A joint, or articulation, is a place where two bones
come together. All of the bones in the human skeleton,
except for one (the hyoid bone in the neck),
form a joint with another bone. In terms of the amount of movement they
allow, there are three types of joints:
Synovial joints (diarthroses)
Most joints in the adult body are diarthroses or synovial joints, which
are freely movable joints. (The singular form is diarthrosis.) In this type
of joint, the ends of the opposing bones are covered with hyaline
cartilage, the articular cartilage, and they are separated by a space
called the joint cavity. The components of the joints are
enclosed in a dense fibrous joint capsule, also called an articular
capsule. The outer layer of the capsule consists of the ligaments
that hold the bones together. The inner layer is the synovial membrane
that secretes synovial fluid into
the joint cavity for lubrication.
| A synovial joint or diarthrosis
| Types of synovial joint
Credit: Lucile Packard Children's Hospital
Because all of these joints have a synovial membrane, they are sometimes
called synovial joints.
Types of diathroses or synovial joints include:
- Ball and socket joints,
such as the hip and shoulder
joints. These are the most mobile type of joint in the human body. They
allow you to swing your arms and legs in many different directions.
- Ellipsoidal joints,
such as the joint at the base of your index finger.
These allow bending and extending, rocking from side to side, but rotation
- Plane joints. These occur
between the surfaces of two flat bones that are held together by ligaments.
Some of the bones in your wrists and ankles
move by gliding against each other.
- Hinge joints, as in the
knee and elbow.
These enable movement similar to the opening and closing of a hinged
- Pivot joints. The pivot
joint in the neck allows the head to turn
from side to side.
- Saddle joints. The only saddle
joints in the body are in the thumbs. The bones in a saddle joint can
rock back and forth and from side to side, but they have limited rotation.
|Simplest of the freely movable or synovial joints
are gliding joints (A), such as those between the carpal bones at
the wrist. Movement is restricted by ligaments. Freely movable joints
(B) contain fluid to lubricate bearing surfaces (1 bone, 2 cartilage,
3 synovial fluid, 4 synovial membrane, 5 tendon, 6 ligament.) Ball-and-socket
joints (C) are found in the hip and shoulder. The socket grips the
ball firmly, assisted by ligaments. The hip joints support the trunk's
full weight. Internal ligaments (D) help to stabilize the knee joint.
They cross within the joint, enclosed in synovial membrane where the
femur and tibia articulate. Pivot joints (E) are found where the atlas
and axis vertebrae articulate and between the radius and ulna. The
former allows the head to turn, the latter the forearm to twist. Hinge
joints (F) allow movement in only one plane, as at the knee, between
the humerus and ulna at the elbow and between the bones of each finger
and toe. The saddle joint (G) at the base of the thumb is so shaped
that combinations of movements in different planes are possible allowing
the pad of the thumb to meet the finger pads.
Fibrous joints (synarthroses)
Fibrous joints, or synarthroses, are immovable joints. (The singular form
is synarthrosis.) In these joints, the bones come in very close contact
and are separated only by a thin layer of fibrous connective
tissue. The sutures in the skull
are examples of immovable joints.
Slightly movable joints are called cartilaginous joints or amphiarthroses.
(The singular form is amphiarthrosis.) In this type of joint, the bones
are connected by hyaline cartilage or fibrocartilage. The ribs
connected to the sternum by costal
cartilages are slightly movable joints connected by hyaline cartilage.
The symphysis pubis in the pelvis is a slightly
movable joint in which there is a fibrocartilage pad between the two bones.
The joints between the vertebrae and the
intervertebral disks are also
of this type.
See separate entry on joint stability.
Nerve supply of joints
The capsule and ligaments receive an abundant sensory
nerve supply. The blood vessels receive autonomic sympathetic fibers.
The cartilage covering the articular surfaces possesses only a few nerve
endings near its edge. Overstretching of the capsule and ligaments produces
reflex contraction of muscles around the joint; excessive stretching produces
pain. The stretch receptors in the capsule and ligaments are continually
sending proprioceptive information up to the central
nervous system, keeping it informed of the position of the joints. The
supplements the information passing to the nervous system from the muscle
and tendon spindles, helps to maintain postural tone, and coordinates voluntary
movements. The sympathetic fibers control the blood supply to the joint.
Hilton's law: A nerve supplying a joint also
supplies the muscles moving the joint and the skin over the insertion of