Bone Homeostatic Imbalance

Bone Homeostatic Imbalance

When imbalances occur between bone deposition and bone resorption, a variety of diseases can affect the skeleton such as rickets (in children), osteo-malacia (in adults), and osteoporosis (primarily in elderly women).

Rickets

Rickets is a disease in children that is nearly identical to osteomalacia in adults. However, rickets is more severe because the bones of children are actively growing­. It usually causes bowing of the legs and deformities of the pelvis (FIGURE 7- 14), rib cage, and skull. Because the epiphyseal plates are unable to calcify, they widen continually. Ends of long bones become abnormally long and visibly enlarged. ­Rickets, as well as osteo-malacia, is caused by insufficient dietary calcium or by vitamin D deficiency. Both disorders can be cured by exposure to sunlight (which stimulates formation of vitamin D by the body) and drinking milk that is fortified with vitamin D. Rickets has become very uncommon in the United States because of improved diets and public education. However, if a mother develops osteomalacia due to sun deprivation, nurs-ing her infant will pass her vitamin D deficiency through the breast milk, with the result of the infant developing rickets.

Osteomalacia

Osteomalacia actually describes a variety of disor-ders involving poor bone mineralization. Although osteoid is produced, there is inadequate deposition of calcium salts. The bones become soft and weak. The affected individual may feel pain when weight is put on affected bones (usually the lower spine, pelvis, hips, legs, and ribs). Soft bones are much more likely to fracture than strong, healthy bones. Osteomalacia is not the same as osteoporosis. When there are no actual symptoms in a person with osteomalacia, the condition may still be apparent on X-rays or other diagnostic procedures. Osteomalacia may also result in an abnormal gait, decreased muscle tone, weak-ness, and immobility.

Osteoporosis

Osteoporosis actually defines a group of diseases involving bone resorption that is quicker than bone deposition. The bones become extremely fragile, able to be fractured by walking down stairs or excessive hard sneezing. In osteoporosis, bone mass declines while the composition of the matrix remains the same. The bones become porous and light, with the spongy bone of the spine being most ­vulnerable. As a result, compression fractures of the vertebrae often occur (FIGURE 7-15). Also, the neck of the femur is very likely to fracture. This is referred to as abroken­ hip.

Osteoporosis is most common in postmenopausal women, although men develop it also. Between the ages of 60 and 70, nearly 30% of American­ women have osteoporosis. By age 80, as many as 70% will develop the disease. The most susceptible group for osteoporosis is Caucasian women, with 30% experi-encing a bone fracture as a result. Normal bone den-sity is maintained by the sex hormones (estrogens in females and androgens in males). Bone density is balanced by promoting deposit of new bone and by restraining the activities of osteoclasts.

In postmenopausal women, estrogen secretion normally slows. Its deficiency greatly contributes to osteoporosis. A postmenopausal woman is most likely to develop osteoporosis when the following factors exist:

■■ She does not exercise sufficiently.

■■ She has a petite body form.

■■ She has abnormal vitamin D receptors.

■■ Her diet is low in calcium and protein.

■■ She has hormone-related conditions such as hyperthyroidism, diabetes mellitus, or low blood levels of thyroid-stimulating hormone.

■■ She smokes, which further reduces estrogen ­levels.

Also, a person at any age can develop osteoporo-sis because of immobility. Men with prostate cancer have a higher risk for osteoporosis because treat-ments for this cancer include androgen-suppressing drugs.