Updated: Feb 26, 2020
What makes the difference among different medical shock waves devices? The following are just a few characteristics.
- The way by which shocks are generated (electrohydraulic, electromagnetic, piezoelectric, ballistic, etc.)
- The construction material of the reflector, which is the element that focuses the shock waves
- The geometry of the reflector (ellipsoidal, paraboloid, spherical, etc.)
- The treatment zone location relatively to the focal point, such as before, in or after the focal point, and also to the depth of penetration in skin or tissue, such as near the skin, inside the tissue, deep inside the tissue, etc.
- The type of treated tissue dictates the optimal location of the focal point in respect to the treatment zone
- Settings such as energy input value, frequency of shock waves, total number of pulses, etc., which is known as “dosage”
Based on the above parameters when using different shock wave devices, there are significant differences in the energy deposited in the tissue from the treatment zone, which differentiates them in performance. In other words, in order to obtain the maximum performance for the targeted application, the above parameters are to be tuned.
Shock waves travel unidirectional without loss via heat, which makes the medical focused shock wave technology a “cold” high energy therapy. Depending on energy setting, the focused shock waves are capable of producing tissue regeneration or tissue ablation in the “focal zone” or “targeted treatment zone” without producing any heat. Also, the focusing of shock waves without significant energy loss makes the focused shock waves efficient and capable to be concentrated superficially or deep inside the human or animal bodies. Any tissue depth penetration can be accomplished with focused shock waves based on reflector’s geometry, either shallower or deeper, and by varying the membrane’s height, as seen in the following figure.
Focused Shock Waves Different Penetration Based on Applicator’s Construction
In the focal zone or the targeted treatment zone, focused shock waves generate a pressure signal that has two major components – the “compressive phase”, where compressive pressures are generated inside the tissue, and the “tensile phase”, where negative pressures produce cavitation in any fluids present in the targeted treatment zone.
Typical Shock Wave Pressure Pulse
Focused shock waves are characterized by intensive compressive waves followed by significant cavitation generation and with very limited and localized and transient heat produced inside the tissue. Thus, one can talk about a “macro-level effect” generated by high compressive forces that can produce tissue micro-tears or tissue strains. During tensile phase, the collapse of large shock wave cavitation bubbles produces powerful high-speed jets with action within a few micrometers, which means at cellular level or “micro-level effect”.
Focused Shock Waves Effects
At macro and micro tissue levels, the synergetic action of the compressive pressures and cavitational high-speed jets seems to give faster and better results in healing wounds, chronic fractures, etc., for focused shock waves. In contrast, the “unfocused” shock waves produce much lower pressures inside the treated tissue and also significantly reduced cavitational action, since cavitation bubbles cannot grow to their full potential, thus producing lower therapeutic effects when compared to focused shock waves.
Unfocused Reflector and Its Targeted Treatment Zone
As seen from the above pictogram, for unfocused shock waves, the treatment zone should be completely before the “Focal Volume”. This is done in order to be able to treat an area larger than the cross section of the focal volume with one position of the applicator, and to avoid high energies found inside the focal volume, as it happens for focused shock waves. Due to lower energy delivered inside the unfocused treatment zone, the unfocused shock waves are also known as “soft shock waves”.
Practically, in the treatment with unfocused shock waves the focal volume should be completely out of the patient, which means that the focal point and its associated focal volume for such device should be far away from the unfocused treatment zone. To better visualize this, imagine a patient being treated in one room with unfocused shock waves with an applicator that has its focal volume positioned in the next room, for avoiding the focal volume to overlap with the patient in any treatment situation. For example, if the abdominal area is treated at the skin level from the left side, then the focal volume should be at all times outside the patient’s right side of the abdomen, which accounts for more than 20 inches or 500 mm for an average-size person. It means that the length of the unfocused treatment zone should be very large in order to accomplish such requirements.
For focused shock waves, the reflectors are approximately a half an ellipsoid, with a surface area depicted as A0 in the following picture. It is also known that the amount of “Energy” that is focused from the point of origin of the shock waves to the treatment zone is directly proportional with the reflective area of the reflector. Thus, the larger the area, the more energy is found in the focal volume.
Dependence of Unfocused Treatment Zone on Reflector’s Geometry
If the reflector area is reduced to 70% or 40% from the initial reflector’s surface area A0, consequently the “Energy” reflected towards focal volume becomes 70% or 40% of the energy for the reflector with A0 reflective surface. The reduced energy translates in reduced pressures and also focal volumes, as depicted in the above pictogram.
In case of unfocused shock waves the half-ellipsoidal reflectors cannot be used even for superficial treatments, due to the fact that the focused zone will be then placed inside the patient body, with unknown consequences. In this case, this is why it is most likely to utilize a reflector with reduced reflective area, where only a small fraction of the ellipsoidal area can be used. Therefore from the start, in the unfocused region, the unfocused applicators will produce significantly lower pressures gradients, when compared with those from half ellipsoid focal volume that represents the optimum for focused shock waves, assuming the same energy setting.
Pressures Comparison in Between Focused and Unfocused Zones
It means that the treatment can be gentler to the tissue when using unfocused shock waves. In the same time, treatments using unfocussed devices lose their therapeutic significance very fast, due to lower energy delivered inside the treatment zone, and thus requiring increased number of treatment sessions to deliver an equivalent amount of energy to the tissue and achieve results relatively close to those produced by fewer focused shock wave treatment sessions.
According to international standard on shock waves, for focused shock waves, the “Focal Volume” is defined as the -6dB Focal Volume that encompasses pressures from maximum pressure (Pmax) generated in geometrical focal point F2 of the treatment zone to half of that maximum pressures (Pmax/2). Another region of importance is the 5MPa Region that incorporates all pressures higher of 5MPa, which is defined as the minimum pressure generating some therapeutic effect inside living tissues.
To better understand the spatial relationship between -6dB Focal Volume and 5MPa Region when a low energy setting is used, the pressure distribution along the reflector’s longitudinal axis is presented in the following pictogram.
Pressure Distribution around Focal Point F2 for Low Energy Setting
In the “Focusing Region”, the shock waves focus towards the second focal point of the ellipsoid, F2, which transforms the sinusoidal waves generated in F1, the first focal point of the ellipsoid, into classic “saw-tooth” distorted pressures signals that are characteristic for shock waves. In this case, the Focusing Region runs from the reflector edge until a pressure of 13 MPa is reached, which represents the half-pressure of 26 MPa from the focal point F2. This is the region where the treatment zone should be place when unfocused shock waves are used.
The “Focused Region” is the region where the shock waves are focused and where the largest pressures and energies are created. The pressure signals from this region have high compressive pressures, very fast rise times, and significant tensile pressures that produce cavitation. The length of the Focused Region is equal with the length of the -6dB Focal Volume. Interesting to note is that for this example, the maximum pressure from F2 (26 MPa) is also the maximum pressure generated in the whole Focused Region.
The “Diffusing Region” is the region where the pressure shock waves are rapidly defocusing and transform themselves in distorted sinusoidal waves that have the positive and negative peak pressures relatively equal in size. The high pressures and energies decrease fast in this region, and generally are at values equivalent to those from the Focusing Region. This can be seen by analyzing the shape of the pressures signals, before and after the geometrical focal point F2.
As presented in the next pictogram, when the energy setting is increased, a higher pressure of 29 MPa is generated in the geometrical focal point F2, which stretches the length of the “Focused Region” or -6dB Focal Volume, when compared to the low energy setting presented before.
The length increase of the -6dB Focal Volume pushes the “Focusing Region” towards the reflector, and limits the available space for unfocused waves region. This shows that the energy setting can severely influence the available unfocused treatment zone length, which limits the treatment options for unfocused devices. This observation brings another important difference between focused and unfocused shock waves.
Pressure Distribution around the Focal Point F2 for High Energy Setting
For high energy setting, it is also interesting to note that the maximum pressure of 31MPa from the Focused Region is found at 6 mm away from F2, which indicates a shift in the focal point of the shock waves from the geometric focal point F2. This finding is common for focused shock wave devices, and it can be produced by the combination of reflector’s geometry and the energy setting for voltage discharge inside the reflector, at the point of origin for the focused shock waves.
For this case of high energy setting, the length of the 5MPa Region also increased significantly. Similarly, it shows that higher energy setting decreases the available length for the unfocused treatment zone, and most importantly it points out the possibility of a larger treatment zone with focused shock waves, which gives a considerable advantage for treatment options with focused shock waves.
Using unfocused shock waves means that living tissue may be treated with all that is not focused on the way to the focus. It looks such as “treatment energy arrows” are all over the place outside a target, due to a target aiming with an unfocused eye, as pictured below.
Focused and Unfocused Effects
The “blurry” aspect of the unfocused notion can easily be transferred to unfocused shock waves’ science and effects, which is in great contrast with the “clear” and “standardized” science behind the focused shock waves.
The “cover story” for unfocused devices benefits is the delivering of lower energies in a possible larger treatment zone, when compared to focused shock wave devices. This can be advantageous in treatment situations where the treatment zone is significantly large and actual low-pressure therapy reduces the patient’s possible pain sensation generated by shock waves. However, the increased number of treatment sessions, necessary to achieve the desired therapeutic results, may be an inconvenience to the patient from logistics, compliance, and financial point of view. In the same time, the construction of the unfocused applicator has its challenges that may reduce treatment possibilities, which can ultimately increase the total number of applicators needed to cover the range of treatments for a specific medical condition.
Therefore, each “cover story” involving shock waves needs to be seen from multiple angles, to understand all consequences, benefits, and disadvantages. Shock waves are never boring!!!
Keywords: shock wave, medical shock wave, focused shock wave, electro-hydraulic, wound care, diabetic foot ulcers, DFU, shockwave therapy, amputation prevention, dermaPACE, chronic wounds, SANUWAVE, PACE Technology
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