Cavitation-effect lithotripsy

Chinese version

Extracorporeal shock wave lithotripsy (SWL) is a currently widely used non-invasive treatment of kidney-, gall- and bladder- stones, using thousands of focused shock waves generated outside body to smash stones into small fragments which can naturally pass through the urethra. The shock wave source with a water-filled coupling cushion contacts the body directly. This technique was initially developed in 1980 by Dornier Medizintechnik GmbH (now Dornier MedTech Systems GmbH), Germany and has been widely spread since the introduction of the first commercial lithotriptor Dornier HM3 in 1983. There are many mechanisms for the stone fragmentation during SWL, e.g. direct stress, cavitation and fatigue. In this article, only cavitation effect during lithotripsy is briefly introduced. The drawbacks of SWL are also discussed and some alternative techniques for stone crush are introduced.

Numerous studies have confirmed the cavitation effect during lithotripsy. Generally speaking, cavitation plays an important role for the generation of small stone fragments during lithotripsy. Micro-size gas bubbles can be generated due to the presence of the weak spots within biological systems or the passage of the previous strong shock waves. The bubble nucleus are compressed by the compressive part of the shock waves and then expand dramatically generating an intense spherical shock wave, which may significantly influence the behavior of surrounding bubbles and stones. Finally, the bubbles near the stone interface collapse, forming high-speed micro jet with strong erosion ability to fragment the stones.

From left to right: a typical shock wave profile for ESWL; kidney stone before and after ESWL. Adapted from Crum et al. (2008, Fig.1).

Details of bubble cluster collapse at the proximal face of a stone. From Pishchalnikov (2003, Fig.3)

It should be emphasized that if the deliver rate of the shock wave is too fast corresponding to high pulse repetition frequency, energy of shock waves could not be delivered to the stones because of the shielding effects of the bubble cloud, i.e. dissipations of energy through bubble oscillations, generated by previous shock waves.

Fragments of stones after ESWL with different pulse repetition frequency. Adapted from Crum et al. (2008, Fig.2c).

Drawbacks of shock wave lithotripsy are:

1. SWL could cause many adverse effects e.g. haemorrhages, hypertension, thrombi. It may also lead to long-term damage of the kidney.
2. The treatment may be uncomfortable and cause pain to the patient if the stone is positioned near a bone or rib because of a mild resonance caused by the shock waves.
3. The further development of this technique is limited during the past several decades. Comparing with the first commercial lithotriptor Dornier HM3, the other lithotriptors do not show remarkable extra effectiveness.
4. For large stones (e.g. >10mm), the fragments after SWL are still too large to pass urethra naturally.
5. Although SWL is effective to treat kidney stones, it has not received general acceptance for treatment of other types of stones (e.g. gallstone, salivary stone).

Nowadays, the use of SWL is waning especially in Europe and USA and many other techniques have been developed e.g. pyeloscopy. As a minimally invasive technique, pyeloscopy inserts a flexible thin fibre-optic telescope (diameter less then 3mm) into the kidney from the bladder via the urethra. The whole kidney system can be visualized. The laser fibers can efficiently smash stones and micro-baskets retrieves the resulting stone fragments. This technique is applicable to the kidney stones up to 20mm in size.

The passage of pyeloscopy. Source

An emerging non-invasive technique using the cavitation generated by the carefully controlled focused ultrasonic waves is being developed (Link in this blog). The strong erosion ability of cavitation cloud during collapse can shatter the stones into much fine powder (approximately <1mm), which can be easily put out of human body.

References
Crum et al. (2008). Cavitation and Therapeutic Ultrasound, Proceedings of WIMRC Cavitation Forum 2008, University of Warwick, UK, pp.10-14.
Leighton, T.G. and Cleveland, R.O. (2009). Lithotripsy, Proc. IMechE Part H: J. Engineering in Medicine, vol. 224, 317-342.
Matsumoto, Y. (2006). Therapeutic application of acoustic cavitation, Proceedings of WIMRC Cavitation Forum 2006, University of Warwick, UK, Chap.3, pp.27-35.
Pishchalnikov (2003). Cavitation Bubble Cluster Activity in the Breakage of Kidney Stones by Lithotripter Shock Waves, J Endourol.September, 17(7): 435–446.