Chapter 59 Osteoporosis and osteomalacia

Questions

What is the difference between osteoporosis and osteomalacia?

What are the causes of osteoporosis and osteomalacia?

Osteoporosis and osteomalacia are common conditions. Osteoporosis is a decrease in the mass of bone in the presence of normal mineralization, whereas osteomalacia is deficient mineralization of bone (Fig. 3.59.1).

Fig. 3.59.1 Bone dysfunction.

Osteoporosis

Bone is constantly being remodelled by osteoblasts producing new bone and osteoclasts breaking it down. The pathogenesis of osteoporosis is an increase in bone resorption by osteoclasts over bone formation by osteoblasts.

Aetiology

There are many causes of osteoporosis, including disuse, loss of bone mass with age, increased circulating corticosteroids, hyperthyroidism, acromegaly, chronic renal failure, chronic liver failure, malnutrition and malabsorption. The first three of these account for the great majority of cases and are discussed below.

Disuse osteoporosis

Normal bone remodelling depends on the bone being subjected to the normal mechanical forces associated with movement and weight bearing. Therefore, if a body part is immobilized, the total mass of bone reduces. This condition is essentially a kind of disuse atrophy and produces localized osteoporosis. For example, there will be significant bone loss after a few weeks if a patient’s arm is immobilized in plaster following a fracture. The mass of bone will return to normal after the fracture has healed, assuming normal activity of the limb is resumed.

A similar process affects astronauts subjected to long periods of weightlessness. In the microgravity of space, the weight that bones must support is reduced to almost zero and movement does not produce the same stresses as it would on Earth. The result is disuse osteoporosis; the high levels of serum calcium found in astronauts’ blood during spaceflight reflects the increase in bone resorption over production.

Age-related osteoporosis

A systemic loss of bone mass with ageing is known as age-related osteoporosis, senile osteoporosis or, in females, postmenopausal osteoporosis.

The pathophysiology of age-related osteoporosis is not fully understood. However, an important factor determining whether it will become clinical problem in an individual is the peak bone mass. During early adulthood, the total mass of bone in the body reaches a peak (Fig. 3.59.2). After the age of approximately 30, it starts to decline as a predictable ageing phenomenon at a rate of approximately 0.7–1% per year. The total mass of bone in old age is thus a function of the peak bone mass that was achieved in early adulthood. Consequently, regular exercise before age 30 can protect against osteoporosis in later life by increasing peak bone mass.

Fig. 3.59.2 Change in total bone mass with age in the human body.

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