Structure and Types of Joints in the Human Body

A joint, also known as articulation is the place where two adjacent bones or cartilages meet.
Although most joints are movable, some are not.
Movable joints provide the mechanism that allows the body to move through coordination of nervous, skeletal and muscular systems.
Joints are classified on two bases:

I. On the basis of their function (Degree of movement):

II. On the basis of their structure:

This classification is based on the presence or absence of a joint cavity and the kind of supporting tissue that binds the bones together.
According to this system, there are three types of joints;
- Fibrous joints
- Cartilaginous joints
- Synovial joints

1. Fibrous joints:

a. Sutures:

b. Syndesmoses:

Articulating bones are held together (without touching each other) by fibrous or interossoeus ligaments.
Allow slight movement: twisting of forearm (pronation and supination)
e.g. Inferior tibiofibular joint, interosseous ligament between shafts of radius and ulna

c. Gomphoses:

A peg fits into a socket.
Mostly immovable and some may have very slight movement of teeth in their sockets.
e. g. roots of teeth in alveolar processes of mandible and maxillae

2. Cartilaginous joints:

The articulating bones are united by a plate of hyaline cartilage or fibro-cartilaginous disk.
They also lack a joint cavity.
Most of them are slightly movable while some are immovable.
Usually amphiarthroses
They are of two types;

a. Synchondroses:

Also called a primary cartilaginous joint.
It is a temporary joint composed of an epiphyseal plate of hyaline cartilage that joins the diaphysis and epiphysis of a growing long bone.
Chief function is to permit growth of the bones, not their movement.
Immovable joints
e. g. epiphyseal plate of femur, union of manubrium and the body of sternum

b. Symphyses:

Also called secondary synchondrosis
The two articulating bony surfaces are covered by thin layers of hyaline cartilage.
Between them are disks of fibro-cartilage (collagenous fibers with cartilage cells) that serve as shock absorbers.
Allow slight movement
e. g. pubic symphysis, manubriosternal joint, intervertebral joints

3. Synovial joints:

The ends of the articulating bones are covered with a smooth hyaline articular cartilage and the joint is enclosed by a flexible articular capsule.
Joint cavity is present which is also called a synovial cavity.
The joint is lubricated by a thick fluid called synovial fluid.
Articular capsules that are reinforced with collagenous fibers are called fibrous capsules, which are further reinforced at certain portions with collagenous fibers to form a ligament.
Allow more free movements than any other type of joint, i.e. usually diarthroses
Synovial joints are classified according to the shape of their articulating surfaces and the type of joint movements those shapes permit.
Six types of synovial joints are recognized;

a. Hinge joints:

The convex surface of one bone fits into the concave surface of another bone.
Allows uniaxial movement like flexion and extension (back and forth movement occurs around a single transverse axis)
e. g. joints in elbow, interphalangeal joints in fingers, knee and ankle

b. Pivot joints:

Central bony pivot is surrounded by a collar of bone and ligament.
Allows uniaxial movement like supination, pronation and rotation around a central axis through the center of the pivot.
e. g. proximal radioulnar joint, atlantoaxial joint

c. Ball and socket joints:

Globe-like head of one bone fits into a cup-like concavity of another bone.
This is the most freely movable of all the joints allowing multiaxial movement
Allows movements like flexion, extension, medial (internal) rotation, lateral (external) rotation, abduction, adduction and circumduction.
e. g. shoulder joint and hip joint

d. Condyloid or Ellipsoidal joints:

They are the modifications of multiaxixal ball and socket joints.
However, because the ligaments and muscles around the joint limit the rotation to two axes, the joint is classified as biaxial joint.
Allows movements like flexion and extension of the hinge joints as well as abduction, adduction and circumduction.
No rotational movement in permitted.
e. g. metacarpophalangeal (knuckle) joints except thumb

e. Gliding joints (Plane joints):

They are almost always small and are formed by essentially flat articular surfaces.
One bone slides on another bone with a minimal axis of rotation, if any.
Allows multiaxial movement like simple gliding within narrow limits.
e. g. between articular processes of vertebrae, acromioclavicular joint, some carpal and tarsal bones

f. Saddle joints:

The opposing articular surfaces of both bones are shaped like a saddle, i.e. they have both concave and convex areas that fit into one another at right angles to each other.
Allows multiaxial movements like abduction, adduction, opposition and reposition.
e. g. carpometacarpal joint of thumb

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