Nothing like a tendon or ligament injury to teach you the meaning of time. It's tough enough for a person to wait out the long months of inactivity it takes for those fibrous tissues to heal. It's even worse for a horse, whose injured leg must continue to bear at least a share of a thousand pounds of body weight. Tendon or ligament injuries are slow to heal and, if they don't lead to outright retirement, they can keep horses out of work for many months or even a year or more. Little can be done to speed the healing process, and for some horses no amount of time is enough for a return to soundness.
That may be changing. A growing number of veterinarians and researchers are becoming convinced that extracorporeal shock wave therapy (ESWT), a noninvasive treatment borrowed from human medicine, can speed the healing of several types of equine orthopedic injuries and conditions. Introduced in the United States about six years ago, ESWT uses a tightly focused beam of energy waves that can penetrate the skin and muscle tissue to reach injured ligaments and bones.
The therapy, adapted from the technology used to break up kidneys stones (a procedure called lithotripsy), has been used in Europe for more than a decade. In fact, European researchers have reported that some 85 percent of people who have received ESWT for chronic conditions like heel spurs and tennis elbow experienced pain relief and improved function. Success rates tend to be lower in more scientifically rigorous, placebo-controlled studies, but they were high enough for the U.S. Food and Drug Administration to approve ESWT for treatment of heel spurs and tennis elbow that fail to respond to conventional therapies for at least six months.
Based on these positive results, more and more veterinary practices around the country have begun offering ESWT. Though no nationwide figures are available, "the number of users has just skyrocketed in the last year or two," says Scott McClure, DVM, PhD, of Iowa State University. "When I first came to Iowa State two years ago, we were the only people offering ESWT between here and Chicago. Now it's in Omaha, Des Moines and every other major city."
As the veterinary use of ESWT continues to rise, research into the effects of the therapy on horses is only just beginning. The preliminary results, however, are encouraging. In multiple studies conducted at Iowa State University, the University of California- Davis and elsewhere, ESWT has been shown to measurably reduce lameness among horses with navicular syndrome, ligament injuries and other chronic orthopedic problems that had failed to respond to conventional treatments.
If ongoing research yields similar results, says Jack Snyder, DVM, PhD, of the University of California-Davis, ESWT may one day become the treatment of choice for some equine musculoskeletal injuries. "It takes a while for research to catch up with practice," says Snyder, "but it's becoming clearer that what research we have so far is showing that there is a positive effect from ESWT."
Not So Shocking
Despite its name, ESWT has more in common with ultrasound imaging than with electroshock therapy. "When people hear 'shock' they think it's electrical, but it's not," says Snyder. "It's a pressure wave." Any action that displaces its surrounding medium causes a shock wave--just as a tossed stone pushes away the water and causes concentric ripples on the surface of a still pond. "Thunder is the shock wave caused by lightning," Snyder explains. "An earthquake sends shock waves through the ground."
Like ultrasound technology, ESWT is the application of acoustic waves that penetrate living tissue. But ultrasound waves are of low intensity compared to those produced in ESWT. "The energy in ESWT is hundreds of times higher," says McClure.
These high-energy waves pass relatively unimpeded through the fluid-filled cavities and cells of soft tissue like skin, fat and muscle. But, says McClure, when they encounter the resistance of denser structures such as ligament or bone, the waves slow to a near halt. This "crash" releases large amounts of energy, which is absorbed by the surrounding tissue. This, in turn, unleashes several physical forces within the tissue. First, the tissue struck by the leading edge of a shock wave experiences compression, just as a torpedo shooting through water pushes on the water in front of its nose, squeezing it as it is pushed out of the way. But the bone or tendon cannot yield the way water does. Instead, sudden, intense shearing forces develop as the shock waves attempt to push aside the denser tissues but cannot. Also, in the wake of each high-energy wave is a trough, where the pressure in the tissue actually drops below its starting point; this sudden drop in pressure causes cavitation, the formation of gas or vacuum bubbles--just as bubbles form in the wake of the speeding torpedo. Those bubbles collapse almost immediately when the surrounding fluid moves back into the area it had just been pushed out of, restoring the original pressure level, but all of that motion causes a series of currents in the fluids within the tissues that continue after each wave has passed.