The success rate of extracorporeal shock wave therapy (ESWT) for fracture nonunions in human medicine (i.e. radiographic union at 6 months after ESWT) is only approximately 75%. Detailed knowledge regarding the underlying mechanisms that induce bio-calcification after ESWT is limited. We analyzed the biological response within mineralized tissue of a new invertebrate model organism, the zebra mussel Dreissena polymorpha, after exposure with extracorporeal shock waves (ESWs). Mussels were exposed to ESWs with positive energy density of 0.4 mJ/mm2 (A) or were sham exposed (B). Detection of newly calcified tissue was performed by exposing the mussels to fluorescent markers. Two weeks later, the A-mussels showed a higher mean fluorescence signal intensity within the shell zone than the B-mussels (P<0.05). Acoustic measurements revealed that the increased mean fluorescence signal intensity within the shell of the A-mussels was independent of the size and position of the focal point of the ESWs. These data demonstrate that induction of bio-calcification after ESWT may not be restricted to the region of direct energy transfer of ESWs into calcified tissue. The results of the present study are of relevance for better understanding of the molecular and cellular mechanisms that induce formation of new mineralized tissue after ESWT.

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BackgroundTo improve understanding of shockwave therapy mechanisms, in vitro experiments are conducted and the correlation between cell reaction and shockwave parameters like the maximum pressure or energy density is studied. If the shockwave is not measured in the experimental setup used, it is usually assumed that the device’s shockwave parameters (=manufacturer’s free field measurements) are valid. But this applies only for in vitro setups which do not modify the shockwave, e.g., by reflection or refraction. We hypothesize that most setups used for in vitro shockwave experiments described in the literature influence the sound field significantly so that correlations between the physical parameters and the biological reaction are not valid.MethodsTo reveal the components of common shockwave in vitro setups which mainly influence the sound field, 32 publications with 37 setups used for focused shockwave experiments were reviewed and evaluated regarding cavitation, cell container material, focal sound field size relative to cell model size, and distance between treated cells and air. For further evaluation of the severity of those influences, experiments and calculations were conducted.ResultsIn 37 setups, 17 different combinations of coupling, cell container, and cell model are described. The setup used mainly is a transducer coupled via water to a tube filled with a cell suspension. As changes of the shockwaves’ maximum pressure of 11 % can already induce changes of the biological reaction, the sound field and biological reactions are mainly disturbed by use of standard cell containers, use of coupling gel, air within the 5 MPa focal zone, and cell model sizes which are bigger than half the −6 dB focal dimensions.ConclusionsUntil now, correct and sufficient information about the shockwave influencing cells in vitro is only provided in 1 of 32 publications. Based on these findings, guidelines for improved in vitro setups are proposed which help minimize the influence of the setup on the sound field.Electronic supplementary materialThe online version of this article (doi:10.1186/s40349-016-0053-z) contains supplementary material, which is available to authorized users.

Cavitation is a major fracture mechanism in extracorporeal shock wave lithotripsy (ESWL). However, it can cause tissue trauma and its effects on kidney stones and surrounding tissue are not fully understood. Therefore experimental setups enabling systematic parameter studies are crucial. We developed and evaluated a testing rig comprising three measuring methods in order to examine this mechanism. Our initial evaluation of this setup based on standard components showed promising results. Primary cavitation was displayed by high-speed photography 195 μs after the shock front had passed the focal zone. The effect of different pulse repetition rates (30, 60, 90, 120 SW/min) on the extension of the cavitation area was determined. The lifetime of secondary cavitation was analysed by B-mode ultrasound imaging. In a post processing progress the images showing bubbles were compared to a reference picture for both types of cavitation and the number of pixels that changed colour was counted. Furthermore stone comminution at different pulse repetition rates (30, 60, 90, 120 SW/min) was investigated by fixed-dose fragmentation. We observed an inverse correlation of cavitation and fragmentation. As the pulse repetition rate increases, the area of primary cavitation grows whereas the fragmentation efficiency decreases. B-mode imaging showed that secondary cavitation bubbles persisted between the shocks and can serve as nuclei. The higher the pulse repetition rate is, the more of these nuclei remain and thus facilitate formation of primary cavitation. The experimental setup provides reproducible results regarding the development of primary and secondary cavitation on the one hand and the fragmentation of phantom stones on the other hand. Therefore it can be utilized to further investigate the effect of different boundary conditions and shock wave parameters on cavitation and stone comminution. The impact of different focal sound fields is subject of ongoing research.

A substantial body of evidence supports the use of extracorporeal shock wave therapy (ESWT) for fracture nonunions in human medicine. However, the success rate (i.e., radiographic union at six months after ESWT) is only approximately 75%. Detailed knowledge regarding the underlying mechanisms that induce bio-calcification after ESWT is limited. The aim of the present study was to analyze the biological response within mineralized tissue of a new invertebrate model organism, the zebra mussel Dreissena polymorpha, after exposure with extracorporeal shock waves (ESWs). Mussels were exposed to ESWs with positive energy density of 0.4 mJ/mm2 or were sham exposed. Detection of newly calcified tissue was performed by concomitantly exposing the mussels to fluorescent markers. Two weeks later, the fluorescence signal intensity of the valves was measured. Mussels exposed to ESWs showed a statistically significantly higher mean fluorescence signal intensity within the shell zone than mussels that were sham exposed. Additional acoustic measurements revealed that the increased mean fluorescence signal intensity within the shell of those mussels that were exposed to ESWs was independent of the size and position of the focal point of the ESWs. These data demonstrate that induction of bio-calcification after ESWT may not be restricted to the region of direct energy transfer of ESWs into calcified tissue. The results of the present study are of relevance for better understanding of the molecular and cellular mechanisms that induce formation of new mineralized tissue after ESWT. Specifically, bio-calcification following ESWT may extend beyond the direct area of treatment.Summary statementThe use of zebra mussels in research on extracorporeal shock wave (ESW) therapy for fracture nonunions allows new insights into the complex process of induction of biomineralization by ESWs.