A joint is the site at which any two or more bones articulate or come together, meaning the ends or edges of the bones are held together by connective tissues. Joints may allow flexibility and movement of the skeleton. In some joints,
however, the participating bones are fastened together so firmly that no movement between them is possible.
These joints are formed by a pad of tough fibrocartilage between the bones that acts as a shock absorber. The joint may be immovable, as in the cartilaginous epiphyseal plates which in the growing child link the diaphysis of a long bone to the epiphysis. Some cartilaginous joints permit limited movement, as between the vertebrae, which are separated by the intervertebral discs, or at the symphysis pubis which is softened
by circulating hormones during pregnancy to allow for expansion during childbirth.
Synovial joints are characterised by the presence of a space or capsule between the articulating bones. The ends of the bones are held close together by a sleeve of fibrous tissue and lubricated with a small amount of fluid. Synovial joints are the most movable of the body.
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Characteristics of a synovial joint
All synovial joints have certain characteristics in common
Articular or hyaline cartilage
The parts of the bones in contact with each other are coated with hyaline cartilage. This provides a smooth articular surface, reduces friction, distributes weight and prevents damaging bone-to-bone contact. The cartilage lining, which is up to 7 mm thick in young people, becomes thinner and less compressible with age. This leads to increasing stress on other structures in the joint. Cartilage has no blood supply and receives its nourishment from synovial fluid.
Capsule or capsular ligament
The joint is wrapped in a sleeve of fibrous tissue that holds the bones together. It is sufficiently loose to allow freedom of movement but strong enough to protect it from injury. The capsule is formed by an extension of the periosteum
covering the participating bones.
This delicate epithelial layer lines the capsule and covers all non-weight-bearing surfaces inside the joint. It secretes synovial fluid.
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This is a thick, sticky fluid of egg-white consistency, which fills the synovial cavity. It:
nourishes the structures within the joint cavity
contains phagocytes, which remove microbes and cellular debris coats and lubricates the moving parts of the joint maintains joint stability
prevents the ends of the bones from being separated, as does a little water between two glass surfaces Little sacs of synovial fluid (bursae) are present in some joints, e.g. the knee. They act as cushions to prevent friction between a bone and a ligament or tendon, or skin where a bone in a joint is near the surface. Other intracapsular structures some joints have structures within the capsule to pad and stabilize the joint, e.g. fat pads and menisci in the knee joint. If these structures do not bear weight, they are covered by synovial membrane.
- Ligaments that blend with the capsule stabilise the joint.
Muscles or their tendons also provide stability and stretch across the joints they move. When the muscle contracts, it shortens, pulling one bone towards the other.
Nerve and blood supply
Nerves and blood vessels crossing a joint usually supply the capsule and the muscles that move it.
Movements at synovial joints Movement at any given joint depends on various factors, such as the tightness of the ligaments holding the joint
together, how well the bones fit and the presence or absence of intracapsular structures. Generally, the more stable the joint, the less mobile it is. The main movements possible
Types Of Synovial Joint
Synovila joints are classified according to the range or movement possible or the shape of the articulation parts of the bones involved
Ball and socket joints
The head of one bone is ball-shaped and articulates with a cup-shaped socket of another. These joints allow a wide range of movement, including flexion, extension, adduction, abduction, rotation and circumduction. Examples include the shoulder and hip.
The articulating ends of the bones fit together like a hinge on a door, and movement is therefore restricted to flexion and extension. The elbow joint is one example, permitting only flexion and extension of the forearm. Other hinge joints include the knee, ankle and the joints between the phalanges of the fingers and toes (interphalangeal joints).
The articular surfaces are flat or very slightly curved and glide over one another but the amount of movement possible is very restricted; this group of joints is the least movable of all the synovial joints. Examples include the joints between the carpal bones in the wrist, the tarsal bones in the foot, and the processes of the spinal vertebrae (note that the joints between the vertebral bodies are the cartilaginous discs)
The articulating bones fit together like a person sitting on a saddle. The most important saddle joint is at the base of the thumb, between the trapezium of the wrist and the first metacarpal bone. The range of movement is similar to that at a condyloid joint but with additional flexibility; opposition of the thumb, the ability to touch each of the finger tips on the same hand, is due to the nature of the thumb joint.
These joints allow a bone or a limb to rotate. One bone fits into a hoop-shaped ligament that holds it close to another bone and allows it to rotate in the ring thus formed. For example, the head rotates on the pivot joint formed by the dens of the axis held within the ring formed by the transverse ligament and the dens (odontoid process) of the atlas
A condyle is a smooth, rounded projection on a bone and in a condyloid joint it sits within a cup-shaped depression on the other bone. Examples include the joint between the condylar process of the mandible and the temporal bone, those between the metacarpal and phalangeal bones of the hand, and those between the metatarsal and phalangeal bones of the foot. These joints permit flexion, extension, abduction, adduction and circumduction.