# Extacorporeal Shockwave Applicator for Spinal Pain and Muscular Contractive: A New Design Approach
## 1 Introduction  
The effect of shockwaves on human tissue was first observed during World War II for severe lung crackles in people exposed to the detonation of bombs in water as the shockwaves transmit high amounts of energy [1]. The transfer of this effect to a medical application has involved years of testing and research. The understanding of how to harness this natural behavior of shockwaves led to the introduction in practice, in 1980, of extracorporeal shockwave therapy (ESWT). First used to destroy kidney stones in humans [2], nowadays, it is considered a safe, nonsurgical, effective treatment [3] for multiple disorders in orthopedics, physical therapy, sports medicine, urology, and veterinary medicine, particularly used in chronic Achilles tendinopathy and plantar fascitis, as well as bone defects [4]. ESWT is considered to be a fast pain relief and mobility restoration treatment while the noninvasive advantage ensures that no painkillers are needed [5]. Numerous investigations in sport-related injuries [6], knee osteoarthritis, upper limb diseases, treatment of plantar fascitis, erectile dysfunction [7; 4; 8; 9], musculoskeletal conditions, bone consolidation [10; 11; 12], human myofasicalpain syndrome, fibromyalgiaalgia [13; 14], and urinary or cardiovascular diseases [15; 16; 17] show that this method leads to a faster healing time and an improvement in functionality and quality of life [5; 18; 19].  
The mechanism producing shockwaves consists of a ballistic device that uses compressed air to move a projectile at high velocity into the applicator, generating radial pressure waves. These shockwaves have a physical phase, which is composed of a maximum of 40 MPa positive pressure that transmits energy to the tissue and cells, lasting as long as 2-3 \(\upmu\)s, and a minimum of \(-15\) MPa negative pressure, as highlighted in Figure 1[20], which induces physical effects (cavitation and increase of permeability in cell membranes) that last for 5 \(\upmu\)s. Besides this mechanism, it is believed that other physicochemical, chemical, and biological phases take place during the treatment [12].  
Although numerous studies have been conducted in the field of ESWT, they are focused on the effects the therapeutic benefits induce, only summary information regarding the applicator optimization being available. Distinct clinical studies mention the shape of the applicator (convex or concave), but not the way it can be improved. Therefore, we consider that present applicators can be developed so that they can be used in more than only one application area at a time. Taking into consideration the spinal pain and muscle contracture treatment, which implies applying the shockwaves successively on each side of the spinal cord, a dual-tip applicator would increase the application area, by targeting both sides of the tissue (in this case, the efficiency in terms of the number of shocks distributed will be double--in comparison with the standard protocol). The main objective is to design a tool that can allow simultaneous passes along both sides of the spinal cord while offering improved results compared to the classical applicator.