6.4 Vertebral Fracture Assessment and Trabecular Bone Score

6.4 Vertebral Fracture Assessment and Trabecular Bone Score

Quick Answer: VFA is a lateral thoracolumbar spine image acquired on the densitometer to find vertebral fractures — which matter enormously, are mostly silent, and are invisible to BMD. TBS is a software texture analysis performed on the PA lumbar spine image you already acquired, estimating trabecular microarchitecture and adding fracture-risk information independent of BMD. Neither replaces BMD; both answer questions BMD cannot.

Vertebral Fracture Assessment

Why a BMD Number Is Not Enough

A prevalent vertebral fracture is one of the strongest predictors of future fracture, and a patient with one is treated regardless of her T-score. Yet roughly two-thirds of vertebral fractures never come to clinical attention: they occur without a recognized traumatic event, produce nonspecific or no back pain, and are never imaged. A patient can present with a T-score of −2.0 — osteopenia, not osteoporosis — and already have two wedge fractures. VFA finds them.

How VFA Is Acquired

VFA is a lateral image of the thoracic and lumbar spine, typically covering approximately T4 through L4, acquired on the DXA system. On fan-beam systems it is a single sweep taking roughly 10 to 30 seconds, with an effective dose on the order of tens of microsieverts — a small fraction of conventional lateral spine radiography.

Acquisition and positioning points:

• Most systems acquire VFA with the patient in the lateral decubitus position, or with the C-arm rotated so the patient remains supine — the supine approach is more reproducible because it avoids spinal sagging.
• The arms are elevated out of the field, typically flexed forward or above the head, so the humeri do not overlie the upper thoracic vertebrae.
• The spine must be parallel to the table and centered; a sagging or rotated spine produces apparent endplate obliquity that mimics deformity.
• Quiet breathing is used; long breath-holds are unnecessary and cause motion.
• Upper thoracic vertebrae above roughly T6–T7 are frequently obscured by the shoulder girdle and mediastinum; this is a known limitation rather than a technique failure.

Who Is Imaged

VFA is considered when knowledge of a vertebral fracture would change management — for example, in patients with a T-score in the osteopenic range, documented historical height loss, self-reported but undocumented prior vertebral fracture, glucocorticoid therapy, or advanced age. Height loss is the single most useful trigger a technologist can capture, which is why height is measured at every visit rather than reported.

Interpreting Deformity: The Genant Method

The widely used Genant semiquantitative approach grades each vertebra by the percentage of height loss relative to adjacent normal vertebrae and by the pattern of deformity (wedge, biconcave, or crush):

Roughly a 20% height reduction is the conventional threshold for calling a vertebral deformity a fracture. Not every deformity is an osteoporotic fracture — Scheuermann disease, degenerative remodeling, congenital variants, and malignancy all deform vertebrae — which is why interpretation belongs to the physician and why equivocal findings prompt conventional radiography.

VFA and BMD Analysis Are Connected

A vertebra identified as fractured on VFA is a vertebra that must be excluded from the PA lumbar spine BMD analysis, because compression concentrates mineral into a smaller projected area and falsely elevates BMD. Acquiring VFA therefore improves the BMD analysis as well as adding fracture information.

Trabecular Bone Score

What TBS Measures

BMD tells you how much mineral is present. It cannot tell you how that mineral is arranged — and microarchitecture is a major component of bone strength. Two patients with identical L1–L4 BMD can have very different trabecular structure and very different fracture risk.

Trabecular bone score is a grey-level texture analysis applied to the pixel intensity variations of the standard PA lumbar spine DXA image. Dense, well-connected trabecular bone produces an image with many small-amplitude grey-level variations, yielding a high TBS. Degraded, perforated trabecular structure produces fewer, larger-amplitude variations, yielding a low TBS.

Key operational facts:

• TBS is computed from the image already acquired. There is no additional scan and no additional radiation dose.
• It is derived from the same L1–L4 region used for BMD, and the same vertebral exclusions apply.
• It is reported as a unitless index, with higher values indicating better-preserved microstructure.
• It is independent of BMD in the statistical sense, which is what allows it to add risk information rather than restate it.

Clinical Use

TBS is used to refine fracture risk when BMD alone is ambiguous — most commonly in osteopenic patients near a treatment threshold. FRAX can be adjusted for TBS, shifting calculated 10-year probabilities up or down. TBS is also of interest in conditions where BMD is known to understate risk, notably type 2 diabetes and glucocorticoid exposure, where bone quality is degraded more than density suggests.

Limitations the Technologist Must Know

• Body habitus dependence. TBS is influenced by soft tissue thickness overlying the spine, and the algorithm is validated within a defined body mass index range. Values in very obese or very thin patients are unreliable, and software versions differ in how they handle this.
• Same artifacts, same rules. Degenerative change, surgical hardware, osteophytes, aortic calcification, and contrast all corrupt the underlying image, so they corrupt TBS. A vertebra excluded from BMD is excluded from TBS.
• Scanner and version specificity. TBS values are tied to the acquisition system and software version, so serial comparison requires the same scanner and consistent software, exactly as BMD does.
• Not a diagnostic criterion. There is no WHO classification based on TBS. It modifies risk estimates; it does not make a diagnosis.
• Image quality determines TBS quality. A motion-degraded or poorly centered spine image produces an unreliable texture analysis even when the BMD number looks acceptable.

TBS and whole body DXA both appear as named topics in the ARRT content specifications effective January 1, 2027, so candidates testing from that date forward should expect explicit items on them.