Short Head of Biceps Brachii Muscle: Anatomy, Function, Imaging

Topic

Short head of biceps brachii muscle

The short head of the biceps brachii is the medial portion of the biceps muscle in the anterior compartment of the arm. It arises from the coracoid process of the scapula, running parallel and medial to the long head before merging into the common biceps tendon that inserts on the radial tuberosity.

This head plays a major role in elbow flexion and forearm supination, particularly when the forearm is pronated, and assists in shoulder flexion. It also helps stabilize the shoulder joint by resisting downward dislocation of the humeral head.

Anatomically and functionally, the short head differs from the long head, which originates from the supraglenoid tubercle and passes through the shoulder joint. The short head remains entirely extracapsular, lying superficial and medial, and is frequently evaluated on imaging for tendon tears, tendinopathy, and coracoid enthesopathy.

Synonyms

Medial head of biceps brachii
Coracoid head of biceps brachii

Origin, Course, and Insertion

Origin: Tip (apex) of the coracoid process of the scapula, via a shared tendon with the coracobrachialis muscle.
Course: Fibers descend vertically along the medial side of the arm, anterior to the brachialis and medial to the long head of biceps.
Insertion: Joins the long head to form a common tendon, which inserts into the posterior rough area of the radial tuberosity. A bicipital aponeurosis (lacertus fibrosus) extends medially to the deep fascia of the forearm.

Tendon Attachments

Proximal tendon: Short, broad, and fused with coracobrachialis origin.
Distal tendon: Common with long head; inserts on radial tuberosity and contributes to the bicipital aponeurosis.
Provides an important attachment point for surgical reference in coracoid transfer procedures.

Relations

Anteriorly: Deep fascia and skin.
Posteriorly: Brachialis muscle.
Laterally: Long head of biceps brachii.
Medially: Coracobrachialis and neurovascular bundle (brachial artery, median nerve).
Superiorly: Coracoid process and pectoralis minor tendon.
Inferiorly: Bicipital tendon and aponeurosis overlying forearm flexors.

Nerve Supply

Musculocutaneous nerve (C5–C7), branch of the lateral cord of the brachial plexus.

Arterial Supply

Brachial artery and its muscular branches.
Contributions from anterior circumflex humeral artery.

Venous Drainage

Brachial veins, draining into the axillary vein.

Function

Elbow flexion: Primary flexor of the forearm at the elbow joint.
Forearm supination: Powerful supinator when the forearm is flexed.
Shoulder flexion: Assists in flexing the shoulder joint.
Shoulder stabilization: Prevents inferior displacement of humeral head during arm abduction.
Clinical note: The short head is less prone to rupture than the long head, but can be affected by overuse or coracoid enthesopathy.

Clinical Significance

Tendinopathy: Common in athletes and weightlifters due to repetitive flexion or overload.
Coracoid enthesopathy: Calcification or inflammation at the origin causing anterior shoulder pain.
Tear or avulsion: Rare, usually due to direct trauma or surgical complications near the coracoid.
Bicipital aponeurosis injury: May coexist with distal tendon pathology.
Surgical relevance: Important in coracoid osteotomy, Latarjet procedure, and biceps tenodesis.
Imaging importance: MRI helps differentiate short head pathology from long head or coracobrachialis lesions.

MRI Appearance

T1-weighted images:
- Muscle belly: Intermediate signal intensity with clear fascicular architecture.
- Tendon: Low signal (dark), continuous from coracoid to radial tuberosity.
- Marrow (radial tuberosity): Bright fatty signal.
- Pathology: Partial tears show focal bright signal at tendon origin or insertion; fatty atrophy seen as diffuse hyperintensity.
T2-weighted images:
- Normal muscle: Intermediate-to-low signal, darker than on T1.
- Normal tendon: Low signal (black).
- Pathology: Tendinopathy, strain, or partial tears appear as bright hyperintense areas near the coracoid origin or musculotendinous junction.
- Bicipital aponeurosis: Thin low-signal band extending medially; inflamed or injured aponeurosis shows increased signal.
STIR:
- Normal muscle: Intermediate-to-dark signal intensity.
- Pathology: Bright hyperintense regions correspond to edema, strain, or inflammation, especially around the coracoid attachment or distal aponeurosis.
- Excellent for detecting early muscle injury or postoperative edema.
Proton Density Fat-Saturated (PD FS):
- Normal muscle: Intermediate-to-dark signal with uniform texture.
- Tendinopathy or tear: Bright hyperintensity along tendon fibers or at coracoid attachment.
- Useful for visualizing subtle myotendinous strains and peritendinous fluid.
T1 Fat-Sat Post-Contrast:
- Normal muscle: Homogeneous mild enhancement.
- Inflammation or partial tear: Focal enhancement at the coracoid origin or myotendinous junction.
- Chronic scarring or fibrosis: Peripheral enhancement with central low signal.

CT Appearance

Non-Contrast CT:

Muscle: Soft-tissue density anterior to humerus.
Coracoid origin: Clearly visualized; may show cortical irregularity or calcific tendinopathy in chronic overload.
Bicipital tuberosity: Normal insertion site visible; periosteal reaction indicates traction stress.
Pathology: Excellent for detecting calcific deposits, enthesophytes, or avulsion fragments at coracoid process.

Post-Contrast CT (standard):

Muscle: Homogeneous enhancement.
Inflamed or injured origin: Focal enhancement at coracoid or along aponeurosis.
Useful for evaluating postoperative changes, mass lesions, and myositis ossificans.