Anconeus Muscle: Anatomy, Function, Relations, and Imaging

Topic

Anconeus muscle

The anconeus muscle is a small triangular muscle located at the posterior aspect of the elbow joint, situated between the lateral epicondyle of the humerus and the olecranon of the ulna. Though small, it plays an important role in elbow extension, joint stabilization, and capsular tension regulation during movement.

It is considered functionally as an accessory extensor of the elbow, aiding the triceps brachii. Its deep position and triangular shape make it an important anatomical landmark in surgical and imaging assessments of the posterior elbow region.

Synonyms

Elbow triangle muscle
Small extensor of the elbow
Accessory triceps muscle

Origin, Course, and Insertion

Origin: Posterior surface of the lateral epicondyle of the humerus (common extensor origin)
Course: Fibers run inferomedially across the posterior aspect of the elbow joint
Insertion: Lateral surface of the olecranon and the proximal posterior surface of the ulna

Relations

Superficially: Skin, superficial fascia, and posterior antebrachial cutaneous nerve
Deeply: Capsule of the elbow joint
Medially: Olecranon process of the ulna
Laterally: Common extensor muscles (extensor carpi ulnaris and extensor digiti minimi)
Anteriorly: Posterior aspect of the elbow joint capsule and synovial recess

Nerve Supply

Branch of the radial nerve (C7, C8, and T1), typically arising from the radial nerve in the radial groove, sometimes directly from the triceps branch

Arterial Supply

Middle collateral artery (branch of the profunda brachii artery)
Recurrent interosseous artery (branch of the posterior interosseous artery)
Minor supply from the posterior branch of the radial collateral artery

Venous Drainage

Deep veins accompanying arteries drain into the radial vein and profunda brachii vein
Ultimately drain into the brachial venous system

Function

Elbow extension: Assists the triceps brachii in extending the forearm at the elbow joint
Joint stabilization: Tightens the capsule of the elbow to prevent impingement during extension
Proprioception: Provides sensory feedback for fine elbow movement and stability
Dynamic support: Helps maintain posterolateral elbow stability during pronation-supination cycles

Clinical Significance

Injury or strain: May occur with overuse, repetitive extension, or valgus stress in throwing athletes
Anconeus epitrochlearis variant: An accessory muscle that can compress the ulnar nerve in the cubital tunnel
Posterolateral elbow pain: Can originate from inflammation or hypertrophy of the anconeus
Surgical relevance: Serves as a local flap in reconstructive elbow surgery and as an anatomical landmark during posterolateral approaches
Imaging role: Important in differentiating between triceps tendon tears and posterior elbow pathology

MRI Appearance

T1-weighted images:
- Muscle belly: intermediate signal intensity with fine fascicular pattern
- Fat planes: bright, clearly defining the muscle boundaries
- Tendon attachments at lateral epicondyle and olecranon: low signal (dark)
- Normal adjacent bone marrow: bright fatty signal
T2-weighted images:
- Normal muscle: intermediate-to-low signal, slightly darker than on T1
- Pathology (strain, edema, inflammation): bright hyperintense signal within muscle fibers or myotendinous junction
- Joint effusion or bursitis: hyperintense fluid in posterior recess of elbow joint
STIR:
- Normal muscle: intermediate-to-dark signal
- Pathologic muscle or edema: bright hyperintense with blurred margins
- Excellent for detecting early strain or inflammatory changes
Proton Density Fat-Saturated (PD FS):
- Normal: intermediate-to-dark homogeneous signal
- Pathology: focal or diffuse bright areas indicating muscle injury or enthesitis
- Provides superior contrast between muscle and fat planes
T1 Fat-Sat Post-Contrast:
- Normal muscle: mild, uniform enhancement
- Myositis or tendinopathy: focal or diffuse enhancement near tendon insertion
- Postsurgical or inflammatory change: peripheral enhancement with central low-signal fibrosis

CT Appearance

Non-Contrast CT:

Muscle: uniform soft-tissue density posterior to the elbow joint
Clear borders between olecranon and lateral epicondyle
Useful for identifying calcification, enthesophyte formation, or post-traumatic changes
Subtle myositis ossificans may appear as early peripheral calcification

Post-Contrast CT (standard):

Muscle shows homogeneous mild enhancement
Inflammation or myositis: localized increased enhancement or swelling
Detects hematoma, soft-tissue edema, and enthesis calcifications near lateral epicondyle

MRI images