Triangular Interval

intervallum triangulare

The triangular interval, sometimes called the lower triangular space, lies just below the shoulder at the back of the arm. It is bounded by teres major above, the long head of triceps medially and the lateral head of triceps or humeral shaft laterally, and it carries the radial nerve and profunda brachii artery into the posterior arm.

Anatomical Data

Boundaries & Contents

Location	Posterior upper arm, below the teres major.
Boundaries	Teres major above Long head of triceps medially Lateral head of triceps and humeral shaft laterally
Contents	Radial nerve Profunda brachii artery and veins

Biomechanics

Function & Actions

Provides the passage of the radial nerve and deep artery of the arm to the posterior compartment.

Clinical Relevance

Clinical Notes

This is where the radial nerve leaves the axilla to reach the spiral groove, so fibrous bands or the lateral head of triceps can rarely compress it here. The interval is a landmark for posterior approaches to the humerus and for radial nerve blocks.

Pathology

Common Injuries & Conditions

Radial Nerve Compression at the Triangular Interval

Rare entrapment of the radial nerve by fibrous or muscular bands in the interval, causing triceps sparing or involving weakness of wrist and finger extension depending on the level.

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