Osteoporosis: bone breakdown outpacing bone formation
Osteoporosis is a loss of bone mass in which bone breakdown by osteoclasts outpaces bone formation by osteoblasts, raising the risk of fracture.
Remember
In osteoporosis bone is broken down faster than it is formed, so bone mineral density (the amount of bone in each unit of volume) falls and fractures follow minor force. In osteomalacia (soft bone) the bone is poorly hardened with mineral rather than lost.
Memory aid
osteoporosis is porous bone (too little of it), whereas osteomalacia is soft bone
Picture osteoporosis as porous bone, with too little of it, and osteomalacia as soft bone.
Osteoporosis is a metabolic bone disease: the trouble lies in how bone is turned over, rather than in an infection, a tumour or a joint. Bone is renewed all the time. Osteoclasts dissolve old bone and osteoblasts lay down new bone, and normally the two are closely balanced. In osteoporosis the balance tips, with breakdown running ahead of formation.
The result is a fall in bone mineral density, meaning the amount of bone per unit volume, and a deterioration of the bone's microarchitecture, the fine scaffolding of struts within it. What remains is still normally mineralised (hardened with calcium and other mineral salts), so there is just less bone, and it is weaker.
Weakened bone can break after a comparatively small amount of force. A fragility fracture is a break that follows a fall from standing height or less. Common sites are the vertebrae (spinal bones), the femoral neck (the neck of the thigh bone) and the forearm. Osteoporosis is usually painless until a bone breaks.
After the menopause oestrogen levels fall, and oestrogen is a protective factor, so the risk of osteoporosis rises. Other risk factors include corticosteroids (steroid medicines that increase osteoclast action), smoking, inactivity and chronic kidney disease.
Osteomalacia is the key contrast: the bone is soft because it is poorly hardened with mineral, rather than because bone has been lost.
Key facts
- Mechanism
- osteoclast breakdown of bone runs ahead of osteoblast formation, so bone mass falls.
- Mineralisation
- the bone that remains is normally mineralised, but osteomalacia has defective mineralisation.
- Typical fractures
- fragility fractures are common at the wrist, hip and spine.
- Presentation
- osteoporosis is usually painless until a bone breaks, and complications of such fractures can include chronic pain and decreased mobility.
- Risk factors
- the menopause, increasing age, corticosteroids, chronic kidney disease, smoking and inactivity.
- Prevention of fractures
- in established osteoporosis this can be done through preventing falls, stopping smoking, reducing alcohol, increasing exercise and improving diet, and bisphosphonate medicines are also useful to decrease the occurrence of future fractures.
Exam traps
- Osteomalacia also weakens bone and raises fracture risk, but its problem is poor mineralisation of the bone framework, so it does not fit a loss of bone with normal mineralisation.
- Osteopetrosis is easily confused with osteoporosis because of the similar name, but it is the mirror image: impaired osteoclasts break bone down too little, so bone mass is increased, not reduced.
- Osteoarthritis has a similar name but is a disease of joint cartilage, which breaks down, causing pain and stiffness, rather than a loss of bone density.
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