Skip to main content

A New Automatically Fixating Stone Basket (2.5 F) Prototype with a Nitinol Spring for Accurate Ureteroscopic Stone Size Measurement



Intraoperative assessment of stone size is crucial for the successful and safe extraction of stones. The first automatically fixating measuring stone basket prototype showed a mismatch between the steel spring and the nitinol basket; therefore, to improve this prototype, the steel spring was replaced with a nitinol spring and a modified scale was implemented on the basket handle for accurate intraoperative stone size measurement.


The proposed tipped basket was composed of nitinol. A standard handle with a spring-supported self-closing mechanism (2.5 F, Urotech®) was used, and a modified nonlinear millimeter scale was established on the handle. The grasping force was provided by the new nitinol spring mechanism in the handgrip. Various colors associated with the stone size were applied on the scale.


The material difference between the basket and the spring was eliminated. The measuring scale ranged from 2 mm (green) through 5 mm (yellow) to 8 mm (red), and the scale was nonlinear because of the nonlinear relationship between the diameter of the stone and the distance marked on the scale.


The proposed automatically fixating stone basket with a nitinol spring has the potential to improve the safety and effectiveness of endourological stone retrieval. Further validation of this new scale and basket should follow.


Ureteroscopy (URS) is currently the first-choice therapy for ureteral stones and is the recommended therapy option for kidney stones smaller than 2 cm [1]. The growing prevalence of kidney stone disease worldwide and the favorable characteristics of URS (i.e., low invasiveness, high stone-free rates, and relatively low risk of intra- and postoperative complications) predict that there will be a growing demand for URS in the future [2, 3].

Low-dose noncontrast computer tomography (CT) is the gold standard for the diagnosis of acute flank pain and therefore for the preoperative planning of endourological treatment [4]; however, in certain settings, the accuracy of CT-based preoperative stone size assessment, especially regarding ureteral stones, may be inaccurate [5]. Moreover, Patel et al. revealed that the CT-based estimation of stone diameter for larger stones (≥ 4 mm) may be less precise than intraoperative visual assessment by the surgeon [6]. Thus, additional reliable stone size assessment during URS would be helpful for making decisions about whether to perform direct stone extraction or laser lithotripsy for larger stone fragments.

Current literature regarding the unique concept of a stone size measuring basket is scarce.

Our research group previously introduced a nonlinear millimeter scale coupled with various self-closing nitinol stone baskets (2.5, 3.0, and 4.0 F) to enhance intraoperative stone size measurements [7]. The nonlinear millimeter basket scale was compared in vitro with the visual estimation of two surgeons, and manual stone measurement was used as the reference method. The 2.5-F nitinol basket was the most accurate for measuring larger stones (> 6 mm) with sensitivity of 56% and specificity of 84%, while the 4.0-F basket was the most accurate for smaller stones (< 3 mm); however, the study showed that the visual ureteroscopic estimation was superior to the basket measurement [8]. The basket system could have been inferior to visual assessment as a result of a mismatch between the steel spring and the nitinol basket, which may have influenced the measurement accuracy depicted on the scale. The different material-specific relationships of steel and nitinol regarding their stress and strain could explain this phenomenon. Initially steel is not flexible and shows increasing stress during engaging the stone, whereas nitinol shows more strain and less stress. Even less stress is present during the reverse action [9].

Therefore, to resolve the described mismatch and to improve the measurement accuracy, a new basket prototype, especially for ureteral and renal pelvic stones, has been proposed in which the steel spring has been replaced with a nitinol spring.


The tipped automatically fixating stone basket was composed of nitinol to provide maximal safety to the surrounding urothelial tissue and best possible performance. The steel spring was replaced with a suitable nitinol spring as a part of the stone-fixating mechanism. The slider on the front side of the handle opens the basket and enables the stone to be grasped (Fig. 1).

Fig. 1

A handle with a new colored millimeter scale and slider

A standard handle (Urotech®) connected to a 2.5-F basket was described previously [7, 8] and was used in this prototype. It was developed in cooperation with Prof. S. Lahme (Pforzheim, Germany). The handle has two unique design elements: firstly, it has a mentioned spring mechanism that enables automatic stone fixation in the basket; secondly, it is equipped with a dis- and reconnectable handle so that the ureteroscope can be fully removed while the retrieval basket with the grasped stone remains in place. The handle can be reconnected again if needed. The handle should be opened on the back side to disconnect the basket (Fig. 2), as this maneuver enables a switch to be made between different URS devices without the need to disengage the stone.

Fig. 2

The opened back of the handle demonstrates the slider in an opened-basket position. The nitinol spring is in the proximal gray part of the handle

A modified nonlinear millimeter scale was established on the handle. The scale was standardized by grasping standardized (DIN “Deutsche Industrie Norm” ISO 281) screws with the basket. Various colors on the millimeter scale were applied, and the color change from green to yellow was based on the study by Abdelrahim et al. They showed that stones greater than 5 mm in width are associated with a statistically significant higher incidence of intraoperative complications [10]. The color change from yellow to red was a proposal by the author and should be further investigated. Extreme caution should be applied during manipulation of the stones situated in the red area because of their size. The development of the presented prototype did not contain any studies with human participants or animals performed by any of the authors.


Figure 1 depicts the front side of the final prototype. The measuring scale ranges from 2 mm (green) through 5 mm (yellow) to 8 mm (red), and the scale is nonlinear because of the nonlinear relationship between the diameter of the stone and the distance marked on the scale. The newly proposed prototype managed to eliminate the material difference between the basket and the spring, which was present in the first prototype [7]. Furthermore, the layout of the scale was improved to make it more comprehensible in comparison with previous scales (Fig. 3).

Fig. 3

Evolution of the measuring scale—current nonlinear version ranges from 2 mm (green) through 5 mm (yellow) to 8 mm (red)


In URS, as in every other surgical procedure, patient safety is the main goal for the surgeon. Despite the many improvements over the years (e.g., invention of ureteral access sheaths, dilatators, safety wires, and especially laser lithotripsy), there is still a need to improve endourological safety, as ureteral injury (of any severity) still occurs in up to 30% of URS cases [11]. Stone diameters greater than 5 mm, a patient history of URS, a dilated proximal ureter, stone location above the ischial spines, and the involvement of a junior urologist are all factors that are known to be associated with a significantly higher incidence of intraoperative complications [10]. The results of our previous study were in line with those of Patel et al. [6] and confirmed that endourologists are able to assess residual stone fragment size accurately enough to make intraoperative decisions about direct extraction or further laser lithotripsy. Conversely, we also showed that it was feasible to measure the stone with the described basket handle scale [8], which could be especially suitable for junior endourologists; however, improvements regarding the accuracy of this new method were required and a possible solution is provided in the current study. It has been already shown that the visual stone size estimation is biased by multiple factors, such as the color of the stone and the experience of the surgeon [8]. Interestingly, a slight tendency to underestimate the size of large stones (> 6 mm) was observed [8]; therefore, the measurement of these stone sizes could be enhanced with the proposed basket prototype.

It is already known that intraoperative visual stone assessment and measurements using regular preoperative diagnostic tools (i.e., CT and ultrasound) are biased [12,13,14]. Other experimental methods such as ultrasound strain sonography have not yet gained clinical application [15]. In our opinion, the modified measuring basket prototype reported here could attempt to level these discrepancies and objectify stone size measurements in the future.

Ludwig et al. recently proposed another approach to improve the intraoperative stone measurement accuracy that was based on additional measuring software calibrated in accordance with the distance of the basket tip in the visual field of the ureteroscope [16]. Future comparison between the “hardware” basket concept and the proposed URS software would be surely interesting to assess their influences on intraoperative outcomes and patients’ safety.


The proposed automatically fixating stone basket with a nitinol spring has the potential to improve the safety and effectiveness of endourological stone retrieval; however, further validation of the proposed prototype regarding measurement accuracy, durability of the device, and patient safety is required.


  1. 1.

    Turk C, Petrik A, Sarica K, Seitz C, Skolarikos A, Straub M, Knoll T. EAU guidelines on interventional treatment for urolithiasis. Eur Urol. 2016;69:475–82.

    Article  Google Scholar 

  2. 2.

    Stamatelou KK, Francis ME, Jones CA, Nyberg LM, Curhan GC. Time trends in reported prevalence of kidney stones in the United States: 1976–1994. Kidney Int. 2003;63:1817–23.

    Article  Google Scholar 

  3. 3.

    Ordon M, Urbach D, Mamdani M, Saskin R, Honey RJ, Pace KT. A population based study of the changing demographics of patients undergoing definitive treatment for kidney stone disease. J Urol. 2015;193:869–74.

    Article  Google Scholar 

  4. 4.

    Worster A, Preyra I, Weaver B, Haines T. The accuracy of noncontrast helical computed tomography versus intravenous pyelography in the diagnosis of suspected acute urolithiasis: a meta-analysis. Ann Emerg Med. 2002;40(3):280–6.

    Article  Google Scholar 

  5. 5.

    Kishore TA, Pedro RN, Hinck B, Monga M. Estimation of size of distal ureteral stones: noncontrast CT scan versus actual size. Urology. 2008;72:761–4.

    CAS  Article  Google Scholar 

  6. 6.

    Patel N, Chew B, Knudsen B, Lipkin M, Wenzler D, Sur RL. Accuracy of endoscopic intraoperative assessment of urologic stone size. J Endourol. 2014;28:582–6.

    Article  Google Scholar 

  7. 7.

    Cordes J, Nguyen F, Pinkowski W, Jocham D. Measurement of stone diameter with three sizes of automatically fixating stone baskets. Open J Urol. 2013;3:58–61.

    Article  Google Scholar 

  8. 8.

    Cordes J, Teske L, Nguyen F, Pinkowski W, Sievert KD, Vonthain R. A comparison between an in vitro ureteroscopic stone size estimation and the stone size measurement with the help of a scale on stone baskets. World J Urol. 2016;34(9):1303–9.

    Article  Google Scholar 

  9. 9.

    Henderson E, Nash DH, Dempster WM. On the experimental testing of fine nitinol wires for medical devices. J Mech Behav Biomed Mater. 2011;4(3):261–8.

    CAS  Article  Google Scholar 

  10. 10.

    Abdelrahim AF, Abdelmaguid A, Abuzeid H, Amin M, Mousa S, Abdelrahim F. Rigid ureteroscopy for ureteral stones: factors associated with intraoperative adverse events. J Endourol. 2008;22:277–80.

    Article  Google Scholar 

  11. 11.

    Karakan T, Kilinc MF, Demirbas A, Hascicek AM, Doluoglu OG, Yucel MO, Resorlu B. Evaluating ureteral wall injuries with endoscopic grading system and analysis of the predisposing factors. J Endourol. 2016;30(4):375–8.

    Article  Google Scholar 

  12. 12.

    Sternberg KM, Eisner B, Larson T, Hernandez N, Han J, Pais VM. Ultrasonography significantly overestimates stone size when compared to low-dose, noncontrast computed tomography. J Urol. 2016;95:67–71.

    Article  Google Scholar 

  13. 13.

    Kadihasanogli M, Marien T, Miller NL. Ureteral stone diameter on computerized tomography coronal reconstruction is clinically important and underreported. J Urol. 2017;102:54–60.

    Article  Google Scholar 

  14. 14.

    Dai JC, Dunmire B, Sternberg KM, et al. Retrospective comparison of measured stone size and posterior acoustic shadow width in clinical ultrasound images. World J Urol. 2018;36(5):727–32.

    Article  Google Scholar 

  15. 15.

    Li Q, Chen L, Halpern EF, Samir AE. Detection and measurement of stones with ultrasound strain elastography: a phantom study. Ultrasound Quart. 2015;31(4):272–8.

    Article  Google Scholar 

  16. 16.

    Ludwig WW, Lim S, Stoianovici D, Matlaga BR. Endoscopic stone measurement during ureteroscopy. J Endourol. 2017;32:34–9.

    Article  Google Scholar 

Download references



No funding or sponsorship was received for this study or publication of this article. Described stone basket prototype has been provided by europ medical GmbH. All authors had full access to all of the data in this study and take complete responsibility for the integrity of the data and accuracy of the data analysis.


All named authors meet the International Committee of Medical Journal Editors (ICMJE) criteria for authorship for this article, take responsibility for the integrity of the work as a whole, and have given their approval for this version to be published.


Jens Cordes, Felix Nguyen, Wolfhard Pinkowski, Axel S. Merseburger and Tomasz Ozimek have nothing to disclose.

Compliance with Ethics Guidelines

This article does not contain any studies with human participants or animals performed by any of the authors.

Data Availability

The manuscript has no associated data.

Open Access

This article is distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International License (, which permits any noncommercial use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Author information



Corresponding author

Correspondence to Jens Cordes.

Additional information

Enhanced digital features

To view enhanced digital features for this article go to

Rights and permissions

This article is published under an open access license. Please check the 'Copyright Information' section either on this page or in the PDF for details of this license and what re-use is permitted. If your intended use exceeds what is permitted by the license or if you are unable to locate the licence and re-use information, please contact the Rights and Permissions team.

About this article

Verify currency and authenticity via CrossMark

Cite this article

Cordes, J., Nguyen, F., Pinkowski, W. et al. A New Automatically Fixating Stone Basket (2.5 F) Prototype with a Nitinol Spring for Accurate Ureteroscopic Stone Size Measurement. Adv Ther 35, 1420–1425 (2018).

Download citation


  • Endourology
  • Nitinol
  • Stone basket
  • Stone measurement
  • Ureteroscopy