Osteoporosis is a metabolic bone disease in which bones lose density and strength, making them fragile enough to break from minor falls or everyday movements. The World Health Organization defines it by a specific measurement: a bone density score (called a T-score) of -2.5 or lower, compared to the bone density of a healthy young adult. About 460,000 deaths worldwide were attributed to osteoporosis and low bone mass in 2021, making it one of the most significant skeletal conditions globally.
How Bone Loss Actually Happens
Your bones are not static. Throughout your life, your body continuously breaks down old bone tissue and replaces it with new bone in a process called remodeling. Two types of cells drive this cycle: one type removes old bone, and another type builds new bone in its place. In healthy bone, these two processes stay roughly in balance, so the total amount of bone remains stable.
Osteoporosis develops when that balance tips. The cells removing bone outpace the cells building it, so more bone is broken down than replaced. Over months and years, this leaves bones increasingly porous and thin. The internal structure of bone, which normally looks like a dense honeycomb, develops larger and larger gaps. That’s what makes the bone weaker and more prone to fracture.
Why It’s Called a Silent Disease
Osteoporosis typically causes no pain or noticeable symptoms while bone is being lost. Most people have no idea their bones are thinning until one breaks. This is why it’s often called a “silent disease.” You can lose significant bone density over years without feeling any different.
Once bones have weakened enough, the signs tend to show up as fractures or their consequences. The most recognizable include back pain from a collapsed vertebra in the spine, gradual loss of height over time, a stooped or hunched posture, and bones that break from surprisingly minor impacts. Spinal vertebrae can weaken so much that they crumple under the body’s own weight, sometimes without any fall or injury at all.
The Most Vulnerable Bones
Not every bone in the body is equally affected. Fragility fractures, defined as breaks caused by low-energy trauma like falling from standing height, occur most often in four areas: the spine, the hip, the wrist, and the upper arm near the shoulder. Hip fractures tend to be the most serious, often requiring surgery and extended recovery. Spinal fractures are the most common and can accumulate silently, each one compressing the spine a little more and contributing to height loss and curvature.
Primary vs. Secondary Osteoporosis
Primary osteoporosis is the most common form and develops as a natural consequence of aging or hormonal changes. It comes in two patterns. The first is tied to menopause: the drop in estrogen accelerates bone loss, particularly in the spongy interior bone tissue of the spine and wrists. This form affects women far more than men and typically appears in the years following menopause. The second pattern, sometimes called senile osteoporosis, shows up after age 70 in both men and women. It affects both the spongy interior and the dense outer shell of bones, and hip and pelvic fractures become more frequent.
Secondary osteoporosis has a different origin. It results from other medical conditions, medications, or lifestyle factors that interfere with bone metabolism. Long-term use of certain anti-inflammatory steroids is one of the most well-known causes. Rheumatoid arthritis, hormonal disorders, and conditions that impair nutrient absorption can also drive bone loss. In younger people, secondary osteoporosis can prevent bones from ever reaching their full potential strength in the first place.
How Osteoporosis Is Diagnosed
Diagnosis relies on a painless scan called a DXA (dual-energy X-ray absorptiometry), which measures bone mineral density at key sites like the hip and spine. The result is reported as a T-score, which compares your bone density to that of a healthy 30-year-old of the same sex. The WHO classifies the results into four categories:
- Normal: T-score of -1.0 or higher
- Low bone mass (osteopenia): T-score between -1.0 and -2.5
- Osteoporosis: T-score of -2.5 or lower
- Severe osteoporosis: T-score of -2.5 or lower plus at least one fragility fracture
A T-score of -2.5 means your bone density is 2.5 standard deviations below the young-adult average. In practical terms, the lower the number, the weaker the bone and the higher the fracture risk. Osteopenia isn’t osteoporosis, but it signals that bone density is heading in the wrong direction.
Assessing Fracture Risk Beyond the Scan
Bone density alone doesn’t tell the full story. A widely used tool called FRAX calculates your 10-year probability of a major fracture by combining your T-score with other risk factors: age, sex, body mass index, history of a prior fracture, whether a parent fractured a hip, smoking status, alcohol intake, long-term steroid use, and rheumatoid arthritis. FRAX can even estimate fracture risk without a bone density scan, which is useful in areas where DXA machines aren’t readily available. This makes it possible to identify people who need treatment even if their T-score alone wouldn’t trigger a diagnosis.
Global Impact
Osteoporosis is not distributed evenly around the world. In 2021, the disease burden (measured as years of healthy life lost) was highest in South Asia, Central Sub-Saharan Africa, and Australasia. India, Saudi Arabia, and Greenland had some of the highest rates per capita. Countries in Central Asia and high-income parts of the Asia Pacific had the lowest rates. These differences reflect a mix of genetics, diet, physical activity patterns, and access to preventive care.
Globally, the condition accounted for an estimated 17.3 million years of healthy life lost in 2021. That number captures not just deaths but the lasting disability that follows fractures, particularly hip fractures in older adults, which often lead to long-term loss of mobility and independence.