The Effects of the Exposure of Musculoskeletal Tissue to Extracorporeal Shock Waves

Extracorporeal shock wave therapy (ESWT) is a safe and effective treatment option for various pathologies of the musculoskeletal system. Many studies address the molecular and cellular mechanisms of action of ESWT. However, to date, no uniform concept could be established on this matter. In the present study, we perform a systematic review of the effects of exposure of musculoskeletal tissue to extracorporeal shock waves (ESWs) reported in the literature. The key results are as follows: (i) compared to the effects of many other forms of therapy, the clinical benefit of ESWT does not appear to be based on a single mechanism; (ii) different tissues respond to the same mechanical stimulus in different ways; (iii) just because a mechanism of action of ESWT is described in a study does not automatically mean that this mechanism is relevant to the observed clinical effect; (iv) focused ESWs and radial ESWs seem to act in a similar way; and (v) even the most sophisticated research into the effects of exposure of musculoskeletal tissue to ESWs cannot substitute clinical research in order to determine the optimum intensity, treatment frequency and localization of ESWT.


Introduction
Extracorporeal shock wave therapy (ESWT) is a safe and effective treatment option for various pathologies of the musculoskeletal system. The beginning of the use of extracorporeal shock waves (ESWs) in medicine was in kidney stone fragmentation; the corresponding method is called Extracorporeal Shock Wave Lithotrypsy (ESWL). After ESWL was performed on dogs for the first time in 1976, four years later, the first human patient was successfully freed from his kidney stone disease using ESWL [1]. Expanded to other stone diseases in the gallbladder [2], pancreas [3], bile duct [4] and salivary glands [5], urologists found (more or less by chance) that the application of ESWs in the area of ureteral stones caused changes in the os ileum [6]. Specifically, when bones were exposed to ESWs, primary osteocyte damage followed by osteoblast stimulation was observed [6]. This resulted in the demonstration of the stimulation of fracture healing with ESWs in animal models [7]. Since these beginnings, the application of ESWs has been expanded to a variety of pathologies of the musculoskeletal system, with the treatment of non-unions (reviewed in [8]) and tendinopathies (reviewed in [9][10][11]) being, by far, the largest groups of indications. The treatment of pathologies of the musculoskeletal system with ESWs is commonly referred to as Extracorporeal Shock Wave Therapy (ESWT) and is thus distinguished from ESWL.
This short description of the history of ESWT demonstrates that the development of this treatment modality has not followed the classical drug discovery process, from initial target identification and validation, through assay development, high throughput screening, hit identification, lead optimization and finally the selection of a candidate molecule for clinical development [12]. Rather, progress in clinical research on ESWT was either accompanied or followed by basic and preclinical research into the potential mechanisms of action of ESWs on the target tissue. The latter was addressed in several recent reviews (e.g., [13][14][15][16][17]). Considering the fact that this study summarizes and discusses 181 studies addressing the effects of the exposure of musculoskeletal tissue on ESWs [6,, the limited number of references in the aforementioned reviews (between 38 [13] and 93 [16]) indicate that these reviews are either outdated or incomplete.
The aim of this study is to provide clinicians, basic science researchers and other stakeholders in healthcare with a comprehensive overview of what is known today regarding the effects of the exposure of musculoskeletal tissue to ESWs. This should help to further understand this fascinating, non-invasive treatment modality that is highly efficient and has a very good safety profile in the treatment of many pathologies of the musculoskeletal system. Because of the variety of different tissues that make up the musculoskeletal system, as well as of the different motivations for performing ESWT (ranging from pain relief to tissue regeneration), we divided our review into three areas focusing on bone and cartilage, connective tissue and muscle/nerve tissue.

Materials and Methods
PubMed and Web of Science were searched for "shock wave OR shock waves OR shockwave OR shockwaves NOT urol* NOT stone NOT review NOT clinical trial" from the days of inception of these databases until 30 September 2021, according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) [198] guidelines. Duplicates were excluded.
For each identified publication, it was determined by reading the title and abstract whether the publication represented a study on the effects of exposure of musculoskeletal tissue to extracorporeal shock waves; the studies only addressing the treatment of skin with ESWT were excluded. All this was independently undertaken by T.W. and C.S. The results were compared and discussed until an agreement was achieved.
Subsequently, all the selected studies were classified with regard to the type of tissue (bone and cartilage, connective tissue or muscle/nerve tissue, respectively) that was exposed to ESWs. Furthermore, it was determined for each selected study whether (i) morphological, functional and radiological findings, (ii) findings of molecular biological investigations and/or (iii) findings of histological investigations were reported. All this was independently undertaken by T.W. and L.J., and the results were compared and discussed until an agreement was achieved.
The strategy of the literature search is summarized in Figure 1.  [198] on 30 September 2021.

Results
The results of this systematic review are summarized in Tables 1-3, with a distinction being made between effects of the exposure of bone and cartilage tissue (Table 1), connective tissue (Table 2) and muscle and nerve tissue (Table 3) to ESWs. Within each table, the results are arranged chronologically, with the most recent findings presented first. More details of the studies listed in Tables 1-3 are provided in Tables S1-S3.  [198] on 30 September 2021.

Results
The results of this systematic review are summarized in Tables 1-3, with a distinction being made between effects of the exposure of bone and cartilage tissue (Table 1), connective tissue (Table 2) and muscle and nerve tissue (Table 3) to ESWs. Within each table, the results are arranged chronologically, with the most recent findings presented first. More details of the studies listed in Tables 1-3 are provided in Tables S1-S3. Table 1. Effects of the exposure of bone and cartilage tissue to extracorporeal shock waves (more details of the studies listed in this table are provided in Table S1).

Ref. First Author
Year   Table S2).

Effects of the Exposure of Bone and Cartilage Tissue to Extracorporeal Shock Waves
Our systematic review revealed 100 studies that addressed the effects of ESWs on bone and cartilage tissue [6,. These studies were published between 1988 and 2021, with 51 (51%) of these studies published during the last ten years (2012-2021). Eighty-five of these studies (85%) applied fESWs, eleven (11%) of these studies applied rESWs, two (2%) of these studies applied both fESWs and rESWs, and in two (2%) of these studies it was not described whether fESWs or rESWs were applied. The majority of these studies (64 of 100, i.e., 64%) described animal experiments; primary or secondary cell-culture experiments were described in 23 (23%) or 7 (7%) of these studies, respectively. Three (3%) of these studies combined animal experiments with primary cell-culture experiments; one (1%) of these studies combined animal experiments with secondary cell-culture experiments, and two (2%) of these studies were conducted ex vivo without animal experiments and cell-culture experiments (details are provided in Table S1). Very different effects of ESWs on bone and cartilage tissue were addressed in these 100 studies; these effects are summarized in Table 1.

Effects of Exposure of Connective Tissue to Extracorporeal Shock Waves
Our systematic review revealed 39 studies that addressed effects of ESWs on connective tissue . These studies were published between 1994 and 2021, with 18 (46.2%) of these studies published during the last ten years (2012-2021). Thirty (76.9%) of these study applied fESWs, and nine (23.1%) of these studies applied rESWs. The majority of these studies (24, i.e., 61.5%) described animal experiments; primary or secondary cell-culture experiments were described in nine (23.1%) or three (7.7%) of these studies, respectively. One study each (2.6% each) described cell-culture experiments (not further specified), experiments on fertilized chicken embryos and a human experiment (details are provided in Table S2). As in case of the exposure of bone and cartilage tissue to ESWs (Table 1), very different effects of ESWs on connective tissue were addressed in these 39 studies. These effects are summarized in Table 2.

Effects of Exposure of Muscle and Nerve Tissue to Extracorporeal Shock Waves
Our systematic review revealed 42 studies that addressed effects of ESWs on muscle and nerve tissue . These studies were published between 1998 and 2021, with 25 (59.5%) of these studies published during the last ten years (2012-2021). Twenty-eight (66.7%) of these study applied fESWs, 10 (23.8%) of these studies applied rESWs, and in four (9.5%) of these studies it was not described whether fESWs or rESWs were applied. The vast majority (39, i.e., 92.9%) of these studies described animal experiments; two (4.8%) of these studies described primary cell-culture experiments, and one (2.4%) of these studies combined animal experiments with primary cell-culture experiments (details are provided in Table S3). As in case of the exposure of bone, cartilage tissue and connective tissue to ESWs (Tables 1 and 2), very different effects of ESWs on muscle and nerve tissue were addressed in these 42 studies. These effects are summarized in Table 3.

Discussion
Based on the results summarized in Tables 1-3, we established ten take-home messages regarding the effects of exposure of musculoskeletal tissue to extracorporeal shock waves. These take-home messages are summarized in Table 4 and discussed below. Table 4. Take-home messages regarding the effects of exposure of musculoskeletal tissue to extracorporeal shock waves.

No.
Take-Home Message 1 Compared to the effects of many other forms of therapy, the clinical benefit of extracorporeal shock wave therapy does not appear to be based on a single mechanism. 2 Different tissues respond to the same mechanical stimulus in different ways.

3
Just because a mechanism of action of extracorporeal shock wave therapy was described in a study does not automatically mean that this mechanism was relevant to the observed clinical effect. 4 Focused and radial extracorporeal shock wave therapy seem to act in a similar way.

5
Extracorporeal shock wave therapy stimulates both progenitor and differentiated cells, and has positive effects on pathologies of bone and cartilage. 6 Extracorporeal shock wave therapy apparently mimics the effect of capsaicin by reducing substance-P concentration.

7
Extracorporeal shock wave therapy apparently mimics effects of injection of Botulinum toxin A by destroying endplates in the neuromuscular junction. 8 Extracorporeal shock wave therapy apparently imitates certain mechanisms of action of neural therapy.

9
Extracorporeal shock wave therapy apparently imitates certain mechanisms of manual therapy treatments.

10
Even the most sophisticated research into the effects of exposure of musculoskeletal tissue to extracorporeal shock waves cannot substitute clinical research in order to determine the optimum intensity, treatment frequency and localization of extracorporeal shock wave therapy.
The first take-home message of this study is that compared to the effects of many other forms of therapy; the clinical benefit of extracorporeal shock wave therapy does not appear to be based on a single mechanism. Most of the basic studies on medical therapies run exactly opposite to the studies on the mode of action of ESWT. In preclinical research, mechanisms are often sought that are later clinically tested for their benefit. However, for the treatment indications of ESWT on the musculoskeletal system, mainly the clinical success is known to date, while, in contrast, the molecular and cellular causes for this success are widely unknown. Thus, studies of the mode of action of ESWT are based on rational considerations of the mechanisms by which clinical success might occur. In the numerous studies, a variety of effects were described, most of which are desirable for the respective indication. Many of these mechanisms are not causally related, so that it is obvious that the combination of different effects leads to the therapeutic success of ESWT.
The second take-home message of this study is that different tissues respond to the same mechanical stimulus in different ways. Based on many years of clinical experience and numerous clinical studies, various pathologies of the musculoskeletal system are known nowadays that can be successfully treated with ESWT [8][9][10][11]. These indications include mainly degenerations and injuries of muscle, bone and cartilage tissue. From basic research, a wide variety of effects at the molecular and cellular levels were described to date, whereby the effects of the ESWs differ in each case from the tissue treated. On the one hand, very tissue-specific reactions were observed. For example, while the enhancement of the osseous differentiation of stem cells occurred in the bone [23,48], the differentiation of stem cells into the osteocytic lineage was not observed in tendon tissue [125]. On the other hand, there are similar effects that were observed, despite the different tissues, such as an increase in the expression of vascular endothelial growth factor (VEGF) after exposure to ESWs in the bone and cartilage tissue [42,58,69,73,90,97], nerve tissue [170,175] and connective tissue [137]. This leads to the conclusion that ESWs generally promote angiogenesis, despite the fact that some studies described no effects after the exposure of tissue to ESWs on the expression of VEGF [118,119,162], or even the reduced expression of VEGF [26]. In addition, the condition of the treated tissue also seems to play a role. For example, healthy tenocytes responded to the exposure to ESWs with a different protein expression pattern than tenocytes from tendinopathic or ruptured tendon tissues [134,141]. This highlights one of the key problems in evaluating studies of the effects of ESWs on the musculoskeletal system: due to differences in design and the prevailing conditions in these studies, comparisons are sometimes difficult to make.
The third take-home message of this study is that just because a mechanism of action of extracorporeal shock wave therapy was described in a study does not automatically mean that this mechanism was relevant to the observed clinical effect. Some of the many effects described include effects that, considered in isolation, would not be desirable for the success of the therapy. However, as clinically a treatment success is mostly shown, other mechanisms must play a greater role for the effect of ESWT. One example is the increased vascularization of tendon tissue after exposure to ESWs [97,133]; although increased vascularization is usually associated with tendon inflammation [199], clinical findings were shown to improve after treatment [148]. Likewise, in the treatment of muscular spasticity by ESWT, it is unlikely that a stimulating effect of ESWs on, for example, stem cells, has anything to do with the reduced muscle tone after ESWT (e.g., [200]). Thus, when deducing the modes of action of ESWT in certain pathologies of the musculoskeletal system, one should always relate certain modes of action to the pathology under investigation in order to not obtain incorrect conclusions.
The fourth take-home message of this study is that focused and radial extracorporeal shock wave therapy seem to act in a similar way. Numerous effects were described for both fESWT and rESWT, however, more effects were described for fESWT (Tables 1-3). This may be due to the fact that fESWT was developed before rESWT [10]. From a physics point of view, these two forms of ESWT appear to differ greatly. Focused ESWs are generated by three methods that are named electrohydraulic, electromagnetic and piezoelectric [10]. Additionally, unlike rESWs, fESWs are generated in water that is inside the applicator [201]. In contrast, rESWs are generated by the acceleration of a projectile in a tube (through compressed air or a magnetic field), and the projectile hits an applicator at the end of the tube. Through contact with the skin via contact gel (to facilitate transmission), the rESWs are transmitted into the treated tissue [201]. As a result of these different mechanisms of ESW generation, rESWT has more of a superficial effect on tissues, while fESWT can also affect deeper tissues [10,201].
Some authors argued that rESWs should not be called shock waves, since they lack the characteristic physical features of true shock waves, including a short rise time in the amount of nanoseconds, a high peak pressure and non-linearity [202]. The physical definition of a "true" shock wave is as follows [203]: a high positive peak pressure (P + ), sometimes more than 100 Megapascal (Mpa), but more often approximately 50 to 80 MPa; a fast initial rise in pressure (T r ) during a period of less than 10 nanoseconds (ns); a low tensile amplitude (P − , up to 10 MPa); a short life cycle (I) of approximately 10 microseconds (µs); and a broad frequency spectrum, typically in the range of 16 Hertz (Hz) to 20 MHz. It is well-known that rESWs are not "true" shock waves in the strict physical sense outlined above [202]. This is because rESWs show a lower positive peak pressure (~10 MPa) and a substantially longer rise time (~600 ns), and have thus been termed radial pressure waves by some authors [204]. However, in 2007, it was already noticed that for treatment protocols at low-energy settings, neither piezoelectric nor electromagnetic fESWT devices generated true shock waves according to the physical criteria set out above [202]. With respect to the various ESWT devices' abilities to generate shock waves as opposed to pressure waves, the initial concept can thus be refined into a concept that considers high-energy settings as a prerequisite for the generation of true shock waves. For clinical applications of ESWT, however, a more feasible concept of therapeutic shock wave technology needs to factor in two more considerations: that biological cells and tissues can differentiate between true shock waves and pressure waves, but cannot differentiate between radial or focused wave forms. As to the former point, it is certainly reasonable to differentiate between shock waves and pressure waves in terms of the differences in positive peak pressure delivered to the pathologic site. However, the question arises whether therapy success in many pathologies of the musculoskeletal system requires "true" shock waves [205]. It appears that this is not the case. With respect to the differentiation between rESWs and fESWs, under plain geometric considerations it is highly unlikely that tissues and cells can differentiate whether they are affected by focused or by radial acoustic waves-the only difference is in the number of affected cells. In consequence, it appears that, clinically, "a wave is a wave" regardless of whether it is generated with an fESWT device or a rESWT device. Much more important is whether sufficient ESWT energy is achieved where it is needed in the body.
Cavitation can be generated only during the shock wave's tensile phase [206]. Of note, both fESWs and rESWs can generate vaporous cavitation [206]. Vaporous cavitation is assumed to play an important role in mediating molecular and cellular mechanisms of action of ESWT in biological tissues, presumably via the mechanical activation of membranebound signaling molecules, which, in turn, elicit cellular responses [206]. Yet, many questions remain open concerning the therapeutic effects of vaporous cavitation during ESWT. For example, it was found that tissues exposed to ESWs show a subsequent decrease in proinflammatory neuropeptides, similar to a "wash-out" effect [193]. This correlates well with the long-term analgesic effect mediated by ESWT in tendinopathies [10]. Yet, it remains unknown which effects vaporous cavitation has on the unmyelinated terminal endings of nociceptive fibers (i.e., C fibers) in the peripheral nervous system. More generally speaking, it is still unknown as to whether the therapeutic benefits of ESWT are due mainly to the positive (i.e., shear stress) or negative (i.e., cavitation) pressures, or a combination of both, in order to optimize treatment protocols [10]. Because of the potentially deleterious side effects of vaporous cavitation on the body, it is imperative to realize that both fESWT devices and rESWT devices can in fact generate vaporous cavitation in the treated tissue.
In summary, it is reasonable to hypothesize that further research into the effects of exposure of musculoskeletal tissue to fESWs and rESWs will demonstrate more similarities than dissimilarities between these modalities. Nevertheless, due to the differing energy distribution of both treatment forms in the target tissue, different energy-dependent effects may occur (e.g., [102]).
The fifth take-home message of this study is that extracorporeal shock wave therapy stimulates both progenitor and differentiated cells and has positive effects on the pathologies of bone and cartilage. A central aspect for the treatment of degenerations and injuries of muscles, tendons, bones and cartilage using ESWT is the activation of the respective tissue-specific cells. The mechanical pressure on the cells themselves leads to an increased expression of cell-specific proteins and cell viability. In bone, for example, there are several mechanisms by which bone growth is promoted and the activity of fully differentiated cells is increased. Numerous studies showed the upregulation of bone morphogenetic protein 2 (BMP-2) after the exposure of bones to fESWs [47,67,104]. BMP-2 plays a major role in osteoblast differentiation by transforming osteoblast precursor cells into mature osteoblasts that form healthy bones [207]. On the other hand, for proteins, such as RANKL, which, in turn, plays a role in osteoclast differentiation [208], a reduced expression was found after exposure to ESWs [19, 63,80]. Furthermore, cavitation induced by ESWs can cause so-called "microcracks", which is a stimulus for bone remodeling and new bone formation [209]. It was demonstrated in the bones of horses that fESWs can induce new microcracks, and rESWs can extend the length of existing microcracks [102]. When observing the effects of ESWT on the activity of different cell types, an increase in activity in tissue-specific cells, such as fibroblasts [68,124] and osteoblasts [39,83], but, at the same time, a reduced activity of osteoclasts [19], was observed. Together with the reduced RANKL expression, this could indicate a positive effect of ESWT on bone formation, as well as an improvement of diseases affecting the skeletal system, such as osteoporosis. In fact, ESWT shows positive effects in the treatment of these indications [8,210].
The sixth take-home message of this study is that extracorporeal shock wave therapy apparently mimics the effect of capsaicin by reducing substance-P concentration. In pathologies of tendons, muscle injuries and dysfunctions, as well as in osteoarthritis, the inflammatory cycle plays a crucial role, as does nociception for the quality of life of the patients. Substance P is a neuropeptide, which, once released after the activation of the TRPV1 receptor on mainly polymodal C-fibers [211], primarily activates the neurokinin-1 receptor (NK1R) [211,212]. Substance P plays an important role in nociception and neurogenic inflammation [213] through several intracellular pathways [212]. Therefore, in recent years, special attention was paid to capsaicin, a naturally occurring alkaloid that has certain reducing effects on substance-P concentration. Specifically, after application to the peripheral nerve, one of the effects of capsaicin was shown in an activation of the TRPV1 channel, mainly in the terminal endings of nociceptive fibers (especially C fibers), which initially does not lead to a reduction in pain and inflammation as an increase in substance-P concentration is to be expected [211,214]. By releasing substance P from the nerve fibers and simultaneously blocking the axoplasmic transport [215], the terminals are then depleted of their substance-P content [211,214]. However, whether this mechanism is (in addition to reducing inflammation [216]) also responsible for the pain relief with local capsaicin application is currently highly debated [217]. With ESWT, on the other hand, there is evidence that one of the analgesic effects is due to a reduction in the substance-P concentration in the tissue under treatment [191,193], thereby removing substance P from the C fibers. The mechanism behind this is probably a detrimental effect of ESWs on the TRPV1 channel. As with capsaicin, a similar time course of alterations in the amount of substance P in the periosteum was found after exposure of the femur of healthy rabbits to fESWs [191,193]. This may break the inflammatory cycle created by substance-P release, and thus has a different mechanism than medications, such as non-steroidal anti-inflammatory drugs (NSAIDs) that inhibit cyclooxygenase [218], but still helps reduce inflammation. In addition, both substance-P and calcitonin gene-related peptide expressions were demonstrated to be reduced in dorsal root ganglia after the exposure of peripheral tissue to ESWs [184,186,192]. The effect on the local inflammatory circuit is probably additionally enhanced by this. Due to the local application of ESWs, this effect is limited to the treatment region and the affected spinal cord segments, proven at least for substance P [184]. An important result of this is that ESWT does not induce the typical adverse events of treatments with NSAIDs, such as gastrointestinal ulcers and renal damage [218].
The seventh take-home message of this study is that extracorporeal shock wave therapy apparently mimics effects of injection of Botulinum toxin A by destroying endplates in the neuromuscular junction. Botulinum toxin A (BTX-A) injections are nowadays widely used for treating spasticity, which mainly affects individual muscle groups. Examples include spasticity induced by stroke [219], spinal cord injury [220] and infantile cerebral palsy [221]. The central problem in muscle spasticity is constant overexcitation at the neuromuscular endplate. BTX-A effectively prevents the formation of a stable SNARE complex by cleaving one of its associated proteins, SNAP-25. Since the SNARE complex is essential for acetylcholine release, a block of the skeletal cholinergic neuromuscular transmission occurs [222]. As reports of potentially serious side effects of BTX-A injections for treating spasticity continue to emerge [223,224] and long-term effects of this treatment modality remain to be established, the question of new treatment options arises. Extracorporeal shock wave therapy, similar to BTX-A injection, can transiently reduce excitatory transmission at the neuromuscular endplate. In this regard, it was shown in a rat model that the exposure of muscles to rESWs reduced the compound muscle action potential while maintaining the latency [157,168]. The key mechanism of ESWs, in contrast to BTX-A, is most likely the destruction of end plates in neuromuscular junctions, whereby the damage was confined to the postsynaptic membrane [168]. In a recent randomized controlled trial, it was found that BTX-A injection was not superior to rESWT in the treatment of plantar flexor muscle spasticity in patients with cerebral palsy [200].
The eighth take-home message of this study is that extracorporeal shock wave therapy apparently imitates certain mechanisms of action of neural therapy. Neural therapy is a treatment commonly used in Europe for pain relief. Its aim is to normalize the nervous system through targeted injections of local anesthetics [225]. Local anesthetics, such as the commonly used procaine, cause a blockade of the voltage-dependent sodium channels of nerve fibers [226]. This causes a reversible blockade of excitation conduction in nerve fibers, i.e., nociceptive afferents are shut down [226]. ESWT may have a similar principle of action in order to reduce pain conditions. Specifically, it was shown that, after the exposure of the femur to fESWs, a selective destruction and decreased number of unmyelinated nerve fibers in the sciatic nerve of rabbits was induced [183]. C fibers, for example, as part of the nociceptive system, belong to the unmyelinated nerve fibers. Furthermore, ESWs were shown to induce disturbed integrity of myelin sheaths combined with reduced nerve conduction velocities in palmar digital nerves in horses [190], as well as a reduced number of epidermal nerve fibers in the skin [189]. In summary, these results suggest that ESWT can reduce peripheral nerve function and conduction, without affecting the performance of professional athletes [227]. This mechanism may be central to the reduction in pain perception following ESWT, given the possibility that the transmission of nociceptive signals via peripheral nerves is impaired. Furthermore, it cannot be excluded that ESWT influences the conduction ability of sensitive nerves through the activation of gate-control mechanisms in the spinal cord [228]. Compared to neural therapy, a recent study demonstrated that, in patients with myofascial trigger points in the upper trapezius, both the repeated injection of 1% lidocaine and rESWT resulted in reduced pain alongside improved muscle elasticity, pressure pain threshold and neck disability index [229].
The ninth take-home message is that extracorporeal shock wave therapy apparently imitates certain mechanisms of manual therapy treatments. Many manual therapy treatments, such as massages, are aimed at achieving effects, including improved blood circulation, angiogenesis and reduced lymph congestion [230]. These effects were also observed after ESWT. For example, the exposure of skin and muscle tissue to both fESWs and rESWs resulted in a significant increase in the local microcirculation [126,131,173]. A positive effect of ESWT was also described on lymphatic drainage [137], and increased angiogenesis after exposure to ESWs was found in both blood vessels [131,165] and lymph vessels [137]. In addition, ESWT has a stimulating effect on the expression of lubricin in fasciae and tendon sheaths [135]. Lubricin was shown to induce an improvement in tendon gliding in vivo, and the absence of lubricin was demonstrated to significantly limit tendon mobility [231]. Of note, tendon gliding plays a major role in the rehabilitation of tendinopathies and tendon injuries [232]. Furthermore, rESWT was shown to significantly improve immobility-related muscle contractures and muscle fibrosis [156] in a rabbit model. A possible mechanism behind this is the reduced collagen deposition that was observed after treatment. However, it is unclear whether ESWT can also improve fascial fibrosis. As this is an alteration within the collagen fiber layers due to large amounts of undirected collagen material deposition [233,234], ESWT could also have a positive effect here.
The tenth take-home message is that even the most sophisticated research into the effects of exposure of musculoskeletal tissue to extracorporeal shock waves cannot substitute clinical research in order to determine the optimum intensity, treatment frequency and localization of extracorporeal shock wave therapy. Since this study was mainly about the different mechanisms of ESWT, no optimal treatment settings can be determined from the results summarized in Tables 1-3. In several studies, certain processes at the cellular level were described at certain points in time, which even contradicted each other in part. For example, while the exposure of cells to ESWs often led to reduced cell viability shortly after exposure, an increase in cell viability was observed in the further course of observation [84]. Therefore, it is reasonable to hypothesize that some biological changes only occur at a certain time, which, however, must be carefully considered in the study protocol and the measurements. In addition, some effects of the exposure of cells and tissue to ESWs were found only at certain energy levels [105,157] and numbers of applied ESWs [42]. Some studies even showed that the exposure of musculoskeletal tissue with ESWs with increasing EFD did not necessarily lead to better outcomes [105,106]. In summary, the only way to further optimize clinical application of ESWT is to perform more and better clinical research on this fascinating treatment modality. It is obvious that the results of basic research may be inspirational in this regard.
This systematic review had three limitations. First, only PubMed and Web of Science were searched. However, considering the fact that, in this review, considerably more studies were considered than in previous reviews on the same topic [13][14][15][16][17], it is reasonable to hypothesize that, in the present investigation, the risk to overlook any relevant study on the effects of exposure of musculoskeletal tissue to extracorporeal shock waves was minimized. Second, no meta-analysis of the presented data was performed. However, as outlined, particularly in the take-home messages 1-3 and 5, this appears to not be possible. Third, this review did not address all the potential indications of ESWT, but was restricted to musculoskeletal tissue. The mechanisms of action of ESWs in the treatment of, e.g., acute and chronic soft tissue wounds (e.g., [235]) or coronary artery disease (e.g., [236]) with ESWT may or may not be the same as discussed in this investigation.

Conclusions
The complementary effects of ESWT in the treatment of musculoskeletal pathologies make it an effective form of therapy that can be used alone or in combination with other therapeutic modalities. Not to be underestimated is the possibility of using ESWT as a supportive measure for any myofascial imbalances and functional movement restrictions underlying the pathologies. This is explained by the effects of ESWT on the myofascial units, such as the reduction in muscle tone, the decreased inflammatory activity and the effect on trigger points. Further studies, especially clinical studies, are needed for the future use of ESWT. To date, there is still minimal evidence on the ideal treatment settings, intensity, duration, localization and applied energy to provide the best possible treatment.
Supplementary Materials: The following supporting information can be downloaded at: https://www. mdpi.com/article/10.3390/biomedicines10051084/s1. Table S1: Details of the studies listed in Table 1. Table S2: details of the studies listed in Table 2. Table S3: Details of the studies listed in Table 3.