The Survival of Flexible Ureterorenoscopes in Terms of the Total Stone Area and Total Usage Time

Abstract

Introduction:

We investigated the survival of a flexible ureterorenoscope (FURS) in regard to the total stone area (TSA) and total usage time (TUT) to determine the cutoff values for its survival.

Materials and Methods:

Data were obtained from 1326 patients who underwent flexible ureterorenoscopy. The stone area and the usage time of the FURS were obtained in each case. The data from each FURS were considered as a group. The TSA was calculated as the sum of the stones for which only one FURS broke, and the TUT was calculated as the sum of the time of use within the body in all cases during the tool's survival. Data from 13 groups of 1258 patients were analyzed in regard to the TSA and TUT.

Results:

We found a positive correlation of the case number with the TSA and TUT. The Pearson correlation coefficients were 0.983 and 0.937 for the TSA and TUT, respectively (p < 0.05). The mean case number, TSA, and TUT where only one FURS was used were 97.38 ± 31.34, 11,886 ± 4567.93 mm², and 5160 ± 1570.52 minutes, respectively. The area under the curve (AUC) of the receiver operating characteristic curve of the FURS survival for the TSA was 91.7% (95% confidence interval [CI]: 0.76–1.07). The best cutoff value for the FURS survival was 6838 mm², with a sensitivity of 91%. The results for the AUC and the best cutoff value in regard to the TUT for using only one FURS were 66.7% (95% CI: 0.4–0.93) and 4617 minutes, respectively, with a sensitivity of 66%. If the cutoff value is taken as the average value of the TSA and TUT, the sensitivity rate drops to ∼58% and 50%, respectively.

Conclusion:

FURS can be safely used when the stone area and TUT are about 9158 mm² and 4617 minutes, respectively, with a sensitivity of 66%.

Introduction

The role of the flexible ureterorenoscope (FURS) has increased in the treatment of kidney and upper ureteral stones, even for stones larger than 2 cm in cases where percutaneous nephrolitotomy and extracorporeal shockwave lithotripsy are not an option. This is possible thanks to technical improvements such as endoscope miniaturization, improved deflection angles, enhanced optical quality and ancillary tools, greater experience, and improved surgical technique.^1,2 Many studies have looked at the tool's cost-effectiveness and durability.^3

–6 It is well known that one of the most important factors that influence the cost of flexible ureterorenoscopy is the survival of the FURS. Despite technical advances in ureteroscopic design, the scopes continue to have problems with durability.⁶ The most common causes of damage are the impairment of the deflection mechanism, inner sheath damage, fiberoptic bundle breaks,⁷ and the surgeon's lack of experience.

Most of the current studies about the durability of the FURS have depended on the average number of the procedures. This study's purpose is to establish the survival of the FURS in terms of the total stone area (TSA) and total usage time (TUT). In this context, we also aimed to determine a cutoff value for the survival with regard to these parameters.

Materials and Methods

Our study was approved by the local ethics committee. All flexible ureterorenoscopic procedures were performed by an experienced surgeon (H.K.) for kidney stones between 2010 and 2019 in a single tertiary center. A total of 16 FURSs with the same brand (7.5F; Karl Storz Flex-X2, Tutlingen, Germany) were used for only urolithiasis. All 16 of the FURS failed within this period. The reason for replacing the FURS with a new one is that it can no longer be used effectively for lithotripsy due to impairment of the deflection mechanism, inner sheath damage, and fiberoptic bundle breaks. Our study uses a retrospective design. Data were collected from 1326 patients who underwent flexible ureterorenoscopy. The information included age, sex, stone location, and stone area by measuring its two dimensions using CT. The usage time of the FURS was also measured. The time was defined as the time that passed between the beginning of the procedure when the device is inserted into the access sheath and the moment of removing the FURS. The data were grouped according to the used FURS in that period. In patients who were scheduled to have a second operation in the control examinations, the stone area in the first operation was corrected by removing the residual area in the first stone area.

The data from the first three FURSs were not used for the statistical evaluation because they consist of learning curve data. The number of cases for the learning curve was determined as more than 50 cases for each group based on the literature.⁸ The data obtained for each reusable FURS until its breakage were considered as a group. The TSA was calculated as the sum of the stones where only one FURS broke. The TUT was calculated as the sum of the time of use within the body in all cases during its survival. Data from 13 groups of 1258 patients were analyzed in terms of the TSA and TUT. Furthermore, the mean stone area and the mean operation time were compared between groups. Another statistical analysis was also performed according to the location of the stones in the kidney in terms of the same parameters. A standardized surgical technique was performed for all procedures to avoid the effect of ancillary equipment on the results.

The cleaning and disinfection of the FURS and ancillary equipment were performed by trained nurses and assistants according to the instructions. The devices were disinfected using a disinfectant solution (Cidex^®) for 20 minutes. At the end of the operation, they were washed and dried according to the instructions, and the FURS was kept in its box.

Surgical technique

A semirigid ureteroscopy was performed to cannulate the ureteric orifice with a safety guidewire (0.035 inch, Microvasive; Boston Scientific Corp., Natick, MA) and to perform active dilatation. A 9.5F 45-cm-long ureteral access sheath (Cook Medical, Inc., Bloomington, IN) was placed under fluoroscopic vision in all cases. A FURS (7.5F; Karl Storz Flex-X2, Tutlingen, Germany) was also used in all procedures. We reached the lower pole easily after deflection of the FURS because we used a 272-μm laser fiber (AMS™; Sureflex) in all cases. We never used any basket catheter so as not to increase the cost of the operation. Upon reaching the stone, a 272-μm laser fiber (AMS; Sureflex) was inserted, and the stone was fragmented with a dusting method using a holmium:YAG laser (Stonelight™; Cooltouch). At the end of the procedure, a Double-J stent was inserted.

Statistical analyses

The results are presented as the mean ± standard deviation (SD). Data were analyzed using SPSS 16.0 for Windows (SPSS, Inc., Chicago, IL). Statistical analyses of the means of continuous variables were performed with one-way analysis of variance (ANOVA), univariate ANOVA, the Mann–Whitney U test, and the Games–Howell test for post hoc multiple comparisons in cases where equal variances are not assumed. The Pearson correlation was used to analyze the correlation of the case number with the TSA and TUT. The area under the curve (AUC) of the receiver operating characteristic (ROC) curve was estimated to evaluate the durability of the FURS. The best cutoff value was determined using the Youden index. The AUC, sensitivity, and specificity were compared using their 95% confidence intervals (CIs). The results were considered significant at p < 0.05.

Results

The demographic characteristics of the groups are summarized in Table 1. We excluded the data of the first three groups from the statistical analysis because they are learning curve data for gaining experience with the procedure. The results with SDs related to the case number, mean stone area, TSA, mean operation time, and TUT are shown in Table 2.

Table 1.

The Demographic Characteristics and Results with Standard Deviations of the Groups

Group	Age ± SD	Sex
Group	Age ± SD	Male, n = 790	Female, n = 536
1^a	42.87 ± 12.4	14	9
2^a	47.26 ± 16.3	10	9
3^a	48.81 ± 15	8	8
4	47.61 ± 14.9	35	22
5	49.84 ± 16.3	75	54
6	47.48 ± 17.3	52	31
7	48.27 ± 19.1	51	38
8	49.08 ± 15.3	35	27
9	42.57 ± 15.3	32	17
10	49.26 ± 15.6	58	20
11	47.92 ± 16.5	77	56
12	50.5 ± 17.5	53	50
13	45.61 ± 18.3	62	47
14	46.7 ± 19.6	85	65
15	51.86 ± 14.7	80	41
16	45.8 ± 14.03	63	42

These groups were considered as learning curve groups.

SD = standard deviation.

Table 2.

The Total Case Number, Mean and Total Stone Area, and Mean and Total Usage Time During the Lifetime of Only One Flexible Ureterorenoscope and Statistical Difference Between Groups in Terms of Mean Stone Area and Total Usage Time

Group	Total case number^a	Stone area (mm²)			Operation time (minutes)
Group	Total case number^a	Mean ± SD^a	p^*	Total	Mean ± SD^a	p^*	Total
1	23	96.00 ± 42.9	9–12	2208	72.78 ± 33.5	13, 15, 16	1674
2	19	77.50 ± 48	8–12, 14, 16	1472	39.47 ± 34.1		750
3	16	121.7 ± 72.3		1948	35.94 ± 31.4		575
4	57	111.6 ± 58		6365	61.23 ± 32.1	13, 15	3490
5	129	124.4 ± 84.4		16,048	62.17 ± 30.1	10, 12–16	8020
6	83	107.39 ± 61.6	10, 12	8914	62.71 ± 35.7	13–16	5205
7	89	105.66 ± 57.7	10–12	9404	64.94 ± 30.7	10, 12–16	5780
8	62	128.22 ± 61.4	2	7950	66.21 ± 28.1	10, 12–16	4105
9	49	146.49 ± 72.2	1, 2, 6, 7	7178	55.41 ± 21.1		2715
10	78	148.41 ± 83.1	1, 2, 6, 7	11,576	47.24 ± 18.4	5, 7, 8	3685
11	133	135.88 ± 64.9	1, 2, 7	18,072	54.92 ± 24.2	13, 15	7305
12	103	141.17 ± 63	1, 2, 6, 7	14,540	49.09 ± 22.8	5, 7, 8	5056
13	109	122.21 ± 58.3		13,321	43.12 ± 18.3	1, 4–8, 11	4700
14	150	133.31 ± 63.2	2	19,997	47.23 ± 21.2	5–8	7084
15	121	120.13 ± 65		14,654	44.17 ± 19.6	1, 4–8, 11	5405
16	105	131.72 ± 78.3	2	13,831	44.23 ± 26.8	1, 5–8	4334

The Pearson correlation is 0.983 and 0.937 for the TSA and TUT, respectively. The first three groups include the learning curve data.

A positive correlation between the case number with mean stone area and mean operation time.

p < 0.05.

TSA = total stone area; TUT = total usage time.

The stone areas of groups 6, 7, 9, 10, and 12 were 107.39 ± 61.6, 105.66 ± 57.7, 146.49 ± 72.28, 148.41 ± 83.1, and 141.17 ± 63 mm², and the numbers of cases were 83, 89, 49, 78, and 103, respectively. Groups 6 and 7 are statistically different from groups 9, 10, and 12 in regard to the mean stone area (Table 2). The stone areas of groups 11 and 14 were 135.88 ± 64.9 and 133.31 ± 63.29 mm², and the numbers of cases were 133 and 150, respectively. These results are not statistically different from those of the other groups (p > 0.05) (Table 2).

The results of the operation time for the groups are shown in Table 2. The operation times in groups 4, 5, 6, 7, 8, and 11 were 61.23 ± 32.1, 62.17 ± 30.1, 62.71 ± 35.7, 64.94 ± 30.7, 66.21 ± 28.16, and 54.92 ± 24.21 minutes, respectively. The operation times in groups 10, 12, 13, 14, 15, and 16 were 47.24 ± 18.42, 49.09 ± 22.85, 43.12 ± 18.31, 47.23 ± 21.29, 44.17 ± 19.6, and 44.23 ± 26.8 minutes, respectively (Table 2). We found a positive correlation of the case number with the TSA and TUT, which had Pearson's correlation coefficients of 0.983 and 0.937, respectively (Table 2) (p < 0.05).

The case number, TSA (mm²), and TUT (minutes) corresponding to when only one FURS was used are 97.38 ± 31.34, 11,886 ± 4567.93, and 5160 ± 1570.52, respectively (Table 3). The AUC of the ROC curve of the FURS survival for the TSA was 91.7% (95% CI: 0.76–1.07). The best cutoff value for the FURS survival was defined by the Youden index to predict the TSA for using only one FURS as 6838 mm², which had a sensitivity of 91% (Table 4). The results for the AUC and the best cutoff value in terms of the TUT for using only one FURS were 66.7% (95% CI: 0.4–0.93) and 4617 minutes, respectively, with a sensitivity of 66% (Table 5). If the cutoff value is taken as the average value of the TSA and TUT, the sensitivity rates drop to ∼58% and 50%, respectively (Tables 4 and 5).

Table 3.

The Mean ± Standard Deviation of the Case Number, Total Stone Area, and Total Usage Time of Only One Flexible Ureterorenoscope, Which Uses Including All Procedures, and Per Case Mean Total Stone Area and Total Usage Time

Mean ± SD
Case number	TSA (mm²)	TUT (minutes)
97.38 ± 31.34	11,886 ± 4567.93	5160 ± 1570.52
Per case	Mean TSA	Mean TUT
Per case	122.05	52.98

Table 4.

The Sensitivity and Specificity Rates of the Two Cutoff Values for the Total Stone Area That was Broken by Only One Flexible Ureterorenoscope

TSA (mm²)	Sensitivity	Specificity
6363	1.00	0.00
6432	0.91	0.00
6838 ^a	0.91	1.00
7563	0.83	1.00
8431	0.75	1.00
9158	0.66	1.00
10,489	0.58	1.00
12,448	0.50	1.00
13,930	0.41	1.00
14,597	0.33	1.00
15,350	0.25	1.00
17,060	0.16	1.00
19,034	0.08	1.00
19,998	0.00	1.00

Bold values may be accepted as a cut-off point according to the reader.

According to the Youden index.

Table 5.

The Sensitivity and Specificity Rates of the Two Cutoff Values for the Total Usage Time of the Only One Flexible Ureterorenoscope

TUT (minutes)	Sensitivity	Specificity
2714	1.0	0.0
3102	0.91	0.0
3587	0.83	0.0
3895	0.75	0.0
4319	0.66	0.0
4617 ^a	0.66	1.0
4878	0.58	1.0
5130	0.50	1.0
5305	0.41	1.0
5592	0.33	1.0
6432	0.25	1.0
7194	0.16	1.0
7662	0.08	1.0
8021	0.00	1.0

Bold values may be accepted as a cut-off point according to the reader.

According to the Youden index.

Table 6 shows the mean operation time and stone area in terms of the stone location in all groups. There is no statistical difference between the number of cases of the subgroups according to stone location (Table 6). The mean operation time in the lower pole subgroup and the others was 51.1 ± 26.07 and 54.3 ± 27.8 minutes (p < 0.05), respectively. The mean stone areas for the same subgroups were 117.54 ± 62.49 and 132.93 ± 72.06 mm² (p < 0.05), respectively (Table 6).

Table 6.

The Case Number According to the Stone Location in the Kidney and the Mean Operation Time and Stone Area with the Standard Deviations in Terms of the Stone Location Including All Groups

Group	Stone location
Group	Lower pole, n = 611	Others, n = 715	p
1	15	8	>0.05
2	11	8
3	4	12
4	29	28
5	47	82
6	39	44
7	38	51
8	31	31
9	15	34
10	37	41
11	59	74
12	49	54
13	46	63
14	91	59
15	50	71
16	50	55
Parameters
Operation time (minutes)	51.1 ± 26.07	54.3 ± 27.8	0.03
Stone area (mm²)	117.54 ± 62.49	132.93 ± 72.06	0.001

Discussion

Considerable development has occurred in the design and application of FURS since its introduction in 1971 by Takayasu and collegues.⁹ The improvements include decreased size, better optics, better maneuverability,¹⁰ and ancillary equipment such as nitinol baskets or triceps, forceps, and smaller caliber laser fiber. The main purpose of these developments is to achieve more cost-effective inventions to use resources more accurately. Economic evaluation methods have also been developed, such as cost-effectiveness analysis (CEA).

There are four possibilities when comparing two modalities: the new modality can be more expensive and more effective (1), more expensive and less effective (2), less expensive and less effective (3), or less expensive and more effective (4).¹¹ Treatments that fall into the second category are said to be dominated by other alternatives because one would not be willing to pay more for less benefit. Treatments that fall into the fourth category are considered dominant and offer health gains at a lower cost. However, CEA is most useful when considering treatments that fall into the other two categories.

CEA helps with the decision by estimating the additional cost per unit of additional gain.¹² In this context, FURS has undergone various assessments as a new method for the treatment of endoscopic kidney stones, especially in terms of durability. The tool is extremely brittle and susceptible to damage. These assessments are performed to achieve more accurate cost-effective analysis results.^3

–6

Different studies have looked at the durability of different FURSs and investigated the mechanism of damage.⁷ According to some studies, the main reason for repair is the failure of the deflection mechanism.^13,14 Within our study period, the FURS became unusable due to only failure of the deflection mechanism in 11 cases and both the breakage of the deflection mechanism and fiberoptic bundles in 4 cases. Some studies are mostly based on the number of cases and time of surgery.^2,10 For instance, according to Thibault and colleagues, the TUT of using only one FURS was 4575 minutes, with a mean of 91.5 minutes and 40 cases.¹⁰ Their TUT is similar to ours, but our mean TSA and mean number of cases are greater than theirs, and our mean operation time is lower (Table 3).

The difference in results may depend on the difference in the stone area. The bigger the stone, the more challenging the case may be. Furthermore, the main difference of our study from other studies is determining a cutoff value for the survival time of a FURS in regard to the TSA and TUT. Another difference is the use of the same brand of FURS to avoid differences in quality due to the device.

This research is the first study with the aim of determining a cutoff value in terms of the TSA and TUT to establish the survival of the FURS. Therefore, we believe that our study could change the perspective of assessments in this context. We believe that these results will help to more accurately evaluate cost-effectiveness. A different statistical analysis could be applied to the CSA according to a different point of view.

The results related to the mean stone area and number of cases show that although the mean stone area in groups 9, 10, and 12 were statistically larger than those in groups 6 and 7, no statistical difference was determined between the number of cases in these groups (Table 2). These results suggest that the mean stone area is much better than the case number for evaluating the durability of the FURS.

Table 2 shows the statistical differences for the mean operation time between groups 4, 5, 6, 7, 8, and 11 and groups 10, 12, 13, 14, 15, and 16. This situation may be explained by the high experience level of the surgeon. The TSA, TUT, and total case numbers during the survival of a FURS were ∼11,886 ± 4567.93 mm², 5160 ± 1570.52 minutes, and 97.38 ± 31.34, respectively (Table 3). The TSA and TUT results were divided by the total number of cases to find the average stone area and usage time per case, and the results were 122.05 mm² and 52.98 minutes, respectively. In every different clinic, the variation of these values according to experience is extremely natural. When we analyzed our results from the group 9 in terms of the TUT, the TSA with which a FURS breaks increases, but the TUT does not change significantly (Table 2). The same interpretation can be made with regard to the mean stone area and the mean operation time as well (Table 2). This situation could be explained by the high experience level of the surgeon.

In groups 11 and 14, the numbers of cases were 133 and 150, and their TSAs were 18,072 and 19,997 mm², respectively. In groups 9, 10, 12, 13, and 16, the total numbers of cases were 49, 78, 103, 109, and 105, whereas the average stone areas (mm²) were 7178, 11,576, 14,540, 13,321, and 13,831, respectively. However, no statistical difference was established between these groups in terms of the mean stone area (Table 2). As expected, there was a significant correlation of the case number with the TSA and TUT, which had Pearson's correlation coefficients of 0.983 and 0.937, respectively (Table 2) (p < 0.05).

The ROC analysis was used to determine the cutoff values for the TSA and TUT. The AUC of the ROC analysis of the FURS survival for the TSA was 91.7% (95% CI: 0.76–1.07). The best cutoff value for the TSA was 6838 mm², which had a sensitivity of 91% (Table 4). If the cutoff value is taken as the mean value of the TSA, the sensitivity rate drops to ∼58% (Table 4). If the stone area with which only one FURS was broken exceeds 6838 mm², the reliability is reduced. Therefore, it is recommended to have a backup device after this point in the operating room in the event that the current instrument is broken so that the operation can be completed without any complications.

The AUC for the TUT was 66.7% (95% CI: 0.4–0.93), and the best cutoff value was defined as 4617 minutes, with a sensitivity of 66% according to the Youden index (Table 5). If the cutoff value is taken as the mean value of TUT, the sensitivity rate drops to ∼50% (Table 5). If the TUT exceeds 4617 minutes, its reliability is reduced. Therefore, it is again recommended to have a backup device in the operating room to finish the operation. If we compare the best cutoff value tables with the TSA and TUT with 66% sensitivity, the values are determined as 9158 mm² and 4617 minutes, respectively (Tables 4 and 5). A chart could be created to be stored in the storage box of the FURS to show the calculated stone area and TUT according to the patient's CT in each case.

In addition to these parameters, we also analyzed the mean operation time and stone area in terms of the stone location in all groups (Table 6). Previous studies have reported that the operation time spent in the lower pole has an effect on the FURS durability.¹⁴ However, according to our results, the operation time in the lower pole of the kidney was significantly lower than in the other locations. We attributed this to the fact that the mean stone area in the lower pole of the kidney was lower than in other locations.

To increase the survival of the FURS, education for nurses and assistants is another important issue. For this purpose, we provide education about the clearance and storage of the brittle FURS devices and other endoscopic devices to prevent failure.

Study limitations

The first main limitation of our study is that the results may not be valid for every situation and patient.

The second limitation is that the number of cases was too small to make significant decisions. The third limitation is that we did not compare the data from the first three groups, which were learning curve data. The fourth limitation is that we did not compare the stone-free rates and complications in each group and between groups. The fifth limitation is that we did not measure the Hounsefiel units of the stones in CT.

Conclusion

Various methods can be used to determine the survival of FURSs. According to the Youden index of the AUC of the ROC analysis, a FURS can be safely used with an efficiency of 66% when the stone area and TUT are about 9158 mm² and 4617 minutes, respectively.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received for this article.

Abbreviations Used

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