I Have An 18mm Kidney Stone...it Is A Solid Mass

I have an 18mm kidney stone...it is a solid mass caused by Topamax. I don't understand why the procedure for removal, entering the back into the kidney through usage of a needle type device, requires a hospital stay. What is draining out of my kideny after the surgry? Why is there no simplar way of removal of this solid mass? Why is the shock therpay not always successful the first time?