Extracorporeal Shock Wave Therapy: Applications in Pain Medicine—Part One
Every once in awhile, clinical rehabilitation and pain practitioners from many disciplines have an opportunity to share in the benefits that a promising new technology can inspire. Such is the case with extracorporeal shock wave therapy (ESWT). This three-part series will focus on the clinical benefits of shock wave therapy in general pain medicine, myofascial disorders, and for chronic enthesopathic conditions that historically have been recalcitrant to conventional treatments.
What Is in a Name?
ESWT is probably best known as a noninvasive treatment for kidney stones (lithotripsy). But the technology has been adopted by a wide variety of practitioners, including pain specialists. When applied to soft tissue, ESWT has appeal for both the manual myofascial therapists as well as the more traditional pain practitioners. For the purpose of this article, I will use the descriptors ESWT and “myotripsy” interchangeably to refer to acoustic pressure or shock wave therapy applied to soft tissue. I prefer the term myotripsy because it is more user friendly. It can be difficult enough to get patients to try a new treatment, or to get a payer to consider reimbursement for a new procedure, without adding any extra barriers to these challenges. The terms “extracorporeal” and “shock wave” connote very frightening images to a patient (or anyone else) and the provider finds himself spending an unusually disproportionate amount of time explaining why this technology is called what it is, and that the treatment is really not harmful to the patient.
How does a treatment whose roots lie in the destruction of renal, biliary, and salivary calculi find a place in musculoskeletal pain medicine? The physiological rationale for using myotripsy might be best explained by using insights derived from studies on cellular mechanotransduction—or the conversion of mechanical forces into chemical signals inside the cell that influence gene expression. We have recently gained an appreciation for how these mechanical forces influence biological control at the cellular and molecular levels. Before we engage in a detailed discussion of mechanotransduction and the corollary concept “tensegrity”—which describes the internal architecture required to stabilize each cell, tissue, and organ in our body—we will describe myotripsy treatment indications.1
ESWT, or myotripsy, has been described as a shock wave because it is fundamentally an acoustic pressure disturbance created by a translation of energy that can be generated in one of three ways: using an electrohydraulic, electromagnetic, and/or a piezoelectric source. Like many forms of sound wave–based treatment, it requires a medium by which energy transfer can occur; therefore, a coupling gel is used. Figure 1 represents a piezoelectric source pressure wave device (WellWave, Richard Wolf Co., Vernon Hills, Illinois) and the actual unit that our company uses when offering myotripsy services to patients.
When an electrical discharge is applied to several piezoelectric crystals mounted inside the generator, a “shock wave” is the result. There are several parameter variables to note such as energy flux density measured in mJ/mm2, which is important depending on the amount of starting discomfort levels. Lower energy flux treatments are usually tolerated well with some discomfort from the patient. The higher energy flux applications might require local anesthesia to the target region prior to myotripsy.2 Total energy output applied per treatment session is the product of the energy flux density by the number of shock waves delivered.
Another noteworthy parameter to consider is frequency or number of shock waves delivered per second. The WellWave unit had a range of frequencies from 1 to 8 cps (cycles per second or Hertz). The intensity parameter also had a range of 1 to 20, with each single unit corresponding to 360 pounds per square inch (psi) with a total psi output near 7,200 psi. Using a piezoelectric model that contains many crystals inside the transducer sphere (>1,000), a rapid electrical discharge will cause the crystals to contract and expand creating a rising pressure pulse that culminates into the shock wave.3 The geometric pattern arrangement of the crystals serve to self-focus the pulse toward the center allowing the clinician precise target control of the transducer with depth of penetration being approximately 30 mm. Stand-off pads allow the user to control depth of penetration into the various tissues. Figure 2 depicts a patient being treated for plantar fasciitis of the foot.
The biologic effects of sound pressure waves on living musculoskeletal tissues are a function of pressure distribution, energy density, and total acoustic energy. We know that when high-pressure sound waves are used to disintegrate kidney stones it appears that these pressure pulses apply a high stress force on the stone surface that exceeds the elastic strength of the stone leading to fracturing. When the first generation of ESWT devices came on the market it became clear that conditions involving a calcification phenomenon such as calcific tendinopathies of the rotator cuff, epicondylitis, or chronic plantar fasciitis would probably respond well to this mechanical type of treatment. In fact, the more material phase transition that had occurred—such as collagen fiber to calcium deposition in a more chronic condition—the more dramatic the results that could be expected using a lithotripsy-type approach. The mechanical pulse waves strike the target repeatedly, causing the calcium or mineral deposit to fracture and dissipate.
This process when applied to soft tissue could be quite painful and explains why the first generation of devices required local anesthesia to be delivered prior to ESWT. Over time, ESWT manufacturers began to differentiate themselves based on parameters such as pressure wave configuration citing enormous gains in peak pressure magnitude and target precision when the pressure wave is focused or converges versus remaining in a more general radial pressure wave.4 These characteristics should be familiar to those who work with the various forms of energy since the ability to focus an energy beam is important to treatment precision. We see it play an important role in other therapy modalities such as cold laser, ultrasound, direct currents, and other forms of electrotherapy.