REVIEWS What is the stone-free rate following flexible ureteroscopy for kidney stones? Khurshid R. Ghani and J. Stuart Wolf Jr Abstract | Flexible ureteroscopy (URS) is increasingly being used as the first-line treatment for patients with renal stones. Despite this increase in use, substantial variations exist in the reported stone-free rates (SFR) following flexible URS. These variations are a result of inconsistencies in the definition of ‘stone-free’, which reflect variations in the type of imaging used to assess the presence of stones postoperatively and the timing of the assessment. Other possible factors such as the importance of residual fragments following stone surgery, and the size and position of the stones might also account for variations in stone-free rates. In order to obtain an accurate estimate of the SFR, especially within subgroups defined by stone characteristics and exact technique, we compare reported SFRs from studies that use imaging other than CT for follow-up and those that use only CT. We also review the evidence on the importance of active retrieval of fragments during flexible URS and whether this technique improves the outcomes of patients with kidney stones. Ghani, K. R. & Wolf, J. S. Jr. Nat. Rev. Urol. 12, 281–288 (2015); published online 14 April 2015; corrected online 2 June 2015; doi:10.1038/nrurol.2015.74


Division of Endourology and Stone Disease, Department of Urology, University of Michigan, 1500 E. Medical Center Drive, Ann Arbor, MI 48109, USA (K.R.G., J.S.W.). Correspondence to: K.R.G. [email protected]

In the USA, kidney stone disease represents a substan­ tial burden on healthcare expenditure with total annual treatment charges now exceeding US$10 billion.1 Two decades ago, one in 20 adults in the US reported a history of kidney stones, whereas now, nearly one in 11 are diagnosed with stones during their lifetime.2 The rising prevalence of kidney stones has been linked to increasing rates of obesity, diabetes and metabolic syndrome.3,4 This increase in prevalence has been accompanied by greater utilization of surgery for treatment of upper urinary tract stones; a similar increase has occurred in most developed nations.5–7 Kidney stones can be monitored or treated based on the presence of symptoms, renal obstruction, renal impair­ ment or known risk factors for kidney stone disease.8 The widespread use of abdominal imaging, especially CT, has led to a ‘stage migration’ effect whereby more patients present with kidney stones early, many of whom might be asymptomatic.9,10 Common and widely available treatment options include three procedures: extracorporeal shock­ wave lithotripsy (SWL), flexible ureteroscopy (URS) and percutaneous nephrolithotomy (PCNL). The availability of holmium lasers to perform intra­ corporeal lithotripsy has enabled substantial improve­ ments in the outcomes that are achievable using flexible URS. Improvements in ureteroscope design, laser deliv­ ery and ancillary instrumentation have enabled all stones, regardless of location or composition, to be treated using retrograde intrarenal surgery (RIRS). Furthermore, URS Competing interests K.R.G. declares that he has acted as a consultant for Boston Scientific and Lumenis. J.S.W. declares no competing interests.

has the unique advantage of a low major complication rate in the setting of ambulatory surgery. These performance characteristics are reflected in population-based patient series published in 2011 and 2014, in which use of URS relative to SWL or PCNL was substantially increased com­ pared with previous reports.6,7 Surgeon training and preferences might influence the selection of kidney stone treatment modalities,11,12 and three factors are broadly considered before making a decision: stone characteristics (such as size or location), patient characteristics (for example obesity, bleeding dia­ thesis or pregnancy) and individual patient preferences. The main objective is to obtain complete clearance of the stone; the degree to which the procedure achieves this is an important criterion when counselling patients and guiding expectations. Many series exist that address the effectiveness of flexible URS for the primary treatment of kidney stones. However, wide variations are reported between different studies. The existence of such variations necessitates a comprehensive review in order to obtain an accurate estimate of the stone-free rate (SFR) following treatment with flexible URS, especially within subgroups defined by stone characteristics and the exact treatment technique. In this Review we explore the current reports of stonefree outcomes of patients with renal stones who were treated with flexible URS. The criteria used to define ‘stone-free’ varies, therefore, we discuss the previous and current consensus on residual fragments following stone surgery. The data regarding stone-free outcomes after flexible URS are divided into studies that used a mixture of imaging modalities to assess residual fragment status and those that used only CT. The influence of stone


VOLUME 12  |  MAY 2015  |  281 © 2015 Macmillan Publishers Limited. All rights reserved

REVIEWS Key points ■■ Use of flexible ureteroscopy (URS) for treating patients with kidney stones has increased compared with shockwave lithotripsy and percutaneous nephrolithotomy ■■ Wide variations in use, and timing of postoperative imaging as well as definitions of stone-free rate (SFR) make accurate assessments of stone clearance after flexible URS challenging ■■ CT provides the most accurate way to assess the presence of residual fragments; however, even when retrieval of fragments is employed, the complete SFR might only approach 55–60% ■■ At the minimum, SFRs should report the zero-fragment rate; residual fragment size >2 mm is associated with significantly increased risk of a stone-related event and greater need for retreatment ■■ Treatment of patients with lower-pole stones using a basket displacement technique can result in higher SFRs ■■ Patients with larger stones receiving treatment with flexible URS might require staged procedures, especially if the cumulative stone size exceeds 1.5–2 cm

Box 1 | Factors influencing the SFR after flexible URS ■■ Imaging modality and time point chosen for follow-up monitoring ■■ Size of stone (>1.5–2.0 cm) ■■ Lower pole location ■■ Use of basket displacement for lower pole stones Abbreviations: SFR, stone-free rate; URS, ureteroscopy

characteristics, such as size and location, on stone-free outcomes is also described (Box 1). We also discuss the importance of active retrieval of fragments during f­lexible URS, and whether use of this technique improves the SFR.

Defining stone-free

Following the introduction of SWL and PCNL in the 1980s it became clear that these minimally invasive procedures, which generate in-situ fragmentation, sometimes resulted in residual fragments that were not subsequently passed. This finding was in contrast to tradi­t ional open surgery where stones were often extracted in toto, albeit with greater morbidity. The term ‘clinically insignificant’ residual fragments was introduced for asymptomatic, noninfectious fragments 3 mm was a signifi­ cant predictor of stone growth, with up to one-third of patients requiring secondary interventions. The timing of postoperative imaging is another factor that is not addressed in many evaluations of the SFR. The wide range of time points at which patients undergo imaging might lead to erroneous measurements of SFR. In particular, if the stone is fragmented to pieces small enough to pass spontaneously, early postoperative imaging could lead to lower SFRs. In a review of 154 studies assessing stone treatment, postoperative imaging for assessment of stone clearance occurred at a mean of 49 days after treatment (range 1–365). Nearly half of the studies assessed outcomes at ≤1 month, and a quarter did not report on the timing.20 Furthermore, the imaging schedule might vary depending on the intervention; it © 2015 Macmillan Publishers Limited. All rights reserved

REVIEWS Table 1 | Studies assessing stone-free rates using a variety of imaging modalities for follow-up monitoring Study characteristics

Mean stone size (mm) and number

Stone-free definition (imaging used) and SFR (timing)

UAS use (%)

Post-op stent use (timing)

Comment on active retrieval of fragments

Perlmutter et al. (2008)22 retrospective, n = 84

6.7,* NS

Zero fragments (KUB, USS, RGP, CT—majority KUB), 94% (3 months)



An attempt to clear all fragments with a basket was made in the vast majority

Gunnar et al. (2011)25‡ retrospective, n = 61

8.5, NS

What is the stone-free rate following flexible ureteroscopy for kidney stones?

Flexible ureteroscopy (URS) is increasingly being used as the first-line treatment for patients with renal stones. Despite this increase in use, subst...
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