Unit 10
resilience
(w-aw)/ w • The greater the loss of work the less the resilience, while the less the loss of work the greater the resilience. • R = 1 is a perfectly resilient material
fragility
-Opposite of toughness -Material absorbs little energy before it breaks
Tendons/ligaments response to rate of stress Rupture
all collagen fibers are broken
type 2 collagen found in
in hyaline and elastic cartilage and functions to resist pressure.
Major GAGs dermatan sulfate is found in
tendon, ligament, fibrocartilage, nerve, arteries and the dermis
type 4 collagen found in
the basement membrane of epithelium and functions to support this tissue and as a filter.
Elastic connective tissue
• Fibers: an abundance of elastic fibers interwoven among collagen fibers • Cells: fibroblasts • Locations: ligamentum flavum, ligamentum nuchae, wall of large (elastic) arteries, vocal ligaments of larynx. • Function: dampen high pressure in arteries; return structures to resting position
Dense regular connective tissue
• Fibers: many densely packed collagen bundles arranged in parallel rows running in the same direction • Cells: few fibroblasts • Locations: tendons, ligaments, fascia, aponeuroses • Function: transmit unidirectional tensile forces, stabilize joints
fibroblasts
• Produce collagen and elastic fibers • Produce glycoaminoglycans which form proteoglycans
hypermobility
- defined as the laxity of ligaments, either locally or systemically, produces an increase in joint mobility -at a joint creates marked joint instability, resulting in sublaxation in which a bone is partially displaced from its normal joint position. - or a dislocation: bone is completely displaced from its normal joint position. -Hypermobility also exposes the opposing joint surfaces to abnormal forces, which produce abnormal wear and degeneration on the articular surfaces, causing pain and inflammation not only of the joint but also of the tendons, ligaments and muscles associated with movement at that joint.
elastic fibers
-Variable quantities in CT contain a specific protein called ELASTIN -abundant in some ligaments, large arteries, the trachea and the dermis of the skin. - These are less stiff than collagen fibers Easily elongate to 1.6 - 2.0 times its length when tension is applied
ligament and tendon replacements
-may be several weeks before the replacement can safely be loaded in tension, because the replacement degenerates initially. New collagen is produced then tension becomes important to align collagen fibers, and strengthen and heal the replacement, just as above.
1. What is the difference between the elastic and plastic regions of a tissue stress vs. strain curve?
An elastic tissue or a tissue in the elastic region of a stress vs. strain curve will return to its original starting position of zero strain (no deformation) when the stress is removed. If a tissue is strained into its plastic region then it is permanently deformed and it will not return to its original starting position when the stress is removed.
4. How does the tendon/ligament load vs. joint displacement (stress vs. strain) curve relate to tendon/ligament injury?
As the tensile load on tendons/ligaments increases during activities, microfailures occur in the tendon as some of the collagen fibers break. This results in pain and inflammation but no instability. When additional numbers of collagen fibers break, tendons/ligaments cross the yield point and are permanently deformed. In this case, there is pain and inflammation as well as instability. When all the collagen fibers break, there is complete failure. There may be no pain with complete failures but there is instability.
1. What are the functional benefits of Loose CT and Dense Irregular CT?
Because loose CT contains few collagen fiber and is composed mainly of cells and the ground substances, it's weak but it permits structures surrounded by loose CT to move and change shape. The loose CT surrounding neurovascular bundles allow these bundles to move during limb & trunk movements w/out imposing large compressive & tensile stresses and strains on these structures that could damage them. The loose CT also allows vascular channels to dilate & constrict. Loose CT between muscles allows muscles to contract individually even though these muscles lie adjacent to each other. The large #'s of cells in loose CT that are involved in the immune & inflammatory responses also contribute to localized infection control & localized repair of injured tissue. Dense irregular CT is composed mainly of strong Type I collagen w/ few cells & little ground substance. As collagen resists tension & is arranged to resist tension in several directions in dense irregular CT, this tissue is located where multidirectional tensile stresses & strain are applied. In the dermis of the skin, it limits tension that occurs when the skin is moved up/down, forward/back, & from side to side. In a joint capsule, different parts of the capsule are tensed w/ different movements & thus the collagen fibers need to be arranged in multiple directions to resist tension associated w/ movement in different directions.
Tissue Changes with Age
Below is a graph showing the general changes in the mechanical properties of bone, tendons, skeletal muscle and cartilage with age. For each of these tissues, a more detailed description of the changes in stress/strain properties with age will be described.
4. What are the three types of joint lubrication and how do they differ?
Boundary lubrication occurs with extreme loading and when the normal lubricating method for that joint fails. It is the extrusion of lubricant from the surface layer of the articular cartilage. Hydrodynamic and squeeze film lubrication involves the distribution of synovial fluid which lies with in the joint cavity and both are normal mechanisms for joint lubrication. The difference between the two is determined by the shapes of the articulating surfaces. If the surfaces are not in parallel to each other than the hydrodynamic mechanism of lubrication is used, but if the surfaces are in parallel then the squeeze film mechanism is used.
Major GAGs hyaluronic acid is found in
CT proper, cartilage and synovial fluid;
3. What is the difference between creep and load (stress) relaxation and how are these properties similar?
Creep is a change in articular cartilage thickness that occurs in time with a constant load. After the cartilage is compressed and the water and metabolic waste products are exuded and the load on the cartilage continues at a constant level, the collagen and proteoglycan reorganize and the result in a further decrease in articular cartilage thickness or creep. Stress relaxation occurs when the thickness of the cartilage after compression remains constant. With stress (load) relaxation, the distribution of the forces from the surface to the deep zone of the cartilage is even prior to loading. When the cartilage is loaded and its thickness decreases because of the loss of fluid, the surfaces stresses increase and are larger superficially than deep. After the fluid loss, the thickness remains constant but the stresses are still larger in the top part of the cartilage than the bottom part. With time and the redistribution of proteoglycans and collagen, the stresses from top to bottom change so that the stresses at the top (surface) and the stresses at the bottom are equivalent. This process reduces the surface stresses and thus these surface stresses are relaxed. With both creep and stress relaxation, there is a redistribution of collagen and proteoglycans to obtain the effects of creep and stress relaxation.
tendon/ ligament repair
Days 2-4: cellular stage • Clot forms • Infiltration of macrophages and fibroblasts • Weak and unstable type III collagen produced • Connection is basically cellular and very fragile • Stretching tears connection Days 5-21: fibroplasia • Still very cellular • Increase in collagen production • High collagen synthesis and degradation • Collagen remodeling • Good time to increase ROM, joint function • Tension helps: increases strength of connection DAY 2 - 21: • Collagen increases in amount and the strength of the scar increases DAY 21 - 60: • Collagen amount levels off and is fairly constant but strength of the scar increases. Increase in strength is due to fiber alignment, large bundles of collagen forming and increase in cross link formation
5. How does tension affect wound healing and when is the most effective time to obtain the best response and why?
In the repaid of tendons/ligaments, tension activates the production of collagen, the production of large collagen bundles, controls the direction of fiber alignment, increases scar strength, & increase the rate & completeness of healing. The most effective time to apply tension is during the fibroplasia stage and maybe early consolidation stage when there is active collagen synthesis and remodeling. It is during this time that tension will align the fibers to resist the appropriate stresses, strengthen the collagen, and increase the rate and completeness of healing.
2. What are the functions of proteoglycans in articular cartilage?
Proteogylcans resist compression forces applied to the cartilage and because they are hydrophilic, proteoglycans are important in maintaining the high water content of the cartilage and in its nutrition. As articular cartilage is avascular, it is the loss of water during compression that removes the waste products of chondrocyte metabolism and the hydrophilic property of the proteoglygans that returns water and nutrients to the cartilage.
3. What is the difference between resilience and toughness?
Resilience is a factor of the amount of mechanical work lost during deformation while toughness is the amount of resistance a material absorbs before it mechanically fails. A highly resilient material will lose only a little mechanical work when it deforms. It will return to its original shape or position when the stress is removes. Articular cartilage is a highly resilient material. On the other hand, a poor resilient material will readily deform permanently when it stressed. Styrofoam is a poor resilient material. A resilient material can also be tough but so can a poor resilient material because toughness is a measure of a tissue ability to absorb the energy of mechanical work before breaking. Toughness is not a measure of mechanical work lost because once the material break, all mechanical work is lost.
7. How are tendons/ligaments strengthened?
Tendons/ligaments are strengthened by adding collagen, forming thick collagen bundles, and increasing cross-links.
1. How are tension and compression resisted in articular cartilage?
Tension is resisted by the type II collagen fibers in the cartilage and compression by proteoglycans. Because the glycoaminoglycans forming a proteoglycan monomer give the monomer a negative charge, the compaction of the monomers during compression results in the negatively charges monomer repelling each other. This repelling force stiffens that ground substance and resists compression of the cartilage.
3. How do tendons/ligaments respond to tension?
Tension on tendons/ligaments stimulates the production of collagen fibers and the formation of cross-links, strengthening the structure. Tension also aids in the repair of tendons/ligaments by accelerating healing, increasing the strength of the scar and orienting the direction of collagen with the lines of stress.
8. What are the affect of aging on the mechanical properties of tendons/ligaments?
The tensile strength and elongation of tendons/ligaments change little (5%) until after age 70 when there is a marked decr (20%) in tensile strength but less of a decr (10%) in elongation.
reticular fibers ( type 3 collagen )
Thin and delicate, and form lace-like networks of fibers around smooth muscle cells, the sarcolemma of striated muscle, and the endoneurium of peripheral nerves
4. What is the difference between toughness and brittleness?
Toughness is the amount of mechanical work a material can absorb before breaking where as brittleness is a factor of the amount of strain that occurs before braking. A brittle material does not deform very much before it breaks. Some brittle material materials are fragile and weak, such as chalk. However, some are strong and tough, such as a drill bit.
6. How does the repair process for a tendon/ligament wound differ from that for a ligament replacement?
When a ligament is replaced, it initially degenerates & weakens significantly so that tension is not advised for may be several weeks. As new collagen is produced so that tension will not damage the replacement, then & only then would tension be applied as the replacement is now in the fibroplasia stage of healing. Tension at this stage would have the same benefits as a tendon/ligament wound.
2. How do tendons/ligaments respond to slow and fast rates of tension and what are the functional implications of these responds?
With a slow rate of tension, low stress produces a large degree of elongation. W/ a fast rate of tension, the tendon stiffens rapidly resulting in high stress and much less elongation than when a slow rate is applied. For stretching tendons/ligaments, low stress applied slowly would be more beneficial for producing elongation than a fast rate of stretch. However, if a rapid movement is needed, the muscle must contract rapidly & this rapid muscle contraction would produce a fast rate of tension on the tendon. If the tendon does not stiffen rapidly w/ the fast application of tension but elongates, the resulting movement would be slowed rather than the rapid movement needed for desired function.
5. What are affects of aging on articular cartilage and fibrocartilage?
With articular cartilage, tensile and compression properties begin to show a noticeable change at about age 40 but decline even further after age 50. The decline is less in compression properties than in tensile properties. With fibrocartilage vertebral discs, tensile and torsional properties decrease slightly by age 40. After age 40, both of these properties continue to decrease with tensile properties decreasing slightly more (20-25%) than torsional properties (15-20%).
Stiffness of tendons/ligaments in tension changes with age.
Young collagen will elongate more w/ less force because it is less stiff in tension than mature collagen. As collagen matures, strength and stiffness incr because of incr collagen production and cross-link formation. • W/ advancing age the tensile strength and elasticity of ligaments and tendons decrease o the amount of collagen decr o the # of large bundles of collagen decr o elastic fibers are damaged • <70 yr: dec in tensile strength is slight o < 50 yr : decr of about 5% o 50-70 yr : dec remains slight at about 5% extra o >70 yr: decrease of about 20% • Elongation of tendons/ligaments to tension changes by only about 5% up to age 70 and then about another 10% thereafter • These changes appear to be greater for ligaments than for tendons with aging
2. What is Young's modulus and how does it relate to tissue stiffness?
Young's modulus is the ration of the amount of stress applied divided by the amount of strain (deformation). A material with a high Young's modulus means that for a lot of stress there is little strain and that the material is stiff. A material with a low Young's modulus means that the material shows a lot of strain with little stress. This material has low stiffness and is thereby more elastic.
Major GAGs chondroitin - 6 - sulfate and chondroitin - 4 - sulfate
are found in hyaline and elastic cartilage, bone, large BV, and the nucleus pulposus
Major GAGs heparin sulfate is found in
basal lamina, aorta, lung, liver, smooth muscle and endoneurium.
Tendons/ligaments response to rate of stress Toe Region
collagen fibers are on slack so that small stress produce a lot of deformation as the fibers tighten
Tendons/ligaments response to rate of stress Linear or Elastic Region
collagen fibers are tightening and as they tighten greater stress is need to produce deformation/strain
Tendons/ligaments response to rate of stress Major failure
most of the collagen fibers are broken and the material is weak and permanently deformed. Maintained or decrease stress continue to produce a proportionately large deformation.
Tendons/ligaments response to rate of stress Progressive failure
some of the collagen fibers break and the material is damaged and permanently deformed. A small increase in stress results in a proportionately large deformation.
Young's modulus of elasticity
stress/ strain -High Young's modulus = high stiffness -Low Young's modulus = low stiffness
type 3 collagen found in
the CT of organs, such as the liver, spleen, lungs, and intestines, and in blood vessels, nerves and muscles. It functions as a structural support system in those structures. Also important in wound closure.
type 5 collagen found in
the basal lamina of smooth and skeletal muscle cells and Schwann and glia cells. It functions as a support system in these structures
Major GAGs keratan sulfate is found in
the cornea, cartilage, the nucleus pulposus and annulus fibrous
type 1 collagen found in
the dermis of skin, bone, tendon, ligament, fibrocartilage, and fascia. It forms 90 % of the collagen in the body and functions to resist tension and stretching.
Ductility
• Ability of a material to deform progressively in tension without breaking • Old bone is more brittle than young bone • Young bone is more ductile than old bone because young bone deforms farther when tension stress is applied than old bone.
Other Tendon/Ligament Response to Stress
• As the magnitude of tensile stress on a tendon or ligament • Increases with time, a tendon or ligament may respond by adding collagen and increasing the number of cross-links, which increases its strength. • Tension on tendons and ligaments at sub-failure levels stimulates fibroblasts to produce new collagen fibers. • In surgically repaired tendons and ligaments, tension accelerates healing, increases the strength of the connective scar at the repair site, and directs the orientation of the collagen fiber along the direction of stress (Cummings and Tillman, 1992). • The addition of new collagen fibers results in an increase in the thickness and stiffness of that structure (Nordinand Frankel, 1989; Noyes, 1977; Woo et al., 1988). • The body strengthens tendons and ligaments to match the demands of increased muscle force and increased joint traction that occur with resistive exercise, increased work loads, and with growth.
Loose (areolar) connective tissue
• Fibers: few loosely arranged collagen fibers, few reticular and elastic fibers • Cells: fibroblasts, myofibroblasts, macrophages, plasma cells, mast cells, eosinophils,basophils, lymphocytes, fat cells • Locations: superficial fascia, epimysium, epineurium • Papillary layer of the dermis, tunica adventia around B.V • Function: permits movement of neurovascular bundles during limb and trunk movements; permits adjacent muscles to contract individually; allows B.V. to enlarge
Dense irregular connective tissue
• Fibers: many densely packed collagen bundles arranged in many different directions, some elastic fibers • Cells: few fibroblasts, macrophages • Locations: dermis of the skin, periosteum, perichondrium, joint capsules, capsules around organs • Function: resist multidirectional tensile forces and shear force; stabilizes joints; protection
collagen producing cell
• Fibroblasts in connective tissues proper • Chondroblasts and chondrocytes in cartilage • Osteoblasts in bone • Skeletal muscle cells in skeletal muscle • Smooth muscle cells in blood vessels and some organs.
myofibroblasts
• Have properties of fibroblast and smooth muscle • Produce collagen but contain myofilaments • Abundant at sites of inflammation • Involved in wound closing
Immobilization
• Loss of tissue stiffness and strength • 7 weeks of immobilization does not tend to affect the ligaments or capsule of a joint if there is no inflammation • If joint inflammation is present, then adhesions and joint involvement can occur within 4 weeks of immobilization • Important to prevent and quickly eliminate joint inflammation when a joint is immobilized
Brittleness
• Materials that strain (deform) very little before failure • Not necessarily related to strength • Both are brittle but #1 is very strong (drill bit) • Both materials #1 and #2 are brittle because they deform very little when stressed, but #1 is very strong (drill bit) compared to #2.
Tendon/Ligament Injury
• Not all collagen and fibers fail at the same time when high levels of tension are applied to tendons and ligaments • Failure of only some of the collagen fibers is referred to as microfailure, which result in some pain and a slight weakening in the tendon/ligament but no joint instability (area 1) • When enough collagen is damaged and tendon passes its yield point and there is permanent deformation then there is noticeable inflammation and joint instability (area 2) • When all the fibers fail, then the tendon/ligament ruptures (area 3) • Collagen damage to a tendon is a STRAIN, while damage to a ligament is a sprain.
damping
• Opposite of resilience • Styrofoam is an excellent damping material D = 1 - R
collagen fibers
• Predominant type of fiber found in CT • Formed from tropocollagen molecules w/ each consisting of 3 polypeptide chains called alpha units that are wound about each other to form a triple helix • Tropocollagen molecules are packed end to end and stacked side to side to form a collagen fibril • Molecules are bound together by intra- and inter-chain bonds or cross-links between lysine and hydroxlysine • Cross-links also bind fibrils together • Cross-links contribute to the high tensile strength, stability and stiffness of collagen so that collagen fibers with many cross-links are stiffer than collagen with few cross-links
Toughness
• Resistance to mechanical failure • Amount of energy a material will absorb before it breaks • Toughness is not necessarily equal to strength • Strength is the magnitude of the force needed to break a material
collagen
• Show very little elongation (< 10 %) in tension, but bend easily in compression • Show a greater resistance to shear stress than elastic fibers • Structures such as tendons that transmit muscular forces and ligaments that provide joint stability are composed mainly of collagen fibers, making them strong and stiff in tension.
creep and load relaxation
• Tendon and ligaments are viscoelastic materials. Such materials show the phenomena for creep and load relaxation • Creep: increase in deformation that occurs over time when the load is constant • Load relaxation: a decrease in force over time when the magnitude of the deformation is held constant • These phenomena are the result of a reorganization of the collagen and proteoglycans in the material
elastic material
• This is a stress/strain curve for an elastic material. • As stress/force is applied, there is deformation. When the stress/force is stopped at the top of the curve, the material goes right back to the beginning points or shape or size. Now there is work lost during this process because the work required to deform the material is not totally recaptured once the forces are removed. Although there is work lost as reflected by the area between the curves, this material is still considered an elastic material because it returns to its starting point.
ground substance
• Water, glycoaminoglycans (GAGs) and proteoglycans • GAGs are polymers of disaccharide units
As the rate of applied tension on a tendon or ligament increases
• a tendon or ligament shows an increase in stiffness and a decrease in the amount of elongation. •This property suggests that elongation of ligaments and tendons is best obtained by applying low force for a long period of time
GAGs
• attach at one end to a protein core and radiate outwardly from this Core to form proteoglycan monomer • Proteoglycan monomers + hyaluronic acid = proteoglycan aggregate • GAGs produce a (-) charge over the periphery of the proteoglycans monomers which repels adjacent (-) charged proteoglycan monomers as they approach each other resulting in increased tissue stiffness • GAGs of proteoglycans are also hydrophilic so that they attract and hold water which is important for cartilage nutrition and in producing the compressive stiffness of cartilage
Stress and Strain Relationships Load - Deformation Curve
• stress/force vs strain/deformation = stress/strain curve • The toe region: the material is slack so that a small amount of stress produces proportionally more strain. • The elastic or linear region: material is tightening so that the amount of stress is proportional to the amount of strain. Release of stress would result in the material returning to its original point of not being deformed. • In the plastic region: material is damaged and the degree of damage increases as the stress increases. This results in the amount of strain/deformation increasing more relative to the increase in stress. At the necking point of the plastic region, the degree of material damage is so extensive that the amount of deformation remains large with a decrease in stress. The material is permanently deformed and as will not return to its original position when the stress is removed. • Failure is the point where the material completely breaks or ruptures.