Focused shockwave therapy uses acoustic waves to produce effects in tissues. For clinicians and patients alike, understanding the physics behind this technology is key to understanding how it works. This post breaks down the core principles of shockwave physics from wave propagation to the different types of devices used in clinical practice.
How a Focused Shockwave Travels
A shockwave is not a continuous wave but a single powerful acoustic pulse. Its journey and characteristics can be broken down into several stages.
It all starts with the Wave Front, which is the leading edge of the shockwave. This front rises to its maximum pressure in no time. The time it takes to get there is called the Rise Time. Immediately after that the pressure starts to fall.
The entire time from the initial rise to the return to zero pressure is called the Positive Pressure Phase. Then the wave enters the Negative Pressure Phase where it dips below baseline before returning to zero, completing the cycle. All of this happens in a microsecond.
The Power of Shockwave Dynamics
The intensity of the shockwave is determined by the energy put into the device, usually measured in kilovolts (kV). A higher kV input means higher peak pressure and faster rise time. These two are the main determinants of the overall power and therapeutic effect of the shockwave.
Once generated the wave is directed into the body’s tissues. The effectiveness of the treatment depends on where this energy is focused.
Focal Point vs Focal Zone: Precision in Practice
Focused shockwave devices are designed to focus the acoustic energy at a specific location in the tissue. This brings us to two important concepts: the focal point and the focal zone.
Focal Point
The focal point is the exact single point where all the converging shockwaves meet. This is where the pressure and energy is at its highest. The clinical implications of this are:
- Maximized Biological Response: The high pressure gradient at the focal point is what triggers mechanotransduction – the process by which cells convert mechanical stimuli into biochemical activity and activate the body’s natural healing factors.* Minimized Collateral Stress: Energy dissipates rapidly outside of this precise point. This protects the surrounding and overlying tissues, such as the skin, from unnecessary stress.
- Accurate Targeting: It allows clinicians to target specific anatomical structures with high precision.
Focal Zone
The focal zone is the 3D volume around the focal point where the energy is strong enough to be therapeutically effective. This is not a single point but an area of treatment. Understanding the focal zone is important for:
- Depth Control: Devices with adjustable focal zones allow practitioners to target tissues at different depths, from superficial to deep.
- Tissue Coverage: A wider focal zone can provide more uniform stimulation across a target area, so the entire region gets a therapeutic dose.
- Reproducibility: Knowing the dimensions of the focal zone ensures consistent treatment outcomes across different sessions and patients.
What does "Power" mean in Acoustic Terms
In shockwave therapy "power" is not measured in watts like an electrical appliance. It’s quantified using acoustic energy parameters from pressure-time profiles.
The key parameter is Energy Flux Density (EFD), which is the amount of energy per unit area. When clinicians talk about the treatment "dose" they are often referring to the total energy delivered, which is the EFD multiplied by the number of pulses.
Note that the relationship between the device’s input voltage and the biological effect is not linear. Doubling the voltage does not simply double the effect, as the energy scales non-linearly.
Typical Parameters and Heat Generation
Focused shockwave devices operate within specific ranges to be safe and effective. While the pulses generate huge pressure, the heat generated in human tissue is minimal. During a typical therapeutic pulse the temperature increase is usually less than 0.1°C which is negligible and ensures the primary effect is mechanical not thermal.
Shockwave Device Technologies
There are several ways to generate focused shockwaves, each with its own design and characteristics.
1. Electrohydraulic Devices
This is the oldest shockwave technology. It uses a spark gap submerged in a saline solution. When the spark fires it creates a pressure bubble that propagates as a shockwave. An elliptical reflector captures and focuses this wave towards the target tissue.* Pros: Can produce high energy wave.
- Cons: Requires maintenance, including the saline solution. The spark gap is a consumable part that corrodes over time and needs to be replaced.
2. Piezoelectric Devices
These use hundreds or thousands of tiny piezoelectric crystals (often lead zirconate titanate, or PZT) on a concave surface. A high-voltage pulse makes the crystals expand and contract rapidly, creating small pressure waves that converge at a precise focal point.
- Pros: Highly focused with minimal maintenance and no consumables.
- Cons: Limited power and penetration depth. Often require different sized standoffs to adjust the treatment depth which can interrupt the workflow.
3. Electromagnetic Devices
This technology has evolved since 1988 and uses electromagnetic principles to generate the shockwave. There are three types:
- Cylindrical: A cylindrical copper coil generates a shockwave that is then focused by an external reflector. This can produce very high power.
- Flat: A flat copper plate generates a planar (unfocused) wave. A separate glass lens is used to focus this wave into the tissue.
- Parabolic: This is the newest. A copper parabola generates a pre-focused wave, eliminating the need for a secondary reflector or lens. This is very efficient, delivering almost all the wave’s energy to the target tissue with minimal loss.
Electromagnetic devices, especially the parabolic type, are known for their reliability, long lifespan (often over 1 million pulses per handpiece) and low maintenance.
Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice. Shockwave therapy should always be applied by a qualified clinician according to the device manufacturer’s guidelines and labeling.
