Osteoporosis: What Happens to Bone and Why Fractures Occur

Osteoporosis is a disease in which bone becomes less able to withstand everyday stress. The outside of a bone can still look normal, but inside, its structure has changed. There is less bone tissue, the internal framework is less connected, and the bone is more likely to fracture under forces that healthy bone would usually tolerate.

That is why osteoporosis can sit quietly in someone’s medical history for years. It does not usually cause pain by itself. A person may only find out they have it after a wrist fracture, a vertebral fracture or a hip fracture following relatively minor trauma.

What osteoporosis changes

Bone is not solid all the way through. The dense outer layer, called cortical bone, gives the skeleton much of its rigidity. Inside many bones is trabecular bone, a fine lattice that helps spread load through areas such as the vertebrae, the ends of long bones and the neck of the femur.

In osteoporosis, that trabecular network becomes thinner and less connected. Some of the supporting struts are lost altogether. Cortical bone can also become thinner and more porous. The result is a skeleton that has less reserve when it is loaded suddenly, twisted or compressed.

This is one reason osteoporosis is more than “low calcium”. Calcium is part of bone mineral, but the problem is also structural. Bone strength depends on how much bone is present, how it is arranged and how well the tissue has been maintained over time.

Bone remodelling is where the story starts

Bone is constantly being renewed. Osteoclasts remove old or damaged bone and osteoblasts lay down new bone matrix in its place. Osteocytes, which sit within the bone itself, help sense mechanical strain and influence this ongoing remodelling process.

In healthy adult bone, breakdown and replacement are closely linked. Small areas are resorbed, then rebuilt. With osteoporosis, that balance shifts. Bone resorption begins to exceed bone formation, so each remodelling cycle leaves a little less bone behind.

That loss is gradual, which is why the condition is often silent. The person does not feel individual trabeculae thinning or cortical bone becoming more porous. What becomes visible clinically is the consequence: a fracture occurring more easily than expected.

Why bone loss accelerates

Ageing affects bone remodelling in both women and men. Bone formation becomes less efficient over time, while the skeleton continues to go through normal cycles of resorption and replacement.

After the menopause, falling oestrogen levels can accelerate bone loss because oestrogen normally helps regulate bone turnover. When that protective effect falls, osteoclast activity increases and resorption can outpace formation more quickly.

Osteoporosis can also be secondary to another condition or treatment. Long-term systemic glucocorticoids are an important example because they can reduce bone formation and increase fracture risk. Other causes include hypogonadism, hyperthyroidism, hyperparathyroidism, malabsorption, chronic inflammatory disease and some treatments that reduce sex hormone levels.

This is important when osteoporosis appears earlier than expected, when fractures are recurrent, or when the degree of bone loss does not fit the person’s age. In those situations, the question is not only whether osteoporosis is present, but what may be driving it.

When bone weakness becomes visible

The classic sites for osteoporotic fragility fractures are the vertebrae, hip and wrist. These fractures may follow trauma that would not normally be expected to break healthy bone, such as a fall from standing height.

Vertebral fractures can be particularly easy to miss. Some cause sudden back pain, but others are less obvious and may present later as loss of height or increasing thoracic kyphosis. A patient with known osteoporosis who develops new back pain does not automatically have a vertebral fracture, but it should be considered rather than dismissed as ordinary musculoskeletal pain.

Hip fractures are often more obvious because they usually cause an abrupt loss of mobility and require urgent treatment. The fracture itself is only one part of the problem, though. It can lead to surgery, deconditioning, loss of confidence and a long period of rehabilitation, particularly in older adults.

DXA is important, but it is not the whole diagnosis

A DXA scan measures bone mineral density, usually at the hip and lumbar spine. In postmenopausal women and older men, results are commonly reported as a T-score. A T-score of −2.5 or below is consistent with densitometric osteoporosis, while a score between −1 and −2.5 is described as low bone mass or osteopenia.

The difficulty with focusing too heavily on that number is that fracture risk is not determined by bone mineral density alone. Someone with a T-score above −2.5 may still have a high fracture risk because they are older, have already had a fragility fracture, are taking long-term glucocorticoids or have other important clinical risk factors.

UK practice therefore combines bone density with clinical fracture-risk assessment. NICE recommends tools such as FRAX or QFracture for people with relevant risk factors, and NOGG uses FRAX-based thresholds to help guide whether DXA, treatment or specialist assessment is appropriate.

Treatment follows the biology

Most drug treatment for osteoporosis is aimed at changing the balance between bone resorption and bone formation.

Bisphosphonates such as alendronate, risedronate and zoledronate reduce osteoclast-mediated bone resorption. Denosumab also reduces bone resorption, but through inhibition of RANKL, a signalling pathway involved in osteoclast development and activity.

For some people at very high fracture risk, treatment may involve drugs that stimulate bone formation. Teriparatide, abaloparatide and romosozumab are examples used within specialist pathways. These are not simply “stronger versions” of bisphosphonates; they work differently and are used in selected patients according to fracture risk, previous treatment and clinical history.

Calcium and vitamin D remain important when intake or vitamin D status is inadequate, but they are not a substitute for osteoporosis treatment when specific therapy is indicated. Exercise, nutrition, smoking cessation and avoiding excessive alcohol all support bone health, but established osteoporosis is still a condition that needs proper fracture-risk assessment and follow-up.

What matters in practice

For nurses, osteoporosis often appears as a background diagnosis rather than the main reason someone is in hospital. It still matters. A history of previous fragility fractures, long-term steroid use, new back pain, loss of height or a recent fall may all add to the clinical picture.

If a patient is taking an oral bisphosphonate, check whether they understand how it should be taken. If someone with osteoporosis has fallen, do not assume that being able to move means a fracture has been excluded. If a patient has sustained one fragility fracture, that should also raise the question of whether their future fracture risk has been assessed and whether bone-protective treatment is in place.

Osteoporosis is easiest to understand when it is kept focused on the bone itself. The remodelling balance shifts, bone architecture deteriorates, and the skeleton becomes less resistant to stress. The fractures, scans and medicines all make more sense once that underlying change is clear.

References

National Institute for Health and Care Excellence (NICE) (2012, updated 2017) Osteoporosis: assessing the risk of fragility fracture (CG146). Available at: https://www.nice.org.uk/guidance/cg146

National Osteoporosis Guideline Group (NOGG) (2024) Clinical guideline for the prevention and treatment of osteoporosis. Updated December 2024. Available at: https://www.nogg.org.uk/full-guideline

Royal Osteoporosis Society (2022) What is osteoporosis? Available at: https://theros.org.uk/information-and-support/about-osteoporosis-and-osteopenia/what-is-osteoporosis/

Florencio-Silva, R., Sasso, G.R.S., Sasso-Cerri, E., Simões, M.J. and Cerri, P.S. (2015) ‘Biology of Bone Tissue: Structure, Function, and Factors That Influence Bone Cells’, BioMed Research International, 2015, 421746.

Bolster, M.B. and Silverman, S.L. (2024) ‘Physiology, Bone’, StatPearls. NCBI Bookshelf. Available at: https://www.ncbi.nlm.nih.gov/books/NBK441968/