11.4 Forearm Regions of Interest: One-Third, Mid, Ultradistal, and Total

11.4 Forearm Regions of Interest: One-Third, Mid, Ultradistal, and Total

Quick Answer: A forearm report typically lists ultradistal, mid-radius, 33% (one-third) radius, and total regions, and on many systems ulna equivalents as well. They differ fundamentally in composition. Only the 33% radius — roughly 95% cortical bone — is used for WHO diagnostic classification. The others are informative but carry no diagnostic threshold.

Why the Forearm Has Multiple Regions

Unlike a vertebral body, the radius is not uniform. Its composition changes continuously from a trabecular-rich metaphysis at the wrist to a thick-walled cortical tube in the shaft. A single "forearm BMD" would average across tissues that behave completely differently, so the scanner segments the bone into regions whose composition is roughly homogeneous.

The regions are defined by position along the measured forearm length, which is why the length measurement in the previous section directly determines what is measured.

The Regions

The 33% Radius

The 33% radius, also called the one-third radius, is the forearm's diagnostic site, and its two properties explain everything about how it is used.

It is cortical. Roughly 95% compact bone. Cortical bone turns over slowly, which makes it:

• Reproducible. Little biological variation between visits and a well-defined dense boundary that edge detection handles reliably. That reproducibility is the main reason it is the diagnostic site.
• Insensitive to short-term change. Antiresorptive therapy that raises spine BMD 2–4% in a year may produce no measurable change here for years, which makes it a poor monitoring site.
• The site that reveals cortical disease. In primary hyperparathyroidism, where parathyroid hormone preferentially resorbs cortical bone, the one-third radius shows loss that a trabecular-rich spine may hide.

It is defined by distance, not anatomy. The region is placed at a computed point along the bone. Get the forearm length wrong, or apply a different convention at follow-up, and the region lands somewhere else on a bone whose cortical thickness varies continuously.

Diagnostic Status

ISCD includes the 33% radius among the sites valid for WHO diagnostic classification, alongside the PA lumbar spine, femoral neck, and total hip. It is used when the spine and hip cannot be measured or interpreted, in primary hyperparathyroidism, and in patients over the table weight limit.

When a forearm T-score is used for classification, it is the 33% radius T-score — not the ultradistal, not the total, not an average.

The Ultradistal Radius

The ultradistal region sits just proximal to the distal radial articular surface, in the metaphyseal flare. It is trabecular-rich, on the order of two-thirds trabecular bone.

That composition gives it the opposite profile to the one-third site:

• More responsive. Early postmenopausal trabecular loss shows here before it shows in the shaft, and therapy effects appear sooner.
• Less reproducible. The region lies where the bone is flaring and changing shape rapidly along its length, so small positioning or length differences shift the region onto measurably different bone. Precision error is higher than at the one-third site.
• Clinically relevant site. It is where Colles fractures occur, so it is of genuine biomechanical interest.
• Not diagnostic. WHO classification thresholds were not validated for it, and its precision does not support a diagnostic threshold.

The ultradistal value is reported and may be commented on, but a patient is not diagnosed with osteoporosis on the basis of an ultradistal T-score of −2.7.

Mid-Radius and Total

• Mid-radius falls between the two in composition and behavior and is reported on some systems. It is not a diagnostic site.
• Total radius combines the measured regions into a composite. Because the cortical shaft contributes most of the mineral, the total tends to track the cortical regions, but it mixes tissue types and is not diagnostic.
• Ulna regions and combined radius-plus-ulna values are reported by some systems. None is a diagnostic site.

Reading a Forearm Report Correctly

Given a report listing all four regions, the interpretive sequence is:

1. Find the 33% radius T-score. That is the diagnostic value.
2. Confirm which side was scanned and that it matches any prior study.
3. Check the entered forearm length against the recorded value from the baseline.
4. Look at the other regions for pattern information, not for diagnosis. A markedly low ultradistal value with a preserved one-third value suggests trabecular-predominant loss; the reverse suggests cortical-predominant loss, as in hyperparathyroidism.
5. For serial comparison, compare absolute BMD at the same region against the facility LSC for that region.

The Composition Logic in One Table

That table is worth memorizing as a unit, because it explains site selection at every level of this exam: why the spine monitors therapy, why the one-third radius diagnoses hyperparathyroid bone disease, and why a single number never tells the whole skeletal story.