A need for strength makes the bones rigid, but if the skeleton consisted of only one solid bone, movement would be impossible. Nature has solved this problem by dividing the skeleton into many bones and creating joints where the bones intersect. Joints, also known as articulations, are strong connections that join the bones, teeth, and cartilage of the body to one another. Each joint is specialized in its shape and structural components to control the range of motion between the parts that it connects.
Joints may be classified functionally based upon how much movement they allow.
- A joint that permits no movement is known as a synarthrosis. The sutures of the skull and the gomphoses that connect the teeth to the skull are examples of synarthroses.
- An amphiarthrosis allows a slight amount of movement at the joint. Examples of amphiarthroses include the intervertebral disks of the spine and the pubic symphysis of the hips.
- The third functional class of joints is the freely movable diarthrosis joints. Diarthroses have the highest range of motion of any joint and include the elbow, knee, shoulder, and wrist.
Joints may also be classified structurally based upon what kind of material is present in the joint.
- Fibrous joints are made of tough collagen fibers and include the sutures of the skull and the syndesmosis joint that holds the ulna and radius of the forearm together.
- Cartilaginous joints are made of a band of cartilage that binds bones together. Some examples of cartilaginous joints include joints between the ribs and costal cartilage, and the intervertebral disks of the spine.
- The most common type of joint, the synovial joint, features a fluid-filled space between smooth cartilage pads at the end of articulating bones. Surrounding the joint is a capsule of tough dense irregular connective tissue lined with synovial membrane. The outer layer of capsule may extend into thick, strong bands called ligaments that reinforce the joint and prevent undesired movements and dislocations. Synovial membrane lining the capsule produces the oily synovial fluid that lubricates the joint and reduces friction and wear.
There are many different classes of synovial joints in the body, including gliding, hinge, saddle, and ball and socket joints.
- Gliding joints, such as the ones between the carpals of the wrist, are found where bones meet as flat surfaces and allow for the bones to glide past one another in any direction.
- Hinge joints, such as the elbow and knee, limit movement in only one direction so that the angle between bones can increase or decrease at the joint. The limited motion at hinge joints provides for more strength and reinforcement from the bones, muscles, and ligaments that make up the joint.
- Saddle joints, such as the one between the first metacarpal and trapezium bone, permit 360 degree motion by allowing the bones to pivot along two axes.
- The shoulder and hip joints form the only ball and socket joints in the body. These joints have the freest range of motion of any joint in the body — they are the only joints that can move in a full circle and rotate around their axis. However, the drawback to the ball and socket joint is that its free range of motion makes it more susceptible to dislocation than less mobile joints.