Synovial joints (diarthroses)

SYNOVIAL JOINTS (DIARTHROSES)
(Classification)
  • These are characterized by the presence of a joint cavity, a synovial membrane and the joint capsule and by their mobility.
  • The opposing ends of bones are free and are enclosed by fibrous capsule with synovial membrane lining. These joints possess a wide range of movement.
  • The following structures enter into the formation of a diarthrodial joint.
    • Articular surfaces are formed of especially dense bone and may present non-articular depressions known as synovial fossae. These may be facets, head, condyle, trochlea or concavities, glenoid cavity, cotyloid cavity etc.
    • Articular cartilages are hyaline in type and cover the articular surfaces. They diminish concussion, reduce friction and may accentuate the curvature of the bone. It is thicker in young ones and thinner in old animals. It is believed that this tissue derives its nutrition from the vascular network of synovial membrane, synovial fluid and from blood vessels of underlying marrow spaces.
    • Besides these articular cartilages, in some joints other varieties of cartilage such as interarticular cartilage, marginal cartilage, etc are present.
    • Inter-articular cartilages or menisci are fibrocartilaginous plates or disc interposed between the two articular surfaces to render the surface congruent. They minimize the pressure and friction, which are produced during the activity of the joint.
    • Marginal or circumferential cartilages are rings of fibro-cartilage encircle the rim of an articular cavity in some joints. They enlarge the cavity and tend to prevent the fracture of the margin.
  • Articular or joint capsule consists of an outer fibrous capsule and inner synovial membrane.
    • The fibrous capsule or capsular ligament is composed of parallel and interlacing bundles of white connective tissue fibres. This capsule is perforated for the passage of blood vessels and nerves. Sometimes a portion of synovial membrane protrudes out through an aperture and forms a pouch. It encapsulates the joint and is attached around the articular ends of the concerned bones.
    • The synovial membrane lines the joint cavity, synovial bursa and synovial sheath. It is a thin membrane richly supplied with blood vessels and nerves and secretes synovia, which lubricates the joints.
    • Synovia resembles white of an egg in consistency but has a yellowish tinge. It contains albumen, mucin, and salts and is alkaline in nature. It also contains hyaluronic acid, which decides the viscosity of the fluid. Articular joint cavity is a potential space enclosed by synovial and articular cartilages.
  • Ligaments are strong strands usually composed of white fibrous tissue, which bind the bones together. In some cases, they are made up of elastic tissue. They may be periarticular or intra-articular. The ligaments may be of following varieties:
    • Capsular ligament or fibrous capsule- already described.
    • Collateral ligaments- Present on either side of the joint.
    • Intra-articular ligament- these are the connecting bands remain within the joint cavity and is covered by synovial membrane.
    • Interosseous ligaments -The opposing surfaces of bones are connected by these ligaments.
    • Annular ligaments-These are in the form of rings or tunnels, over or near the joints for the passage and the protection of tendons.
  • Vessels and nerves: The arteries form anastomoses around the larger joints and give branches to the extremities of the bones and joint capsule. Nerve fibres are numerous in and around the synovial membrane and specialized nerve endings (Pacinian corpuscles, articular end bulbs of Krause) are present.

Classification of diarthrodial joints

The diarthrodial joints are classified according to the axes of movement. This classification assumes the existence of three mutually perpendicular axes. They are

  • Uniaxial-When a joint permits movement in one plane only. Hinge joints and pivot joints belong to this group. E.g. ginglymus, pivot and condyloid joints.
  • Biaxial-These joints permit movement in two planes at right angles. E.g extension-flexion and abduction–adduction. A combined circumduction movement may also available. E.g. saddle and ellipsoidal joints.
  • Multiaxial-These joints permit angular, rotation and circumduction movements. E.g. ball and socket joints.
    • Synovial joints may also be classified according to the shape of the articular surfaces of the constituent bones. These shapes determine the type of movement and are partly responsible for determining the range of movement. The more common types are gliding, hinge and condyloid. Ball and socket, ellipsoidal, pivot and saddle joints are less common.
      • Arthrodia or Gliding joints: The articular surfaces of bones are flat and admit only gliding movements.E.g. carpus, tarsus.
      • Ginglymus or Hinge joints: The condyles or convexities of one bone articulate with corresponding concavities of another and permit only extension and flexion.E.g. elbow.
      • Condyloid or imperfect hinge joints: The elliptical concavities of one bone atriculate with other and except rotation all movements are permitted to varying degrees. E.g. radio-carpal.
      • Enarthroses or Ball and socket joint: The head of one bone is received into a glenoid or cotyloid cavity and shows extensive and various movements including circumduction. E.g. hip. 
      • Trochoid or Pivot joints: One segment rotates around another.E.g. atlanto-axial.
      • Ellipsoidal joint resembles the ball and socket joint; the articulating surfaces are much longer in one direction than in the direction at right angles. The circumference of the joint thus resembles ellipse. E.g. carpal joint.
      • Saddle or Sellar joint: The opposing surfaces are convexo-concave or saddle shaped. E.g. interphalangeal articulations of the dog.

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Last modified: Friday, 13 April 2012, 10:27 AM