Incidence and Causes
About 5 % of all women and 10 % of men have a kidney stone once in a life time. Kidney stones usually appear in people between the ages of 20 – 45, and occasionally develop in children. After the age of 50 the incidence rate declines. Kidney stones are especially common in dry, hot countries and are seen more often in Caucasians than people of African descent.
When the urine becomes too concentrated with impurities, small crystals may form and develop into stones. Calcium oxalate stones are most common. They often develop due to insufficient fluid intake. Other types of stones include calcium phosphate, struvite, uric acid and cystine stones.
The most common causes of calculi are dehydration and poor nutrition. Certain medications such as antacids and protein supplements have also been linked to calculi formation. Cystine stones are linked to heredity and although rare, have been seen in children.
Kidney stones are usually undetected during formation. As the stone moves into the urinary tract they are often accompanied by the following symptoms:
- ● Renal colic
- ● Frequent, burning or slow urination
- ● Blood in the urine
- ● Fever, nausea and/or vomiting
Open surgical treatment – With the invention of ESWL, open surgical procedures on the kidney are rare. There are, however, specific indications, which make open surgery necessary.
• ● Failure of less invasive treatments due to size, composition and location of the stone
• ● Certain anatomic abnormalities of the urinary tract
Minimally Invasive Treatments – Ureteroscopy (URS) can be performed to remove stones located in the lower part of the ureter. During this procedure, an ureteroscope is inserted through the urethra to gain access to the stone. Once the stone is located, it is either removed by means of a specialized basket or by laser lithotripsy. The advantage of Holmium laser lithotripsy over other endoscopic treatments is decreased stone movement and decreased bleeding during treatment.
Percutaneous Nephrolithotomy (PCNL) – This method is often used for calculi larger than 2 centimeters in size or for hard stones. General anesthesia is required. A small incision is made in the back and a nephroscope is passed directly into the kidney. Direct fragmentation of the stone is performed using an ultrasonic, electrohydraulic, or laser device through the nephroscope under direct vision. This treatment can also be performed using the Dornier Medilas H20.
Conservative treatment – Diet, hydration, medications or a combination of these treatments may assist the natural passing of the calculi. The conservative treatment is only successful for stones smaller than 5 mm in diameter. The effectiveness of medications is dependent on the composition of the stone.
Stones can either be localized with ultrasound or X-ray. The main benefits of ultrasound are real time monitoring of the disintegration process and the absence of ionizing radiation. Fluoroscopic imaging usually is very fast and precise. It visualizes areas that are not seen with ultrasound. Dornier lithotripters are designed for dual-mode imaging offering the choice to apply both imaging modalities simultaneously.
Stone fragmentation is primarily caused by high local tensile and shear waves created by the focused shock waves hitting the stone. Spherical shock wave fronts contribute to compression-induced tensile cracks or spalling at the posterior surface of the stone. Cavitation is also important for stone comminution. The rapid collapse of cavitation bubbles on the surface of the stone or in liquid-filled cracks within the stone produces shock waves that cause microfractures in the stone.
Stone fragmentation correlates quite well with the shock wave energy delivered into the focal zone. The acoustic energy of a shock wave pulse is determined within an area having a diameter that corresponds to the average size of urinary stones. The effective energy contributing to stone disintegration is generally defined as energy delivered to an area of 12 mm diameter in the focal plane. The 12-mm area corresponds to most stones indicated for ESWL monotherapy. Provided the stone is precisely targeted, most of the effective energy contributes to fragmentation except for the portion that does not hit the stone. The surrounding tissue absorbs the energy that misses the stone. For larger stones, the full effective energy is applied to the stone for fragmentation. This is important when selecting a safe energy dose, i.e., the number of shock waves times the effective energy, to disintegrate the stone.
The energy dose Etot (12mm) is defined as Etot(12mm) = n * Eeff(12mm) where n is the number of applied shocks and Eeff(12mm) is the disintegration energy, i.e. the acoustic energy per shock wave delivered to an area of 12 mm diameter in the focal plane. The applied energy dose determines treatment success in terms of fragmentation and side effects. Thus, the success of ESWL treatments at different intensity levels seems to remain the same when the number of shots is in the range receiving equivalent energy dose. The energy dose for kidney stones typically is in the range of 100 to 200 J per treatment
Cavitation bubbles in the shock wave path have a noticeable effect on fragmentation efficacy. They attenuate the shock wave and fragmentation efficacy is reduced. Studies performed in vitro suggest that the PRF influences fragmentation efficacy due to cavitation effects. At higher PRFs shock waves become less effective. For this reason PRF should be kept as low as possible.
Most patients only experience minimal side effects after the procedure. Side effects may include mild discomfort in the abdominal region, along with redness or bruising at the treatment site, and blood in the urine. Patients can return to a normal routine within twenty-four hours of the treatment.
Even though it has not been scientifically proven, the risk of side effects from a shock wave exposure may increase due to factors such as medication, age, hypertension, cardiac rhythm dysfunction, vascular debility, diabetes or obesity. Tender vessels in the renal parenchyma are particularly endangered, and high shock wave intensities can lead to a serious hematoma if the patient presents a combination of these risk factors. Clinically relevant perirenal hematomas are observed within 1 - 3% after ESWL as confirmed by ultrasound imaging, and most are treated conservatively.