the study of those structures that hold bones together, allowing them to move to varying
degrees — or fixing them in place — depending on the design and function of the
joint. The term articulation, or joint, applies to any union of bones, whether it moves
freely or not at all.
Inside some joints, such as knees and elbows, are fluid-filled sacs called bursae that
help reduce friction between tendons and bones; inflammation in these sacs is called
bursitis. Some joints are stabilized by connective tissue called ligaments that range
from bundles of collagenous fibers that restrict movement and hold a joint in place to
elastic fibers that can repeatedly stretch and return to their original shapes.
The three types of joints are as follows:
Fibrous: Fibrous tissue rigidly joins the bones in a form of articulation called
synarthrosis, which is characterized by no movement at all. The sutures of the
skull are fibrous joints.
Cartilaginous: This type of joint is found in two forms:
• Synchondrosis articulation involves rigid cartilage that allows no movement,
such as the joint between the ribs, costal cartilage, and sternum.
• Symphysis joints occur where cartilage fuses bones in such a way that
pressure can cause slight movement, called amphiarthrosis. Examples
include the intervertebral discs and the symphysis pubis.
Synovial: Also known as diarthrosis, or freely moving, joints, this type of articulation
involves a synovial cavity, which contains articular fluid secreted from the
synovial membrane to lubricate the opposing surfaces of bone. The synovial
membrane is covered by a fibrous joint capsule layer that’s continuous with the
periosteum of the bone. Ligaments surrounding the joint strengthen the capsule
and hold the bones in place, preventing dislocation. In some synovial joints,
such as the knee, fibrous connective tissue called meniscus develops in the
cavity, dividing it into two parts. In the knee, this meniscus stabilizes the joint
and acts as a shock absorber.
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