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13 Articles in Volume 12, Issue #9
PROMPT Challenges PROP’s Petition
PROP Answers Questions Raised About Opioid Label Changes
PROP vs PROMPT: Who Speaks for the Pain Doctor?
PROP’s Petition: PPM’s Editorial Board Weighs in
Assessment of Long-term Outcomes Of Opioid Treatment: How to Set Goals and Objectives
Extracorporeal Shock Wave Therapy: Applications in Pain Medicine—Part One
Neck Pain: Diagnosis And Management
Part Two: Trigeminal Neuralgia: A Closer Look at This Enigmatic and Debilitating Disease
Reducing Musculoskeletal Disorders Through Ergonomics
Risk Evaluation and Mitigation Strategy Compliance
Treating the Opioid-addicted Chronic Pain Patient: The Role of Suboxone
Electromagnetic Devices: A New Partner in Pain Management
Methadone Management in a Patient With Pain and History Of Addiction

Extracorporeal Shock Wave Therapy: Applications in Pain Medicine—Part One

Part one of a three-part series examining the clinical role of shock wave therapy in the treatment of musculoskeletal pain conditions.

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.

Clinical Rationale
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.

Figure 1. WellWave machine

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.

Figure 2. Treatment of patient with plantar fascitis.

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.

High-energy ESWT has developed a reputation for being able to resolve difficult strains within the tendons near their insertion site (insertional tendinopathies). Research studies have recognized the value of high-energy ESWT in effectively treating recalcitrant calcific conditions, but the majority of our musculoskeletal patient population does not always have a condition that will warrant high-energy ESWT. The prevalence of calcific conditions will vary within populations, but as a proportion of the total susceptible population, and might be insignificant enough to justify the financial investment required for this technology (machines range from $20,000 to $60,000). That would be a fair conclusion if it were not for the fact that ESWT has reinvented itself into a much more versatile treatment modality, and that it can now demonstrate efficacy throughout the entire range or spectrum of pathology—from acute to chronic pain, including calcific conditions.

Table 1. Common Terms Used in Mechanobiology

Mechanobiology
Perhaps underscored by the genomic revolution, there has been a renaissance in the field of mechanobiology. Individual cells comprise tissues and organs that are held together by an extra cellular matrix (ECM) made up of collagen, glycoproteins, and proteoglycans. Furthermore, each individual cell contains a surface membrane, intracellular organelles, a nucleus, and a filamentous cytoskeleton that connects all these elements, which are all in a viscous cytosol. The important point is the interconnectivity between cells that acts as a communication conduit from one cell to another.

Mechanical loads are transmitted through each cell via their structural elements, which are physically interconnected within the ECM. These molecular scaffolds that connect cells to the ECM are comprised of cells that bind to cell surface receptors known as integrins. A cluster of these integrins is referred to as a focal adhesion and acts as a preferred site for mechanical signal transfer across a cell surface. Cell surface integrins function to not only provide support and stiffness to a cell, but also act as the first-line molecular sensors of mechanical stress imparted to a cell. The intracellular stress is transmitted across the cell plasma and to the cytoskeleton creating a mechanical coupling system. The cell-to-cell coupling is performed by adhesion molecules referred to as cadherins and selectins, which transmit mechanical forces from cytoskeleton to adjacent cells. The internal molecular framework or lattice of a cell is composed of three different types of filaments: microfilaments, microtubules, and intermediate filaments that provide both the shape and internal stability in a cell. What is very important to remember is that all cells demonstrate an inherent tendency to generate tensional forces through filament sliding within the cytoskeleton. These internal tensional forces are counterbalanced by external adhesions to the ECM and neighboring cells. This type of force balance is known in architecture as a “tensegrity system” (Figure 3). This type of system can predict very complicated mechanical behaviors in cells.5 Table 1 defines some of the terminology commonly used in mechanobiology.

Tensegrity and Mechano-transduction
The critical and complex cell functions and interactions found in the tensegrity system can be controlled by cytoskeletal forces, which ultimately influence cellular homeostasis. This is a powerful piece of information. The physical distortion of cells can influence cell growth, differentiation, and apoptosis. Tensional forces on nerve cells have been shown to facilitate the extension of their nerve processes when the tension is applied in the direction of the nerve process.6 There are numerous other examples of various cell types that demonstrate positive unique responses to physical stresses. The most well-known mechanical force induction models that clinicians recognize are those governed by the laws of both Wolff and Davis.7,8 They refer to bone and soft tissue respectively and essentially state that both biomaterials will adapt according to the loads imposed on them. What we didn’t realize back then was that most, if not all, cells respond to mechanical loading in a very profound and measureable way—not just bone and collagen cells. Furthermore, research evidence suggests that the composition of the ECM, combined with the degree of cell distortion, are the primary determinants of cell behavior irrespective of the presence of hormones, cytokines, or other regulatory factors.9 Chondrocytes respond to compressive loading10; fibroblasts to mechanical strain along with skin, epithelium, bone cells, and embryonic heart muscle cells11; skeletal muscle cells to stretch/strain cycles12; kidney epithelial cells to fluid shear13; and pulmonary epithelial cells increase surfactant when stretched.14 When integrins on the surface membrane of cultured cells are mechanically deformed, the cells respond by increasing recruitment of focal adhesion proteins (cytoskeletal linker proteins) in an effort to strengthen itself against additional stress.15 This same stress applied to non-transmembrane adhesion molecules has little to no effect. In fact, when macroscale forces are applied to integrins, cytoskeletal filaments and linked intranuclear structures can be seen to realign along the lines of tension.16 This is clear evidence of the dynamic nature and resilience of the ECM and cytoskeletal network and has profound implications on how we might understand injury, disease, adaptation, interventions, and the healing process in general. A detailed presentation of mechanotransduction is beyond the scope of this article and the reader is encouraged to search the many other sources that exist on the topic including some of the references listed at the end of this article.

Figure 3. Illustrative example of tensegrity system.

Myotripsy for Pain Conditions
The use of ESWT, or myotripsy, on human tissue to treat clinical disorders has a foundation based on a reasonable amount of in vivo, in vitro, experimental research, and empirical observations. In vitro research has shown that pathologic Achilles tendon tissue cell cultures treated with myotripsy demonstrated a reduction of inflammatory cytokines and matrix metalloproteinase (MMP) production, with the authors speculating that this mechanism may be important in the treatment of clinical tendinopathy.17 Another study examined the in vivo effects of myotripsy application on perfusion levels in muscle flaps of mouse hind legs and found that acoustic energy as applied by ESWT does indeed induce upregulation of proangiogenic factors and chemokine gene expression in the muscle.18 An intriguing study examined the interaction between myotriptic treatment and effects on lubricin levels in rat tendons. Lubricin is a lubricating glycoprotein that is important to tendons and how well they slide in their sheath. Both low- and higher-dose ESWT was found to increase lubricin expression. It is postulated that improved lubricin expression could be beneficial in clinical conditions that would benefit from improved tendon lubrication for sliding efficiency and reduction of erosive wear.19

In musculoskeletal medicine, few conditions rival chronic tendinopathy in terms of incidence, prevalence, and the total amount of attributed disability in the workplace. The translational research body of evidence for ESWT encompasses well more than 250 studies being published from all parts of the globe. The clinical studies are not as clear and decisive as the basic research body of literature; however, the overwhelming pattern supports a positive clinical effect for this modality. Research evidence is available that links ESWT-myotripsy treatment to improvement in status of conditions such as hamstring tendinopathy,20 chronic calcifying rotator cuff tendonitis,21,22 Achilles tendinopathy,23,24 patellar tendonitis,25,26 patellar tendinopathy,26,27 and non-calcific rotator cuff tendinosis,28,29 to name a few.

The clinical manifestations of myotripsy treatment are that the patient will typically feel some soreness post-treatment not unlike other therapies that treat collagen-based myofascial structures, including prolotherapy. The treatment itself can cause some mild to moderate discomfort and the patient has had all this explained to him/her prior to treatment. After treatment we advise that no anti-inflammatory medication is taken for 1 to 2 days and no ice should be applied on the target area. The reason for this is that either intervention may negate the inflammatory process, which in these cases is a desirable physiological event. In much the same way that prolotherapy-injected dextrose/saline might irritate a ligamentous structure and cause irritation/inflammation in turn leading to a healing response, the ESWT treatments work in a similar manner. It is through a mechanotransduction-mediated cascade of biochemical events working in a tensegritous architectural system that these events can lead to tissue normalization and subsequent symptom reduction. It is the recognition that physical forces play a vital role in cell-to-cell function that provides a practical framework from which therapies such as myotripsy can be explained in more reductionist terms. There is a large body of information available on the scientific underpinnings of mechanobiology, an evolving paradigm that will help link the genomic revelations of molecular biology testing with our current understanding of biochemistry-based pharmacotherapies.

The WellWave ESWT can also be used in diagnostic mode. This device emits the more focused pressure or shock wave versus a generalized radial shock wave that the first-generation devices emitted. Myotripsy devices use a focused pressure wave that can be directed or guided by the user to the target area with depth of penetration controlled by the clinician. The pressure distribution, energy density, and total acoustic energy are the most important physical parameters for the treatment of musculoskeletal disorders.3 The frequency and intensity of pulses will determine the energy applied to any specific area. The optimal number of pulses emitted by a myotripsy device depends on a number of factors but generally a treatment consists of 500 to 2,500 pulses per target site with some patient conditions requiring greater energy emissions.30

This report has focused on the science of myotripsy, but there is clearly an art to optimizing the benefits this therapy can bring to a patient. The WellWave device we tested has a “sono-isolate” function whereby the myofascial problem can actually be located with the transducer probe of this unit. It is quite fascinating to observe the reaction in a patient’s face as this probe passes over a generally painful area and as it focuses the acoustic beam onto an area, the patient feels the pulsations as an achy pressure, helping to confirm the precise location of a trigger point or myofascial restriction. It is unusual that a therapy device also assists in the diagnosis.

Conclusion
The discoveries surrounding the viscous environment of each one of our trillions of cells is critically important to better understand new approaches to healing, as well as to help provide the rationale underlying the many years of empirical observations clinicians have experienced with manual therapies. It is evident that you can target the cell, its environment, or both to effect a change. Pharmacotherapies have traditionally targeted cells via their surface receptors, but science is elucidating another intricate and complementary pathway to effect change, and one perhaps more powerful than chemical induction alone. The fascial system—which surrounds every cell, tissue, and organ—relies on a healthy ground substance, and in turn, a healthy ground substance environment ensures proper transportation of nutrients, oxygen, hormones, medication, blood factors, energy, and cell-to-cell communication. Cell-to-cell electromagnetic coupling for signaling and communication is reliant on a healthy environment including an optimum viscosity and functional tensegrity. Events that thwart or interfere with this system such as injury, surgery, or disease will disrupt this environment and healing potential. Areas of dehydration in our ground substance could very well be the areas that manifest with pain/tenderness and become target regions for mechanical therapies such as myotripsy.

Mechanobiology may also help explain why so many therapies—ie, microcurrents, manual soft tissue techniques such as myofascial release, and craniosacral treatments—can have a very profound effect despite the clinician applying only a small amount of force (energy) into the target tissue (Arndt-Schultz law). Mechanobiology operating through an extracellular network organized within a tensegrity architectural system brings plausibility to many of our treatment methods, including drug transportation and metabolism. It appears this framework for understanding our cellular infrastructure offers an intriguing explanation for the benefits of an acoustically driven therapy such as myotripsy. The second and third parts of this report will focus on more specific applications and clinical conditions.

Read part two of this series.

Last updated on: December 20, 2012
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