MNB.8 Friction in Synovial Joints PDF

Musculoskeletal System, Nervous System & Bioelectricity MNB.8 Friction in Synovial Joints D r A ndy Ma Learning outcomes At the end of this lecture, the learner will be able to Differentiate between static and kinetic friction. Describe the structure and composition of cartilage in relation t...

Musculoskeletal System, Nervous System & Bioelectricity MNB.8 Friction in Synovial Joints D r A ndy Ma Learning outcomes At the end of this lecture, the learner will be able to Differentiate between static and kinetic friction. Describe the structure and composition of cartilage in relation to its mechanical behaviour. Explain the mechanical role of synovial fluid in normal joints. Describe the current state of understanding of joint lubrication, including fluid film and boundary lubrication. Describe modes of mechanical failure of cartilage. Discuss the clinical relevance of permeability and tension in cartilage function. Describe the aetiology of osteoarthritis in terms of mechanical factors. Differentiate between rheumatoid arthritis and osteoarthritis. MN B.8 Fri cti o n in Syn o via l Jo in ts 2 Articulation of Synovial Joints When two surfaces which form a joint move relative to each other: Frictional forces exist which resist the relative motion. Wear occurs resulting in a progressive loss of material due to mechanical or chemical action. Synovial joints are the most common and movable joint in the body. Examples include the shoulder and knee. They contain articular cartilage and synovial fluid which are encased in a watertight fibrous joint capsule. MN B.8 Fri cti o n in Syn o via l Jo in ts 3 Friction Friction – force exerted by a surface as an object moves across it or tries to move across it. Lubrication can be used to reduce these forces within the body, and biological lubricants include saliva, mucus, tears and synovial fluid. MN B.8 Fri cti o n in Syn o via l Jo in ts 4 Static and Kinetic Friction Static and kinetic friction are both present in synovial joints. Static friction is effective between surfaces that are at rest with respect to each other. This friction must be overcome, by either muscle tension or gravity, for the joint to move. Once moving, kinetic friction is effective between the surfaces that are in relative motion. This friction acts to oppose motion, cause wear on joint surfaces, generate thermal energy, and make the body less efficient. MN B.8 Fri cti o n in Syn o via l Jo in ts 5 Static and Kinetic Friction 𝐹𝑠 = 𝜇𝑠 𝑚𝑔 𝐹𝑘 = 𝜇𝑘 𝑚𝑔 The knee joint is lubricated by synovial fluid so that the coefficient of static friction between the bones is 0.020 and the coefficient of kinetic friction is 0.016. MN B.8 Fri cti o n in Syn o via l Jo in ts 6 Joint Lubrication Mechanisms MN B.8 Fri cti o n in Syn o via l Jo in ts 7 Articular (Hyaline) Cartilage Articular cartilage is a thin layer (2 to 4 mm thick) of dense, translucent, connective tissue covering the ends of articulating bones. Articular cartilage does not have blood vessels, nerves, or lymphatics therefore it has a limited ability to repair itself. Function of Cartilage: 1. Helps to distribute the loads between opposing bones by increasing the contact area across a synovial joint. 2. Provides a bearing surface to allow movement with minimum friction and wear. μs of 0.01 and μk of 0.003 (μk of rubber on glass is 2). MN B.8 Fri cti o n in Syn o via l Jo in ts 8 Articular (Hyaline) Cartilage The extracellular matrix is principally composed of water (~75% wet weight), and a solid phase composed of type II collagen (20%) interwoven with proteoglycans (5%,primarily aggrecan), and