Therapeutic ultrasound and shockwave therapy for tendinopathy: a narrative review

Abstract

Tendon injury is prevalent and costly in the United States, comprising 45% of the 66 million musculoskeletal injuries and costing $114 billion annually. Surgical and therapeutic methods, such as arthroscopic surgery, dry needling, and physical therapy, produce mixed success in re-introducing a healing response in tendinopathy due in part to inconsistent dosing and monitoring. Ultrasound is one therapeutic modality that has been shown to noninvasively induce bioeffects in tendon that may help promote healing. However, results from this modality have also been mixed. This review compares the current state of the field in therapeutic ultrasound and shockwave therapy, including low-intensity therapeutic ultrasound (LITUS), extracorporeal shockwave therapy (ESWT), and radial shockwave therapy (RSWT), and evaluates the efficacy in treating tendinopathies with ultrasound. We found that the mixed successes may be attributed to the wide variety of achievable parameters within each broader treatment type and the lack of standardization in measurements and reporting. Despite mixed outcomes, all three therapies show potential as an alternative therapy with lower risk side-effects than more invasive methods like surgery. There is currently insufficient evidence to conclude which ultrasound modality or settings are most effective. More research is needed to understand the healing effects of these different therapeutic ultrasound and shockwave modalities.

Introduction

Tendon injury accounts for 45% of the 66 million musculoskeletal injuries that occur in the United States totaling $144 billion annually¹. The etiology of tendon injury, or tendinopathy is not well understood². When imaging or histological information is available, tendinopathy can be more specifically referred to as tendinosis based on degeneration and disorientation of collagen fibers, absence of inflammatory cells, and poor healing². Of note, this should not be confused with the term “tendinitis”, or inflammation of the tendon, which occurs from abrupt trauma. Intrinsic factors such as collagen alignment and extrinsic factors such as biomechanical flaws have been associated with the onset of approximately 2/3 of Achilles tendinopathies in athletes². Researchers suggest these physical flaws result from repetitive strain, leading to microscopic tears which initiate a reparative process in acute injuries². If repetitive strain continues, the normal healing event is hindered and results in chronic tendinosis². Fibers become disorganized, and type I collagen is replaced by a weaker type III collagen, thus undermining the strength of the tendon and causing localized pain². The healing process can result in the formation of scar tissue, calcifications, and avascular zones that prevent nutrients from reaching the injured fibers². Even after long periods of healing, chronically injured tendons are mechanically inferior to the pre-injured tendon².

Tendinopathy surgeries and therapies aim to remove scar tissue and induce microdamage; this can re-introduce a healing response and increase the delivery of nutrients to the tendon. However, most therapies have mixed success rates. Invasive methods can cause joint defects from damage to the surrounding tissue, which can decrease joint stability and increase unwanted formation of adhesions at the repaired site. They also carry an increased risk of infection³. In less invasive therapies, like dry needling or physical therapy, mixed success rates can stem from a lack of standardization, including inconsistent monitoring or dosing variations. Particularly in dry needling, or repeated fenestrations with a filiform needle, practitioners vary in the use of image guidance, location, direction, frequency, depth of needle insertion, and treatment frequency⁴. The mixed success of these surgical and therapeutic approaches to tendinopathy suggests a need for a treatment that is easier to standardize and monitor in real time.

Ultrasound as a therapeutic has also been used to improve tendon healing. It is a non-invasive technology that is often paired with ultrasound imaging for alignment and monitoring⁵. Various biological effects can be achieved with ultrasound through thermal and non-thermal mechanisms. Bioeffects can range from mild reversible effects to irreversible destructive effects⁵. Generally, as the acoustic intensity increases, evoked bioeffects change from enhancement of normal physiological responses, like increased blood perfusion, to increasingly destructive mechanisms like cell necrosis, which is typically achieved by focusing the energy (Figure 1). Along with intensity, variations in frequency, amplitude, treatment time, and wave modes (continuous versus pulsed) influence the mechanisms elicited by the acoustic pressure wave. However, the thermal and non-thermal effects of ultrasound are impossible to isolate as even the lowest amplitude pressure wave causes both heat deposition and radiation forces from the propagation into tissues. As the amplitude increases, cavitation also occurs and heat is still deposited. Thus, ultrasound parameter combinations can only emphasize, not isolate, specific bioeffects. This still allows for significant variation in the effects produced by ultrasound therapies.

Figure 1.

Therapeutic ultrasound can elicit various mechanisms. (a) Diagram showing that in general as acoustic intensity increases, mechanisms become more destructive and induce irreversible bioeffects. Multiple mechanisms, like periodic motion and forces from the displacement of the tissue from ultrasound propagation, thermal effects from acoustic absorption, and bubble effects (in brackets), can occur at the same intensity. The dotted outline indicates secondary mechanisms, like radiation forces, microstreaming, and microjets, can form from primary bubble mechanisms, which can elicit net motion of the tissue and/or surrounding fluids. (b) Summary table showing which bioeffects can occur from the therapeutic ultrasound mechanisms.

Knowing the ultrasound pressure or intensity only gives some indication of the therapeutic influence. Key characteristics of the acoustic waveform, like nonlinearities or peak negative pressures, will significantly influence the types of bioeffects that may be produced. For this reason, measuring the acoustic field is very important and can be achieved by using hydrophones or radiation force balances⁵. Reporting peak pressures, spatial peak temporal average intensity (I_SPTA), spatial average temporal peak intensity (I_SATP), and spatial average temporal average (I_SATA) can aid in the interpretation of results. It is uncommon in the tendinopathy literature to detail the complete pressure field or convey verification that the device performs per manufacturer specifications. Often, researchers report only I_SATA or other metrics, like energy flux density (EFD), which lack key characteristics of ultrasound pressure waveforms.

The objective of this paper is to summarize and compare ultrasound tendinopathy therapies, specifically low-intensity therapeutic ultrasound (LITUS), extracorporeal shockwave therapy (ESWT), and radial shockwave therapy (RSWT) in a narrative review (Figure 2). Study-specific acoustic parameters, timing protocols, and reported success rates and efficacy will be compared within and between therapies. The purpose is to provide an improved understanding of therapeutic ultrasound and shockwave therapy efficacy in tendinopathy treatment and highlight gaps in the literature to promote improved reporting of clinical results.

Figure 2.

General characteristics of low-intensity therapeutic ultrasound (LITUS), extracorporeal shockwave therapy (ESWT), and radial shockwave therapy (RSWT). ESWT achieves nonlinear shock waves at the focus, while LITUS operates at much lower acoustic energy to produce linear waves. RSWT energy lies in between the other two therapies. (I_SATA = spatially-averaged temporally-averaged intensity; EFD = energy flux density)

Ultrasound Tendinopathy Treatments

LITUS

Definition & Rationale

LITUS, commonly referred to as therapeutic ultrasound, is an unfocused sinusoidal acoustic modality (Figure 2) (I_SATA < 3 W/cm², continuous or pulsed mode), that has been used in both home and clinical settings to treat tendon injuries for decades^5,6. LIPUS, or low-intensity pulsed ultrasound, is sometimes used interchangeably for pulsed LITUS. Though the amount of ultrasound energy is relatively low compared to other ultrasound therapies, LITUS exposure can induce non-destructive thermal and/or non-thermal effects to accelerate healing (Figure 2). Known factors of healing from LITUS include altering tissue biomechanical properties (e.g. treating transected rat Achilles tendon, 1 MHz, 0.5 W/cm², continuous mode, 5 min daily for 9 days)⁷, stimulating fiber proliferation and improving collagen alignment (e.g. treating partially ruptured rat Achilles tendon, 1 MHz, 1.5 W/cm², continuous mode, 4 min daily for 8 days and every other for 13 more days⁷; treating partially ruptured rat Achilles tendon, 1 MHz, 0.5 W/cm², continuous mode, 5 min daily for 9 days⁸). Ultrasound-induced hyperthermia, or mild heating, occurs from acoustic energy absorption within the tendon (0.95 dB/cm at 1 MHz), which can create a regional increase in blood perfusion. In turn, this has been shown to reduce swelling and increase collagen synthesis, thus advancing the rate of repair of injured tendon in small animal models⁷. To minimize the discomfort associated with rapid heating and the possibility of skin burns, LITUS is administered for several days over multiple weeks, and the transducer is typically moved in circular motions around the injured site. Mechanical effects from the pressure waves have also been claimed to induce a healing response through modifications in cell signaling and may improve collagen alignment; microstreaming, radiation forces, periodic motion of the tissue, or surrounding fluid in response to the pressure wave may alter the extracellular concentration gradients at boundary layers or deform cell membranes and influence cell transport⁵. The relative contributions of potential bioeffects elicited by LITUS are difficult to discern because most publications lack detail in the reported acoustic measurements. Small differences in the acoustic field or parameter choices in individual publications may contribute to the mixed success rates reported from LITUS.

Animal Studies

Small animal studies evaluating LITUS compared to mock sonication on tendon have reported mixed outcomes depending on their metric of success. Some studies show characteristics of accelerated healing, including increases in rupture strength, tensile strength, extensibility, collagen synthesis, and collagen alignment compared to control groups^7–10. Others report insufficient evidence to support a beneficial effect of LITUS at clinical doses (machine output typically limited to ≤ 3 W/cm²)^5,6. Among the many LITUS studies, parameters and methodology vary in many ways. Generally, studies using LITUS therapy with 1 MHz continuous pressure waves at 0.5–1.5 W/cm² intensity (I_SATA or unknown) with an effective radiation area of 5 cm^{2 7} or 0.8 cm^{2 8} for 4–5 min/day for 8+ days found significantly accelerated healing compared to sham after treating rat Achilles tendons that were fully transected, sutured, and immobilized⁷, or partially ruptured via 18G needle puncture^7,8. These reports speculate that the contributing mechanisms may include: micro-massages (tissue oscillations by acoustic waves) that induce changes to membrane permeability⁸; relative motion of fluid that agitates and shortens the diffusion boundary layer near cells, thus modulating intracellular calcium concentrations⁷; and/or other unknown non-thermal ultrasound effects that augment protein synthesis⁷ and enhance rates of molecular processes⁷.

When the frequency was increased to 3 MHz at slightly lower intensities of 0.2 – 0.75 W/cm² (I_SATA or unknown; continuous or pulsed mode; 3–4 min daily for up to 20 days), and effective radiation area of 0.5–5 cm², no significant difference was found in mechanical strength between sonication and control groups after treating fully transected, sutured or un-sutured immobilized hen flexor tendons⁹. Compared to the results at 1 MHz (0.5–1.5 W/cm²), these results at 3 MHz (0.2–0.75 W/cm²) suggest either the lower frequencies or higher intensities might be important to enhance tendon healing. It is important to note that the absorption of acoustic energy in tendon increases with frequency (tendon absorption ∝f^0.8), which reduces the acoustic intensities within the tendon and increases heating. For transducers of the same size, the full-width half-maximum focal volume will also decrease as the frequency increases, thus reducing the volume of tissue exposed to ultrasound. These are important considerations when evaluating the translation of LITUS from small animals to humans.

Differences in success rates are also reported between continuous wave LITUS (LICUS) and pulsed LITUS (LIPUS) applications. Although, only a few have shown direct comparisons between the two methods⁷. Some studies have shown that pulsed LITUS induced a better healing response in small animal tendon compared to continuous LITUS^9,11. For example, da Cunha et al. (2001)⁹ applied 1 MHz 20% duty cycle pulsed treatments at I_SATA of 0.5 W/cm², I_SATP 2.5 W/cm², effective radiation area of 0.5 cm², for 5 min daily for 12 days (resting on day 6 and 7) to rat Achilles tendon. Results showed improvements in collagen synthesis, fiber alignment, and rate of tissue regeneration⁹. Although, with the same intensity, frequency, and transducer, continuous-wave treatments showed no beneficial effects compared to sham⁹. However, other researchers have shown no difference in the healing induced between continuous and pulsed mode (1–3 MHz, ≤ 1.5 W/cm², continuous or 20% pulsed modes, rat or rabbit Achilles or patellar tendons, partially or fully transected)¹⁰.

Clinical Studies

When successful LITUS parameters tested in small animal studies are translated into humans, the results are generally poor for both acute and chronic tendinopathy cases. Randomized controlled trials have consistently shown no beneficial effect from LITUS compared to placebo in pain (as determined by visual analog scales), functional disability, or range of motion^6,11. Of the trials with more promising results, some were considered “low quality” as they were not truly randomized controlled trials^10,11. Others showed short-term improvements for <9 months before performing similar to placebo in Constant-Murley scores of pain management and daily task performance. Best et al. (2016)¹⁰ showed tendon pain was significantly decreased in elbow and Achilles “tendinopathy” patients upon application of 3 MHz LITUS at 0.132 W/cm² in continuous mode. However, this study had a small sample size, high drop-out rate, and lack of a placebo comparison. Studies comparing LITUS to other tendinopathy therapies show LITUS performs worse than those other therapies^11,12. Giombini et al. (2007)¹² showed that microwave-induced hyperthermia was more successful than ultrasound-induced hyperthermia with two different parameter sets (3.2 MHz continuous mode, 1.5 W/cm², 5 cm² effective radiation area; 1 MHz continuous mode, 2 W/cm², 10 cm² effective radiation area) in athletes’ pain reduction at the end of 4 weeks treatment of patellar and Achilles tendinopathies. At 6-week follow-up, overall patient satisfaction was 77% from microwave-induced hyperthermia treatment compared to 33% from ultrasound treatment¹². Though microwave-induced hyperthermia was calculated using the device software, ultrasound-induced hyperthermia was not measured or calculated at the target depth. This makes it difficult to directly compare the efficacy of different hyperthermia modalities¹². It is speculated that ultrasound-induced hyperthermia may be less effective in larger tendons because of the limited heating area for a single element transducer¹². The mixed success rates in LITUS therapies that have been translated to humans highlight the importance of increased knowledge and reporting of acoustic parameters. Acoustic information can then be directly linked to bioeffects and allow for comparison between LITUS protocols and more successful translation into humans.

Summary

In summary, LITUS is a low-risk ultrasound therapy due to its low intensity and non-destructive, reversible mechanisms. As one of the most frequently used ultrasound therapies, LITUS treatment protocols and respective bioeffects have been studied extensively in small animal models. However, reports of clinical success are mixed and show few positive clinical results, perhaps due to inconsistent dosages and reporting of acoustic metrics. More work is needed in parameter optimization to successfully translate from pre-clinical models into clinical adoption. Therefore, it is difficult to make specific recommendations toward certain tendinopathies based on the current state of the field.

ESWT

Definition & Rationale

Originally designed to fragment kidney stones, ESWT has gained traction in treating tendinopathies, like epicondylopathy, plantar fasciopathy, rotator cuff tendinopathy, trochanteropathy, Achilles tendinopathy, and patellar tendinopathy¹³. ESWT differs from LITUS in that the wavefront is a pressure shock wave that is created by a piezoelectric, electromagnetic, or electrohydraulic source that is focused on a target (Figure 2). In general, ESWT achieves a shock front at the focus of 50–80 MPa with a fast initial rise time < 10 ns followed by a relatively long (~ms) negative pressure tail up to 10 MPa¹⁴ (Figure 2). Because of the high shock amplitude at the focus, ESWT can be painful and is sometimes administered with anesthesia¹⁴. ESWT dosage is often reported by physicians in terms of energy flux density (EFD) to describe the shock wave energy flowing through an area perpendicular to the source¹³. Similar to LITUS, this time-averaged intensity lacks detail including the peak pressures and duration of the pulse, which could help determine which mechanisms contribute to the healing response. Even after clinical use of ESWT to treat tendinopathy for more than 10 years, there is little research about the biological effects of ESWT on tendons. Lack of information may explain why reported success rates range from 48–81% and are highly dependent on dosage¹³.

Animal Studies

Acoustic cavitation has been proposed as the dominant mechanism of ESWT healing stimulation because it can induce localized microdamage, increasing perfusion, and thus healing^13,15. Treatments with a higher EFD of 0.3–0.6 mJ/mm² have shown some success in treating tendinopathy calcifications as the increased cavitation activity can fragment the mineral deposits^2,13. A non-cavitation mechanism that can occur with the high acoustic pressure is nerve hyperstimulation, which causes an analgesic effect due to the intense sensory input applied at the target of greatest discomfort¹⁶. Low- (< 0.08 mJ/mm²) and mid- (< 0.28 mJ/mm²) EFD treatments have shown success in treating chronic tendinopathy in animals, as they can release nitric oxide and promote angiogenesis (e.g. treating transected rabbit Achilles tendon, 500 shockwaves of 0.12 mJ/mm² at 0.42 Hz), and thus reduce inflammation and increase growth factor levels (e.g. treating collagenase-degraded rat Achilles tendon, 200 shockwaves of 0.16 mJ/mm² at 1 Hz)².

Clinical Studies

Gerdesmeyer et al. (2003)¹⁷ showed in a double-blind random chronic calcific shoulder tendinopathy clinical trial that both high- (0.32 mJ/mm², 2 treatment sessions of 1500 shockwaves at 2 Hz) and low-EFD (0.08 mJ/mm², 2 treatment sessions of 6000 shockwaves at 2 Hz) ESWT resulted in reduced pain and calcifications compared to sham. This result is supported in several other studies treating shoulder tendinopathy and plantar fasciopathy with these EFD levels^2,11,17. However, some studies of the same tendinopathy protocol show no added benefits compared to sham^2,11,17. Since peak pressures, pulse durations, and rise times are not reported, it is difficult to determine the differences in these studies, especially when comparing pressure fields between different source sizes and transduction methods. Focal size, depth, and alignment of these transducers are also important as the broad focal volumes typical in ESWT can damage tissue surrounding the tendon^13,15. Peak negative pressure amplitude, pulse duration, and pulse repetition are of particular importance if cavitation is the predominant mechanism, because these values contribute to the production of cavitation bubbles in tendons and bubble shielding between pulses that can decrease the efficiency of subsequent pulses.

Another possible reason behind the reported mixed success of ESWT in tendinopathies stems from placebo methodology. Application of ESWT for placebo groups typically is achieved by: (1) modifying the device so it absorbs shock fronts and inhibits most energy transmission; (2) reflecting the shock waves so they do not enter the body; or (3) applying a smaller number of shock waves². The first option offers the best patient blinding, as it uses the same device and treatment time. Although, as was noted in LITUS treatment, beneficial effects can still result from low-intensity pulses, making this an ineffective placebo treatment. The second option allows therapists and possibly patients knowledge of the treatment because the device is physically modified. Though all placebo options may result in less pain than ESWT treatment, the complete lack of sonication would be apparent due to neither pain nor stimulation. The third option also allows therapists and patients potential knowledge of their group because of the reduced treatment time, resulting in a corrupt blind study. For example, Buchbinder et al. (2002)¹⁸ performed a placebo treatment on chronic plantar fasciopathy of 100 shockwaves of 0.02 mJ/mm² at 1 Hz (< 2 min), while the ESWT treatment applied 2000+ shockwaves of 0.02–0.33 mJ/mm² up to 4 Hz (>10 min), gradually increasing to the highest tolerable level of pain per patient^14,18. Therefore, the placebo group received less pain and shorter treatment, which was vastly different than the active group. More consideration is needed to develop a suitable placebo for ESWT treatment.

In general, ESWT has shown improvements in pain relief, with the reported mixed success rates occurring when comparing to placebo or other therapies. Unlike LITUS, which was shown to provide only short-term, if any, relief, ESWT has shown beneficial maintenance of pain relief long-term. With ESWT, some studies have shown symptom improvements within 3–12 weeks, and the pain relief has been reported to persist up to 2 years compared to sham^2,16. Lizis (2015)¹⁶ directly compared ESWT treatment in lateral epicondylopathy (5 weekly sessions of 1000–2000 shockwaves at 8 Hz, 0.4 mJ/mm²) to LITUS treatment (10 sessions 3 times per week of 1 MHz continuous wave, 0.8 W/cm²) and showed significantly greater pain relief in ESWT, which persisted for > 3 months. However, other studies in lateral epicondylopathy have shown decreases in pain that are not significantly different than placebo^2,17. This may stem from differences in the type of tendinopathy, as it has been speculated that ESWT treatment is not as effective in acute tendinopathy patients¹⁴. Compared to other tendinopathy treatments, like nonsteroidal anti-inflammatory drugs (NSAIDs), physical therapy, and patellar straps, Wang et al. (2007)¹⁹ showed significant improvements from ESWT (1500 shockwaves of 0.18 mJ/mm² at 1–2 Hz) after treating patellar tendinopathy. Furthermore, Rebuzzi et al. (2008)²⁰ and Louwerens et al. (2016)³ showed no significant differences between ESWT (3 biweekly sessions of 1500 shockwaves of 0.1–0.13 mJ/mm² at 4 Hz²⁰; 1–4 sessions of 1000–2400 shockwaves of 0.2–0.55 mJ/mm^{2 3}) and arthroscopic surgery of chronic calcific supraspinatus tendinopathy, indicating ESWT has potential as a noninvasive alternative to surgery.

Summary

In brief, ESWT focuses high amplitude shockwaves to potentially create cavitation within the tendon. This, in turn, may induce microdamage to promote the release of healing factors at that site, which is a more aggressive approach to treat tendon than LITUS mechanisms. As a result, ESWT treatment can be painful, which makes randomized, controlled studies difficult to perform because a painless placebo will indicate the patient is not receiving the ESWT treatment. Animal models have not been as extensively studied in ESWT compared to LITUS. Focal pressures of clinical devices are not monitored, so it is unclear whether cavitation is achieved or whether it is even inside the targeted region in clinical studies. Though ESWT shows mixed potential in human studies, more research should be done in animal models to better understand the biological effects involved in healing to better tailor the acoustic parameters of the treatment. From the current state of the field, ESWT has better success in long-term pain management over LITUS. Although more research is needed to make specific recommendations toward certain tendinopathies, ESWT treatment has shown some success toward calcific tendinopathies over acute tendinopathies, likely due to fragmentation of mineral deposits from cavitation.

RSWT

Definition & Rationale

RSWT is a subset of ESWT developed around the year 2000 as an “out-of-bath” treatment²¹. The therapy induces an impulse superficially that propagates radially into the tendon (Figure 2). Radial waves are generated by accelerating a projectile via compressed air, which hits an applicator, or bullet. The bullet makes direct contact with the skin, which has a more superficial effect compared to focused ESWT²² (Figure 2). Because the highest pressure and EFD are at the applicator tip rather than in deep tissue, RSWT can be administered without anesthesia^14,22. Despite the name, RSWT treatment should not be considered a shock wave treatment, as the applicator does not actually produce a shock wave, or vertical spike in the pressure-time profile, on the skin. The radial geometric spreading attenuates the pressure to the 1/3^rd power as the acoustic wave penetrates the tissue, so a shock wave is also not present in the tendon²². Because of this, the waveform lacks characteristic features of ESWT shock waves, like short rise time (90x longer), high peak positive pressures (<10 MPa), and nonlinearities in the tendon¹⁵ (Figure 2). Penetration depth is also hypothesized to be limited to approximately 3.5 cm, which limits the utility of RSWT for treatment of deeper tendons, like at the rotator cuff²². Skin thickness between patients plays a pivotal role in the underlying mechanism as well, as Cirovic et al. (2016)²³ measured peak negative pressures at the site of plantar fascia in two cadaver models to be 0.9 and 2 MPa, whereas the onset of cavitation in soft tissue is suggested to be at 1.5 MPa²³.

Clinical Studies

Similar to other therapies, RSWT has reported mixed success of 10–90% in calcific shoulder tendinopathy treatments (3 sessions of 2000 impulses of 0.191–0.12 mJ/mm² at 8 Hz every 7–10 days)²². Since peak pressures are low at the target site, RSWT is often considered only to induce pain relief possibly from direct stimulation of a healing process and release of growth factors near the site, or suppressive effects that increase pain tolerance from hyperstimulation^22,24. Additionally, it comes with many potential side effects, including transient swelling, injury to nerves, or flattening of the arch in plantar fasciopathy¹⁴. Near the applicator tip, the radial waves are not yet attenuated, which can create cavitation bubbles near the skin as opposed to deep within the tendon. Subsequently, the bubble collapses can induce secondary shock waves and cause superficial damage¹⁴. While these side effects may be attributed to cavitation, bioeffects induced in the tendon remain a question since the pressures are much lower and are unlikely to produce cavitation.

Success rates of trials in humans with RSWT have been mixed. Some have shown a greater decrease of pain compared to placebo with even more pronounced differences over placebo at both 6 months²⁵ and 12 months (e.g. plantar fasciopathy, 2000 impulses at 0.16 mJ/mm² at 8 Hz¹⁴; lateral epicondylopathy, 4 sessions of 2000 impulses at 0.11–0.12 MPa at 4–10 Hz¹⁶; calcific shoulder tendinopathy, 3 sessions of 2000 impulses of 0.191–0.12 mJ/mm² at 8 Hz every 7–10 days²²). However, others have shown no difference compared to sham (e.g. plantar fasciopathy, 3 sessions of 500–2000 impulses of 0.16 mJ/mm² every 3 days)²⁶. Challenges exist in comparing these studies, as patient pathologies ranged from acute to chronic, which may explain the large standard deviation of symptom duration (35.6 +/− 43.2 days for RSWT; 21.0 +/− 16.4 days for placebo)^14,26. Compared to other tendinopathy therapies, like LITUS (10 biweekly sessions of 1.2 W/cm² at 1 Hz, operating frequency unknown)²⁷ in combination with physical therapy, or LITUS alone (10 triweekly sessions of 2 W/cm² at 3 MHz, continuous or pulsed mode unknown)^24,28, pain relief from RSWT occurred sooner (plantar fasciopathy, 3 weekly sessions of 2000 impulses of 3 MPa at 6 Hz)³⁰ or ranked higher scores in UP-PFQ (“University of Peloponnese Pain, Functionality and Quality of Life Questionnaire”) after a 4-week follow-up from RSWT treatment (lateral elbow tendinopathy²⁴ and plantar fasciopathy²⁸, 3–5 weekly sessions of 1500–2000 impulses of 0.16–0.18 MPa at 1–21 Hz)^24,28. Compared to focused ESWT, van der Worp et al. (2014)²⁹ showed no statistically significant difference between treatments of patellar tendinopathy (3 weekly sessions of 2000 pulses; focused ESWT of 0.12 mJ/mm² at 4 Hz, and RSWT of 0.24 MPa at 8 Hz), with both ESWT and RSWT methods significantly reducing pain and improving function at 14-week follow-up. Li et al. (2021)³⁰ showed ESWT to be significantly superior in long-term (24-week follow-up) management of pain level and functionality to RSWT after treating non-calcific rotator cuff tendinopathies (4 weekly sessions of 3000 pulses; focused ESWT of 0.01–0.15 mJ/mm² at 4–16 Hz, and RSWT of 0.1–0.5 MPa at 3–5 Hz).

Summary

RSWT physiological effects may fall between LITUS and ESWT based on little data of pressure measurements²³. The pseudo-shockwave mechanism is unlikely to produce cavitation in the tendon like focused ESWT, but the acoustic pressures may exceed that of LITUS due to the ballistic transducer design. Because it is not used as widely as the other ultrasound therapies and is relatively newest, studies investigating healing of tendinopathies in animal models are lacking. The primary benefit of RSWT over ESWT in clinical studies is the lack of a need for anesthesia, but RSWT comes with higher risk of side effects than LITUS. Although more research is needed to clarify which mechanisms are involved in the healing process and to determine the pressures generated within the tendon, RSWT is likely to perform better on superficial tendinopathies, e.g. patellar, over deeper tendinopathies, e.g. rotator cuff, due to its limiting penetration depth. Currently, there is not enough evidence to strongly recommend RSWT over other therapeutic ultrasound and shockwave therapies, but RSWT has shown some promise in performing similar to ESWT short-term, and produce more rapid results than LITUS in pain management of patellar tendinopathies.

Discussion

This review shows that LITUS, ESWT, and RSWT produce mixed success rates in the treatment of tendinopathy. These mixed success rates are, in part, due to the wide variety of methodologies and parameters that can be achieved within each broader treatment type. Lack of standardization in measurements and reporting make interpretation of the treatment success rates difficult. Many studies suggest these ultrasound treatments improve healing, function, and pain, with some proving more effective than sham. However, in numerous randomized controlled studies, patients receiving the ultrasound treatment showed no benefit compared to a placebo. When ultrasound is compared to other therapies, such as microwave-induced hyperthermia or physical therapy, some studies have shown that non-ultrasound therapies were more effective. If ultrasound can be tuned to consistently achieve the positive results, it has major advantages over many other therapies by being noninvasive with only minimal side effects.

Among the many variables mentioned in producing different results, two common gaps among all treatments are the absence to report pressure field measurements and in-house calibration. This makes it difficult to evaluate which bioeffects are most effective in enhancing healing. High intensity focused ultrasound (HIFU) is known to conventionally report calibration methods and focal peak pressures to determine what mechanisms are involved in the treatment. Acoustic lessons learned with HIFU should be applied in the ultrasound physiotherapy literature.

This paper compared differing therapeutic ultrasound and shockwave therapies in tendinopathy literature to provide improved understanding of parameter and mechanism efficacy, and highlight gaps in the field to promote better reporting. LITUS, ESWT, and RSWT all produce improvements in some patients with chronic tendinopathies. LITUS has been tested extensively in animal models and uses low intensities, thus LITUS has the lowest risk of side effects from treatment. ESWT is more painful, with high acoustic energies increasing the risk of unintended side effects. However, it has produced better long-term pain management than LITUS. RSWT is less painful than ESWT, but healing mechanisms are unknown, as are the acoustic pressures that reach the tendon. There is currently insufficient evidence to unequivocally state which therapeutic ultrasound or shockwave therapy modality is best and which treatment parameters are most efficient in treating different tendinopathies.

Therapeutic ultrasound and shockwave modalities for tendinopathies have the potential to be an alternative treatment with lower risk side-effects compared to other treatments like invasive surgery, and the modalities have potential for standardization compared to dry needling. Further research is needed, with emphasis on improving parameter reporting of peak pressures and intensity types. Through these recommendations, ultrasound treatments can be improved to create the most effective therapeutic for tendinopathy in humans.