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Bone densitometry scan

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Understanding Osteoporosis and Bone Densitometry

Bone densitometry is a non-invasive medical imaging technique used to measure bone mineral density (BMD). It is the gold standard for diagnosing osteoporosis and assessing fracture risk. This section explores the role of bone densitometry in the context of osteoporosis, a widespread skeletal disease.

 

Osteoporosis: The "Silent Epidemic"

Osteoporosis is a systemic skeletal disease characterized by a decrease in bone mass and a deterioration of bone tissue microarchitecture. These changes lead to increased bone fragility and a consequently heightened risk of fractures, even from minor trauma or everyday activities. The core pathology of osteoporosis is the progressive loss of bone mineral density and overall bone mass. This loss often occurs gradually and latently (without obvious symptoms) over many years. Consequently, osteoporosis is frequently diagnosed only after a fragility fracture (a fracture resulting from a fall from standing height or less) occurs. This insidious nature has led to osteoporosis being termed a "silent epidemic."

Densitometry is a crucial diagnostic procedure for accurately measuring bone density, primarily used when osteoporosis is suspected, to assess fracture risk and guide treatment decisions.

Global Impact and Prevalence

Globally, the prevalence of diseases affecting the musculoskeletal system, including osteoporosis, is on the rise, partly due to aging populations. Fractures of bones commonly affected by osteoporosis, such as the femoral neck (hip), vertebral bodies (spine), and distal forearm (wrist), have significant public health implications, leading to pain, disability, loss of independence, and increased mortality. The number of hip fractures among the world's inhabitants, a common and severe consequence of osteoporosis, has practically doubled over the past few decades. Mathematical modeling estimated 1.7 million hip fracture cases in 1990; this number has risen to approximately 2.5 million in recent years, and projections suggest this figure could reach as high as 6 million by 2050 if preventive measures are not widely implemented.

In many regions, including the Russian Federation historically, a lack of sufficient bone densitometers has hindered the collection of complete statistical data on osteoporosis prevalence based on the World Health Organization (WHO) criteria, which rely on densitometry measurements. However, available data indicate a significant burden: among individuals over 50 years of age, estimates suggest that approximately one in ten may have a compression fracture of a vertebral body, one in two hundred a fracture of the distal forearm, and one in a thousand a fracture of the femoral neck, many of which are attributable to underlying osteoporosis.

 

Bone Densitometry: Measuring Bone Mineral Density (BMD)

The Densitometry Procedure

The procedure for measuring bone mineral density (BMD) when osteoporosis is suspected is called **bone densitometry**. This test quantifies the amount of calcium and other minerals packed into a segment of bone. It is performed using specialized measuring equipment that typically utilizes low-dose X-rays or ultrasound waves.

During a common type of densitometry scan, like Dual-Energy X-ray Absorptiometry (DXA or DEXA), the patient lies on a padded table while a scanner passes over specific areas of the body, usually the hip (femoral neck) and lumbar spine. These sites are chosen because they are common locations for osteoporotic fractures and provide a good indication of overall skeletal health. Peripheral sites like the forearm, wrist, or heel can also be measured, particularly for screening purposes or if central sites are not accessible.

The procedure is quick (usually 10-30 minutes), painless, and involves minimal radiation exposure (significantly less than a standard chest X-ray).

 

Types of Bone Densitometry Techniques

Several techniques can measure bone density:

  • Dual-Energy X-ray Absorptiometry (DXA or DEXA): This is the most widely used and validated method for measuring BMD at central sites (hip and spine) and for diagnosing osteoporosis according to WHO criteria. It uses two X-ray beams with different energy levels to differentiate bone from soft tissue.
  • Quantitative Computed Tomography (QCT): Can measure volumetric BMD (g/cm³) and separately assess cortical and trabecular bone. It is often used for spinal measurements but involves higher radiation exposure than DXA.
  • Peripheral DXA (pDXA): Measures BMD at peripheral sites like the wrist, heel, or finger. Used for screening but not as definitive as central DXA for diagnosis or monitoring treatment.
  • Quantitative Ultrasound (QUS): Uses sound waves to assess bone density, typically at the heel or finger. It is portable and radiation-free but measures different bone properties than DXA and is primarily used for fracture risk screening rather than definitive osteoporosis diagnosis.
  • Radiographic Absorptiometry (RA): Uses standard X-rays of the hand with an aluminum wedge for comparison. Less common now.

 

Interpreting Bone Densitometry Results: T-scores and Z-scores

Bone densitometry results are typically reported using two main scores:

  • T-score: This score compares the patient's BMD to the average BMD of a healthy young adult of the same sex and ethnicity at their peak bone mass (usually around age 25-30). The T-score is expressed in standard deviations (SD) from this young adult mean. It is the primary score used for diagnosing osteoporosis in postmenopausal women and men over 50.
  • Z-score: This score compares the patient's BMD to the average BMD of individuals of the same age, sex, and ethnicity. The Z-score is more commonly used for premenopausal women, men under 50, and children. A Z-score of -2.0 or lower is considered below the expected range for age and may warrant further investigation for secondary causes of osteoporosis.

 

WHO Diagnostic Criteria for Osteoporosis (Based on T-scores at Hip or Spine)

The World Health Organization (WHO) has established the following diagnostic categories based on BMD T-scores in postmenopausal women and men aged 50 and older:

  • Normal Bone Mass: T-score is greater than or equal to -1.0 SD.
  • Low Bone Mass (Osteopenia): T-score is between -1.0 SD and -2.5 SD. This indicates bone density lower than normal peak bone mass but not yet at the level of osteoporosis.
  • Osteoporosis: T-score is less than or equal to -2.5 SD.
  • Severe or Established Osteoporosis: T-score is less than or equal to -2.5 SD, AND the individual has experienced one or more fragility fractures.

It's important to note that while BMD is a major determinant of bone strength, other factors also contribute to fracture risk, such as bone quality, fall risk, and clinical risk factors (e.g., age, previous fracture, family history, smoking, low body weight, certain medications).

Video illustrating the bone densitometry scan procedure.

 

Indications for Bone Densitometry Scan

A doctor may order a bone densitometry scan for various reasons, including:

  • Screening for Osteoporosis:
    • Women aged 65 and older.
    • Men aged 70 and older.
    • Postmenopausal women younger than 65 with risk factors for osteoporosis.
    • Men aged 50-69 with risk factors for osteoporosis.
  • Individuals with a History of Fragility Fracture: A fracture occurring with minimal trauma.
  • Individuals with Medical Conditions Associated with Bone Loss: Such as rheumatoid arthritis, hyperparathyroidism, hyperthyroidism, malabsorption syndromes (e.g., celiac disease, inflammatory bowel disease), chronic kidney disease, chronic liver disease, hypogonadism.
  • Individuals Taking Medications Known to Cause Bone Loss: Long-term glucocorticoid (steroid) use (e.g., prednisone), certain anticonvulsants, aromatase inhibitors, androgen deprivation therapy.
  • Monitoring Osteoporosis Treatment: To assess the effectiveness of medications prescribed to slow bone loss or increase bone density. Scans are typically repeated every 1-2 years or as clinically indicated.
  • Assessing Fracture Risk: BMD results are often incorporated into fracture risk assessment tools (e.g., FRAX®) to estimate the 10-year probability of major osteoporotic fractures.
  • Unexplained bone pain or suspicion of bone thinning.

 

Pathological Fractures: A Complication of Osteoporosis

The primary danger of osteoporosis lies in its major complication: pathological fractures, also known as fragility fractures. These occur with minimal or no trauma, such as a fall from standing height, or even from simple activities like coughing, sneezing, or bending over, due to the weakened state of the affected bone or group of bones.

The most common and well-known osteoporotic fractures include:

  • Vertebral Compression Fractures: Thinning of bone density in one or several vertebrae can lead to a decrease in vertebral height or the formation of a wedge-shaped compression fracture. For an elderly person with osteoporosis, such a fracture can occur from something as simple as awkwardly sitting down on a hard surface or lifting a moderate weight. These can cause back pain, loss of height, and kyphosis (stooped posture).
  • Hip Fractures (Femoral Neck Fractures): These are particularly serious, often requiring surgery and leading to significant disability, loss of independence, and increased mortality. An elderly person with osteoporosis might sustain a hip fracture from an accidental stumble and fall, or even from a sudden twist or turn in bed.
  • Distal Forearm (Wrist) Fractures (Colles' Fracture): Often occur when trying to break a fall with an outstretched hand.

Side view (lateral X-ray) illustrating a compression fracture of a thoracic vertebral body, a frequent and often debilitating consequence of underlying osteoporosis.

 

Limitations and Considerations of Bone Densitometry

While bone densitometry is a valuable tool, it has some limitations:

  • Measures Density, Not Necessarily Quality: BMD is a major component of bone strength, but it doesn't fully capture bone quality (microarchitecture, turnover, damage accumulation).
  • Radiation Exposure (for DXA/QCT): Although low for DXA, QCT involves higher doses. Ultrasound (QUS) is radiation-free.
  • Variability Between Machines and Operators: Results can vary slightly depending on the densitometer model and technician technique. Serial measurements should ideally be done on the same machine.
  • Artifacts: Conditions like severe osteoarthritis, vertebral fractures, scoliosis, or vascular calcifications can sometimes artificially inflate BMD readings at the spine.
  • Not a Perfect Predictor of Fractures: While low BMD increases fracture risk, many fractures occur in individuals with osteopenia rather than osteoporosis, and some with osteoporosis never fracture. Clinical risk factors are also crucial.
  • Limited Usefulness in Certain Populations: Interpretation in children and very elderly individuals can be more complex.

The "harmfulness" of a bone densitometry scan, particularly DXA, is very low due to the minimal radiation dose. The benefits of identifying individuals at high risk of fracture generally far outweigh the small radiation risk.

 

Differential Diagnosis of Low Bone Density and Pathological Fractures

While osteoporosis is the most common cause of low bone density leading to fragility fractures, other conditions can also result in weakened bones or fractures with minimal trauma:

Condition Key Differentiating Features
Osteoporosis (Primary/Secondary) Low BMD on densitometry (T-score ≤ -2.5). Primary (age-related, postmenopausal) or secondary to other diseases/medications.
Osteomalacia / Rickets Defective bone mineralization due to vitamin D deficiency or phosphate depletion. Bones are soft. Symptoms include bone pain, muscle weakness, skeletal deformities (in children). Normal or low BMD, but characteristic biochemical findings (low vitamin D, low/normal calcium, low phosphate, high alkaline phosphatase, high PTH).
Hyperparathyroidism (Primary or Secondary) Excess parathyroid hormone (PTH) leads to increased bone resorption. High serum calcium (primary), high PTH. BMD may be low, particularly at cortical sites.
Multiple Myeloma Malignancy of plasma cells. Causes lytic bone lesions, bone pain, pathological fractures, hypercalcemia, renal insufficiency, anemia. Monoclonal protein in serum/urine. Bone marrow biopsy confirms.
Metastatic Bone Disease Cancer spread to bones from a primary site (e.g., breast, prostate, lung, kidney). Causes lytic or blastic lesions, bone pain, pathological fractures. History of cancer. Bone scan, imaging help identify.
Paget's Disease of Bone Disordered bone remodeling leading to enlarged, deformed, and fragile bones. Localized bone pain, warmth, deformities. Elevated alkaline phosphatase. Characteristic X-ray findings.
Osteogenesis Imperfecta ("Brittle Bone Disease") Genetic disorder of collagen synthesis leading to extremely fragile bones, frequent fractures from minimal trauma, blue sclerae, hearing loss, dental problems. Onset typically in childhood.
Renal Osteodystrophy Bone disease occurring in chronic kidney disease due to disturbances in mineral and hormone metabolism.

 

Management and Prevention of Osteoporosis

Management of low bone mass or osteoporosis aims to prevent fractures and involves:

  • Lifestyle Modifications: Adequate calcium and vitamin D intake, regular weight-bearing and muscle-strengthening exercise, smoking cessation, moderation of alcohol intake, fall prevention strategies.
  • Pharmacological Treatment: For individuals with osteoporosis or high fracture risk, medications may be prescribed. These include:
    • Bisphosphonates (e.g., alendronate, risedronate, zoledronic acid)
    • Denosumab
    • Teriparatide or Abaloparatide (anabolic agents)
    • Romosozumab (anabolic agent)
    • Selective Estrogen Receptor Modulators (SERMs) (e.g., raloxifene)
    • Hormone Replacement Therapy (HRT) in some postmenopausal women.

The choice of treatment depends on the severity of bone loss, fracture risk, and individual patient factors.

 

When to Consider Bone Densitometry

A doctor would typically order a bone densitometry scan based on established screening guidelines (age, sex) or if a patient presents with specific risk factors or conditions that increase their likelihood of having low bone density or experiencing a fragility fracture. It is a key tool in the proactive management of bone health and osteoporosis prevention.

References

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