📌 Anatomy of the Distal Biceps Tendon and Lacertus Fibrosus

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1. Basic Structure

• Distal biceps tendon is a flat, paratenon-lined, extrasynovial structure
• No true tendon sheath
• Formed by contributions from the short head (SHB) and long head (LHB)
• Tendon formation begins approximately 7 cm proximal to the elbow

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2. Myotendinous Configuration

• SHB and LHB gradually coalesce distally
• Interdigitating fibers create a “goose quill appearance”
• Individual myotendinous junctions are often difficult to separate, especially on imaging

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3. Course and Insertion

• Tendon enters the antecubital fossa laterally
• Inserts on the ulnar/posterior margin of the radial (bicipital) tuberosity
• Anterior fibers → short head
• Posterior fibers → long head

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4. Distal Tendon Rotation

• Flattened tendon undergoes an approximate 90° distal rotation
• Rotation pattern:
• Results in:

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5. Distal Tendon Morphology and Variants

• Anatomically separable into two components on dissection
• On imaging, usually appears as a single distal tendon
• Bifurcate distal tendon may be seen → normal variant
• Additional variants include accessory slips to:
• Tendon is generally wider and thicker in males

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6. Lacertus Fibrosus (Bicipital Aponeurosis)

• Formed by superficial fibers of the distal biceps tendon
• Usually from both heads, occasionally short head only
• Fans medially across the antecubital fossa
• Blends with antebrachial fascia over the superficial flexors
• Covers and protects the median nerve and brachial artery
• May limit tendon retraction in distal biceps rupture

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7. Radial (Bicipital) Tuberosity

• Approximate size: 22 mm (length) × 15 mm (width)
• Tendon inserts on a semilunar footprint along the ulnar/posterior aspect
• Ulnar ridge morphology may be single, bifid, or absent
• A prominent ridge may predispose to tendon attrition

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🔑 MRI Take-Home Points

• Distal biceps usually appears as a single tendon with predictable internal rotation
• Anterior fibers = short head, posterior fibers = long head
• Lacertus fibrosus is best appreciated on non–fat-suppressed images

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📌 Anatomy of the Distal Biceps Tendon: What Really Matters