Residency · Residency · Urology
Shock Wave Lithotripsy: Principles, Indications, and Limitations
Physics of Shock Wave Generation
Extracorporeal shock wave lithotripsy (SWL) employs focused acoustic shock waves generated outside the body to fragment urinary stones. These shock waves are precisely focused on the stone using imaging guidance such as fluoroscopy or ultrasound. Stone fragmentation occurs through several mechanisms. The spall effect involves tensile stress applied to the far surface of the stone, while the squeezing effect compresses the stone circumferentially. Cavitation, the collapse of microbubbles on the stone surface, is the most important mechanism contributing to fragmentation. Additionally, dynamic fatigue results from cumulative microcrack propagation within the stone structure.
Generator Types
There are three main types of shock wave generators used in SWL. The electrohydraulic generator, exemplified by the original Dornier HM-3, produces shock waves via a spark gap. It has a broad focal zone, making it effective but associated with more tissue injury. This system requires a water bath (as in the HM-3) or coupling gel to transmit the shock waves. Electromagnetic generators, the most common in modern lithotripters, use a coil to generate a pressure wave that is focused by an acoustic lens, producing a tight focal zone. Piezoelectric generators consist of ceramic elements that generate focused waves through a self-focusing design; however, they have limited power and are less effective for large or hard stones.
Key Technical Parameters
The focal zone size influences treatment precision: a smaller focal zone yields higher peak pressure but demands precise targeting, whereas a larger focal zone is more forgiving. Energy levels, measured in kilovolts (kV), are adjustable and typically start low with gradual ramping to reduce tissue injury. The shock wave rate ranges from 60 to 120 shocks per minute, with slower rates (around 60/min) improving fragmentation efficiency and reducing renal injury. The number of shocks per session usually ranges from 2,000 to 3,500, with a maximum of approximately 4,000. Proper coupling between the shock head and patient skin using gel is critical; the presence of air bubbles in the coupling interface drastically reduces treatment efficacy.
<image>Diagram showing the three types of shock wave generators (electrohydraulic, electromagnetic, piezoelectric) with their focusing mechanisms and focal zone characteristics</image>
Patient Selection and Indications
Ideal Candidates
SWL is most effective for renal stones smaller than 20 mm, with optimal results seen in stones under 10 mm. Proximal ureteral stones less than 10 mm also respond well. Stones located outside the lower pole are preferred because lower-pole clearance is impaired due to gravity-dependent drainage. Stones with a density less than 1,000 Hounsfield units (HU) on CT, indicating softer composition, are better candidates. Additionally, a skin-to-stone distance under 10 cm and a body mass index (BMI) below 30 favor successful shock wave delivery. Absence of distal obstruction is also necessary for effective treatment.
Favorable Stone Characteristics
Calcium oxalate dihydrate, struvite, and uric acid stones fragment well with SWL. Stones smaller than 15 mm located in the renal pelvis or upper calyx are ideal. Visibility on fluoroscopy is important for accurate targeting during the procedure.
Contraindications
Absolute contraindications include pregnancy, uncorrected bleeding diathesis or anticoagulation, untreated urinary tract infection, distal ureteral obstruction below the stone, and the presence of aortic or renal artery aneurysms in the shock wave path. Relative contraindications encompass morbid obesity with skin-to-stone distance exceeding 10 cm, stones larger than 20 mm due to a high risk of steinstrasse (which may require pre-stenting), and stone types resistant to SWL such as cystine, calcium oxalate monohydrate, and brushite. Lower-pole stones larger than 10 mm, horseshoe kidney, or calyceal diverticulum are also less favorable due to impaired drainage. Patients with pacemakers or defibrillators require coordination with cardiology and may need disabling of tachyarrhythmia detection during treatment.
Technique and Optimization
Pre-Procedure
Before SWL, it is essential to confirm a negative urine culture and hold anticoagulation according to guidelines. Pre-stenting is recommended for stones larger than 15-20 mm to prevent steinstrasse. Reviewing imaging studies allows confirmation of stone location, measurement of stone density in HU, and assessment of skin-to-stone distance.
During Procedure
Anesthesia options include intravenous sedation or general anesthesia, with the patient required to remain still throughout. Positioning is typically supine for renal and proximal ureteral stones, although prone positioning may improve clearance of lower-pole stones. Coupling requires generous application of gel and meticulous elimination of air bubbles to ensure effective shock wave transmission. The shock wave rate should start at 60 shocks per minute, as this slower rate improves fragmentation efficiency compared to 120/min. Energy is ramped up gradually, beginning at low levels to reduce renal vasoconstriction and parenchymal injury. Frequent re-imaging during treatment is necessary to maintain accurate targeting because stones move with respiration. A total of 2,000 to 3,500 shocks are delivered per session, with retreatment intervals of at least 7 to 14 days.
<image>Patient positioned for SWL treatment with coupling membrane, fluoroscopic targeting crosshairs on the stone, and lithotripter head alignment</image>
Outcomes
Stone-Free Rates (Single Session)
Stone-free rates (SFR) vary by stone location and size. For renal stones under 10 mm, SFR ranges from 70 to 80%. Stones between 10 and 20 mm have a 50 to 65% SFR, while stones larger than 20 mm have rates of 30 to 40%. Upper ureteral stones smaller than 10 mm achieve 65 to 80% SFR. Lower-pole stones under 10 mm have a 60 to 70% SFR, but this drops to 35 to 55% for stones between 10 and 20 mm. Multiple treatment sessions, typically up to three, may be required, with a minimum interval of 7 to 14 days between sessions.
| Stone Location/Size | Single-Session Stone-Free Rate |
|---|---|
| Renal, <10 mm | 70-80% |
| Renal, 10-20 mm | 50-65% |
| Renal, >20 mm | 30-40% |
| Upper ureter, <10 mm | 65-80% |
| Lower pole, <10 mm | 60-70% |
| Lower pole, 10-20 mm | 35-55% |
Factors Predicting Failure
Factors associated with SWL failure include stone density greater than 1,000 HU, indicating hard stones, and a skin-to-stone distance exceeding 10 cm. Lower-pole stones with a narrow infundibulum (less than 5 mm) or a steep infundibulopelvic angle (greater than 70 degrees) are less likely to clear fragments effectively. Large stone size over 15 mm, BMI above 30, and stone composition such as cystine or calcium oxalate monohydrate also predict poorer outcomes.
Complications
Steinstrasse, or "stone street," refers to a column of stone fragments obstructing the ureter and is more likely with stones larger than 15 mm. Management options include placement of a JJ stent, ureteroscopy, or observation depending on severity. Renal or subcapsular hematomas occur in less than 1% of cases but risk increases with anticoagulation, uncontrolled hypertension, and high shock numbers. Infection and urosepsis are concerns, especially with struvite stones or preexisting bacteriuria. Perinephric hematomas are symptomatic in fewer than 1% of patients and are managed with bed rest and serial imaging. Renal colic from fragment passage is common and expected. There is some controversy regarding hypertension as a long-term complication, with older data suggesting a mild risk. Loss of renal function is minimal when appropriate technique is used, though cumulative damage can occur with repeated treatments.
<image>CT image demonstrating steinstrasse (column of stone fragments) in the distal ureter following SWL treatment</image>
SWL vs. URS: Comparative Considerations
When comparing SWL to ureteroscopy (URS), SWL is less invasive and can be performed under sedation or general anesthesia, whereas URS requires general anesthesia and is invasive. Single-session stone-free rates are generally lower with SWL, and retreatment rates are higher. Stent placement is usually unnecessary with SWL unless treating large stones, while URS may require variable stenting. Both procedures allow rapid recovery. SWL is less effective for hard stones such as calcium oxalate monohydrate and cystine, lower-pole stones, and in obese patients, whereas URS is effective in these scenarios. SWL is contraindicated in anticoagulated patients, while URS can be performed safely with appropriate management.
Clinical Pearls
Shock wave lithotripsy is a non-invasive and well-tolerated treatment modality but generally achieves lower single-session stone-free rates compared to ureteroscopy for most stone types. Employing a slow shock wave rate of 60 shocks per minute combined with energy ramping represents the most evidence-based technique to enhance outcomes. The presence of air bubbles in the coupling gel is a major, yet preventable, cause of treatment failure. CT stone density measured in Hounsfield units is the single best predictor of SWL success; stones exceeding 1,000 HU strongly favor ureteroscopy instead. Lower-pole stones pose a significant challenge for SWL due to impaired fragment clearance caused by gravity and infundibular anatomy. Pre-stenting for large stones over 15-20 mm reduces the risk of steinstrasse but does not improve stone-free rates. The classic Dornier HM-3 electrohydraulic lithotripter, with its large focal zone, still demonstrates the best published outcomes, although modern machines prioritize convenience at some cost to efficacy.
References
- AUA/Endourology Society Guideline on Surgical Management of Stones, 2016 (amended 2022)
- EAU Guidelines on Urolithiasis, 2024 Update
- Lingeman JE, et al. "Shock wave lithotripsy: advances in technology and technique." Nat Rev Urol. 2009;6(12):660-670.
- Pareek G, et al. "Hounsfield units on CT predict stone fragmentation by SWL." Urology. 2003;62(4):596-601.
- Pace KT, et al. "Shock wave lithotripsy at 60 or 120 shocks per minute." J Urol. 2005;174(2):595-599.
- Campbell-Walsh-Wein Urology, 12th Edition, Chapter on Shock Wave Lithotripsy
