Osteoporosis

Osteoporosis is metabolic skeletal disease defined as a reduction of bone mineral density (BMD) below a defined lower limit of normal.

The World Health Organisation (WHO) defines osteoporosis as a T score less than -2.5 SD. However, Z scores are more reliable than T scores (which are defined against adult white females) as it compares with normal people of same age and gender.

Clinical presentation

Non specific features predominate. Osteoporosis is often diagnosed incidentally in most cases butasymptomatic patients may also presentevaluated on the basis of risk factors or following presentation with pathological fractures.

Pathology

Osteoporosis is essentially decreased bony tissue per unit volume of bone. There is no microstructural and biochemical change as occurs in osteomalacia or rickets. Hence the mineral to osteoid ratio is normal (c.f. osteomalacia in which mineral/osteoid is decreased).

Osteoporosis can be localised or diffuse and be divided into:

• primary: no cause is identifiable (i.e. involutional or age-related osteoporosis) of which there are two types
  • postmenopausal: occurs in 50-65 year old females; disproportionate loss of cancellous bone as compared to cortical bone resulting in more involvement of cancellous bone rich areas, like vertebra and ends of long bones
  • senile: occurs in the elderly; proportionate loss of cortical and cancellous bones affecting long bones
  • juvenile: juvenile osteoporosis
• secondary: occurs due a range of causes including
  • endocrine disease
  • trauma
  • inflammation
  • medications (e.g. steroids), etc
  • thalassaemia major ⁴

Radiographic features

Decreased bone density can be appreciated by decreased cortical thickness, loss of bony trabecula in early stages in radiography. Bones like vertebra, long bones (proximal femur), calcaneum and tubular bones are usually looked for evidence of osteoporosis.

Plain radiograph

• not a sensitive modality, as more than 30-50% bone loss is required to appreciate decreased bone density on radiograph
• vertebral osteoporosis manifests as:
  • pencilling of vertebrae
  • loss of cortical bone (picture frame vertebra) and trabecular bone (ghost vertebra)
  • compression fractures and vertebra plana
• loss of trabecula in proximal femur area which is explained by Singh's index (which can also be seen in the calcaneum
• in tubular bones (especially metacarpals), there will be thinning of cortex
  • cortical thickness <25% of whole thickness of metacarpal signifies osteoporosis (normally 25-33%)

Bone mineral density measurement

BMD is the method of estimation of calcium hydroxyapatite. Multiple x-ray based, gamma-ray based and ultrasonic methods are available:

• radiographic absorptiometry (RA)
• single photon and x-ray absorptiometry(SPA)
• dual energy x-ray absorptiometry (DEXA)
  • most commonly used and most reliable
• quantitative computed tomography can be used

Treatment and prognosis

As osteoporosis decreases bone strength, patients are at an increased risk of fracture, often with minimal trauma, and commonly at the pelvis, hip and wrist.

Oral bisphosphonates are the most commonly prescribed medication but they have their own complications including bisphosphonate related atypical femoral fractures and bisphosphonate related osteonecrosis of the jaw. There are a range of other medications that can also be used.