11.8: Joints

Why does his knee hurt?

As you age, you might start noticing pain in your knees or elbows. These are examples of joints. Joints are the part of the skeletal system that connect your bones. Joint pain is a common problem as people age.

Joints and How They Move

A joint is a point at which two or more bones meet. There are three main types of joints in the body:

1. Fixed joints do not allow any bone movement. Many of the joints in your skull are fixed (Figure below). There are eight bones that fuse together to form the cranium. The joints between these bones do not allow movement, which helps protect the brain.

Figure \(\PageIndex{1}\): The skull has fixed joints. Fixed joints do not allow any movement of the bones, which protects the brain from injury.

2. Partly movable joints allow only a little movement. Your backbone has partly movable joints between the vertebrae (Figure below).

Figure \(\PageIndex{2}\): The joints between your vertebrae are partially movable.

3. Movable joints allow the most movement.

Movable joints are also the most common type of joint in your body. Your fingers, toes, hips, elbows, and knees all provide examples of movable joints. The surfaces of bones at movable joints are covered with a smooth layer of cartilage. The cartilage reduces friction between the bones. Ligaments often cross a joint, holding two nones together. For example, there are numerous ligaments connecting the leg bones across the knee joint.

Types of Movable Joints

Four types of movable joints are discussed here.

1. In a ball-and-socket joint, the ball-shaped surface of one bone fits into the cup-like shape of another. Examples of a ball-and-socket joint include the hip (Figure below) and the shoulder.

Figure \(\PageIndex{3}\): Your hip joint is a ball-and-socket joint. The “ball” end of one bone fits into the “socket” of another bone. These joints can move in many different directions.

2. In a hinge joint, the ends of the bones are shaped in a way that allows motion in two directions, forward and backward. Examples of hinge joints are the knees (Figure below) and elbows.

Figure \(\PageIndex{4}\): Hinge Joint. The knee joint is a hinge joint. Like a door hinge, a hinge joint allows backward and forward movement.

3. The pivot joint (Figure below) only allows rotating movement. An example of a pivot joint is the joint between the radius and ulna that allows you to turn the palm of your hand up and down.

Figure \(\PageIndex{5}\): Pivot Joint. The joint at which the radius and ulna meet is a pivot joint. Movement at this joint allows you to flip your palm over without moving your elbow joint.

4. A gliding joint is a joint which allows only gliding movement. The gliding joint allows one bone to slide over the other. The gliding joint in your wrist allows you to flex your wrist. It also allows you to make very small side-to-side motions. There are also gliding joints in your ankles.

Summary

• Joints, a point at which two or more bones meet; they can be fixed, partly movable, or movable.
• Types of movable joints include the ball-and-socket joint, hinge joint, pivot joint, and gliding joint.

Explore More

Use the resource below to answer the questions that follow.

• Joints of the Skeleton at http://www.youtube.com/watch?v=VsBJ4oUff10 (0:45)

1. What is an example of a fixed joint? How would an organism be affected if this joint was moveable?
2. What are the best joints for movement?
3. What is the function of synovial fluid? Where is it located?
4. What is the function of the knee cap?

Review

1. What's the difference between a fixed joint and a movable joint? Give examples of each.
2. Describe the four types of movable joints.
3. What type of joint are each of the following?
   1. the shoulder
   2. the wrist
   3. the knee