In Vitro Evaluation of Nitinol Stone Retrieval Baskets for Flexible Ureteroscopy

Introduction

R etrograde intrarenal surgery (RIRS) for urinary stone disease has been established in the management of urolithiasis over the past decade and has created a challenging environment for the manufacturer of working tools for RIRS. Since Dormia ¹ presented his experience with a spiral basket for stone retrieval in 1982, many attempts have been made to improve basket design with different wires (number, material, shape), shaft (size, material), and configuration (spherical, helical, paired wire, tipless). ^2,3 Previously used stainless steel gave way to the soft “memory metal” nitinol, a flexible and kink-resistant alloy of nickel and titanium, which is also less rigid than stainless steel and imposes less limitation on ureteroscope deflection. ² Thus, stone baskets have particular physical characteristics that facilitate optimal stone extraction in different clinical situations. ³ These differences may have consequences in performance and efficacy of RIRS.

We evaluated the in vitro performance of multiple, commercially available stone retrieval baskets.

Materials and Methods

10 manufacturers of baskets for RIRS were invited to participate in this evaluation. All tested baskets were new, packaged, and were sent by the manufacturers for testing.

One flexible fiber ureterorenoscope (URF-P5, Olympus Europe, Hamburg, Germany), which was not in clinical practice before measurements, was used to analyze the impact of each basket on renoscope deflection. The deflection angle of the renoscope was measured empty and loaded (with baskets) using a protractor. The basket function (open/close) was tested in maximum deflection of the renoscope.

A measurement of the basket's own resistance toward deflection was performed. Therefore, each basket was closed and fixed on a 90-degree workbench edge, leaving 4 cm of the basket's tip on air (Fig. 1). A 0.5 g weight was positioned at the very edge of the tip. The vertical deflection at the basket's tip was measured (mm) for each basket. The basket resistence (mm/g) was calculated. The measurements were performed five times for each basket.

OPEN IN VIEWER

FIG. 1.

Measurement of the basket's internal resistance toward deflection.

Results

Of 10 invited manufacturers, 9 decided to participate in this evaluation, providing a total of 15 baskets. A total of 14 tipless (1 helical, 13 nonhelical) baskets, and 1 nontipless basket (nonhelical, Rüsch Stone ex 2.5) for RIRS were analyzed (Table 1). The maximum deflection of the empty URF-P5 ureterorenoscope was 275 degrees as stated by the manufacturer. Maximum deflection was achieved in 12 baskets and reduced in 3. In particular, maximum deflection was reduced with IMP Tipless 2.5, Rüsch Stoneex 2.5, and Boston Optiflex 1.3 to 231, 244, and 270 degrees, respectively (Table 1).

In maximum deflection, complete opening of the basket was achieved in 13 samples. A partial opening of the basket was possible with Bard Dimension 2.4, whereas Rüsch Stoneex 2.5 failed in maximum deflection (Table 1).

The measurement of the basket's internal resistance toward bending showed wide variations (5.97 to 38 mm/g). Of the tipless nonhelical baskets ≥2F, Cook N-Circle 2.2 (35 mm/g) and for those <2F, the Cook N-Circle 1.5 (38 mm/g) provided the lowest resistance of the basket tip. Conversely, highest resistance was observed with IMP Tipless 2.5 (tipless, helical, 5.97 mm/g), Rüsch Stoneex 2.5 (nontipless, nonhelical, 7.08 mm/g), and Cook N-Gage (tipless, nonhelical, 10 mm/g), respectively (Table 1). With closure of the basket, an increase of the inner radius of the deflected renoscope was observed in 10 of the 15 tested samples (Table 1).

Discussion

With this study, we investigated the physical characteristics of 15 commonly used stone retrieval baskets—14 tipless (1 helical, 13 nonhelical) and 1 nontipless nonhelical stone extractor—from 9 major manufacturers. Resistance (mm/g) toward bending showed wide variations among the baskets. Higher resistance led to an increased inner radius of the deflected scope in 10 of the 15 tested samples, which may alter functionality and thereby reduces the effectiveness of RIRS. ^4,5 The scope deflection was reduced in 3 of 15 baskets. Highest reduction of renoscope deflection, however, was observed in those baskets with the highest resistance.

Increased deflection of flexible ureteroscopes provides the surgeon the ability to treat lower caliceal stones and horseshoe kidneys effectively. ⁶ Traxer and associates ⁷ demonstrated deterioration of maximal dorsal/ventral deflection from 270 degrees to 133 degrees and 270 degrees to 208 degrees after 50 consecutive RIRS with the same renoscope (Flex-X, K. Storz, Tuttlingen, Germany). In contrast, in our series, all measurements were performed with a new flexible ureterorenoscope that was not in clinical use before. Therefore, minor reduction of renoscope deflection because of the use of stone retrieval baskets with higher internal resistance might contribute to a significant reduction of renoscope deflection if the renoscope is a used one.

Few studies have focused on efficacy or physical characteristics of different basket designs using in vitro and ex vivo models. Honey ⁸ demonstrated faster, less traumatic, and more effective caliceal stone extraction with a two-loop 3.2F N-Circle^® nitinol tipless basket than with a 3.0F four flatwire Segura-design basket (Atlas Wire™) in a porcine kidney model. El-Gabry and Bagley ⁹ compared the ability of five baskets to retrieve metal beads from an in vitro ureteral and caliceal model. In the caliceal model, all baskets failed to remove any beads with the exception of the 3.2F N-O-tip (Cook Urological, Spencer, IN). Using a similar model, they currently found an advantage of an articulating basket in releasing beads in both ureteral and caliceal models. ¹⁰ The same working group recently evaluated three tipped and four tipless stone retrieval baskets in an ureteral and caliceal model. The tipped designed baskets failed to retrieve any stones from the caliceal model. ²

Ptashnyk and colleagues ¹¹ studied the efficacy and associated mucosal damage of four basket and three grasper designs (flat-wire vs helical vs graspers) in ex vivo porcine ureteral models. They found that the design of baskets and graspers affects the time of stone retrieval in different situations and causes various grades of tissue damage. ¹¹ Lukasewycz and coworkers ¹² compared 10 commonly used stone extractors, 6 tipless and 4 helical stone extractors, in a commercially available ureteral model. All baskets were able to retrieve the ureteral stones successfully without differences between tipless and helical basket extractors. The Cook N-Circle 3.0F basket provided the most expeditious means to stone extraction.

In most of these studies, tipless nitinol baskets were shown to be superior with regard to stone extraction in the ureter and the calix, although physical specifications of stone retrieval baskets that confer these advantages were not examined.

Monga and colleagues ³ investigated the role of basket actuation dynamics in 17 commercially available stone baskets. The only basket to exhibit linear opening was the Cook N-Circle (2.2F, 3.0F, and 3.2F), while all other baskets opened in an exponential fashion. Tipless baskets opened more rapidly to the target basket width than flat-wire and helical baskets. They concluded that linear basket opening necessitates a shorter basket excursion and would therefore be easier to maintain in the field of view, providing the surgeon greater control to engage a stone and a more controlled capture sequence.

The effective value of basket actuation dynamics for functionality and effectiveness of RIRS is, however, difficult to determine. We therefore evaluated one physical aspect, the basket's internal resistance toward bending, of multiple baskets in nearly the same design category (nonhelical and tipless, except one helical tipless basket) that might have a direct benefit for clinical practice.

The clinical effectiveness of ureteroscopic stone removal is strongly influenced by factors such as operator skill, intraoperative visualization, and ureteral anatomy (eg, strictures, steinstrassen, bleeding sources). Conversely, Bach and associates ⁴ demonstrated recently a considerable decrease of irrigation volume in five different flexible ureterorenoscopes ranging from 62.24% (1.5F) to 87.2% (2.4F) using nitinol baskets. Therefore, knowledge about a basket's internal resistance toward bending and its impact on renoscope deflection might help to choose the adequate stone retrieval basket for RIRS.

Conclusions

Understanding the benefits and limitations of different basket designs and performance is essential for effective ureteroscopic stone extraction with regard to complications and cost reduction. Resistance of the tested baskets showed wide variations in our study, but impact on renoscope deflection seems minor. Higher internal resistance, however, leads to increased inner radius of the deflected renoscope, which may alter functionality in RIRS by reducing the scope deflection. Knowledge about the basket's key features and differences in basket function may therefore be crucial to choose the best tool for different surgeons and indications.