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Orthopedics => Bone


Bone (anatomy), hard connective tissue, the major component of almost all skeletal systems in adult vertebrate animals. Bone appears to be nonliving-in fact, the word skeleton is derived from a Greek word meaning dried up. However, bone actually is a dynamic structure composed of both living tissues, such as bone cells, fat cells, and blood vessels, and nonliving materials, including water and minerals.

Bones are multipurpose structures that play diverse, vital roles in vertebrates. They provide a framework for the body, supporting it and giving it shape. They also provide a surface for the attachment of muscles and act as levers, permitting many complex movements. Many bones protect softer internal organs; for example, skull bones protect the brain, and rib bones form a cage around the lungs and heart. In addition to these structural and mechanical functions, bones also participate in the body's physiology. They store calcium, a mineral essential for the activity of nerve and muscle cells. The soft core of bone, the bone marrow, is the site of formation of red blood cells, certain white blood cells, and blood platelets.
An adult human has 206 bones, which account for 14 percent of the body's total weight. The longest and strongest bone is the thighbone, which at maturity is about 50 cm (20 in) long and 2.5 cm (1 in) wide. The smallest bone, the stirrup bone, is one of three tiny bones buried within the middle ear; it is only 0.18 cm (0.07 in) long.

Bone consists of living cells widely scattered within a nonliving material called the matrix. The matrix is formed by osteoblasts, cells that are constantly renewed in the bone. Osteoblasts make and secrete the protein collagen, which makes bones elastic so that they can give under the stresses generated by walking, lifting, and other activities. Osteoblasts also secrete mineral salts formed from calcium and phosphorus, which impart hardness so that bones do not break easily. If more bone is needed, new osteoblasts carry out the task of building it. As bone tissue matures, osteoblasts transform into osteocytes, mature bone cells that carry out daily cellular activities.
There are two main types of bone. Compact bone, which makes up most of the bone of arms and legs, is very dense and hard on the outside. The structural units of compact bone are osteons, elongated cylinders that act as weight-bearing pillars, able to withstand any mechanical stress placed on the bone. The center of each osteon contains a hollow canal that acts as a central passageway for blood vessels and nerves.

In some bones, internal to the compact bone is spongy bone, also known as cancellous bone, composed of a honeycomb network of bones called trabeculae that act as supporting beams. Spongy bone is designed to bear stress from several directions, such as that exerted on the pelvis in bending or stretching. The spaces between the trabeculae are filled with red bone marrow containing the blood vessels that nourish spongy bone. Spongy bone is found in bones of the pelvis, ribs, breastbone, vertebrae, skull, and at the ends of the arm and leg bones.

Surrounding both compact and spongy bone is a thin membrane, the periosteum. The outer layer of this membrane contains nerves and blood vessels that branch and travel into the bone. The inner layer of the periosteum consists mainly of osteoblasts.

The point where two or more bones come together is called a joint, or articulation. Different kinds of joints enable different ranges of motion. Some joints barely move, such as those between the interlocking bones of the skull. Other bones, held together by tough connective tissues called ligaments, form joints such as the hinge joint in the elbow, which permits movement in only one direction. The pivot joint between the first and second vertebrae allows the head to turn from side to side.

Intimately associated with bone is another type of connective tissue called cartilage. Cartilage is softer, more elastic, and more compressible than bone. It is found in body parts that require both stiffness and flexibility, such as the ends of bones, the tip of the nose, and the outer part of the ear.

During the early development of a baby within its mother's body, the skeletal structure consists of cartilage. At about the eighth week of fetal development, calcium and phosphorous salts begin to deposit around the cartilage. At 40 weeks of development, however, the fetal bones still consist primarily of soft cartilage. The skull consists of several cartilage plates that are not completely joined. The spaces between the cartilage plates are called soft spots, or fontanels. The soft cartilage and the fontanels enable the skull to be compressed as it passes through the birth canal. During childhood, cartilage gradually is replaced by bone through the activity of osteoblasts. More than 300 bones are present in an infant, several of which fuse as the infant matures.

Throughout life, bone tissue undergoes continual breakdown and restoration in response to the body's demands. For example, calcium must always be present in blood at a certain concentration. If calcium blood levels drop, cells called osteoclasts break down bone to release calcium into the bloodstream. If exercise increases muscle mass, bones must thicken so that the pull of the stronger muscle does not break the bone. In this case, osteoblasts create new bone.

During childhood and adolescence, much more bone tissue is deposited than broken down, so the skeleton grows in size and strength. During early adulthood, breakdown slowly begins to exceed deposits. As a person ages, bone tissue is depleted, and bones are weakened and increasingly susceptible to breaks. Exercise and proper diet are critical for maintaining healthy bone growth at all stages of life. Nutrients-particularly sufficient calcium, phosphorus, and vitamin D, and hormones-including growth hormone, parathyroid hormone, calcitonin, and sex hormones-all influence bone growth.

Fractures, or breaks, are very common injuries to bones. The repair process requires the interplay of several processes. About a week after a fracture occurs, cells from the periosteum invade the damaged area and produce a fibrous network. Then, other cells produce cartilage in the network. Finally, osteoblasts enter the network and convert the cartilage to bone. Complete healing may take weeks or even months, depending on the individual's general health, age, and other factors. Some fractures are treated with a splint, a firm object that supports the area surrounding the broken bone and restricts motion. Other fractures must be completely immobilized to heal because movement can cause a new fracture in the area. These fractures may be immobilized with a cast, plastic or plaster wrapped around the area that surrounds the broken bone.

A variety of diseases affect bones. One of the most common bone diseases is osteoporosis, which is characterized by a thinning of bone tissue, causing bones to become weak, brittle, and prone to fractures. Many factors can cause osteoporosis, including menopause, lack of exercise, low calcium intake, smoking, use of steroid drugs, and excessive consumption of alcohol.

Dietary deficiencies of calcium, phosphorus, and vitamin D cause rickets, a disease characterized by abnormal bone formation and skeletal deformities. Rickets is most common in children. Dietary deficiencies of these nutrients in adults-or metabolic disorders that cause poor absorption of the nutrients-can result in an abnormal softening of bone, a condition called osteomalacia.

Infections of bones called osteomyelitis usually are caused by bacteria, especially Staphylococcus, which enters the body through open wounds and may destroy bone tissues. Tumors, or abnormal growths, occur in bone tissue, though most are benign. Cancerous tumors can be caused by excessive radiation; many radioactive substances have an affinity for bone, particularly the marrow, and are readily stored there. Most cancerous tumors in bones, however, are tumors that spread from cancer in other parts of the body. Cancers that arise in bone, cartilage, and other connective tissues are called sarcomas.