Soft tissueconnects and surrounds or supports internal organs and bones, and includesmuscle,tendons,ligaments,fat,fibrous tissue,lymph andblood vessels,fasciae, andsynovial membranes.[1][2] Soft tissue istissue in the body that is nothardened by the processes ofossification orcalcification such asbones andteeth.[1]
It is sometimes defined by what it is not – such as "nonepithelial, extraskeletalmesenchyme exclusive of thereticuloendothelial system andglia".[3]
The characteristic substances inside theextracellular matrix of soft tissue are thecollagen,elastin andground substance. Normally the soft tissue is very hydrated because of the ground substance. Thefibroblasts are the most common cell responsible for the production of soft tissues' fibers and ground substance. Variations of fibroblasts, likechondroblasts, may also produce these substances.[4]
At smallstrains, elastin confersstiffness to the tissue and stores most of thestrain energy. The collagen fibers are comparatively inextensible and are usually loose (wavy, crimped). With increasing tissue deformation the collagen is gradually stretched in the direction of deformation. When taut, these fibers produce a strong growth in tissue stiffness. Thecomposite behavior is analogous to anylon stocking, whose rubber band does the role of elastin as thenylon does the role of collagen. In soft tissues, the collagen limits the deformation and protects the tissues from injury.
Human soft tissue is highly deformable, and its mechanical properties vary significantly from one person to another. Impact testing results showed that the stiffness and the damping resistance of a test subject's tissue are correlated with the mass, velocity, and size of the striking object. Such properties may be useful for forensics investigation when contusions were induced.[5] When a solid object impacts a human soft tissue, the energy of the impact will be absorbed by the tissues to reduce the effect of the impact or the pain level; subjects with more soft tissue thickness tended to absorb the impacts with less aversion.[6]
.png)
Soft tissues have the potential to undergo large deformations and still return to the initial configuration when unloaded, i.e. they arehyperelastic materials, and theirstress-strain curve isnonlinear. The soft tissues are alsoviscoelastic,incompressible and usuallyanisotropic. Some viscoelastic properties observable in soft tissues are:relaxation,creep andhysteresis.[7][8] In order to describe the mechanical response of soft tissues, several methods have been used. These methods include: hyperelastic macroscopic models based on strain energy, mathematical fits where nonlinear constitutive equations are used, and structurally based models where the response of a linear elastic material is modified by its geometric characteristics.[9]
Even though soft tissues have viscoelastic properties, i.e. stress as function of strain rate, it can be approximated by ahyperelastic model afterprecondition to a load pattern. After some cycles of loading and unloading the material, the mechanical response becomes independent of strain rate.
Despite the independence of strain rate, preconditioned soft tissues still present hysteresis, so the mechanical response can be modeled as hyperelastic with different material constants at loading and unloading. By this method the elasticity theory is used to model an inelastic material. Fung has called this model aspseudoelastic to point out that the material is not truly elastic.[8]
In physiological state soft tissues usually presentresidual stress that may be released when the tissue isexcised.Physiologists andhistologists must be aware of this fact to avoid mistakes when analyzing excised tissues. This retraction usually causes avisual artifact.[8]
Fung developed aconstitutive equation for preconditioned soft tissues which is
with
quadratic forms ofGreen-Lagrange strains and, and material constants.[8] is thestrain energy function per volume unit, which is the mechanical strain energy for a given temperature.
The Fung-model, simplified with isotropic hypothesis (same mechanical properties in all directions). This written in respect of the principal stretches ():
where a, b and c are constants.
For small strains, the exponential term is very small, thus negligible.
On the other hand, the linear term is negligible when the analysis rely only on big strains.
where is the shear modulus for infinitesimal strains and is a stiffening parameter, associated with limiting chain extensibility.[10] This constitutive model cannot be stretched in uni-axial tension beyond a maximal stretch, which is the positive root of
Soft tissues have the potential to grow and remodel reacting to chemical and mechanical long term changes. The rate the fibroblasts producetropocollagen is proportional to these stimuli. Diseases, injuries and changes in the level of mechanical load may induce remodeling.[11][12] An example of this phenomenon is the thickening of farmer's hands. The remodeling of connective tissues is well known in bones by theWolff's law (bone remodeling).Mechanobiology is the science that study the relation between stress and growth at cellular level.[7]
Growth and remodeling have a major role in the cause of some common soft tissue diseases, like arterialstenosis andaneurisms[13][14] and any soft tissuefibrosis. Other instance of tissue remodeling is the thickening of thecardiac muscle in response to the growth ofblood pressure detected by thearterial wall.
There are certain issues that have to be kept in mind when choosing an imaging technique for visualizing soft tissueextracellular matrix (ECM) components. The accuracy of the image analysis relies on the properties and the quality of the raw data and, therefore, the choice of the imaging technique must be based upon issues such as:
The collagen fibers are approximately 1–2 μm thick. Thus, the resolution of the imaging technique needs to be approximately 0.5 μm. Some techniques allow the direct acquisition of volume data while other need the slicing of the specimen. In both cases, the volume that is extracted must be able to follow the fiber bundles across the volume. High contrast makessegmentation easier, especially when color information is available. In addition, the need forfixation must also be addressed. It has been shown that soft tissue fixation informalin causes shrinkage, altering the structure of the original tissue. Some typical values of contraction for different fixation are: formalin (5% - 10%),alcohol (10%),bouin (<5%).[15]
Imaging methods used inECM visualization and their properties.[15][16]
Transmission Light | Confocal | Multi-Photon Excitation Fluorescence | Second Harmonic Generation | ||
Resolution | 0.25 μm | Axial: 0.25–0.5 μm Lateral: 1 μm | Axial: 0.5 μm Lateral: 1 μm | Axial: 0.5 μm Lateral: 1 μm | Axial: 3–15 μm Lateral: 1–15 μm |
Contrast | Very High | Low | High | High | Moderate |
Penetration | N/A | 10 μm–300 μm | 100–1000 μm | 100–1000 μm | Up to 2–3 mm |
Image stack cost | High | Low | Low | Low | Low |
Fixation | Required | Required | Not required | Not required | Not required |
Embedding | Required | Required | Not required | Not required | Not required |
Staining | Required | Not required | Not required | Not required | Not required |
Cost | Low | Moderate to high | High | High | Moderate |
Soft tissue disorders are medical conditions affecting soft tissue.Soft tissue injuries are some of the most chronically painful and difficult conditions to treat because it is very difficult to see what is going on under the skin with the soft connective tissues, fascia, joints, muscles and tendons.[citation needed]
Musculoskeletal specialists, manual therapists, neuromuscular physiologists and neurologists specialize in treating injuries and ailments in the soft tissue areas of the body. These specialized clinicians often develop innovative ways to manipulate the soft tissue to speed natural healing and relieve the mysterious pain that often accompanies soft tissue injuries. This area of expertise has become known assoft tissue therapy and is rapidly expanding as technology continues to improve the ability of these specialists to identify problem areas.[citation needed]
A promising new method of treating wounds and soft tissue injuries is viaplatelet-derived growth factor.[17]
There is a close overlap between the term "soft tissue disorder" andrheumatism. Sometimes the term "soft tissue rheumatic disorders" is used to describe these conditions.[18]
Soft tissue sarcomas are many types ofcancer that can develop in the soft tissues.
Media related toSoft tissues at Wikimedia Commons