Among the methods of intracorporeal lithotripsy discussed in this chapter, EHL has the highest risk of acute tissue damage when used to treat ureteral stones. Although considered safe and effective for the treatment of bladder stones, early studies on the ureters of cadavers demonstrated the Urat-1 was capable of rapidly inflicting tissue damage and ureteral perforation [11]. Direct application of the Urat-1 to the abdominal wall and bladder mucosa of anesthetized canines was found to cause blistering and hemorrhage through the full thickness of the dermis and epidermis of the abdomen, and similar effects through the mucosa and detrussor muscle of the bladder [12]. High speed video of spark discharges at the tip of EHL probes revealed that the blast energy of the cavitation bubbles cause only small areas of trauma, limited in size to the diameter of the probe, even with the probe in direct contact with sections of opened human ureter. However, when confined within in the lumen of the ureter, the blast energy cannot be effectively dispersed and it causes rapid expansion of the ureteral wall and complete disruption of the ureter in a longitudinal fashion, as far as 1 cm from the tip of the probe [43]. To better characterize the immediate and potential long-term ureteral damage of EHL, the probe of a second generation EHL lithotrite was directly applied in a perpendicular fashion to the ureters of two anesthetized pigs. Histological samples were then analyzed at the time of injury, 1 day, and 6 days after injury. Within the ureter EHL produced an immediate partial abrasion of the urothelium and caused edema within the underlying lamina propria. By day one the urothelium was totally abraded and the edema in the lamina propria was increased and extended into the ureteric smooth muscle. Analysis of the ureteral sections 6 days after injury revealed worsening damage and reactive hyperplasia with excessive mucous secretion. This extensive damage can be partially attributed to the initial blast injury caused by the rapid expansion and collapse of the plasma bubble. To an even greater degree, full-thickness ureteral damage is due to the heat that is rapidly generated at the probe tip [44].

In an ex vivo model utilizing sections of pig ureter, application of EHL at the lowest power setting produced visible perforation of the ureter within approximately 25 pulses [45]. To reduce the risk of collateral tissue damage, a plasma shield was developed for the EHL probe. Composed of a stainless steel coil and a perforated end cap, the plasma shield was designed to fit the distal end of the EHL probe and confine the plasma within the coil. The energy was then focused through a hole in the end of the cap that reduced energy transfer to the tissue. In a study employing rabbit bladders, an unshielded EHL probe perforated the bladder in two pulses, compared to a mean of 35 pulses required to perforate with a shielded EHL probe [46].

While the EHL remains an efficient and economical tool for the treatment of some bladder stones, the propensity for tissue damage has limited its role for treatment of stones within the ureter.

In contrast to the early outcomes of EHL, PL the novel Swiss Lithoclast appeared to be effective and safe for use in the treatment of ureteral stones. Deliberate attempts to perforate the ureters of anesthetized female swine with 0.8 and 1.0 mm rigid Lithoclast probes resulted in perforation with the 0.8 mm probe, but not until after delivery of 250 consecutive shocks. Perforation of the ureter with the 1.0 mm probe was deemed impossible. Although no macroscopic lesions were visible on the ureteral epithelium, abrasion, and edema were seen on histologic analysis of treated sections of ureter [44]. Similar areas of transmural edema, abrasion, and hemorrhage were identified microscopically in sections of pig ureter subjected to direct firing of a 3.5 F Lithoclast probe for 5–7 s. In the same experiment, animals sacrificed 3 weeks later had no evidence of ureteral perforation on retrograde pyelography, and treated areas of ureteral tissue showed no microscopic evidence of damage. Animals sacrificed at 6 weeks posttreatment also had normal retrograde pyelograms and no direct evidence of microscopic tissue trauma to treated ureteral sections [47]. The safety of PL was further confirmed when the direct application of a 3 F rigid probe for 6 min of continuous activation failed to perforate ex vivo sections of pig ureter, even with “vigorous attempts to impale the ureter” [45].

An extensive study of the safety of PL, including its use to treat plaster-of-Paris stone phantoms in swine ureters and stones in the kidneys, ureters, and bladders of human patients, revealed that even direct and deliberate application to soft tissues results in minimal trauma. Among the 37 ureteral stones treated in the human portion of the study, one patient suffered a minor ureteral perforation attributed to “overenthusiastic ureteroscopy.” Follow-up intravenous urograms of 25 patients obtained 1 month after PL demonstrated no ureteral injuries [48].

Some of the earliest experiments to assess the effects of USL on the tissue of the ureter were performed in dogs. A 30-s application of the USL probe to human stones implanted in the canine ureter was followed by direct application of the probe to the ureteral mucosa for 5, 10, and 15-s intervals. Gross and histological examination of the ureteral tissue was performed after 48 and 72 h. No gross trauma to the ureteral wall was noted during applications of 10 s or less. Applications of the USL probe directly to the ureteral wall for 15 s caused visible and histologic changes consistent with thermal damage. The investigators theorized that this thermal damage could easily be prevented during routine use of the USL when pulses of 10 s or less are used and the buildup heat at the probe tip is avoided. Because this study was conducted prior to the introduction of ureteroscopy and direct vision intracorporeal lithotripsy, the main cause of most ureteral perforation was actually excessive force applied to the probe, and not ultrasound energy or the resultant heat [34]. A second study examining the effect of USL on the canine bladders revealed that direct contact application of the probe to the urothelium for 10 min produced only modest long-term histologic changes in sections examined 1 month after treatment [31]. A test of the ability of USL probe to cause perforation of swine ureter after 12 pulses of 3 s each resulted in no perforations and no macroscopic changes in the urothelium. Histologic examination of treated tissue on the day of treatment revealed no changes to the urothelium. Microscopic changes seen 1 and 6 days after treatment were moderate and less extensive than those seen in tissue samples of ureters treated with PL, EHL, or LL. The authors concluded that USL was among the safest of the lithotripsy technologies available at the time [44].

Because the energy of USL is produced by the generation of sound waves, its clinical use presented a unique safety concern regarding the effects of USL on the hearing of patients, physicians, and operating room personnel. In the US workplace noise exposure limits are established and monitored by the Occupational Safety and Health Administration (OSHA). The allowable continuous exposure is 8 h at 90 decibels (dBA), with a 50% decrease in duration mandated for each 5 dBA above 90 dBA. Measurements of the sound levels of the three commercially available USL units during actual cases of intracorporeal lithotripsy showed both decibel levels and exposure durations well below OSHA standards. Additionally, an audiogram performed on a urologist with a 3-year total of more than 40 cases of ultrasonic lithotripsy did not detect any hearing threshold sensitivity deficit [49]. An experimental model used to simulate the use of USL in the ureter during pregnancy concluded that the peak pressure of sound emitted during lithotripsy was unlikely to harm the hearing of the fetus [50].