Extracorporeal shock wave therapy: minimally invasive therapy of osteoarthritis (Proceedings)

Background and history

Lameness and pain caused by osteoarthritis (OA) is one of the most common presenting complaints in small animal practice.  Despite a plethora of pharmaceutical, neutraceutical and nutritional interventions chronic discomfort is frequent in these patients.  Extracorporeal shock wave therapy (ESWT) provides a simple, alternative, non-invasive treatment to assist in the multimodality management of dogs with OA.

ESWT was first used for fragmentation of renal calculi (lithotripsy) in humans in the 1970's, and remains the state of the art for this application.  Musculoskeletal uses in humans include treatment of plantar fasciitis (where it is FDA approved), lateral epicondylitis, non-union fracture, calcifying tendinitis, and Legg-Calve-Perthes disease.  ESWT is approved for human use as a class III medical device, suggesting likely safety and efficacy of shock wave technology, but also potential for risk.

Mechanism of action

"Shock wave" is a physics term used to describe a rapid increase in pressure, such as the energy from an explosion or thunder resulting from lightening.  In the case of ESWT, shock waves are generated in a therapy head outside the body by electrohydraulic, electromagnetic or piezoelectric means, and transmitted to the tissue.  When the shock wave encounters a change in impedance (such as at a ligament or tendon attachment) energy is released.  Typical pressures generated by ESWT are in the range of 500 BAR, which for reference represents the pressure on the seafloor at a depth of 3.125 miles!  Energy delivered to a defined area of tissue (focal point) is defined as "energy flux density (EFD)", and given in mJ/mm2.

The therapeutic mechanism of action of shock waves on tissue is poorly understood, but is almost certainly multi-factorial.  Potential mechanisms include stimulation of local nociceptors that in turn inhibit afferent pain signals, induction of nitric oxide synthesis, simulation of osteoblast activity, microrupture of avascular or poorly vascular tissue to encourage revascularization and recruitment of appropriate stem cells, and stimulation of angiogenic cytokines.  The biological effect of ESWT may vary with EFD.

Veterinary unit

Unlike early human units, modern shock wave generators are portable and extremely versatile. The most common small animal veterinary unit is the Versatron 4Paws™, sold by PulseVet, (Alpharetta, GA).  While multiple therapy heads are available for this unit, the R05 provides a range of effective treatment depth of between 0 and 25mm, which is adequate for almost all small animal applications.  The R05 is capable of delivering six different levels of energy (EFD) at a pulse rate of between 60 and 360 per minute.  The Versatron™ creates a relatively large focal size compared to other units, which is advantageous as the potential for local tissue damage is considerably less.  The unit weighs 16kg.

Performing the ESWT procedure

ESWT requires sedation, as it is mildly painful, and definitely LOUD. I often use a combination of dexdomitor and butorphanol, for this purpose, but any suitable short-acting agent is appropriate. Energy level and number of shock waves to be applied is a subject of fair debate, but in general can be determined from available literature (Table 1). It should be stressed that more is not necessarily better with ESWT; in fact it is becoming apparent that treatment protocols using fewer pulses and lower EFD are at least as effective as those that are more aggressive.

Table 1: Suggested energy level and # pulses for common conditions in dogs Site Condition Pulses Energy Level (Versatron) Hip OA, dysplasia 1000 E4-E6 Tarsus OA, tendonitis 600 E3-E5 Stifle OA, tendonitis 1000 E5-E6 Shoulder Tendonitis 1000 E5-E6 Elbow OA, dysplasia 800 E3-E5 Carpus OA, tendonitis 600 E3-E5

From: PulseVet; Pulse Veterinary Technologies, LLC.  www.pulsevet.com

(E1=EFD of 0.11mJ/mm2, E4=EFD of 0.14mJ/mm2, E6=EFD of 0.15mJ/mm2)

The skin surface of the area to be treated is clipped, and ultrasound gel applied. Hematoma can occur if air is present between the treatment probe and skin. The probe is placed firmly against the skin over the desired treatment area and slowly pivoted to direct the focal zone throughout the desired treatment region.  If possible, the coupling surface between the probe and skin should be maintained in the same spot. When treating larger areas it may be necessary to reposition the probe.  Most treatments can be completed in 2-5 minutes and are usually done on an outpatient basis.  Some patients are somewhat more painful for the first several days after ESWT, so clients are warned of this possibility and appropriate analgesics dispensed.  Interestingly, some patients are actually less painful immediately after therapy. Treatment is repeated every 2-3 weeks for a total of 2-3 treatments.  If effective, ESWT can then be repeated as necessary.

Complications

Specific contraindications for ESWT include patients that are pregnant, have coagulopathies, or sepsis.  ESWT should not be used for treatment of conditions involving the head, eyes or spinal cord.  Shock waves can cause damage to the heart, large nerves and vessels, intestines, and lungs, so it is critical to avoid inadvertent direction of shock waves at these structures.  Overall, ESWT has an excellent long-term safety profile.

Clinical data

Data confirming the benefits of ESWT is abundant in horses and humans, but still  limited in small animal patients.  Researchers at CSU evaluated the efficacy of ESWT in reducing lameness associated with OA in horses compared to no treatment and intramuscular polysulfated glycosaminoglycans (PSGAG's)1. In the study, ESWT performed better than both control groups in reducing lameness, and significantly reduced synovial fluid total protein, a marker of synovitis.  ESWT also has been shown to down regulate levels of TNFa and IL-10 in osteoarthritic chondrocytes, likely preventing MMP activation and cartilage breakdown2.

Dahlberg evaluated weight bearing on a force plate after ESWT in 7 dogs with stifle OA, and found that while treatment dogs did not improve significantly, control dogs deteriorated3.  Range of motion "trended" toward significant improvement, and subjective data provided by client questionnaire did not show significant difference between groups. Similar results have been noted by Millis (unreported data) in dogs with elbow OA.  Clearly additional studies on larger groups of patients are necessary.

Financial analysis

ESWT is cost efficient for both the client and veterinary practitioner.  The Versatron 4Paws costs in the range of $40,000 but can be leased to purchase if desired. Treatment probes cost about $1500 and can be used for about 20,000 total pulses before replacement is required.  At Tufts we charge $400 per treatment and usually each patient receives 3 treatments. Assuming an average of 800 pulses per treatment this means the unit is paid for in about 120 treatments (or 40 patients).

References

Orthopaedic Bioengineering Laboratory 2002-2003 Report. Evaluation of Extracorporeal Shock Wave Therapy for Osteoarthritis. Orthopaedic Research Center, Colorado State University. 2003:94-95.

Moretti B, et al. Extracorporeal shock waves down-regulate the expression of interleukin-10 and tumor necrosis factor-alpha in osteoarthritic chondrocytes. BMC Musculoskelet Disord. 2008;9:16.

Dahlberg J, Fitch G, Evans RB, et al. The evaluation of extracorporeal shockwave therapy in naturally occurring osteoarthritis of the stifle joint in dogs. Vet Comp Orthop Traumatol 2005;18:147-152.