Early studies of shock waves (SWs) for ureteral stones resulted in radiographically evident remodeling of the pelvis.1 These findings sparked the initial studies investigating the use of SWs in orthopedic applications. These studies and
anecdotal clinical use resulted in the initial 5 standard indications used in human medicine: 1) calcifying tendonitis of
the shoulder (tendinosis calcarea), 2) tennis elbow (lateral epicondylitis), 3) golfer's elbow (medial epicondylitis), 4)
heel spurs (plantar fasciitis), and 5) pseudarthrosis. The utilization of focused shock wave therapy (SWT in horses) started
in Germany in 1996. Applications were based on the human indications. Because of positive experiences treating people with
insertional desmopathies, the first equine application was proximal suspensory desmitis. Initial clinical responses were
positive, therefore SWT was attempted in numerous other equine conditions. In 1997, horses with navicular syndrome were
first treated. Initially, the probe was positioned behind the navicular bone from the heel, but when ultrasonographic imaging
showed the distal impar ligament could be visualized through the frog, this location was then used to administer SWT to the
distal part of the navicular region.
In 1998, horses with distal hock joint and navicular pain were first treated in the United States with SWT. The first equipment
was large, requiring water cycling and degassing. Its use was limited to anesthetized horses. The development of more affordable,
portable and durable equipment has led to an expansion of its use and applications. Associated with the expansion has been
a contraction of applications to those that have consistently provided good clinical outcomes.
Shock Wave Generators
Two distinctly different pressure waves have been lumped together into SWT. True SWs are pressure waves that meet specific
physical parameters including a rapid rise time (within nanoseconds), high peak pressures, and a more gradual decrease in
pressure over a few milliseconds, often with a negative pressure component. Shock waves occur naturally associated with lightening,
planes breaking the sound barrier or explosions. Shock waves used for medical purposes utilize electricity as the driving
energy source. Pneumatically powered sources drive a metal rod to strike a plate in contact with the skin surface which create
radial pressure waves (RPWs) Radial pressure waves have slower rise times and lower peak pressures than SWs. The importance
of recognizing the differences between these modalities is that they may not affect tissue similarly; consequently one must
consider the equipment being used when evaluating the effect on tissues and efficacy of treatment and how to use it. A complete
review of SWs and RPWs, along with all of the equipment currently available, has recently been published.
Dose Dependence
There is a dose effect of SWT which is a combination of the energy flux density (EFD) and the number of pulses. The EFD is
the energy (millijoules) in the focal area (mm2) resulting in the EFD (mJ/mm2). There is a lower EFD that must be reached to be effective and a point where the EFD is high enough to create damage at
any number of pulses. The range is dependent upon a number of factors including species, tissue being treated, waveform and
generator.
It is difficult to transpose doses from other species to horses. When 1000 pulses at an EFD of 0.28 mJ/mm2 was applied to rabbit Achilles tendons there was an inflammatory reaction and at 0.6 mJ/mm2 there was inflammation and tendon necrosis. Energy levels at this or higher have been used in the horse without complications.
However, the minimal dose to achieve the desired affect is not known, and we may not need to utilize levels this high. There
is a trend toward lower EFD's for a number of applications. One thousand pulses at 0.18 mJ/mm2 stimulated neovascularization of the tendon bone junction in dogs. A dose of 200 pulses at 0.12 mJ/mm2 was better than 500 pulses in an Achilles tendonitis model in rats. At this time, the ideal energy levels and pulse numbers
remain empirical in most equine applications.
The frequency (number of pulses per second) of the pulse application does not seem important except for the time required
to complete the treatment. All are relatively slow frequency from a biological standpoint and do not result in tissue heating
or other biological effects seen at high rates.
