Trabecular Bone Score (TBS) is a specialized measurement focusing on bone quality rather than quantity. It provides information about the microarchitecture of the inner, spongy bone tissue. By analyzing this structural integrity, TBS offers a more complete picture of fracture risk than traditional bone density tests alone. This helps physicians identify patients who have seemingly healthy bone density but possess underlying structural weaknesses.
Understanding Trabecular Bone Score
The Trabecular Bone Score is a software application that analyzes the texture of existing Dual-Energy X-ray Absorptiometry (DEXA) images of the lumbar spine. This technique extracts information from standard bone density test data, requiring no separate scan or additional radiation exposure. The software specifically examines the trabecular bone, which is the spongy, porous tissue found inside the vertebrae and at the ends of long bones.
The score is an indirect measure of the three-dimensional microarchitecture of this inner bone structure. It assesses the connectivity and distribution of the tiny, rod-like structures within the bone matrix. A high TBS indicates a dense, well-connected network, signifying good bone quality, while a low score suggests a fragmented and porous structure.
The Role of DEXA Scans and T-Scores
The standard for evaluating skeletal health is the DEXA scan, which measures Areal Bone Mineral Density (aBMD). This measurement quantifies the mineral content in a given area, assessing bone quantity. A limitation is that the DEXA scan provides a two-dimensional image of a three-dimensional structure.
DEXA scan results are primarily reported using a T-score, which compares a patient’s BMD to that of a healthy young adult. A T-score of -1.0 or higher is considered normal bone density. Scores between -1.0 and -2.5 indicate osteopenia (low bone mass), and a score of -2.5 or lower is the diagnostic threshold for osteoporosis.
The Z-score compares a patient’s BMD to the average density of individuals of the same age, sex, and size. A Z-score significantly below -2.0 suggests bone loss may be due to factors other than typical aging, such as an underlying medical condition. While these scores are effective for diagnosis, they do not fully capture the structural arrangement measured by the Trabecular Bone Score.
Interpreting Your TBS Results
The Trabecular Bone Score is reported as a numerical index, where higher numbers correlate to better bone microarchitecture and lower fracture risk. For clinical interpretation, scores are categorized into three groups to guide risk assessment. A TBS value of 1.350 or greater is associated with normal or optimal bone microarchitecture.
Scores ranging between 1.200 and 1.350 suggest a partially degraded microarchitecture, indicating a moderate decline in bone quality. This means the internal structure is becoming less robust. A score of 1.200 or lower is indicative of a degraded microarchitecture, representing a significant loss of structural integrity and a higher risk of fracture.
These categories allow clinicians to assess bone quality independently of bone density. For instance, two individuals might have the same T-score, but the one with the lower TBS score has a greater underlying structural vulnerability, which aids in treatment decisions.
How TBS Improves Fracture Risk Prediction
The utility of the Trabecular Bone Score lies in its ability to enhance the accuracy of fracture risk prediction beyond standard BMD measurements. TBS is a risk factor independent of bone density and other clinical risk factors, such as age or prior fractures. It can be integrated into comprehensive tools like the FRAX (Fracture Risk Assessment Tool) to adjust the estimated 10-year probability of a major fracture.
This combination is beneficial for patients near an intervention threshold, such as those with osteopenia. A patient considered moderate risk based on their T-score alone may be reclassified as high risk if their TBS score is in the degraded range, prompting proactive treatment.
Conversely, a patient with mild osteoporosis might have a good TBS, which could lower their calculated risk and influence the intensity of their treatment plan. TBS provides a quantitative measure of bone quality, allowing physicians to fine-tune a patient’s risk profile for personalized management.