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.
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.
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.
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.
Synarthrotic joints allow little or no movement. Their main function is to provide stability, and they also join bones to form a larger unit. Diarthrotic joints provide stability and mobility. Joints can be affected by injury, increased demand, immobilization, or long-term bed rest, and diseases, such as osteoarthritis. Injury can occur if a large stress or load is placed on a joint. Constant excessive loading over time can also cause joint structure to break down. Immobilization or long-term bed rest causes muscles around joints to weaken. Furthermore, joints and articular surfaces need some load, such as gravity, to maintain proper integrity. Over time, if load is not present, articular surfaces will weaken and degenerate due to lack of stimulus. Disease processes such as osteoarthritis can also disrupt the integrity of the joint. All of the above problems can affect joint structure and eventually disrupt functions such as walking. Severe joint degradation can lead to disability.
Increased demand or trauma placed on a joint can cause tearing or even rupture of the ligaments, joint capsule, or hyaline cartilage. Furthermore, immobilization and disease can degrade the joint surfaces. Any one of these complications can disrupt the integrity of the joint. If the integrity of a joint is compromised, there could be decreased motion at the joint and possibly pain. Thus, pain and decreased joint mobility can lead to decreased function and eventual disability.
Other pathologies such as osteoarthritis, rheumatoid arthritis, trauma, and gout can all negatively affect joint integrity and function. In the acute phases of gout, joint effusion secondary to injury, and rheumatoid arthritis the joint capsule of diarthroidal joints becomes distended due to over production of synovial fluid. Because of this distention, joint receptors are impaired and may provide inaccurate information on position and movement. Furthermore, there is pain associated with these conditions. If treatment is not effective in reducing pain and inflammation, joint integrity and eventual function will be compromised. In situations where injury has occurred to a joint, such as ligament tear or rupture, the joint is unstable. This instability leads to further stresses placed on other structures within the joint. Eventually, if the ligament is not healed or repaired, further damage to the joint will occur because of the increased demand on other structures. An example is tearing of a ligament in the knee or ankle.
Osteoarthritis is a disease process that negatively affects the integrity and function of a joint. In this degenerative disease the articular surfaces of the joint are degrading. As time passes, the degradation of the joint continues. The most common joints affected by osteoarthritis are the knee and the hip. Conservative treatments such as medications and rehabilitation may be used to decrease pain and restore mobility. However,
there are instances where conservative treatments fail and joint replacement is a viable option, i.e. total knee arthroplasty.
Joint structures need optimum motion and stress to maintain proper function. Moreover, proper cartilage and joint nutrition are required for proper joint function. Therefore, it is important to have synovial fluid maintaining nourishment to the joint surfaces and cartilage in an effort to maintain the integrity of the joint. Since joints are not well supplied by blood flow it is important that movement and weight-bearing are encouraged to promote fluid flow between joint surfaces and disks, i.e., menisci.
Femur—The large upper bone, also known as the thigh bone.
Gait—Refers to walking, i.e. ambulation.
Gomphosis—A joint where a bony structure is implanted deep into another bony structure. An example would be the joint between a tooth and the mandible (jaw bone).
Immobilization—Keeping a joint from moving, i.e. when an individual breaks the lower leg; a cast may be used that covers the knee, thus preventing motion.
Knee joint—A lower limb joint connecting the tibia to the femur. It allows for straightening and bending of the knee.
Stance phase—The point where, when walking, one foot is in contact with the ground.
Suture—A joint where two bony structures are united by dense fibrous tissue. An example is the sutures of the skull.
Symphyses—Joints where bones are connected by a fibrocartilage disk. An example is the symphesis pubis.
Synchondroses—Joints that connect two bony structures by hyaline cartilage. Examples are the ends of long bones, where the bone growth is not yet complete. As the skeleton matures, the hyaline cartilage eventually ossifies.
Syndesmosis—A joint where two bones are joined by a ligament or membrane. An example is the membrane that joins the shaft of the tibia to the shaft of the fibula in the lower leg.
Tibia—The large lower leg bone between the knee and ankle.
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American Physical Therapy Association, 1111 North Fairfax St., Alexandria, VA 22314-1488. Phone: (703) 684-APTA or (800) 999-APTA. TDD: (703) 683-6748. Fax: (703) 684-7343. <http://www.apta.org>.
Sports Medicine and Orthopedic Center, University of Washington. <http://www.orthop.washington.edu>.
Mark Damian Rossi, Ph.D., P.T., C.S.C.S.