Christiansen FE et al, 2015: Internal Structure of Kidney Calculi as a Predictor for Shockwave Lithotripsy Success.
Christiansen FE, Andreassen KH, Osther SS, Osther PJ.
Department of Urology, Urological Research Center, Lillebaelt Hospital, University of Southern Denmark , Fredericia, Denmark.
INTRODUCTION: The internal structure of renal calculi can be determined on CT using bone windows and may be classified as homogeneous or inhomogeneous with void regions. In vitro studies have shown homogeneous stones to be less responsive to extracorporeal shockwave lithotripsy (SWL). The objective was to evaluate whether the internal morphology of calculi defined by CT bone
window influences SWL outcome in vivo.
MATERIALS AND METHODS: One hundred eleven patients with solitary renal calculi treated with SWL were included. Treatment data were registered prospectively and follow-up data were collected retrospectively. All patients had noncontrast computed tomography (NCCT) performed before SWL and at 3-month follow-up. The stones were categorized as homogeneous or inhomogeneous. At follow-up, the patient's stone status was registered. Stone-free status was defined as no evidence of calculi on NCCT. Treatment was considered successful if the patient was either stone free or had clinically insignificant residual fragments.
RESULTS: Using simple logistic regression, the odds for being stone free 3 months post-SWL were significantly reduced in the patients with inhomogeneous stones compared with patients with homogeneous stones (odds ratio 0.43 [95% confidence interval 0.20, 0.92; p < 0.05]). However, when adjusting for stone size by multiple logistic regression, including stone size (area) as a covariate, this difference became insignificant.
CONCLUSION: The internal structure of kidney stones did not predict the outcome of SWL in vivo.
J Endourol. 2015 Dec 11. [Epub ahead of print]
This article presents the unexpected observation that the internal morphology of stones treated with SWL was of no or little importance for the final treatment result. The finding is surprising because the general idea is that stones with many reflecting surfaces disintegrate better than those with more homogeneous structure. This is also my personal experience and harness factors for COM and COD were previously determined to 1.3 and 1.0, respectively [ 1]. The authors concluded that only stone size was important for the outcome.
Approximate calculation of STI (stone treatment index)  also give support to the authors’ conclusion when the hardness index for homogeneous stones was set to 1.30 and that for inhomogeneous stones was set to 1.15. The STI-values are shown in the Table.
The interpretation is, however, influenced by the difference in stone size (surface area; SA). That inhomogeneous stones had a median SA twice that of homogeneous stones is problematic for definite conclusion. One note of caution is that so many stones were located in the lower calyx and it is my own assumption that the calyx physiology is negatively affected by large stones in that position. In case of disturbed contraction of the calyx, fragment clearance might be reduced more for large than for small stones. Therefore, despite inhomogeneous structure of stones, residuals can be expected to occur more commonly in those patients.
Nevertheless, the conclusion seems to be that the stones in the size range included in this study were disintegrated to similar extent irrespective of morphology. It is suggested by the authors that only differences in stone size might have influenced disintegration. This assumption is, however, contradicted by the fact that re-treatment rates were similar in the two groups (25% and 26%). But it is obvious that despite the smaller size of homogeneous stones the quotient SMLI / median SA, was 3.63 for those stones, compared with1.67 for inhomogeneous stones.
It is not mentioned if the authors used the standard focus for all patients or if the large focus was used for some patients.
The bottom-line of all this is, however, that good disintegration with the lithotripter used can be expected for both homogeneous and inhomogeneous stones and probably that different forces affect the smaller and larger stones differently.
1. Ringdén I, Tiselius HG. Composition and clinically determined hardness of urinary tract stones. Scand J Urol Nephrol. 2007;41:316-323.
2. Tiselius HG, Ringdén I. Stone treatment index: a mathematical summary of the procedure for removal of stones from the urinary tract.
J Endourol. 2007 ;21:1261-269