a ureteral catheter before PCNL has traditionally been performed with the patient in the dorsal lithotomy position before repositioning the patient prone. The prone split-leg approach was introduced as a modification to prone positioning to increase efficiency and decrease the number of operative interventions required for patients with both upper and lower tract pathology. This approach has become widely accepted for PCNL because it obviates patient repositioning, thereby decreasing operative time and need for operative staff for multiple patient transfers.

Another extension of the prone position is the prone-flexed position. Ray and colleagues conducted an anatomic survey of subjects in the prone, prone-flexed (30°), and supine positions using triphasic CT. They found that the distance from the posterior iliac crest to the 12th and 11th ribs was increased by 2.9 cm and 3.0 cm, respectively ( P <.001), with prone-flexed positioning compared with prone positioning. Consequently, in 5 of 11 subjects (45.5%) upper pole access above the 11th rib was converted to access above the 12th rib or access above the 12th rib was converted to infracostal access by using the prone-flexed position. This same group later reported on 318 subjects who underwent PCNL in the prone-flexed position and found that most (>85%) of single tract punctures of upper pole calyces could be accomplished below the 11th rib, thereby reducing morbidity.

Although prone positioning in general has proven successful, it does have notable drawbacks. Obese patients and those with cardiopulmonary comorbidities often do not tolerate being in the prone position for long periods of time. In addition, repositioning the patient from lithotomy to prone, if a split-leg table is not used, is time consuming. In an effort to overcome these drawbacks and streamline the procedure, supine PCNL was introduced.

Valdivia and colleagues first described PCNL in the supine position in 1987 and later published a series of 557 consecutive subjects undergoing supine PCNL with favorable outcomes, including a transfusion rate of 1%, no colonic injuries, and greater than 90% of subjects describing little or no pain. Since that time several other series of supine PCNL have been published with similar results.

A Cochrane-based systematic review and meta-analysis comparing supine and prone PCNL included two RCTs and two case-controlled studies that met inclusion criteria. Operative time was shorter for supine PCNL, but complication, transfusion, and fever rates were similar for the two approaches. Only a single colonic injury was reported in the supine setting. Another subsequent meta-analysis comparing the two approaches included the previous four studies in addition to 27 case series (8 supine and 19 prone). This analysis also found a significant difference in operative time between the supine and prone groups (65 ± 15 minutes vs 90 ± minutes, respectively, P = .0009) but detected no significant differences between the two groups with respect to stone-free (82% vs 82%, respectively) and transfusion rates (9% vs 4%, respectively).

Overall, the safety and efficacy of supine and prone PCNL seems to be equivalent and, at this time, there is no demonstrable advantage of one technique over the other. However, both the sample sizes and methodological quality of the studies included in the meta-analyses were limited and, therefore, a large, prospective, multicenter, RCT is needed to more reliably compare the two approaches.

In addition to the supine position, other modifications to the traditional prone position have been proposed over the years. Published series have reported on the supine oblique, semisupine, lateral decubitus, split-leg modified lateral, flank, and flank prone position. Many of these positions have demonstrated the advantage of allowing for simultaneous antegrade and retrograde access to the kidney, enabling access to stones remote from the nephrostomy tract without the need for additional percutaneous punctures.

Access

The key to a successful percutaneous procedure is well-placed access into the kidney. The percutaneous puncture can be performed under fluoroscopic, ultrasound, MRI, or CT guidance, and it can be obtained from an antegrade or retrograde approach. In 2003, three-quarters of the practicing urologists in the North Central Section of the AUA who responded to a survey reported feeling comfortable performing PCNL, but only 11% of that group routinely obtained percutaneous access without the assistance of a radiologist. Today, it remains true that most practicing urologists, including endourologists, in the United States do not obtain their own percutaneous renal access. Despite this, several studies have demonstrated that access obtained by a urologist is comparable or favorable to access obtained by an interventional radiologist.

Watterson and colleagues retrospectively reviewed 103 PCNL procedures at a single institution in which access was obtained by a radiologist in 54 subjects and by a urologist in 49 subjects. They found fewer complications (5 vs 15, respectively) and higher stone-free rates (86% vs 61%, respectively) in the urologist-directed access group compared with the radiologist-directed access group. Similar retrospective reviews with larger numbers of subjects have corroborated these findings. Tomaszewski and colleagues also compared 195 subjects with urologist-directed access with 38 subjects with radiologist-directed access and found comparable complication rates between the two groups. However, the stone-free rate was higher with urologist-obtained access (99% vs 92%, respectively), and access obtained by a radiologist was considered unsuitable in 37% of subjects, necessitating access by a urologist at the time of surgery. Of note, however, access obtained by a radiologist was often done solely for the purpose of renal decompression, without the foresight or opportunity to communicate with the urologist about the suitability of the access for future surgical intervention. These data underscore the advantage of the urologist over the radiologist in obtaining appropriate access, which is that the urologist can apply his or her knowledge and expertise in renal anatomy and surgical technique to select a puncture site that minimizes the number of procedures and improves outcomes.

Intraoperative percutaneous access is most commonly performed using fluoroscopic guidance. Inherent to this modality is exposure to ionizing radiation for the surgeon, operating room staff, and patient. The dose of radiation in this setting is not inconsequential to the physicians or staff who perform these surgeries daily, or to the recurrent stone formers who often undergo repeated surgical interventions. Kumari and colleagues sought to quantify the radiation exposure received by the urologist, operating room staff, and patient during PCNL using lithium fluoride thermo-luminescent dosimeter chips to monitor incident radiation exposure in 50 consecutive PCNL procedures. With a mean operating time of 75 minutes and mean fluoroscopy time of 6 minutes (1.8–12.16 minutes), the mean incident radiation exposure to the finger of the operating urologist and assisting resident were 0.28 mSv and 0.36 mSv, respectively, and to the subject was 0.56 mSv. Although the mean radiation dose at the level of the trunk of the operating urologist was 60 μSv, the radiation dose to the anesthesiologist and floor nurse was minimal. Of note, the National Commission on Radiation Protection states that the maximum permissible dose equivalent for occupational exposure in 1 year for the combined whole body is 50,000 mSv. Although the average radiation exposure during PCNL described by Kumari and others is well under the permissible yearly amount, it is important to keep in mind that radiation dose can vary greatly depending on complexity and length of procedure and can accumulate quickly, particularly for those urologists and/or staff largely dedicated to endourology and for those patients afflicted with chronic stone-forming conditions.

An alternative to fluoroscopy is ultrasound-guided percutaneous access. The advantages of ultrasound-guidance include lack of ionizing radiation to the patient and physician, the ability to identify nearby organs such as bowel, spleen or liver, widespread availability, and low cost. However, ultrasound-guidance is quite operator dependent and has limited ability to delineate fine detail of renal anatomy, particularly in obese patients or those with nondilated collecting systems. Despite these limitations, ultrasound-guided percutaneous puncture of the collecting system is ideal for pregnant and pediatric patients and for those in whom retrograde placement of a ureteral catheter is difficult or impossible (eg, those with urinary diversions or renal transplants ).

Several groups of investigators have examined the safety and efficacy of ultrasound-guided percutaneous renal access, all of which concluded that ultrasound-guided access has satisfactory outcomes with few complications and less radiation exposure compared with conventional fluoroscopic-guided access. Agarwal and colleagues prospectively evaluated 224 subjects undergoing PCNL who were randomized to fluoroscopic or ultrasound-guided access. They found a shorter mean access time (1.8 minutes vs 3.2 minutes, P <.01) and fewer mean puncture attempts (1.5 vs 3.3, P <.01) to achieve access into the desired calyx in the ultrasound group compared with the fluoroscopy group. Furthermore, mean duration of radiation exposure was shorter for ultrasound compared with fluoroscopic guidance (14.4 seconds vs 28.6 seconds, respectively, P <.01) and all subjects in both groups were stone free at 1 month based on noncontrast CT.

MRI has recently been described as an alternative means of nonionizing radiation that can be used to guide percutaneous renal access. Kariniemi and colleagues prospectively evaluated eight subjects undergoing percutaneous access using an open-configuration, C-arm shaped MRI and reported success in seven of the eight attempts, with the only failed attempt occurring in a nondilated system. The investigators noted that a significant drawback of this technique, however, is difficulty visualizing the guidewire with MRI.

For patients with unusual anatomy, such as spinal cord deformity in spina bifida or scoliosis, percutaneous access under fluoroscopic guidance can predispose to a higher likelihood of adjacent organ injury. In these instances, CT-guided percutaneous renal access obtained at a setting separate from PCNL provides an advantage over the traditional fluoroscopic approach. Matlaga and colleagues reported that 3% of 154 percutaneous nephrolithotomies underwent CT-guided access because of spinal deformity, retrorenal colon, or transplant kidney, and subsequently underwent successful PCNL.

Because percutaneous renal access is not a skill set that is routinely included in the curriculum of urologic residency, some practitioners have favored using the more familiar retrograde approach to the kidney. Lawson and colleagues, and Hunter and colleagues, described directing a steerable catheter in a retrograde fashion into the desired calyx, then advancing a puncture wire out through the catheter to the skin, creating through-and-through access. Since their first reports, contemporary modifications of the technique have been described, including the use of endoscopic assistance to visually direct the puncture wire out through the targeted calyx.

Endoscopic assistance has also been used to facilitate antegrade access to the kidney when a percutaneous puncture cannot be successfully performed or when a wire cannot be navigated out of an obstructed calyx. The initial report in 1995 described the use of simultaneous retrograde URS and fluoroscopic percutaneous renal access. Kidd and Conlin reported three clinical scenarios, morbid obesity, renal ptosis, and a large staghorn calculus, in which endoscopic assistance was a valuable adjunct to traditional fluoroscopic or ultrasound-guided antegrade renal access. Other investigators have advocated for the routine use of ureteroscopic-guided percutaneous renal access because they argue that endoscopic-assisted access provides for safer, more precise placement of the working sheath, potentially translating into less bleeding, and that the use of a ureteroscopic access sheath facilitates drainage, passage of fragments, and easy access to calyces that are inaccessible via the nephroscope. Although this technique adds to troubleshooting options available to assist with difficult access, routine use is arguably unnecessary, time-consuming, and cost-inefficient.

One of the advantages of PCNL in treating large or complex stones is the large caliber of the access tract, which allows for intact removal of large stones and the use of fragmentation devices that incorporate suction capability. However, morbidity of the procedure is often attributed to the large size of the nephrostomy tract (typically 30F). A review of data derived from the Clinical Research Office of the Endourology Society (CROES) global database of over 5000 PCNLs showed that the probability of bleeding was higher with larger caliber working sheaths (odds ratio 1.42, P = .0001). In an effort to decrease the morbidity of PCNL, there has been interest in reducing the size of the working sheath and/or the size of the nephrostomy tube.

The technique of using a small caliber working sheath, known as the mini-perc, was originally an extension of a pediatric technique that used an 11-15 F peel-away vascular access sheath. Jackman and colleagues later substituted a 13 F ureteral access sheath for the flimsy vascular access sheath because of it offered superior stability. The technique involves conventional percutaneous access to the collecting system using an 18-gauge needle. After passage of two guide wires down the ureter, a 13 F sheath is passed directly over the working wire, without the need for sequential dilation. In a small series of nine mini-percs on stones with an average cross-sectional area of 1.5 cm ² , Jackman and colleagues reported stone-free rates comparable to standard PCNL with no transfusions and modest pain medication requirements.

Three prospective trials compared the safety and efficacy of mini-perc with standard PCNL. Knoll and colleagues randomized 50 subjects with solitary lower pole or renal pelvic calculi to mini-perc (25 subjects, 18 F outer sheath) or standard PCNL (24 subjects, 26 F outer sheath). Of note, mean stone size in the standard group was slightly larger than in the mini-perc group (22 mm vs 18 mm) and postoperative management differed between the two groups (subjects undergoing uncomplicated mini-percs were left tubeless, while all subjects undergoing standard PCNL were left with 22 F nephrostomy tubes). Operative times, stone-free rates, and complication rates were comparable between the two groups, but the mini-perc group reported slightly lower postoperative pain scores and had a significantly shorter hospital stay (3.8 vs 6.9 days) compared with the standard PCNL group. Cheng and colleagues performed an RCT that compared 72 subjects undergoing mini-perc using a 16 F sheath with 115 subjects undergoing standard PCNL using a 24 F sheath. Although mini-perc was associated with a lower transfusion rate ( P <0.05), the procedure was significantly longer than standard PCNL. Finally, Mishra and colleagues prospectively compared mini-perc with standard PCNL in subjects with 1 to 2 cm renal stones and found longer operative times but significantly less blood loss, lower analgesic use, and reduced hospital stay for mini-perc compared with standard PCNL.

Mini-perc is the predecessor of the microperc, a technique that furthers the concept of downsizing renal access. Bader and colleagues initially described using an “all-seeing needle,” which allowed visualization of the punctured calyx and stone before proceeding with dilation to 30 F for standard PCNL. The group subsequently used this needle to obtain access and perform PCNL in a single-step. After obtaining percutaneous access with the 16-gauge all-seeing needle under optical guidance, the inner bevel of the needle is removed, leaving the 4.85 F outer sheath in place. A three-way connector is attached to the proximal end of the sheath to allow for irrigation and passage of a 200 μm laser fiber and a micro-optic ( Fig. 1 ). This technique may have an advantage compared with SWL for the treatment of intermediate-sized lower pole stones because it allows for direct visualization of the stone and active clearance of fragments using pressurized irrigation. Micro-perc is applicable to stones in calyceal diverticuli and in horseshoe and ectopic kidneys. Further studies, ideally in a randomized, controlled setting, are needed to define the safety, efficacy, and applicability of this innovative technique.

Fully assembled microperc set-up, including. 4.85 F needle sheath fitted with a 3-way adapter accommodating irrigation tubing above, telescope cable below, and laser fiber passing through the center of the sheath.

( From Desai MR, Sharma R, Mishra S, et al. Single-step percutaneous nephrolithotomy (microperc): the initial clinical report. J Urol 2011;186:140–1, Fig. 2D; with permission.)

For complex stones, multiple percutaneous accesses are often needed to removal stones from disparate locations. However, the use of multiple accesses carries a higher risk of bleeding and complications, including potentially a detrimental effect on renal function, compared with single access. Accordingly, measures that reduce the need for multiple accesses are desirable. Wong and Leveillee made liberal use of flexible nephroscopy and holmium:YAG laser lithotripsy in their series of 45 subjects with >5 cm partial or complete staghorn calculi in which a single upper percutaneous renal access was obtained. With a mean of 1.6 procedures per subject and a transfusion rate of 2.2%, they achieved a 95% stone-free rate. Other investigators have described the use of same-procedure URS to access and retrieve calyceal stones inaccessible from the percutaneous nephrostomy tract. Landman and colleagues reported on nine subjects with partial or compete staghorn calculi in whom a ureteral access sheath was placed before lower pole percutaneous puncture. Inaccessible stones were retrieved ureteroscopically and placed in a position where they could be removed percutaneously. They achieved a stone-free rate of 78% with an estimated blood loss of only 290 cc. Likewise, Marguet and colleagues performed initial URS and treated or displaced stones they anticipated would be remote from the planned percutaneous access tract before repositioning the subject prone and proceeding with standard PCNL using a single percutaneous access.

Finally, in another effort to reduce the number of percutaneous accesses, Miller and colleagues described the use of a nondilated puncture to facilitate access to a stone that is remote from the nephrostomy tract for which the associated infundibulum is either not identifiable or is inaccessible. The technique involves percutaneous needle puncture directly into the stone-bearing calyx with subsequent injection of contrast-stained saline or air that can be identified endoscopically with a flexible nephroscope ( Fig. 2 ). In some cases, the nephroscope can then be advanced into the stone-bearing calyx to fragment or retrieve the stone or, in some cases, the stone may be able to be irrigated out of the calyx via saline injection through the needle.

Nondilated percutaneous access to facilitate identification of a stone-bearing calyx that could not be located endoscopically. ( A ) Percutaneous needle puncture into a stone-bearing calyx visualized fluoroscopically but not accessible with a flexible nephroscope via either of the two established nephrostomy tracts. ( B ) The flexible nephroscope has been advanced into the previously inaccessible calyx via an upper pole access tract after air or contrast had been injected through the needle, enabling endoscopic identification of the infundibulum.

Tract dilation

Historically, a variety of methods have been used to dilate the nephrostomy tract, including metal telescoping Alken dilators, sequential Amplatz dilators, and balloon dilation. In general, balloon dilation is thought to be quicker than fascial dilation, which requires numerous passes of metal or plastic dilators. Additionally, the numerous passes that are required during dilation with sequential dilators increase the likelihood of wire dislodgement or perforation of the collecting system, which can increase the risk of hemorrhage. On the other hand, balloon dilation is more costly and arguably less effective in the setting of a previously operated kidney.

The introduction of the X-Force N30 dilating balloon (Bard Medical, Covington, GA, USA), with a burst pressure of 30 ATM (atmospheres) instead of the standard 17 ATM, offers the potential for greater efficacy in the setting of the patient undergoing reoperation. Hendlin and Monga reported a 100% success rate in dilating 60 consecutive percutaneous tracts with a 30 ATM balloon compared with the historic 5% to10% failure rate with a 17 ATM balloon.

Despite a general consensus in the literature regarding the superior safety and efficacy of balloon dilation over metal or plastic dilators, a recent analysis of the CROES database assessing PCNL operative times and bleeding complications in 5537 tract dilations (2277 balloon dilations and 3260 telescopic or serial dilations) demonstrated higher median operating time, bleeding, and transfusion rates in the balloon dilation group compared with the telescoping-serial dilation group. Furthermore, on multivariate analysis, independent predictors of bleeding complications included sheath size, operating time, stone size, and caseload but did not include dilation method. Of note, this was not a randomized comparison, and it is possible that there are other unidentified confounding factors.

In an effort to streamline traditional two-step dilation (balloon dilation and passage of the working sheath over the balloon), a novel device called the Pathway Balloon Expandable PCNL Sheath (Onset Medical, Irvine, CA, USA) was developed that is comprised of a polyester balloon housed in an expandable Teflon access sheath. The device allows for simultaneous tract dilation and sheath placement in one step. Pathak and Bellman compared the safety and efficacy of this dilation system (Pathway Access Sheath [PAS]) to standard balloon dilation in 21 randomly assigned subjects undergoing PCNL and found a shorter insertion time with the PAS system compared with standard balloon dilation (3 minutes vs 5.7 minutes, respectively) but no significant difference in blood loss or cost. The investigators admitted that potential drawbacks to the design include a less stiff body compared with the Amplatz sheath and a slightly oblong shape, which may hinder the removal of very round stones.