Smith NB et al, 2013: A heuristic model of stone comminution in shock wave lithotripsy
Smith NB, Zhong P
Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, USA
A heuristic model is presented to describe the overall progression of stone comminution in shock wave lithotripsy (SWL), accounting for the effects of shock wave dose and the average peak pressure, P+(avg), incident on the stone during the treatment. The model is developed through adaptation of the Weibull theory for brittle fracture, incorporating threshold values in dose and P+(avg) that are required to initiate fragmentation. The model is validated against experimental data of stone comminution from two stone types (hard and soft BegoStone) obtained at various positions in lithotripter fields produced by two shock wave sources of different beam width and pulse profile both in water and in 1,3-butanediol (which suppresses cavitation). Subsequently, the model is used to assess the performance of a newly developed acoustic lens for electromagnetic lithotripters in comparison with its original counterpart both under static and simulated respiratory motion. The results have demonstrated the predictive value of this heuristic model in elucidating the physical basis for improved performance of the new lens. The model also provides a rationale for the selection of SWL treatment protocols to achieve effective stone comminution without elevating the risk of tissue injury.
J Acoust Soc Am. 2013 Aug;134(2):1548-58. doi: 10.1121/1.4812876
PMID:23927195 [PubMed - in process]
PMCID:PMC3745501 [Available on 2014/8/1]. FREE ARTICLE
After more than 30 years the mechanism of stone fragmentation in lithotripsy is still not completely understood. Understanding basic physical facts would help to develop criteria for lithotripter improvements without increasing side effects. The authors are devoted to this goal and have done impressive and comprehensive scientific research in the past. An important step was to differentiate between the mechanism of first fractures and final disintegration to fine fragments. The heuristic model presented here provides fairly good agreement between theory and experimental results on artificial stones.
The second part of the paper focuses on the development of a new lens design for an electromagnetic lithotripter which is supposed to be superior to the original lens of that specific lithotripter. The inner part of the lens is unmodified whereas the outer ring shaped zone features a cut similar to a Fresnel lens design. This cut reduces the thickness of the lens material in the absorbing outer area and, through this, increases transmission of the primary acoustic energy to the focus. Against expectation however, the primary electrical energy input to the novel shock head has to be increased almost 50%, in order to achieve fragmentation results similar to the original lens.
Another feature of the new lens design is the generation of a delayed shock wave pulse interfering with the central pulse in the focal area. Why this is beneficial, is not clear. It may influence cavitation but increased cavitation may not only increase fragmentation efficiency but also possible side effects such as micro lesions and bleedings.
The peripheral cut reduces the effective aperture of the lens and the -6dB-focus is widened. The fragmentation efficiency is claimed to be increased under respiratory motion due to an increased hit rate. In this respect the new lens may offer an advantage over the original lens but under the acceptance of a higher primary energy dose transmitted through a downsized coupling area. Increased pain and possible skin lesions are probable. It would be interesting to compare performance data of the new lens and the original lens with 50% increased primary input.
Final proof of superiority in clinical use would require larger clinical studies. Usually, higher fragmentation efficiency is accompanied by higher side effects. We wait for the clinical results.