Pricop C et al, 2016: Has the pelvic renal stone position inside the upper loop of JJ stent any influence on the extracorporeal shock wave lithotripsy results?
Pricop C, Serban DN, Serban IL, Cumpanas AA, Gingu CV.
Department of Urology, "C. I. Parhon" Hospital, "Gr. T. Popa" University of Medicine and Pharmacy, Iasi, Romania.
Department of Physiology and Center for Study and Therapy of Pain, "Gr. T. Popa" University of Medicine and Pharmacy, Iasi, Romania.
Department of Urology, University Hospital, "Victor Babes" University of Medicine and Pharmacy, Timisoara, Romania.
Center of Urological Surgery and Renal Transplantation, Fundeni Clinical Institute, Bucharest, Romania.
BACKGROUND: JJ stents are often encountered in patients with pelvic renal stones referred for shock wave lithotripsy, most of them being placed either for obstructive renal pelvic stones or for ureteric stones mobilized retrograde during the JJ stent insertion. The aim of the study was to determine whether the relative stone position in the upper loop of the JJ stent during extracorporeal shock wave lithotripsy (SWL) influences the efficiency of the procedure. The study was designed as a prospective cohort study on 162 patients addressing the same urological department, with single renal pelvic stone (primary or mobilized to the renal pelvis during the insertion of JJ stent), smaller than 15 mm, with JJ stent, treated by SWL using a second generation spark gap lithotripter, 18 kV, 3000 waves/session. Patients were divided in three groups according to the relative position of the stone to the upper loop of the JJ stent as appears on plain X-ray: stone-inside-loop, loop-crossing-stone and stone-outside the loop. The SWL success rate was the primary outcome of the study. p Value, Chi square and Kruskal-Wallis tests were used for statistical analysis.
RESULTS: For stone-inside-loop cases, SWL efficiency was 22.7 versus 42 % for all the other cases (p = 0.002). Other factors for decreased SWL success rate were: higher stone radio-opacity, larger JJ of stent and obese patients. Study limitation is represented by the relative small study group and by the evaluation of stone density using plain X-ray instead of computer tomography.
CONCLUSIONS: For pelvic renal stones having the same density characteristics studied by plain X-ray, the SWL efficiency is lower in stone-inside-loop cases comparing with the other positions. The overall stone free rate for renal pelvic stones could be explained by the second generation lithotripter used for all procedures.
Springerplus. 2016 Aug 8;5(1): 1284. doi: 10.1186/s40064-016-2954-2. eCollection. 2016
Three situations were considered by the authors: stone inside stent-loop, stone outside stent-loop and stone crossed by the stent. The conclusion was that stones located inside the stent-loop were less well disintegrated than those in other locations relative to the stent. I cannot remember that I personally have encountered that problem and theoretically a stone surrounded by a stent-loop could be expected to disintegrate better as a result of fluid expansion around the stone. Obviously that was not the case and the explanation provided by the authors is that the stent-loop is interposed between the shockwave front and the stone. Thereby the amount of energy is reflected by the stent material and the stone hit by significantly less energy than normally would be the case.
It is likely that in this regard the geometrical relationship between the shockwave, stent and stone plays the major role. Nothing is mentioned about the angle between the stone and the shockwave path but the outcome reported in this study can only occur when the stent, located close to the shockwave-focus absorbs a significant part of the energy.
Three different alternatives are shown below.
The lesson learnt from this report is that when patients with stents are treated it is wise to consider the geometrical relationships and when necessary adjust patient position.