A joint, also called an articulation, is any place where adjacent bones or bone and cartilage come together (articulate with each other) to form a connection. Joints are classified both structurally and functionally. Structural classifications of joints take into account whether the adjacent bones are strongly anchored to each other by fibrous connective tissue or cartilage, or whether the adjacent bones articulate with each other within a fluid-filled space called a joint cavity. Functional classifications describe the degree of movement available between the bones, ranging from immobile, to slightly mobile, to freely moveable joints. The amount of movement available at a particular joint of the body is related to the functional requirements for that joint. Thus immobile or slightly moveable joints serve to protect internal organs, give stability to the body, and allow for limited body movement. In contrast, freely moveable joints allow for much more extensive movements of the body and limbs.
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The structural classification of joints is based on whether the articulating surfaces of the adjacent bones are directly connected by fibrous connective tissue or cartilage, or whether the articulating surfaces contact each other within a fluid-filled joint cavity. These differences serve to divide the joints of the body into three structural classifications. A fibrous joint is where the adjacent bones are united by fibrous connective tissue. At a cartilaginous joint, the bones are joined by hyaline cartilage or fibrocartilage. At a synovial joint, the articulating surfaces of the bones are not directly connected, but instead come into contact with each other within a joint cavity that is filled with a lubricating fluid. Synovial joints allow for free movement between the bones and are the most common joints of the body.
The functional classification of joints is determined by the amount of mobility found between the adjacent bones. Joints are thus functionally classified as a synarthrosis or immobile joint, an amphiarthrosis or slightly moveable joint, or as a diarthrosis, which is a freely moveable joint (arthroun = 'to fasten by a joint'). Depending on their location, fibrous joints may be functionally classified as a synarthrosis (immobile joint) or an amphiarthrosis (slightly mobile joint). Cartilaginous joints are also functionally classified as either a synarthrosis or an amphiarthrosis joint. All synovial joints are functionally classified as a diarthrosis joint.
An immobile or nearly immobile joint is called a synarthrosis. The immobile nature of these joints provide for a strong union between the articulating bones. This is important at locations where the bones provide protection for internal organs. Examples include sutures, the fibrous joints between the bones of the skull that surround and protect the brain (Figure 1), and the manubriosternal joint, the cartilaginous joint that unites the manubrium and body of the sternum for protection of the heart.
An amphiarthrosis is a joint that has limited mobility. An example of this type of joint is the cartilaginous joint that unites the bodies of adjacent vertebrae. Filling the gap between the vertebrae is a thick pad of fibrocartilage called an intervertebral disc (Figure 2). Each intervertebral disc strongly unites the vertebrae but still allows for a limited amount of movement between them. However, the small movements available between adjacent vertebrae can sum together along the length of the vertebral column to provide for large ranges of body movements.
Another example of an amphiarthrosis is the pubic symphysis of the pelvis. This is a cartilaginous joint in which the pubic regions of the right and left hip bones are strongly anchored to each other by fibrocartilage. This joint normally has very little mobility. The strength of the pubic symphysis is important in conferring weight-bearing stability to the pelvis.
A freely mobile joint is classified as a diarthrosis. These types of joints include all synovial joints of the body, which provide the majority of body movements. Most diarthrotic joints are found in the appendicular skeleton and thus give the limbs a wide range of motion. These joints are divided into three categories, based on the number of axes of motion provided by each. An axis in anatomy is described as the movements in reference to the three anatomical planes: transverse, frontal, and sagittal. Thus, diarthroses are classified as uniaxial (for movement in one plane), biaxial (for movement in two planes), or multiaxial joints (for movement in all three anatomical planes).
A uniaxial joint only allows for a motion in a single plane (around a single axis). The elbow joint, which only allows for bending or straightening, is an example of a uniaxial joint. A biaxial joint allows for motions within two planes. An example of a biaxial joint is a metacarpophalangeal joint (knuckle joint) of the hand. The joint allows for movement along one axis to produce bending or straightening of the finger, and movement along a second axis, which allows for spreading of the fingers away from each other and bringing them together. A joint that allows for the several directions of movement is called a multiaxial joint (polyaxial or triaxial joint). This type of diarthrotic joint allows for movement along three axes (Figure 3). The shoulder and hip joints are multiaxial joints. They allow the upper or lower limb to move in an anterior-posterior direction and a medial-lateral direction. In addition, the limb can also be rotated around its long axis. This third movement results in rotation of the limb so that its anterior surface is moved either toward or away from the midline of the body.
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Joints hold the skeleton together and support movement. There are two ways to categorize joints. The first is by joint function, also referred to as range of motion. The second way to categorize joints is by the material that holds the bones of the joints together; that is an organization of joints by structure.
Joints in the human skeleton can be grouped by function (range of motion) and by structure (material). Here are some joints and their categorizations.
Joint
Range of Motion and Material
Skull Sutures
Immovable fibrous joints
Knee
Full movement synovial capsule hinge joint
Vertebrae
Some movement cartilaginous joint
Immovable joints (called synarthroses) include skull sutures, the articulations between the teeth and the mandible, and the joint found between the first pair of ribs and the sternum. Examples of joints allowing slight movement (called amphiarthroses) include the distal joint between the tibia and the fibula and the pubic symphisis of the pelvic girdle. Joints allowing full movement (called diarthroses) include many bone articulations in the upper and lower limbs. Examples of these include the elbow, shoulder, and ankle.
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Type of Joint Function
Examples
Synarthrosis (range of joint motion: no movement)
Skull Sutures, articulations of bony sockets and teeth in facial skeleton
Amphiarthrosis (range of joint motion: little Movement)
distal joint between the tibia and the fibula and the pubic symphysis
Diarthrosis (range of joint motion full movement)
Elbow, shoulder, ankle
Fibrous Joints. Between the articulations of fibrous joints is thick connective tissue, which is why most (but not all) fibrous joints are immovable (synarthroses). There are three types of fibrous joints:
(1) Sutures are nonmoving joints that connect bones of the skull. These joints have serrated edges that lock together with fibers of connective tissue.
(2) The fibrous articulations between the teeth and the mandible or maxilla are called gomphoses and are also immovable.
(3) A syndesmosis is a joint in which a ligament connects two bones, allowing for a little movement (amphiarthroses). The distal joint between the tibia and fibula is an example of a syndesmosis.
Cartiliginous Joints. Joints that unite bones with cartilage are called cartilaginous joints. There are two types of cartilaginous joints:
(1) A synchrondosis is an immovable cartilaginous joint. One example is the joint between the first pair of ribs and the sternum.
(2) A symphysis consists of a compressable fibrocartilaginous pad that connects two bones. This type of joint allows for some movement. The hip bones, connected by the pubic symphysis, and the vertebrae, connected by intervertebral discs, are two examples of symphyses.
Synovial Joints. Synovial joints are characterized by the presence of an articular capsule between the two joined bones. Bone surfaces at synovial joints are protected by a coating of articular cartilage. Synovial joints are often supported and reinforced by surrounding ligaments, which limit movement to prevent injury. There are six types of synovial joints:
(1) Gliding joints move against each other on a single plane. Major gliding joints include the intervertebral joints and the bones of the wrists and ankles.
(2) Hinge joints move on just one axis. These joints allow for flexion and extension. Major hinge joints include the elbow and finger joints.
(3) A pivot joint provides rotation. At the top of the spine, the atlas and axis form a pivot joint that allows for rotation of the head.
(4) A condyloid joint allows for circular motion, flexion, and extension. The wrist joint between the radius and the carpal bones is an example of a condyloid joint.
(5) A saddle joint allows for flexion, extension, and other movements, but no rotation. In the hand, the thumb's saddle joint (between the first metacarpal and the trapezium) lets the thumb cross over the palm, making it opposable.
(6) The ball-and-socket joint is a freely moving joint that can rotate on any axis. The hip and shoulder joints are examples of ball and socket joints.
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