Bone Mineral Density (BMD) test

-Dr.Apoorva.E

PG,DCMS

STRUCTURE OF A NORMAL BONE

• The majority of bone is made of the matrix.

• Difference between matrix of bone and that of other cells is that the matrix of bone is hard.

• It has inorganic and organic parts.

MOLECULAR STRUCTURE :

MATRIX

INORGANIC

ORGANIC

• The inorganic part/bone mineral consists of calcium hydroxyapatite (Ca10(PO4)6(OH)2).

• The organic part of matrix is mainly composed of Type I collagen.

It is also composed of various growth factors like glycosaminoglycans,osteocalcin, osteonectin,osteopontin and Cell Attachment Factor.

CELLULAR STRUCTURE :

• There are several types of cells constituting the bone :

-Osteoblasts are bone-forming cells.They are the immature bone cells which lay down the matrix as unmineralised osteoid.They produce alkaline phosphatase which helps in mineralisation of bone.

-Osteocytes originate from osteoblasts.Their functions incude matrix maintenance and calcium homeostasis.They are mature bone cells.

-Osteoclasts are the cells responsible for bone resorption,thus they break down bone. New bone is then formed by the osteoblasts.

ELECTRON MICROGRAPHIC PICTURE OF BONE MINERAL

STRUCTURE OF BONE

• In osteopenia and osteoporosis,there is an imbalance between osteoblastic and osteoclasticactivity.Existing bone is reabsorbed faster than new bone is made.

• As this occurs,the bones lose minerals, heaviness(mass) and structure,making them less dense,weaker and increasing the risk of fractures.

PATHOGENESIS OF OSTEOPENIA AND OSTEOPOROSIS

What is a Bone Mineral Density (BMD) test?

• A bone mineral density test is an easy,reliabletest that measures the density or thickness of bones.

• It measures the amount of mineral (calcium) in a specific area of the bone.

• The more mineral in the bone measured, the greater is the bone density or bone mass.

• A BMD test can:

-Measure the density of bones

-Detect osteoporosis before a fracture occurs

-Help to predict chances of fracturing in the future

-Monitor the effectiveness of treatments for osteoporosis and osteopenia.

• There are several different ways to measure BMD

1.Dual-energy X-ray absorptiometry (DEXA)

2.Peripheral dual-energy X-ray absorptiometry (P-DEXA).

3.Dual photon absorptiometry (DPA)

4.Quantitative computed tomography (QCT)

5.Quantitative ultrasound

• This is the most accurate and standardized way to measure BMD.

• It uses two different X-ray beams to estimate bone density in the spine and hip.

• It is a quick,easy and painless test where nothing is injected or swallowed.

• A low-dose x-ray is taken which is is only 10% of the radiation exposure of a chest x-ray.

Dual-energy X-ray absorptiometry(DEXA)

DEXA SCANNER

Peripheral dual energy X-ray absorptiometry (P-DEXA)

• P-DEXA is a type of DEXA test.

• It measures the density of bones in the arms or legs,such as the wrist.

• P-DEXA machines are portable units.

• The results are quicker than standard DEXA measurements.

• P-DEXA has a disadvantage of not being able to monitor the treatment of osteoporosis.

Four Informative Skeletal Sites 1.Radius -The distal one-third of the radius

(wrist) is efficacious in predicting fracture risk.

2.Phalanx-The proximal phalanx.

3.Metatarsus-The 5th metatarsus.Measurementat this site is particularly important because weight-bearing bone may lose strength at a different rate than non-weight-bearing bone.

4.Tibia-The mid-shaft of the tibia.It is useful in the monitoring of treatment for osteoporosis.

INDICATIONS FOR BMD TEST

BMD is done in :

• All women aged 65 or older and men over 70 years of age regardless of risk factors

• Postmenopausal women under age 65 and men in the age group of 50 – 70 years who have one or more risk factors

RISK FACTORS FOR OSTEOPOROSIS1.Having a current or previous fracture

2.On steroid medications for more than three months

3. Chronic anticonvulsant therapy

3.Men with low testosterone,alcoholism or any other secondary cause of osteoporosis

4.Chronic rheumatoid arthritis

5.Chronic kidney disease

6.Early menopause

5.History of hormone treatment for prostate cancer or breast cancer

6.Smoking

7.Strong family history of osteoporosis

8. Significant loss of height (vertebral compression fractures)

CONTRAINDICATIONS FOR BMD TEST

• Pregnancy

• Recent gastrointestinal contrast studies(recommend waiting for at least 72 hours before central DEXA Scan)

• Body weight exceeding limit for DEXA scanners(>120-130kgs)

• Bilateral hip replacements or bilateral hip pins or screws would prevent the hip sites from being scanned.Metallic rods or spinal fusion devices in the lumbar spine prevent scanning at this site.

WHAT DOES THE TEST DO?

• A DEXA test measures the bone mineral density

and compares it to that of an established standard .

• The results of the test are usually reported as a "T score" and "Z score."

• In either score, a negative number means you have thinner bones than the standard.

• The more negative the number, the higher your risk of a bone fracture.

T-SCORE

• The T score compares your bone density with that of healthy young adult.

• A score of 0 means your BMD is equal to the standard for a healthy young adult.

• Differences between your BMD and that of the healthy young adult standard are measured in units called standard deviations (SDs). The more standard deviations below 0, indicated as negative numbers, the lower your BMD and the higher your risk of fracture.

• As shown in the table below, a T-score between +1 and −1 is considered normal or healthy.

• A T-score between −1 and −2.5 indicates that you have low bone mass.

• A T-score of −2.5 or lower indicates that you have osteoporosis.

• The greater the negative number, the more severe the osteoporosis.

Z-SCORE

• The Z score compares your bone density with that of other people of same age and gender.

• A low Z-score (below —2.0) is a warning sign that you have less bone mass or that you are losing bone more rapidly than expected for someone of your age.

• Z-score = T-score - Reference T-score

How is a "Bone Scan" different from a BMD test?

• Unlike a BMD test, a bone scan is an invasive test.

• The patient is injected with a dye that allows a scanner to look at the condition of bone tissue.

• A bone scan can diagnose inflammation, fractures, bone lesions and cancer.

• It cannot predict the risk of osteoporosis or diagnose the condition.

Osteopenia• Osteopenia refers to bone mineral density that is

lower than normal peak BMD (i.e,between -1.0 and -2.5) but not low enough to be classified as osteoporosis.

• It is a sign of normal ageing,in contrast to osteoporosis which is a sign of pathologic ageing.

• It occurs more frequently in post-menopausal women,can be exacerbated by lifestyle factors such as lack of exercise,alcoholism,smoking or prolonged use of glucocorticoid medications for asthma.

• Osteopenia has no symptoms.There is no pain as the bone becomes thinner but the risk of fracture increases as the bone becomes less dense.

• Diagnosis is by a bone mineral density test. A standard X-ray is not useful in diagnosing because it is not sensitive enough to detect small amounts of bone loss or minor changes in bone density

• Osteopenia is treated by taking steps to prevent it from progressing to osteoporosis like –

1.Lifestyle changes

2.Dietary modifications : Foods rich in calcium(milk,dairy products,green leafy vegetables) and vitamin D(eggs,fish,fish oils)

3.Vitamin D and calcium supplements

Osteoporosis

• Osteoporosis = "porous bones."

• Osteoporosis is defined by the WHO as a bone mineral density that is 2.5 standard deviations or more below the mean bone mass (average of young,healthy adults).

• The underlying mechanism in osteoporosis is an imbalance between bone resorption and bone formation.

• Osteoporosis is classified as

1. primary type 1 - postmenopausal osteoporosis

2. primary type 2 - senile osteoporosis occuring after age 75

3. secondary - resulting from chronic predisposing medical conditions or prolonged use of glucocorticoids,when the disease is called steroid-or glucocorticoid-induced osteoporosis.

• The risk factors are same as that of osteopenia.

• Osteoporosis has no symptoms but its main consequence is the increased risk of bone fractures.

• Osteoporotic fractures are those that occur in situations where healthy people would not normally break a bone;they are therefore called fragility fractures.

• Bones involved are the vertebral column,ribs,hipand wrist.

• Acute and chronic pain in the elderly is often due to osteoporotic fractures .

• The symptoms of a vertebral collapse (compression fracture) are sudden back pain,shooting pain due to nerve root compression.

• Multiple vertebral fractures lead to a stooped posture, loss of height, and chronic pain with resultant reduction in mobility.

• Hip fractures require immediate surgery as there are risks of having a deep vein thrombosis and pulmonary embolism.

stooped posture

• The diagnosis of osteoporosis can be made by using conventional radiography and by measuring the BMD using DEXA which is the gold standard for diagnosis.

• It also requires blood tests to find out the underlying cause.

• The main radiographic features of generalized osteoporosis are cortical thinning and increased radiolucency.

X-RAY SHOWING CURVED SPINE DUE TO OSTEOPOROSIS

BMD TEST :NORMAL OSTEOPOROSIS

• Prevention of osteoporosis-related complications is by lifestyle changes,taking calcium and vitamin D rich foods and supplements.

• Bisphosphonates can be prescribed in cases of very high risk. Other medicines include raloxifene, a selective estrogen receptor modulator(SERM).

• Teriparatide (a recombinant parathyroid hormone) can be given for postmenopausal osteoporosis.Strontium ranelate is effective in decreasing the risk of fractures.

• Estrogen replacement therapy after menopause can be started if indicated.

• In hypogonadal men,testosterone has been shown to improve bone quantity and quality.