**2.5 In chronic phase on B mode US**

The US can observe the healing process of injured tissues depending on the nature of the original injury. The B mode US may appear hyperechogenic during the healing phase. Normal tissue healing is considered a reduction in size or resolution of the region of increased echogenicity [30]. Even though the B mode US can evaluate the chronic phase of injured tissue, it is less sensitive than MRI to residual morphologic changes after MSI for the higher soft-tissue contrast and high to extracellular fluid in MRI [31]. In clinical practice, the detection of small echogenic scar tissue by using the B mode US is difficult for a less experienced practitioner. As mentioned above, the

relationship between demonstration of scar tissue and re-injury is still controversial. However, excessive scar tissue may be symptomatic that is described as "feeling tight." It may disturb neural tension, which leads to re-injury [32].

Scar tissue is often shown as irregular thickening of the fascial tissue compared with the uninjured side on the B mode US [33].

Skeletal muscles are also composed of connective tissue, which resists and transmits the force generated by myofibrils to the tendon and bone structures to generate physical movement. When skeletal muscles are injured, any one of these components including fascial tissue can be damaged [18].

#### **2.6 In chronic phase on SWE**

SWE can take an advantage to evaluate tissue properties in chronic phase compared with B mode US. SWE can evaluate the absolute elasticity value of soft tissue structures and obtain useful quantitative information about the mechanical properties in the chronic phase.

In the chronic phase of MSI, the stiffness is significantly higher in the chronic phase compared with the acute phase [25].

In the chronic phase of tendon rupture injury, the stiffness of the tissue gradually increased following the healing process with or without surgical repair [34].

From these results, SWE can be a useful tool for evaluating in the phase of transition of acute to chronic phase.

Tendinopathy is considered to occur from mechanical, degenerative, and overuse diseases. It is associated with degeneration and disorganization of the collagenous structure, changes in the proteoglycan and water contents, increased cellularity, fatty infiltration, and neovascularization due to repetitive mechanical stress [35]. A study of tendinopathy shows tendon stiffness is correlated with the patients' symptom scores, demonstrating the promise of shear wave elastography during follow-up for tendinopathies [36].

Interestingly, by evaluating SWE, injured area of fascial tissue increased stiffness between injured leg and uninjured leg in 11 injured professional rugby players, mean average of shear wave modulus on injured side (17.34 ± 9.04 kPa) and maximum shear wave modulus on injured side (33.53 kPa) compared with mean average of shear wave modulus on uninjured side (12.7 ± 4.96 kPa) and maximum shear wave modulus on uninjured side (20.86 kPa) (**Figures 5** and **6**) [37].

Chronic cumulative injury can affect the fascial tissue in addition to the chronic phase of direct trauma. Cumulative mechanical stress leads to fibrotic tissue, thickness of tendinous tissue could be related to the injury [38]. Repetitive cumulative stress, especially eccentric contraction, causes microscopic tissue damage and increases inflammation. ECM of tissue changes plays an important role in tissue stiffness changes [39]. Change in property of ECM by cumulative stress may affect the stiffness in the chronic musculoskeletal injury.

Considering the results, in chronic musculoskeletal injury, it affects not only the muscle tissue but also a wide variety of tissues including fascial tissue. Even though a wide variety of ultrasound imaging has been used in fascial tissue, there is a lack of standardization [40]. SWE can be a more accurate diagnostic tool compared with B-mode, and the combination of SWE and B-US can be a strong diagnostic tool for fascial pathology [41].

To measure the fascial tissue, SWE provides the images reflecting the shear wave value as a tightness of the area of interest.


#### **Figure 5.**

*The stiffness of fascial tissue of injured side by using Q box trace mode, and the unit was given automatically by machine in kilopascal units. Injured side stiffness is higher than that of uninjured side.*

#### **Figure 6.**

*The stiffness of fascial tissue of uninjured side by using Q box trace mode, and the unit was given automatically by machine in kilopascal units.*

As considering the tissue property depending on viscoelastic property, utilization of SWE can be a useful tool for evaluating a wide variety of tissues in chronic musculoskeletal injury.

To explain fascial tissue, the term Fascia is used to be recognized as "a sheet or band of soft connective tissue that attaches, surrounds and separates internal organs and skeletal muscles." However, according to the recognition of physiological and

pathophysiological behaviors of a range of connective tissues, the definition is widely considered. With current understanding of mechanical aspects of connective tissue function, fascia is considered in the view of micro to macro as fibril to fascial system. From a morphological view, fascia is described as a sheet or any other dissectible aggregations of connective tissue that forms beneath the skin to attach, enclose, and separate muscles and other internal organs.

There are several types of fascial tissues in the fascial system. The fascial system consists of adipose tissue, adventitia, neurovascular sheaths, aponeuroses, deep and superficial fasciae, dermis, epineurium, joint capsules, ligaments, membranes, meninges, myofascial expansions, periostea, retinacula, septa, tendons. The fascial system is also considered to be included endotendon, peritendon, epitendon, and paratenson, visceral fasciae, and all the intramuscular and intermuscular connective tissues, including endomysium, perimysium, epimysium. The fascial system consists of various components, and it is built on three-dimensional soft seamless collagenous fibers. The loose and dense fibrous connective tissue fills the whole body and allows the integration of body systems.

With injured fascial tissues, it will have a very similar healing process to muscle injury. Micro or macro changes occurred by excessive or repetitive loading or direct trauma of fascial tissue. The pathological changes will modify mechanical function that compromises initial tissue or function. In the acute inflammation phase of fascial tissue, immune response proceeds by phagocytose from the injured cell. It releases proinflammatory cytokines and macrophages to promote immune cell infiltration. If the excessive loading is chronically prolonged, continuing inflammation develops, which leads to the presence of cytotoxic cytokines affected tissues. From this reaction, interleukin-1β, tumor necrosis factor (TNF) and transforming growth factor beta (TGFβ-1)) can promote fibrosis by excessive fibroblast proliferation and collagen matrix deposition that consequently develops fibrotic tissue. A study indicates that substance P stimulates TGFβ-1, which leads to fibrotic tissue development [42]. That phenomenon shows in the chronic phase of fascial injury.

Most pathological cases of fascial tissue demonstrated that a decreased tissue stiffness is present, while some cases demonstrated an increased stiffness due to fibrotic tissues.

However, the viscoelasticity is varied from tissue to tissue. The stiffness of tissue can be affected by the viscoelastic properties of ECM, especially the aponeurotic tissue containing loose connective tissue in which the ECM has ground substances, such as glycosaminoglycans (GAGs) especially hyaluronan (HA)-containing fluid between each layer [43]. The fascial component of the ECM is the main site of the inflammatory responses that occur in tissues. Thus, when the tissue reacts to an inflammatory response, the viscosity of the tissue can be increased, which could lead to increased viscoelasticity of the fascial tissue.

Evaluation of the stiffness of fascial tissue using SWE is considered as viscoelastic, inhomogeneous tissues [44]. The shear modulus value, stiffness, of fascial tissue is affected not only by the fibrotic tissue itself, but also ground substances and fluid components [45]. Therefore, stiffness is affected not only by pure elastic properties, but also by viscosity properties in the fascial tissue [46].

Fascial tissue can be affected by viscoelastic properties more than muscle [47].

Fascial tissue should include loose connective tissue, which contains rich ground substances between each layer. These properties affect the movement of loose connective tissue within and under the tissues [48].

In the chronic condition of MSI, the concentration and molecular weight of HA are altered. In this regard, binding interactions with other macromolecules may affect the sliding movement of fascial tissues [49]. Generally speaking, elastic tissues are hydrophilic and function using a tissue sliding system. However, fibrotic tissues exhibit an altered tissue sliding system, which affects the rehydration and expansion [50]. Therefore, in chronic MSI with scar tissue, there may be less function of rehydration; consequently, it will be stiffer tissue than healthy tissue. Even though SWE is considered not operator-dependent, the viscosity component will affect the results of measurement. Therefore, viscoelastic tissue such as fascial tissue must take special consideration in chronic musculoskeletal conditions.

This phenomenon may affect our daily activities for some reasons.

First, fascial tissues are rich in nerve receptors and free and encapsulated nerve endings including Pacinian corpuscles and Ruffini endings. Those receptors detect and react to mechanical stimulations [51]. As the tissue is stimulated, the nerve endings react and provide sensory feedback that translates into the human ability to detect and coordinate movement and achieve neuromuscular control. Chronic musculoskeletal issues, especially with fibrotic scar tissue, can alter the movement in daily activities.

Secondly, changes in the viscoelasticity of the tissues, basically modulated by ground substances, alter pain sensitivity as activation of nociceptors [48]. The more adhered tissue such as an inflamed tissue, the less lubricated that leads to the alteration of the tissue sliding. Thus, nociceptors can translate mechanical stimuli into pain sensation; consequently, incorrect sensory feedback will modify proprioceptors to nociceptors. Finally, myofascial network transmits to other tissues for muscle force [52]. Stiffened tissue affects this transmission and may change muscle mechanics [53]. Therefore, impaired myofascial force transmission by stiffened tissue may have a negative effect on the proper muscle biomechanics.
