THE JOINTS OR ARTICULATIONS
A JOINT is the site at which any two or more bones come together. Some joints have no movement, some only slight movement and some are freely movable. Before going on to the classification of joints we must first consider the movements which can occur.
Flexion or bending, usually forward but occasionally backward as in the case of the knee joint.
Extension means straightening or bending backward. Abduction is movement away from the midline of the body. Adduction is movement towards the midline of the body. Rotation is movement round the long axis of a part. Pronation, that is, turning the palm of the hand down. Supination means turning the palm of the hand up. Circumduction is the combination of flexion, extension, abduction and adduction.
Inversion is the turning of the sole of the foot towards the midline.
Eversion consists of the opposite movement to inversion, that is, turning the sole of the foot outwards.
CARTILAGINOUS OR SLIGHTLY MOVABLE JOINTS
In this case there is a pad of white fibrocartilage between the ends of the bones taking part in the joint, which allows for very slight movement. Movement is only possible because of compression of the pad of cartilage. Examples of cartilaginous joints include the symphysis pubis and the joints between the bodies of the vertebrae.
SYNOVIAL OR FREELY MOVABLE JOINTS
Synovial joints are characterised by the presence of synovial membrane. A considerable amount of movement is permitted at all synovial joints and the types are subdivided according to the movements possible. Ball and Socket.—These are the most freely movable of all joints. The movements possible are flexion, extension, abduction, adduction, rotation and circumduction. Examples are the shoulder and the hip joints. Hinge joints.—This type permits movement in one plane only. The movements are flexion and extension and examples include the elbow, knee, ankle, the joints between the atlas and the occipital bone and the interphalangeal joints of the fingers and toes. Gliding joints.—In this case the articular surfaces glide over each other. Examples of this type include the sterno-clavicular joints, the acromio-clavicular joints and the joints between the carpal bones and between the tarsal bones. Pivot joints.—These joints allow movement round one axis only, that is, a rotatory movement. The classical examples are the superior and inferior radio-ulnar joints and between the atlas and the odontoid process of the axis. Condyloid and saddle joints.—In these joints the movements take place round two axes thus permitting the movements of flexion, extension, abduction, adduction and the combination of these is circumduction. Examples include the wrist joint, the temporomandibular joint, metacarpo-phalangeal and metatarso-phalangeal joints. Characteristics of a Synovial Joint All synovial joints have certain characteristics in common and when describing them it is convenient to do so under the following headings. The type of synovial joint. The names of the parts of the bones concerned in its formation. Articular or hyaline cartilage. The parts of the bones forming the joint are always covered with hyaline cartilage. This tissue is strong enough to bear the weight of the body, as is necessary in the ankle joint, and at the same time provides a smooth surface for articulation. Capsular ligament. The joint is surrounded and enclosed by a sleeve of fibrous tissue which joins the bones together. It is sufficiently loose to allow for the range of movement necessary at the joint. Intra-capsular structures. Some joints have structures within the joint capsule which are necessary to ensure their stability. Synovial membrane. This is composed of secretory epithelial cells, which secrete a thick sticky fluid, of the consistency of white of egg, which acts as a lubricant to the joint, provides nutrient materials for the structures within the joint cavity and helps to maintain the stability of the joint. It prevents the ends of the bones from being separated as does a little water between two glass surfaces. Synovial membrane is found:— Lining the capsular ligament. Covering those parts of the bones within the joint capsule which are not covered with hyaline cartilage. (c) Covering all intra-capsular structures. Little sacs of synovial fluid or bursae are found in some joints. Their position is such that they act as cushions to prevent friction between a bone and a ligament or tendon. Extra-capsular structures.—Most joints have ligaments outside the capsular ligament which strengthen and lend stability to the joint. Muscles and movements.—The contraction of muscles is responsible for producing the movements at individual joints, therefore, they are best considered together. Only the major joints of the body, that is, the joints of the limbs, will be described in detail.
CARTILAGINOUS OR SLIGHTLY MOVABLE JOINTS
In this case there is a pad of white fibrocartilage between the ends of the bones taking part in the joint, which allows for very slight movement. Movement is only possible because of compression of the pad of cartilage. Examples of cartilaginous joints include the symphysis pubis and the joints between the bodies of the vertebrae.
SYNOVIAL OR FREELY MOVABLE JOINTS
Synovial joints are characterised by the presence of synovial membrane. A considerable amount of movement is permitted at all synovial joints and the types are subdivided according to the movements possible. Ball and Socket.—These are the most freely movable of all joints. The movements possible are flexion, extension, abduction, adduction, rotation and circumduction. Examples are the shoulder and the hip joints. Hinge joints.—This type permits movement in one plane only. The movements are flexion and extension and examples include the elbow, knee, ankle, the joints between the atlas and the occipital bone and the interphalangeal joints of the fingers and toes. Gliding joints.—In this case the articular surfaces glide over each other. Examples of this type include the sterno-clavicular joints, the acromio-clavicular joints and the joints between the carpal bones and between the tarsal bones. Pivot joints.—These joints allow movement round one axis only, that is, a rotatory movement. The classical examples are the superior and inferior radio-ulnar joints and between the atlas and the odontoid process of the axis. Condyloid and saddle joints.—In these joints the movements take place round two axes thus permitting the movements of flexion, extension, abduction, adduction and the combination of these is circumduction. Examples include the wrist joint, the temporomandibular joint, metacarpo-phalangeal and metatarso-phalangeal joints. Characteristics of a Synovial Joint All synovial joints have certain characteristics in common and when describing them it is convenient to do so under the following headings. The type of synovial joint. The names of the parts of the bones concerned in its formation. Articular or hyaline cartilage. The parts of the bones forming the joint are always covered with hyaline cartilage. This tissue is strong enough to bear the weight of the body, as is necessary in the ankle joint, and at the same time provides a smooth surface for articulation. Capsular ligament. The joint is surrounded and enclosed by a sleeve of fibrous tissue which joins the bones together. It is sufficiently loose to allow for the range of movement necessary at the joint. Intra-capsular structures. Some joints have structures within the joint capsule which are necessary to ensure their stability. Synovial membrane. This is composed of secretory epithelial cells, which secrete a thick sticky fluid, of the consistency of white of egg, which acts as a lubricant to the joint, provides nutrient materials for the structures within the joint cavity and helps to maintain the stability of the joint. It prevents the ends of the bones from being separated as does a little water between two glass surfaces. Synovial membrane is found:— Lining the capsular ligament. Covering those parts of the bones within the joint capsule which are not covered with hyaline cartilage. (c) Covering all intra-capsular structures. Little sacs of synovial fluid or bursae are found in some joints. Their position is such that they act as cushions to prevent friction between a bone and a ligament or tendon. Extra-capsular structures.—Most joints have ligaments outside the capsular ligament which strengthen and lend stability to the joint. Muscles and movements.—The contraction of muscles is responsible for producing the movements at individual joints, therefore, they are best considered together. Only the major joints of the body, that is, the joints of the limbs, will be described in detail.