DEFINITION

Osteoporosis is a disease characterized by decreased bone mass and strength along with microarchitectural deterioration of bone tissue leading to bone fragility. Osteoporosis affects the entire skeleton. These changes in bone mass and strength ultimately lead to an increased susceptibility to fractures.

Osteoporosis can be classified into two broad categories: primary and secondary. Primary osteoporosis is the most common form of the disease and includes:

• postmenopausal osteoporosis (Type I); 
• age-associated osteoporosis (Type II), affecting a majority of individuals age 70 and older; and 
• idiopathic osteoporosis affecting premenopausal women and middle-aged men.

Secondary osteoporosis is a disease in which an identifiable agent or disease process causes loss of bone tissue and includes:

• inflammatory disorders, 
• disorders of bone marrow cellularity, and 
• endocrine disorders of bone remodeling.

Bone has three components: a collagenous fiber component, a ground substance component, and a cellular component. The collagenous fiber component is organic and is approximately 40% of bone. The ground substance is an inorganic mineral component consisting mostly of calcium-phosphate (Ca++-P). The mineral phase gives bone stiffness, rigidity and hardness. The cellular component consists of osteoblasts and osteoclasts. Osteoblasts form new bone and osteoclasts remove or resorb bone.

There are two major types of bone: compact or cortical bone, and cancellous or trabecular bone. Cortical bone forms the outer shell and cancellous bone forms the inner portion. Some bones have the inner trabecular bone replaced by a medullary cavity or an air space. Cortical bone is 5% to 30% porous, with cancellous bone being 30% to 90% porous.

Bone remodeling occurs throughout life. Osteoclasts resorb bone in microscopic cavities and osteoblasts then reform the bone surfaces to fill in the cavities. Peak bone mass is achieved in the third decade of life. Bone mineralization increases with age causing a decrease in cortical bone density and an increase in average bone porosity. This relates to an overall decrease in bone material strength after age 30. It is estimated that from age 70 on, up to 25% of bone surface may be occupied by resorption. The decline in bone mass that is associated with aging is a remodeling imbalance that results in excess resorption.

The mechanical properties of bone also decrease with age. The strain characteristics of the elderly decreases to approximately one half the amount of strain tolerated in the bones of younger adults.