Urolithiasis is a common and growing health problem, with an estimated 106 million incident cases worldwide in 2021, 26.7% more than in 2000.1 Prevalence is higher in tropical and subtropical regions,2 and a Karachi tertiary-centre experience recorded more than 83,000 adult stone patients over four decades, with a marked rise in numbers alongside wider access to ESWL, percutaneous nephrolithotomy (PCNL) and ureteroscopy.3 Stones in the lower pole are the most difficult to clear, because the dependent position of the calyx limits drainage of fragments and a steep infundibulopelvic angle predisposes to failure of retrograde intrarenal surgery.4 For stones of 1-2 cm, treatment must therefore balance clearance against morbidity, repeat procedures and hospital stay. Evidence on the best option is inconsistent. A systematic review of the last decade compared PCNL with retrograde intrarenal surgery (RIRS) for mid-sized lower calyx stones,5 and in a randomized trial of 414 patients with hard lower pole stones up to 2 cm, success at two weeks was 90.2% after flexible ureteroscopy, 83.3% after mini-PCNL and 61.5% after ESWL.6 In contrast, a retrospective comparison of 10-20 mm lower pole stones found no significant difference between mini-PCNL (81.5%) and RIRS (89.7%),7 whereas small Pakistani studies reported higher clearance with mini-PCNL than RIRS.8,9 The UK PUrE trials compared PCNL, flexible ureterorenoscopy and ESWL for lower pole stones on clinical and cost-effectiveness.10 Success definitions, imaging modality and timing differ between these studies, and local comparative data on all three treatments are limited. The objective of this study was to compare single-session treatment success among mini-PCNL, flexible ureteroscopy and ESWL for 1-2 cm lower pole renal stones, and to compare complete stone clearance, 30-day complications, repeat or auxiliary procedures within 90 days and index hospital stay. We hypothesised that single-session success differs between at least two of the three treatments.

Study Design: Retrospective comparative cohort study using routinely collected clinical records. Settings: Department of Urology, The Kidney Centre Postgraduate Training Institute, Karachi. Duration: Index treatments performed from 1 May 2020 to 31 May 2022 were included. Follow-up records extended to 29 August 2022 to capture 90-day outcomes. This was a historical treatment window and not prospective recruitment. Sample Size: The sample size was calculated using the WHO approach for two independent proportions, with expected success proportions of 0.902, 0.833 and 0.615 from a published randomized trial,6 80% power and a two-sided α of 0.0167 (0.05/3 pairwise comparisons). The largest requirement was 505 evaluable records per group; allowing 10% unevaluable primary outcomes gave 562 per group (1,686 in total). All eligible records in the study window were included. Sampling Technique: Non-probability consecutive inclusion of every eligible record in the fixed period, without selecting records by outcome or forcing equal groups. Inclusion Criteria: Adults aged ≥18 years of either sex with a solitary, unilateral lower pole stone of 10-20 mm maximum diameter on pretreatment non-contrast CT who underwent one of three initial treatments: mini-PCNL using a documented 12-22 Fr tract, flexible ureteroscopic laser lithotripsy, or ESWL. Only the first eligible episode per patient was included, and any later crossover remained attributed to the initial treatment. Exclusion Criteria: Additional ipsilateral stones, concurrent bilateral treatment, staghorn stones, calyceal diverticula or major renal anatomical anomalies, simultaneous reconstructive surgery, previous treatment of the same episode before index assessment, and records that could not establish eligibility or exposure. Missing follow-up alone did not exclude an otherwise eligible record. Data Collection Procedure: Theatre, ESWL and imaging registers were used to identify records. A coded chart-abstraction proforma captured age, sex, body mass index, diabetes, baseline creatinine, stone side, CT stone diameter and density (HU), hydronephrosis, lower pole anatomy (infundibulopelvic angle, infundibular length and width), recorded reason for treatment selection, treatment details (tract size, ureteric stent, nephrostomy tube), admission and discharge times, outcome imaging, repeat or auxiliary procedures and complications. A second reviewer verified 10% of forms and disagreements were adjudicated. Identifiers were kept in a separate protected linkage file. Outcome definitions. Treatment success was no residual fragment >4 mm on the earliest evaluable imaging 28-84 days after the first session (CT preferred when equally timed), with no additional stone-directed procedure before that assessment. Complete clearance was no detectable fragment on the same imaging. Records with neither evaluable imaging nor evidence of retreatment were classified as unknown, not success. A complication was a documented treatment-related adverse event within 30 days, graded by the Clavien-Dindo classification; routine stent removal was excluded. Repeat treatment was another session of the initial modality and auxiliary treatment was another modality for the index stone within 90 days. Hospital stay was the time in hours from index admission to discharge, including day-case care. Data / Statistical Analysis: Data were analysed using IBM SPSS Statistics (version [insert version]). Categorical variables are presented as frequencies and percentages and continuous variables as mean±standard deviation or median (interquartile range) according to distribution. Baseline characteristics were compared using one-way ANOVA (confirmed by the Kruskal-Wallis test) and the chi-square test. Success proportions are reported with Wilson 95% confidence intervals and were compared with the chi-square test; the three pairwise comparisons were adjusted using the Holm method. Binary logistic regression with ESWL as the reference category estimated odds ratios; the prespecified model adjusted for stone diameter, stone density (per 100 HU), age and diabetes, with additional models including lower pole anatomy and calendar year. Model fit was assessed with the Hosmer-Lemeshow test. Sensitivity analyses treated unknown outcomes first as failures and then as successes, applied a strict zero-fragment definition and restricted the analysis to CT-assessed patients. Hospital stay was compared using the Kruskal-Wallis and Mann-Whitney U tests with Bonferroni correction. A two-sided p-value <0.05 was considered significant; secondary and subgroup analyses were exploratory. Ethical Approval: This is retrospective study and an exemption is given by TKC-ERC, as long as there will be no patient contact and confidentiality is assured.

Table 1: Baseline characteristics of patients by initial treatment (n=1,686) Characteristic Mini-PCNL (n=562) Flexible URS (n=562) ESWL (n=562) Total (n=1,686) p-value Age, years 43.8±11.8 43.4±12.5 43.4±12.5 43.5±12.3 0.861 Male sex, n (%) 359 (63.9) 348 (61.9) 369 (65.7) 1076 (63.8) 0.427 BMI, kg/m² 25.9±3.8 26.1±4.1 26.0±4.0 26.0±4.0 0.527 Diabetes mellitus, n (%) 96 (17.1) 85 (15.1) 89 (15.8) 270 (16.0) 0.664 Baseline creatinine, mg/dL 0.92±0.22 0.92±0.23 0.93±0.23 0.92±0.23 0.363 Right-sided stone, n (%) 283 (50.4) 268 (47.7) 267 (47.5) 818 (48.5) 0.564 Stone diameter, mm 15.0±2.5 15.0±2.4 14.9±2.5 14.9±2.5 0.814 Stone density, HU 939±268 919±269 926±279 928±272 0.476 Moderate/severe hydronephrosis, n (%) 188 (33.5) 195 (34.7) 199 (35.4) 582 (34.5) 0.980* Infundibulopelvic angle, degrees (n=1,207) 66.4±16.7 65.3±17.3 63.5±16.8 65.1±17.0 0.046 Ureteric stent placed, n (%) 380 (67.6) 504 (89.7) 74 (13.2) 958 (56.8) <0.001 Nephrostomy tube, n (%) 175 (31.1) 0 (0.0) 0 (0.0) 175 (10.4) <0.001 *p-value for the full four-category distribution (none, mild, moderate, severe). Mean±SD or n (%); ANOVA or chi-square test. URS: ureteroscopy. A total of 1,686 eligible records were analysed (562 per treatment). Mean age was 43.5±12.3 years, 1,076 (63.8%) patients were male and 270 (16.0%) had diabetes. Mean stone diameter was 14.9±2.5 mm and mean density 928±272 HU. Baseline characteristics were comparable across groups (Table 1); the infundibulopelvic angle was marginally narrower in the ESWL group (63.5° versus 66.4° with mini-PCNL; ANOVA p=0.046, Kruskal-Wallis p=0.073). Ureteric stents were placed in 89.7% of fURS, 67.6% of mini-PCNL and 13.2% of ESWL patients (p<0.001), and a nephrostomy tube was left in 175 (31.1%) mini-PCNL patients. The mean mini-PCNL tract size was 18.1±2.8 Fr. Outcome imaging was obtained at a median of 56 days (IQR 41-70) and was CT in 612 (36.3%) patients. The primary outcome could not be determined in 146 (8.7%) records, more often after mini-PCNL (65, 11.6%) than fURS (47, 8.4%) or ESWL (34, 6.0%; p=0.004). These patients were slightly older (45.6 versus 43.3 years; p=0.033) but did not differ in stone diameter, density, sex or diabetes. Single-session treatment success was achieved in 1,254 of 1,540 patients with a known outcome (81.4%). Success was 83.7% (95% CI 80.2-86.7) after mini-PCNL, 80.4% (76.7-83.6) after fURS and 80.3% (76.7-83.5) after ESWL (χ²=2.51, p=0.285; Table 2, Figure 1). No pairwise comparison was significant after Holm correction (adjusted p≥0.472; Table 3), so the null hypothesis of equal success was not rejected. Table 2: Single-session treatment success and sensitivity analyses by initial treatment Analysis Mini-PCNL n/N (%) Flexible URS n/N (%) ESWL n/N (%) p-value Primary analysis (known outcomes) 416/497 (83.7) 414/515 (80.4) 424/528 (80.3) 0.285 Unknown outcome counted as failure 416/562 (74.0) 414/562 (73.7) 424/562 (75.4) 0.770 Unknown outcome counted as success 481/562 (85.6) 461/562 (82.0) 458/562 (81.5) 0.139 Strict zero-fragment definition 302/497 (60.8) 327/515 (63.5) 320/528 (60.6) 0.563 CT-assessed patients only 164/195 (84.1) 155/204 (76.0) 171/213 (80.3) 0.127 Chi-square test. Primary analysis n=1,540; bounds analyses n=1,686; CT-only n=612. Table 3: Pairwise comparisons of single-session success (n=1,540) Comparison Risk difference, % (95% CI) χ² p-value Holm-adjusted p Mini-PCNL vs Flexible URS +3.3 (-1.4 to 8.0) 1.88 0.170 0.472 Mini-PCNL vs ESWL +3.4 (-1.3 to 8.1) 2.00 0.157 0.472 Flexible URS vs ESWL +0.1 (-4.7 to 4.9) 0.00 0.972 0.972 Figure 1: Primary and secondary outcomes by initial treatment (percentage with 95% CI) Table 4: Logistic regression for single-session treatment success (n=1,540) Variable Unadjusted OR (95% CI) p-value Adjusted OR (95% CI) p-value Mini-PCNL (vs ESWL) 1.26 (0.91-1.74) 0.158 1.30 (0.94-1.80) 0.113 Flexible URS (vs ESWL) 1.01 (0.74-1.36) 0.972 1.02 (0.75-1.38) 0.922 Stone diameter (per mm) - - 0.86 (0.81-0.90) <0.001 Stone density (per 100 HU) - - 0.95 (0.91-1.00) 0.046 Age (per year) - - 1.00 (0.99-1.02) 0.421 Diabetes (yes versus no) - - 1.03 (0.71-1.49) 0.869 OR: odds ratio. Hosmer-Lemeshow p=0.824; Nagelkerke R²=0.043; area under ROC curve=0.62. On logistic regression adjusted for stone diameter, density, age and diabetes, neither mini-PCNL (aOR 1.30, 95% CI 0.94-1.80) nor fURS (aOR 1.02, 95% CI 0.75-1.38) differed from ESWL (Table 4). Each additional millimetre of stone diameter reduced the odds of success by 14% (aOR 0.86, 95% CI 0.81-0.90; p<0.001) and each 100 HU by 5% (aOR 0.95, 95% CI 0.91-1.00; p=0.046). The treatment estimates were unchanged after adding lower pole anatomy (n=1,109; mini-PCNL aOR 1.14, 95% CI 0.79-1.65) or calendar year (Figure 2). Figure 2: Adjusted odds ratios for single-session success relative to ESWL across models and sensitivity analyses The findings were robust to sensitivity analyses (Table 2). Success ranged from 73.7% to 75.4% when unknown outcomes were treated as failures and from 81.5% to 85.6% when treated as successes, and from 60.6% to 63.5% with the strict zero-fragment definition (all p>0.10). Among 612 CT-assessed patients, success was 84.1%, 76.0% and 80.3% (p=0.127). The treatment effect did not differ by stone density (interaction p=0.246) or diameter (interaction p=0.741). Among stones >1,000 HU, mini-PCNL had higher unadjusted success than ESWL (83.7% versus 75.0%, p=0.029), which did not remain significant after Holm correction (p=0.086). Table 5: Secondary outcomes by initial treatment Outcome Mini-PCNL Flexible URS ESWL p-value Complete imaging clearance 316/496 (63.7) 343/515 (66.6) 339/525 (64.6) 0.611 Complications within 30 days (n=1,613) Any complication 62/543 (11.4) 75/529 (14.2) 59/541 (10.9) 0.213 Major complication (Clavien-Dindo ≥III) 12/543 (2.2) 22/529 (4.2) 43/541 (7.9) <0.001 Fever or urinary tract infection 30 (5.5) 53 (10.0) 16 (3.0) Sepsis requiring intensive care 12 (2.2) 22 (4.2) 0 (0.0) Transfusion 20 (3.7) 0 (0.0) 0 (0.0) Steinstrasse requiring nephrostomy 0 (0.0) 0 (0.0) 43 (7.9) Further treatment within 90 days (n=1,578) Any repeat or auxiliary procedure 53/530 (10.0) 55/524 (10.5) 59/524 (11.3) 0.799 Index hospital stay (n=1,686) Median (IQR), hours 42.5 (31.6-56.8) 22.7 (16.7-30.1) 3.0 (2.3-3.8) <0.001 Discharged within 24 hours, % 7.8 54.8 100.0 - n/N (%) unless stated; denominators exclude unknown outcomes. Chi-square test; hospital stay by Kruskal-Wallis test (H=1296.8). Complete imaging clearance was achieved in 998 of 1,536 patients (65.0%) and did not differ between treatments (p=0.611). Any 30-day complication occurred in 196 of 1,613 patients with available follow-up (12.2%; p=0.213). However, major complications differed significantly (p<0.001; Table 5): 43 (7.9%) after ESWL, all steinstrasse requiring nephrostomy (grade IIIa), versus 22 (4.2%) after fURS and 12 (2.2%) after mini-PCNL, all sepsis requiring intensive care for single-organ dysfunction (grade IVa). After Holm correction, ESWL differed from mini-PCNL (p<0.001) and fURS (p=0.019). Transfusion occurred only after mini-PCNL (20 patients, 3.7%). Compared with ESWL, the adjusted odds of a major complication were lower after mini-PCNL (aOR 0.26, 95% CI 0.14-0.50) and fURS (aOR 0.50, 95% CI 0.29-0.85). Further stone treatment within 90 days was needed in 167 of 1,578 patients (10.6%) without difference between treatments (p=0.799); larger stone diameter predicted retreatment (aOR 1.22 per mm, 95% CI 1.14-1.30; p<0.001). Median hospital stay was 42.5 hours (IQR 31.6-56.8) after mini-PCNL, 22.7 hours (16.7-30.1) after fURS and 3.0 hours (2.3-3.8) after ESWL (Kruskal-Wallis p<0.001; all pairwise comparisons p<0.001; Figure 3). All ESWL patients and 54.8% of fURS patients were discharged within 24 hours, compared with 7.8% of mini-PCNL patients. Figure 3: Index hospital stay (hours) by initial treatment

In this institutional cohort of 1,686 patients with 1-2 cm lower pole stones, single-session success was similar for mini-PCNL, flexible ureteroscopy and ESWL (80.3-83.7%), and remained similar after adjustment for stone size, density, age and diabetes. Our ESWL success was higher than in an Egyptian randomized trial of hard stones (61.5% versus 83.3% for mini-PCNL and 90.2% for fURS at two weeks)6 and in a recent trial of high-density stones in which SWL cleared 51.0% compared with 96.2% for mini-PCNL and 91.3% for RIRS.11 Several factors may explain the difference. Our stones were not selected for hardness (mean 928 HU), success allowed residual fragments up to 4 mm assessed at 4-12 weeks, and ESWL was chosen in routine practice, which may favour more suitable stones. In addition, nearly two thirds of outcome imaging used ultrasound or plain radiography, which may miss small fragments; success fell to 60.6-63.5% with a strict zero-fragment definition and was 76.0-84.1% among CT-assessed patients. The comparable results of mini-PCNL and fURS agree with Gokalp et al., who found no significant difference for 10-20 mm lower pole stones (81.5% versus 89.7%, p=0.101),7 and with a matched analysis in which lower pole location did not reduce RIRS success (79.2% versus 83.6%) compared with other locations,12 similar to our 80.4% for fURS. Pakistani series have reported higher clearance with mini-PCNL, 90.0% versus 74.0% for RIRS in solitary 1-2 cm stones8 and 95% versus 82% for PCNL versus RIRS in 2-3.5 cm stones,13 but these studies were small (100-150 patients), used investigator allocation or larger stones, and so are not directly comparable. The point estimate in our data (3.3 percentage points higher for mini-PCNL, 95% CI -1.4 to 8.0) suggests a modest advantage that this study cannot exclude, although differences larger than about 8 percentage points are unlikely. Stone diameter was the strongest determinant of outcome: each additional millimetre reduced the odds of success by 14% and increased the odds of retreatment by 22%. Stone density had a smaller effect (aOR 0.95 per 100 HU), in keeping with its recognised role as a determinant of shock wave fragmentation.14 Lower pole anatomy was not independently associated with success, similar to an ESWL series in which the infundibulopelvic angle and infundibular dimensions were not significant,15 although anatomical factors16 and relocation of lower pole stones17 continue to be studied in RIRS. Anatomy was documented in only 71.6% of our patients, so a smaller effect cannot be excluded. The clearest differences concerned safety. Major complications were less frequent after mini-PCNL (2.2%) and fURS (4.2%) than after ESWL (7.9%), driven by steinstrasse, a recognised complication of SWL for 10-20 mm stones whose risk relates to stone burden and shape.18 Major events after the endoscopic procedures were sepsis. Infectious complications after ureteroscopy vary widely between series (0.8-18.2%),19 and our fURS rates (fever or urinary tract infection 10.0%, sepsis 4.2%) fall within that range. Overall complication rates (10.9-14.2%) were comparable between treatments and lower than the 17.6-27.1% reported in the Egyptian trial6 and the 21.3% reported after PCNL at the Institute of Kidney Diseases, Peshawar,20 and close to the 11.4% reported for mini-PCNL by Ahmed et al.11 Transfusion was required only after mini-PCNL (3.7%). Hospital stay followed the expected gradient. ESWL was an exclusively day-case treatment (median 3.0 hours), fURS required about one night (22.7 hours) and mini-PCNL about two days (42.5 hours), partly reflecting nephrostomy drainage in one third of patients. The Egyptian trial reported a similar order but much longer stays for the endoscopic procedures (60 hours for fURS, 72 hours for mini-PCNL),6 and Pakistani series also found shorter stays after RIRS than mini-PCNL.8,9 Retreatment within 90 days was needed in about one in ten patients irrespective of modality, which is relevant to counselling, because shorter stay with ESWL has to be weighed against its higher rate of major complications and the similar probability of needing another procedure. These findings add locally derived comparative data for all three modalities under shared clinical conditions, in a country where stone disease is highly prevalent but single-institution comparisons remain sparse.3,21 Strengths include prespecified endpoints, a large sample, adjustment for case mix and sensitivity analyses for missing outcomes and imaging modality. In practice, the choice may reasonably rest on stone size, patient preference, the acceptability of a longer admission and the local capacity to manage sepsis or steinstrasse, rather than on expected clearance alone. This study has limitations. Its retrospective design and non-random treatment allocation leave residual confounding and treatment-selection bias, so associations should not be read as causal. The primary outcome was unknown in 8.7% of patients, more often after mini-PCNL, although bounding analyses did not change the conclusions. Imaging modality was heterogeneous (CT in 36.3%) and may have detected residual fragments differently between groups. Lower pole anatomy was documented in 71.6% of records, and surgeon, equipment and laser settings could not be adjusted. The study was conducted at a single centre, follow-up was limited to 90 days, and cost, pain and quality of life were not measured. Prospective multicentre studies with CT-based outcome assessment are needed.

Mini-PCNL, flexible ureteroscopy and ESWL achieved similar single-session success for 1-2 cm lower pole renal stones, with stone diameter the main predictor of failure. Mini-PCNL and flexible ureteroscopy were associated with fewer major complications than ESWL, whereas ESWL required the shortest hospital stay, so treatment should be individualised accordingly. CONFLICT OF INTEREST / DISCLOSURE The authors declare no conflict of interest. SOURCE OF FUNDING This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. ACKNOWLEDGEMENTS The authors thank the departmental records and imaging staff for assistance with data retrieval.

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