Which Is More Important in Predicting the Outcome of Extracorporeal Shockwave Lithotripsy of Solitary Renal Stones: Stone Location or Stone Burden?

Abstract

Purpose:

To assess the effect of stone location and burden on the outcome of extracorporeal shockwave lithotripsy (SWL) as a primary treatment of solitary renal stone.

Patients and Methods:

The study included 438 patients with a solitary renal stone who underwent SWL as a primary treatment for their stones. All were evaluated by plain radiography of the kidneys, ureters, and bladder (KUB), ultrasonography, intravenous urography, or noncontrast enhanced CT before SWL and followed up for 3 months after treatment by KUB radiography and/or ultrasongraphy. Patients were classified into four groups according to stone location (renal pelvis, lower, middle, and upper calix) and three groups according to stone burden (≤1 cm², 1.1–2 cm², and >2 cm²). Treatment outcome was considered successful if no residual fragments (stone free) or clinically insignificant nonobstructing residuals less than 4 mm remained after 3 months of follow-up.

Results:

The mean age of the patients was 45.1±12.5 years. The mean stone burden, number of sessions, and shockwaves for the whole study were 1.3±0.49 cm², 2.1±0.7 sessions, and 5616.6±2017.4 shockwaves, respectively. The stone-free rate of the study was 65.1%. The stone-free rates of the stones in the renal pelvis, lower, middle, and upper calices were 72.4%, 56%, 55.6%, and 69%, respectively. The stone-free rate of the stones ≤1 cm², 1.1 to 2 cm², and >2 cm² was 50.2%, 39.6%, and 10.2%, respectively (P<0.05).

Conclusion:

Stone burden rather than stone location is considered as a predicting factor for the outcome of SWL in a solitary renal stone, especially in the renal pelvis and lower calix.

Introduction

E xtracorporeal shockwave lithotripsy (SWL) has radically changed the management of upper urinary tract calculi since its first application by Chaussy and associates¹ in 1980. In the last two decades, it became the primary treatment modality in the management of the majority of renal stones and minimized the indications for surgical interventions.^2,3 SWL takes the upper hand because of its advantages of being an effective, noninvasive, and outpatient procedure with minimal morbidity and absence of anesthesia.⁴ There are several factors that affect the success rate of SWL; these factors are related to the stone (size, composition, number, location), patient habitus (renal anomalies, bony deformities, obesity), and efficacy of the lithotripters.^5,6 The current study evaluated the effect of stone location and burden on the outcome of SWL as a primary treatment for solitary renal stone.

Patients and Methods

Between October 2007 and February 2011, 438 patients (438 renal units) with a solitary renal stone underwent SWL as a primary treatment for a stone at Mubarak Al-Kabir Hospital in Kuwait. All patients were evaluated by radiography of the kidneys, ureters, and bladder (KUB), ultrasonography, intravenous urography (IVU), or noncontrast enhanced CT when the stone was radiolucent or not visualized by IVU, serum biochemistry, coagulation profile, and urine culture and sensitivity. In cases of positive urine culture, patients were treated first by the appropriate antibiotic, and SWL was postponed until a negative urine culture was achieved. Patients with definite metabolic stone diseases, congenital anomalies of the urinary tract, multiple stones, and those treated with combined procedures such as percutaneous nephrolithotomy were excluded from the study.

All patients were treated in the supine position by the same lithotripter (Siemens Lithostar Multiline). The treatment began with a low voltage (11–12 kv), then increased gradually to a maximum that could be tolerated by the patients. The treatment session was terminated when the stone fragmentation was observed on the screen, until reaching 3000 shocks if no fragmentation was observed, or in case of intolerance of the procedure because of pain in some patients. Analgesia in the form of 20 mg nalbuphin was administered intravenously in the second sessions to the patients who reported pain or could not tolerate the pain in their first sessions.

Patients were followed after 1 week by KUB radiography and ultrasonography, if needed. If there was no or inadequate fragmentation observed, re-treatment was performed with a maximum of three total sessions. Patients were followed by KUB radiography and ulrasonography every 2 weeks for 3 months. The treatment outcome was considered successful if patients were stone free or had clinically insignificant residual fragments (CIRF) less than 4 mm without obstruction.

According to stone location, patients were classified into four groups (renal pelvis, lower calix, middle calix, and upper calix). Stone burden was expressed by measuring stone surface area in cm²,⁶ and according to stone burden, patients were stratified into three groups (≤1 cm², 1.1–2 cm², and >2 cm²).

Data of the study were collected, tabulated, and statistically analyzed by many tests such as chi-square, and one-way analysis of variance using SPSS software ver. 17. P values were estimated and considered statistically significant if <0.05.

Results

The mean age of the 438 patients was 45.1±12.5 years, and there was no statistically significant difference in the mean age in different stone locations (P=0.6). The study included 320 males (73.1%) and 118 females (26.9%) with no statistically significant difference between patients in different stone locations (P=0.084). Stones were located in the renal pelvis, lower calix, middle calix, and upper calix in 210, 159, 27, and 42 patients, respectively.

The mean stone burden for the whole study was 1.3±0.49 cm². According to stone burden, patients were stratified into three groups, ≤1 cm², 1.1 to 2 cm², and >2 cm²; each group included 195, 176, and 67 patients, respectively (P=0.4). There was no statistically significant difference in the mean stone burden for the stones in the renal pelvis, lower calix, middle calix, and upper calyx, which was 1.3±0.51 cm², 1.2±0.47 cm², 1.2±0.52 cm², and 1.2±0.5 cm², respectively (P=0.5). Characteristics of patients and stones are presented in Table 1.

Table 1.

Characteristics of Patients and Stones

	Renal pelvis	Lower calix	Middle calix	Upper calix	Total	P value
Mean age	45.6±11.08	45.2±13.7	34.1±12.6	43.3±12.2	45.1±12.5	0.6
Sex Male: No (%)	144 (45%)	118 (36.9)	22 (6.9)	36 (11.2)	320 (100)	0.084
Female: No (%)	66 (55.9)	41 (34.8)	5 (4.2)	6 (4.1)	118 (100)
No. of stones (%)	210 (47.9)	159 (36.3)	27 (6.2)	42 (9.6)	438 (100)
Mean stone burden (cm²)	1.3±0.51	1.2±0.47	1.2±0.52	1.2±0.5	1.3±0.49	0.5
No. of patients acc. to stone burden						0.4
≤1 cm²: No (%)	83 (42.6)	81 (41.5)	13 (6.7)	18 (9.2)	195 (100)
1.1–2 cm²: No(%)	90 (51.1)	60 (34.1)	10 (507)	16 (9.1)	176 (100)
>2 cm²: No (%)	37 (55.2)	18 (26.9)	4 (6)	8 (11.9)	67 (100)

The auxiliary procedures in the form of ureteral Double-J stents were placed in 11 patients before SWL because of calcular obstructive uropathy in 8 patients and single kidney in 3 patients. There was no statistically significant difference in the placement of ureteral Double-J stents among different stone locations (P>0.05) and also did not significantly affect the treatment outcome.

The mean number of treatment sessions for all cases was 2.1±0.7 sessions. The mean number of sessions for the stones in the renal pelvis, lower calix, middle calyx, and upper calix was 1.9±0.7, 2.4±0.8, 1.8±0.6, and 1.9±0.8 sessions, respectively. The mean voltage used in the study was 15.9±1.9 kv with a range from 14 to 20 kv. It was relatively low because intravenous sedation was not routinely used from the start; when the patients could not tolerate the procedure, the treatment session was terminated at low voltage, and sedation was administered intravenously from the second session. The mean number of sessions for the stones in the lower calix was statistically significantly higher than that for the stones in the other locations. The re-treatment rate of the whole study was 78.1%. The re-treatment rate of the stones in different locations did not reach a statistically significant difference being 78.1, 79.2%, 74.1%, and 76.2% for renal pelvis, lower calix, middle calix, and upper calix, respectively (P>0.05).

The mean number of shockwaves applied for all patients was 5616.6±2017.4. The mean number of shockwaves applied to the stones in the lower calix was 6130.8±2160.9, which was statistically significantly higher than that applied to the stones in the renal pelvis, middle calix and upper calix (P<0.05). The stone-free rate, the rate of CIRF, and failure rate of the entire study was 65.1%, 25.5% and 9.4%, respectively. There was no statistically significant difference with regard to the previous items of treatment outcome between stones in the renal pelvis, lower calix, middle calyx, and upper calix (P>0.05).

The effectiveness quotient (EQ) was estimated by the following formula: EQ=percent of stone free×100/(100%+percent of re-treatment+percent of auxiliary procedures). The EQ of the study was 36. The EQ of the stones in the renal pelvis, lower calix, middle calyx, and upper calix was 40, 31, 31, and 38, respectively. Features and outcome of treatment according to stone location are summarized in Table 2.

Table 2.

Features and Outcome of Treatment According to Stone Location

	Renal pelvis	Lower calix	Middle calix	Upper calix	Total
	(n=210)	(n=159)	(n=27)	(n=42)	(N=438)	P value
Mean no. of sessions	1.9±0.7	2.4±0.8	1.8±0.6	1.9±0.8	2.1±0.7	<0.05
Mean no. of shockwaves	5339±1849.6	6130.8±2160.9	5029.6±1490.6	5316.7±2036	5616.6±2017.4	<0.05
Re-treatment rate (%)	78.1	79.2	74.1	76.2	78.1	>0.05
Auxiliary procedure no. (%)	6 (2.9)	2 (1.3)	1 (3.7)	2 (4.8)	11 (3.2)	>0.05
Stone-free rate no. (%)	152 (72.4)	89 (56)	15 (55.6)	29 (69)	285 (65.1)
CIRF no. (%)	42 (20)	49 (30.8)	9 (33.3)	12 (28.6)	112 (25.5)	>0.05
Failure no. (%)	16 (7.6)	21 (13.2)	3 (11.1)	1 (2.4)	41 (9.4)
EQ	40	31	31	38	36

CIFR=clinically insignificant residual fragments; EQ=effectiveness quotient.

The difference between the three groups of stone burden with regard to the mean number of sessions and shockwaves was statistically significant (P<0.05), and the same difference in the re-treatment rate was also found to be statistically significant (P<0.05) as shown in Table 3. Generally, there was a statistically significant difference in the stone-free rate between the stone burden ≤1 cm², 1.1 to 2 cm², and >2 cm²; it was 50.2%, 39.6%, and 10.2%, respectively (P<0.05). This difference in stone-free rate was statistically significant between the same groups of stone burden within the renal pelvis and lower calix (P<0.05); however, in the middle and upper calix, it did not reach a significant difference (P>0.05). Treatment outcomes according to stone burden are presented in Table 4.

Table 3.

Features of Treatment According to Stone Burden

	≤1 cm ²	1.1–2 cm ²	>2 cm ²	Total	P value
Mean no. of sessions	1.9±0.7	2.3±0.7	2.4±0.8	2.1±0.7	<0.05
Mean no. of shockwaves	5056.9±1933.2	5912.5±1886.8	6394±2101.1	5616.6±2017.4	<0.05
Re-treatment rate (%)	62.8	76.2	83.7	78.1	<0.05

Table 4.

Treatment Outcome According to Stone Burden

	≤1 cm ²	1.1–2 cm ²	>2 cm ²	Total	P value
Renal pelvis
Stone free No. (%)	70 (46.1)	64 (42.1)	18 (11.8)	152 (100)	<0.01
CIRF No. (%)	12 (28.6)	19 (45.2)	11 (26.2)	42 (100)
Failure No. (%)	1 (6.3)	7 (43.8)	8 (50)	16 (100)
Lower calyx
Stone free No. (%)	51 (57.3)	32 (36)	6 (6.7)	89 (100)	<0.05
CIRF No. (%)	25 (51)	19 (38.8)	5 (10.2)	49 (100)
Failure No. (%)	5 (23.8)	9 (42.9)	7 (33.3)	21 (100)
Middle calyx
Stone free No. (%)	8 (53.3)	6 (40)	1 (6.7)	15 (100)	0.25
CIRF No. (%)	4 (44.4)	3 (33.4)	2 (22.2)	9 (100)
Failure No. (%)	1 (33.3)	1 (33.3)	1 (33.3)	3 (100)
Upper calyx
Stone free No. (%)	14 (48.3)	11 (37.9)	4 (13.8)	29 (100)	0.09
CIRF No. (%)	4 (33.3)	5 (41.7)	3 (25)	12 (100)
Failure No. (%)	0 (0)	0 (0)	1 (100)	1 (100)
Total
Stone free No. (%)	143 (50.2)	113 (39.6)	29 (10.2)	285 (100)
CIRF No. (%)	45 (40.2)	46 (41.1)	21 (18.7)	112 (100)	<0.05
Failure No. (%)	7 (17)	17 (41.5)	17 (41,5)	41 (100)

CIRF=clinically insignificant residual fragments.

Discussion

Although the development in the technology of endourologic procedures and SWL increases the management options of renal stones, it also increases the need for more evaluation of their efficacy and indications.⁷ In the current study, there was no statistically significant difference in the mean age and sex percentage of the patients between different stone locations, which was favored to avoid the effect of any of them on the study results. There was no statistically significant difference in the mean stone burden between different stone locations, which made the distribution of the stones in different stone locations homogeneous. The mean number of sessions and shockwaves applied for the lower caliceal stones was statistically significantly higher than that applied for the stones in other locations, and this matches with the results of Obek and colleagues⁸ and Turna and coworkers⁹; however, they did not include pelvic stones in their studies.

The re-treatment rate in the whole study was 78.1%, and it was higher for the stone in the lower calix than in other locations but without statistically significant difference. The re-treatment rate in the study was higher than that reported by Obek and colleagues⁸ in which the re-treatment rate was 58%, 46%, and 61% for the stones in the lower, middle, and upper caliceal stones, respectively. In the study of Al-Ansary and associates,⁷ it was 53% for the entire study; however, in the study of Turna and coworkers,⁹ they reported similar or even higher re-treatment rate than in the current study, being 71.2% for the entire study and 57.1%, 72%, and 82% for the upper, middle, and lower caliceal stones, respectively. The results of EQ in the current study were comparable to the results of Obek and colleagues,⁸ because they reported that EQ in the lower, middle, and upper calix was 36, 46, and 41, respectively.

In the current study, the stone-free rate of the entire study was 65.1%. The stone-free rates of the stones in the renal pelvis (72.4%) and upper calix (69%) were higher than that of the stones in the lower calix (56%) and middle calix (55.6%); however, this difference was not statistically significant (P>0.05). In general, the clearance rate of the stone in the lower calices was lower than that of the middle and upper calices,^10,11 and the stone clearance decreased from 80% in the renal pelvic stones to 70% in upper caliceal stones and reached approximately 50% in the lower caliceal stones.²

The stone-free rate in many studies was comparable to that in the current study; however, the stone-free rate of the middle caliceal stones was not relatively low as in the current study. In the study of Kosar and associates,¹² the stone-free rate for the renal pelvis, upper calix, middle calyx, and lower calix was 66.3%, 69.4%, 76.5%, and 47.8%, respectively, and in the study of Coz and colleagues,¹³ the stone-free rate of the stones in the upper calix, middle calix, lower calix, and renal pelvis was 98.2%, 90.5%, 84.8%, and 86%, respectively. Also, in the studies of Obek and colleagues,⁸ the stone-free rate of the stones in the lower, middle and upper calix was 63%, 73%, and 71%, respectively, and Turna and coworkers⁹ in their study reported that the stone-free rate for the stones in the lower, middle, and upper calix was 60%, 72%, and 77.1%, respectively. The relatively lower stone-free rate of the middle caliceal stones in the current study in comparison with the previous two studies might be attributed to the larger stone burden of the middle caliceal stones that was 1.27±0.47 cm² in comparison to 75.2 mm in the Turna and coworkers⁹ study and 1.1±0.8 cm² in the study of Obek and colleagues.⁸

In the current study, there was a statistically significant difference in the mean number of the applied sessions and shockwaves between stones in different locations and also between different stone burden groups (P<0.05). The re-treatment rate was significantly increased with increased stone burden (P<0.05); however, this significant difference in re-treatment rate was not found between the stones in different locations within the kidney (P>0.05).

In the current study, there was a statistically significant increase in the stone-free rate with the increase of the stone burden; it was 50.2%, 39.6%, and 10.2%, for the stone burden of ≤1 cm², 1.1 to 2 cm², and >2 cm², respectively (P<0.05). The stone-free rate in different calices was also found to be higher with increasing stone burden; however, this was of statistically significant difference only in the renal pelvis and lower calix.

Stone burden is perhaps the most important single factor in determining the best treatment modality for patients with simple renal calculi.¹⁴ It is an important factor to predict the outcome of SWL in patients with nonstaghorn renal calculi; as the stone burden increases, the likelihood of successful outcome decreases with an unsatisfactory response when the stone size is larger than 2 cm.^15,16 Many studies supported the role of the stone size as a significant factor in predicting the outcome of SWL in management of renal stones.^17,18 Lalak and colleagues¹⁹ reported that the stone-free rate of the stone size <10 mm, 10 to 20 mm, and >20 mm was 76%, 66%, and 47%, respectively. Abdel-Khalek and associates²⁰ in their multivariate analysis model determined the stone size as a highly significant factor in predicting the outcome of SWL in management of renal stones (P<0.001).

The main limitation of the study is the relatively small number of patients; however, by collecting more cases that meet the criteria of the study, it was noticed that the new cases would be added mainly to the renal pelvis and lower caliceal groups, which would affect the significant distribution of cases among different locations.

Conclusion

The overall stone-free rate of SWL as primary management of solitary renal stones was 65.1%. The effect of stone location on the outcome of SWL was not significant. The stone burden significantly affects the outcome of SWL, especially in the renal pelvis and lower calix.

Footnotes

Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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