Combining Lithoclast and ultrasound power in one device for percutaneous nephrolithotomy: in vitro results of a novel and highly effective technology

Material and methods 

Instrument 

The new device (Fig. 1) consists of a Lithoclast Master (EMS, Nyon, Switzerland) with probes 0.8, 1.0, and 1.2 mm in diameter. The pulse repetition rate varies from 1 to 12 Hz, and the power of the ballistic impulse is adjustable between 0% and 100%. The second component is a newly developed US handpiece with a 3.3-mm hollow US probe. US energy can be adjusted from 0% to 100% at a frequency of 24 to 26 kHz. The two units are combined, with the LC probe positioned off center in the US tube. The tip of the LC probe is advanced about 1 mm out of the US tube. Suction is applied through the hollow US tube.

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FIGURE 1. Handpiece of the novel lithotriptor combining LC and US generator. (Inset) Instrument tip with US tube and LC rod shown in detail.

Foot switch 

A two-paddle foot switch, one for the LC and one for the US unit, activates the new lithotriptor. The paddles can be activated separately or together. The US switch has a two-step mechanism, activating suction only with step one and suction plus US with step two.

To compare the efficacy of the LC, US, and combined application, we performed the following, standardized, in vitro lithotripsy procedure. Five artificial, cube-shaped stones of the same size but of different hardness and density levels (Table I) were used. A 5-L metal bowl was filled with water and a soft textile pad lined the bottom to keep the stone in a stable position while performing disintegration. Three stones of each hardness were evaluated. All stones had the same volume (0.5 cm3). For in vitro disintegration, a 1.0-mm LC probe inside a 3.3-mm US tube was used. For both US and LC, 80% power was used, the impulse frequency of the LC was 6 Hz. This frequency and power setting showed the best clinical results and reliable function of the instrument in our clinical experience.

TABLE I. Mean lithotripsy time until first fragmentation and complete disintegration

	Stone Material	Hardness (n/m2)	First Fragmentation/Complete Disintegration (s)
			US	LC	COMB
	Moldabaster	9	254 (220–278)*	257 (228–274)*	11 (10–12)†/298 (270–320)
	Moldano	27	254 (215–280)*, ‡	582 (563–618)*	3 (2–4)†/760 (745–790)
	Fuji Rock White	85	600 (550–650)*	600 (577–632)*	3 (2–4)†/660 (600–710)
	Fuji Rock	110	526 (498–550)*	497 (466–515)*	5 (4–6)†/990 (900–1060)
	Bongranit	120	265 (245–290)*	270 (250–300)*	3†/295 (270–315)

Key: US = ultrasonography; LC = Lithoclast; COMB = combination. Numbers in parentheses are the range.

* No complete disintegration after 1200 s. † P <0.001 vs. LC and US. ‡ P <0.05 vs. LC.

To provide consistent results, one investigator (J.W.) performed all lithotripsy procedures. After placing the stone on the pad inside the water-filled bowl, three stones of one hardness were disintegrated either by LC or US, or combined application. The probe was positioned vertically on the stone or the fragments, respectively. Pressure was applied on the stone only by the weight of the instrument (350 g), with the investigator holding the instrument in position and repositioning it if necessary. The time until the first fragmentation of each stone and the time until complete disintegration, defined as all fragments evacuated by suction through the US tube (ie, smaller than 2.2 mm), were measured. Furthermore, the percentage of stone mass disintegrated after 1 minute was determined. By weighing the disintegrated partition of the stone every 30 seconds, the 50% disintegration time was evaluated.

Results 

The efficacy of the combination device was compared with LC and US alone. The time until first stone fragmentation and until disintegration of the complete stone mass to fragments small enough for evacuation through the US tube was measured. The combined application of US and LC had a 25 to 200-fold shorter time until first stone fragmentation that was statistically significant for all stones (Table I). Complete disintegration was achieved by US and LC combination in 298 and 295 seconds in the softest and hardest stones, respectively and in 660, 760, and 990 seconds in medium-hard stones. With neither US nor LC disintegration alone could we achieve complete disintegration within a 20-minute time limit (Table I).

After 1 minute, the disintegrated stone mass was 1.5 to 4 times larger with the combined application of US and LC compared with US or LC lithotripsy alone. The difference reached the level of significance for three of five tested stone materials. The time until 50% disintegration was significantly shorter with combined lithotripsy compared with separate activation (Table II).

TABLE II. Comparison of mean percentage of disintegrated stone mass after 1 minute and time until 50% disintegration

	Stone Material	Hardness (n/m2)	% Disintegration at 1 min			Time Until 50% Disintegration (s)
			US	LC	COMB	US	LC	COMB
	Moldabaster	9	15 (10–19)	10 (6–15)	40* (30–50)	270 (240–295)	360 (320–385)	120* (110–127)
	Moldano	27	10 (8–13)	5 (3–7)	15 (10–20)	920 (850–1010)	600 (525–655)	330* (315–340)
	Fuji Rock White	85	5 (4–6)	5 (3–7)	20* (12–30)	1050 (950–1100)	710 (684–745)	300* (275–320)
	Fuji Rock	110	5 (4–6)	5 (4–6)	10 (5–16)	†	1200 (1150–1270)	420‡ (390–442)
	Bongranit	120	5 (3–7)	15 (12–17)	30* (25–33)	†	850 (800–910)	120‡ (110–135)

Abbreviations as in Table I. Numbers in parentheses are the range.

* P <0.05 vs. LC and US. † No 50% disintegration after 1200 s. ‡ P <0.05 vs. LC.

The lithotripsy procedures were performed on 3 consecutive days for 3 to 6 hours and with almost continuous activation of at least one of the instrument’s components. No technical defect in the probes, US generator, pneumatic lithotriptor, control unit, or foot switch ensued. All experiments (disintegration of 45 stones of 0.5 cm3) were performed using the same US tube and LC probe.

Comment 

Therapeutic management of urolithiasis has been revolutionized by the development of extracorporeal and intracorporeal lithotripsy. Different techniques of stone disintegration, a direct approach to the stone under direct visualization of the urinary tract, and auxiliary modalities such as ureteral stenting and percutaneous nephrostomy have reduced the need for open stone surgery. Before evaluating different intracorporeal and extracorporeal lithotripsy techniques in clinical practice, the disintegration efficacy, safety, and reproducibility have to be tested in artificial stones under defined in vitro conditions.

The physics, microhardness, and comparability of natural and artificially produced stones and the importance of artificial stones for experimental models of extracorporeal and intracorporeal lithotripsy have been demonstrated.14, 15 By special fabrication techniques, artificial stones have become comparable to their natural counterparts, and the physical events resulting in fragmentation and finally disintegration of the stones seem to be similar.16, 17 Earlier experimental work on SWL could prove the important role of microfragmentation and cavitational activity for effective stone disintegration.18, 19 Although a number of reports on in vitro or phantom lithotripsy in SWL have been published, experimental work on the different techniques of direct stone disintegration is limited.20, 21, 22, 23 Teh et al.12 compared electrohydraulic, pneumatic, and US lithotripsy in a stone phantom model, as well as in a porcine model, judging the LC technique as effective and clinically safe. In their model, as in our studies, a stone fragment size of 2 mm was used to define successful disintegration. Animal experiments concerning the safety of the LC technique showed no acute or long-term harmful effects to the surrounding tissue, especially compared with electrohydraulic and laser lithotripsy.22, 24

In our experimental model, we tried to create reproducible conditions to compare the efficacy of two established lithotripsy techniques with the newly developed combination of both. Systemic mistakes were eliminated by using standardized artificial stones of defined volume, size, and hardness, defining pressure on the stone at 350g, which seems to be in the range of that used in percutaneous lithotripsy. One investigator performed all lithotripsy procedures.

To our knowledge, no in vitro comparison of pneumatic and US lithotripsy has been done other than the work of Teh and coworkers12 who showed the LC to be superior. From our own clinical experience, we had the impression that the LC might be more efficient in harder stones, and US lithotripsy might have advantages in softer stones because of the possibility to evacuate debris, small fragments, and sludge immediately. With the new US and LC probes, these properties stay the same. Combined lithotripsy also fragments hard and soft stones more effectively than those of intermediate hardness. Neither US nor LC was able to achieve complete disintegration within 20 minutes of continuous activation. US showed the best results in all investigated parameters in the softer stones. LC seems to be highly effective in very soft and very hard stones. A synergistic effect in combining both techniques can be found in calculi otherwise difficult to fragment.

Conclusions 

Combining LC and US in one device for percutaneous nephrolithotomy shows promising in vitro results in an artificial stone model. The novel technology seems to provide superior efficacy in disintegration parameters important for clinical practice, especially the time to first fragmentation, the time to complete disintegration, and small particle size. All artificial stones could be disintegrated completely, independent of their composition and hardness.