Porth's Essentials of Pathophysiology, 4e

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Musculoskeletal Function

U N I T 1 2

Osteopenia Osteopenia is a condition that is common to all meta- bolic bone diseases. It is characterized by a reduction in bone mineral density greater than expected for age, race, or gender, and occurs because of a decrease in bone formation, inadequate bone mineralization, or excessive bone deossification. 1 Osteopenia is not a diagnosis but a term used to describe an apparent loss of bone density seen on x-ray studies. 2 Approximately 60% of bone is mineral content, approximately 30% is organic matrix, and the remainder is living bone cells. Osteopenia can involve a decrease in bone matrix due to an imbalance between bone formation and destruction, or a decrease in mineralization. The major causes of osteopenia are osteoporosis, osteomalacia, malignancies such as mul- tiple myeloma, and endocrine disorders such as hyper- parathyroidism and hyperthyroidism. Osteoporosis Osteoporosis is a metabolic bone disease characterized by decreased bone density (i.e., increased porosity) and strength in which both the bone matrix and mineraliza- tion are decreased. 3–6 The World Health Organization has defined osteoporosis as a bone mineral density (BMD) value greater than 2.5 standard deviations (SD) below the mean for a young adult reference population. 7 The most useful methods of estimating fracture risk are BMD testing and consideration of clinical risk factors for fracture. 8 Fracture risk assessment tests, such as the International Osteoporosis Foundation one-minute osteoporosis risk test, are available online to estimate the fracture probability. 9 Osteoporosis can occur as the result of a number of disorders, but is most often associated with the aging process. In the United States alone, osteoporosis affects approximately 10 million persons aged 50 years or older, and an additional 34 million have low bone mass (osteopenia) and are potentially at risk for development of osteoporosis and its complications. 10 Etiology and Pathogenesis The cause of osteoporosis remains largely unknown, but most data suggest an imbalance between bone resorp- tion and formation such that bone resorption exceeds bone formation. Although both of these factors play a role in most cases of osteoporosis, their relative contri- bution to bone loss may vary depending on age, gen- der, genetic predisposition, activity level, and nutritional status. Under normal conditions, bone mass increases steadily during childhood, reaching a peak in the young adult years. The peak bone mass, or BMD, is an impor- tant determinant of the subsequent risk for osteoporo- sis. It is determined in part by genetic factors, hormone (estrogen) levels, exercise, calcium intake and absorp- tion, and environmental factors (Chart 44-1). Genetic factors are linked, in largest part, to the maximal amount of bone in a given person, referred to as peak

CHART 44-1   Risk Factors Associated with Osteoporosis* Personal Characteristics Advanced age Female Gender Ethnicity (white or Asian) Small bone structure/low body weight Postmenopausal Family history

Lifestyle Sedentary Calcium/Vitamin D deficiency High-protein diet Excessive alcohol intake Excessive caffeine intake Smoking

Drug and Disease Related Aluminum-containing antacids Anticonvulsants Heparin Corticosteroids or Cushing disease Gastrectomy Celiac disease Diabetes mellitus Anorexia nervosa/female athlete triad Hyperthyroidism

Hyperparathyroidism Rheumatoid arthritis

*Not exclusive

bone mass. Race is a key determinant of BMD and the risk of fractures. Incidence rates obtained from stud- ies among racial and ethnic groups demonstrate that although women have higher fracture rates compared with men overall, these differences vary by race and age. White and Asian women had higher rates for all age groups older than 50 years. 10 The highest BMD values and lowest fracture rates have been reported for black women. 10 Body size is another factor affecting the risk of osteoporosis and risk of fractures. Women with smaller body builds are at increased risk of hip fracture because of lower hip BMD. Hormonal factors play a significant role in the devel- opment of osteoporosis, particularly in postmeno- pausal women. 11 Postmenopausal osteoporosis, which is caused by an estrogen deficiency, is manifested by a loss of cancellous bone and a predisposition to frac- tures of the vertebrae and distal radius. The loss of bone mass is greatest during early menopause, when estrogen levels are withdrawing. Several factors appear to influ- ence the increased loss of bone mass associated with an estrogen deficiency, including an increased secretion of cytokines by monocytes and bone marrow cells. These cytokines stimulate osteoclast recruitment and activity by increasing the levels of RANK ligand (RANKL) while