Joint is a junction between two or more bones or cartilages. It is a device to permit movements in a hard and rigid skeleton. Joints are formed in such a way that they keep a balance between the movement, stability and strength of human skeleton. However there are some types of joints where movements are sacrificed for stability and in other cases the stability is sacrificed for movements. For example the shoulder joint and hip joint are both ball and socket joints but shoulder joint is more mobile and less stable while hip joint is more stable and less mobile. Also there are certain types of joints where motion does not occur. These joints are called immovable joints and are primarily meant for growth and they permit molding during child birth.
There are more joints in a child then in an adult because as growth proceeds some of the bones fuse together e.g. the ischium, ilium and pubis fuse together to form the pelvic bone (hip bone). Similarly the two halves of the infant frontal bone and of the infant mandible fuse together, five sacral vertebrae form one sacrum and four coccygeal vertebrae form one coccyx.
Defining some basic terms related to joints:
Male surface: An articulating surface which is larger in surface area and always convex in all directions.
Female surface: An articulating surface which is smaller and concave in all directions.
Simple joints: Joints with only two articulating surfaces; male and female.
Compound joints: Joints possessing more than two articulating surfaces.
Degrees of freedom: Number of axes at which the bone in a joint can move.
Uni-axial movement: Movement of bone at a joint limited to one axis only that is with one degree of freedom
Bi-axial movement: Movement of a bone at a joint with two degrees of freedom
Multi-axial movement: Movement of a bone at a joint limited to three axes with intermediate positions as well.
Terms used for movements of joints:
Angular movements: Movements in which the angle between two adjoining bones is changed. They are of two types;
- Flexion and extension: These are bending and straightening respectively.
- Adduction and abduction: Movement towards and away from the midline of the body respectively:
Circumduction: When a long bone circumscribes a conical space.
Rotation: Bone moves around a longitudinal axis.
Adjunct rotation: Independent rotation
Conjunct rotation: Rotation which accompany other movements.
Spin: Simple rotation around the bone’s stationary mechanical axis.
Swing: Any displacement of the bone and its mechanical axis other than spin. Swing may be pure or impure (swing + element of spin).
Ovoid of motion: This represents an imaginary surface which will include all possible paths of a point on the mechanical axis of the bone at some distance from its related joint.
Cardinal swing: When the mechanical axis moves in the shortest pathway when bone moves
Arcuate swing: When the mechanical axis moves in the longest pathway when bone moves.
Co-spin: When the effect of adjunct rotation is additive to the normal spin
Anti-spin: When the effect of adjunct rotation is nullifying on the normal spin.
Shapes of articular surfaces:
Ovoid: When the ovoid is convex it is called male ovoid and when the ovoid is concave it is called female ovoid.
Saddle shaped: These are convex in one plane and concave in the other perpendicular plane.
Applied anatomy of joints:
Dislocation of joint:
This is a condition in which the articular surfaces of the joint are abnormally displaced so that one surface loses its contact completely with the other. If a partial contact is still retained, it is better called subluxation rather than dislocation. Dislocation is commonly caused by trauma and is characterized by pain, deformity and loss of function. X-ray is used for confirmation.
It is the severe pain in the joint caused by ligamentous tear, but without any associated dislocation or fracture. The tear leads to effusion into the ligament and joint causing great pain.
It is the inflammation of one or more joints. It can be caused by a variety of diseases but the common types of arthritis are rheumatic, rheumatoid, osteoarthritis and tuberculoses. The involved joint is commonly swollen and its movements are restricted and painful.
It represents the ageing process. In old age the articular cartilage shows degenerative changes in the center (fibrillation of cartilage) and proliferative changes around the edges. Due to this lips are formed around the edges of joints.
Stiffness of joints related to weather:
The viscosity of synovial fluid increases with fall in temperature. This accounts for stiffness of the joints in cold weather. Mobility of the joint itself is an important factor in promoting lubrication. Thus the stiffness of the joints experienced in the morning gradually passes off as the movements are resumed.
It is the result of complete denervation of the joint so that all reflexes are eliminated and the joint is left unprotected and liable to mechanical damage. A neuropathic joint shows painless swelling, excessive mobility and bony destruction. It is commonly caused by leprosy, tabes dorsalis and syringomyelia.