Extracorporeal shock-wave lithotripsy (ESL) as a medical device was initiated by cocktail party speculation in Munich concerning the origin of small pits on the leading surfaces of airplane wings. When proof became available that the pits were the product of cavitating bubbles that imploded and released enormous energy in a tiny region, speculation turned to using the energy of cavitation for medical purposes.

Cavitating bubbles arise at the surface of many objects subjected to intense, focused energy. Although every bubble is minute, the energy of each one as it implodes approximates the temperature at the surface of the sun: 5000°C. The collective energy can be transmitted to flaws in an appropriate target and reflected through it, just as a diamond cutter seeks cleavage planes along which to cut his stone (Fig. 1) 1277. A laser or other high-energy source can be used for this purpose: the shock waves specifically tailored for medical lithotripsy include electrohydraulic, acoustic, and piezoelectric waves.

The electrohydraulic lithotripter generates an intense underwater spark gap discharge at the F&sub1; focus of an ellipsoidal reflector. This energy is concentrated at the F2; focus.

Transmission of energy without loss obviously demands a medium for coupling. Since the body is largely water, a water bath provides a good coupling. This usually takes the form of a Silastic bag filled with water, applied to the patient's skin with ultrasound gel.

The advantage of an electrohydraulic impulse is its intense energy. Its disadvantage is that the impact is of such high energy that it is uncomfortable for most patients unless they receive intravenous analgesia.

The acoustic generator works on the principle of the diaphragm of a home loudspeaker, but is many times more powerful. It can be focused, in the same way as a spark-gap device, but seems to be more endurable for patients than is the electrohydraulic generator.

Deformation of an array of piezoelectric crystals generates shock waves proportional to the magnitude of incoming voltage. Like a dish for receiving electromagnetic waves from space, the piezoelectric array collects and distributes energy over a broad area. Discomfort during treatment is reduced, but multiple treatments are required to achieve good fragmentation of stones.

Renal calculi are particularly attractive targets for lithotripsy. The physical nature of many is such that they can be pulverized easily and flushed through the ureters. Surgical removal of renal calculi formerly required a hospital stay of 10 to 12 days, and was associated with considerable morbidity. With lithotripsy, the hospital stay can be as short as 2 days.

Pure cholesterol gallstones or those containing only a nidus of non-cholesterol material are a good target for the lithotripter. In the hands of surgeons and physicians skilled in ultrasonic imaging excellent results can be achieved: 91 per cent disintegration of stones in 12 to 18 months. Nonetheless, elimination of the rubble produced is unpredictable unless the bile acid cholesterol solvent ursodeoxycholic acid is taken by mouth so that the gallstone dust becomes completely solubilized (Fig. 4) 1280. Except for a slightly increased frequency of bowel movement, there are no side-effects from use of this adjuvant. There are, however, hazards. For example, fragments of gallstones might drop from the gallbladder and into the pancreatic duct, inciting pancreatitis.

In the United States gallstone lithotripsy is not yet approved as a bona fide treatment. Only one of 10 American groups was able to duplicate the results of the German lithotripters. Thus, when laparoscopic cholecystectomy was introduced in 1988, lithotripsy for gallstones was largely abandoned. However, a single, large cholesterol gallstone precipitating recurrent biliary colic is suitable for lithotripsy as the primary form of treatment. Relief of pain is almost immediate, and the patient can return to work the next day without fear of a catastrophic event. The treatment can be repeated if required, or converted into a laparoscopic cholecystectomy. It could be argued that stones are likely to recur because the gallbladder itself is abnormal if stones were formed in the first place.

Stones in the common bile duct that cannot be removed by radiologically guided stone-crushing instruments in patients who still have a biliary T-tube are other appropriate targets for lithotripsy. The problems are that ductal stones can be up to 2 cm in diameter and multiple, patients are often old and weak, and a nasobiliary tube must be placed within the common duct to provide space for reverberation (Fig. 5) 1281. If all goes well, however, the fragments can be extracted following an endoscopic sphincterotomy of the bile duct, performed before or after the attempt at lithotripsy.

Although gallstone pancreatitis would seem to be a likely hazard of this form of treatment, pancreatitis is rare, but may eventuate long after anyone can remember that lithotripsy treatment was done. Bacteraemia is more common.

The alternative form of treatment if the patient has a T-tube still in place is laser lithotripsy. This involves passage of a fine fibreoptic bundle capable of transmitting laser energy through the T-tube into the area of the common bile duct where the stone is located. A coumarin green dye-tuned laser is used because its optical spectrum does not overlap that of haemoglobin, which is inevitably present in the lining of the bile duct: absorption of laser energy by haemoglobin would cause a ductal perforation. Provided that a choledochoenterostomy or an endoscopic sphincterotomy is in place, stone fragments normally pass into the gut without problems.

Pancreatic stones are even more fragile than are common duct stones. Only 400 shockwaves, rather than the 2000 normally used to fragment stones within the gallbladder, cause these stones to disintegrate. Pancreatic lithotripsy should ideally precede a Puestow procedure (longitudinal pancreaticojejunostomy) routinely.

Stones in the submandibular duct may also be disintegrated by lithotripsy, but whether or not this effort is worthwhile awaits proof. Stones are normally easily extractable under local anaesthesia.

Barkun ANG, Ponchon T. Extracorporeal biliary lithotripsy: review of experimental studies and a clinical update. Ann Intern Med, 1990; 122: 126–37.

Sackmann M, Ippisch E, Sauerbruch T, Holl J, Brendel W, Paumgartner G. Early gallstone recurrence rate after successful shock-wave therapy. Gastroenterology, 1990; 98: 392–6.

Sackmann M, et al. Efficacy and safety of ursodeoxycholic acid for dissolution of gallstone fragments: comparison with the combination of ursodeoxycholic acid and chenodeoxycholic acid. Hepatology, 1991; 14: 1136–41.

Sackmann M, Pauletzki J, Sauerbruch T, Holl J, Schelling G, Paumgartner G. The Munich gallbladder lithotripsy study: results of the first 5 years with 711 patients. Ann Intern Med, 1991; 114: 290–6.

Sass W, Dreyer HP, Kettermann S, Seifert J. The role of cavitational activity in fragmentation processes by lithotripters. J Stone Dis, 1992; 4: 193–207.

Sauerbruch T, Holl J, Sackmann M, Paumgartner G. Extracorporeal shock wave lithotripsy of pancreatic stones. Gut, 1989; 30: 1406–11.

Sauerbruch T, Holl J, Sackmann M, Paumgartner G. Fragmentation of bile duct stones by extracorporeal shock-wave lithotripsy: a five-year experience. Hepatology, 1992; 15: 208–14.

Schneider HT, et al. Pain in extracorporeal shockwave lithotripsy: a comparison of different lithotripters in volunteers. Gastroenterology, 1992; 102: 640–6.

Suslick KS. The chemical effects of ultrasound. Sci Am, 1989; Feb: 80–6.

Tint G, et al. Ursodeoxycholic acid: a safe and effective agent for dissolving cholesterol gallstones. Ann Intern Med, 1982; 97: 351–6.