Answer
The most common bone disease is osteoporosis (literally, “porous
bones”), a severe loss of bone density. It affects especially spongy bone, since this is more metabolically active than dense bone and has more surface area exposed to bone-dissolving osteoclasts (fig. 7.20a). As a result, the bones become brittle and highly subject to pathological fractures from stresses as slight as sitting down too quickly. Fractures occur especially in the hip, wrist, and vertebral column. As the vertebrae lose bone, they become compressed and the spine is often deformed into a condition called kyphosis (“widow’s hump”) (fig. 7.20b, c). Hip fractures are especially serious. Each year, about 275,000 elderly Americans fracture their hips, and about 1 in 5 of these die within a year from complications of the resulting loss of mobility, such as pneumonia and thrombosis.
Work Step by Step
FIGURE 7.20 Spinal Osteoporosis
