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JOINTS. Joints. Joints or articulations : sites where two or more bones meet Gives our skeleton mobility and holds it together Weakest parts of the skeleton: Yet, their structure resists various forces, such as crushing or tearing, that threaten to force them out of alignment.
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Joints • Joints or articulations: sites where two or more bones meet • Gives our skeleton mobility and holds it together • Weakest parts of the skeleton: • Yet, their structure resists various forces, such as crushing or tearing, that threaten to force them out of alignment
Structural Classification • Focuses on the material binding the bones together and whether or not a joint cavity is present • In fibrous joints the bones are joined together by fibrous tissue and lack a joint cavity • In cartilaginous joints the bones are joined together by cartilage and they lack a joint cavity • In synovial joints, the articulating bones are separated by a fluid-containing joint cavity
Functional Classification • Is based on the amount of movement allowed at the joint: • Synarthroses are immovable joints • Amphiarthroses are slightly movable joints • Diarthroses are freely movable joints
Fibrous Joints • Bones joined by fibrous tissue • No joint cavity • Amount of movement allowed depends on the length of the connective tissue fibers uniting the bones • A few are slightly movable, most are immovable • Three types: • Sutures • Syndesmoses • Gomphoses
Fibrous JointsSutures • Occur only between bones of the skull • Wavy articulating bone edges interlock, and the junction is completely filled by a minimal amount of very short connective tissue fibers that are continuous with the periosteum • During middle age, the fibrous tissue ossifies and the skull bones fuse into a single unit • At this stage, the sutures are more precisely called synostoses (bony junctions) • Because movement of the cranial bones would damage the brain, the immovable nature of sutures is a protective adaptation
Fibrous JointsSyndesmoses • Bones are connected by a ligament (cord or band of fibrous tissue) • Connecting fibers vary in length: amount of movement allowed depends on the length of the connecting fibers
Fibrous JointsSyndesmoses • Slight to considerable movement is possible • Examples: • 1. Ligament connecting the distal ends of the tibia and fibula is short, and this joint allows only slight movement • True movement is still prevented, so the joint is classed functionally as an immovable joint, or synarthrosis (amphiarthrosis) • 2. Ligament (interosseous membrane) connecting the radius and ulna along their length are long enough to permit rotation of the radius around the ulna
Fibrous JointsGomphoses • Gompho: Greek for nail/bolt • Peg-in-socket fibrous joint • Refers to the way teeth are embedded in their sockets (as if hammered in) • Only example is the articulation of a tooth with its bony alveolar socket • Fibrous connection is the short periodontal ligament
Cartilaginous Joints • Articulating joints are united by cartilage • Lack joint cavity • Two types: • Synchondroses • Symphyses
Cartilaginous JointsSynchondroses • A bar or plate of hyaline cartilage unites the bones at a synchondrosis (junction of cartilage) • Virtually all synchondroses are synarthrotic (immovable) • Examples: • Epiphyseal plates connecting diaphysis and epiphysis regions in long bones of children • (a):Epiphyseal plates are temporary joints and eventually become synostoses (bony junctions) • (b):Immovable joint between the costal cartilage of the first rib and the manubrium of the sternum
Cartilaginous JointsSymphyses • Symphyses (growing together) • Articular surfaces of the bones are covered with articular (hyaline) cartilage, which in turn is fused to an intervening pad, or plate, of fibrocartilage • Since fibrocartilage is compressible and resilient, it acts as a shock absorber and permits a limited amount of movement at the joint • Amphiarthrotic joints (synarthrosis: immovable) designed for strength with flexiblity: • Examples: • Intervertebral joints • Pubic symphysis of the pelvis
Synovial Joints • Articulating bones are separated by a fluid-containing joint cavity • This arrangement permits substantial freedom of movement, and all synovial joints are freely movable diarthroses (freely movable) • All joints of the limbs—indeed, most joints of the body—fall into this class
Synovial JointsGeneral Structure • Contains five distinguishing features • 1.Glassy-smooth articular (hyaline) cartilage covers the ends of the opposing articulating bones • Thin but spongy cushions absorb compression placed on the joint and thereby keep the bone ends from being crushed • 2.The joint (synovial) cavity is a space that is filled with synovial fluid
Synovial JointsGeneral Structure • 3.The two-layered articular capsule encloses the joint cavity • The external layer is a tough fibrous capsule, composed of dense irregular connective tissue, that is continuous with the periostea of the articulating bones • It strengthens the joint so that the bones are not pulled apart • The inner layer of the joint capsule is a synovial membrane composed of loose connective tissue • Besides lining the fibrous capsule internally, it covers all internal joint surfaces that are not hyaline cartilage
Synovial JointsGeneral Structure • 4.Synovial (joint egg) fluid is a viscous egg-white consistency, slippery fluid that fills all free space within the joint cavity • Derived largely by filtration from blood flowing through the capillaries in the synovial membrane • Viscous egg-white consistency due to its content of hyaluronic acid secreted by cells in the synovial membrane, but thins, becoming less viscous, as it warms during joint activity
Synovial JointsGeneral Structure • 4.Synovial fluid: • Also found within the articular cartilageproviding a slippery weight-bearing film that reduces friction between the cartilages • Forced from the cartilage when a joint is compressed: weeping lubrication(lubricates the free surfaces of the cartilages and nourishes their cells) • Seeps back into the articular cartilages like water into a sponge, ready to be squeezed out again the next time the joint is put under pressure • Contains phagocytic cells that rid the joint cavity of microbes and cellular debris
Synovial JointsGeneral Structure • 5.Reinforcing ligaments cross synovial joints to strengthen the joint • Capsular, or intrinsic ligaments: thickened parts of the fibrous capsule • Extracapsular ligaments: outside the capsule • Intracapsular ligaments: deep to the capsule • Since they are covered with synovial membrane, they do not actually lie within the joint cavity • Double jointed: • People whose joint capsules and ligaments are more stretchy and looser than average • They have the same number of joints
Synovial JointsGeneral Structure • Richly supplied with sensory nerve endings that monitor joints position and help maintain muscle tone • Richly supplied with blood vessels, most of which supply the synovial membrane • Other specific structures: • Hip, knee joints: • Fatty pads between the fibrous capsule and the synovial membrane or bone • Knee, jaw, sternum and clavicle, shoulder, distal radioulnar: • Wedge of fibrocartilage separating the articular surfaces • Called articular dics, or menisci • Extends inward from the articular capsule and partially or completelydivides the synovial cavity in two • Improve the fit between articulating bone ends, making the joint more stable and maintaining wear and tear on the joint surfaces
Bursae and Tendon Sheaths • Not strictly part of the synovial joint, BUT often found closely associated with the joint • Bags of lubricant that reduce friction at synovial joints during joint activity • Bursae (purse): flattened fibrous sacs lined with synovial membrane and containing a thin film of synovial fluid • Common where ligaments, muscles, skin, tendons, or bones rub together • Example: • Bunion: enlarged bursa at the base of the big toe, swollen from rubbing of a tight or poorly fitting shoe
Bursae and Tendon Sheaths • Tendon sheath: • Essentially an elongated bursa that wraps completely around a tendon subjected to friction • Like a bun around a hot dog
Factors Influencing the Stability of Synovial Joints • Because joints are constantly stretched and compressed, they must be stabilized so that they do not dislocate (come out of alignment) • Stability of a synovial joint depends chiefly on three factors: • 1.The shapes of the articular surfaces of bones found at a synovial joint : • Determines the movements that occur at the joint • Play a minimal role in stabilizing the joint • Example:ball and deep socket of the hip joint provides the best example of a joint made extremely stable by the shape of its articular surfaces
Factors Influencing the Stability of Synovial Joints • 2.Number and positioning of ligaments: • Ligament: band of regular fibrous tissue that connects bones • Capsules and ligaments of synovial joints unite the bones and prevent excessive or undesirable motion • As a rule: the more ligaments a joint has, the stronger it is • When other stabilizing factors are inadequate, undue tension is placed on the ligaments and they stretch • Stretched ligaments stay stretched (like taffy) • A ligament can stretch only about 6% of its length before it snaps • When ligaments are the major means of bracing a joint, the joint is not very stable
Factors Influencing the Stability of Synovial Joints • 3.Muscle tone: low levels of contractile activity in relaxed muscles • Keeps the muscles healthy and ready to react to stimulation • For most joints, the muscle tendons that cross the joint are the most important stabilizing factor • Tendon: cord of dense fibrous tissue attaching muscle to bone • Kept taut at all times by the tone of their muscles • Extremely important in reinforcing the shoulder and knee joints and the arches of the foot
Movements Allowed by Synovial Joints • Every skeletal muscle of the body is attached to bone or other connective tissue structures at no fewer than two points • The muscle’s origin is attached to the immovable (or less movable) bone • The other end, the insertion, is attached to the movable bone • Body movement occurs when muscles contract across joints and their insertion moves toward their origin • Movements can be described in directional terms relative to the lines, or axes, around which the body part moves and the planes of space along which movement occurs, that is, along the transverse, frontal, or sagittal plane
Movements Allowed by Synovial Joints • Range of motion: • Nonaxial: slipping movements only • No axis around which movement can occur • Uniaxial: • Movement in one plane • Biaxial: • Movement in two planes • Multiaxial: • Movement in or around all three planes of space and axes
Movements Allowed by Synovial Joints • Three general types of movement: • 1. Gliding • 2. Angular • 3. Rotation
Movements Allowed by Synovial Joints • In gliding movements: • Also known as translation • One flat, or nearly flat, bone surface glides or slips over another (back-and-forth or side-to-side) without appreciable angulation or rotation • Examples: • Intercarpal and intertarsal joints • Flat articular processes of the vertebrae
Movements Allowed by Synovial Joints • Angular movements: • Increase or decrease the angle between two bones • May occur in any plane of the body and include: • Flexion • Extension • Hyperextension • Abduction • Adduction • Circumduction
Movements Allowed by Synovial Joints • Angular Flexion: • Bending movement, usually along the sagittal plane • Decreases the angle of the joint and brings the articulating bones closer together • Examples: • (b): Bending the knee from a straight to an angled position • (b): Arm flexed at the shoulder when the arm is lifted in an anterior direction • (c): Bending the body trunk from a straight to an angled position • (d): Bending the head forward on the chest
Movements Allowed by Synovial Joints • AngularFlexion: • (b): Bending the knee from a straight to an angled position • (b): Arm flexed at the shoulder when the arm is lifted in an anterior direction
Movements Allowed by Synovial Joints • AngularFlexion: • (c): Bending the body trunk from a straight to an angled position • Lateral Flexion: lateral bending of the trunk away from the body midline in the frontal plane
