Shock wave therapy (Proceedings)

Early studies of shock waves (SWs) for ureteral stones resulted in radiographically evident remodeling of the pelvis.1 These findings sparked the initial studies investigating the use of SWs in orthopedic applications. These studies and anecdotal clinical use resulted in the initial 5 standard indications used in human medicine: 1) calcifying tendonitis of the shoulder (tendinosis calcarea), 2) tennis elbow (lateral epicondylitis), 3) golfer's elbow (medial epicondylitis), 4) heel spurs (plantar fasciitis), and 5) pseudarthrosis. The utilization of focused shock wave therapy (SWT in horses) started in Germany in 1996. Applications were based on the human indications. Because of positive experiences treating people with insertional desmopathies, the first equine application was proximal suspensory desmitis. Initial clinical responses were positive, therefore SWT was attempted in numerous other equine conditions. In 1997, horses with navicular syndrome were first treated. Initially, the probe was positioned behind the navicular bone from the heel, but when ultrasonographic imaging showed the distal impar ligament could be visualized through the frog, this location was then used to administer SWT to the distal part of the navicular region.

In 1998, horses with distal hock joint and navicular pain were first treated in the United States with SWT. The first equipment was large, requiring water cycling and degassing. Its use was limited to anesthetized horses. The development of more affordable, portable and durable equipment has led to an expansion of its use and applications. Associated with the expansion has been a contraction of applications to those that have consistently provided good clinical outcomes.

Shock Wave Generators

Two distinctly different pressure waves have been lumped together into SWT. True SWs are pressure waves that meet specific physical parameters including a rapid rise time (within nanoseconds), high peak pressures, and a more gradual decrease in pressure over a few milliseconds, often with a negative pressure component. Shock waves occur naturally associated with lightening, planes breaking the sound barrier or explosions. Shock waves used for medical purposes utilize electricity as the driving energy source. Pneumatically powered sources drive a metal rod to strike a plate in contact with the skin surface which create radial pressure waves (RPWs) Radial pressure waves have slower rise times and lower peak pressures than SWs. The importance of recognizing the differences between these modalities is that they may not affect tissue similarly; consequently one must consider the equipment being used when evaluating the effect on tissues and efficacy of treatment and how to use it. A complete review of SWs and RPWs, along with all of the equipment currently available, has recently been published.

Dose Dependence

There is a dose effect of SWT which is a combination of the energy flux density (EFD) and the number of pulses. The EFD is the energy (millijoules) in the focal area (mm2) resulting in the EFD (mJ/mm2). There is a lower EFD that must be reached to be effective and a point where the EFD is high enough to create damage at any number of pulses. The range is dependent upon a number of factors including species, tissue being treated, waveform and generator.

It is difficult to transpose doses from other species to horses. When 1000 pulses at an EFD of 0.28 mJ/mm2 was applied to rabbit Achilles tendons there was an inflammatory reaction and at 0.6 mJ/mm2 there was inflammation and tendon necrosis. Energy levels at this or higher have been used in the horse without complications. However, the minimal dose to achieve the desired affect is not known, and we may not need to utilize levels this high. There is a trend toward lower EFD's for a number of applications. One thousand pulses at 0.18 mJ/mm2 stimulated neovascularization of the tendon bone junction in dogs. A dose of 200 pulses at 0.12 mJ/mm2 was better than 500 pulses in an Achilles tendonitis model in rats. At this time, the ideal energy levels and pulse numbers remain empirical in most equine applications.

The frequency (number of pulses per second) of the pulse application does not seem important except for the time required to complete the treatment. All are relatively slow frequency from a biological standpoint and do not result in tissue heating or other biological effects seen at high rates.

Applications

Bone

Early studies of SWs on bones yielded variable results and were confusing. Very high energy treatment of rodent and rabbit bones resulted in physical disruption of the bone and periosteum resulting in the thought that bone remodeling was the result of this physical damage. Studies in more appropriate tissue and species have resulted in a stimulation of bone formation without physical disruption. The ability to stimulate bone healing resulted in the interest in utilizing SWs for the treatment of nonunion fractures. Nonunion fractures were created in 10 beagles with segmental radial osteotomies. Five of the dogs with nonunions were treated with 4000 SWs at 0.54 mJ/mm2 and 5 served as untreated controls. All of the treated dogs had osseous union by 12 weeks after treatment compared to only 1 dog in the control group. The application of SWs to nonunion fractures in people was the first large number of applications of musculoskeletal SWT in people. Clinical trials have demonstrated bone healing following SWT in delayed and nonunions. The success rates are fairly consistent between investigators. Wang reported 80% success 12 months after treatment in 72 nonunions of long bones, Schaden had a 75% success rate in delayed unions and nonunions in 115 patients and Rompe reported on 43 patients with femoral and tibial nonunions with 72% healed in 4 months.

The mechanisms of SW induced osteogenic stimulation are being investigated. There is an increase in osteoprogenitor colony forming units from the marrow of rat femora 24 hours after treatment with 500 SWs at 0.16 mJ/mm2. A dose response was seen with the highest TGF-Β1 concentrations in the supernatant after 500 SWs with lower numbers of pulses ineffective and higher numbers inhibitory. The same treatment protocol (500 SWs at 0.16 mJ/mm2) was administered to segmental defects of rats. This resulted in an increased expression of TGF-β1 and VEGF-A mRNA and an increase in osteoprogenitor cells in shock wave treated femora. In both of these studies, the SWT appeared to produce osteogenic stimulation without physical disruption of the bone as was initially thought. Subsequent studies have shown that SWT increases concentrations of VEGF and BMP-2 as well as neovascularization and ultimately an increase in load to failure in a rabbit femoral fracture model.

The application of SWT to bone was the initial musculoskeletal application in people and horses. The applications and outcomes in human medicine have grown and are positive. The applications of SWs in horses and Veterinary Medicine have not been pursued as aggressively as may have been expected. An initial concern was the potential creation of microfractures that could weaken the bone following treatment. One study indicated it was possible to increase microfractures. The application of 9000 pulses of RPWs at 0.175 mJ/mm2 or 9000 SWs at 0.15 mJ/mm2 to cadaver metacarpi from racehorses that suffered catastrophic injuries of the contralateral limb did find that SWT had a small but significant increase in microcrack density and RPWs were found to result in increased microcrack length. In dorsal cortical bone specimens of the third metacarpal bone (McIII), 2000 pulses administered in 500 pulse increments of both SWT (0.15 mJ/mm2) and RPWs (0.16 mJ/mm2), there were no changes in modulus of elasticity detectable. Histologically, there were no increases in microfractures demonstrated with either method. In vivo, studies utilizing 1000 pulses at 1.8 mJ/mm2, which is a very high EFD, did not lead to microfracture formation.

In an in vivo pilot study in horses that used 2000 SWs at 0.89 mJ/mm2, 2 treated MCIII had 30% more activated osteons than the opposite untreated control. A comparison of 2 SW treated third metatarsal bone (MT III) to 2 with periosteal irritation stimulated by scarifying with a needle was done to evaluate if this response was solely due to periosteal irritation. The SW treated MTIII had 56% more double labeled osteons than those with only periosteal irritation. Additionally, this pilot study demonstrated that the osteonal response was endosteal as well as periosteal. Scintigraphic and histological examination of the fourth metatarsal bone after 2000 pulses at 0.15 mJ/mm2 resulted in an increased number of osteoblasts and correlated with an increased radiopharmaceutical uptake in the treatment area.

Twenty-two Thoroughbred racehorses with dorsal metacarpal disease consisting of pain on palpation, a thickened dorsal cortex, and longitudinal radiolucent lines, that were unresponsive to conventional therapy were treated with RPWs. Three treatments at 2 week intervals were administered and a modified training program instituted. The mean time to the first race was 4.5 months. Similar results have been seen with focused shockwave therapy. We treat horses with periostitis with SWT (0.14 mJ/mm2 , 800 pulses), give 1 week of walking, then resume training for 2 weeks; the cycle is repeated 2 more times. When horses are treated early in the disease process most can continue in training.

Horses with stress fractures of the, tibia, humerus and MCIII are amenable to treatment with SWT. Without controlled studies, it is difficult to compare SWT to healing of non-treated fractures or treatments such as osteostixis. It is our impression that Thoroughbred racehorses with dorsal cortical fractures of the MCIII can return to training in a similar time period as those which have received osteostixis. Six of 10 horses with metacarpal stress fractures were pain free and/or showed radiographic healing by 90 days. Two horses took over 120 days and 2 were retired for other reasons. In 20 horses with a single oblique dorsal cortical stress fracture treated with RPWs, the mean time from the last treatment to the first race was 5 months.

Splint bone fractures and exostoses are frequently treated with SWT. In 4 horses with closed and 3 with open, infected, comminuted fractures of the proximal splint bones were managed with focused SWT. The infected horses were first treated locally and systemically with antibiotics, local wound curettage and ostectomy of small loose fragments. Shock wave therapy began when signs of infection were resolved and the skin was healed. The horses were treated with three treatments at 10 – 14 day intervals. They were administered 600 to 900 pulses, depending on the size of the lesion, at an EFD of 0.15 mJ/mm�. Radiographs were taken after the third treatment and about 4 weeks later. The horses were started hand walking after the second or third treatment. In horses without an infection, training began 10 to 12 weeks after fracture, whereas in those with infected fractures training began 13-16 weeks after fracture. All the horses were sound and returned to normal use.

Fractures of the distal phalanx that can be treated through the frog are good candidates for shock wave treatment. It is not possible to get the SWs to penetrate through the hoof wall on the sole, so they must be near the center of the distal phalanx and accessible through the frog. Individual clinical cases have shown positive outcomes.

Long bone fractures and arthrodesis stabilized with internal fixation have not been routinely treated by SW's. Expense may be a consideration, however access is a major limiting factor. Most horses have some type of external coaptation such as a cast limiting routine access. Shock wave therapy has been shown to improve callus formation in the immediate post-operative period. In dogs, bilateral 3 mm osteotomy gaps in the mid tibia were stabilized with a plate and treated with 2000 pulses at 0.18 mJ/mm2. At 12 weeks there was more radiographically visible callus and more cortical bone formation evident histologically than control untreated dogs. Early treatment of fractures may be beneficial in the horse.

Tendons and Ligaments

In vivo research of SWs on the healing of tendons and ligaments has provided support for the use of SWT for tendon and ligament injuries. In rabbit patellar tendonopathy induced by collagenase, the tendon was treated two times with 1500 pulses at 0.29 mJ/mm2 . Tendons were harvested 4 and 16 weeks after SWT. Shock wave treated tendons had a 7% and 10% increase in strength at 4 and 16 weeks respectively compared to untreated controls. This corresponded with increased hydroxyproline concentrations in treated tendons and more blast-like tenocytes after 4 weeks. When 500 SWs at 0.12 mJ/mm2 were applied to long digital tendon grafts placed to stabilize surgically disrupted anterior cruciate ligaments in rabbits, the treated bone-tendon interface had more trabecular bone ingrowth than controls. The contact between the bone and tendon was improved and subsequently the strength of the graft was increased 24 weeks postoperatively. Growth factors known to affect tendon repair including TBF-± and IGF 1 are increased after shock wave treatment in rats.

Explants have been used to evaluate the effect of SWs on healthy pony tendons and ligaments. Six hundred pulses at 0.14 mJ/mm2 increased glycosaminoglycan and protein synthesis in explants harvested 3 hours after treatment. However by 6 weeks there was a decrease in GAG and collagen synthesis. In a similar study, samples harvested at 3 hours and 6 weeks after treatment revealed histologic findings of disorganization of matrix structure and increase in degraded collagen at 3 hours after treatment. Gene expression of collagen type 1 and matrix metalloproteinase 1 were increased at 6 weeks after shockwave therapy. In these studies the degraded collagen and disorganization of the collagen at 3 hours after treatment warrants some concern. However these studies were done in normal pony tendon and ligament. In injured tendons/ligaments one would expect these findings to already be present.

Suspensory Desmitis

Two studies have evaluated the ultrasonographic healing of suspensory ligaments following collagenase-induction of lesions. In both studies the ultrasonographic measurements showed the defects healed faster in the SW treated groups. Different outcomes were evaluated histologically, but a positive effect was found in both studies. Forelimb collagenase-induced desmitis showed increased metachromasia consistent with increased production of extracellular matrix in the treated group. In the hind limb, after collagenase-induced desmitis, treated subjects exhibited more newly formed small collagen fibrils. The exact mechanisms by which these outcomes occur are not fully understood. Immunocytochemical evaluation of shock wave treated suspensory ligaments showed increased TGF-β1 following treatment.

Clinical response to SWT is consistently better in fore than hindlimbs. Lischer's retrospective study 30 proximal suspensory desmitis with 2000 pulses at 0.15 mJ/mm2 and had 61.8% of forelimbs in full work by 6 months and 55.9% still working at 1 year. Treatment of hindlimb suspensory desmitis resulted in 40.9% in full work at 6 months and dropped to 18.2% by one year. Results using RPW therapy in horses with forelimb or hindlimb lameness of at least 3 months prior to treatment associated with proximal suspensory desmitis were remarkably similar. Response was related to lesion severity determined ultrasonographically. At a recent equine practitioners meetinga the consensus on outcome was similar to Lischer's report. Many practitioners do not consider hindlimb suspensory desmitis as a good candidate for SWT alone. A combination of SW and plantar fasciotomy to relieve the pressure on the proximal aspect of the suspensory ligament and to stimulate healing of the defect may provide a better prognosis. Body and branch lesions are also commonly treated and reported to have acceptable outcomes.

Superficial Digital Flexor Tendonitis

There are two studies that evaluate the effect of SWT on collagenase-induced SDF tendonitis. Both studies utilized collagenase-induced lesions in the SDF tendon at the mid- metacarpal region. Ultrasonographically, the treated and control groups were similar throughout the study periods. Histologically the treated tendons were more "mature" indicating that the healing process was occurring at a faster rate in the shock wave treated tendons. The treated tendons had more parallel collagen fibers when evaluated in one study, and in the other study, an increase in neovascularization was found. They did show an initial decrease in inflammation associated with the start of SWT. Unfortunately, neither of these studies evaluated the strength of the repaired tendons.

A notable finding following SWT of tendonitis is the rapid improvement in clinical appearance. The peritendonous inflammation that results in the "bowed" appearance decreases quickly after SWT. One must be careful not to interpret that as tendon healing. The echogenicity and fiber alignment of the tendon injury must be monitored with ultrasonography.

Osteoarthritis

Osteoarthritis of the distal hock joints was one of the first applications of SWs in the United States. Seventy-four horses with 'bone spavin' refractory to medical therapy were included. A single high energy (0.89 mJ/mm2 ) treatment of 2000 pulses resulted in 80% (59/74) of the horses improving by 90 days. There are horses with osteoarthritis of multiple joints that anecdotally have improvemed following treatment. Responses are mixed. Some horses have decreased lameness for a few months following treatment and others do not change. I have not been able to identify the reasons why. Shock wave therapy was evaluated in the osteochondral fragment model of osteoarthritis of the middle carpal joint. There was a decrease in protein concentration and less lameness in the SW treated horses on day 28.

Navicular Syndrome

Horses with "navicular syndrome" will improve following SWT. In one group of horses treated with 1000 pulses through the frog and 1000 pulses from between the heel bulbs under general anesthesia at 0.89 mJ/mm2, 16 horses had complete follow-up. Masked video analysis from before and 6 months after treatment showed 56% of horses improved at least 1 lameness grade. Owners reported 69% of the horses returned to their previous level of activity. Another study included horses treated under general anesthesia with 3000 pulses at 0.56 mJ/mm2. and 100% of 26 horses were free of lameness at 6 months.

The disease however is complex and as diagnostic techniques have improved, it is difficult to tell if the lameness was associated with the deep digital flexor tendon, navicular bursa or navicular bone or other structures. Shock wave therapy is indicated for horses that do not adequately respond to medical therapy and shoeing. However it must be recognized that 'navicular syndrome' is a progressive disease and SWT is not a cure.

Therapy Planning

When considering the application of SWs, obviously an accurate diagnosis is imperative. Next, one needs to consider if you can focus SWs on the pathology. Shock waves will not penetrate the hoof wall. You can treat bone, however 70% of the energy is lost in the bone, so little energy is available on the opposite side of the bone. Most focused generators have a series of stand-offs that tell the user the depth of penetration. Any site that can be visualized with ultrasonography can be treated with SWs, and ultrasonography can be utilized to measure the required depth if necessary. Radial pressure waves are different in that the energy of the wave decreases proportional to the square of the depth, so the depth of penetration is limited.

The 3-dimensional treatment concept is to treat the full width and depth of the treatment area. An example would be that during treatment of a proximal suspensory ligament a combination of the 20 and 35 mm depth probes of one commonly used generatorb is used from the palmar aspect and the 20 mm probe from both oblique angles just palmar to the splint bones. In addition to the change in the depth, the probes are slowly moved around the treatment area. This should expose the entire proximal suspensory ligament and bone to the SWs.

Shock waves will completely reflect at an air interface. It is important to avoid air interfaces during treatments near the chest and abdomen. The pressure wave will be reflected and can cause hemorrhage at the interface. It is important to have the generator in good contact with the skin surface. Typically, the hair is clipped with a number 40 blade. Ultrasound coupling gel is the best agent, but water, mineral oil and petroleum jelly can all be used. It is best to slowly move the generator in the direction of the clipped hair then move back up and down again, with care taken to recoat the area with the gel. The convex radial generators may get air bubbles in the concavity, making coupling difficult.

Combination therapies are frequently used. The use of platelet-rich plasma and pig's urinary bladder matrix (A-Cell) with SWT may result in some additive effects. Personal experience of using A-Cell in superficial digital flexor tendonitis at the first of 3 shock wave treatments has resulted in good clinical outcome. The effect of SWs on stem cells in vivo is unknown. For this reason, when used together, the site should be shock waved before the stem cells are injected and then the next SW treatment is done about 3 weeks later.

Potential Complications

Very few complications are seen with SWT. There are dose related effects and it is possible to over-treat an area. With most commonly available devices there are general guidelines to assist with appropriate dosing. Transient swelling at the treatment site is occasionally noted. This is seen more commonly with RPW devices. White hairs may develop at the treatment site. As noted above, air or gas filled structures are avoided. There are many questions about the effect of SW's on the physis. Some say it stimulates growth, others say it inhibits growth. Studies in rabbits showed SWs can lead to premature closure of the physis, however this has not been examined in larger species.

Analgesia

Analgesia has been a welfare concern for the horse since the beginning of shock wave application to the horse. There are concerns of analgesia following treatment resulting in worsening of an injury when the horse returns to work. In vivo evaluation of skin sensation following both focused SWT and RPW therapy showed some analgesia for 48 hours after treatment. There was no analgesia of the nerve field following the application of SWs and RPWs directly to the palmar nerve, so it was concluded that the analgesia was a local effect. In clinical cases of unilateral navicular syndrome and osteoarthritis where analgesia was measured as an increase in peak vertical force following treatment, analgesia was identified that peaked at 48 hours after shock wave treatment as measured. A similar study utilizing unilateral navicular syndrome horses and RPWs, no analgesia was found. The RPWs may not reach the depth necessary to affect the pain from the palmar foot structures. Many racing and horse show jurisdictions have specific rules about shock wave therapy and they should be consulted.

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