The intricate movements of a human, such as those performed in dance and athletics, are accomplished by using a wide variety of joints. Though joints enable the skeleton to be dynamic, they also play an important role in stability and protection. In fact, the mobility of a joint is often inversely proportional to its stability. For example, the sutures of the bones of the cranium are basically immovable in relationship to one another, but due to their stable nature, they serve to protect the brain throughout daily life and during incidents of trauma. On the other hand, the ball-and-socket of the shoulder enables a wide variety of complex movements. This increase in the amount of mobility leads to instability, which is why the shoulder is susceptible to injury.
A joint (articulation) occurs wherever bones meet. Joints are classified both structurally and functionally, as shown in Table 1.
Structural classification is based on the materials that hold the joint together and whether or not a cavity is present in the joint. There are three structural classes:
- Fibrous joints are held together by fibrous connective tissue. No joint cavity is present. Fibrous joints may be immovable or slightly movable.
- Cartilaginous joints are held together by cartilage (hyaline or fibrocartilage). No joint cavity is present. Cartilaginous joints may be immovable or slightly movable.
- Synovial joints are characterized by a synovial cavity (joint cavity) containing synovial fluid. Synovial joints are freely movable and characterize most joints of the body. Figure 1 lists other features of a synovial joint, including the following:
- Articular cartilage (hyaline cartilage), which covers the end of each bone.
- A synovial membrane, which surrounds the synovial cavity. Its areolar connective tissue secretes a lubricating synovial fluid into the synovial cavity.
- A fibrous capsule outside the synovial membrane, which surrounds the joint. It often contains bundles of dense, irregular, connective tissue called ligaments. The ligaments provide strength and flexibility to the joint.
- The articulate capsule, composed of the synovial membrane and fibrous capsule.
- Accessory ligaments, which lie outside the articular capsule (extracapsular ligaments) or inside the synovial cavity (intracapsular ligaments).
Figure 1.A synovial joint.
Functional classification is based on the degree to which the joint permits movement. There are three types:
- A synarthrosis joint permits no movement. Structurally, it may be a fibrous or cartilaginous joint.
- An amphiarthrosis joint permits only slight movement. Structurally, it may be a fibrous or cartilaginous joint.
- A diarthrosis joint is a freely movable joint. Structurally, it is always a synovial joint.