The dual wavelength Nd:YAG (multi-YAG) laser has excellent stone ablating properties. To investigate the safety of this laser for fragmenting ureteral calculi, an ex vivo animal study was undertaken to study acute tissue interactions associated with multi-YAG laser lithotripsy. Human ureteral calculi were implanted in ureters harvested from swine (n equals 42). Direct vision ureteroscopic laser lithotripsy was performed while varying pulse energy (0.3 to 1.5 Joules) and pulse frequency (5 to 15 Hz). All ureters were then examined histologically, graded for tissue injury, and compared to controls. Photofragmentation was associated with mucosal denudation and/or focal mucosal necrosis (grade 0 and 1) in 37/42 cases. Four treatments caused necrosis involving up to two thirds of the ureteral wall (grade 2) and 1/42 had grade 3 changes (transmural necrosis). Ureteroscopic examination alone produced grade 0 to 1 injuries. Logistic regression analysis revealed that pulse energy (p equals 0.47), total energy used for fragmentation (p equals 0.82), and stone weight (p equals 0.64) were not significant predictors of higher grade tissue injury. Pulse frequency (p equals 0.14) began to approach significance. Of the five ureters with grade 2 or greater injury, four were associated with a pulse frequency of 15 Hz. Our findings show that multi-YAG laser lithotripsy is associated with acute tissue changes ranging from mucosal denudation to different levels of coagulative necrosis. Low grade injury is most common and would have minimal potential for strictures. Although higher grade tissue injury is possible, all grade 2 and 3 injuries were microscopically focal in nature. Therefore, multi-YAG laser lithotripsy can be performed with acceptable levels of acute tissue injury at pulse energies up to 1.5 Joules and at pulse frequencies less than 15 Hz.
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