Joints

Skeletal System – Part 8

Joints  Every bone in the body

(except the hyoid bone of the neck) forms a joint with at least one other bone.  Joints (Articulations) – Sites where two or more bones meet.

Functions of Joints  Joints have two functions: 1. Hold the bones together securely 2. Give the rigid skeleton mobility  Joints are classified in two ways: 1. Functionally (focuses on the amount of movement allowed by the joint) 2. Structurally

Functional Classification of Joints 1. Synarthroses: Immoveable joints; Allows no movement. - Example: Bones in the skull

2. Amphiarthroses – Slightly moveable joints; Allow a small amount of restricted movement. - Example: Vertebrae and the joints between the two bones of the lower leg.

3. Diarthroses - Freely Moveable; Permit movement in one or more directions - Example: Shoulder, neck, and knee

Structural Classification of Joints  Structurally,

there are three types of joints: 1.

2.

3.

Fibrous Joints Cartilagenous Joints Synovial Joints

Fibrous Joints  Fibrous Joints – The

bones are united by fibrous tissue; As a general rule, these are immoveable.  Bones

are bound tightly together by CT fibers, allowing essentially no movement. Example: Sutures of the skull.

Cartilagenous Joints  Cartilagenous

Joints – Bone ends are connected by cartilage.

Cartilagenous Joints  Slightly Moveable

Examples:  



Pubic symphysis of the pelvis Intervertebral joints of the spinal column (connected by discs of fibrocartilage) Most cartilagenous joints are slightly moveable

 Immoveable Examples: 



The Epiphyseal plates of growing long bones Joints between the first ribs and the sternum.

Synovial Joints  Synovial Joints – Joints in

which the articulating bone ends are separated by a joint cavity containing synovial fluid.

All synovial joints have four distinguishing characteristics: 1. Articular cartilage 

Covers the ends of the bones forming the joint.

2. Fibrous articular capsule Joint surfaces are enclosed by a sleeve or capsule of fibrous CT  The capsule is lined with a smooth synovial membrane (the reason these joints are called synovial joints). 

All synovial joints have four distinguishing characteristics (continued): 3. Joint cavity 

The articular capsule encloses a cavity, called the joint cavity, which contains lubricating synovial fluid.

4. Reinforcing ligaments 

The fibrous capsule is usually reinforced with ligaments.

Bursae  Bursae are often found

closely associated with synovial joints.  Bursae

– Flattened fibrous sacs lined with synovial membrane and containing a thin film of synovial fluid.  Common

where ligaments, muscles, skin, tendons, or bones rub together.

Tendon Sheaths  Tendon sheaths are often found closely

associated with synovial joints.  Tendon

Sheath – An elongated bursae that wraps completely around a tendon subjected to friction.  Like

a bun around a hot dog.

Types of Synovial Joints: Based on Shape 1. Plane 2. Hinge 3. Pivot 4. Condyloid 5. Saddle

6. Ball-and-

socket

Plane Joint  Plane Joint – The articular

surfaces are essentially flat, and only short slipping or gliding movements are allowed.  Movements

are nonaxial (does not involve rotation around any axis).  Examples: Intercarpal joints of the wrist.

Hinge Joint  Hinge Joint – The

cylindrical end of one bone fits into a trough-shaped surface on another bone. 

Angular movement is allowed in just one plane, like a mechanical hinge.  Classified

as uniaxial (they allow movement around one axis only).



Examples: elbow joint, ankle joint, and the joints between the phalanges of the fingers.

Pivot Joint  Pivot Joint – The

rounded end of one bone fits into a sleeve or ring of bone. Because the rotating bone can turn only around its long axis, pivot joints are also uniaxial joints.  Examples: 

 Proximal

radioulnar joint  Joint between the atlas and the dens of the axis

Condyloid Joint  Condyloid Joint – The

egg-shaped articular surface of one bone fits into an oval concavity in another.

Both of these articular surfaces are oval.  Allow the moving joint to travel (1) from side to side and (2) back and forth. 

 But

the bone cannot rotate around its long axis.  Movement occurs around two axes, hence these joints are biaxial.

Saddle Joint  Saddle Joint – Each

articular surface has both convex and concave areas, like a saddle. These biaxial joints allow essentially the same movements as condyloid joints.  Example: Carpometacarpal 

joints in the thumbs

Ball-and-Socket Joint Ball-and-Socket Joint – The spherical head of one bone fits into a round socket in another.







These multiaxial joints allow movement in all axes, including rotation and are the most freely moving synovial joints. Examples: Shoulder and hip

Dislocations  A dislocation happens when

a bone is forced out of its normal position in the joint cavity. 

Reduction – The process of returning the bone to its proper position. Should be done only by a physician.  Attempts by an untrained person to “snap the bone back into its socket” are usually more harmful than helpful. 

Sprains  Sprains – The ligaments or tendons reinforcing a

joint are damaged by excessive stretching, or they are torn away from the bone. 

Since tendons and ligaments get poor blood supply, sprains heal slowly and are extremely painful.