203 Diseases and Disorders of the Skeletal System


Osteoporosis, the result of reduced bone mineral density, can lead to an increased risk for fractures.

Learning Objectives

Differentiate among primary type 1, primary type 2, and secondary osteoporosis

Key Takeaways

Key Points

  • The three main mechanisms by which osteoporosis develops are an inadequate peak bone mass (the skeleton develops insufficient mass and strength during growth), excessive bone resorption, and inadequate formation of new bone during remodeling.
  • Primary type 1 osteoporosis is the most common type in post-menopausal women. Primary type 2 or senile osteoporosis is common in both men and women over age 75.
  • Secondary osteoporosis may arise at any age as the result of predisposing medical problems or prolonged use of of medications such as glucocorticoids.
  • Primary type 2 osteoporosis is twice as prevelent in females as in males, but secondary osteoporosis affects men and women equally.
  • Risk of osteoporosis can be reduced by exercises such as walking and by taking vitamin D supplements.
  • Fragility fractures are most common in the vertebrae, hip and wrist joints, and ribs.

Key Terms

  • peak bone mass: Peak bone mass is the average bone mass of healthy, young adults.
  • fragility fractures: Fragility fracture is a type of pathologic fracture that occurs as result of normal activities, such as a fall from standing height or less, suggesting a weakness of the skeleton.


Women who are thin are at higher risk of developing osteoporosis than are heavier women. Weight-bearing exercise such as walking helps maintain bone strength. Heavier women require their skeletons to bear more weight, so their bones are stronger than is the case for thin women. Ballet dancers are at particular risk, as are women who diet to be as thin as these dancers. Gwyneth Paltrow is an example of such a woman. In 2010 at age 37, she found out that she had osteopenia, the early stage of osteoporosis.

This image shows an elderly woman walking. You can tell she has osteoporosis because of the way she is hunched over, colloquially referred to as a "dowager's hump."

Woman with Osteoporosis: Hyperkyphosis, colloquially called a “dowager’s hump,” refers to an anteriorly concave curvature of the thoracic region of the spine.

Osteoporosis is a disease of the bones that leads to an increased risk of fracture. In osteoporosis, the bone mineral density (BMD) is reduced, bone microarchitecture deteriorates, and the amount and variety of proteins in bone is altered. The three main mechanisms by which osteoporosis develop are an inadequate peak bone mass (the skeleton develops insufficient mass and strength during growth), excessive bone resorption, and inadequate formation of new bone during remodeling. An interplay of these three mechanisms underlies the development of fragile bone tissue. The form of osteoporosis most common in women after menopause is referred to as primary type 1 or postmenopausal osteoporosis. Primary type 2 osteoporosis or senile osteoporosis occurs after age 75 and is seen in both females and males at a ratio of 2:1. Finally, secondary osteoporosis may arise at any age and affect men and women equally. This form of osteoporosis results from chronic predisposing medical problems or disease, or prolonged use of medications such as glucocorticoids.

Treatment of Osteoporosis

Osteoporosis risks can be reduced with lifestyle changes and sometimes medication, or treatment may involve both. Lifestyle change includes diet, exercise, and preventing falls. Medication includes calcium, vitamin D, bisphosphonates, and several others. Fall-prevention advice includes exercise to tone deambulatory muscles, proprioception-improvement exercises, and equilibrium therapies. Exercise, with its anabolic effect, may at the same time stop or reverse osteoporosis.

Risks of Osteoporosis

Osteoporosis itself has no symptoms. Its main consequence is the increased risk of bone fractures. Osteoporotic fractures are those that occur in situations where healthy people would not normally break a bone. They are, therefore, regarded as fragility fractures. Typical fragility fractures occur in the vertebral column, rib, hip, and wrist.

Paget’s Disease

Paget’s disease is a chronic bone disorder that causes affected bones to become large and misshapen.

Learning Objectives

Differentiate Paget’s disease from osteoporosis

Key Takeaways

Key Points

  • Men are more commonly affected by Paget’s disease than women (ratio of 3:2) and people are not usually diagnosed before the age of 40.
  • The process of bone remodeling is disrupted so that excessive bone resorption is followed by chaotic bone growth.
  • Patients require vitamin D and calcium supplementation, along with moderate exercise (in the sun) to maintain joint mobility, but there is no cure.

Key Terms

  • Paget’s disease: Paget’s disease of bone is a chronic disorder that can result in enlarged and misshapen bones.


Along with osteomalacia, rickets, and osteoporosis, Paget’s disease is associated with vitamin D deprivation. If one chooses to avoid exposing unprotected skin to the UVB radiation of the sun, then one must compensate for this lack by taking vitamin D supplements or risk the development of one of these diseases.

Paget’s disease of bone is a chronic disorder that can result in enlarged and misshapen bones. The excessive breakdown and formation of bone tissue causes affected bone to weaken, resulting in pain, misshapen bones, fractures, and arthritis in the joints near the affected bones. Paget’s disease typically is localized, affecting just one or a few bones, as opposed to osteoporosis, for example, which affects all the bones in the body. Although there is no cure for Paget’s disease, medications (bisphosphonates and calcitonin) can help control the disorder and lessen pain and other symptoms.

This image is an X-ray of the pelvis with Paget's disease. While the right pelvis looks normal, the left pelvis has much more visible outlines and definition, which indicates Paget's disease.

Paget’s disease of the bone: Paget’s disease of bone is shown in the left pelvis. Notice the seeds in the prostate after brachytherapy.

Diagnosis of Paget’s Disease

Paget’s disease is rarely diagnosed in people less than 40 years of age. Men are more commonly affected than women (ratio of 3:2). Prevalence of Paget’s disease ranges from 1.5 to 8.0 percent, depending on age and country of residence. Because early diagnosis and treatment is important, after age 40, siblings and children of someone with Paget’s disease may wish to have an alkaline phosphatase blood test every two or three years. If the alkaline phosphatase level is above normal, other tests such as a bone-specific alkaline phosphatase test, bone scan, or X-ray can be performed.

Initially, in Paget’s disease there is a marked increase in the rate of bone resorption at localized areas caused by large and numerous osteoclasts. These localized areas of osteolysis are seen radiologically as an advancing lytic wedge in long bones or osteoporosis circumscripta in the skull. The osteolysis is followed by a compensatory increase in bone formation induced by osteoblasts recruited to the area. This is associated with accelerated deposition of lamellar bone in a disorganized fashion. This intense cellular activity produces a chaotic picture of trabecular bone (“mosaic” pattern), rather than the normal linear lamellar pattern. The resorbed bone is replaced and the marrow spaces are filled by an excess of fibrous connective tissue with a marked increase in blood vessels, causing the bone to become hypervascular. The bone hypercellularity may then diminish, leaving a dense “pagetic bone,” also known as burned-out Paget’s disease.

This image is a highly magnified look at a bone with Paget's disease. Intense cellular activity produces the jigsaw puzzle-like pattern shown in this image, rather than the regular linear lamellar pattern in healthy bones.

Paget’s disease bone tissue: High magnification micrograph of Paget’s disease of the bone. Notice the prominent jigsaw-puzzle like pattern.


In general, patients with Paget’s disease should receive 1000–1500 mg of calcium, adequate sunshine, and at least 400 units of vitamin D daily. Exercise is very important in maintaining skeletal health, avoiding weight gain, and maintaining joint mobility. Since undue stress on affected bones should be avoided, patients should discuss any exercise program with their physicians before beginning.


Icon for the Creative Commons Attribution 4.0 International License

Boundless Anatomy and Physiology Copyright © by Lumen Learning is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Share This Book