The Effect of Stone Composition on the Efficacy of Retrograde Intrarenal Surgery: Kidney Stones 1

Abstract

Purpose:

The goal of this study was to analyze the effect of stone composition on the efficacy of retrograde intrarenal surgery (RIRS) with kidney stones of 1–3 cm, 1–2 cm, and 2–3 cm in diameter.

Methods:

We undertook a retrospective analysis of 74 patients with kidney stones who underwent RIRS. The patients were divided into two groups based on stone composition: Group I (n=47) (calcium oxalate monohydrate and calcium phosphate) was the hard to fragment stone group and group II (n=27) (calcium oxalate dihydrate, magnesium ammonium phosphate, and uric acid) was the easy to fragment stone group. Forty-six patients with kidney stones 1 to 2 cm in diameter were divided into group A (n=30) (smaller than 20 mm, hard to fragment stones) and group B (n=16) (smaller than 20 mm, easy to fragment stones). Twenty-eight patients with stones 2 to 3 cm in diameter were divided into group C (n=17) (larger than 20 mm, hard to fragment stones) and group D (n=11) (larger than 20 mm, easy-to-crush stones).

Results:

The stone clearance rates of group I and group II were 66.0% and 88.9%, respectively (P<0.05). The stone clearance rates of group A and group B were 73.3% and 100% (P<0.05). The stone clearance rates of group C and group D were 52.9% and 72.7%, respectively.

Conclusion:

Stone composition has a significant impact on the efficacy of RIRS in the management of 1 to 3 cm kidney stones. For 2–3 cm calcium oxalate dihydrate stones, uric acid stones, and magnesium ammonium phosphate stones, the outcome of RIRS treatment was relatively good, and RIRS is recommended.

Introduction

Percutaneous nephrolithotomy (PCNL) is considered the most effective treatment for patients with kidney stones greater than 20 mm. Bleeding is a major complication of PCNL.^1,2 The improvement of optical and laser technology has led to the increased use of retrograde intrarenal surgery (RIRS).^3,4 The European Association of Urology (2014) recommends: “Depending on operator skills, stones up to 3 cm can be treated efficiently by RIRS. RIRS can be successful in experienced hands in high volume centers.” A few studies have confirmed that RIRS is efficacious in managing stones larger than 2 cm.^5,6 The choice of RIRS is largely based on physician experience. The effect of stone composition on the outcome of RIRS is not known.

Stone composition affects the hardness of stone. PCNL allows the use of multiple stone fragmentation modalities, including pneumatic lithotripsy, ultrasonic lithotripsy, and laser lithotripsy, so the effect of stone composition is relatively small.^7,8 Only lasers are used to fragment stones in RIRS. Although the holmium laser can fragment all kinds of stones, the time for fragmentation varies widely.⁹ As we know, stone composition has a significant effect on the treatment of extracorporeal shockwave lithotripsy. In the common kinds of calculi, calcium oxalate monohydrate and calcium phosphate stones are very hard stones and are difficult to be fragmented; calcium oxalate dihydrate, magnesium ammonium phosphate, and uric acid stones are more easily be fragmented.^10,11 In our clinical work of RIRS, we also found that calcium oxalate phosphate stones are difficult to be fragmented; calcium dihydrate magnesium ammonium phosphate, and uric acid stones are easily fragmented.

Previous studies of RIRS have only evaluated the effect of stone size on treatment. We evaluated the effect of stone composition on the management of 1 to 3 cm kidney stones to establish better guidelines for treatment.

Methods

The research protocol was approved by the Ethics Committee of the Second Affiliated Hospital of Xi'an Jiaotong University. A retrospective analysis was performed of 74 patients with kidney stone who received RIRS in the urology department of the Second Affiliated Hospital of Xi'an Jiaotong University from January 2013 to June 2014. All patients were informed of alternative treatment options and chose RIRS treatment.

All patients underwent routine preoperative blood and urine testing and urine culture. Urinary tract infections were treated before RIRS. Preoperative CT imaging was performed to obtain three-dimensional images of the stones and to determine stone size and location. All patients had a 6F Double-J stent left in place for 2 to 3 weeks before operation. All surgical procedures were performed by the same group of surgeons, with all patients in the dorsal lithotomy position. A guidewire (Cook Medical, Bloomington, IN) was inserted into the ureteral orifice; a 14F or 12F access sheath (Cook Medical) was passed over the guidewire; a 7.5F flexible ureteroscope (URF-P5, Olympus, Japan) was inserted through the access sheath.

Laser fiber was used for stone fragmentation; stones were fragmented with 20 W holmium laser (Lumenis, San Jose, CA). Holmium laser power was set to 10 W. The repetition rates were 15 Hz and 8 Hz for 365- and 200-mm laser fibers. PCNL was used to fragment stones when RIRS was not successful. Cases converted to the use of PCNL were considered RIRS failures.

Stone fragments were collected during and after surgery and subjected to stone composition analysis using an infrared spectroscopy analyzer (LIIR-20, Lanmode Scientific Instrument Co. Ltd, Tianjin, China). All patients underwent unenhanced CT imaging 4 weeks after the surgical procedure for evaluation of residual stone burden. Patients were considered to have retained stones if there were residual stone fragments larger than 4 mm.

Calcium oxalate monohydrate and calcium phosphate stones are hard to fragment and calcium oxalate dihydrate, magnesium ammonium phosphate, and uric acid stones are relatively easy to fragment. The 74 patients were divided into two groups according to ease of stone fragmentation. Group I (n=47) included patients with calcium oxalate monohydrate and calcium phosphate stones and group II (n=27), patients with calcium oxalate dihydrate, magnesium ammonium phosphate, and uric acid stones. The stone clearance rates, operative times, duration of postoperative hospitalization, and complications of the two groups were compared.

The effect of stone composition on stone fragmentation was examined. Forty-six patients with stones 1 to 2 cm in diameter were divided into group A (smaller than 20 mm, hard to fragment group) and group B (smaller than 20 mm, easy to fragment group). Twenty-eight patients with stones 2 to 3 cm in diameter were divided into group C (larger than 20 mm, hard to fragment group) and group D (larger than 20 mm, easy to fragment group). The stone clearance rates, operative times, length of the postoperative hospitalization and complications of the different groups were compared.

SPSS19.0 software was used for data analysis. An independent sample t test was used to compare patient age, stone diameter, operative time, and length of the postoperative hospitalization of the groups. The chi-square test was used to compare stone clearance rates of the groups. P less than 0.05 was considered statistically significant.

Results

The average age of the 74 patients was 45.9±15.9 years. There were 47 male patients and 27 female patients. The overall stone clearance rate was 74.3%. The mean operative time was 75.81±17.5 minutes. The mean postoperative stay was 4.1±1.6 days. A total of 14 complications occurred in 10 patients—a complication rate of 13.5%. The 14 complications included 7 cases of hematuria, 4 of postoperative fever, and 3 urinary tract infections. There was no complication greater than Clavien grade II (Table 1).

Table 1.

Treatment Results of All Patients

Parameter	Results
Stone-free rate (n=55)	55/74 (74.3%)
Residual stone rate (n=19)	19/74 (25.7%)
Complications (n=10)	10/74 (13.5%)
Hematuria (n=7)	7/74 (9.5%)
Fever-sepsis (n=4)	4/74 (5.4%)
Urinary infection (n=3)	3/74 (4.1%)
Surgical time (min)	75.81±17.5
Hospital stay (d)	4.08±1.6

Ten patients had fourteen complications. Four of the patients had two complications.

Group I consisted of 47 patients, including 33 males and 14 females with an average age of 44.9±15.3 years. The mean stone diameter was 19.1±4.7 mm. Group II consisted of 27 patients, including 14 males and 13 females with an average age of 47.6±16.7 years. The mean stone diameter was 19.9±6.4 mm. There was no difference in the stone diameter of the two groups (Table 2).

Table 2.

Baseline Characteristics of Group I and Group II

Parameter	Group I (n=47)	Group II (n=27)	P value
Sex
Male	33	14	0.45
Female	14	13	0.29
Age (years)	44.9±15.3	47.6±16.7	0.68
Stone diameter (mm)	19.1±4.7	19.9±6.4	0.55
Stone location
Left/right	25/22	8/19	0.05
Stone position
Pelvis	28	21	0.48
Upper calix	5	1
Middle calix	5	4
Lower calix	9	2
Stones composition			<0.05
Oxalate monohydrate	40	0
Oxalate dehydrate	0	15
Calcium phosphate	7	0
Struvite	0	6
Uric acid	0	6

Group I: hard stone, 1 to 3 cm; group II: fragile stone, 1 to 3 cm.

The stone clearance rates of group I and group II were 66.0% and 88.9%, respectively (P<0.05). The operative times of group I and group II were 74.89±17.30 min and 67.85±17.71 minutes, respectively (P<0.05). The duration of postoperative hospitalizations of group I and group II were 4.1±1.6 d and 4.0±1.5 days, respectively (P>0.05). In group I, there were four procedures converted to PCNL because of extreme difficulty in fragmenting stones with the holmium laser. In group II, there was one conversion. In group I, a total of 10 complications occurred in seven patients. The complication rate was 14.9%. Complications included five hematuria, three fever, and two urinary tract infections. In group II, four complications occurred in three patients. The complication rate was 11.1%. Complications included two hematuria, one fever, and one urinary tract infection. There was no complication over Clavien grade II in either group (Table 3).

Table 3.

Treatment Results of Group I and Group II

Parameter	Group I (n=47)	Group II (n=27)	P value
Stone diameter (mm)	19.1±4.7	19.9±6.4	>0.05
Stone-free rate (%)	31 (66.0%)	24 (88.9%)	<0.05
Surgical time (min)	74.89±17.30	67.85±17.71	<0.05
Hospitalization time (d)	4.1±1.6	4.0±1.5	>0.05
Change to PCNL	4/47	1/27
Complications	7	3
Hematuria	5	2
Fever-sepsis	3	1
Urinary Infection	2	1

Group I: hard stone, 1 to 3 cm); group II: fragile stone, 1 to 3 cm.

In group I, seven patients had ten complications and three patients had two complications. In group II, three patients had four complications, and one of these patients had two complications.

The 46 patients with 1 to 2 cm stones were divided into group A (hard to fragment stones, n=30) and group B (easy to fragment stones, n=16). The mean stone diameter of the two groups was similar (group A, 16.1±2.1 mm vs Group B, 15.4±2.9 mm; P>0.05). The stone clearance rates of group A and group B were 73.3% and 100%, respectively (P<0.05). The operative times of group A and group B were 74.8±15.2 minutes and 59.0±10.3 minutes, respectively (P>0.05). The duration of the postoperative hospitalization of group A and group B was 4.1±1.7 days and 3.2±0.8 days, respectively (P>0.05).

The 28 patients with stones 2 to 3 cm in diameter were divided into group C (hard to fragment stones, n=17) and group D (easy to fragment stones, n=11). The stone diameters of group C and group D were 24.6±2.4 mm and 26.5±3.9 mm, respectively (P>0.05). The stone clearance rates of group C and group D were 52.9% and 72.7%, respectively. The stone clearance rate of group D was higher than that of group C, but the difference was not significant because of the small number of patients in each group. The operative times of group C and group D were 90.2±11.5 minutes and 80.7±18.7 min, respectively. The duration of postoperative hospitalization of group C and group D was 5.2±1.7 days and 4.2±1.5 days, respectively (P>0.05). The stone clearance rate of group D was comparable to that of group A (72.7% vs 73.3%). There was no significant difference because of the small number of patients in each group (Table 4).

Table 4.

Treatment Results of Groups A, B, C, and D

Parameter	Group A (n=30)	Group B (n=16)	Group C (n=17)	Group D (n=11)
Stone diameter (mm)	16.07±2.05	15.43±2.87	24.59±2.40	26.45±3.91
Stone-free rate (%)	22 (73.3%)	16 (100%)^a	9 (52.9%)^b	8 (72.7%)
Surgical time (min)	74.80±15.23	59.00±10.29^a	90.24±11.49^b	80.73±18.70^b
Hospital duration (d)	4.1±1.7	3.2±0.8	5.18±1.66	4.18±1.48
Change to PCNL	3	0	1	1
Complications	3	2	4	1
Hematuria	2	1	2	1
Fever-sepsis	2	0	2	1
Urinary infection	1	1	0	0

Group A: hard stone, 1 to 2 cm; group B: fragile stone, 1 to 2 cm; group C: hard stone, 2 to 3 cm; group D: fragile stone, 2 to 3 cm.

Significantly different from group A.

Significantly different from group A and group B.

A significant difference for the stone clearance rate was not obtained for group D because of the small sample size. Only the percentage was used to compare stone clearance rate with other groups.

In group A, three patients had five complications and two of these patients had two complications. In group B, two patients had two complications. In group C, four patients had five complications and one of these patients had two complications. In group D, one patient had two complications.

Group A and group C were easy to fragment stones, The stone diameters of Group A and Group C were 16.1±2.1 mm and 24.6±2.4 mm, respectively (P>0.05). The treatment results of group A were significantly better than that of group C. Group B and group D were hard to fragment stones. The stone diameters of Group B and group D were 15.4±2.9 mm and 26.5±3.9 mm, respectively (P>0.05). The treatment results of group B were significantly better than that of group D.

Discussion

It has been reported that the most important factor affecting the efficacy of RIRS is stone size. The efficacy of RIRS decreases significantly with stones larger than 20 mm, treatment of smaller stones being associated with shorter operative times and higher stone-free rates.^12,13 The comparison of group A and group C, as well as group B and group D results show, when the stone composition was similar, stone diameter is the an important factor to affect the RIRS. The number and position of stones also have obvious influence on RIRS treatment; because of the small sample size, we cannot count the effect of stone number and location on RIRS treatment. It has been reported that¹⁴ RIRS is not as efficacious as PCNL, but better than minimally invasive PCNL.

Stone composition is an important factor that affects the outcome of any stone fragmentation procedure. Because PCNL involves many methods to fragment stones, including pneumatic, ultrasonic, and laser techniques, the efficacy is not greatly affected by stone composition. In contrast, RIRS uses only one method to fragment stones, so the outcome obviously is affected by stone composition. Although the holmium laser can fragment all kinds of stones, stone hardness is still the most important factor limiting its use in RIRS. The speed of calcium oxalate monohydrate and calcium phosphate stone fragmentation with RIRS is substantially slower than that of calcium oxalate dihydrate, magnesium ammonium phosphate, and uric acid stones. This is especially true for stones larger than 2 cm.

Our comparative study demonstrated that the stone clearance rate of calcium oxalate dihydrate, magnesium ammonium phosphate, and uric acid stones 1 to 3 cm was significantly higher than that of calcium oxalate monohydrate and calcium phosphate stones. RIRS had a considerable clearance rate with calcium oxalate dihydrate, magnesium ammonium phosphate, and uric acid stones that were 2 to 3 cm than with calcium oxalate monohydrate and calcium phosphate stones 1 to 2 cm. These findings support the use of stone composition as a factor in choosing stone treatment modality, especially for stones larger than 2 cm.

Considering the high blood transfusion rate of PCNL, which is 7%, RIRS has a notable advantage.¹⁴ In the present study, no serious complication occurred in any patient, and there was not a single patient who needed a blood transfusion.

Preoperative stone composition analysis using CT or dual-energy CT has gained use in recent years. Giovanni and associates¹⁵ confirmed that unenhanced CT imaging was able to discriminate between calcium-containing stones and uric acid stones. Several studies^16
–18 have supported the use of dual-source dual-energy CT in characterizing stone composition before surgery. Mostafavi and colleagues¹⁹ reported that stone composition could be predicted with Hounsfield units (HU). Motley and coworkers²⁰ reported that HU density was more effective than HU alone. Patel and associates²¹ reported that HU values could be used for differentiating among calcium oxalate monohydrate and dehydrate stones. Prospective randomized trials are needed to confirm the use of preoperative stone composition analyses and stone size as factors important in patient selection for RIRS.

Footnotes

Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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The Effect of Stone Composition on the Efficacy of Retrograde Intrarenal Surgery: Kidney Stones 1–3 cm in Diameter