BASIC PHYSICS AND DEFINITION OF PHYSICAL PARAMETERS

Author:

DIPL. PHYS. ANNA TÓTH-KISCHKAT ISMST, 5/00

Introduction:

This presentation is aiming to give you a basic understanding of the physical phenomena involved in Muscolosceletal Shockwave Therapy. We will also look into quantification of the wave field and develop an understanding of the parameters published for all the approved devices currently on the market. Measurement of well defined parameters are the basis of all quantitative research.

Basic Physics:

A Shockwave is defined as a Sonic Pulse characterised by:

In shockwave therapy the pulse energy is focused in order to be applied where treatment is needed.

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

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 that act upon interfaces and tensile forces that 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 = _ E n

The variables being: V for the disintegrated volume; __ 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 orthopaedic 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 characterising 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 volume from the energy flux density ED being the energy transmitted to a single point within this volume. Going back to stone disintegration in urology may help to visualise this difference: while the total energy is a measure of the disintegrated volume, the energy flux density corresponds to the depths of the crater.

Definition of Physical Parameters:

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 with reproducible physical parameters. Therefore we need to quantify the parameters involved.

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

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 a 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 volume defined by 3 different conditions:

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 regions are compared in Figures 4 and 5 for high and low energy settings.

This set of parameters was measured - using unified standards - for most of the shockwave devices on the market. The results are published on the Internet site of DIGEST - German Society for Shockwave Therapy - http://www.DIGEST-eV.de.

Outlook:

Now it will be up to medical research to use the technical data and correlate them to biological events.
The correlation between pressure, energy flux density and energy of the shock waves with the medical impacts can finally be analysed. The ball is in your court now and you can play it back to our side if and when you know more specificly what additional information the medical community needs. From there we can enter into the next round. This far we have looked into pressure and energy distribution; but we can also go back and analyse lifecycle, rise time or other specification of the shockwaves.
It is solid research of biological mechanisms in orthopaedics and interrelation with the physical data that is needed to put the debate over shockwave therapy on a factual and rational basis. This will widen the acceptance of this proven and extraordinarily successful treatment.

 

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