The assessment of urinary stone fragility

Field: Medicine (Health Sciences)

Extracorporeal shock wave lithotripsy (ESWL) is the treatment of choice for the majority of patients presenting with symptomatic urinary tract calculi. However, at the present time it remains impossible to predict with any degree of certainty those patients in whom ESWL is likely to be successful. Although ESWL remains a successful treatment in the majority of patients the expanding use of ESWL to more difficult cases means that in some patients successful stone fragmentation will not occur. The ability to correctly predict in advance those patients in whom ESWL is likely to succeed would allow Urologists to accurately plan treatments and avoid wastage of precious time and resources. The aim of this Thesis was to investigate four techniques to determine whether any of them were able to predict stone fragmentation as measured either by in vitro ESWL fragility, or by microhardness testing on a Shimadzu indenter. The four techniques were; o Porosity estimation o Density estimation o Low-angle X-ray Diffraction (LAXD) o Magnetic Resonance Imaging (MRI) Of these four techniques only density and porosity showed any identifiable trends between stones of different chemical type. The "actual" density (density of the stone material excluding the effect of the pore spaces) was shown for the first time to vary according to the chemical type of stone under consideration. The lithotripsy experiment allowed quantification of ESWL fragility for the first time, with the development of the ESWL score. This will be useful for future studies as it also incorporates the fragmentation pattern of the calculus. Both the ESWL fragility results and the Vickers Hardness number showed clear differences between stones of different chemical type although these two techniques, when compared with each other, showed only a loose correlation. None of the four techniques under investigation showed a significant correlation with Vickers Hardness number. Of the four techniques only porosity and density results were shown to be significantly correlated with ESWL fragility. In conclusion; the prediction of ESWL fragility is not yet possible by available techniques. Porosity and density measurements showed some promise, and these should form the basis of future study. Research is also in progress to improve the results of LAXD. The results of this Thesis have contributed towards an understanding of the difficulty of predicting the results of ESWL. Current views on shock wave physics and the way shock waves are thought to cause stone fragmentation are also discussed. Recent evidence suggests that chemical composition may affect the results of ESWL through the effect that the former has on stone porosity and microstructure, and literature is cited to support these views. Further work on the microstructure of calculi is required to determine the effect of stone composition on stone structure and, in turn, the effect this has on stone fragility. Ultimately it is hoped that this research will yield more information about the way in which stone fragmentation occurs, where in the stone it occurs, and how we can develop techniques to predict with certainty when it will occur.