374 CHAPTER 8 Stone disease
Intracorporeal techniques of stone fragmentation (fragmentation within the body)
Electrohydraulic lithotripsy (EHL)
EHL was the first technique developed for intracorporeal lithotripsy. A high voltage applied across a concentric electrode under water generates a spark. This vaporizes water, and the subsequent expansion and collapse of the gas bubble generates a shock wave.
EHL is an effective form of stone fragmentation. The shock wave is not focused, so the EHL probe must be applied within 1 mm of the stone to optimize stone fragmentation.
EHL has a narrower safety margin than that of pneumatic, ultrasonic, or laser lithotripsy and should be kept as far away as possible from the wall of the ureter, renal pelvis, or bladder to limit damage to these structures, and at least 2 mm away from the cystoscope, ureteroscope, or nephroscope to prevent lens fracture.
Principal uses are for bladder stones (wider safety margin than in the narrower ureter).
Pneumatic (ballistic) lithotripsy
A metal projectile contained within the handpiece is propelled backward and forward at great speed by bursts of compressed air (see Fig. 8.3). It strikes a long, thin, metal probe at one end of the handpiece at 12Hz (12 strikes/second) transmitting shock waves to the probe, which, when in contact with a rigid structure such as a stone, fragments the stone.
This technique is used for stone fragmentation in the ureter (using a thin probe to allow insertion down a ureteroscope) or kidney (a thicker probe may be used, with an inbuilt suction device—Lithovac—to remove stone fragments).
Pneumatic lithotripsy is very safe, since the excursion of the end of probe is about a millimeter, and it bounces off the pliable wall of the ureter. Ureteric perforation is therefore rare.
The device is low cost and requires low maintenance. However, its ballistic effect has a tendency to cause stone migration into the proximal ureter or renal pelvis, where the stone may be inaccessible to further treatment. The metal probe cannot bend around corners, so it cannot be used for ureteroscopic treatment of stones within the kidney or with a flexible ureteroscope.
Its principal use is for ureteric stones.
Ultrasonic lithotripsy
An electrical current applied across a piezoceramic plate located in the ultrasound transducer generates ultrasound waves of a specific frequency (23,000–25,000 Hz). The ultrasound energy is transmitted to a hollow metal probe, which in turn is applied to the stone (see Fig. 8.4).
INTRACORPOREAL TECHNIQUES OF STONE FRAGMENTATION 375
Metal projectile
Probe (1–3.5mm)
Jackhammer effect
Connection to generator
Figure 8.3 The Lithoclast: a pneumatic lithotripsy device. This figure was published in Walsh PC, et al. Campbell’s Urology, 8th edition, pp. 3395–979. Copyright Elsevier 2002.
Ultrasound generator
Piezoceramic elements
Longitudinal vibration
Ultrasound |
|
probe |
Acoustical end parts |
Suction |
Foot |
|
piece |
Figure 8.4 The Calcuson: an ultrasonic lithotripsy device. This figure was published in Walsh PC, et al. Campbell’s Urology, 8th edition, pp. 3395–979. Copyright Elsevier 2002.
The stone resonates at high frequency and this causes it to break into small fragments (the opera singer breaking a glass) that are then sucked out through the center of the hollow probe. Soft tissues do not resonate when the probe is applied to them and thus are not damaged.
This technique can only be used down straight, rigid instruments. Principal uses include fragmentation of renal calculi during PCNL.
376 CHAPTER 8 Stone disease
Laser lithotripsy
The holmium:YAG laser is principally a photothermal mechanism of action, causing stone vaporization. It has minimal shock-wave generation and therefore less risk of causing stone migration. The laser energy is delivered down fibers that vary in diameter from 200 to 360 microns. The 200-micron fiber is very flexible and can be used to gain access to stones even within the lower pole of the kidney (see Figs. 8.5 and 8.6).
The zone of thermal injury is limited to 0.5–1 mm from the laser tip. No stone can withstand the heat generated by the Ho:YAG laser. Laser lithotripsy takes time; however, since the thin laser fiber must be “painted” over the surface of the stone to vaporize it.
Principal uses are for ureteric stones and small intrarenal stones.
INTRACORPOREAL TECHNIQUES OF STONE FRAGMENTATION 377
Figure 8.5 A laser fiber.
Figure 8.6 Access to the lower pole of the kidney with a flexible ureteroscope.
378 CHAPTER 8 Stone disease
Kidney stone treatment: flexible ureteroscopy and laser treatment
The development of small-caliber ureteroscopes with active deflecting mechanisms and instrument channels, in combination with the development of laser technology, small-diameter laser fibers, and stone baskets and graspers, has opened the way for intracorporeal, endoscopic treatment of kidney stones.
Access to virtually the entire collecting system is possible with modern instruments. The holmium:YAG laser has a minimal effect on tissues at distances of 2–3 mm from the laser tip and so collateral tissue damage is minimal with this laser type.
Flexible ureteroscopy and laser fragmentation offers a more effective treatment option than ESWL, with a lower morbidity than PCNL, but usually requires a general anesthetic (some patients will tolerate it with sedation alone). It can also allow access to areas of the kidney where ESWL is less efficient or where PCNL cannot reach. It is most suited to stones <2 cm in diameter.
Indications for flexible ureteroscopic kidney stone treatment
-ESWL failure
-Lower pole stone (reduces likelihood of stone passage postESWL— fragments have to pass uphill)
-Cystine stones
-Obesity such that PCNL access is technically difficult or impossible (nephroscopes may not be long enough to reach stone)
-Obesity such that ESWL is technically difficult or impossible. BMI >28 is associated with lower ESWL success rates. Treatment distance may exceed focal length of lithotriptor.
-Musculoskeletal deformities such that stone access by PCNL or ESWL is difficult or impossible (e.g., kyphoscoliosis)
-Stone in a calyceal diverticulum (accessing stones in small diverticulae in upper and anterior calyces is difficult and carries significant risks)
-Stenosis of a calyceal infundibulum or tight angle between renal pelvis and infundibulum. The flexible ureteroscope can negotiate acute angles and the laser can be used to divide obstructions.
-Bleeding diathesis where reversal of this diathesis is potentially dangerous or difficult
-Horseshoe or pelvic kidney. ESWL fragmentation rates are only 50% in such cases1 due to difficulties of shock-wave transmission through overlying organs (bowel). PCNL for such kidneys is difficult because of bowel proximity and variable blood supply (blood supply derived from multiple sources).
-Patient preference
1 Kupeli B, Isen K, Biri H, et al. (1999). Extracorporeal shockwave lithotripsy in anomalous kidneys.
J Endourol 13:39–52.
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Disadvantages
Efficacy diminishes as stone burden increases—it simply takes a long time to paint the surface of the stone with laser energy, causing fragmentation.
A dust cloud is produced as the stone fragments, and this temporarily obscures the view until it has been washed away by irrigation. Stone fragmentation rates for those expert in flexible ureteroscopy are ~70–80% for stones <2 cm in diameter and 50% for those >2 cm in diameter,2 and ~10% of patients will require two or more treatment sessions.
2 Dasgupta P, et al. (2004). Flexible ureterorenoscopy: prospective analysis of the Guy’s experience. Ann R Coll Surg 86:367–370.