Another word for anatomical joints is articulations from the Latin word articularis, meaning "jointed." There are many types of joints, and they tend to be classified by the amount of movement they permit.
Seeing which joints jump
You probably think of joints as the hingelike type in your knees, but a joint is simply a connection between two bones. Some joints move freely, some move a little, and some never move.
Joining immovable bones
Synarthroses are joints that do not move. Examples of joints in the body that never move are the synarthroses in the skull. The sutures between the different bones in the cranium do not move. A thin layer of connective tissue joins them together. The sutures in the cranium include the following:
- Coronal suture: Joins the parietal bones and the frontal bone
- Lambdoidal suture: Joins the parietal bones and the occipital bone
- Sagittal suture: Between the parietal bones
- Squamosal sutures: Between the parietal and temporal bones
Joining bones that move a little
Amphiarthroses are slightly moveable joints connected by fibrous cartilage (fibrocartilage) or hyaline cartilage. Examples include the vertebrae of the spinal column. The intervertebal disks join each vertebrae and allow slight movement of the vertebrae.
Joining bones that move freely
Diarthroses are freely moveable joints (see Table 1). Diarthroses are also synovial joints because a cavity between the two connecting bones is lined with a synovial membrane and filled with synovial fluid, which helps to lubricate and cushion the joint.
Diarthroses are joined together by ligaments, which are made of fibrous connective tissue. Tendons are fibrous connective tissues that join muscles to bones. Tendons also help to stabilize joints, but they do not form joints. Bursae are fluid-filled sacs that help to reduce the friction between the tendons and ligaments and between the tendons and bones. The knee contains 13 bursae; inflammation in these sacs is called bursitis. What is commonly called "tennis elbow" is bursitis in the elbow.
Table 1: Types of Diarthroses (Synovial Joints)
Type of Joint | Description | Movement | Example |
Ball-and-socket joint | The ball-shaped head of one bone fits into a depression (socket) in another bone | Circular movements; joints can move in all planes, and rotation is possible | Shoulder, hip |
Condyloid joint | Oval-shaped condyle of one bone fits into oval-shaped cavity of another bone. | Can move in different planes but cannot rotate | Knuckles (joints between metacarpals and phalanges) |
Gliding joint | Flat or slightly curved surfaces join | Sliding or twisting in different planes | Joints between carpal bones (wrist) and between tarsal bones (ankle) |
Hinge joint | Convex surface joins with concave surface | Up and down motion in one plane | Elbow, knee |
Pivot joint | Cylinder-shaped projection on one bone is surrounded by a ring of another bone and ligament | Rotation is only movement possible | Joint between radius and ulna at elbow and joint between atlas and axis at top of vertebral column |
Saddle joint | Each bone is saddle shaped and fits into the saddle-shaped region of the opposite bone | Many movements are possible | Joint between carpal and metacarpal bones of the thumb |
Knowing what your joints can do
You know that certain types of joints can perform certain kinds of movements. The following list is a quick overview of those special movements. The two basic types of movements are angular and circular.
Angular movements make the angle formed by two bones larger or smaller. Examples of these include the following:
- Abduction moves a body part to the side, away from the middle of the body. When you make a snow angel, and you move your arms and legs out and up, that's abduction.
- Adduction moves a body part from the side toward the middle of the body. When you're in snow angel position, and you move your arms and legs back down, that's adduction.
- Extension makes the angle larger. Hyperextension occurs when the body part moves beyond a straight line (180 degrees).
- Flexion decreases the joint angle. When you flex your arm, you move your forearm to your upper arm.
Circular movements occur only at ball-and-socket joints, such as in the hip or shoulder. Examples include the following:
- Circumduction is the movement of a body part in circles.
- Depression is the downward movement of a body part.
- Elevation is the upward movement of a body part, such as shrugging your shoulders.
- Eversion happens only in the feet, when the foot is turned so the sole is outward.
- Inversion also happens only in the feet, when the foot is turned so that the sole is facing inward.
- Rotation is the movement of a body part around its own axis, such as shaking your head to answer, "No."
- Supinationand pronation refer to the arm and stem from the terms supine and prone. Supination is the rotation of the lower arm to make the palm face upward or forward. Pronation is the rotation of the lower arm to make the palm face downward or backward.