Osteoporosis

Last revised by Dr MT. Niknejad on 03 Jul 2022

Osteoporosis is a metabolic bone disease characterized by decreased bone mass and skeletal fragility.

The World Health Organization (WHO) operationally defines osteoporosis as a bone mineral density T-score less than -2.5 SD (more than 2.5 standard deviations under the young-adult mean), which is measured by dual-energy x-ray absorptiometry (DEXA), in postmenopausal women and men at least 50 years old. The reference standard site of bone mineral density analysis is the femoral neck, but other sites such as lumbar spine can be used. A clinical diagnosis of osteoporosis may also be established without bone mineral density measurement by the presence of a fragility fracture, particularly at typical sites (spine, hip, pelvis, wrist, humerus, or rib).

The WHO diagnostic criterion for osteoporosis is not sufficient to identify patients who are at high risk of fracture. The following risk factors, in addition to femoral neck bone mineral density, are used in FRAX (Fracture Risk Assessment Tool), which calculates a 10-year probability of major osteoporotic fracture (hip, clinical spine, humerus, or wrist fracture) and of hip fracture 11:

  • sex (females have higher risk)
  • age (older adults have higher risk)
  • body mass index (lower body mass carries higher risk)
  • prior fragility fracture
  • parental history of hip fracture
  • current tobacco smoking
  • daily alcohol consumption of at least 3 units
  • ever long-term use of oral glucocorticoids (more than 3 months at a dose equivalent to as least 5 mg daily prednisolone)
  • rheumatoid arthritis
  • other causes of secondary osteoporosis, including
    • type I (insulin dependent) diabetes
    • osteogenesis imperfecta in adults
    • untreated long-standing hyperthyroidism
    • hypogonadism
    • premature menopause (<45 years)
    • chronic malnutrition
    • malabsorption
    • chronic liver disease
    • HIV/AIDS, especially with some antiretroviral therapy (ART) (e.g. tenofovir disoproxil fumarate) 12

Osteoporosis per se is asymptomatic and is most often diagnosed when individuals are evaluated on the basis of risk factors or following presentation with fragility fracture.

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 (cf. osteomalacia in which the mineral-to-osteoid ratio is decreased).

Osteoporosis can be localized or diffuse and be divided into:

There is a different list of secondary causes for juvenile osteoporosis with some overlap with adult causes. 

Decreased bone density can be appreciated by decreased cortical thickness and loss of bony trabeculae in the early stages in radiography. Bones like the vertebra, long bones (proximal femur), calcaneum and tubular bones are usually looked at for evidence of osteoporosis. Nevertheless, dual energy x-ray absorptiometry (DEXA) is the gold standard of diagnosing osteoporosis 10

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

Bone mineral density (BMD) measurement is the method of estimation of calcium hydroxyapatite. Multiple x-ray based, gamma-ray based and ultrasonic methods are available:

Based on DEXA BMD can fall into three categories 10:

  • normal (low risk of fracture)
  • osteopenic (medium risk)
  • osteoporotic (high risk)

Quantitative CT can measure bone mineralization and BMD, which is usually done in the lumbar spine 10

Quantitative ultrasound of the calcaneal bone quality has recently emerged as a cost-efficient screening tool for osteoporosis 10

Bone marrow signal takes on a heterogeneous appearance with rounded focal fatty lesions replacing normal marrow with coalescence often occurring 5:

  • T1: heterogeneously hyperintense
  • T2: variable signal

Osteoporotic wedge compression fractures will alter in signal characteristics depending on age.

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 medications and are effective in reducing the risk of further osteoporotic fracture. There are a range of other medications that can also be used, including intravenous bisphosphonates, selective estrogen receptor modulators (e.g. raloxifene), denosumab, strontium ranelate, calcitonin, and parathyroid hormone-based treatments (e.g. teriparatide) 8.

Bisphosphonates and denosumab have been associated with rare, but serious, side effects including bisphosphonate-related atypical femoral fractures and bisphosphonate-related osteonecrosis of the jaw

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Cases and figures

  • Case 1
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  • Case 2
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  • Case 3
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  • Case 4
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  • Case 5: on DEXA
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  • Case 6: severe osteoporosis
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