Lecture 3.docx

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Exercise Science and Sport Studies
Sara Campbell

LECTURE 3: BIOMECHANICS OF ARTICULAR CARTILAGE I. biomechanics of articular cartilage ■ three joints in the body *exam 1. fibrous- fibrous joints lack a synovial cavity, have the articulating bones held together by fibrous CT, and permit little or no movement, ex cranial sutures 2. cartilaginous joints- lacks a synovial cavity, has the articulating bones connected by either fibrocartilage or hyaline cartilage, allows little or no movement, ex intervertebral discs or the pubic symphysis 3. synovial or diarthrodial- synovial joints have a synovial (joint) cavity between the articulating bone and are freely movable (diarthrotic) ex shoulder and knee ■ craniosacral therapy- alternative form of medicine used to normalize the craniosacral rhythm; craniosacral rhythm is 6-8 rhythms/ min during which the head elongates and rounds out ■ intervertebral discs ○ annulus fibrosis- fibrous material ○ nucleus pulposus- nuclear material ○ herniated disc- nuclear material stretches and breaks through the annular fibrosis ■ pubic symphysis- not a lot of movement occurs here, of there is movement it can create a problem at the SI joint, in PT can use the adductor muscles or pull the leg to slide it back into place II. structure of synovial joints A. articular cartilage ○ the articular cartilage covers the bones at synovial joints ○ it reduces friction at the joint, allows for movement, and helps absorb shock by distributing loads over a larger area ○ it is absent of blood vessels, lymph vessels, and nerves (therefore does not heal well- synovial fluid can help when injured) ○ it is the least dense of any tissue in the body B. articular capsule ○ the articular capsule surrounds the diarthrosis, encloses the synovial cavity, and unites the articulating bones ○ composed of two layers: a) the outer fibrous capsule- which may contain ligaments (helps support joint) b) the inner synovial membrane- which secretes a lubricating and joint-nourishing synovial fluid (important for protection) II. composition of articular cartilage ■ chondrocytes ○ superficial tangential zone (STZ)- chondrocytes are oblong with their long axis parallel to the articular surface ○ middle zone- chondrocytes are round and randomly distributed ○ deep zone- chondrocytes are arranged in a columnar fashion and perpendicular to the bone ■ collagen ○ the collagen in hyaline cartilage is layered into three zones rooting into the underlying bone ○ this layered system allows the cartilage to distribute the load within the joint more evenly ➢ if not layered could expose the periosteum and cause pain ○ type II collagen in cartilage is different from type I in bone, tendons, and ligaments ○ type II has thinner fibrils than type I permitting maximum dispersion of collagen throughout the cartilage tissue ○ most important property of collagen is its tensile strength but it cannot resist compressive loads ■ proteoglycans ○ these molecules provide structural stability and rigidity to the extracellular matrix of the cartilage ■ water ○ the most abundant component of articular cartilage ○ most concentrated at the joint level and less concentrated toward the deeper networks ○ most of the water occupies the interfibrillar space of the ECM and is free to move when a load or pressure gradient is applied ○ 70% of water is displaced upon a compression load (problem in wb areas) II. biomechanical behavior of articular cartilage ■ it is a biphasic material ○ interstitial fluid phase ○ porous permeable solid phase ■ viscoelastic material III. nature of articular cartilage viscoelasticity ■ two responses of articular cartilage: creep and stress relaxation ■ creep occurs when a viscoelastic solid is subjected to a constant load, this load responds with a rapid initial deformation followed by a slow progressively increasing deformation until an equilibrium state is reached ■ stress relaxation occurs when a viscoelastic solid is subjected to a constant deformation, this solid responds with a high initial stress and then a slow progressive decreasing stress to maintain the deformation ■ *this is important because it allows for proper fluid pressurization to occur for compression of a joint ■ figure 3.9- a load is applied, exudate comes out (water, fluid properties), redistributes and equilibrium is reached and no more exudate is released IV. behavior of articular cartilage under uniaxial tension (change in vol) ■ the outer layer of the articular cartilage is filed with collagen providing the joint with a string wear and tear resistance shell ■ articular cartilage stiffens with increased strain (reduces wear and tear) ■ figure 3.12- stress strain curve for articular cartilage ○ shows collagen fibrils at various stages of loading ○ in the toe region collagen fibril pull out occurs as the fibrils align themselves in the direction of the tensile load ○ in the linear region the aligned collagen fibers are stretched until failure occurs ■ note: *when compressive or tensile forces are applied there are volume changes, when shear loading there are no volume changes V. behavior of articular cartilage in pure shear (no change in vol) ■ no interstitial flow during shearing because there are no volume changes which usually occur with tensile or compressive loads ■ collagen is the main substance that is able to control the shearing forces that occur within the articular cartilage ■ proteoglycans do not provide shear stiffness for articular cartilage - are in the middle and shear forces are located at the surface ■ figure 3.15- cartilage subject to pure shear ○ no volume or pressure gradient changes are produced therefore no interstitial fluid flow occurs ○ demonstrates the function of collagen fibrils in resisting shear deformation VI. lubrication of articular cartilage *exam ■ two types of lubrication 1. boundary lubrication- covers the articular surfaces, allows creep and stress relaxation to occur 2. fluid film lubrication- creates a separation between the bones ■ joint lubrication during, walking, standing, extended period of standing ■ walking distributes the fluid between the femoral condyles and tibial plateau ○ midstance- increase in load (wb) redistributes the fluid ○ swing through- release load (non wb) fluid fills back in space ■ ACL- fx to prevent anterior translation of the tibia on the femur and creates joint separation within the knee ○ no ACL cant refill joint with fluid in no wb because there is no joint space II. viscosupplementation ■ two hyaluronic acid products are currently available in the US: 1. naturally occurring hyaluronan (hyalgan) 2. synthetic hylan G_F 20 (synvisc) ■ hylans are cross linked hyaluronic acids, which gives them a higher molecular weight and increased elastoviscous properties ■ used to help redistribute fluid to reduce pain and restore function, comes from a comb of a rooster, can last up to a year III. wear of articular cartilage: two types *exam A. interfacial: bearing surfaces come into direct contact without lubricant film, this can lead to adhesive and abrasive wear, creates
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