Page 25 - Spring 2006

hoop stress, dubbed “squeezing,” around the stone that helps
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break the stone.
Squeezing has since been captured innate-
ly in a linear elastic model of stress within a stone37 and test-
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ed with the model and experiment. Figure 7 includes a
sequence of images showing the shock-wave-induced stress within a cylindrical stone (a common shape used for indus- try and research stone phantoms). First, a longitudinal wave enters the stone. Eventually this reaches the back end of the stone and reflects and inverts from the “acoustically soft” stone-water interface. The inverted wave adds to the trailing negative trough of the incident wave and creates a locally maximum tensile stress and transverse fracture. This frac- ture mechanism is termed spallation. Following the longitu- dinal wave is a wake that is generated at the surface of the stone where the wave travels faster than the sound speed of water. Traveling at the sound speed in water along the stone surface and encircling the stone is the shock wave, and it cre- ates squeezing. In addition, the shock wave incident on the corners of the stone generates shear waves that focus in the distal half of the stone. Because the shear wave speed in the stone is close to the sound speed in water, the squeezing wave reinforces the shear wave and together they create the high-
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break stones supports the concept of a broad focus lithotripter. This may be an important positive trend for the future, one in line with the national concern for constructive regulatory oversight, as beam width and peak pressure are the two primary parameters used to characterize and differ- entiate devices for approval by the US Food and Drug Administration (FDA).
When improvements take a step backward
Sometimes even well intentioned improvements in technology do not work out for the better, and the evolution of the lithotripter may be a good example.
Currently SWL is attempting to recover
from an industry-wide miscalculation in
which modifications to lithotripter design
reduced both the efficacy and the safety of
SWL. As mentioned above, in the original
clinical lithotripter (Dornier HM3), the
anesthetized patient lay in a water bath.
The Dornier HM3 was an electrohydraulic
lithotripter that produced shock waves of
moderate peak positive pressure (~35
MPa) delivered to a relatively wide focal
zone (~15 mm). The HM3 was a great suc-
cess, yielding stone-free rates of nearly
The location agrees with the location of the fracture of the stone. These mechanisms— shear, squeezing, spallation, and cavitation (described above)—all participate to varying degrees in the breakage of stones of different shapes. Cracks in the surface (as may be produced by cavitation) lead to stress concentrations at the crack tip that, when connected linearly to the site of maxi- mum stress in the stone, trace the conical fracture pattern seen in stone experiments (Fig. 8). The model predicts a decrease in stress in the stone with decreasing beam width
est tensile stress within the stone.
and with decreasing stone size.
This mechanistic understanding of how shock waves
90%. To improve convenience and ease of use, effort was made in subsequent lithotripters to forego the water bath and anesthesia.
Early-on, lithotripters were so large that they occupied a dedicated suite in the hospital, and lithotripsy units oper- ated as referral centers serving a substantial geographic area. Urologists wanted to bring SWL to their patients and make lithotripsy more accessible. The principal change needed to make lithotripters transportable was to eliminate the water bath, so dry head lithotripters were created. Indeed, all lithotripters currently in production use dry- head technology. However, recent in vitro studies39 have shown that routine coupling with a dry treatment head is decidedly inefficient, and can pose a substantial barrier to transmission of shock waves. Air pockets get trapped at the coupling interface, and this can reduce the breakage of model stones by 20-40%. Breaking and re-establishing con- tact, as when a patient is repositioned during treatment can reduce the focal pressure by 50% (Fig. 9). It is not known if the acoustic energy is focused elsewhere in the tissue. The quality of coupling is highly variable, and it seems quite fea- sible that this variability could contribute to variability in clinical outcomes, and that poor coupling could lead to treatment with an excessive number of shock waves, increasing the potential for adverse effects.
There was also great interest in making lithotripsy an anesthesia-free procedure, so that SWL could be performed on an outpa- tient basis. The effort to reduce pain sensed in the patient’s skin resulted in refinement of electromagnetic shock sources in which the aperture was large and highly focused. The design produced low pressure over a broad skin surface and still produced high peak pressures in the kidney. Such focusing yielded small focal volumes. As discussed above, the stone drifts in and out of the focal zone during respiratory motion. Thus, by narrowing the focus, fewer shock waves hit
“Numerical modeling and simulation can help explain mechanisms of shock wave action, investigate parameter spaces, and predict outcomes.”
Fig. 9. Poor coupling in dry-head lithotripters blocks the acoustic transmission of the shock pulse. An in vitro experiment was performed in which the water cushion of the treatment head of an EML was wetted to the acoustic window of a test tank with coupling gel. Waveforms show mean values for pulses when air pockets were trapped at the coupling interface (blue and red), or when all air pockets were removed manually (black). Photographs show air pockets (dark shadows) at the interface. The quality of coupling was highly variable and reduced pulse amplitude in some cases by more than 50%.
Progress in Lithotripsy Research 23