Joint
Introduction
A joint is the site at which any two or more bones articulate or come together. Joints allow flexibility and movement of the skeleton and allow attachment between bones.
Types of Joint
They are devided into three terms-
Fibrous joints
The bones forming these joints are linked with tough fibrous material. Such as arrangement often permits no movement. For example, the joints between the skull bones, the sutures, are completely immovable, and the healthy tooth is cemented into the mandible by the periodontal ligament. The tibia and fibula in the leg are held together along their shafts by a sheet of fibrous tissue called the interosseous membrane. This fibrous joint allows a limited amount of movement and stabilise the alignment of the bones.
Cartilaginous joints
These joints are formed by a pad of tough fibrocartilage that acts as a shock absorber. The joint may be immovable, as in the cartilaginous epiphyseal plates, which in the growing child links 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
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 moveable of the body.
Characteristics of a synovial joints
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 and is strong enough to absorb compression forces and bear the weight of the body. The cartilage lining, which is up to 7mm 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 surrounded and encloses by a sleeve of fibrous tissue which holds the bones together. It is sufficiently loose to allow freedom of movement but strong enough to protect it from injury.
Synovial membrane
This epithelial layer lines the capsule and covers all non-weight-bearing surfaces inside the joint. It secrets synovial fluid.
Synovial fluid
- 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 joints
- Acts as a lubricant
- Maintain 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 are present in some joints, e.g. the Knee.they acts as a 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 stables 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.
Extra capsular structures
Ligaments - that blend with the capsule stabilise the joint. Muscles or their tendons also provides 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.
Types of synovial joints
It consists of the following terms-
Ball and socket joints
The head of one bone is ball-shaped and articulates with a cup-shaped socket of another. The joint allows for a wide range of movement, including flexion, extension, adduction, abduction, rotation and circumduction.
Examples include the shoulder and hip.Hinge joint
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.
Gliding joints
The articular surface 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 moveable of all the synovial joints. Examples include the joints between the carpal bones in the wrist, the tarsal bones in the foot, and between the processes of the spinal vertebrae
Pivot joints
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 rings 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 odontoid process of the atlas.
Condyloid joint
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, and the joints between the metacarpal and phalangeal bones of the hand, and between the metatarsal and phalangeal bone of the foot. These joints permit flexion, extension, abduction and circumduction.
Saddle joint
The articulating bones fir together like a man 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 metatarsal 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 fingertips on the same hand, is due to the nature of the thumb joint.
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