# Effects of Mechanical Vibration in Equine Osteoarthritis: A Pilot Study
## 4 Discussion  
TUs are used both in companion animals and athletes to treat various diseases and injuries affecting tendons, ligaments, muscles, joints, and bones. There is limited information about the use of TUs in horse joints in the literature, particularly regarding the treatment of OA. To date, few veterinary medicine studies on the topic have been published. Moreover, the little scientific knowledge on the effects of TUs on dogs OA comes from a single cohort study without a control group and with a high risk of bias [24]. In particular, there are no studies about OA in horses, although there are two studies on traumatic arthritis in donkeys [25; 26]. In the four donkeys assigned to TU, the authors observed earlier improvement of lameness, faster reduction of joint swelling, and less pain at flexion than in the other four, non-treated donkeys. Synovial cytological and biochemical analyses indicated less inflammation in the TUs group. In addition, the histomorphological examination revealed signs of improved healing and reduced signs of cartilage degeneration in TU-treated donkeys [25; 26]. The present study was mainly designed starting from the need to clarify the use of TUs in horses and to better understand its clinical effects on spontaneous onset OA. To begin with, we inquired about the operating mechanism of TUs. We know from experiments conducted on healthy horses that the temperature changes in the tendons of treated horses [27]. In fact, in the present study, at the end of a 10- to 20-minute treatment session, the temperature had increased by 2.5-5.2 \({}^{\circ}\)C in the superficial and deep digital flexor tendons of the thoracic limb (one 10 min session, continuous mode of 3.3 MHz). Another study showed that a 1\({}^{\circ}\) increase in temperature triggers metabolic activity, 2\({}^{\circ}\) to 3\({}^{\circ}\) decreases muscle spasms and increases blood flow, and 4\({}^{\circ}\) changes the viscoelastic properties of collagen [1; 13]  
US propagation in biological tissue is fundamentally a non-linear process. This non-linearity becomes more dominant with the increase in intensity, where secondary phenomena, which are not predicted from a purely linear propagation model, begin to emerge [28]. These effects include cavitation, radiation forces, and micro-streaming [29]. Except for cavitation, the magnitudes of the other mechanisms are often insignificant [30]. The cavitation effect is the most widely known. It consists of the formation, growth, and implosion of gas bubbles within the fluid subjected to an ultrasonic field. The gas bubbles are generated and then expand and contract in tissue when ultrasonic pressure goes from a positive peak to a negative peak [31]. Despite the fact that cavitation nuclei are not present in human and animal tissues and blood, we assume a priori that nuclei exist, and we focus instead on the dynamics of an acoustically small gas bubble in a Newtonian fluid undergoing radial oscillations--all key approximations intended to simplify the model while retaining most of the essential underlying physics [4; 32]. In particular, the action of a specific intense ultrasonic field within a liquid is called \"acoustic cavitation\" and is generally divided into two categories: stable and transient. Stable cavitation occurs from low to moderate pressure amplitudes (less than w1 MPa with an inverse dependence on frequency). A radially oscillating bubble inside a liquid medium can grow in an acoustic pressure field as a result of rectified diffusion. In this process, during the expansion half cycle, due to the decreasing pressure inside the bubble, some of the liquid surrounding the bubble can diffuse through the boundary layer, turning into vapor inside the bubble. In the subsequent contraction half cycle, as a result of the increase in pressure inside the bubble, some of this vapor will condense and diffuse out.