M01.03.003 The Triangular Interval

The triangular interval is an anatomical space located immediately below the axilla. It serves as a conduit for structures passing between the anterior and posterior compartments of the upper arm, and between the posterior compartment and the axilla. This guide explores its borders, contents, and clinical relevance.

Borders of the Triangular Interval

The triangular interval is oriented with a superior base and inferior apex. It is defined by three distinct borders:

Visual Aid

• Fig 1: Illustration of the borders of the triangular interval.

Contents of the Triangular Interval

The triangular interval facilitates the movement of structures between:

1. The anterior and posterior compartments of the upper arm.
2. The posterior compartment of the upper arm and the axilla.

Structures Within

Visual Aid

• Fig 2: Diagram of the triangular interval contents.

Clinical Relevance

Triangular Interval Syndrome

This condition arises from compression of the radial nerve within the triangular interval.

Causes

• Hypertrophy of triceps brachii or teres major.
• Presence of fibrous bands between these muscles.

Clinical Features

Points to Remember

• Orientation: The triangular interval has a base superiorly and an apex inferiorly.
• Key Contents: Radial nerve and profunda brachii artery.
• Clinical Concern: Radial nerve compression can lead to pain, paraesthesia, and motor deficits.

Suggested Reading

• Gray’s Anatomy Textbook
• Clinical Neuroanatomy and Neuroscience