Wess O et al., 2018: Fragmentation of brittle material by shock wave lithotripsy. Momentum transfer and inertia: a novel view on fragmentation mechanisms
Othmar J. Wess, Juergen Mayer
Storz Medical AG, Lohstampfestrasse 8, 8274, Taegerwilen, Switzerland.
Shock wave lithotripsy is the only non-invasive stone therapy and in clinical use since 1980. In spite of decades with millions of patients treated, the mechanism of fragmentation is still under debate. Detailed knowledge of the fragmentation process is required for improvements regarding safety and efficiency. The purpose of this paper is to gain a deeper insight into the mechanism of fragmentation by drawing attention to basic physical laws of inertia and momentum transfer. Many fragmentation
experiments are based on the overall efficiency of multiple shock waves in crushing kidney stones or artificial model stones utilizing small baskets or latex pouches. Due to the high dynamic nature of the fragmentation process, in vitro and in vivo, a detailed action of a single shock wave on a particular stone target is difficult to investigate. We utilized a bifilar stone suspension, which allowed us to observe horizontal movements of model stones, their return to the initial position and orientation for repeated exposure of separate identical shocks. The method does not describe the entire fragmentation process in detail but elucidates a mechanism, which may be effective throughout shock wave lithotripsy in general. Measurements on model stones in water revealed forces in the range of 370 N, acceleration values of 100,000–200,000 m/s2 (≈ 10,000 g) and gained momentum of 3.7 × 10− 4 kg m/s we consider sufficient to break most human urinary stones. Fracture patterns of repeated identical shock waves show typical features supporting spallation (Hopkinson effect) and the mechanism of momentum transfer. Schlieren and photo-elastic images provide a visual impression of spatial stress in a transparent acrylic glass cylinder, cavitation fields outside and at the surface of the cylinder, which are compatible with the inertia model. The proposed mechanism covers coarse as well as fine fragmentation. Collapsing cavitation bubbles may have an impact on the fragmentation process but although expected, a direct action of micro-jets on sample surfaces could not be detected.
Received: 22 May 2018 / Accepted: 27 November 2018 © The Author(s) 2018
Original Paper: https://doi.org/10.1007/s00240-018-1102-6. FREE ARTICLE
Understanding the mechanism of shock wave fragmentation is of essential relevance for the design of a shock wave lithotripter. Safety and efficiency of shock wave exposure is significantly dependent on field parameters of the shock waves. In the early days of shock wave lithotripsy (SWL) the maximum peak pressure was considered the most important feature. Currently, a more complex theory of circumferential squeezing, certain wave reflections and collapsing cavitation bubbles are made responsible for efficient fragmentation. Accordingly, a wide-focus- low-pressure strategy is often favoured.
For the first time this paper throws a light on the momentum of a shock wave as the dominant factor for stone fracture. A simple but original experiment revealed unexpected high forces and acceleration values in the range of some hundred Newton (N) and 100,000–200,000 m/s2 (≈ 10,000 g) respectively. These values are sufficient to break most of the known urinary stones.
The mechanism behind is well-founded on the basic laws of Newton of momentum transfer and inertia forces.
The paper shows no evidence of circumferential squeezing and little influence of cavitation.
The stringent demand for a wide focus is not supported by the results of the paper. The new theory is applicable to small as well as wide focussing.
Interestingly, the paper challenges current and well-accepted theories of fragmentation with different mechanisms for coarse and fine fragmentation. The new momentum transfer and inertia model covers both phases (coarse and fine) with one theory since inertia is an intrinsic feature of small and bulky masses.
Due to the new and differing approach to fragmentation process, a critical dispute of the presented mechanism will be stimulating.