Joining everything together with joints

Categorizing the types of joints

Joints, which vary greatly in their size and shape, can be classified by the amount of movement they permit or by their structure.

 

Structural groupings

Joints fall into three categories based on the type of connective tissue present where the bones meet:

• Fibrous: Bones held tightly together by dense connective tissue containing numerous collagen fibers
• Cartilaginous: Bones held together with either hyaline or fibrocartilage
• Synovial: Bones, which are lined with hyaline cartilage, are held together by a connective tissue capsule

 

Immovable joints

Synarthroses are joints that don’t move, such as those between the bones of the skull. A thin layer of fibrous connective tissue, called a suture, joins them together. The sutures in the cranium are named as follows:

• 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 Most fibrous joints are synarthroses.

 

Slightly movable joints

Amphiarthroses are slightly movable joints connected by fibrocartilage or hyaline cartilage. Examples include the intervertebral disks, which join each vertebrae and allow slight movement of the vertebrae.

Most cartilaginous joints are amphiarthroses.

 

Freely movable joints

Diarthroses are joints that are freely movable. The numerous types of diarthroses are shown in Table 5-3.

TABLE 5-3 Types of Diarthroses (Synovial Joints)

Type of Joint	Description	Movement	Example
Ball-and-socket joint	A joint in which the ball-shaped head of one bone fits into a depression (socket) in another bone	Circular movements; can move in all planes, and rotation is possible.	Shoulder, hip
Condyloid joint	A joint in which the oval-shaped condyle of one bone fits into the oval-shaped cavity of another bone	Can move in all planes, but can’t rotate.	Knuckles (joints between metacarpals and phalanges)
Gliding joint	A flat or slightly curved surfaces joint	Sliding or twisting; movement in two planes.	Joints between carpal bones (wrist) and between tarsal bones (ankle)
Hinge joint	A joint in which a convex surface joins with a concave surface	Up and down motion in one plane; can bend (flex) or straighten (extend).	Elbow, knee
Pivot joint	A joint in which a 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 atlas and axis at top of vertebral column
Saddle joint	A joint in which each bone is saddle shaped and fits into the saddle-shaped region of the opposite bone	Many movements are possible; can move in different planes but can’t rotate.	Joint between carpal and metacarpal bones of the thumb

All diarthroses are also synovial joints. The joint capsule creates a cavity between the two connecting bones which is filled with synovial fluid, to help lubricate and cushion the joint. The ends of the bones are cushioned by hyaline cartilage and the range of movement allowed depends greatly on their shape.

 

Knowing what your joints can do

You know that certain types of joints can perform certain kinds of movements. The movement of a body part — say, raising your hand — often has an opposing movement to return it to its original position, like putting your hand down in frustration when you don’t get called on. Here’s a quick overview of those special movements:

• Abduction: Moves a body part to the side, away from the body’s middle. 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 body’s middle. When you’re in snow angel position and you move your arms and legs back down, that’s adduction — you’re "adding" your body back together.
• Flexion: Decreases the joint angle. When you flex to show off your biceps, you move your forearm to your arm, decreasing the angle at the elbow.
• Extension: Makes the angle larger. Returning your arm from the flexed position increases the angle and the elbow and is, thus, extension. Hyperextension occurs when the body part moves beyond a straight line (180 degrees) like tilting your head back in exasperation.
• Elevation: The upward movement of a body part, such as shrugging your shoulders.
• Depression: The downward movement of a body part, such as the downward movement after shrugging your shoulders.
• Eversion: Happens only in the feet when the foot is turned so the sole is facing outward.
• Inversion: Happens only in the feet when the foot is turned so that the sole is facing inward.
• Supination: Happens only in the arm, when the forearm is rotated to make the palm face upward or forward (think about holding a bowl of soup).
• Pronation: Happens only in the arm, when the forearm is rotated to make the palm face downward or backward.
• Rotation: The movement of a body part around its own axis, such as shaking your head to answer, "No". The partnered motions are medial rotation (movement toward the midline) and lateral rotation (movement away from the midline).
• Circumduction: The movement of a body part in circles, like doing arm circles in gym class.