Osteoporosis

Osteoporosis is a skeletal disorder of low bone mass and deteriorated bone microarchitecture that reduces bone strength and predisposes to fragility fractures — fractures from forces that would not break a healthy bone. It is the most common metabolic bone disease and a leading cause of fracture in older adults, especially postmenopausal women.

Definition

Osteoporosis is a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture (MeSH: Osteoporosis).

Scope

This entry covers what osteoporosis is, how it develops, who it affects, and how it is recognized as a clinical entity. It explains the loss of bone mass and quality, the central importance of fragility fracture, and the place of osteoporosis within metabolic bone disease. It is reference-educational and does not provide individualized diagnostic or treatment recommendations; pharmacotherapy is treated in a separate entry.

Core questions

• What distinguishes osteoporotic bone from normal bone?
• Why does bone mass decline with age and after menopause?
• What makes a fracture a fragility (osteoporotic) fracture?
• Which skeletal sites are most affected, and why do they matter?
• How is osteoporosis defined and recognized as a clinical entity?

Key concepts

• Low bone mass and bone microarchitecture
• Bone strength and bone quality
• Fragility (low-trauma) fracture
• Hip and vertebral fracture
• Postmenopausal estrogen deficiency
• Age-related bone loss
• Secondary osteoporosis
• T-score and densitometric definition

Mechanisms

Osteoporosis arises when bone resorption persistently exceeds bone formation during remodeling, so that bone mass falls and the trabecular and cortical architecture is degraded. Estrogen deficiency after menopause accelerates this imbalance by increasing osteoclast activity, in part through the RANK/RANKL/osteoprotegerin system that governs osteoclast formation (Hofbauer et al., 2000). With aging, declining bone formation and accumulating microdamage further reduce bone strength. Because bone strength reflects both mass and quality, fractures can occur even when density loss appears modest, and they characteristically involve the hip, spine, and wrist.

Clinical relevance

Osteoporosis is clinically important because it is common, often silent until a fracture occurs, and a major source of disability and dependence in older people. Recognizing it as a systemic disease of bone strength, rather than simply low density on a scan, frames how risk is understood. This description is educational and is not a substitute for clinical assessment or individualized care.

Epidemiology

Osteoporotic fractures impose a large and growing public-health burden: in the United States more than two million such fractures were estimated for 2005, with substantial associated costs projected to rise as the population ages (Burge et al., 2007). The condition disproportionately affects postmenopausal women and older adults of both sexes, and hip and vertebral fractures carry the greatest morbidity and mortality (Compston et al., 2019).

Evidence & guidelines

Authoritative clinician guidance, such as the National Osteoporosis Foundation Clinician's Guide (Cosman et al., 2014), frames osteoporosis as a disease to be identified through bone density, clinical risk factors, and fracture history together. Such guidance synthesizes observational epidemiology and trial evidence; the present entry summarizes concepts and does not reproduce specific clinical recommendations.

History

Osteoporosis was long regarded as an inevitable feature of aging until twentieth-century work distinguished it as a definable disease of bone strength. The introduction of bone densitometry and a densitometric (T-score) definition gave it operational criteria, and the later elucidation of estrogen's role and of the RANKL/osteoprotegerin pathway clarified its biology and pointed toward targeted therapies.

Key figures

• Juliet Compston
• B. Lawrence Riggs
• Lorenz Hofbauer
• Felicia Cosman

Seminal works

• compston-2019
• burge-2007
• hofbauer-2000

Frequently asked questions

Is osteoporosis the same as having low bone density on a scan?

Low bone mineral density is the densitometric hallmark, but osteoporosis is a disease of reduced bone strength that also involves bone microarchitecture and quality, which is why fragility fractures can occur across a range of density values.

What is a fragility fracture?

It is a fracture that results from a force, such as a fall from standing height, that would not normally break healthy bone; the hip, spine, and wrist are characteristic sites.

Why are postmenopausal women especially affected?

The fall in estrogen after menopause accelerates bone resorption relative to formation, producing rapid bone loss in the years following menopause.