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Chapter 8

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Anatomy and Physiology
Jacqueline Carnegie

Chapter 8: Joints -involves the sites where bones meet -weakest part of the skeleton -classified by structure and function -structural classification focuses on the material binding the bones together and whether or not a joint cavity is present 1) Fibrous Joints -generally immovable 2) Cartilaginous Joints -has both rigid and slightly movable examples 3) Synovial Joints -freely movable -functional classification is based on the amount of movement allowed at the joint 1) Synarthroses -immovable joints -found in axial skeleton 2) Amphiarthroses -slightly movable joints -found in axial skeleton 3) Diarthroses -freely movable joints -predominate in the limbs Fibrous Joints -bones are joined by fibrous tissue (i.e.: dense fibrous connective tissue) -no joint cavity is present -movement allowed depends on the length of connective tissue fibres uniting the bones -are mostly movable -3 types of fibrous joints: 1) Sutures -seams that occur only between the bones of the skull -filled with minimal amounts of short connective tissue fibre -immovable 2) Syndesmoses -bones are connected by ligaments, cords, or bands of fibrous tissue -connecting fibres are longer than those in sutures -length of connecting fibres vary -limited amount of movement and is dependant on the length of connecting fibres -i.e.: the interosseous membrane connecting the radius and ulna 3) Gomphoses -a "peg-in-socket" fibrous joint -the only example is the articulation of a tooth with its bony alveolar socket -"nail and bolt" -the fibrous connection is the short periodontal ligament between the tooth and the socket Cartilaginous Joints -articulating bones are united by cartilage -lack joint cavity -not very movable -2 types: 1) Synchondroses -a bar or plate of hyaline cartilage that unites the bones at a synchondrosis (junction of cartilage) -all synchondroses are synarthrotic -involve areas of growth -e.g.: epiphyseal plate in long bones of children, the immovable joint between the costal cartilage of the first rib and the manubrium of the sternum 2) Symphyses -"growing together" -the articular surfaces of the bones are covered with hyaline cartilage which fuse into a plate of fibrocartilage (the main connecting material) -compressible and resilient -acts as a shock absorber -some degree of joint movement -designed for strength and flexibility -e.g.: intervertebral joints and the pubic symphysis of the pelvis Synovial Joints -articulating bones are separated by a fluid-containing joint cavity -allows for freedom of movement -include almost all limb joints and most joints in the body -6 distinguishing features: 1) Articular Cartilage -hyaline cartilage covers the opposing bone surfaces as articular cartilage -act as spongy cushions that absorb compression so the ends of the bones don't crush each other 2) Joint (Synovial Cavity) -has a joint cavity -a potential space that contains a small amount of synovial fluid 3) Articular Capsule -the joint cavity is enclosed by a 2-layered articular capsule / joint capsule -the external layer is a tough fibrous capsule -made up of dense irregular connective tissue -continuous with the periostea of the articulating bones -strengthens the joint so bones are not pulled apart -the inner layer of the joint capsule is a synovial membrane composed of loose connective tissue -lines the fibrous capsule internally -covers all internal joint surfaces that have no hyaline cartilage 4) Synovial Fluid -occupies free spaces within the joint capsule -viscous, egg-white consistency -provides a slipper, weight-bearing film that reduces friction between cartilages -"weeping lubrication" -synovial fluid is forced out by pressure and seeps back in when there is less pressure -nourishes cartilage cells -has phagocytic cells that rid the joint cavity of microbes and cellular debris -warms up during movement 5) Reinforcing Ligaments -synovial joints are reinforced and strengthened by ligaments 6) Nerves and Blood Vessels -synovial joints are richly supplied with sensory nerve fibres that innervate the capsule -richly supplied with blood vessels which supply the synovial membrane -some synovial joints have cushioning fatty pads between the fibrous capsule and the synovial membrane or bone (i.e.: hip and knee joints) -some have articular discs or menisci that separate articular surfaces -they extend inward from the articular capsule and divide the synovial cavity into 2 -they improve the fit between articulating bone ends -found in the knee, jaw, etc. Bursa and Tendon Sheaths -closely associated with synovial joints -bags of lubricants or "ball bearings" that reduce friction between adjacent structures during joint activity -bursae are flattened fibrous sacs lined with synovial membrane and containing a thin film of synovial fluid -found with ligaments, muscles, skin, tendons, articulations of bones -a tendon sheath is an elongated bursa that wraps completely around a tendon subjected to friction -common where several tendons are crowded together in narrow spaces (i.e.: wrist) Factors Influencing the Stability of Synovial Joints -joints must be stabilized so that they do not dislocate -the stability of synovial joints depends on 3 factors: 1) Articular Surfaces -the shapes of articular surfaces determines what movements are possible at a joint -many joints have a shallow socket or noncomplementary articulating surfaces -large articular surfaces or balls with deep sockets have improved stability (i.e.: hip joint) 2) Ligaments -capsules and ligaments of synovial joints unite the bones and prevent excessive or undesirable motion -the more ligaments a joint has, the stronger it is -if ligaments are the major means of bracing a joint, it is unstable -ligaments can only stretch 6% of its length 3) Muscle Tone -the muscle tendons that cross the joint are the most important stabilizing factor -tendons are kept taut at all times by their muscle tone -muscle tone helps reinforce the shoulder, knee joints, and arches in the foot Types of Movements Allowed by Synovial Joints 1) Gliding Movements -occurs when one flat bone surface glides or slips over another -occur at the intercarpal and intertarsal joints and the flat articular processes of the vertebrae 2) Angular Movements -increase or decrease the angle between 2 bones a) Flexion -bending movement, usually along the sagittal plane, that decreases the angle of the joint and brings the articulating bones closer together -i.e.: bending the head forward on the chest, bending the knee from a straight to angled position b) Extension -bending movement along the sagittal plane that increases the angle between the articulating bones and typically straightens a flexed limb or body part -hyperextension involves extending the joint beyond anatomical position c) Abduction -movement of a limb away from the midline or median plane of the body, along the frontal plane -i.e.: raising the arm or thigh laterally, spreading the fingers apart -bending the trunk away from the body midline is lateral flexion d) Adduction -movement of a limb toward the body midline -i.e.: putting your fingers closer together e) Circumduction -moving a limb so that it describes a cone in space -the distal end of the limb moves in a circle -consists of flexion, abduction, extension, and adduction performed in succession 3) Rotation -turning a bone around its own long axis -i.e.: medial rotation: the femur's anterior surface moves toward the median plane of the body -lateral rotation: the femur's anterior surface moves toward the lateral plane of the body 4) Supination -rotating the forearm laterally so that the palm faces anteriorly or superiorly 5) Pronation -rotating the forearm medially so the palm faces posteriorly or inferiorly 6) Dorsiflexion and Plantar Flexion of the Foot -up and down movement of the foot at the ankle -dorsiflexion: lifting the foot so that its superior surface approaches the shin -plantar flexion: depressing the foot and pointing the foot downward 7) Inversion and Eversion -inversion: the sole of the foot turns medially -eversion: the sole of the foot turns laterally 8) Protraction and Retraction -nonangular anterior and posterior movements in a transverse plane 9) Elevation and Depression -elevation: lifting a body part superiorly -depression: moving the elevated part inferiorly 10) Opposition -touching the thumb tips to the other fingers on the same hand Types of Synovial Joints 1) Plane Joints -involves 2 flat opposing surfaces -allows for gliding movement -e.g.: intercarpal joints 2) Hinge Joints -a cylindrical end of one bone conforms to a trough-shaped surface on another -motion is along a single plane -e.g.: elbow 3) Pivot Joints -the rounded end of one bone conforms to a sleeve or ring composed of bone of another -e.g.: the joint between the atlas and dens of the axis; allows for the "no" head movement 4) Condyloid Joints -oval articular surface of one bone fits into a complementary depression in another -permits all angular motions (i.e.: flexion, extension, abduction adduction, circumduction) -e.g.: the wrist and knuckle joints 5) Saddle Joints -allow greater freedom of movement than condyloid joints -each articular surface is concave and convex areas; articular surfaces fit together (concave to convex surfaces) -shaped like a saddle -e.g.: carpmetacarpal joints of the thumbs (seen through the twiddling of thumbs) 6) Ball and Socket Joints -the spherical or hemi-spherical head of one bone articulates with the cuplike socket of another -universal movement -e.g.: shoulder and hip are the only examples Common Joint Injuries 1) Sprains -partially torn ligaments -can repair themselves slowly (due to poor vascularization) -completely torn ligaments need surgery 2) Cartilage Injuries -usually in the knee -cannot repair itself because it is avascular -pieces can break off and interfere with joint function 3) Dislocations -bone is forced out of the normal position at a joint -easy to get dislocated again because ligaments get stretched Knee Joint -has the 6 distinguishing characteristics of synovial joints -the largest and most complex joint in the body -has a single joint cavity but consists of 3 joints in one: -femeorpatellar joint between the patella and the lower end of the femur -lateral tibiofemoral joint and medial tibiofemoral joint make up the lateral and medial joints between the femoral condyles above and the C-shaped menisci (semilunar cartilages) of the tibia below -the menisci deepens the shallow tibial articular surfaces and helps to prevent side-to-side rocking of the femur on the tibia and absorb shock transmitted to the knee joint -menisci are at
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