High bone mass, or HBM, is a condition where an individual’s bone mineral density is significantly higher than the average for their age and sex. It is an uncommon finding, often identified unexpectedly. This condition represents a deviation from the typical bone structure, where the amount of mineral in the bone tissue is substantially increased.
Causes of High Bone Mass
The primary driver behind many cases of hereditary high bone mass is genetics. Specific variations in the LRP5 gene are a well-documented cause. This gene provides instructions for making a protein that is involved in a signaling pathway that regulates bone formation. Mutations can lead to an overactive form of this protein, resulting in increased bone production by cells called osteoblasts. This phenomenon has been popularly nicknamed the “unbreakable” gene, as it leads to bones that are denser and more robust than usual.
This genetic predisposition is the most studied cause of familial HBM, where multiple family members exhibit unusually strong skeletons. The process involves the Wnt/β-catenin signaling pathway, where the LRP5 protein acts as a co-receptor. When mutated, it enhances this pathway, tipping the balance towards bone building and away from bone breakdown. This constant state of net bone gain leads to the characteristic high bone density seen in affected individuals.
Beyond genetics, certain acquired factors can also contribute to higher bone mass. Lifelong participation in high-impact sports, for example, can stimulate bones to become denser over time as they adapt to repeated stress. Similarly, obesity is associated with increased bone density, partly because the skeleton must support a greater body weight.
Identifying High Bone Mass
High bone mass is typically an asymptomatic condition, meaning it does not produce noticeable symptoms in most individuals. Consequently, it is often discovered incidentally. The most common scenario for identification is during a dual-energy X-ray absorptiometry (DXA) scan performed for other reasons, such as routine screening for osteoporosis. The unexpected result of exceptionally dense bones can be a surprise for both the patient and the clinician.
A DXA scan is a non-invasive imaging test that measures bone mineral density. The results are reported using T-scores and Z-scores, which compare an individual’s bone density to a reference population. A T-score compares density to that of a healthy 30-year-old adult, while a Z-score compares it to people of the same age and sex. A Z-score of +2.5 or higher in at least two skeletal sites is often used to define HBM.
Health Implications
The most significant health implication of high bone mass is a markedly lower risk of fractures. The increased density and structural integrity of the bones provide a natural defense against breaks, even from significant trauma. This characteristic offers lifelong protection from osteoporosis, a common condition of bone fragility that affects many people as they age. For individuals with HBM, the concern of developing weak and brittle bones is substantially reduced.
While largely beneficial, HBM is not without potential downsides, though they are uncommon. Some individuals may develop benign bony growths, such as a torus mandibularis (a growth on the inner side of the lower jaw) or a torus palatinus (on the roof of the mouth). In very rare and extreme cases, excessive bone growth in the skull can lead to the compression of cranial nerves, potentially causing facial paralysis or hearing loss.
The density of the bone can also present challenges during certain medical procedures. Orthopedic surgeons may find it more difficult to drill into or cut through the bone during operations like a joint replacement. The hardness of the bone can complicate the placement of surgical implants and may require specialized instruments or techniques.
Associated Conditions
It is important to distinguish benign, familial high bone mass from other serious genetic disorders that also cause dense bones. Conditions like osteopetrosis, sometimes called “marble bone disease,” and sclerosteosis are distinct diseases. While they share the feature of increased bone density, they are separate and more severe skeletal dysplasias. These disorders have significant health consequences not typically found in the more common forms of HBM.
Osteopetrosis, for example, results from a failure of osteoclasts, the cells responsible for breaking down bone. This leads to bones that are not only dense but also brittle and prone to fracture. It can also cause bone marrow failure, leading to anemia and infections, as well as nerve compression. Sclerosteosis is another severe disorder characterized by progressive and often disabling bone overgrowth.
A finding of high bone density on a DXA scan does not automatically indicate the presence of a severe skeletal disease. Benign HBM, particularly the form linked to LRP5 gene mutations, generally has a much more favorable outlook with fewer and less severe complications.