A joint is a site where two bones of the body connect. Joints can be mobile or immobile. Continuous joints are relatively immobile joints wherein different types of rigid connecting tissues join two bones. These joints provide skeletal stability at the junction of the attached bones. Several types of continuous joints, also known as synarthroses or fixed joints, are found in the human body.
Fibrous Joints
A fibrous joint is one in which two bones are joined by ligaments or other inflexible connective tissue, explains Dr. Werner Platzer in the medical reference text "Color Atlas of Human Anatomy." Because ligaments have limited elasticity, little movement occurs at a fibrous joint, also known as a syndesmosis.
Examples of fibrous joints include the radioulnar joint, which holds together the radius and ulna bones of the forearm; the tibiofibular syndesmosis, which unites the tibia and fibula bones of the lower leg; the joints connecting the bones of the skull; and the tympanostapedial junction, which connects two of the minute bones of the middle ear. Gomphoses are specialized fibrous joints that hold the teeth in place in the jawbone. Loosening of the gomphoses causes abnormal mobility of the teeth.
Cartilaginous Joints
Cartilaginous joints, or synchondroses, unite two bones with an intervening piece of cartilage. Cartilage is a hard but somewhat flexible tissue. The consistency of cartilage can be appreciated by feeling the outer ear and the tip of the nose, which derive their shapes from underlying cartilage.
During the growth years of human development, the long bones on the arms and legs contain growth plates composed of cartilage. The growth plates, or epiphysial disks, occur toward the ends of the long bones. Bone growth emerges from these cartilaginous plates. The growth plates serve as a joint, uniting the pieces of bone on either side of the intervening cartilage. Upon completion of growth during adolescence, solid bone replaces the growth plates.
The pubic symphysis is a cartilaginous joint that unites the pelvic bones in the midline at the front of the body. Tough fibrocartilage holds the two pelvic bones together. The pubic symphysis loosens during pregnancy to permit passage of the baby through the pelvis during delivery. Loosening of the pubic sympysis can cause painful instability of the pelvic bones in some pregnant women, explains the Kaiser Permanente Medical Group.
Bony Joints
A bony joint is the connection of two bones with intervening bone. Bony joints, or synostoses, are the sturdiest of the many types of joints of the human body. After full growth occurs, the cartilaginous joints formed by the growth plates of the long bones are replaced by synostoses. Similarly, the bones that form the hips unite via synostoses.
References
- "Color Atlas of Human Anatomy"; Werner Platzer, M.D.; 2008
- Kaiser Permanente Medical Group: Pubic Symphysis Dysfunction
- "Grant's Atlas of Anatomy, Tenth Edition"; Anne M. R. Agur, Ph.D., et al., Editors; 1999
