General properties of connective tissues.pptx

general properties of connective tissue

1. General properties of connective tissues.

2. Connective tissues change their structure in response to applied forces; that is, they adapt. This adaptive behavior illustrates the dynamic nature of connective tissue and the strong relationships among structure, composition, and function. The remarkable ability of connective tissues to respond to load alterations is often referred to as the SAID principle (specific adaptation to imposed demand).

3. Mechanical Behavior Load, Force, and Elongation: Load refers to a force or forces applied to a structure. The magnitude, direction, and rate of force application, as well as the size and composition of the tissue, will all affect the tissue’s response to load. When a force acts on an object, it produces a deformation. A tensile load produces elongation ; a compressive force produces compression .

4. The load-deformation curve is the result of plotting the applied load (force) against the deformation, providing information about the strength properties of a particular material or structure. The load-deformation curve shows the elasticity, plasticity, ultimate strength, and stiffness of the material, as well as the amount of energy that the material can absorb before it fails.

5. The portion of the curve between point A and point B is the elastic region. If the response to loading is confined to the elastic region, the deformation of the material will not be permanent; the structure will return to its original dimensions immediately after the load is removed After point B, the yield point at the end of the elastic region, the material will no longer immediately return to its original state when the load is removed, although it may recover in time.

6. The portion of the curve between B and C is the plastic region. Although the structure will appear to be intact, after the load is removed the material will not recover its original length—the deformation is permanent If loading continues through the plastic region, the material will continue to deform until it reaches the ultimate failure point, C. The load being applied when this point is reached is the failure load.

7. Force values on the load-deformation curve depend on both the size of the structure and its composition. A structure with a greater cross-sectional area can withstand more force with less deformation than a structure of the same original length with less cross-sectional area. A longer structure deforms more when a force is applied than does a shorter structure of similar cross-section

8. Stress and Strain When loads (forces) are applied to a structure or material, forces within the material are produced to oppose the applied forces. These forces within the material depend on the composition of the material. When the applied force is tensile, we calculate the stress on the tissue. Stress, the force per cross-sectional unit of material, can be expressed mathematically with the following formula, where S = stress, F = applied force, and A = area: S = F/A

9. The percentage change in the length or cross-section of a structure or material is called strain. Strain = (L2 – L1) ÷ L1 The types of stress and strain that develop in human tissues depend on the material, the type of load applied, the point at which the load is applied, the direction and magnitude of the load, and the rate and duration of loading.

10. If two applied forces act along the same line but in opposite directions, they create a distractive or tensile load and cause tensile stress and tensile strain in the structure or material

11. If two applied forces act in a line toward each other, they constitute compressive loading and compressive stress and, as a result, compressive strain will develop in the structure.

12. If two applied forces are parallel and are applied in opposite directions but are not in-line with one another, they constitute shear loading Because stress and strain are independent of size of the material, the stress-strain curve is said to reflect the material properties of the tissue. With size accounted for, only changes in the material constituting the tissue will alter the stress-strain curve.

13. The stress-strain curve can be used to compare the strength properties of one material with that of another material or to compare the same tissue under different conditions (e.g., ligaments before and after immobilization). The stress-strain curve contains the same defining points (A, B, and C) as the load deformation curve, but the shape of the curve and the amount of stress and strain will vary with the composition of the material The curve will be flatter in more elastic materials and steeper in stiffer materials.

14. Young’s Modulus modulus of elasticity, of a material under compressive or tensile loading is represented by the slope of the linear portion of the curve between point A and point B The modulus of elasticity is a measure of the material’s stiffness (its resistance to external loads). A value for stiffness can be found by dividing the change in (Δ) stress by the change in (Δ) strain for any two consecutive sets of points in the elastic range of the curve.

15. The inverse of stiffness is compliance. If the slope of the curve is steep and the modulus of elasticity is high, the material exhibits high stiffness and low compliance. If the slope of the curve is gradual and the modulus of elasticity is low, the material exhibits low stiffness and a high compliance

16. Load Deformation and Stress-Strain Curves Each material has its own unique stress-strain curve. The first region of the curve (0 to A) is called the toe region. Very little force is required to deform the tissue In this region, a minimal amount of force produces a relatively large amount of deformation (elongation); stress is low, and the strain is typically in the 1% to 2% range.

17. The second portion of the curve A to B is the elastic region, in which elongation (strain) has a linear relationship with stress. Each additional unit of applied force creates an equal stress and strain in the tissue. In this region of the curve, collagen fibrils are being stretched and are resisting the applied force. When the load is removed, the ligament or tendon will return to its prestressed dimensions, although this return will take some time. This level of loading includes the stresses and strains that occur with normal activities and typically extends to about 4% strain

18. In the third region (B to C, the plastic region), the failure of collagen fibers ( microfailure ) begins, and the ligament or tendon is no longer capable of returning to its original length after the force is removed. Clinical examples include grade I and II ligament sprains and tendon strains. Recovery after this level of loading requires considerable time because it involves aspects of healing such as synthesis of new tissue and cross-linking of collagen molecules.

19. If force continues to be applied beyond the plastic region, the remaining collagen fibrils experience increased stress and rapidly rupture sequentially, creating overt failure ( macrofailure ) of the tissue. In the case of a ligament or tendon, if the failure occurs in the middle of the structure through a disruption of the connective tissue fibers , it is called a rupture. If the failure occurs at the bony attachment of the ligament or tendon, it is called an avulsion. When failure occurs within bony tissue, it is called a fracture

20. Each type of connective tissue is able to withstand a different percentage of strain before failure. In general, ligaments and tendons are able to deform more than cartilage, and cartilage is able to deform more than bone. However, the total deformation also depends on the size (length, width, or depth) of the structure.

21. Viscoelasticity All connective tissues are viscoelastic materials: they combine the properties of elasticity and viscosity, making their behavior time-, rate-, and history-dependent. Elasticity refers to the material’s ability to return to its original length or shape after the removal of a deforming load

22. Viscosity refers to a material’s resistance to flow. It is a fluid property, and depends on the PG and water composition of the tissue. A tissue with high viscosity will exhibit high resistance to deformation, whereas a less viscous fluid will deform more readily When forces are applied to viscous materials, the tissues exhibit time-dependent and rate-dependent properties. Viscosity diminishes as temperature rises or loads are slowly applied and increases as pressure increases or loads are rapidly applied.

23. Time-Dependent and Rate-Dependent Properties Viscoelastic materials are capable of undergoing deformation under either tensile or compressive forces and returning to their original state after removal of the force. However, their viscous qualities make the deformation and return time-dependent. A viscoelastic material possesses characteristics of creep, stress-relaxation, strain-rate sensitivity, and hysteresis.

24. Creep If a force is applied to a tissue and maintained at the same level while the deformation produced by this force is measured, the deformation will gradually increase. Force remains constant while length changes. For example, if you hang a weight on the end of an elastic band, you will get an immediate elastic deformation. However, it will also gradually elongate further over time.

25. Connective tissues also will gradually elongate (creep) after an initial elastic response to a constant tensile load and then gradually return to their original length (recovery) after the load is removed. In a clinical setting, this might apply to stretching shortened tissue: The clinician applies a constant force and the tissue gradually elongates. For cartilage and bone, compressive loading is used to test creep, and so the depth of indentation represents creep and recovery

26. Stress-Relaxation If a tissue is stretched to a fixed length while the force required to maintain this length is measured, the force needed will decrease over time. Length remains constant while force decreases. In a clinical setting, a therapist may perceive this as a reduced resistance to stretch (less force is required to maintain tissue length).

27. Hysteresis When the force and length of the tissues are measured as force is applied (loaded) and removed (unloaded), the resulting load-deformation curves do not follow the same path. Not all of the energy gained as a result of the lengthening work is recovered during the exchange from energy to shortening work. Some energy is lost, usually as heat

28. Strain-Rate Sensitivity Most tissues behave differently if loaded rapidly or slowly. When a load is applied rapidly, the tissue is stiffer, and a larger peak force can be applied to the tissue than if the load was applied slowly. The subsequent stressrelaxation also will be larger than if the load was applied slowly. Creep will take longer to occur under conditions of rapid loading