Joints are locations where bones make contact with each other. Because their function is movement and support, the joints have been modified accordingly to serve the required function.
The joints are broadly classified into three categories – fibrous, cartilaginous and synovial. Each category has further subcategories
These are also called synarthroses or immovable articulations. In these joints, the bone surfaces are in almost direct contact. The bones are fastened together by intervening connective tissue. Often in such joints, there is no appreciable motion.
Sutura, gomphosis, and syndesmosis are further subtypes of these joints
Sutura is the joint contiguous margins of the bones are united by a thin layer of fibrous tissue. This kind of joint is found in the skull only.
When the margins of the bones are connected by a series of processes, and indentations interlocked together, the articulation is termed a true suture or sutura vera. When the articulation is formed by roughened surfaces placed in apposition with one another, it is termed a false suture or sutura notha.
True Suture or Sutura vera is of three types sutura dentata, serrata, and limbosa.
It has got its name from is so called from the tooth-like form of the projecting processes. An example is the suture between the parietal bones.
The edges of the bones are serrated like the teeth of a fine saw, as between the two portions of the frontal bone.
Besides the interlocking, a certain degree of beveling of the articular surfaces, so that the bones overlap one another. This is found between parietal and frontal bones.
Sutura notha are of two kinds
It is formed by the overlapping of contiguous bones by broad beveled margins, as in the squamosal suture between the temporal and parietal.
There is simple apposition of contiguous rough surfaces. This is found in the articulation between the maxillae, or between the horizontal parts of the palatine bones.
Gomphosis is articulation in which the surfaces of bony components are adapted to each other like a peg in a hole. This is illustrated by the articulations of the roots of the teeth with the alveoli of the mandible and maxillae.
A type of joint in which two bony components are joined directly by a ligament, cord or aponeurotic membrane.
The shaft of tibia joins shaft of fibula by a membrane to form tibiofibular syndesmosis which allows a slight amount of motion with movements of knee and ankle joints.
Another example of a syndesmotic joint is inferior tibiofibular articulation where the bones are connected by an interosseous ligament.
Cartilaginous joints are also called amphiarthroses or slightly movable articulations.
In these joints, the contiguous bony surfaces are either connected by broad flattened discs of fibrocartilage or hyaline growth cartilages.
In this kind of joint the cartilage directly unites one bony structure to another ( bone – cartilage – bone).
Cartilaginous joints are of two types
It is a type of joint where the intervening cartilage which connects the two bones is fibrocartilage in form of disk or plate.
The symphysis pubis is one such joint where two pubic bones are joined by fibrocartilage.
Vertebral bodies connected by intervertebral discs is another example
The connecting material between two bones is hyaline cartilage. It is a temporary form of joint, for the cartilage is converted into bone before adult life.
The function of this joint is to permit bone growth, provide stability and allow a small amount of mobility.
Such joints are found between the epiphyses and bodies of long bones and skull bones.
Also called Diarthroses or diarthrodial joint.
This class of joints includes the majority of the joints in the body.
In these joints, the ends of the bones are free to move in relation to each other and there is no cartilaginous tissue connects the adjacent bony surfaces.
The bone ends are indirectly connected to each other by a joint capsule that covers and encloses the joint. Ligaments play important role in stabilization of these joints.
All synovial joints have following features
- A joint capsule formed by fibrous tissue
- A joint cavity enclosed by a joint capsule
- Inner surface of the capsule is lined by synovial membrane
- Synovial fluid which forms a thin film over the joint surfaces
- Hyaline cartilage covering the joint surface
The synovial joints may have also have accessory structures. The joints may be divided, completely or incompletely, by an articular disk or meniscus as in knee or labrum as in hip.
Menisci or disks and synovial fluid help to prevent excessive compression of opposing joint surfaces. Ligaments and tendons assist in guiding motion and also have an important role in keeping joint surfaces together.
Joint surfaces are checked by active tension in muscles and passive tension of the ligaments.
These joints are further classified into three subtypes depending upon planes of movement of the joint.
The visible motion occurs only in one plane of the body around a single axis which usually is located near or center of the joint. They are said to have 1° of freedom of motion.
Two types of uniaxial diarthrodial joints are found in the human body
Ginglymus or Hinge joint
This joint is called so because it resembles a door hinge. In this type, the articular surfaces are molded to each other in such a manner as to permit motion only in one plane, forward and backward, the extent of motion at the same time is considerable. The articular surfaces are connected together by strong collateral ligaments, which form their chief bond of union. Examples are the interphalangeal joints and the joint between the humerus and ulna.
Trochoid or Pivot-joint
It is a type of joint where the movement is limited to rotation. The joint is formed by a pivot-like process turning within a ring, or a ring on a pivot, the ring being formed partly of bone, partly of the ligament. The proximal radioulnar joint is an example where the ring is formed by the radial notch of the ulna and the annular ligament and the head of the radius rotates within the ring.
In the articulation of the odontoid process of the axis with the atlas the ring is formed in front by the anterior arch, and behind by the transverse ligament of the atlas; here, the ring rotates around the odontoid process.
A biaxial diarthrodial joint permits the motion in two planes around two axes and has 2° degree of freedom. Condyloid and saddle joints are two subtypes.
The joint surfaces are shaped in such a manner that the concave surface of one bony component is allowed to slide over the convex surface of another component in two directions.
The wrist joint and metacarpophalangeal joint are examples of this form of articulation. Another example is
In this variety, the opposing surfaces are reciprocally concavo-convex. The movements flexion, extension, adduction, abduction, and circumduction are allowed but no axial rotation. The best example of this form is the carpometacarpal joint of the thumb.
Also called or multiaxial joints These joints permit movement in three planes around three axes. Thus these joints have 3° of freedom of motion. Motion at these joints may also occur in oblique planes. The two types of joints in this category are plane joints and ball and socket joints.
Plane or Gliding joints
These joints permit gliding movement only it is formed by the apposition of plane surfaces, or one slightly concave, the other slightly convex, the amount of motion between them is limited by the ligaments or osseous processes surrounding the articulation.
Joints between the articular processes of the vertebrae, the carpal joints [except that of the capitate with the navicular and lunate] and the tarsal joints [except the talus and the navicular] are examples
Ball and socket joints
These are the joints in which the distal bone is capable of motion around an indefinite number of axes, which have one common center. It is formed by the reception of a globular head into a cup-like cavity, hence the name ball-and-socket.
Hip and shoulder joints are the examples
Factors Responsible for Joint Stability
Joint stability is maintained by the structure of the joint and soft tissue structures around it. A joint need to be stable for carrying out its functions. Apart from their structure, the various factors maintaining joint stability are described below in the order of their importance.
Muscles have an inherent tone. The tone of different groups of muscles acting on the joint is the most important and indispensable factor in maintaining stability. Without muscles, the knee and shoulder would be unstable, and arches of the foot would collapse.
Ligaments are important in preventing any over movement, and in guarding against sudden accidental stresses. However, they do not help against a continuous strain, because once stretched, they tend to remain elongated. In this respect, the elastic ligaments are superior to the common type of white fibrous ligaments.
Bones help in maintaining joint stability only in the firm type of joints, like the hip and ankle. Otherwise, in most of the joints (shoulder, knee, sacroiliac, etc.), their role in joint stability is negligible.
Dislocation of Joint
This a condition in which the articular surfaces of the joint are abnormally displaced and the two articular surfaces have no contact in between. If a partial contact is still retained, it is called subluxation.
Dislocation is commonly caused by trauma and is characterized by pain, deformity, and loss of function. It can be diagnosed by clinical examination and X-ray is confirmatory.
Neuropathic joint is the result of its complete denervation to 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, tubes dorsalis, and syringomyelia.
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