Shock-wave therapy should only be applied by a veterinarian experienced in its use. Photo courtesy of Dr. Scott McClure

Shock-wave therapy, as a form of equine pain relief therapy, has received a lot of press, both good and bad.  Since shock-wave therapy's  introduction a few years ago, this equine pain relief therapy been called everything from a near miracle to the most dangerous thing you can do to a performance horse. Trying to figure out the truth was severely hampered by the fact that most of the information was anecdotal. We now have better studies to learn from.

What It Is
Without getting overly technical, “shock waves” are waves of extremely high-pressure ultrasound, like the force that travels over distance from an explosion. Shock waves travel smoothly through many different mediums but disrupt and release their energy when they come in contact with a material of a different density.

Shock waves from an explosion can travel smoothly great distances through the air. Any glass that might be in their path would be shattered. The glass is shattered because it gets in the way of the forward progress of the wave (called impedance), because it has a different density from air and because it’s relatively fragile. The waves also impact the surrounding wood or metal in the window frame but wouldn’t damage it unless the explosion was close.

In medical shock-wave therapy, mini impacts and explosions occur inside the body. High-powered shock waves can shatter human kidney and bladder stones because the difference in impedance between the body tissues, fluids and the stone itself is so great. Some, but much less, cellular damage also occurs in blood vessels because of the difference in impedance between the blood fluid and the blood vessel wall, and in the walls of organs like the bladder (bladder wall vs. urine) for the same reason.

Elsewhere in the body, the same principles apply. The effects of the waves will be concentrated in areas where there is a difference in how easily they can pass, such as junctions between tendon and bone. The more different the tissues are, the greater the effects.  The properties of the shock waves raise some interesting questions about uses other than in shattering urinary-tract stones, plus some safety concerns with its use.

Put It To Use • This is a veterinarians-only procedure. • Shock wave is worth considering for nonunion fractures. • Immediate pain relief may not mean healing. • Follow-up exams are necessities.

Usefulness
Shock-wave therapy was first tried for orthopedic problems in human medicine.  The first reports, from Europe, began to appear in the scientific literature about 13 years ago, with trials starting about 20 years ago. It’s been used to treat fractures that have failed to heal (nonunion fractures, also called pseudarthroses), and elbow, shoulder or heel problems related to pain and inflammation in areas where tendons or ligaments attach to bone. 

Today, there is generally good agreement only that shock-wave therapy is effective for nonunions. Its usefulness for other bone and tendon problems continues to be questioned. Well-controlled studies comparing it to either no treatment or other types of treatment are few and give conflicting results, ranging from no positive effects to 80% positive effects.

The therapy has been used for a much shorter time in horses. As in people, the clearest indication is nonunion fractures and problems involving the insertion sites of ligaments or tendons onto bone such as splints (a ligament attaches the splint bone to the cannon bone), high/blind suspensory/splint problems, sesamoiditis (which occurs where the sensory ligament inserts), tears of joint capsule or ligaments surrounding and stabilizing joints, and problems inside joints related to damage to the tiny ligaments between the bones. One particularly promising application is for treatment of foot pain originating from damage to the insertion of the deep flexor tendon on the coffin bone.

Shock-wave effects have been specifically studied in:
• Experimentally induced suspensory lesions: Shock-wave therapy (three treatments, three weeks apart) caused more rapid healing than what was observed in untreated legs (Caminoto, Brazilian study; McClure, Iowa State study)

• Dr. Scott R. McClure, Iowa State, also did a study where horses with navicular syndrome were treated and followed for six months. At six months, 81% of horses improved when evaluated trotting in a straight line, 70% improvement in lameness on circling.  When these same horses were evaluated by videotape by a veterinarian who did not know they had been treated, 56% were felt to have improved. X-rays actually worsened slightly. All treatments had been carefully focused on the navicular area, so it would only have been of possible benefit to horses that actually had navicular-area disease as a cause of their pain. (See our September article on navicular.)

What To Expect If Your Horse Has Shock-Wave Therapy 1) Because the shock waves have to be directed precisely to the problem area, before even considering using it, you should expect the horse to have a thorough and detailed workup to accurately diagnose the problem and localize the area to be treated. This will include a full lameness exam, with nerve blocks; X-rays and/or fluoroscopy; and ultrasound. 2) One to three treatment sessions will be needed for the initial round of treatments. 3) Depending on the type of equipment used and the preference of the treating veterinarian, it may be done with the horse sedated and standing, using local anesthesia by nerve blocks or the horse may be put under with a short-acting general anesthetic. 4) Some low-level tenderness and swelling may or may not be present at the treatment site but resolves over a few days. 5) Because of the analgesic effect that many horses experience, it’s extremely important to follow the veterinarian’s detailed instructions regarding exercise and turn out to avoid further injuring the horse. Reduced pain doesn’t mean the horse has healed. 6) Follow-up examinations are necessary to follow healing and make safe adjustments to the horse’s level of activity.

• Dr. G.D. McCarrol and Dr. McClure reported that 80% of horses with lower-hock-joint arthritis (“spavin”) had improved at least one lameness grade when evaluated three months after treatment, but only 18% were actually sound. Again, radiographs were unchanged.

• The equine orthopedic team at Colorado State used a surgically created osteoarthritis model (in the knees/carpus) to compare the effects of time alone, Adequan, and two shock-wave treatments on pain relief, cartilage metabolism/repair and joint fluid characteristics. Horses were followed for 42 days and were exercised starting the fifth day after surgery.

Both the Adequan-treated and shock-wave treated horses showed significant improvement compared to untreated horses in terms of lameness and markers of cartilage repair or breakdown. Synovial-fluid evidence of inflammation was also reduced in those two groups compared to control, but microscopic examination of the joint cartilage showed no difference between the groups when the experiment ended at 42 days, so the long-term outcome may not have differed.

There have also been case history and anecdotal reports of decrease in healing time for serious cannon bone fractures, healing of incomplete suspensory fractures and healing of high-suspensory lesions.

Safety
Use of shock waves over the eyes, intestines or the lungs can and does cause serious organ damage and hemorrhage. No veterinarian familiar with shock-wave therapy would ever do this—nor is it indicated for treatment of any problems in those organs—but careless use by non-veterinarians could be a problem. Using higher-than-recommended settings for number of pulses or the strength of the pulses could also cause considerable tissue damage anywhere on the body. This should definitely be a “vets only” procedure.

Safety issues among vets refer primarily to microfracturing of bone and the pain-relieving effects. Shock wave can to worsen stress fractures. These tiny fractures are a normal part of bone remodeling in horses in heavy training but can become a problem if the horse is pushed too hard.

Common examples are bucked shins and more serious fractures of the cannon bone. These fractures can sometimes be difficult to see on a routine X-ray.

Studies have shown that shock-wave therapy can worsen microfractures, with the potential increasing with the dose.  However, correct commonly used dosages didn’t change the mechanical properties of bone in one study. Nevertheless, experts agree that the potential is there to further damage bone.

The Weird Pain-Relief Effect Up to about 60% of human patients receiving shock-wave therapy report an analgesic effect afterward. This may be temporary, while in some cases the pain remains reduced or even is gone. Laboratory animals show higher pain thresholds in areas that have been treated with shock waves, too, and reduced lameness can be seen in horses after a treatment, again with a full range from no improvement to dramatic and/or long-lasting changes. Dr. McClure, a pioneer of equine shock-wave therapy, did a study where skin samples were taken from treated areas and the local nerves examined. There was no change found in pain-signaling neurotransmitter proteins, like substance P, but the nerves themselves showed some direct damage. They also found a higher skin threshold for pain from electrical current for three to four days after the treatments. How this might relate to pain coming directly from an injured/diseased area is not clear. Bolt et al at Louisiana State reported last year that digital nerves (“heel nerves”) treated with shock-wave-conducted sensory-nerve impulses slow for as long as 35 days (the longest they tested) after treatment. The nerves themselves weren’t damaged except for damage to the sheath surrounding them. While there’s still no explanation for how the shock waves influence nerve function, long-lasting changes in the nerve’s ability to transmit pain sensations may prove to be why.

The pain-relieving effects caused quite a stir a few years ago when top Thoroughbred jockey Chris McCarron announced he felt all shock-wave therapy should be banned because of the potential to block pain from serious injuries and result in catastrophic accidents on the track. That’s probably an exaggeration of the pain-relieving effectiveness of correct therapy but, unfortunately, these machines can find themselves in the wrong hands. There’s some risk it could be used to keep horses going that shouldn’t be worked.

As a result, many racing jurisdictions have ruled that all shock-wave therapy machines on the track must be in the hands of vets only and have to be registered. Treated horses must be reported and are prohibited from racing for from six to 14 days after treatment. Even that interval may be too short.

Forms Of Shock-Wave Therapy
The common acronym for shock-wave therapy in medical circles is ESWT - Extracorporeal Shock Wave Therapy. Extracorporeal means outside (extra) the body (corpus), indicating the waves are generated outside the body. It produces a shock wave that has its peak effects at a narrow focus.

Another form is called RPWT - Radial Pressure Wave Therapy. The shock waves generated by this equipment have a wider focus point. Both are reported to be effective. Although there’s more research behind ESWT, there’s no clear indication whether one or the other would be better in specific situations.

Bottom Line
Only a veterinarian thoroughly familiar with the treatment can advise you whether your horse’s specific condition is appropriate for shock wave. Most veterinarians prefer to exhaust more time-tested therapies before going to shock wave. There’s simply not enough information from large-scale studies to really tell if shock wave is the best treatment option for some conditions.

However, the types of problems that would probably be most likely to be helped are:
• Fractures that haven’t healed with conventional treatment or that can’t be effectively treated by those methods because of their location or size.

• Problems involving the insertion site of a tendon or ligament on bone (e.g. head of the suspensory ligament, insertion of suspensory on the sesamoids, insertion of the deep flexor tendon on the coffin bone).

• Suspensory inflammation/damage, possibly bowed tendons. 

• Some  success, although possibly only related to pain relief, has been reported for navicular disease and other arthritic conditions, with good results in an experimentally produced arthritis in the knees.