Osteoporosis is a disorder in which bone mass and bone microarchitecture both deteriorate. Microarchitecture is how the bone is built inside. In osteoporosis the skeleton becomes fragile and fractures more easily than expected.
What makes bone strong?
Fragile means the bone breaks too easily, so the question is what makes a bone strong in the first place. Bone strength depends on how much bone there is and on how well it is built, as the fracture-risk note explained. Osteoporosis affects both: it reduces how much bone there is and how well it is built.
How does the loss begin?
If strength rests on both, the question is what takes them away. Osteoporosis usually develops when bone remodelling becomes unbalanced. Bone is renewed all the time by remodelling, as the pathophysiology note explained: old bone is resorbed and new bone is laid down in a continuous cycle. Because bone is renewed all the time, a tilt in the balance between removing old bone and laying down new bone is what usually leads to less bone.
What tips the balance?
The tipping has more than one source, and the reader needs to know the two settings before going further. The source of the imbalance separates two main settings: primary and secondary osteoporosis.
Primary osteoporosis: ageing and estrogen loss
The first setting needs no outside cause. Primary osteoporosis arises from the biology of ageing and from the loss of estrogen after the menopause. Estrogen loss after the menopause contributes to primary bone loss, as the pathophysiology note explained. Primary osteoporosis has no external cause.
Secondary osteoporosis: another cause
The second setting has a cause that can be found and named. Secondary osteoporosis arises from another disease, a nutritional deficiency or a drug.
Where to start
Each part of this picture has its own note, so the list follows the order in which a student should read them. Start with the first.
- Epidemiology of osteoporosis: how common osteoporotic fractures are, who they affect most, how their numbers have changed, and why the burden is often underestimated.
- Risk factors for osteoporotic fracture: what raises fracture risk, from low bone density and hip-specific risks to measurable bone-quality measures and the FRAX tool.
- Physiology and pathophysiology of osteoporosis: how peak bone mass is built and lost, how remodelling is controlled, and what shifts the balance toward bone loss in primary and secondary osteoporosis.
- Clinical manifestation of osteoporosis: the consequences of osteoporosis, and how the medical history is used to assess future fracture risk, look for secondary causes and guide the choice of treatment.
- Diagnosis and risk assessment of osteoporosis: the examination, imaging and criteria used to diagnose osteoporosis and to estimate fracture risk.
- Secondary causes of osteoporosis: the diseases, deficiencies and drugs that cause bone loss, and why finding one changes management.
- Treatment of osteoporosis: the measures and drugs that reduce fracture risk, who is offered them, and how treatment is sequenced, monitored and stopped.
So how common are these fractures, and who has them most often?