Shock Wave Lithotripsy for the Treatment of Ureteral Stones

Localization

Size (mm)

SWL

Ureteroscopy

No. of patients

SFR (95 % CI)

No. of patients

SFR (95 % CI)

Distal ureter

<10

1,684

86 % (80–91)

2,013

97 % (96–98)

>10

966

74 % (57–87)

668

93 % (91–95)

Mid ureter

<10

84 % (65–95)

116

93 % (88–98)

>10

76 % (36–97)

110

79 % (71–87)

Proximal ureter

<10

967

89 % (87–91)

318

84 % (80–88)

>10

481

70 % (66–74)

338

81 % (77–85)

The stone free rates after primary treatment with SWL are significantly lower than the results of ureteroscopy for distal ureteral stones <10 mm and >10 mm and proximal ureteral stones >10 mm. There are no significant differences between primary treatment outcomes of SWL and ureteroscopy for mid-ureteral calculi and proximal ureteral stones <10 mm [16].

Using more than one SWL treatment session, high stone free rates of 96.1 % for proximal, 97.8 % for mid-ureteral and 97.9 % for distal ureteral calculi have been shown. Average number of SWL procedure was 1.37, 1.47 and 1.22 for proximal, mid-ureteral and distal ureteral stones [17].

Kanao et al. presented preoperative nomograms for predicting stone free rates after extracorporal shock wave lithotripsy. Stone length, stone location and number of stones were identified as independent predictors for stone clearance in their analysis. The predicted stone free rates 3 months after single SWL for ureteral calculi are shown in Table 6.2 [18].

Table 6.2

Nomograms predicting stone free rates 3 months after single SWL for solitary ureteral calculi [18]

	Length of stone (mm)
	≤5	6–10	11–15	16–20	>20
Proximal ureter	93.8 (86–91)	86.6 (81–91)	76.4 (65–85)	56.7 (37–73)	42.2 (19–64)
Mid/distal ureter	92.3 (81–98)	83.6 (77–90)	71.8 (59–82)	51.1 (29–71)	36.9 (16–60)

% (95 % confidence interval)

Resit-Goren and co-workers calculated the time to stone clearance after SWL for ureteral stones: Mean time to stone clearance was 2.2 days for stones <10 mm, 7.7 days for stones 11–15 mm and 12.2 days for stones >15 mm. Regarding the localization of the stone, clearance was achieved significantly faster for distal and mid-ureteral stones compared to proximal stones, however, the proximal stones were significantly larger than the stones in the two other groups and no multivariable analysis was performed. 93.1 % of all patients were stone-free 20 days after primary SWL treatment, 19.7 patients needed more than one SWL session (2–4), 6.8 % of the patients had ureteroscopy for residual calculi [19].

Parameters Influencing SWL of Ureteral Calculi

Technical parameters:

Type of lithotripter:

The first generation lithotripter (HM3, Dornier) was developed in the 1980s [20]. Today, four generations of lithotripters are on the market. The results of studies comparing different generations of lithotripters concerning treatment of ureteral calculi are mixed. Gerber et al. compared the results of the first-generation lithotripter to a second- and a third-generation lithotripter showing higher stone-free rates with the first-generation machine [21]. In contrast, Tiselius published higher rates using two third-generation lithotripters, however, this study included no direct comparison to the HM3 [17]. Nomikos and co-workers presented a comparison of a fourth-generation lithotripter with the HM3: the demonstrated comparable results, but fewer analgesics were required using the fourth-generation machine [22]. In summary, evidence suggests that lithotripters of every generation can be used for treatment of ureteral stones with reasonable efficacy.
Shock rate:

A meta-analysis of Semis and co-workers showed that outcomes of SWL with 60 shocks per minute are superior compared to 120 shocks per minute [23]. A review by Weizer et al. and the randomized, controlled, double-blind study of Honey et al. including 163 patients with proximal ureteral stones come to the same result [24, 25].
Energy of shock waves:

The energy of shock waves produced by the lithotripter is proportional to the kilovoltage (kV) of the shock wave generator. The technique of voltage stepping (beginning with lower kilovoltage and stepping towards the maximum) has been shown superior compared to a fixed kilovoltage during the whole treatment: In vitro studies and clinical trials demonstrated improved stone fragmentation and less shock wave-induced damage of renal tissue [26–29].

Influence of stone location on mechanisms of stone fragmentation:

Using SWL, it is more difficult to treat a stone within the ureter than within the renal pelvis or a renal calyx. The physical basics of stone fragmentation provide the explanation for this clinical observation. Four effects of SWL on stones can be distinguished [30]:

Compressive fracture: Shock waves generate stress of the stone which cracks mainly in areas with imperfections or defects of its molecular structure. As a result of his in vitro studies, Eisenmenger proposed a circumferential squeezing effect of shock waves on a calculus due to the fact that shock waves travel faster in stone than in water [31].
Spallation: Due to the change in impedance at the stone-water-interface, parts of a shock wave are reflected at the posterior surface of the stone where the wave leaves the stone. A negative pressure results which leads to tensile stress within the stone.
Cavitation: The shock wave creates bubbles within the water at the stone-water-interface. Collapse of these bubbles leads to disposal of energy affecting the stone.
Dynamic squeezing: This theory combines the squeezing and spallation hypothesis. Parts of the shock wave travel around the stone and the remaining will travel inside the calculus. The outer part travels slower than the inner part which will be also reflected at the backside of the stone. Therefore the outer part reinforces the spallation process by squeezing the stone from outside.

All mechanisms of stone fragmentation during shock wave lithotripsy arise at the stone-water-interface. In stones impacted in the mucosa of the ureter, there is less contact of the stone surface with water. Additionally, in case of cracking of the stone, the ureteral walls may hold the fragments together not allowing them to shift.

Size of the stone:

The size of a ureteral stone is an independent predictor of stone clearance after SWL as shown earlier (section on: Stone free rates after SWL for ureteral calculi).

Stone composition:

SWL of calcium oxalate monohydrate stones and cystine stones are more likely to fail compared to SWL of other types of stones due to difficulties in fragmenting these stones because of their relative hardness compared to other stones. Uric acid stones may be difficult to shock as their relative radiolucency make them difficult to localize on fluoroscopy.