Date Published: December 19, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Yajun Liu, Xiaodong Chen, Anyi Guo, Sijin Liu, Guoqing Hu.
Although radial extracorporeal shock wave therapy (rESWT) has been widely used to treat orthopedic disorders with promising clinical results, rESWT largely relies on clinicians’ personal experiences and arbitrary judgments, without knowing relationships between administration doses and effective doses at target sites. In fact, practitioners lack a general and reliable way to assess propagation and distribution of pressure waves inside biological tissues quantitatively. This study develops a methodology to combine experimental measurements and computational simulations to obtain pressure fields from rESWT through calibrating and validating computational models with experimental measurements. Wave pressures at the bottom of a petri dish and inside biological tissues are measured, respectively, by attaching and implanting flexible membrane sensors. Detailed wave dynamics are simulated through explicit finite element analyses. The data decipher that waves from rESWT radiate directionally and can be modeled as acoustic waves generated from a vibrating circular piston. Models are thus established to correlate pressure amplitudes at the bottom of petri dishes and in the axial direction of biological tissues. Additionally, a pilot simulation upon rESWT for human lumbar reveals a detailed and realistic pressure field mapping. This study will open a new avenue of personalized treatment planning and mechanism research for rESWT.
As a noninvasive technology, extracorporeal shock wave therapy (ESWT), was developed from the extracorporeal shock wave lithotripsy that is conventionally used to destroy kidney stones or biliary calculi using an externally applied, focused, and high‐intensity acoustic pulse.1 Further, ESWT was also customized to treat orthopedic diseases, for example, pseudarthrosis,2 plantar fasciitis,3 lateral epicondylitis,4 shoulder calcific tendinitis,5 Achilles tendinopathy,6 and osteonecrosis of the femoral head.7 There are two types of ESWT according to their wave patterns, namely, focused and radial. By definitions, focused ESWT creates a pressure field with a focal zone in the treatment region, while radial ESWT (rESWT) generates radially expanding pressure. In the rESWT device, the projectile is driven by compressed air or a magnetic field to impact onto the applicator at the head end of a handpiece for wave generation. As an economic and noninvasive physical therapy with good clinical effects, applications of rESWT have gradually increased in recent years.8
In this paper, we have established a combined experimental and numerical methodology to quantify pressure waves generated by a ballistic rESWT device. Direct measurements were carried out by placing a thin and flexible film sensor on the bottom of a petri dish and inside biological tissues. Numerical simulations were based on explicit dynamic analyses considering realistic geometrical and mechanical properties. 3D evolutions of pressure waves were obtained for three configurations, as well as axial and radial distributions of peak pressures. Simulation results were compared with experimental measurements to calibrate and validate numerical models. Pressure waves can be regarded to be generated from a circle piston with a directional distribution of wave energy. The amplitudes of wave pressures along the axial direction are inversely proportional to the axial distance approximately. Finally, numerical simulations were carried out for a more realistic clinical practice, treatment of protrusion of intervertebral disc. Proper applied position and direction were identified from reconstructed tissues based on CT images. Simulations suggested that the pressure wave can arrive at the intervertebral disc. However, the amplitudes were most probably too small to generate effective biological responses. This study combined practicable technologies of pressure measures and simulations. Thin and flexible film sensors were robust for measuring wave pressure inside biological tissues. CT‐based reconstruction of human tissues and explicit dynamics analysis were useful tools to evaluate wave pressures in the target sites. Combined with clinical practices, the methodology can provide a route toward personalized treatment planning and mechanism study for rESWT.
Numerical simulations are capable of obtaining comprehensive information of the pressure field, which is difficult for experimental measurements, especially inside biological tissues with complex geometries.[[qv: 12b,30]] Finite element method has been used to simulate the generation and propagation of the ballistic type rESW.18, 20 However, the finite element method has not been widely used in studying rESWT. One reason is the lack of calibration between the impact velocity of the projectile U and the driven pressure Pin of the rESWT device. Another reason is that numerical and material models have not been validated by experiments. We thus carried out detailed finite element simulations with calibration and validation from experimental measurements.
Experiments were designed to measure the pressure propagation either in the water within a petri dish or inside porcine tissues. Pressure waves were generated by a ballistic shock wave therapy device with a standard 15 mm applicator (Masterpuls MP100, STORZ Medical AG, Switzerland), which created radial shock waves in the medium contacting with the applicator. The collision of the projectile and the applicator caused movement of the headpiece during clinical treatment. To avoid the uncertainty caused by this motion, the handpiece of the ESWT device was firmly clamped by three circular rings of a supporting frame (Figure S4a, Supporting Information). The vertical distance between the tip of the applicator and the measuring position was tuned through the vertical positioning of the rings. This adjustable feature also allowed controlling the contact between the applicator and the porcine skin.
The authors declare no conflict of interest.