Human Body Joints
Bones come together at places in the body called , which enable us to move our bodies in different ways. A joint or articulation (or articular surface) is the connection made between bones in the body which link the skeletal system into a functional whole. They are constructed to allow for different degrees and types of movement. Some joints, such as the knee, elbow, and shoulder, are self-lubricating, almost frictionless, and are able to withstand compression and maintain heavy loads while still executing smooth and precise movements. Other joints such as sutures between the bones of the skull permit very little movement (only during birth) in order to protect the brain and the sense organs.The connection between a tooth and the jawbone is also called a joint, and is described as a fibrous joint known as a gomphosis. Joints are classified both structurally and functionally.
Types of Joints in the Body:
There are three types of joints in the body. are freely movable and allow for motion at the location where bones meet. They provide a wide range of motion and flexibility. Other joints provide more stability and less flexibility. Bones at connected by cartilage and are slightly movable. Bones at are immovable and connected by fibrous connective tissue. Joints can be classified by either their structure or function. are based on how the bones at joints are connected. Fibrous, synovial, and cartilaginous are structural classifications of joints. Classifications based on consider how movable bones are at joint locations. These classifications include (synarthrosis), (amphiarthrosis), and (diarthrosis) joints.
Immovable (Fibrous) Joints:
are those that do not allow movement (or allow for only very slight movement) at joint locations. Bones at these joints have no joint cavity and are held together structurally by thick fibrous connective tissue, usually collagen. These joints are important for stability and protection. There are three types of immovable joints: sutures, syndesmosis, and gomphosis.
- These narrow fibrous joints connect bones of the skull (excluding the jaw bone). In adults, the bones are held tightly together to protect the brain and help shape the face. In newborns and infants, bones at these joints are separated by a larger area of connective tissue and are more flexible. Overtime, cranial bones fuse together providing more stability and protection for the brain.
- This type of fibrous joint connects two bones that are relatively far apart. The bones are linked by ligaments or a thick membrane (interosseous membrane). A syndesmosis can be found between the bones of the forearm (ulna and radius) and between the two long bones of the lower leg (tibia and fibula).
- This type of fibrous joint holds a tooth in place in its socket in the upper and lower jaw. A gomphosis is an exception to the rule that joints connect bone to bone, as it connects teeth to bone. This specialized joint is also called a peg and socket joint and allows for limited to no movement.
Slightly Movable (Cartilaginous) Joints:
permit some movement but provide less stability than immovable joints. These joints can be structurally classified as , as bones are connected by cartilage at the joints. Cartilage is a tough, elastic connective tissue that helps to reduce friction between bones. Two types of cartilage may be found at cartilaginous joints: and . Hyaline cartilage is very flexible and elastic, while fibrocartilage is stronger and less flexible. Cartilaginous joints formed with hyaline cartilage can be found between certain bones of the rib cage. located between spinal vertebrae are examples of slightly movable joints composed of fibrocartilage. Fibrocartilage provides support for bones while allowing for limited movement. These are important functions as it relates to the spinal column as spinal vertebrae help to protect the spinal cord. The pubic symphysis (connects the right and left hip bones) is another example of a cartilaginous joint that unites bones with fibrocartilage. The pubic symphysis helps to support and stabilize the pelvis.
Freely Movable (Synovial) Joints:
are classified structurally as . Unlike fibrous and cartilaginous joints, synovial joints have a (fluid-filled space) between connecting bones. Synovial joints allow for greater mobility but are less stable than fibrous and cartilaginous joints. Examples of synovial joints include joints in the wrist, elbow, knees, shoulders, and hip.
Three main structural components are found in all synovial joints and include a synovial cavity, articular capsule, and articular cartilage.
- This space between adjacent bones is filled with synovial fluid and is where bones can move freely in relation to each another. Synovial fluid helps to prevent friction between bones.
- Composed of fibrous connective tissue, this capsule surrounds the joint and connects to adjacent bones. The inner layer of the capsule is lined with a synovial membrane that produces the thick synovial fluid.
- Within the articular capsule, the rounded ends of adjacent bones are covered with smooth articular (relating to joints) cartilage composed of hyaline cartilage. Articular cartilage absorbs shock and provides a smooth surface for fluent movements.
Additionally, bones at synovial joints may be supported by structures outside of the joint such as ligaments, tendons, and bursae (fluid-filled sacs that reduce friction between supporting structures at joints).
Types of Synovial Joints:
allow for a number of different types of body movements. There are six types of synovial joints found at different locations in the body.
- This joint permits rotational movement around a single axis. One bone is encircled by a ring formed by the other bone at the joint and a ligament. The bone that pivots may either rotate within the ring or the ring may rotate around the bone. The joint between the first and second cervical vertebrae near the base of the skull is an example of a pivot joint. It allows the head to turn from side to side.
- This joint permits bending and straitening movements along one plane. Similar to a door hinge, movement is limited to a single direction. Examples of hinge joints include the elbow, knee, ankle, and joints between the bones of the fingers and toes.
- Several different types of movements are allowed by this type of joint including bending and straightening, side-to-side, and circular movements. One of the bones has an oval-shaped, or convex end (male surface) that fits into the depressed oval-shaped, or concave end (female surface) of another bone. This type of joint can be found between the radius bone of the forearm and bones of the wrist.
- These distinct joints are very flexible, allowing for bending and straightening, side-to-side, and circular movements. The bones at these joints form what look like a rider on a saddle. One bone is turned inward at one end, while the other is turned outward. An example of a saddle joint is the thumb joint between the thumb and palm.
- Bones at this type of joint slide past each other in a gliding motion. The bones at plane joints are of similar size and the surfaces where the bones meet at the joint are nearly flat. These joints can be found between bones of the wrist and foot, as well as between the collar bone and shoulder blade.
- These joints allow the greatest degree of motion permitting bending and straitening, side-to-side, circular, and rotational movement. The end of one bone at this type of joint is rounded (ball) and fits into the cupped end (socket) of another bone. The hip and shoulder joints are examples of ball-and-socket joints.
Each of the different types of synovial joints allow for specialized movements that permit different degrees of motion. They may allow movement in a single direction only or movement along multiple planes depending on the type of joint. The range of motion of a joint is therefore limited by the type of joint and by its supporting ligaments and muscles.