Joint Integrity and Function Health Article

Definition

Joints serve as links between structures; in this case, bones in the human body. There are numerous joints in the body that act to stabilize and control bony segments. One example is the knee joint, which joins the femur and tibia. This joint allows the lower leg to swing freely, but also to be stable during the stance phase of gait. Some joints provide the body with stability, while others provide it with mobility. However, most joints provide both stability and mobility.

Description

There are two major types of joints: synarthroses and diarthroses. Synarthroses are joints connected by fibrous tissue. Diarthroses are synovial joints, where two bones are bound together by a joint capsule, forming a joint cavity. In synovial joints, there is a nourishing lubricating fluid called synovial fluid.

Synarthoses

There are two types of synarthroses: fibrous joints and cartilaginous joints.

FIBROUS JOINTS. In fibrous joints, bones are united by fibrous tissue. There are three types of fibrous joints: gomphosis, suture, and syndesmosis. A gomphosis joint occurs where one bone fits into another bone. The articulating edges are bound together by connective tissue, and the bony surfaces in the articulation are close together. An example of a gomphosis joint is a tooth in the jawbone. An example of a suture is the fibrous joints between the bones of the skull of an infant. Before birth fibrous tissue forms soft spots on the skull, called fontanelles. As growth and development occurs the sutures ossify. A syndesmosis joint connects two bones through connective tissue and is found throughout the human body. An example is the tibio-fibular syndesmosis, the connective tissue that binds the distal ends of the fibula and tibia. A syndesmosis allows the fibula and tibia to work in unison as part of the lower leg. The limited motion available at this joint allows the tibia and fibula to move about each other, yet still function as a unit.

CARTILAGINOUS JOINTS. In cartilaginous joints, bones are connected by either fibrocartilage or hyaline cartilage. There are two types of cartilaginous joints: symphyses and synchondroses. A symphysis is a cartilaginous joint where the connecting entity is fibrocartilage. The symphysis is stable but it allows limited motion. An example of a symphysis joint is the attachment of one vertebra to another by an intervertebral disk, a fibrocartilage ring, in the vertebral column. In this symphysis joint only minimal motion occurs between vertebrae, thus maintaining stability. The combination of small movements between each successive vertebral attachment is what allows the vertebral column to flex and extend. A synchondrosis is a joint where the articulating surfaces are close together, yet are bound by hyaline cartilage. An example of a synchondrosis is the two distinct portions of long bone separated by a hyaline cartilaginous plate. This typically occurs at the ends of long bones, where a cartilaginous plate separates the diaphysis from the epiphysis. This plate allows the end of bones to grow throughout early human development. As growth and development continues, the hyaline cartilage ossifies and by adulthood the joint is gone. Another example of a synchondrosis in the human body is the articulation between the first rib and the manubrium, the upper portion of the sternum.

Diarthroses

A diarthroses has a synovial component. The bones are connected to a joint capsule that surrounds the bones and creates a joint cavity. Ligaments also attach bone-to-bone stabilizing the joint and making the diarthrotic joint stable, yet mobile. Again, the knee joint is a good example of a diarthroses; two bones (tibia and femur) that are attached by ligaments called the anterior and posterior cruciate ligaments. An extensive joint capsule also surrounds the knee joint. In synarthroses there are also disks or menisci that aid in maintaining congruency between bones, i.e., the medial and lateral menisci of the knee joint. Making the diarthroses even more unique from the synarthroses is the addition of synovial fluid. The synovial fluid provides lubrication within the joint. In summary, the diarthroses is complex, with ligaments and capsule providing stability, disks or menisci aiding in congruency, and synovial fluid providing lubrication.

Synovial-type joints can be further classified into three categories: uniaxial, biaxial, and triaxial.

UNIAXIAL JOINTS. Uniaxial joints can be further categorized into hinge and pivot joints. Examples of hinge joints are the joints of the fingers, i.e. interphalangeal joints. An example of a pivot-type joint is the articulation between the axis and atlas in the cervical region, allowing true rotation of the head. In a uniaxial joint the motion is in one plane or is said to have one degree of freedom.

BIAXIAL JOINTS. In a biaxial joint, motion occurs in two planes; thus, there are two degrees of freedom. There are two types of biaxial joints: saddle and condyloid. An example of a saddle joint is the carpmetocarpal joint of the thumb, where bones fit together like an individual riding a horse while sitting on a saddle. One bone is concave, the other is convex. Examples of condyloid joints are the metacarpophalangeal joints of the fingers.

TRIAXIAL JOINTS. Triaxial joints have three degrees of motion and can move in three planes. There are two types of triaxial joints: ball and socket, and plane joints. An example of a ball and socket joint is the hip. The attachment of the carpal bones in the hand are considered plane joints where gliding is permitted between bones.