Classification of Joints

Contents:

Classification of Joint

Arthrology:

It is the branch of science concerned with the study of anatomy, function, dysfunction, and treatment of joints and articulations.

Joint (Articulation):

It is a point of contact between two bones, between bone and cartilage, or between bone and teeth.
A joint is the site at which any two or more bones articulate or come together.
Some joints have no movement (fibrous), some have only slight movement (cartilaginous) and some are freely movable (synovial).

Classification of Joint

Joints are classified on the basis of their structure and function.

(I) Structural Classification of Joints:

Structural classification is based on the materials that hold the joint together and whether or not a cavity is present in the joint.
Fibrous joints: There is no synovial cavity and the bones are held together by fibrous tissue.
Cartilaginous joints: There is no synovial cavity and the bones are held together by cartilage.
Synovial joints: There is a synovial cavity and the bones are held together by the dense irregular connective tissue of an articular capsule and accessory ligaments.

(II) Functional classification of joints:

Functional classification is based on the degree to which the joint permits movement.

Synarthrosis:

It is an immovable joint.
Structurally, it may be a fibrous or cartilaginous joint.
The articular surfaces are connected by tough fibrous tissue.
The edges of the bones are devoted to one another as in the sutures of the skull.

Amphiarthrosis:

It is a slightly movable joint.
Structurally, it may be a fibrous or cartilaginous joint.
A pad of cartilage lies between the bone surfaces.
There is a fibrous capsule to hold the bone and cartilage in place.
The cartilage of such joints acts as shock absorbers.

Diarthrosis:

It is a freely movable joint.
Structurally, it is a synovial joint.
The movement of these joints is restricted by the shape of the articulating surfaces and ligaments which hold them together.
These ligaments are of elastic connective tissue.

Based on the structural differences joints are of the following types:

Fibrous Joint or Fixed Joint

It lacks a synovial cavity.
The articulating bones are held together by a dense irregular connective tissue.
It permits little or no movement.
The three types of fibrous joints are:
- Sutures
- Syndesmoses
- Interosseous membranes

(i) Sutures:

It is a fibrous joint composed of a thin layer of dense irregular connective tissue.
It occurs only between the bones of the skull. E.g. Coronal suture between the parietal and frontal bones.
The irregular and interlocking edges of sutures give them additional strength and decrease the chance of fracturing.

(ii) Syndesmoses:

In syndesmoses, a greater distance is present between the articulating surfaces.
It contains denser irregular connective tissue than in a suture.
The dense irregular connective tissue is typically arranged as a bundle and the joint permits limited movement. E.g. The distal tibiofibular joint.

(iii) Interosseous Membranes:

It contains a sheet of dense irregular connective tissue that binds neighboring long bones.
It permits slight movement.
There are two interosseous membrane joints in the human body.
One occurs between the radius and ulna in the forearm and the other occurs between the tibia and fibula in the leg.

Cartilaginous Joints

The characteristics of cartilaginous joints are:

It lacks a synovial cavity.
It allows little or no movement.
The articulating bones are tightly connected by either hyaline cartilage or fibrocartilage.
The two types of cartilaginous joints are:
- Synchondroses
- Symphysis

(i) Synchondroses:

It is a cartilaginous joint in which hyaline cartilage is the connecting material. E.g. The epiphyseal (growth) plate that connects the epiphysis and diaphysis of a growing bone.
It is a synarthrosis, an immovable joint.

(ii) Symphysis:

It is a cartilaginous joint in which the ends of the articulating bones are covered with hyaline cartilage, and abroad, a flat disc of fibrocartilage connects the bones.
All symphysis occur in the midline of the body. E.g. The pubic symphysis between the anterior surfaces of the hip bones.
It is an amphiarthrosis, a slightly movable joint.

Synovial Joint

The exclusive characteristic of the synovial joint is the presence of space called a synovial cavity between the articulating bones.
All synovial joints are classified as diarthrosis or movable joints.
A synovial joint shows the presence of the following structures:
- Articular or hyaline cartilage
- Capsule or capsular ligament
- Synovial membrane
- Synovial fluid
- Other intracapsular structures
- Extracapsular structures

Articular or Hyaline Cartilage:

The parts of the bones that are in contact are covered with hyaline cartilage.
It provides a smooth articular surface that is strong enough to absorb compression forces and bear the weight of the body.
This leads to increased stress on other structures in the joint.
It has no blood supply and receives its nourishment from synovial fluid.

Capsule or Capsular Ligament:

The joint is surrounded by a sheath of fibrous tissue that holds the bones together.
It is sufficiently loose to allow movement but strong enough to protect from injury.

Synovial Membrane:

It is composed of epithelial cells.
This membrane is found in:
The lining of the capsule
The parts of bones within the joints not covered by articular cartilage
All intracapsular structures

Synovial Fluid:

It is a thick, sticky fluid of egg-white consistency, secreted by synovial membranes into the synovial cavity.
Functions of synovial fluid:
- It provides nutrients for the structures within the joint cavity.
- It contains phagocytes, which remove microbes and cellular debris.
- It acts as a lubricant.
- It maintains joint stability.
- It prevents the ends of bones from being physically separated.
- The little sacs of synovial fluid 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.

Intracapsular Structures:

Some joints have structures within the capsule, but outside the synovial membrane, that assist in the maintenance of stability. E.g. Fat pads in the knee joint.
These structures are covered by a synovial membrane.

Extracapsular Structures:

Ligaments that blend with the capsule provides additional stability at most joints.
Muscles or their tendons also provide stability and stretch across the joints they move.

Nerve and blood supply:

Nerves and blood vessels crossing a joint usually supply the capsule and the muscles that move it.
Synovial joints contain many nerve endings that are distributed to the articular capsule and associated ligaments.
Arteries and their numerous branches penetrate the ligaments and articular capsule to deliver oxygen and nutrients.
Veins remove the carbon dioxide and wastes from the joints.

Table 1: Movements of Synovial Joints

Types of Synovial Joints:

Gliding Joint:

The articulating surfaces are flat or slightly curved.
They permit back-and-forth and side-to-side movements between the flat surfaces of bones.
Many planar joints are biaxial because they permit movement around two axes.
An axis is a straight line around which a rotating bone moves.
Examples of planar joints are:
- Intercarpal joints (between carpal bones at the wrist),
- Intertarsal joints (between tarsal bones at the ankle),
- Sternoclavicular joints (between the manubrium of the sternum and the clavicle),
- Acromioclavicular joints (between the acromion of the scapula and the clavicle),
- Sternocostal joints (between the sternum and ends of the costal cartilages at the tips of the second through seventh pairs of ribs),
- Vertebrocostal joints (between the heads and tubercles of ribs and transverse processes of thoracic vertebrae).

Hinge Joint:

The convex surface of one bone fits into the concave surface of another bone.
It produces an angular, opening-and-closing motion.
In most joint movements, one bone remains in a fixed position while the other moves around an axis.
These are monaxial because they typically allow motion around a single axis.
They permit only flexion and extension.
Examples of hinge joints are:
- Knee joint
- Elbow joint
- Ankle joint
- Interphalangeal joint

Pivot Joint:

In a pivot joint, the rounded or pointed surface of one bone articulates with a ring formed by another bone and ligament.
It is a monaxial joint because it allows rotation only around its own longitudinal axis.
Examples of pivot joints are;
- Atlanto-axial joint, in which the atlas rotates around the axis and permits the head to turn from side to side.
- Radioulnar joints that enable the palms to turn anteriorly and posteriorly.

Condyloid Joint:

In a condyloid joint or ellipsoidal joint, the convex oval-shaped projection of one bone fits into the oval-shaped depression of another bone.
It is a biaxial joint because the movement it permits is around two axes.
Examples of condyloid joints are the wrist and metacarpophalangeal joints for the second through fifth digits.

Saddle Joint:

In a saddle joint, the articular surface of one bone is saddle-shaped, and the articular surface of the other bone fits into the saddle.
It is a modified condyloid joint.
These are triaxial joints, permitting the movements around three axes.
An example of a saddle joint is the carpometacarpal joint between the trapezium of the carpus and the metacarpal of the thumb.

Ball and Socket Joint:

It consists of a ball-like surface of one bone fitting into a cuplike depression of another
bone.
Such joints are triaxial, permitting movements around three axes.
Examples of ball-and-socket joints are;
- Shoulder joints: The head of the humerus fits into the glenoid cavity of the scapula.
- Hip joints: The head of the femur fits into the acetabulum of the hip bone.

Table 2: Different types of Synovial Joints