Abstract
Plantar fasciitis is a chronic pain condition caused by the inflammation of the connective tissue along the sole of the foot (the plantar fascia). The condition affects up to 10% of the total population during the course of their lives. While non-invasive treatment methods are effective in most cases, surgical procedure is considered if the pain persists for several months. Shock wave therapy has been used as a last resort to avoid surgical intervention after other conservative treatment methods have proved to be ineffective. Radial shock wave therapy uses a ballistic method to generate high-amplitude pressure in the tissue; a metal object of 6-15 mm in diameter (an applicator) is applied superficially to the location that has to be treated. The applicator is impacted by a small cylindrical metal object (projectile) which is driven by compressed air. The understanding of the mechanical effects of shock wave therapy is not complete, and the insight into the mechanisms through which these effects may promote the healing process is at the level of speculation. The objectives of this research are to gain a better understanding of: the physical mechanisms through which the ballistic shock wave source operates; the mechanical stimuli that this method produces in the context of plantar fasciitis treatment; and the potential biological effects of these stimuli on the tissues of the foot. A finite element model of the pressure wave source was constructed based on the geometry of an actual device. The model consisted of the applicator, the projectile, parts of the casing, and the o-rings on which the applicator is suspended in the casing. A finite element model of the foot was also constructed. The model was based on the geometry reconstructed from MRI images of a volunteer and it comprised of bones, cartilage, soft tissue, plantar fascia, and Achilles tendon. The material properties for the model components were taken from the literature. Simulations were conducted both to characterise the behaviour of the shock wave source and to simulate the effect of shock wave therapy on the foot. When the shock wave device is “fired in the air”, the bulk movement of the applicator is in the form of highly damped oscillations with a maximum displacement of 0. 2 mm and a period of about 0. 5 ms. The collision between the projectile and the applicator lasts for about 10us which is the time needed for the stress wave to traverse the applicator in both directions. After the collision, there is a standing stress wave in the applicator with amplitude of the order of 10 MPa. When the device is applied to the soft tissue of the foot, pressure waves are generated that propagate in all directions. The waves consist of positive and negative pressure phases with the wave length of approximately 10 mm. The magnitude of the pressure generated in the soft tissue is of the order of several MPa, which is consistent with data reported for experiments conducted in water. The pressure amplitude at the surface of the soft tissue can be related in a simple way to the speed of the projectile and acoustic impedance of the soft tissue. The pressure magnitude and energy density decrease rapidly with the distance from the source, so that the effect of the treatment is localised to the region where the device is applied. The negative pressure at the plantar fascia origin reaches values of up to 3 MPa which should be sufficient to generate cavitation in the tissue. This result supports the hypothesis that cavitation-induced micro-trauma may be one of the mechanisms that enhance the healing process. Simulations of multiple pulses delivered at 20 Hz show that energy transferred to the foot with a pulse is not dissipated before the subsequent pulse. This result suggests that multiple pulses may lead to accumulation of energy in the foot.