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PH-MS03

Osteoporosis

How continuous bone remodelling tips towards resorption after the menopause and with age, how that thins and disconnects the trabecular lattice, and why the result is silent until a bone breaks from a minor fall.

By the end, you can

  • Describe bone remodelling by osteoclasts and osteoblasts and the idea of peak bone mass.
  • Explain how oestrogen loss, ageing and other causes tip remodelling towards net bone loss.
  • Describe how trabecular bone changes in osteoporosis and why that weakens it disproportionately.
  • Relate vertebral, hip and wrist fractures to the bones most affected.
  • Explain how bone density and fracture risk are assessed, and distinguish osteoporosis from osteomalacia.
  • Recognise the trabecular and vertebral changes, the WHO T-score bands and the main tests of osteoporosis, and explain how each main treatment acts on remodelling.

What it is

Osteoporosis is a skeletal disorder of low bone mass and deterioration of bone microarchitecture, which make bone fragile and increase the risk of fracture. The bone that remains is normally mineralised: there is simply less of it. A fragility fracture is one caused by a force that would not normally break a bone, such as a fall from standing height.

2 types to tell apart
Primary osteoporosis
Related to the menopause and to ageing, without another cause.
Secondary osteoporosis
Caused by medicines (especially long-term glucocorticoids), endocrine disease (hyperthyroidism, hyperparathyroidism, low sex hormones, Cushing’s syndrome), malabsorption, chronic inflammation, immobility, alcohol or smoking.

Normal function

Bone is constantly rebuilt

Bone is living tissue, replaced throughout life by remodelling. Osteoclasts dissolve small pits of old bone; osteoblasts then fill them with new matrix, which mineralises. About a tenth of the adult skeleton is replaced each year, repairing microscopic damage and releasing calcium when needed.

Inside the ends of long bones and the vertebral bodies, cancellous (trabecular) bone forms a lattice of braced along the lines of load, with in the spaces, under a shell of . Its large surface area means it remodels faster than compact bone.

Bone mass rises through childhood and adolescence to a peak around the late twenties or early thirties. After that, resorption slightly exceeds formation each year. A higher peak, built by genetics, nutrition, sex hormones and weight-bearing exercise, leaves more in reserve.

Point to it on the model
The remodelling cycleSchematic
NormalActivationSignals (RANKL) recruit osteoclasts
NormalResorptionOsteoclasts dissolve a pit of bone
NormalFormationOsteoblasts lay new matrix
NormalMineralisationNew matrix hardens
weeks
months
months

Schematic. In osteoporosis, resorption removes more bone than formation replaces at each site.