The musculoskeletal system comprises a family of related tissues that interact with each other to perform work and also serve to support and protect the other organ systems of the body. The functional demands of normal human activities impose a broad spectrum of mechanical problems. Human joints permit motion to occur between adjacent bones. Fibrous joints, or amphiarthroses, are the second major type of union between adjacent bones. These joints meet a different functional need. The intervertebral disks provide the third major example of a fibrocartilaginous joint. The stability of the ankle and humeroulnar joints do not depend solely on their shape. Like other “hinge” joints, these articulations are reinforced on both sides by strong collateral ligaments. A final contribution to joint stability is that provided by the synovial fluid. Opposing surfaces of articular cartilage are normally separated by only a film of this “boundary layer” lubricant.