Understanding Mechanical Properties & Behavior of Soft Tissues

 

 

Mechanical Properties of Noncontractile Soft Tissue

The body is made up of noncontractile "soft tissue," which is arranged into different kinds of connective tissue to support the body's structures. Adhesions and contractures can arise from the connective tissue properties of ligaments, tendons, joint capsules, fascia, noncontractile muscle tissue, and skin. Each of these traits may become less extensible and result in less mobility. Understanding how these tissues react to the magnitude and duration of stretch forces is crucial when these tissues limit joint range of motion and necessitate stretching. It's also critical to realize that the only approach to enhance connective tissue's extensibility is to completely redesign its basic structure.

Connection Tissue Composition

Collagen, elastin, and reticulin are the three forms of fiber that make up connective tissue. Nonfibrous ground substance, on the other hand, is made up of glycoproteins and proteoglycans.

Fibers of collagen

Tissue's strength and stiffness are attributed to collagen fibers, which also withstand tensile deformation. Collagen microfibrils are constructed from tropocollagen crystals. Every extra layer of the fibers' composition is put in a structured relationship and dimension. Collagen fibers attach to one another by an unstable hydrogen bonding process that eventually changes to a stable covalent bond as they mature. The tissue's mechanical stability increases with the strength of these bonds. Increased collagen content in tissue results in increased stability.

Fibers made of elastic

Extensibility is provided by elastin fibers. At lower loads, they exhibit a large deal of elongation, and at greater loads, they fail suddenly and without distortion. Greater elastin content results in more flexible tissues.

Fibers of Reticulin

Tissue is given bulk by the fibers of reticulin. ground matter. Ground substance is an organic gel composed of glycoproteins and proteoglycans (PGs) that contains water. In cartilage and intervertebral discs, in particular, the PGs' functions of hydrating the matrix, stabilizing the collagen networks, and fending off compressive stresses are crucial. The kinds of compressive and tensile stresses that the tissue is functionally exposed to determine the kind and quantity of PGs. Glycoproteins establish connections between the major elements of the tissue matrix and between the matrix's adversaries and the cells. The ground substance keeps space between fibers to assist prevent excessive cross-linking between them, transfers nutrients and metabolites within the tissue, and lessens friction between fibers.

Behavior of Noncontractile Tissue in Mechanical Terms

The ratio of PGs to collagen and elastin fibers, as well as the structural orientation of the fibers, govern the mechanical properties of the different noncontractile tissues. Collagen fiber concentrations are higher in tissues that can tolerate high tensile strains than in tissues that can handle higher compressive loads. Variations in the loading environment alter the tissue's composition. The structural component that takes up the majority of the tensile stress is collagen. In the paragraphs that follow, the stress-strain curve is used to explain its mechanical behavior. Collagen fibers extend rapidly in response to an externally imposed load, while wavy fibers align and straighten. Tension rises and the fibers stiffen as loading increases. Fibre strain will steadily rise with further loads until the collagen fiber linkages start to deteriorate. The fibers themselves will eventually break when a significant amount of connections are broken. Less than 10% of a fiber's length increase causes collagen to fail under tensile loading, but elastin can extend by 150% without failing. But five times as strong as elastin is collagen.

Collagen Fiber Orientation in Tissues

The normal tensile stress pattern operating on a given tissue is reflected in the orientation and alignment of the collagen fibers inside that tissue. Collagen fibers in tendons are parallel and have the highest tensile load resistance. To transfer muscle forces to the bone, their fiber alignment is in series with muscle fibers. Because of their erratic orientation, collagen fibers in skin are unable to withstand greater tensions. Collagen fiber orientation varies in ligaments, joint capsules, and fascia to enable them to withstand multidirectional stresses. Collagen fibers in ligaments that withstand significant joint loads are oriented more parallelly and have a greater cross-sectional area.

Understanding Connective Tissue's Mechanical Behavior: The Stress-Strain Curve

The stress-strain curve is used to describe the behavior of connective tissue under stress caused by an externally applied load and shows the mechanical strength of structures. A structure will elongate when a tensile load is applied; the stress-strain curve shows the stiffness, strength, and amount of energy the material can hold before the structure fails.

Force (or load) per unit area is known as stress. The internal resistance to an externally applied load is known as mechanical stress.

The amount of deformation or lengthening that happens to a structure when an external load, like a stretch force, is applied is known as strain.

Stress Types

When a load is applied, a structure can experience one of three types of stress:

• Tension the tissue to withstand a force applied in a way that will cause it to lengthen. Tension stress is the outcome of a stretching force.
• Compression resistance in response to a force applied in a tissue-approximating manner. Compression strains are created when weight presses against a joint.
• Shear resistance is the result of two or more forces acting against one another.

Stress and Strain Curve Regions

The Toe Area

Collagen fibers initially straighten and align when loaded because they are wavy and rest within a three-dimensional matrix. This reaction happens with very little force exerted, and it causes slight increases in tissue tension. This behavior is shown by the toe region of the stress-strain curve, which is the range in which the majority of functional activity typically occurs.

linear Phase/Elastic Range.

Stress and strain are directly correlated in the area next to the toes, meaning that a change in stress causes a corresponding change in strain. The slope of this area, known as the linear region, is determined by how a particular tissue responds to loading. A tissue, like bone, that experiences a sharp increase in strain in response to loading would have a steeper slope and be more rigid than a tissue that has a more gradual rise in strain. Collagen fibers are aligned in the direction of applied force when tissue tension is applied beyond the toe region with respect to stretching. Water may be displaced from the ground substance as a result of some microfailure between the collagen bonds occurring as strain increases. When the load is released, the tissue recovers to its initial size and shape if the strain does not increase beyond the linear zone or is not maintained.

Elastic Boundary

The elastic limit is the boundary of the linear region and the point at which tissue can no longer revert to its initial dimensions.

Plastic Assortment

When strain exceeds the elastic limit, tissue distortion becomes irreversible. Tissue that is strained into the plastic range, which stretches from the elastic limit to the point of rupture, will permanently distort once the external load is removed. The successive failure (micro failure) of the bonds separating collagen fibrils and, finally, of the collagen fibers itself causes plastic deformation. Collagen's crystalline structure causes individual strands to rupture as opposed to stretch, which could lead to an increase in length.

Supreme Power

The tissue's ultimate strength is the utmost strain it can withstand. After reaching this threshold, additional strain increases do not cause increasing stress because of tissue macro failure. When the necking region is reached, the tissue begins to significantly deteriorate and fails quickly. In experiments, single collagen fibers may endure strains of up to 7% to 8% before failing, but whole ligaments can withstand forces of up to 40% before collapsing.

Not Succeeding

When the tissue breaks and loses its structural integrity, failure occurs. Structural stiffness. The stiffness of the tort is represented by the modulus of elasticity, also known as Young's modulus, which is the slope of the linear section of the curve (elastic range). Greater stiffness in tissues results in a steeper elastic area slope and less elastic deformation as stress increases. Due to a higher degree of attachment between collagen fibers and their surrounding matrix, contractures and scar tissue are more rigid. Under comparable external strains, tissues with less stiffness will elongate more than those with more stiffness.

Effects of Time and Rate on Tissue: Deformation

Because connective tissue is viscoelastic, how long and how quickly an external force is given to it will determine how the tissue responds.

Rate sensitivity

The stress-strain curve's slope will be greater when a load is applied quickly to a viscoelastic tissue than when it is applied slowly. In other words, when the load is applied quickly, the tissue gets stiffer. By limiting failure potential and preventing deformation below the plastic range, this heightened stress response safeguards the tissue. Because it enables the body to withstand heavy loads applied over brief periods of time a combination frequently encountered during high-velocity events or activities—viscoelasticity is an incredibly beneficial tissue feature. This rate-dependent reaction can be reduced during a stretch by applying a tissue stress gradually.

Creep

Creep is the term used to describe the slow elongation that occurs during a maintained strain in viscoelastic tissue under gradually increasing external load. This gradual adaptation of a viscoelastic tissue to prolonged loading is a time-dependent characteristic. Both the force and the velocity of application determine how much tissue deformation occurs. Long-term application of low-magnitude loads that approach the clastic range causes connective tissue deformation, allowing for the gradual remodeling of collagen fiber connections and the redistribution of water to neighboring tissues. Increasing the temperature of the tissue encourages creep and extensibility. While full recovery from creep could take some time, it won't happen as quickly as recovery from a short-term load. Stretches with a long duration used for chronic contractures benefit from this tissue characteristic.

Relaxation After Stress

A viscoelastic tissue experiences a progressive decrease in the force needed to sustain the degree of deformation when a sub-failure load is applied and maintained constant. The redistribution of water content and the connective tissue's viscoelastic properties both contribute to this response. The fundamental idea behind protracted stretching methods, which involve holding the stretched position for several hours or days, is stress-relaxation. The degree of deformation and the duration of the deformation determine whether the length changes permanently or recovers.

Exhaustion of the Connective Tissue and Cyclic stress

When tissue is repeatedly loaded in a brief amount of time, heat is produced more quickly and failure may occur at strain values lower than those required for a single load. In a similar vein, fewer cycles are required to attain failure the higher these recurring loads. We call this syndrome connective tissue fatigue. Stress fractures and overuse syndromes are two instances of cyclic loading-induced connective tissue fatigue. These scenarios show instances where the number of repetitions and the load intensity above the endurance threshold, below which an apparently endless series of modest loading cycles won't result in failure. Research on animals demonstrates that cyclic loading, as opposed to static loading, will cause a reduction in ligament stiffness, which is a sign of tissue injury sooner.