High Potency Ultrasound for the Treatment of Connective Tissue Disorders
Extracorporeal shockwave therapy (ESWT) is a form of ultrasound energy waves that was first introduced in the United States in 2000. It was called “extracorporeal,” since it was administered outside the body. “Shockwave” referred to the high potency energy wave that produces a number of therapeutic benefits. Over the years since introduction of this high energy device, many practitioners consider ESWT to be the premier treatment for chronic tendon and ligament disorders often referred to as enthesopathic conditions. These conditions have challenged clinicians over the years and no universal “best” therapy has emerged from the various interventions available.
Mechanical Properties of Ultrasound
Ultrasound waves, which are the basis for ESWT, have found numerous therapeutic medical applications over the years including kidney stone ablation (litho-tripsy), bone growth stimulation, as a sclerosing and anti-inflammatory agent in physical therapy and, most recently, as a treatment for recalcitrant enthesopathies including plantar fasciitis (see Figure 1), achilles tendonopathy and chronic epicondylitis (see Figure 2). Conventional ultrasound physical therapy consists of applying high frequency sound waves (acoustic radiation) to a target area for the reduction of pain and inflammation. There is a certain amount of heat generated (thermal effects) as ultrasonic waves collide with tissue interfaces of varying densities. The practitioner can adjust the frequency of the sound waves emitted from the transducer, or sound head, to target superficial or deeper structures with higher and lower frequency adjustments, respectively. There is a piezoelectric crystal that vibrates within the casing of a transducer or sound head causing a conversion from electrical to mechanical energy. The ultrasound waves usually require a couplant gel that acts as an energy transfer medium from the sound head to the target tissue.
In physiotherapeutic applications, the “piezoelectric effect” caused by the crystal creates a “micro-massaging effect” in the tissue that can facilitate fluid re-absorption in inflamed areas and help to physically break down tissue in arthrofibrosis or cheloid scarring. Ultrasound is primarily a mechanical wave and secondarily a form of thermal energy. Heat is a byproduct of sound wave interaction with tissues of varying densities and not inherent in the wave itself.
ESWT is different than conventional ultrasound in that it is delivered as a more focused and intense beam of energy usually to a small area considered the focal point of the problem. ESWT is a shock wave or high-pressure acoustic wave with very high amplitude, rapid rise time, and short pulse duration. In the early days of this technology, patients were anesthetized prior to having high energy ESWT performed on them. Today these treatments are performed by a number of different practitioners with no anesthesia. In fact, localized pain during the treatment can act as a guide to where the most effective irradiation should occur. Often, the more pain elicited during the treatment may mean better post treatment outcomes.
Mechanism of Action
A precise mechanism of action with respect to ESWT has not been identified with certainty but a couple of competing hypothesis have been put forth. One theory suggests that chronic pain is simply a function of the central nervous system “forgetting” that there is a physiological problem somewhere in the body and so healing efforts are no longer directed to that area—hence, persistent pain and symptoms. It is difficult to reconcile this notion with a condition such as phantom limb pain where the body part in question has been removed yet symptoms persist.
There is some evidence to suggest that pre-amputation anesthesia to abolish pain well in advance of the surgery can abate and/or minimize the chance of developing phantom limb disorder. This would support the idea that there is a hardwiring element to pain and that both central and peripheral mechanisms need to be considered. Proponents of this theory might consider that ESWT works by reminding the brain that there continues to be a problem and thereby reactivating the healing mechanism of the body. There is little evidence that ESWT works in this manner, in fact, the scientific literature supports the likelihood of a true “mechanical effect” as being the most obvious and measurable tissue response.
In vivo studies done in rabbits supports the second and more plausible explanation for positive effects, namely that of neo-vascularization or increased local blood supply along with concomitant tissue regeneration, as the primary mechanism by which tendon-bone junction enthesopathies actually heal.1 A recent European study examined the use of shock wave therapy to stimulate hypertrophic long bone non-unions and concluded that not only is ESWT as effective as surgery in stimulating union of long bones but that it has better short-term outcomes than surgery.2 Another similar study published a short time later supports this same premise when applied to non-union fractures of the metaphyseal-diaphyseal region of the fifth metatarsal. The investigators found that screw fixation has more associated complications and these often lead to additional surgeries and further complications. These authors conclude that ESWT is the preferred method of treatment for non-union fractures of the fifth metatarsal bone.3