Biomechanics of Articular Cartilage C1 Articular cartilage is a 1-5 mm thick dense covering of articular surfaces. It is composed of “hyaline cartilage” as opposed to _______________________________________. Purpose: 1. To decrease ___________________on articular surfaces by increasing ______________. 2. To decrease ______________on bones and allow relative movement of articular surfaces. Composition: (Articular cartilage is devoid of _________________, lymph channels and ___________!) 1. Solid matrix __________ by weight a. Collagen fibers (mostly type II) _______ b. Interfibrillar proteoglycan gel (PG) ______ c. cells (chondrocytes) _______ 2. water _________ by weight water can be squeezed out under load (part of “forced circulation”) C2 Collagen Collagen fibril is very resistant to tensile loads, but offers little resistance to compression.C3 How strong is collagen compared with other materials? Tested in tension Tendons (80% collagen) Steel Aluminum Distribution of collogen: C4 Note: collagen provides very little resistance to __________ or ____________________ stresses. However, compressive loads on cartilage surface tend to stretch the underlying collagen fibers, thereby creating ____________ stresses in these fibers. Surface of articular cartilage is anisotropic: Split lines indicate strength and stiffness of AC are _________________________________. What is the functional significance? Split-line pattern of AC on distal femurC5 Proteoglycan gel This portion of the solid matrix is closely tied with the flow of water in and out of the cartilage. “Bottlebrush” structure: Building blocks: glycosaminoglycans (_______) The “bristles” of the bottlebrush are ___________________ and tend to _______ each other. Compressive loads on the cartilage surface tend to “cramp” these bristles and move negative charges closer together. The bristles then try to push each other apart to reduce the ______________ , hence resisting the compressive loads. This is the first of two mechanisms in which AC provides cushioning under load. This first mechanism does not require any fluid flow to work. The second mechanism (requiring fluid flow) is as follows: C6 Within the GAGs the negative charges tend to attract _________________ (e.g. Na+, Ca++, K+). The gel then tries to dilute itself via ___________ to reduce the ionic concentrations. This produces a _____________ within the cartilage as water flows into the cartilage from the joint synovial fluid. This produces a “swelling pressure” within the cartilage of about ___________ even with no load applied. Now, when you apply external loads to the cartilage surface, water gets squeezed out into the joint space. This tends to ________________________________ (of Na+, Ca++, K+). The cartilage then tries to __________ itself by drawing water back in. Eventually equilibrium is reached between the external pressure and the internal pressure and the loads are supported. These are the two mechanisms by which AC can resist ___________________on its surface. Cells ( __________ )C7 Water Most abundant component of AC ( ___________ of total weight). Varies from ______ water at surface reducing to approximately ________ in deep zone. Water contains Na+, Ca++, K+ ions as mentioned previously. About 30% of the total water is tied up with __________. This leaves ~70% of the water free to move in and out of the cartilage. This movement of water is crucial to the cushioning properties of cartilage, but also to the general health of the cartilage. __________ to the cartilage and ______________________ are all done through the movement of water in and out of the cartilage through the _____________________. Biomechanical Behavior of AC Permeability: This relates to the ability of water to flow in and out of the articular cartilage. As load ___________, permeability ____________ so as to prevent total removal of water within the AC. C8 Under pathological conditions, this relationship can be disrupted, and the normal cushioning properties fail—tending to create high ___________ and abnormally high _________ of the tissue. Rate dependence of AC material behavior 1. Elastic behavior (time ______________ ) occurs only with very rapid loading, no time for measurable fluid flow. 2. Viscoelastic behavior (time _____________ ) for slower rates of loading, involves fluid flowing in and out of cartilage. Note: this is a simplification (i.e., making two categories). In reality, there is a continuum from fast to slow loading.C9 a. Creep test _______ is held constant, ___________ changes over time. Can use creep test to determine the strength and stiffness of the ________________________. Stiffness of this solid matrix is called the __________________ of the cartilage. b. Stress-relaxation test _____________ is held constant and, after an initial peak, ___________ decreases over time. This behavior is consistent with creep response mentioned above, just a different way of measuring the viscoelastic behavior. C10 Behavior of AC under uniaxial tensionC11 Cartilage is ____________________________ when tested at the surface as opposed to deeper (closer to the bone). Also, cartilage is stronger and stiffer when tested _____________ _____ split lines. Overall, the surface of AC can be described as a “ ____________________________________________.” Lubrication of AC Basic mechanisms: 1. A relatively thick fluid film exists (under normal conditions) that acts to lubricate the joint for the sliding and squeezing of one surface against the other. Thickness: _____________ . 2. Under ____________________ loads or under __________ gliding conditions, the thickness of the film decreases (_______). There is evidence that suggests that fluid squeezed out of the cartilage matrix us the mail contributor of this fluid film. Dynamically here’s what happens: the fluid is squeezed out (extruded) in front of and beneath the loaded contact area. Once the area of peak stress has passed any given point on the cartilage, the fluid will start to be reabsorbed, ready for the next C12 cycle of movement. This ___________________ is good for the cartilage. It gets its nutrients from the synovial fluid. 3. Under extreme loading conditions, a thin boundary layer exists on the surfaces that acts as a lubricant. Thickness: ___________ (1 nm = _____ m). This layer is believed to be 1 molecule