Osteoporosis is a metabolic skeletal disease defined as a reduction of bone mineral density below a defined lower limit of normal.
The World Health Organization (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 they compare with normal people of same age and gender - see dual-energy x-ray absorptiometry (DEXA) for additional discussion of T and Z-scores.
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:
- primary: no cause is identifiable
- postmenopausal (type 1): occurs in 50-65-year-olds females; disproportionate loss of cancellous bone as compared to cortical bone resulting in more involvement of cancellous bone-rich areas, like vertebrae and ends of long bones
- senile (type 2): occurs in the elderly; proportionate loss of cortical and cancellous bones affecting long bones
- idiopathic juvenile osteoporosis
- secondary (type 3): occurs due to a range of causes including
- endocrine disease (e.g. Cushing syndrome, hyperthyroidism, hyperparathyroidism, diabetes mellitus)
- chronic illness (e.g. COPD, chronic liver disease, multiple sclerosis, celiac disease)
- medications (e.g. steroids, phenytoin)
- thalassemia major
- nutritional disturbances (e.g. malnutrition, anorexia)
- muscular dystrophies (e.g. Duchenne muscular dystrophy) - can be due to inherent derangement in calcium metabolism or due to steroid treatment.
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 .
- 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 measurement
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:
- 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
Based on DEXA BMD cann fall into three categories :
- 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 .
Quantitative ultrasound of the calcaneal bone quality has recently emerged as a cost-efficient screening tool for osteoporosis .
Bone marrow signal takes on a heterogeneous appearance with rounded focal fatty lesions replacing normal marrow with coalescence often occurring :
- T1: heterogeneously hyperintense
- T2: variable signal
Osteoporotic wedge compression fractures will alter in signal characteristics depending on age.
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 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) .
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.