Osteoporosis is a skeletal disorder in which bones become weaker and more fragile, making them more likely to fracture. Bone is living tissue that is continually remodelled: specialised cells remove old bone while other cells form new bone. Healthy bone maintains a balance between resorption and formation and adapts to mechanical loading. Osteoporosis develops when bone mass and structural strength become insufficient relative to the stresses placed on the skeleton. The result is increased fracture susceptibility rather than simply a reduction in bone size. The condition develops gradually, often over years, and may remain clinically silent until a fracture occurs.
Bone density is an important but not complete measure of bone strength. A dual-energy X-ray absorptiometry scan, usually called a DEXA or DXA scan, measures bone mineral density at commonly assessed sites such as the hip and spine. A T-score compares measured density with that of a healthy young-adult reference population. NHS guidance describes a T-score above −1 standard deviation as normal, between −1 and −2.5 as osteopenia, and below −2.5 as osteoporosis. However, fracture risk depends on more than density alone, including age, previous fractures, falls, family history and other clinical risk factors.
Osteopenia means that bone density is lower than the young-adult reference level but does not meet the densitometric definition of osteoporosis. It is not synonymous with osteoporosis and does not inevitably progress to it. Fracture risk exists across a continuum, so a person with osteopenia can still have clinically important fracture risk depending on other factors. Conversely, bone density measurements should not be interpreted as an exact prediction of whether an individual will fracture. Clinicians may therefore combine DXA results with validated fracture-risk tools such as FRAX or Q-Fracture when deciding whether additional investigation or treatment is appropriate.
Osteoporosis commonly affects the hip, wrist and vertebrae, although fractures can occur in many other bones. Vertebral compression fractures may occur after relatively minor forces and can cause persistent back pain, loss of height and progressive forward curvature of the spine. Hip fractures are particularly consequential because they can impair mobility and independence and may require surgery and prolonged rehabilitation. A fracture caused by minimal trauma can therefore be an important clinical clue to previously unrecognised osteoporosis. In some cases, even coughing or sneezing can contribute to a rib or vertebral fracture when bones have become sufficiently fragile.
Osteoporosis is often described as a silent condition because the bone loss itself usually does not cause pain. Pain generally results when a fracture occurs, especially a vertebral fracture. This distinguishes osteoporosis from conditions such as osteoarthritis, which commonly produces joint pain and stiffness. The absence of pain therefore cannot be used to exclude osteoporosis. A person can lose substantial bone density without knowing that anything is wrong until an otherwise minor fall or impact produces a fracture. This silent progression is one reason risk assessment and targeted investigation are important in people with significant risk factors.
The condition results from an imbalance in bone remodelling. Peak bone mass is generally achieved in early adulthood, after which bone gradually becomes less dense with age. The rate of loss differs considerably between individuals. Hormones, nutrition, physical activity, genetics, medications and medical conditions all influence this trajectory. Oestrogen has a particularly important role in maintaining bone density, so bone loss can accelerate around and after menopause. Testosterone also contributes to skeletal health, meaning that men with low testosterone can have increased risk. Osteoporosis can therefore occur in men and women and is not exclusively an older women's disease.
Diagnosis involves assessment of fracture risk and, where indicated, bone density. A clinician may consider age, sex, previous fractures, family history, body mass index, medicines and medical conditions before arranging a DXA scan. Treatment is not determined by the scan result alone; the overall probability of future fracture is important. Bone-strengthening medicines include bisphosphonates and, in selected circumstances, denosumab, raloxifene, teriparatide or romosozumab. Adequate calcium and vitamin D are also important, although supplements are not a universal substitute for pharmacological treatment when fracture risk is high.
Prevention and treatment overlap because both aim to preserve skeletal strength and reduce fractures. Weight-bearing and resistance exercise stimulate bone adaptation, while adequate calcium and vitamin D support mineralisation and normal bone physiology. Smoking, heavy alcohol consumption, prolonged inactivity and certain medicines can increase risk and may need modification where clinically appropriate. Preventing falls becomes increasingly important once fracture risk is elevated because stronger bones alone cannot eliminate the consequences of a fall. Osteoporosis is therefore best understood as a disorder of skeletal strength and fracture susceptibility influenced by ageing, hormones, genetics, lifestyle and disease rather than simply as "thin bones".
