What is a high bone mass? A Guide to Understanding the Causes and Implications

While most bone health conversations focus on low bone mass and osteoporosis, an estimated 0.2% of DXA scans reveal the opposite: unusually high bone mass (HBM). This uncommon finding, characterized by bone mineral density significantly higher than average, is not always the 'good news' it might seem and requires careful investigation by a healthcare professional.

Quick Summary

High bone mass is a condition where an individual's bone mineral density is markedly elevated compared to what is considered normal for their age and gender. It can stem from various underlying factors, including rare genetic disorders, chronic diseases, or sometimes as a benign anomaly, necessitating thorough medical evaluation to understand its cause and potential health implications.

Key Points

• DXA Scan Finding: High bone mass (HBM) is identified on a dual-energy X-ray absorptiometry (DXA) scan, where a Z-score is significantly elevated.

• Diverse Causes: HBM can be caused by rare genetic disorders, acquired medical conditions like hematological diseases or Paget's disease, or be an artefactual reading from other medical issues.

• Not Always Protective: While high peak bone mass in youth is beneficial, an unusually high BMD later in life may signal an underlying issue and is not universally protective.

• Associated Osteoarthritis: Certain types of HBM are associated with a 'bone-forming' phenotype of osteoarthritis, characterized by increased bone spurs in joints.

• Requires Evaluation: A finding of HBM warrants a thorough medical investigation to rule out serious underlying causes and identify any associated health risks.

• Management is Key: Treatment focuses on addressing the specific underlying cause and managing symptoms like joint pain, rather than attempting to reduce bone density.

What Exactly is High Bone Mass?

High bone mass (HBM) is a medical finding defined by a bone mineral density (BMD) that is significantly higher than the average for a person's age, sex, and ethnicity. It is typically identified using a dual-energy X-ray absorptiometry (DXA) scan, the same tool used to diagnose osteoporosis. While a high peak bone mass achieved in young adulthood is generally protective against later osteoporosis, an unusually high BMD in an older adult can point to several underlying conditions rather than simply a stronger skeleton.

Unlike low bone mass (osteopenia) or osteoporosis, which are characterized by brittle bones prone to fracture, the effects of high bone mass are more complex and depend entirely on the underlying cause. In some cases, dense bones can still be prone to fractures, a paradoxical situation seen in certain genetic disorders. In others, HBM can be linked to other health problems, such as joint issues.

Potential Causes of High Bone Mass

High bone mass is not a single disease but a descriptive term for a finding that can have multiple origins. These are broadly categorized into genetic, acquired, and artefactual causes.

Genetic Causes

Genetic factors play a significant role in determining bone density, with some rare inherited disorders causing a dramatic increase in bone mass. These include:

• Sclerosing Bone Dysplasias: A group of rare disorders that affect bone growth, leading to increased density at various skeletal sites. Examples include sclerosteosis and van Buchem disease.
• LRP5 Mutations: Mutations in the LRP5 gene can lead to either high or low bone mass. Activating mutations can enhance bone formation, leading to significantly higher BMD.
• Osteopetrosis: A rare inherited condition where bones become abnormally dense and hard but are also prone to fracturing.
• Polygenic Inheritance: For most people, bone density is influenced by a combination of many genes, each with a small effect. Individuals with HBM may have an enrichment of these 'high BMD' variants.

Acquired Causes

Various medical conditions that develop over a lifetime can also result in increased bone density:

• Hematological Disorders: Conditions affecting the bone marrow, such as myelofibrosis, mastocytosis, and certain types of leukemia, can lead to increased bone density.
• Paget's Disease of Bone: A chronic disease that disrupts the normal bone remodeling process, leading to thickened and enlarged bones.
• Renal Osteodystrophy: A complication of chronic kidney disease that can alter bone metabolism and sometimes lead to increased bone density.
• Diffuse Idiopathic Skeletal Hyperostosis (DISH): A condition characterized by the hardening and calcification of ligaments, particularly in the spine.
• Exposure to Heavy Metals or Fluoride: Excessive intake or exposure can lead to osteosclerosis.

Artefactual Causes

It is critical to distinguish a true increase in bone mass from a false reading caused by other factors. A careful review of the DXA scan images is necessary to exclude issues such as:

• Degenerative Spinal Disease: Arthritis in the spine is a very common cause of false-positive high BMD readings on DXA scans, as the degenerative changes artificially inflate the density measurements.
• Vascular Calcifications: Calcification of blood vessels near the measurement site can also lead to an artificially high reading.
• Implanted Material: Surgical hardware or other implanted materials can interfere with the scan.

Diagnosis and Evaluation

The initial finding of HBM typically occurs during a routine DXA scan, where a Z-score (a comparison to age-matched peers) greater than +2.5 or +4 is noted. The next steps involve a comprehensive workup to identify the root cause.

A diagnosis typically involves:

• Detailed Clinical History: Doctors will ask about family history of bone disorders, previous health issues, and medication use.
• Physical Examination: This helps identify any outward signs of a skeletal dysplasia, such as an enlarged jaw.
• Visual Inspection of Scans: A trained clinician will review the DXA images to rule out artefactual causes like spinal arthritis.
• Further Imaging: Additional imaging, such as X-rays, CT, or MRI scans, can provide a more detailed look at the bone structure.
• Blood Tests: Checking for conditions affecting bone metabolism, such as problems with the parathyroid gland, vitamin D levels, or signs of hematological disease.
• Genetic Testing: For younger patients or those with a strong family history, genetic testing may be used to look for known mutations associated with HBM.

Potential Health Implications of HBM

While it's not a guarantee, some forms of HBM have been linked to an increased risk of other health problems. Research suggests a link between HBM and a higher prevalence of osteoarthritis, particularly a 'bone-forming' phenotype characterized by osteophyte (bone spur) formation in joints. This has been observed in both weight-bearing (knee, hip) and non-weight-bearing (hand) joints.

Common implications may include:

• Increased risk for joint pain and osteoarthritis, potentially requiring pain management or joint replacement in severe cases.
• Risk of nerve entrapment due to enlarged bone, especially in rare genetic conditions.
• In some severe inherited cases, even with dense bone, the bone may be paradoxically weaker and more prone to fracture.

Treatment and Management

Managing high bone mass is not about reducing bone density but addressing the underlying cause and any associated symptoms. For many, simply understanding the diagnosis is sufficient. Treatment plans are highly individualized and depend on the specific condition causing the HBM.

Feature	Low Bone Mass / Osteoporosis	High Bone Mass
Bone Mineral Density (BMD)	Lower than average	Higher than average (Z-score > +2.5)
Primary Risk	Fragility fractures, bone breaks	Increased risk of osteoarthritis, specific genetic complications
Cause	Age-related loss, hormone changes, lifestyle factors	Genetics, chronic diseases (hematological, metabolic), artifacts
Initial Diagnosis	DXA Scan (Low T-score)	DXA Scan (High Z-score)
Treatment Focus	Increasing or slowing bone loss	Managing underlying condition and symptoms like joint pain
Long-Term Outlook	Can be managed with lifestyle, medication	Varies significantly depending on cause; some are benign

Conclusion: Navigating an Uncommon Diagnosis

High bone mass is a unique and often unexpected medical finding. While the instinctive reaction might be that a higher bone density is a good thing, a DXA reading of HBM is a signal to investigate further. A thorough medical evaluation can distinguish between an artefactual reading, a benign anomaly, or an underlying medical condition.

For many, especially when HBM is the result of a genetic variant with no significant health impact, management may be limited to ongoing monitoring. For others, identifying the cause, such as a hematological disorder or genetic syndrome, is the first step toward a targeted treatment plan focused on preventing associated complications, such as severe osteoarthritis. A proactive, informed approach, guided by a healthcare provider, is the best path forward for anyone with this diagnosis. For more information on bone health, consult resources like the National Institute of Arthritis and Musculoskeletal and Skin Diseases: https://www.niams.nih.gov/.

Frequently Asked Questions

What is the difference between high bone mass and bone spurs?

High bone mass refers to an overall elevated bone mineral density across the skeleton. Bone spurs, or osteophytes, are specific areas of bony overgrowth, often developing in response to inflammation or stress in joints. Some forms of high bone mass are associated with an increased risk of forming bone spurs.

Does high bone mass protect against fractures?

Not necessarily. While a naturally high bone mass in young adulthood can reduce later fracture risk, an unusually high BMD found later in life may be due to an underlying condition. Paradoxically, some genetic disorders that cause high bone mass (like osteopetrosis) result in bones that are dense but brittle and more prone to fracture.

Is high bone mass hereditary?

Yes, it can be. In some cases, high bone mass is caused by a genetic mutation passed down through families, such as mutations in the LRP5 gene. Other times, it is a polygenic trait, influenced by a combination of multiple genes. A detailed family history is often part of the diagnostic process.

How is high bone mass diagnosed?

High bone mass is typically detected during a DXA scan, where the Z-score is found to be unusually high. A doctor will then conduct a full evaluation, which may include further imaging (X-rays, CT scans), blood work, and, in some cases, genetic testing to determine the cause.

Can diet and exercise cause high bone mass?

Diet and weight-bearing exercise can help a person reach their genetic potential for peak bone mass during youth. However, they are not typically the cause of a pathologically high bone mass. The significant elevation seen in HBM is usually linked to other medical factors, either genetic or acquired.

What are the treatment options for high bone mass?

There is no standardized treatment to lower high bone mass. Instead, the focus is on addressing the underlying cause and managing any associated symptoms. Depending on the condition, this might involve medication, physical therapy, or surgery to address complications like severe osteoarthritis.

Can high bone mass be an artifact on a DXA scan?

Yes. Degenerative changes in the spine, such as arthritis, are a common artefactual cause of high bone mass readings on DXA scans. Calcium deposits in nearby blood vessels can also cause a falsely high reading. For this reason, a careful visual review of the scan images is a critical part of the diagnosis.