Lateral Head of Triceps Brachii: Anatomy, Function, Imaging

Topic

Lateral head of triceps brachii muscle

The lateral head of the triceps brachii is one of the three heads of the triceps muscle, forming the posterolateral aspect of the upper arm. It lies superficial and lateral to the long and medial heads and constitutes the bulk of the upper arm’s posterior contour. This head provides significant power in forearm extension and plays a critical role in stabilizing the elbow joint during upper limb movements.

It is the strongest head of the triceps and is particularly active during resisted extension, such as in pushing, throwing, or weightlifting.

Synonyms

Lateral triceps head
External head of triceps

Origin, Course, and Insertion

Origin: Posterior surface of the humerus, superior to the radial (spiral) groove, and lateral intermuscular septum.
Course: Fibers run downward and medially, forming a thick muscular belly on the upper lateral arm. They converge into a broad aponeurosis that joins the long and medial heads to form the common triceps tendon.
Insertion: Posterior surface of the olecranon process of the ulna via the common triceps tendon.

Relations

Superficially: Deep fascia and skin of the posterior arm.
Deeply: Radial nerve and profunda brachii vessels in the spiral groove, medial head of triceps below the groove.
Medially: Long head of triceps.
Laterally: Lateral intermuscular septum and brachialis near its origin.
Inferiorly: Olecranon and elbow joint capsule.

Nerve Supply

Radial nerve (C6–C8) — branch to the lateral head arises proximal to the spiral groove.

Arterial Supply

Profunda brachii artery (deep brachial artery), a branch of the brachial artery.
Minor supply from posterior circumflex humeral artery.

Venous Drainage

Accompanying veins of the profunda brachii artery drain into the brachial vein, then into the axillary vein.

Function

Forearm extension: Primary action — extends the elbow joint.
Elbow stabilization: Maintains joint integrity during fine movements of the hand and forearm.
Force generation: Contributes to powerful pushing, throwing, and striking actions.
Synergistic coordination: Works with long and medial heads to produce smooth extension and deceleration during flexion.

Clinical Significance

Muscle tears: Lateral head can be strained or partially torn in powerlifting or throwing sports.
Radial nerve injury: Since the nerve passes beneath the lateral head, trauma or entrapment causes triceps weakness and sensory loss over posterior forearm.
Tendinopathy: Chronic overuse leads to triceps insertional tendinopathy or olecranon bursitis.
Intramuscular hematoma: Common after direct trauma due to dense muscle fibers and vascularity.
Imaging importance: MRI differentiates between partial tears, nerve compression, and muscle strain.

MRI Appearance

T1-weighted images:
- Muscle belly: Intermediate signal intensity with clear fascicular pattern.
- Tendon: Low signal (dark), merging into the common triceps tendon.
- Marrow of humerus: Bright signal due to fatty content.
- Pathology: Partial tears appear as focal high-signal discontinuities; hematomas as intermediate-to-bright areas within the muscle.
T2-weighted images:
- Normal muscle: Intermediate-to-low signal, darker than on T1.
- Tendon: Uniformly low signal.
- Pathology: Strain, edema, or partial tear shows bright hyperintense areas at musculotendinous junction or within fibers.
- Radial nerve region: Normal nerve appears as small round low-intermediate structure lateral to humerus.
STIR:
- Normal muscle: Intermediate-to-dark signal.
- Injury or edema: Bright hyperintense signal indicating acute strain or hemorrhage.
- Excellent for detecting subtle fiber disruption and perimuscular fluid.
Proton Density Fat-Saturated (PD FS):
- Normal: Homogeneous intermediate-to-dark muscle signal.
- Pathologic: Bright hyperintensity at musculotendinous junction or around tendon in strain or tendinitis.
- Useful for grading muscle tears and detecting small hematomas.
T1 Fat-Sat Post-Contrast:
- Normal: Uniform mild enhancement.
- Inflamed or injured muscle: Focal enhancement at myotendinous junction or around hematoma.
- Chronic tendinopathy: Patchy or peripheral enhancement due to fibrosis and vascular granulation.

CT Appearance

Non-Contrast CT:

Muscle: Uniform soft-tissue density lateral to humerus.
Tendon: Slightly higher density linear structure inserting into olecranon.
Humerus: Cortical outline clear; radial groove and attachment sites visible.
Pathology: Detects muscle atrophy, calcifications, intramuscular hematoma, or avulsion fractures of olecranon.

Post-Contrast CT (standard):

Normal muscle: Mild homogeneous enhancement.
Injury: Enhancing perimuscular or intramuscular soft tissue indicates hematoma or inflammation.
Useful for: Evaluating triceps tendon rupture, postoperative scarring, or calcific tendinitis.

MRI image