Bone health assessment now incorporates a deeper understanding of bone structure to more accurately predict fracture risk. Dual-Energy X-ray Absorptiometry (DEXA) has long been the standard tool, but it focuses only on bone mass. The Trabecular Bone Score (TBS) offers a complementary measure of bone quality. This dual approach provides a more comprehensive view of skeletal strength, moving toward a personalized and precise evaluation of fracture vulnerability. The interplay between these two measurements is changing how healthcare providers diagnose and manage conditions like osteoporosis.
Dual-Energy X-ray Absorptiometry Explained
Dual-Energy X-ray Absorptiometry (DEXA) is the foundational imaging technique for assessing skeletal health by measuring Bone Mineral Density (BMD). The scan passes two different low-dose X-ray beams through the body, typically focusing on the lumbar spine and hip, which are sites often affected by osteoporotic fractures. The differential absorption of these beams by the bone mineral allows for a precise calculation of BMD, representing the quantity of mineralized tissue present.
The results are expressed in two primary scores: the T-score and the Z-score. The T-score compares the patient’s BMD with the average peak bone mass of a healthy young adult of the same sex, measured in standard deviations (SDs). The T-score is the metric used for the formal diagnosis of osteoporosis, as defined by the World Health Organization (WHO) criteria. A T-score of -2.5 or lower is classified as osteoporosis, while a score between -1.0 and -2.5 indicates low bone mass (osteopenia). The Z-score compares the patient’s BMD to that of an age-matched and sex-matched reference population, which is useful for identifying secondary causes of bone loss.
Trabecular Bone Score Explained
The Trabecular Bone Score (TBS) measures bone quality, or microarchitecture, which is distinct from the quantity measured by DEXA. TBS is derived from a specialized software analysis of the existing lumbar spine DEXA image, requiring no separate scan or additional radiation exposure. This software analyzes the pixel gray-level variations within the image to create an index reflecting the underlying structure of the trabecular bone.
Trabecular bone is the spongy, inner layer of bone tissue; its microarchitecture is the complex lattice-like network of rods and plates (trabeculae). A higher TBS score indicates a denser, more interconnected structure, characteristic of stronger bone. Conversely, a lower score suggests a degraded, less connected, and more porous structure, indicating poorer bone quality. TBS provides information independent of the BMD T-score, meaning two patients with the same BMD can have different TBS values and fracture risks.
The Difference Between Density and Microarchitecture
Understanding the distinction between bone density and bone microarchitecture is central to appreciating the value of both DEXA and TBS. DEXA measures bone density, analogous to measuring how much building material is present in a structure. It provides a two-dimensional area measurement of the total mineral content, which is a determinant of overall bone strength. However, bone strength is not solely determined by mass; it is also influenced by the internal arrangement of that mass.
Microarchitecture, assessed by TBS, represents how well structured the material is. Consider a brick wall: BMD is like the number of bricks, but TBS is like the quality and arrangement of the mortar holding them together. A wall with high BMD can still be weak if the mortar is crumbling and the bricks are poorly connected (low TBS). Similarly, a patient with a normal BMD might still face a high fracture risk if their trabecular network is poorly connected, making the bone fragile. This structural arrangement determines bone mechanical strength independent of total bone mass.
Using Both Scores for Comprehensive Fracture Risk
Combining the information from DEXA and TBS provides a more accurate and comprehensive assessment of fracture risk than using BMD alone. The two measurements are complementary, with TBS adding predictive power independent of the standard T-score. This combined data is frequently integrated into advanced risk assessment tools, such as the Fracture Risk Assessment Tool (FRAX).
The FRAX tool uses an algorithm that calculates the 10-year probability of a major osteoporotic fracture. Including the TBS value allows for an adjustment that refines this probability. This is particularly useful for individuals whose BMD scores fall into the osteopenia range, where the clinical decision to begin treatment is often uncertain. Adjusting the FRAX score based on TBS helps healthcare providers better classify patients near the intervention threshold, identifying those with low BMD but poor bone quality who require treatment. TBS is also informative in conditions such as type 2 diabetes or in patients taking glucocorticoids, where BMD readings can be misleading because the disease or medication degrades bone quality disproportionately to density.