Osteoporosis and Fracture Risk

Osteoporosis is a systemic skeletal disorder of low bone mass and microarchitectural deterioration that weakens bone and raises the likelihood of fragility fractures. In older adults it is a leading cause of hip, vertebral, and wrist fractures, and fracture risk depends not only on bone mineral density but also on age, falls, and other clinical factors.

Definition

Osteoporosis is a skeletal disorder characterized by reduced bone mineral density and deterioration of bone microarchitecture, leading to increased bone fragility and susceptibility to low-trauma (fragility) fracture; operationally it is often defined by a bone mineral density T-score at or below -2.5 standard deviations relative to a young-adult reference.

Scope

The entry covers the definition and diagnostic basis of osteoporosis, the biology of bone loss with aging, and the concept of fracture risk as a combination of bone density and clinical risk factors. It is a reference topic within common geriatric conditions and does not provide treatment or screening instructions.

Core questions

• How is osteoporosis defined and measured?
• Why does bone mass decline with age and after menopause?
• Why is fracture risk more than bone mineral density alone?
• Which fractures carry the greatest burden in older adults?

Key concepts

• Bone mineral density and the T-score
• Bone remodeling imbalance (resorption exceeding formation)
• Fragility (low-trauma) fracture
• Clinical risk factors beyond density
• Postmenopausal versus age-related bone loss
• Fracture risk assessment

Mechanisms

Bone is continuously remodeled by osteoclast-mediated resorption and osteoblast-mediated formation. With aging and, in women, estrogen withdrawal at menopause, resorption outpaces formation, reducing bone mineral density and degrading trabecular and cortical microarchitecture. The resulting loss of bone strength, compounded by an increased tendency to fall in older adults, raises the probability of fragility fracture. Because density captures only part of bone strength, fracture risk also reflects clinical factors such as age, prior fracture, and propensity to fall.

Clinical relevance

Osteoporosis and fracture risk are central to geriatric care because fragility fractures, especially of the hip and spine, are major contributors to disability, loss of independence, and mortality in older adults. Understanding how density and clinical risk factors combine helps in appraising evidence on screening and prevention. This entry is for reference and education and is not a basis for individual diagnostic or treatment decisions.

Epidemiology

Osteoporosis prevalence rises steeply with age and is higher in women, largely owing to accelerated bone loss after menopause. Fragility fractures are common in older populations, and hip fractures in particular are associated with substantial morbidity and excess mortality. Vertebral fractures are frequent but often clinically silent, so their prevalence is underestimated.

History

The modern operational definition of osteoporosis based on bone mineral density was framed by a World Health Organization working group in 1994, which set the T-score thresholds still used to classify osteopenia and osteoporosis. Subsequent work emphasized that density alone underestimates who will fracture, leading to integrated fracture-risk assessment that combines density with clinical risk factors, as synthesized by Kanis and colleagues.

Debates

Should fracture risk be based on bone density or on absolute risk?

Density-based diagnosis identifies low bone mass but misclassifies many who fracture; combining density with clinical risk factors to estimate absolute fracture probability is argued to better target risk, though thresholds and inputs remain debated.

Key figures

• John A. Kanis
• Sundeep Khosla
• Dennis M. Black

Seminal works

• who-1994
• kanis-2002
• rachner-2011

Frequently asked questions

Is osteoporosis the same as low bone density?

Low bone mineral density is the central measurable feature and is used to define osteoporosis, but osteoporosis also involves deterioration of bone microarchitecture, and fracture risk depends on clinical factors beyond density.

Why is fracture risk not determined by bone density alone?

Many fractures occur in people whose density is not in the osteoporotic range, because age, prior fracture, and tendency to fall also influence the chance of a fragility fracture; risk is therefore assessed by combining density with clinical factors.