11.1 Forearm Anatomy, Landmarks, and Adjacent Structures

11.1 Forearm Anatomy, Landmarks, and Adjacent Structures

Quick Answer: The forearm holds two bones: the radius laterally (thumb side) and the ulna medially (little-finger side). The ulnar styloid distally and the olecranon proximally are the landmarks that define forearm length, which the software uses to place regions of interest. The distal radius is trabecular-rich metaphysis; the shaft at the one-third site is predominantly cortical. The carpal bones and the radioulnar gap are the adjacent structures that determine analyzability.

Why the Forearm Has Its Own Anatomy Section

The forearm is not simply a smaller spine. Its regions of interest are located by measured distance along a bone, not by recognizing a vertebral body or a femoral neck. That makes landmark identification and length measurement into primary determinants of the result, in a way that has no equivalent at the other two sites.

Osteology

The Radius

• Lateral bone of the forearm, on the thumb side.
• Small proximally (the radial head at the elbow) and large distally at the wrist — the opposite of the ulna.
• The distal radius carries most of the load transmitted from the hand through the wrist, which is why a fall on an outstretched hand fractures the distal radius (Colles fracture) rather than the ulna.
• Radial styloid process: the distal lateral projection, palpable at the base of the thumb.
• The distal metaphysis is trabecular-rich; the mid and proximal shaft are cortical.

The Ulna

• Medial bone, on the little-finger side.
• Large proximally (the olecranon at the elbow) and small distally.
• Olecranon process: the prominent point of the elbow — the proximal landmark for forearm length measurement.
• Ulnar styloid process: the small distal projection, palpable on the little-finger side of the wrist — the distal landmark.

The Relationship Between Them

The radius and ulna lie side by side, separated by the interosseous space occupied by the interosseous membrane. Distally they converge; proximally they diverge. That geometry matters because:

• The gap between the two bones widens proximally, and analysis software uses the point at which the radius and ulna are separated by a defined distance as a reference for locating the ultradistal region on some systems.
• Pronation and supination rotate the radius around the ulna. In full supination the bones lie parallel; in pronation the radius crosses over the ulna. A pronated, rotated forearm projects the two bones overlapping, which defeats the software's ability to locate regions.

Landmarks Used in DXA

Forearm length — measured from the ulnar styloid to the olecranon on most systems — is entered into the scanner so the software can compute where along the bone to place the percentage-defined regions. A length entered incorrectly places the ROI at the wrong point along a bone whose cortical thickness changes continuously with position, so the reported BMD is a measurement of a different piece of bone than intended.

Radiographic Appearance on DXA

A correctly acquired forearm image shows:

• The radius and ulna running parallel along the long axis of the image, without overlap.
• The distal ends — radial and ulnar styloids — at one end of the field, with the carpal bones just beyond.
• Clear separation between the two bones through the region of interest zone.
• Soft tissue on both sides of the forearm for the baseline estimate.
• The cortical shell of each bone visible as a dense rim, thick in the shaft and thin distally.

Density changes visibly along the bone: the distal metaphyseal region appears less dense (trabecular) and the shaft appears denser (cortical). That gradient is precisely why ROI position matters.

Significant Adjacent Structures

Skeletal Composition and Site Choice

The cortical composition of the one-third radius explains both of its uses. It is reproducible, which makes it the diagnostic forearm site. And it is cortical, which makes it the preferred site in primary hyperparathyroidism — a disease in which chronically elevated parathyroid hormone preferentially resorbs cortical bone, so the one-third radius shows the disease while the trabecular-rich spine may look deceptively normal.