Bone is living tissue that carries load and resists the forces of daily life. Its strength depends on how much mineral it holds and on how its internal structure is built. Osteoporosis is a disorder of low bone mass and deteriorated bone structure. When both fall, bone breaks under less force than a healthy bone survives, so osteoporosis matters clinically mainly through the fractures it causes.
The disorder is silent, so it usually shows itself through a fracture. Counting fractures is therefore how its burden is measured, and its epidemiology is largely the epidemiology of those fractures.
What counts as a case?
Fractures are the harm, so the count has to say which fractures and which measurement it includes. The number of people affected depends on how the disorder is defined.
Osteoporosis is defined by the presence of either or both of two things: osteoporotic fractures or low bone mineral density. Osteoporotic fractures are hip, vertebral compression, wrist, humerus or tibial fractures. Low bone mineral density is the amount of mineral in a given area or volume of bone, measured by a scan.
Both count because both mark a weak bone. A bone with low mineral density and disrupted structure is weaker, and the risk of fragility fracture rises with that weakness. So either a fracture or low mineral density is a sign that the bone is weak.
Why is the hip the clearest marker?
Fractures come at several sites, and one site stands out in the count. Hip fractures are the clearest marker of the disorder.
Accidental falls and trips can cause hip fractures. These magnify force on a small area of the upper femur, so a femur weakened by osteoporosis can give way there. Virtually all hip fractures are attributable to osteoporosis, whether primary or secondary.
Current estimates are about 1.7 million hip fractures a year in Europe and about 0.3 million a year in the United States.
Who is affected most?
Once the hip stands out, the next question is who breaks it. Hip fractures are about twice as common in women as in men, and vertebral fractures are also about twice as common in women as in men.
Most postmenopausal women with osteoporosis have bone loss linked to estrogen deficiency. That is why the disorder falls more heavily on women. Men are far from spared: more than 5 million American men are estimated to be affected by osteoporosis.
How have the numbers changed?
The count of who breaks is not fixed in time, and the trend in these fractures has not been steady. Annual hip fracture rates declined through the early 2000s. The decline occurred when effective treatments and clear prevention messaging became widely available.
From about 2015 hip fracture rates flattened and began trending upwards again. Part of that reversal is attributed to widespread reporting of atypical femoral fractures in patients treated with bisphosphonates and denosumab. That reporting was followed by a fall in prescribing.
Why is the burden easy to underestimate?
Even the counts that exist fall short of the harm. The disorder is silent, and a vertebral fracture can be seen on imaging without prompting anyone to look for the disorder behind it, so the harm is easy to miss. The burden is easy to underestimate relative to the harm it causes.
In women, fragility fractures (fractures occurring with little force) are more numerous than myocardial infarction, stroke and invasive breast cancer combined.
What follows a fracture, and what happens to treatment?
The harm does not end at the fracture. Hip fracture carries a mortality of 20–24% within the first year.
Treatment is frequently abandoned. More than 70% of individuals diagnosed with osteoporosis do not adhere to treatment beyond one year.
Which people are most likely to fracture?
Gaps in treatment lead to a practical question: which people are most likely to fracture, and what makes them so?