Good day everyone, Today we are going to discuss fully on joints:
After studying this section, you should be able to:
- state the characteristics of fibrous and cartilaginous joints
- list the different types of synovial joint
- outline the movements possible at five types of synovial joint
- describe the structure and functions of a typical synovial joint.
- describe the structure and movements of the following synovial joints shoulder, elbow, wrist, hip, knee and the ankle.
What is are joints?
A joint is the site at which any two or more bones articu- late or come together. Joint allow flexibility and move- ment of the skeleton and allow attachment between bones.
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The bones forming these joints are linked with tough, fibrous material. Such an 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 stabilises the alignment of the bones.
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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 hor mones 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 moveable of the body.
Characteristics of a synovial joint
All synovial joints have certain characteristics in common.
Articules or hyaline cartilage
The part of the bones in contact with each other are coated with hyaline cartilage. This provides a smooth articular surface, reduces and is strong enough to absorb compression forces and bear the weight of the body. 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 surrounded and enclosed 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.
This epithelial layer lines the capsule and covers all non weight-bearing surfaces inside the joint. It secretes 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 debris
- acts as a lubricant
- 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 or 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 Stabilise 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.
Types of synovial joints
Synovial joints are classified according to the range of movement possible or to the shape of the articulating parts of the bones involved.
Ball and socket joint
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.
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 joint 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 between the processes of the spinal vertebrae (note that the joints between the vertebral bodies are the cartilaginous discs.
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 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, and the joints between the metacarpal and phalangeal bones of the hand, and between the metatarsal and phalangeal bones of the foot. These joints permit flexion, extension, abduction, adduction and circumduction.
The articulating bones fit 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 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 fingertips on the same hand, is due to the nature of the thumb joint.