Studies On Mechanical Mechanism About Stone Comminution And Tissue Trauma In Extracorporeal Shock Wave Lithotripsy

Extracorporeal Shock Wave Lithotripsy (ESWL) invented in 1980 is one of the most significant achievements of modern medical treatment technology. The kidney stone inside the patient can be broken down into small fragments, by firing shock waves with high energy density and cavitation effect by underwater discharge or explosion during the ESWL treatment. Since ESWL emerged, due to its convenience of the treatment, high efficiency of the stone comminution, without invasive damages of surgery, less clinical costs and etc., it has rapidly become the most common clinical treatment of kidney stone disease. However, ESWL still should be improved, because the side effects, different short- and long-term damages, are observed by the clinical use and laboratory researches of ESWL.A common side effect of the ESWL treatment is the presence of blood in the urine (hematuria). There are others too, such as chronic hemorrhage, hematoma in kidney parenchyma, kidney edema and etc.. Therefore, both efficiency and security of the treatment are very important for the shock wave therapy. In order to maximize the efficiency of stone comminutions and minimize the side effects in the ESWL treatment, it is especially meaningful to locate exactly the dynamical focus of shock waves and to gain good insight in the evolution of the negative pressure as well as the cavitation effect. With the support of National Natural Science Foundation of China(20472316)and Doctoral Fund of Ministry of Education of Chin(a20070611004), we completed the numerical simulations of the process about shock wave focusing and negative pressure evolution, and the numerical simulations of cavitation. In this paper some important scientific data to maximize stone comminution and decrease tissue damage of the piezoelectric and the electro-hycaulic ESWL are shown. The data are relevant to design, manufacture and clinical applications of ESWL.Shock wave is a nonlinear wave, for which some physical laws of the linear waves, such as Snell reflection law, are valid no more. For the numerical research of focusing of underwater shock waves the compressibility of water must be considered. In this paper, the characters of water were described by Tait's state equation. It was also supposed that water was inviscid and there was no heat conduction or radiation during the movement. So by the help of the methods of shock wave of gas dynamics, we could investigate the propagating, reflecting and focusing of the underwater shock waves in ESWL. The negative pressure waves of the underwater shock wave in ESWL are generally considered as the main cause of the cavitations.CCW (Chester-Chisnell-Whitham) method of Whitham's shock wave dynamics is an approximation. It is simple but effective in the tracing the shock wave fronts under some conditions. CCW method can be utilized to describe all the nonlinear process of the shock wave front. In this paper, a numerical code of the geometric shock wave dynamics method CCW was made and applied to track the successive positions of the fronts of shock waves of the spherical piezoelectric ESWL. Some important scientific results to design, manufacture and clinical application of the piezoelectric lithotripter were given.CE/SE (space-time conservation element and solution element) method, proposed in 1995, is already become one of the primary methods in the second generation of the CFD scheme of NASA, due to its good stability, calculation accuracy and high resolution. Based on the appropriate transformation to Zhang's modified CE/SE method in order to discrete Euler equations, in this paper a two-dimensional numerical model of the studying to the underwater focusing shock wave of ESWL was supposed, and the flow fields of shock waves were numerically simulated for one-side source and both-side wave sources of both piezoelectric and electric-hydraulic ESWL. In this paper, moreover, by the CE/SE method the cavitation flow was described with the continuous model, using Liu model and Cut-off model as the state equation respectively, to the numerical simulation of the pressure, speed, and density distribution of the one-dimensional channels in different initial conditions when there were cavitation flow.Numerical results show that: 1. The dynamic (non-linear) focus of the shock wave is generally different from the geometric (linear) focus in ESWL, there is often some deviation. Therefore, in the clinical treatment of spherical piezoelectric ESWL, the actual focus should be defined as the greatest pressure point and located as the target of firing, rather than the geometric focus. 2. It is the first time to apply CE / SE method to the problems of cavitation in this paper. The numerical results show that the CE / SE method is an effective method to study of cavitation. 3. The cavitations are inevitable during the focusing of shock waves in EWSL, therefore, for the design, manufacture and clinical applications of EWSL cavitation can not be ignored.All the numerical simulation programs except graphic software are written by the author with the FORTRAN.