Int Urol Nephrol DOI 10.1007/s11255-014-0812-0
UROLOGY - ORIGINAL PAPER
Does the nephrostomy tract length impact the outcomes of percutaneous nephrolithotomy (PNL)? Gaston M. Astroza • Andreas Neisius • Matvey Tsivian • Agnes J. Wang • Glenn M. Preminger Michael E. Lipkin
•
Received: 13 May 2014 / Accepted: 31 July 2014 Ó Springer Science+Business Media Dordrecht 2014
Abstract Purpose Different factors can determine the outcomes of percutaneous nephrolithotomy (PNL). We analyzed the effect of tract length (TL) on outcomes after PNL. Methods We performed a retrospective review of patients undergoing PNL between 2006 and 2011. Patients with preoperative computed tomography (CT), one percutaneous access tract and follow-up imaging within 3 months were included. TL was defined as distance between the skin to the calyx of puncture as measured on preoperative CT. Measurements were independently performed by two urologists and the average was used for analysis. Stone-free rate (SFR) was defined as zero fragments on follow-up imaging. Factors independently associated with the likelihood of being stone-free after PNL were determined using multivariable analysis adjusted for TL, location of access, the presence of incomplete or complete staghorn calculi and type of follow-up imaging. Complications (Clavien score) were independently assessed.
Results A total of 222 patients were included. Median stone burden and body mass index (BMI) was 239.4 mm2 and 30.5 [interquartile range (IQR): 25.7–36.2]. The median TL was 85.0 mm (IQR: 70.3–100.0) and highly correlated with BMI (q = 0.66, p \ 0.001). A total of 101 patients (45.5 %) were stone-free. TL was not associated with SFR (p = 0.53). Clavien 1 and 2 complications occurred in 38 (17 %) while Clavien 3 and 4 complications occurred in 17 (8 %) patients. Multivariable analysis revealed no association between complications and TL even when adjusted for gender. Conclusions Percutaneous TL is not associated with outcomes of PNL. PNL is a safe and effective treatment for stones in patients with differing body habitus. Keywords Percutaneous nephrolithotomy Percutaneous tract PNL Stone-free rate
Introduction
Gaston M. Astroza and Andreas Neisius contributed equally to this work and also shared first authorship. G. M. Astroza A. Neisius (&) M. Tsivian A. J. Wang G. M. Preminger M. E. Lipkin Division of Urologic Surgery, Duke University Medical Center, DUMC 3167, Durham, NC 27710, USA e-mail: [email protected] G. M. Astroza Department of Urology, Universidad Cato´lica de Chile, Santiago, Chile A. Neisius Department of Urology, Universita¨tsmedizin Mainz, Johannes- Gutenberg University, Mainz, Germany
Percutaneous nephrolithotomy (PNL) remains the first-line treatment for renal calculi C2 cm and for lower pole stones C15 mm [1, 2]. PNL is also indicated for treatment of smaller stones in obese patients since shock wave lithotripsy has been demonstrated to have inferior success rates in this cohort of patients [3, 4]. Outcomes of PNL are related to different factors such as stone burden, stone location, anatomical factors and not necessarily obesity [5]. Yet, different types of obesity have been defined showing that body fat distribution is not constant among people and races [6, 7]. Therefore, the length of the percutaneous tract in patients with similar BMI values undergoing PNL could differ based on various body types and different quantities of retroperitoneal fat.
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Scoring systems or nomograms that include the length of the percutaneous tract as a variable have been recently reported [8]. However, it is unclear whether tract length impacts stone-free and complication rates after PNL. We analyzed the effects of tract length on stone-free and complication rates of patients undergoing PNL in the prone position.
Methods After institutional review board approval, a retrospective review of patients undergoing prone PNL between June 2006 and June 2011 was performed. Only patients with data on preoperative body mass index (BMI in kg/m2), preoperative computerized tomography (CT) and follow-up imaging within at least 3 months after PNL were included in this analysis. To decrease the number of confounding factors, patients with multiple percutaneous tracts during the procedure and staged procedures were excluded from this analysis. Tract length (TL) was determined on the preoperative CT as the distance between the skin and the surface/lateral edge of the calix (in millimeters) which was utilized for the puncture. Preoperative imaging was performed using the institutional stone protocol CT which is a lowdose CT in prone position. Surgical procedures were performed in a similar fashion (prone). The site of puncture and the calix of choice were recovered from the surgical report and intraoperative fluoroscopy images. Tract length was uniformly measured at an angle of 25–30° from a vertical line drawn between the anterior wall of the abdomen and the spinous process. The distance between the skin surface and the punctured posterior calix was established as TL (Fig. 1). All punctures were performed by the interventional radiologist in association with an endourologist at the time of surgery in the operating room with the patient prone, using the most posterior calix at the moment of the surgery. All procedures were performed by two experienced surgeons (MEL and GMP). The measurements were obtained independently from two fully trained urologists (GMA and AN) and both measurements were compared for concordance. The averaged distance between the two original measurements was used for the final analysis. Stone burden was calculated by multiplying the anteroposterior and latero-medial dimensions of the stone on axial images from the preoperative CT. In cases of multiple stones, the three largest stones were measured and the stone burden was considered the sum of the three. Complications were assessed from the medical records of each patient and classified using the Clavien–Dindo
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Fig. 1 Example of an axial image to demonstrate the method used to determine the length of the percutaneous tract
system [9]. Clavien grades 1 and 2 were defined as minor, whereas grades 3 through 5 were considered severe complications. Stone-free rates (SFRs) were determined with intravenous pyelography (IVP), CT and KUB and tomograms at 3 months postoperatively. Stone-free was defined as the complete absence of any residual fragments. Correlation of the measurements between both urologists and between TL and the BMI was determined using the Pearson correlation statistics. Comparisons between groups were carried out using parametric and nonparametric tests for continuous variables and Fisher’s exact tests for categorical variables, as appropriate. Odds of becoming stone-free after PNL were determined using crude and multivariable-adjusted logistic regression controlling for gender, BMI, type of imaging to assess stonefree status, tract length, location of puncture (lower vs. mid vs. upper pole) and the presence of staghorn calculi (nonstaghorn vs. incomplete vs. complete staghorn). Staghorn stones were defined as a stone in the renal pelvis and filling multiple calyces, incomplete staghorn stones were defined as stones filling the renal pelvis and only one single calyx. All analyses were conducted using R v2.13 (the R Foundation for Statistical Computing, Vienna, Austria) with ‘Hmisc’, ‘rms’ and ‘gmodels’ libraries. All tests were twosided and p values \0.05 were considered statistically significant. Data are presented as median [interquartile range (IQR)] or number (percent) unless otherwise specified.
Results A total of 510 patients who underwent PNL at our institution from 2006 to 2011 were identified, and of these 222
Int Urol Nephrol Table 1 Distribution of patients with different BMI and respective nephrolithotomy tract length adjusted by gender
Variable BMI (kg/m2)
Overall
Male
30.5 (25.7–36.2)
Female
29.4 (25.4–34.1)
p value
31.3 (26.3–37.4)
0.095
BMI, categories
0.251
\25
46 (20.7 %)
22 (22.7 %)
24 (19.2 %)
25–29.9
60 (27.0 %)
29 (29.9 %)
31 (24.8 %)
30–34.9
55 (24.8 %)
26 (26.8 %)
29 (23.2 %)
C35
61 (27.5 %)
20 (20.6 %)
41 (32.8 %)
Length of tract (mm)
85.0 (70.3–100.0)
79.2 (66.6–97.0)
89.6 (77.1–101.9)
0.007
BMI \25
63.3 (55.1–76.1)
70.7 (56.0–80.2)
60.9 (48.1–68.4)
0.115
BMI 25–29.9
79.3 (70.0–86.9)
85.5 (79.8–95.0)
74.7 (65.6–79.1)
\0.001
BMI 30–34.9
85.9 (74.1–98.1)
89.9 (82.6–100.3)
84.4 (68.2–90.9)
0.046
106.2 (94.1–119.4)
114.7 (103.6–140.0)
104.5 (93.2–113.2)
0.005
BMI C35
Table 2 Univariable and multivariable analyses of factors associated with stone-free status Variable
Univariable
Multivariable
OR (95 % CIs)
p value
OR (95 % CIs)
p value
Female
ref
–
ref
–
Male
1.79 (1.05–3.08)
0.033
2.05 (1.10–3.89)
0.026
1.00 (0.97–1.03)
0.828
1.00 (0.96–1.05)
0.854
CT
ref
–
ref
–
IVP
2.26 (1.22–4.30)
0.011
2.70 (1.37–5.45)
0.005
KUB
1.03 (0.32–3.06)
0.965
0.90 (0.26–2.89)
0.868
Tract length
1.00 (0.99–1.01)
0.538
1.00 (0.98–1.01)
0.587
Inferior
ref
–
ref
–
Mid
0.60 (0.32–1.09)
0.094
0.61 (0.31–1.19)
0.151
Superior
0.51 (0.23–1.09)
0.086
0.68 (0.29–1.58)
0.368
Gender
BMI Imaging
Fig. 2 Association between BMI and tract length by gender
patients met inclusion criteria. Ninety-seven (43.7 %) were males and 125 (56.3 %) were females. The median age of the group was 53.5 years (43–63). When stratified by gender, the median age was 56 (54–65) years for men and 51 (42–60) years for women, p = 0.081. The median BMI was 30.5 (25.7–36.2) overall with no statistical difference between men and women. Table 1 summarizes the distribution of patient characteristics and tract lengths by gender and BMI in four groups (\25, 25–29.9, 30–34.9 and C35 kg/m2). Measurement of the tract length was significantly associated between the two urologists (q = 0.88; p \ 0.001) with a median tract length of 85.0 mm (70.3–100.0). There was a statistically significant association between tract length and BMI (q = 0.69; p \ 0.001) and a significant different TL between males [79.2 mm (66.6–97.0) and females 89.6 (77.1–101.9)], p = 0.007 (Fig. 2). Access was obtained through the upper (19 %), mid (48 %) or lower pole (33 %) with a significantly different TL of 85.7 (69.1–100.7), 81.8 (65.5–93.1) and 92.6 (76.3–106.1), respectively (p = 0.025).
Tract location
Staghorn Not staghorn
ref
–
ref
–
Incomplete
0.40 (0.17–0.91)
0.034
0.30 (0.12–0.72)
0.009
Complete
0.13 (0.02–0.46)
0.007
0.13 (0.02–0.51)
0.010
ref reference value
Seventeen patients (7.7 %) had a complete staghorn stone, 30 (13.5 %) an incomplete staghorn stone, and 175 (78.8 %) were classified as non-staghorn stones. The median stone burden was 239.5 mm2 (150–422). Higher stone burden was strongly associated with staghorn calculi
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Int Urol Nephrol Table 3 Specific complications Number (%) Clavien grade 1 Postoperative fever
Description
Table 4 Multivariable analyses of risk factors for overall and severe complications Variable
25 (11) 12 (5)
Overall complications
Severe complications
OR (95 % CIs)
OR (95 % CIs)
p value
p value
Gender
Pain
1 (0.5)
Prolonged hospital stay
Female
ref
–
ref
–
Urinoma
2 (1)
Conservative management
Male
0.126
3 (1)
0.79 (0.25–2.41)
0.680
Pleural effusion
0.58 (0.28–1.16)
Perforation
4 (2)
Prolonged stent required
BMI
w/o transfusion Perioperative
0.99 (0.90–1.08)
0.886
2 (1) 1 (0.5)
0.97 (0.91–1.02)
0.228
Gross hematuria SPO2 low
Tract length
1.01 (0.99–1.03)
0.618
1.01 (0.98–1.04)
0.390
w/o embolization or intervention
Location Inferior
Clavien grade 2
13 (6)
Transfusion
10 (5)
Atrial fibrillation
2 (1)
UTI
1 (0.5)
Clavien grade 3
3 (1) 2 (1)
Bleeding requiring embolization
6 (3)
Fragments requ URS DVT ? PE MI Sepsis Total complications
ref
–
0.818
2.90 (0.81–14.0)
0.130
Superior
1.19 (0.46–2.98)
0.710
2.54 (0.51–14.0)
0.252
Not staghorn
ref
–
ref
–
Incomplete
0.85 (0.29–2.19)
0.749
2.09 (0.43–7.89)
0.306
Complete
3.26 (1.13–9.57)
0.028
1.75 (0.25–7.84)
0.507
Staghorn
2/6 Required a blood transfusion
2 (1) 4 (2) 1 (0.5) 1 b (0.5)
Required blood transfusion
ref reference value
2 (1) 55 (25)
[non-staghorn 199 mm2, (138–334), incomplete 415 mm2 (298–697) and complete 604 mm2 (480–963), p \ 0.001]. A total of 101 patients (45.5 %) were stone-free (the absence of any fragment on follow-up imaging) postoperatively at 3 months. Patients with single stones showed a trend to be more likely to become stone-free (56/109) with 51.4 % compared to patients with multiple stones (45/113) with 39.8 % but without reaching statistical significance, p = 0.106. The majority of patients (n = 143) received an IVP, n = 61 patients underwent CT imaging and n = 18 had a KUB and tomograms for follow-up after 3 months. Patients receiving an IVP were twice more likely to achieve stone-free status (OR 2.26, 95 % CIs 1.22–4.30, p = 0.011) compared to those evaluated with a CT. There was no association between the length of the percutaneous tract and likelihood of being stone-free (OR 1.00, 95 % CIs 0.99–1.01, p = 0.538). Moreover, BMI and tract location (superior, middle or inferior calyx) were not associated with SFRs on unadjusted analysis. Male gender and the presence of incomplete or complete staghorn calculi showed an association with SFRs using univariate testing with p = 0.033, 0.034 and 0.007, respectively (Table 2).
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–
1.09 (0.52–2.33)
13 (6)
Persistent leakage Pleural effusion
Clavien grade 4
Requiring prolonged iv antibiotics
ref
Mid
These findings remained essentially unchanged on multivariable analyses showing male gender, staghorn stones and type of imaging, but not TL, to be associated with stone-free status (Table 2). On univariable analyses, transfusion rates (transfusion of packed RBCs) were associated with lower BMI [23.6 (19.3–38.5) in transfused vs. 30.6 (26.5–32.0) in nontransfused, p = 0.030]. Shorter TL revealed a similar trend whereby patients receiving transfusions had shorter TL [63.6 mm (55.0–92.7) vs. 85.4 mm (71.3–100.1), p = 0.055]. Overall, 55 patients (25 %) experienced postoperative complications (Table 3). Thirteen patients (6 %) had Clavien grade 3 complications and four patients (2 %) had grade 4 events. In multivariable analyses the presence of staghorn calculi was associated with increased odds of overall complications [OR 3.26 (1.13–9.57), p = 0.028], whereas we could not identify any factors statistically associated with severe complications (Table 4).
Discussion The length of a percutaneous nephrostomy tract is principally determined by the amount of subcutaneous fat, back
Int Urol Nephrol
muscles and retroperitoneal fat. Yet, corporal fat distribution is not evenly distributed among different sexes and races [6, 7, 10]. In this sense, retroperitoneal fat is not constant between people with the same BMI of different races and gender. In the current study, we demonstrated that there is a positive association between the tract length and BMI, meaning that the percutaneous tract length increases with increasing BMI. Women had a significantly higher median TL overall which is probably due to the fact that more women were morbidly obese ([35 kg/m2) in our cohort when compared to men, although the morbidly obese men presented the highest median TL with 114.7 mm. Allard et al. evaluated in a cohort of 79 stone patients with a mean BMI of 30.02 kg/m2 on pre-interventional CT scans that women have a greater mean skin to stone distance when compared to men (112.6 ± 2.1 vs. 98.6 ± 3.3 mm, respectively, p = 0.025). They defined the skin to stone distance as standardized measurements from the skin to the most lateral midpole calyx. These results support our finding of a significantly greater TL for female versus male patients of 89.6 versus 79.2 mm. In contrast to this previous study, we analyzed only PNL patients (n = 222) and measured the distance to the calyx which was utilized for the procedure. While the previous study reported skin to stone distance, no analysis of related outcomes had been performed [11]. Different factors such as stone burden, stone complexity and stone composition have been related to the outcomes of PNL [4]. The length of the percutaneous tract has been incorporated as a variable for predicting SFR after PNL in a preoperative scoring system [8]. Given the variable distribution of fat in obese patients, tract length may be a more accurate measurement for predicting outcomes after PNL than BMI. In theory, increased tract length could enhance the degree of difficulty of the PNL procedure by limiting the range of motion for rigid instruments through the tract. By traversing more tissue, the risk of bleeding could be increased as well. In the current study, higher transfusion rates were associated with lower BMI; tract length did reveal a similar trend though not statistically significant. This finding may be explained by reduced retroperitoneal fat to tamponade bleeding from the tract. Using multivariable analysis, we could not demonstrate any association between PNL outcomes and the length of the percutaneous tract. Specifically, there was no correlation in SFRs or complications with tract length. Only the presence of staghorn stones was associated with higher overall complications when using multivariable analysis but no factors predicting severe complications could be identified. Treating complex stones is often a more difficult procedure, often requiring multiple tracts and resulting in
decreased in SFRs [12, 13]. In the current study, we only analyzed patients with a single tract approach. Notwithstanding, the presence of partial or complete staghorn stones was significantly related to the risk of failure to achieve stone-free status after PNL in univariable and multivariable analysis. Different reports have been published on the impact of outcomes for PNL in obese or morbidly obese patients. These studies did not describe any difference in the outcomes between the different groups when stratified by BMI [3, 14, 15]. If we consider that TL is highly associated with BMI, these reports are concordant with our findings that there is no correlation between the length of the tract and the outcomes of PNL. The overall stone-free rate (SFR) described in our series is lower than what is typically reported as SFR in other contemporary studies. This finding is related to the strict definition that we used to define stone-free rates (the absence of any residual fragments on the follow-up imaging) and with the large percentage of patients with complex stones (21 % of our patients had incomplete or complete staghorn stones). Additionally, stone-free status was mostly determined using CT imaging and IVPs ([90 %) which are more sensitive than traditional KUB and tomograms for detecting residual fragments [16]. Another contributing factor for this low overall stone-free rate could be the fact that we did not include any patient undergoing auxiliary procedures such as shock wave lithotripsy, staged PNL or secondary ureterorenoscopy. This study has several limitations. It is a retrospective review. The tract length was measured on preoperative CT as a straight line between the skin and the calix of choice in an axial plane. Clinically this does not necessarily represent the exact tract used in each of these procedures, which could be associated with some differences in the real tract length, although we measured the distance to the calyx which was used for the procedure. The stone burden was calculated multiplying two dimension of the stone in the axial plane and not as stone volume. Some previous studies have reported differences in the calculated stone burden between various observers [17]. This finding has been put into perspective by analyzing the outcomes and complications for non-, incomplete and complete staghorn stones, instead of using stone burden exclusively.
Conclusion The length of the percutaneous tract is not related with the outcomes of stone-free rates or complication rates after PNL. Lower BMI and shorter tract lengths were associated with higher transfusion rates.
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Int Urol Nephrol Acknowledgments Gasto´n M. Astroza was supported by the Endourological Society Research Fellowship (sponsored by Cook Medical. Andreas Neisius was supported by a Ferdinand Eisenberger Grant of the Deutsche Gesellschaft fu¨r Urologie (German Society of Urology), Grant ID NeA1/FE-11. Leah Gerber assisted in conducting the initial medical record screening. Conflict of interest of interest.
The authors declare that they have no conflict
Ethical standard This study has been approved by the appropriate ethics committee (IRB) and has therefore been performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments (as revised in Tokyo 2008) as stated in the ‘‘Methods’’ section.
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8. Okhunov Z, Friedlander JI, George AK, Duty BD, Moreira DM, Srinivasan AK, Hillelsohn J, Smith AD, Okeke Z (2013) STONE nephrolithometry: novel surgical classification system for kidney calculi. Urology 81(6):1154–1160. doi:10.1016/j.urology.2012. 10.083 9. Dindo D, Demartines N, Clavien PA (2004) Classification of surgical complications: a new proposal with evaluation in a cohort of 6336 patients and results of a survey. Ann Surg 240(2):205–213 10. Vega GL, Adams-Huet B, Peshock R, Willett D, Shah B, Grundy SM (2006) Influence of body fat content and distribution on variation in metabolic risk. J Clin Endocrinol Metab 91(11):4459–4466. doi:10.1210/jc.2006-0814 11. Allard CB, Shuster A, Pinthus JH, Farrokhyar F, Raees A, Patlas M, Matsumoto ED, Whelan JP (2012) Obesometric factors associated with increased skin-to-stone distances in renal stone patients. Can J Urol 19(6):6554–6559 12. Desai M, De Lisa A, Turna B, Rioja J, Walfridsson H, D’Addessi A, Wong C, Rosette J, On Behalf Of The Croes Pcnl Study Group (2011) The clinical research office of the endourological society percutaneous nephrolithotomy global study: staghorn versus nonstaghorn stones. J Endourol 25(8):1263–1268. doi:10.1089/ end.2011.0055 13. Netto NR Jr, Ikonomidis J, Ikari O, Claro JA (2005) Comparative study of percutaneous access for staghorn calculi. Urology 65(4):659–662. doi:10.1016/j.urology.2004.10.081 discussion 662–653 14. Bagrodia A, Gupta A, Raman JD, Bensalah K, Pearle MS, Lotan Y (2008) Impact of body mass index on cost and clinical outcomes after percutaneous nephrostolithotomy. Urology 72(4):756–760. doi:10.1016/j.urology.2008.06.054 15. Koo BC, Burtt G, Burgess NA (2004) Percutaneous stone surgery in the obese: outcome stratified according to body mass index. BJU Int 93(9):1296–1299. doi:10.1111/j.1464-410X.2004.04862. x 16. Lipkin ME, Preminger GM (2013) Imaging techniques for stone disease and methods for reducing radiation exposure. Urol Clin N Am 40(1):47–57. doi:10.1016/j.ucl.2012.09.008 17. Patel SR, Stanton P, Zelinski N, Borman EJ, Pozniak MA, Nakada SY, Pickhardt PJ (2011) Automated renal stone volume measurement by noncontrast computerized tomography is more reproducible than manual linear size measurement. J Urol 186(6):2275–2279. doi:10.1016/j.juro.2011.07.091
UROLOGY - ORIGINAL PAPER
Does the nephrostomy tract length impact the outcomes of percutaneous nephrolithotomy (PNL)? Gaston M. Astroza • Andreas Neisius • Matvey Tsivian • Agnes J. Wang • Glenn M. Preminger Michael E. Lipkin
•
Received: 13 May 2014 / Accepted: 31 July 2014 Ó Springer Science+Business Media Dordrecht 2014
Abstract Purpose Different factors can determine the outcomes of percutaneous nephrolithotomy (PNL). We analyzed the effect of tract length (TL) on outcomes after PNL. Methods We performed a retrospective review of patients undergoing PNL between 2006 and 2011. Patients with preoperative computed tomography (CT), one percutaneous access tract and follow-up imaging within 3 months were included. TL was defined as distance between the skin to the calyx of puncture as measured on preoperative CT. Measurements were independently performed by two urologists and the average was used for analysis. Stone-free rate (SFR) was defined as zero fragments on follow-up imaging. Factors independently associated with the likelihood of being stone-free after PNL were determined using multivariable analysis adjusted for TL, location of access, the presence of incomplete or complete staghorn calculi and type of follow-up imaging. Complications (Clavien score) were independently assessed.
Results A total of 222 patients were included. Median stone burden and body mass index (BMI) was 239.4 mm2 and 30.5 [interquartile range (IQR): 25.7–36.2]. The median TL was 85.0 mm (IQR: 70.3–100.0) and highly correlated with BMI (q = 0.66, p \ 0.001). A total of 101 patients (45.5 %) were stone-free. TL was not associated with SFR (p = 0.53). Clavien 1 and 2 complications occurred in 38 (17 %) while Clavien 3 and 4 complications occurred in 17 (8 %) patients. Multivariable analysis revealed no association between complications and TL even when adjusted for gender. Conclusions Percutaneous TL is not associated with outcomes of PNL. PNL is a safe and effective treatment for stones in patients with differing body habitus. Keywords Percutaneous nephrolithotomy Percutaneous tract PNL Stone-free rate
Introduction
Gaston M. Astroza and Andreas Neisius contributed equally to this work and also shared first authorship. G. M. Astroza A. Neisius (&) M. Tsivian A. J. Wang G. M. Preminger M. E. Lipkin Division of Urologic Surgery, Duke University Medical Center, DUMC 3167, Durham, NC 27710, USA e-mail: [email protected] G. M. Astroza Department of Urology, Universidad Cato´lica de Chile, Santiago, Chile A. Neisius Department of Urology, Universita¨tsmedizin Mainz, Johannes- Gutenberg University, Mainz, Germany
Percutaneous nephrolithotomy (PNL) remains the first-line treatment for renal calculi C2 cm and for lower pole stones C15 mm [1, 2]. PNL is also indicated for treatment of smaller stones in obese patients since shock wave lithotripsy has been demonstrated to have inferior success rates in this cohort of patients [3, 4]. Outcomes of PNL are related to different factors such as stone burden, stone location, anatomical factors and not necessarily obesity [5]. Yet, different types of obesity have been defined showing that body fat distribution is not constant among people and races [6, 7]. Therefore, the length of the percutaneous tract in patients with similar BMI values undergoing PNL could differ based on various body types and different quantities of retroperitoneal fat.
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Int Urol Nephrol
Scoring systems or nomograms that include the length of the percutaneous tract as a variable have been recently reported [8]. However, it is unclear whether tract length impacts stone-free and complication rates after PNL. We analyzed the effects of tract length on stone-free and complication rates of patients undergoing PNL in the prone position.
Methods After institutional review board approval, a retrospective review of patients undergoing prone PNL between June 2006 and June 2011 was performed. Only patients with data on preoperative body mass index (BMI in kg/m2), preoperative computerized tomography (CT) and follow-up imaging within at least 3 months after PNL were included in this analysis. To decrease the number of confounding factors, patients with multiple percutaneous tracts during the procedure and staged procedures were excluded from this analysis. Tract length (TL) was determined on the preoperative CT as the distance between the skin and the surface/lateral edge of the calix (in millimeters) which was utilized for the puncture. Preoperative imaging was performed using the institutional stone protocol CT which is a lowdose CT in prone position. Surgical procedures were performed in a similar fashion (prone). The site of puncture and the calix of choice were recovered from the surgical report and intraoperative fluoroscopy images. Tract length was uniformly measured at an angle of 25–30° from a vertical line drawn between the anterior wall of the abdomen and the spinous process. The distance between the skin surface and the punctured posterior calix was established as TL (Fig. 1). All punctures were performed by the interventional radiologist in association with an endourologist at the time of surgery in the operating room with the patient prone, using the most posterior calix at the moment of the surgery. All procedures were performed by two experienced surgeons (MEL and GMP). The measurements were obtained independently from two fully trained urologists (GMA and AN) and both measurements were compared for concordance. The averaged distance between the two original measurements was used for the final analysis. Stone burden was calculated by multiplying the anteroposterior and latero-medial dimensions of the stone on axial images from the preoperative CT. In cases of multiple stones, the three largest stones were measured and the stone burden was considered the sum of the three. Complications were assessed from the medical records of each patient and classified using the Clavien–Dindo
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Fig. 1 Example of an axial image to demonstrate the method used to determine the length of the percutaneous tract
system [9]. Clavien grades 1 and 2 were defined as minor, whereas grades 3 through 5 were considered severe complications. Stone-free rates (SFRs) were determined with intravenous pyelography (IVP), CT and KUB and tomograms at 3 months postoperatively. Stone-free was defined as the complete absence of any residual fragments. Correlation of the measurements between both urologists and between TL and the BMI was determined using the Pearson correlation statistics. Comparisons between groups were carried out using parametric and nonparametric tests for continuous variables and Fisher’s exact tests for categorical variables, as appropriate. Odds of becoming stone-free after PNL were determined using crude and multivariable-adjusted logistic regression controlling for gender, BMI, type of imaging to assess stonefree status, tract length, location of puncture (lower vs. mid vs. upper pole) and the presence of staghorn calculi (nonstaghorn vs. incomplete vs. complete staghorn). Staghorn stones were defined as a stone in the renal pelvis and filling multiple calyces, incomplete staghorn stones were defined as stones filling the renal pelvis and only one single calyx. All analyses were conducted using R v2.13 (the R Foundation for Statistical Computing, Vienna, Austria) with ‘Hmisc’, ‘rms’ and ‘gmodels’ libraries. All tests were twosided and p values \0.05 were considered statistically significant. Data are presented as median [interquartile range (IQR)] or number (percent) unless otherwise specified.
Results A total of 510 patients who underwent PNL at our institution from 2006 to 2011 were identified, and of these 222
Int Urol Nephrol Table 1 Distribution of patients with different BMI and respective nephrolithotomy tract length adjusted by gender
Variable BMI (kg/m2)
Overall
Male
30.5 (25.7–36.2)
Female
29.4 (25.4–34.1)
p value
31.3 (26.3–37.4)
0.095
BMI, categories
0.251
\25
46 (20.7 %)
22 (22.7 %)
24 (19.2 %)
25–29.9
60 (27.0 %)
29 (29.9 %)
31 (24.8 %)
30–34.9
55 (24.8 %)
26 (26.8 %)
29 (23.2 %)
C35
61 (27.5 %)
20 (20.6 %)
41 (32.8 %)
Length of tract (mm)
85.0 (70.3–100.0)
79.2 (66.6–97.0)
89.6 (77.1–101.9)
0.007
BMI \25
63.3 (55.1–76.1)
70.7 (56.0–80.2)
60.9 (48.1–68.4)
0.115
BMI 25–29.9
79.3 (70.0–86.9)
85.5 (79.8–95.0)
74.7 (65.6–79.1)
\0.001
BMI 30–34.9
85.9 (74.1–98.1)
89.9 (82.6–100.3)
84.4 (68.2–90.9)
0.046
106.2 (94.1–119.4)
114.7 (103.6–140.0)
104.5 (93.2–113.2)
0.005
BMI C35
Table 2 Univariable and multivariable analyses of factors associated with stone-free status Variable
Univariable
Multivariable
OR (95 % CIs)
p value
OR (95 % CIs)
p value
Female
ref
–
ref
–
Male
1.79 (1.05–3.08)
0.033
2.05 (1.10–3.89)
0.026
1.00 (0.97–1.03)
0.828
1.00 (0.96–1.05)
0.854
CT
ref
–
ref
–
IVP
2.26 (1.22–4.30)
0.011
2.70 (1.37–5.45)
0.005
KUB
1.03 (0.32–3.06)
0.965
0.90 (0.26–2.89)
0.868
Tract length
1.00 (0.99–1.01)
0.538
1.00 (0.98–1.01)
0.587
Inferior
ref
–
ref
–
Mid
0.60 (0.32–1.09)
0.094
0.61 (0.31–1.19)
0.151
Superior
0.51 (0.23–1.09)
0.086
0.68 (0.29–1.58)
0.368
Gender
BMI Imaging
Fig. 2 Association between BMI and tract length by gender
patients met inclusion criteria. Ninety-seven (43.7 %) were males and 125 (56.3 %) were females. The median age of the group was 53.5 years (43–63). When stratified by gender, the median age was 56 (54–65) years for men and 51 (42–60) years for women, p = 0.081. The median BMI was 30.5 (25.7–36.2) overall with no statistical difference between men and women. Table 1 summarizes the distribution of patient characteristics and tract lengths by gender and BMI in four groups (\25, 25–29.9, 30–34.9 and C35 kg/m2). Measurement of the tract length was significantly associated between the two urologists (q = 0.88; p \ 0.001) with a median tract length of 85.0 mm (70.3–100.0). There was a statistically significant association between tract length and BMI (q = 0.69; p \ 0.001) and a significant different TL between males [79.2 mm (66.6–97.0) and females 89.6 (77.1–101.9)], p = 0.007 (Fig. 2). Access was obtained through the upper (19 %), mid (48 %) or lower pole (33 %) with a significantly different TL of 85.7 (69.1–100.7), 81.8 (65.5–93.1) and 92.6 (76.3–106.1), respectively (p = 0.025).
Tract location
Staghorn Not staghorn
ref
–
ref
–
Incomplete
0.40 (0.17–0.91)
0.034
0.30 (0.12–0.72)
0.009
Complete
0.13 (0.02–0.46)
0.007
0.13 (0.02–0.51)
0.010
ref reference value
Seventeen patients (7.7 %) had a complete staghorn stone, 30 (13.5 %) an incomplete staghorn stone, and 175 (78.8 %) were classified as non-staghorn stones. The median stone burden was 239.5 mm2 (150–422). Higher stone burden was strongly associated with staghorn calculi
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Int Urol Nephrol Table 3 Specific complications Number (%) Clavien grade 1 Postoperative fever
Description
Table 4 Multivariable analyses of risk factors for overall and severe complications Variable
25 (11) 12 (5)
Overall complications
Severe complications
OR (95 % CIs)
OR (95 % CIs)
p value
p value
Gender
Pain
1 (0.5)
Prolonged hospital stay
Female
ref
–
ref
–
Urinoma
2 (1)
Conservative management
Male
0.126
3 (1)
0.79 (0.25–2.41)
0.680
Pleural effusion
0.58 (0.28–1.16)
Perforation
4 (2)
Prolonged stent required
BMI
w/o transfusion Perioperative
0.99 (0.90–1.08)
0.886
2 (1) 1 (0.5)
0.97 (0.91–1.02)
0.228
Gross hematuria SPO2 low
Tract length
1.01 (0.99–1.03)
0.618
1.01 (0.98–1.04)
0.390
w/o embolization or intervention
Location Inferior
Clavien grade 2
13 (6)
Transfusion
10 (5)
Atrial fibrillation
2 (1)
UTI
1 (0.5)
Clavien grade 3
3 (1) 2 (1)
Bleeding requiring embolization
6 (3)
Fragments requ URS DVT ? PE MI Sepsis Total complications
ref
–
0.818
2.90 (0.81–14.0)
0.130
Superior
1.19 (0.46–2.98)
0.710
2.54 (0.51–14.0)
0.252
Not staghorn
ref
–
ref
–
Incomplete
0.85 (0.29–2.19)
0.749
2.09 (0.43–7.89)
0.306
Complete
3.26 (1.13–9.57)
0.028
1.75 (0.25–7.84)
0.507
Staghorn
2/6 Required a blood transfusion
2 (1) 4 (2) 1 (0.5) 1 b (0.5)
Required blood transfusion
ref reference value
2 (1) 55 (25)
[non-staghorn 199 mm2, (138–334), incomplete 415 mm2 (298–697) and complete 604 mm2 (480–963), p \ 0.001]. A total of 101 patients (45.5 %) were stone-free (the absence of any fragment on follow-up imaging) postoperatively at 3 months. Patients with single stones showed a trend to be more likely to become stone-free (56/109) with 51.4 % compared to patients with multiple stones (45/113) with 39.8 % but without reaching statistical significance, p = 0.106. The majority of patients (n = 143) received an IVP, n = 61 patients underwent CT imaging and n = 18 had a KUB and tomograms for follow-up after 3 months. Patients receiving an IVP were twice more likely to achieve stone-free status (OR 2.26, 95 % CIs 1.22–4.30, p = 0.011) compared to those evaluated with a CT. There was no association between the length of the percutaneous tract and likelihood of being stone-free (OR 1.00, 95 % CIs 0.99–1.01, p = 0.538). Moreover, BMI and tract location (superior, middle or inferior calyx) were not associated with SFRs on unadjusted analysis. Male gender and the presence of incomplete or complete staghorn calculi showed an association with SFRs using univariate testing with p = 0.033, 0.034 and 0.007, respectively (Table 2).
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–
1.09 (0.52–2.33)
13 (6)
Persistent leakage Pleural effusion
Clavien grade 4
Requiring prolonged iv antibiotics
ref
Mid
These findings remained essentially unchanged on multivariable analyses showing male gender, staghorn stones and type of imaging, but not TL, to be associated with stone-free status (Table 2). On univariable analyses, transfusion rates (transfusion of packed RBCs) were associated with lower BMI [23.6 (19.3–38.5) in transfused vs. 30.6 (26.5–32.0) in nontransfused, p = 0.030]. Shorter TL revealed a similar trend whereby patients receiving transfusions had shorter TL [63.6 mm (55.0–92.7) vs. 85.4 mm (71.3–100.1), p = 0.055]. Overall, 55 patients (25 %) experienced postoperative complications (Table 3). Thirteen patients (6 %) had Clavien grade 3 complications and four patients (2 %) had grade 4 events. In multivariable analyses the presence of staghorn calculi was associated with increased odds of overall complications [OR 3.26 (1.13–9.57), p = 0.028], whereas we could not identify any factors statistically associated with severe complications (Table 4).
Discussion The length of a percutaneous nephrostomy tract is principally determined by the amount of subcutaneous fat, back
Int Urol Nephrol
muscles and retroperitoneal fat. Yet, corporal fat distribution is not evenly distributed among different sexes and races [6, 7, 10]. In this sense, retroperitoneal fat is not constant between people with the same BMI of different races and gender. In the current study, we demonstrated that there is a positive association between the tract length and BMI, meaning that the percutaneous tract length increases with increasing BMI. Women had a significantly higher median TL overall which is probably due to the fact that more women were morbidly obese ([35 kg/m2) in our cohort when compared to men, although the morbidly obese men presented the highest median TL with 114.7 mm. Allard et al. evaluated in a cohort of 79 stone patients with a mean BMI of 30.02 kg/m2 on pre-interventional CT scans that women have a greater mean skin to stone distance when compared to men (112.6 ± 2.1 vs. 98.6 ± 3.3 mm, respectively, p = 0.025). They defined the skin to stone distance as standardized measurements from the skin to the most lateral midpole calyx. These results support our finding of a significantly greater TL for female versus male patients of 89.6 versus 79.2 mm. In contrast to this previous study, we analyzed only PNL patients (n = 222) and measured the distance to the calyx which was utilized for the procedure. While the previous study reported skin to stone distance, no analysis of related outcomes had been performed [11]. Different factors such as stone burden, stone complexity and stone composition have been related to the outcomes of PNL [4]. The length of the percutaneous tract has been incorporated as a variable for predicting SFR after PNL in a preoperative scoring system [8]. Given the variable distribution of fat in obese patients, tract length may be a more accurate measurement for predicting outcomes after PNL than BMI. In theory, increased tract length could enhance the degree of difficulty of the PNL procedure by limiting the range of motion for rigid instruments through the tract. By traversing more tissue, the risk of bleeding could be increased as well. In the current study, higher transfusion rates were associated with lower BMI; tract length did reveal a similar trend though not statistically significant. This finding may be explained by reduced retroperitoneal fat to tamponade bleeding from the tract. Using multivariable analysis, we could not demonstrate any association between PNL outcomes and the length of the percutaneous tract. Specifically, there was no correlation in SFRs or complications with tract length. Only the presence of staghorn stones was associated with higher overall complications when using multivariable analysis but no factors predicting severe complications could be identified. Treating complex stones is often a more difficult procedure, often requiring multiple tracts and resulting in
decreased in SFRs [12, 13]. In the current study, we only analyzed patients with a single tract approach. Notwithstanding, the presence of partial or complete staghorn stones was significantly related to the risk of failure to achieve stone-free status after PNL in univariable and multivariable analysis. Different reports have been published on the impact of outcomes for PNL in obese or morbidly obese patients. These studies did not describe any difference in the outcomes between the different groups when stratified by BMI [3, 14, 15]. If we consider that TL is highly associated with BMI, these reports are concordant with our findings that there is no correlation between the length of the tract and the outcomes of PNL. The overall stone-free rate (SFR) described in our series is lower than what is typically reported as SFR in other contemporary studies. This finding is related to the strict definition that we used to define stone-free rates (the absence of any residual fragments on the follow-up imaging) and with the large percentage of patients with complex stones (21 % of our patients had incomplete or complete staghorn stones). Additionally, stone-free status was mostly determined using CT imaging and IVPs ([90 %) which are more sensitive than traditional KUB and tomograms for detecting residual fragments [16]. Another contributing factor for this low overall stone-free rate could be the fact that we did not include any patient undergoing auxiliary procedures such as shock wave lithotripsy, staged PNL or secondary ureterorenoscopy. This study has several limitations. It is a retrospective review. The tract length was measured on preoperative CT as a straight line between the skin and the calix of choice in an axial plane. Clinically this does not necessarily represent the exact tract used in each of these procedures, which could be associated with some differences in the real tract length, although we measured the distance to the calyx which was used for the procedure. The stone burden was calculated multiplying two dimension of the stone in the axial plane and not as stone volume. Some previous studies have reported differences in the calculated stone burden between various observers [17]. This finding has been put into perspective by analyzing the outcomes and complications for non-, incomplete and complete staghorn stones, instead of using stone burden exclusively.
Conclusion The length of the percutaneous tract is not related with the outcomes of stone-free rates or complication rates after PNL. Lower BMI and shorter tract lengths were associated with higher transfusion rates.
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Int Urol Nephrol Acknowledgments Gasto´n M. Astroza was supported by the Endourological Society Research Fellowship (sponsored by Cook Medical. Andreas Neisius was supported by a Ferdinand Eisenberger Grant of the Deutsche Gesellschaft fu¨r Urologie (German Society of Urology), Grant ID NeA1/FE-11. Leah Gerber assisted in conducting the initial medical record screening. Conflict of interest of interest.
The authors declare that they have no conflict
Ethical standard This study has been approved by the appropriate ethics committee (IRB) and has therefore been performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments (as revised in Tokyo 2008) as stated in the ‘‘Methods’’ section.
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