Minimally invasive surgical therapies for stone removal have largely obviated the need for open stone extraction. However, the minimally invasive nature of these procedures prevents the intact extraction of many stones, requiring in situ fragmentation to allow extraction or passage of the stone fragments. Despite continued advancements in our surgical therapies and devices, urologists and patients continue to contend with remnant stone fragments. The clinical relevance of these residual fragments remains debated and may be impacted by many factors including fragment size, location, and even composition. Since the introduction of the term “clinically insignificant residual fragment (CIRF),”¹ the natural history of residual fragments has been the subject of intense study, and the term itself has become a point of contention. Contributing to the debate is the lack of standardized thresholds regarding the amount or size of residual stones to be deemed “stone-free” or “insignificant,” which is further compounded by the lack of standardization in postoperative imaging technique.² Herein, we briefly review the fate of these postoperative residual stone fragments following the most common stone treatment procedures, including percutaneous nephrolithotomy (PCNL), ureteroscopy (URS), and extracorporeal shock wave lithotripsy (SWL).

The CIRF nomenclature originated from the early SWL literature and was largely applied to fragments <4 mm.^1,3,4 Initially presumed insignificant, Streem et al later showed that 43.1% of patients who underwent SWL had a symptomatic episode or required an additional intervention within 2 years for residual fragments ≤4 mm.⁵ Similarly, Osman et al demonstrated that 20% of patients with residual fragments ≤4 mm after SWL developed additional stones in the ipsilateral kidney within 5 years.⁶ It is important to note that these studies utilized a combination of intravenous pyelography with renal ultrasound and radiographic imaging of the kidneys, ureters, and bladder, which may have underestimated their reported incidence of residual fragments. While CT imaging serves as the gold standard to evaluate for treatment outcomes within study protocols, clinical practice does not uniformly follow this.

PCNL is a unique modality that provides the opportunity to obtain immediate access to the kidney, which favors high rates of stone clearance ranging from 40%-90%.⁷ Despite this more direct approach, the risk for residual fragments still persists, in particular due to stone migration into a remote calyx, collecting system abnormalities, or intraoperative bleeding. Osman et al showed that within 2.5 years of follow-up for fragments ≤5 mm, 33.3% of stone fragments had interval growth with an additional 4% presenting as symptomatic ureteral stones.⁸ Altunrende et al reported that in the setting of fragments ≤4 mm, 26% of patients suffered a subsequent symptomatic episode within 2 years.⁷

With advancements enabling access to virtually any ureteral or renal stone, URS is the most frequently utilized surgical option for stone disease.⁹ A study by Rebuck et al demonstrated that after fragmentation of renal stones and basket extraction of all identified fragments >1 mm, 19.6% of patients with renal stone fragments ≤4 mm had a recurrent stone event at a mean follow-up of 26.8 months; 67% of the events required repeat URS, while the remainder had colic requiring hospital visit but successfully treated with presumed passage.¹⁰ In contrast to planned fragment extraction, dusting techniques seek to reduce stone to submillimeter particles appropriate for evacuation or spontaneous passage. As such, residual stone material is anticipated, but anticipated to clear. In such a cohort, Iremashvili et al similarly reported that approximately 20% of patients undergoing dusting-only URS required repeat surgery within 4 years of follow-up, with the greatest risk of repeat intervention linked to a fragment size greater than 2 mm.¹¹ A report from the EDGE research consortium reported that 29% with residual fragments after URS ultimately required intervention, most closely associated with fragments >4 mm. Of note, there was no significant difference associated with initial dusting or basket extraction techniques.¹²

By way of summary, a recent systematic review and meta-analysis of the literature assessing residual stone fragments following any of the modalities of SWL, PCNL, and URS concluded that all residual fragments may have important clinical implications. While larger fragments were more likely to require future intervention, the risk of disease progression and subsequent intervention increased over time for fragments of all sizes.¹³ Therefore, the authors suggested abandoning the term “CIRF” as no fragment is invariably insignificant.

If clinical relevance is presumed for residual fragments, it then behooves the treating urologist to approach intervention with a goal of rendering a patient stone-free. However, the bulk of the urological literature confirms that regardless of intention, many will have postoperative residual fragments. Once identified, data suggest residual fragments should at least be followed due to their risk of progression. Whether long-term monitoring or pre-emptive intervention is preferable remains to be definitively determined and is likely patient dependent. Data—including those summarized above—will thus be invaluable to counsel patients and to ultimately contribute to well-informed shared decision making.