A Shockwave is defined as a Sonic Pulse characterized by

• high peak-pressure (500 bar)
• a short lifecycle (10 ms)
• fast pressure rise (< 10 ns)
• a broad frequency spectrum (16 Hz-20 MHz).

The pulse energy needs to be focused in order to be applied where treatment is needed.

In industry, there are three methods applied in shockwave generation:

• Electrohydraulic principle
• Electromagnetic principle
• Piezoelectric principle

Those three methods represent different technical philosophies which are widely discussed in the shockwave literature.

Instead of dealing with the technical differences in shockwave generation we want to take a look at the physical effects of shockwaves.

Shockwaves are generating high stress forces which act upon interfaces and tensile forces which cause cavities. We know these forces from applications in urology. In those applications, measurements of the disintegrative power acting on artificial stones follow the relationship

V = eE n

The variables being: V for the disintegrated volume; e the specific disintegration capability for a certain material; E the total energy of a pulse; n the number of pulses.

However, this equation - as powerful as it may be in urology - does not allow for adequate prediction of orthopedic effects, where disintegration is not the issue. We do not yet fully understand the curative mechanism of shockwaves in musculosceletal therapy, but in order to research them, we need to take into consideration not only the total energy, but also the other parameters characterizing the shockwaves. We need to investigate pressure distribution, energy flux density and total energy in the focal region. In particular we want to distinguish the total energy E absorbed within the focal area from the energy flux density ED being the energy transmitted to a single point within this area. Going back to stone disintegration in urology may help to visualize this difference: while the total energy is a measure of the disintegrated volume, the energy flux density corresponds to the depths of the crater.

As mentioned above, we do not yet fully understand the processes induced in the biological tissue. We do not understand how shockwaves induce bone-healing. That is why it is particularly important to be able to correlate medical results to reproduceable physical parameters. Therefore we need to quantify the parameters involved.

Within a well defined focal region, information is required on:

• pressure
• energy flux density
• energy

Let us first define a focal region. In theory pressure and energy are concentrated within a point, the focus. In this case we would not need to distinguish between energy and energy flux density. In reality our focus has finite dimensions. The pressure field is at its highest in the focal centre, but the pressure does have decreasing finite values in the neighbouring regions as well.

We decide to be interested in the field within focal areas defined by 3 different conditions:

• -6dB-Area the boundaries of the focus are defined by the pressure having decreased to half of its peak value and measured in mm along the x, y and z direction (see Figures 2 and 3)
• -5 MPa-Area the boundaries of the focus are defined by the pressure having decreased to 5 MPa, again measured in mm along the x, y and z direction
• 5 mm-Area the focal area is simply the 5 mm sphere around the focus

In order to compare the fields within these boundaries, it is necessary to measure the pressure field. In order to be able to distinguish the ranges of individual devices it is also necessary to give parameter values of maximum, minimum and intermediate energy-settings. The different focal areas are compared in Figure 4 and 5 for high and low energy settings.

These requirements result in the table of parameters introduced in Table 1.

As it is technically challenging - and therefore financially demanding - to measure tensile forces, it took some time to fill in the values for the different devices. Apart from this, different technical methods of measurement lead to different findings. But we are happy to state that finally measurements have been almost completed using unified standards. The individual values of the devices on the market are now published in the internet at http://www.digest-ev.de and will be presented in London. The few missing values will be added by July 1999.

graph: peak positive pressure vs energy level


table: Energy Flux Density Output versus SONOCUR Basic Energy setting