ecallenHow Your Shock Waves Are Generated – It Makes a Difference!
By T.J. Barclay, DVM
Extracorporeal shock wave therapy (ESWT) is a common treatment modality for a variety of orthopedic conditions in both animals and humans. In veterinary medicine, ESWT was first developed for use in horses over two decades ago. More recently, usage of ESWT has expanded to small animal species. Several different shock wave generators are marketed for use in veterinary medicine, leading to the erroneous assumption that all are equally effective in producing the desired results. Let us explore the differences between these devices, and what they mean for our patients.
Understanding Shock Wave Generation Methods
Shock waves are a specific form of sound wave, characterized by an extremely rapid, high amplitude peak followed by a slower and lower amplitude trough. Current therapeutic devices generate focused shock waves by using one of three methods:
- Electrohydraulic: An electric spark within a fluid medium generates a shock wave that is then focused by a metal reflector.
- Electromagnetic: These devices function similarly to a loudspeaker, wherein an electromagnetic coil produces the shock wave.
- Piezoelectric: An electric current causes an array of piezoelectric crystals to vibrate, producing the shock wave.
While all these devices produce shock waves, there are significant differences in how those waves are focused into the target tissues. These different focusing methods result in varying volumes of tissue that are subjected to shock waves. The electrohydraulic method produces a focal zone that is several orders of magnitude larger than either of the other two methods. This means that for every individual shock wave (aka “pulse”), more cells are affected. Put another way, for any given volume of tissue to be treated, an electrohydraulic device requires fewer pulses to effectively treat it.
Energy Flux Density: A Misleading Comparison
The energy delivered by shock waves can be measured, and it is termed “energy flux density” (EFD). While on its face, EFD would seem to be an effective way to compare the different shock wave devices, this is a common misconception. Energy flux density is defined as the energy delivered across the focal plane and is reported in units of energy per unit of area (mJ/mm2). Total energy delivered is thus dependent on the size of the focal zone of the particular device. As stated before, electrohydraulic devices have a much larger focal zone, and therefore deliver a much larger amount of energy. Saying that electromagnetic or piezoelectric devices are equivalent to their electrohydraulic counterparts based on EFD delivered, is akin to saying that a teacup and a swimming pool hold the same amount of water!
Shock Wave Clinical Efficacy and Research
The effects of ESWT on soft tissue injuries, bone injuries, osteoarthritis, wounds, and other lesions have been extensively studied. However, given the differences in shock wave generation methods, care must be taken when extrapolating protocols and expected results from one method to another. The results seen in studies performed with electrohydraulic devices cannot be assumed to be achievable with the other devices. Ideally, the efficacy of each particular device would be proven through its own clinical trials. Keep that in mind when deciding on a shock wave device to incorporate into your practice!
