What is osteoporosis according to WHO?: A Comprehensive Definition and Classification

The World Health Organization (WHO) formally recognized osteoporosis as a disease in 1993 and provided an official diagnostic classification. So, what is osteoporosis according to WHO's criteria? It is defined by low bone density and the structural degradation of bone tissue, which consequently increases bone fragility and susceptibility to fracture.

Quick Summary

The World Health Organization defines osteoporosis as a disease causing low bone mass and deterioration of bone tissue, leading to enhanced bone fragility and increased fracture risk. Diagnosis is based on T-scores and, in some cases, the presence of fragility fractures.

Key Points

• Low Bone Mass: According to the WHO, osteoporosis is characterized by low bone mass, which is a major factor in bone fragility.

• Microarchitectural Deterioration: The definition also specifies the deterioration of bone tissue's internal structure, further weakening the bone.

• Increased Fracture Risk: The low bone mass and poor microarchitecture lead to a significantly higher risk of fractures, which is the primary clinical consequence of the disease.

• T-Score of -2.5 or Lower: Diagnosis of osteoporosis is typically based on a bone mineral density (BMD) T-score of -2.5 or lower, as measured by a DXA scan.

• Severe Osteoporosis: The WHO distinguishes 'severe' osteoporosis for patients with a T-score of -2.5 or lower and a history of one or more fragility fractures.

• Standardized Classification: The WHO's criteria, first established in 1994, provide a standardized global framework for diagnosing osteoporosis.

• Risk Assessment Tool: The WHO developed the FRAX tool, which integrates T-scores with other clinical risk factors to predict a person's 10-year fracture probability.

The WHO's Comprehensive Definition

The World Health Organization's (WHO) classification for osteoporosis is primarily based on bone mineral density (BMD) measurements using dual-energy X-ray absorptiometry (DXA). The WHO defines osteoporosis as a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue. This ultimately leads to increased bone fragility and a higher risk of fractures. The WHO established specific T-score thresholds for diagnosis, initially for postmenopausal Caucasian women, which are now widely used.

WHO's T-Score Classification

The WHO utilizes T-scores to classify bone density, comparing an individual's BMD to that of a healthy young adult. The classifications are:

• Normal: A T-score of -1.0 or higher.
• Osteopenia: A T-score between -1.0 and -2.5, indicating lower than normal bone mass.
• Osteoporosis: A T-score of -2.5 or lower, indicating significantly low bone density and high fracture risk.
• Severe (Established) Osteoporosis: A T-score of -2.5 or lower with one or more fragility fractures.

Clinical and Risk-Based Diagnostic Considerations

The WHO recognizes that a diagnosis isn't solely based on T-scores and includes clinical factors and risk assessment. Bone strength involves more than just BMD. The WHO's Fracture Risk Assessment Tool (FRAX) combines clinical risk factors with BMD to estimate a 10-year fracture risk. A diagnosis may also be made based on certain low-trauma fractures, defined as a fracture from a fall from standing height or less, even if the T-score is above -2.5. Hip or vertebral fragility fractures are often sufficient for a diagnosis regardless of BMD.

The Importance of the WHO's Classification

The WHO's standardized diagnostic criteria have significantly impacted osteoporosis management globally by providing a consistent framework for diagnosis and shifting the focus towards fracture prevention. This standardization supports international research, guideline development, and tailored management based on individual risk.

Comparison of WHO Criteria vs. Clinical Diagnosis

Feature	WHO BMD-Based Criteria	Clinically-Driven Diagnosis
Primary Basis	T-score from DXA scan.	Fracture history and risk factors, possibly with BMD.
Diagnostic Threshold	T-score of -2.5 or lower.	Can be diagnosed with T-scores above -2.5 if a low-trauma fracture occurs.
Key Tool	DXA.	Risk factor evaluation, fracture history, potentially FRAX.
Population Focus	Initially postmenopausal Caucasian women, now adapted.	Broader population, including men, based on clinical judgment.
Diagnosis Type	Standardized, numerical.	Can involve subjective clinical assessment based on specific fractures.
Associated Risk	High fracture risk due to low bone density.	Fracture proves bone fragility regardless of measured density.

Risk Factors and Symptoms Recognized by WHO

The WHO acknowledges various risk factors, including non-modifiable ones like age, gender, race, and family history, and modifiable factors such as diet, exercise, and lifestyle. Osteoporosis is often asymptomatic until a fracture occurs, earning it the nickname "silent disease". Symptoms, when present, usually result from a fracture and can include back pain, height loss, and stooped posture.

Conclusion

The World Health Organization defines osteoporosis as a skeletal disease characterized by reduced bone mass and architectural integrity, increasing fracture risk. Its standardized classification, primarily using T-scores from DXA, is a global framework for diagnosis and risk assessment. This definition has advanced osteoporosis management by promoting proactive prevention. Tools like FRAX and considering fracture history highlight the complexity of fracture risk and the need for a comprehensive approach.

Frequently Asked Questions

How does the WHO define osteoporosis using a T-score?

The World Health Organization (WHO) defines osteoporosis based on a T-score of -2.5 or lower as measured by a bone mineral density (BMD) test like a DXA scan. This T-score compares an individual's bone density to that of a healthy young adult.

What is the difference between osteoporosis and osteopenia according to the WHO?

According to the WHO, osteopenia is a precursor state to osteoporosis, with a T-score between -1.0 and -2.5. Osteoporosis is diagnosed at a lower threshold, with a T-score of -2.5 or below, indicating more significant bone loss and higher fracture risk.

Can you be diagnosed with osteoporosis without a T-score below -2.5?

Yes. The WHO and other medical organizations acknowledge that a diagnosis can be made clinically if an older individual (e.g., a postmenopausal woman or man over 50) experiences a fragility fracture, particularly of the hip or spine, regardless of their T-score.

What is a fragility fracture in the context of a WHO diagnosis?

A fragility fracture is a broken bone that occurs from a fall from standing height or less, or due to minimal trauma. The presence of one or more fragility fractures in a person with a T-score below -2.5 constitutes severe or established osteoporosis, according to WHO criteria.

What is the Fracture Risk Assessment Tool (FRAX)?

The FRAX tool is a WHO-sponsored algorithm that helps predict a person's 10-year probability of a major osteoporotic fracture. It combines BMD results with other clinical risk factors, such as age, BMI, prior fractures, and lifestyle choices, for a more comprehensive risk assessment.

Which populations do the WHO T-score criteria apply to?

The original WHO criteria were developed for postmenopausal Caucasian women. While they are now widely applied to men aged 50 and older and other ethnic groups, it is important to consider these initial population limitations and use clinical judgment in diagnosis.

Why is the WHO definition of osteoporosis so important?

The WHO definition established a global standard for diagnosing osteoporosis, enabling international research and consistent clinical guidelines. It shifted the focus toward preventing fractures by identifying bone weakness early, rather than simply responding to fractures after they occur.