Scientists recently confirmed that shock wave therapy has an analgesic (pain-killing) effect on horses that had been observed anecdotally, and they also found that the therapy does not have a harmful effect on bone. Scott McClure, DVM, PhD, Dipl. ACVS, assistant professor of surgery at Iowa State University, presented the results of these studies at the 2003 American Association of Equine Practitioners' convention. 

There's been a lot of speculation about the analgesic effects of extracorporeal shock wave therapy (ESWT) and radial shock wave therapy (RSWT)–no one has identified the duration or mechanism of analgesia. Some have thought that nerves and/or nerve receptors might be destroyed, but further study on the nerves of frogs (not horse feet) showed that nerves were not affected directly, but that small gas bubbles produced by the shock waves interacted with the nerves. Scientists weren't sure if this theory was applicable in live animals, particularly in the horse's distal limb.  

Early studies of ESWT in lab rodents showed microfracture of bones, but consequential studies in other species have not supported those findings. McClure his colleagues recognized that a change in elasticity or microstructure of equine bone, the bone might be weakened and more prone to catastrophic failure. 

The scientists showed that there is a potential risk associated with analgesia in the horse, which supports the post-treatment withdrawal periods enacted by racing jurisdictions and the Federation Equestre Internationale. "These data indicate that a horse should not be subjected to strenuous activities where local analgesia might pre-dispose the horse to injury for at least four days after ESWT or RSWT treatment," wrote the researchers in their summary paper. Exercise should be limited in animals under shock wave treatment to protect them from re-injury, and most horses under treatment require rest for the underlying problem anyway