Joints (articulations) occur wherever two bones contact one another. The function of a joint depends on its anatomy. Joints may permit (1) no movement, (2) slight movement, or (3) extensive movement.
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In a synarthrosis, bony edges are close together and may interlock. Examples of synarthroses include a suture between skull bones, a gomphosis between teeth and jaws, a synchondrosis between bone and cartilage in an epiphyseal plate, and a synostosis, where two bones fuse and the boundary between them disappears.
A wide range of movement is permitted at a diarthrosis, or synovial joint. These joints possess seven common characteristics: a joint capsule, articular cartilages, a fluidfilled synovial cavity, a synovial membrane, accessory capsular ligaments, sensory nerves, and blood vessels that supply the synovial membrane. The articular cartilages are lubricated by synovial fluid. Other accessory structures can include menisci, fat pads, ligaments, tendons, bursae, and tendon sheaths.
Hyperextension is a term applied to any movement in which a limb is extended beyond its normal limits.A special type of angular motion is circumduction, movement of an extremity in a circular direction.Rotation of the head may involve left rotation or right rotation. Rotation of the limbs may involve internal rotation (or medial rotation) and external rotation (or lateral rotation)Special terms apply to specific joints or unusual types of movements. These include eversion, inversion, dorsiflexion, plantar flexion, lateral flexion, protraction, retraction, opposition, reposition, elevation, and depressionThe temporomandibular joint (TMJ) involves the mandibular fossa of the temporal bone and the condylar process of the mandible. This joint has a thick pad of fibrous cartilage, the articular disc. Supporting structures include the lateral ligament, the stylomandibular ligament, and the sphenomandibular ligament. This relatively loose hinge joint permits small amounts of gliding and rotation.The zygapophysial joints are plane joints that are formed by the superior and inferior articular processes of adjacent vertebrae. The bodies of adjacent vertebrae form symphyseal joints.The vertebrae are separated by intervertebral discs containing an inner soft, elastic gelatinous core, the nucleus pulposus, and an outer layer of fibrous cartilage, the anulus fibrosusNumerous ligaments bind together the bodies and processes of all vertebrae.The joints of the vertebral column permit anterior flexion and posterior extension, lateral flexion, and rotation.The shoulder complex is composed of the clavicle, scapula, and humerus and their associated joints and supporting structures. These bones link the upper limb to the thorax.The sternoclavicular joint is a plane joint that lies between the sternal end of each clavicle and the manubrium of the sternum. An articular disc separates the opposing surfaces. The anterior and posterior sternoclavicular ligaments, along with the interclavicular and costoclavicular ligaments, reinforce the joint capsule.The shoulder joint, or glenohumeral joint, formed by the glenoid fossa and the head of the humerus, is a loose, shallow joint that permits the greatest range of motion of any joint in the body. It is a ball-andsocket diarthrosis. Strength and stability are sacrificed to gain mobility.The ligaments and surrounding muscles and tendons provide strength and stability. The shoulder has a large number of bursae that reduce friction as large muscles and tendons pass across the joint capsule.The elbow joint is composed of the joints between (1) the humerus and the ulna, (2) the humerus and the radius, (3) between the humerus and the ulna (humero-ulnar joint) and (4) between the humerus and the radius (humeroradial joint).Radial and ulnar collateral ligaments and anular ligaments stabilize this joint.The proximal radio-ulnar and distal radio-ulnar joints allow for supination and pronation of the forearm. The head of the radius is held in place by the anular ligament, whereas the distal radio-ulnar articulating surfaces are held in place by a series of radio-ulnar ligaments and the antebrachial interosseous membrane.The wrist joint is formed by the radiocarpal joint and the intercarpal joints. The radiocarpal joint is a condylar joint that involves the distal articular surface of the radius and three proximal carpal bones (scaphoid, lunate, and triquetrum). The radiocarpal joint permits flexion/extension, adduction/abduction, and circumduction. A connective tissue capsule and broad ligaments stabilize the positions of the individual carpal bones. The intercarpal joints are plane joints.Five types of diarthrotic joints are found in the hand: (1) carpal bone/ carpal bone (intercarpal joints), plane diarthrosis; (2) carpal bone/ first metacarpal bone (carpometacarpal joint of the thumb), saddle diarthrosis, permitting flexion/extension, adduction/abduction, circumduction, opposition; (3) carpal bones/metacarpal bones II-V (carpometacarpal joints), plane diarthrosis, permitting slight flexion/ extension and adduction/abduction; (4) metacarpal bone/phalanx (metacarpophalangeal joints), condylar diarthrosis, permitting flexion/extension, adduction/abduction, and circumduction; and (5) phalanx/phalanx (interphalangeal joints), hinge diarthrosis, permitting flexion/extension.The hip joint is a ball-and-socket diarthrosis between the head of the femur and the acetabulum. The joint permits flexion/extension, adduction/abduction, circumduction, and rotation.The articular capsule of the hip joint is reinforced and stabilized by four broad ligaments: the iliofemoral, pubofemoral, ischiofemoral, and transverse acetabular ligaments.The ligament of the femoral head helps stabilize the hip joint.The knee joint functions as a hinge joint, but is more complex than standard hinge joints such as the elbow. Structurally, the knee is composed of two joints: (1) one between the tibia and femur and (2) one between the patella and the patellar surface of the femur. The joint permits flexion/extension and limited rotation.The articular capsule of the knee is not a single unified capsule with a common synovial cavity. It contains (1) fibrous cartilage pads, called the medial and lateral menisci, and (2) fat pads.Seven major ligaments bind and stabilize the knee joint: the patellar, tibial collateral, fibular collateral, popliteal (two), and anterior and posterior cruciate ligaments (ACL and PCL, respectively).The knee joint typically "locks" in the extended position, thereby stabilizing the knee. At full extension, a slight lateral rotation of the tibia tightens the anterior cruciate ligament and jams the meniscus between the tibia and femur, thereby "locking" the knee.The ankle joint, or talocrural joint, is a hinge joint formed by the inferior surface of the tibia, the lateral malleolus of the fibula, and the trochlea of the talus. The primary joint is the tibiotalar joint. The tibia and fibula are bound together by anterior and posterior tibiofibular ligaments. With these stabilizing ligaments holding the bones together, the medial and lateral malleoli prevent lateral or medial sliding of the tibia across the trochlear surface. The ankle joint permits dorsiflexion/plantar flexion. The medial deltoid ligament and three lateral ligaments further stabilize the ankle joint.Four types of diarthrotic joints are in the foot: (1) tarsal bone/ tarsal bone (intertarsal joints, named after the participating bone), plane diarthrosis; (2) tarsal bone/metatarsal bone (tarsometatarsal joints), plane diarthrosis; (3) metatarsal bone/ phalanx (metatarsophalangeal joints), condylar diarthrosis, permitting flexion/extension and adduction/abduction; and (4) phalanx/phalanx (interphalangeal joints), hinge diarthrosis, permitting flexion/extension.Problems with joint function are relatively common, especially in older people. Rheumatism is a general term for pain and stiffness affecting joints or other parts of the skeletal system, the muscular system, or both; several major forms exist. Arthritis is inflammation of the joints and is a type of rheumatic disease that affects synovial joints. Both conditions become increasingly common with age.