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Accessory collateral ligament

The accessory collateral ligament (ACL) of the finger is a thin, fan-shaped ligament located on each side of the proximal interphalangeal (PIP) and metacarpophalangeal (MCP) joints, lying volar (anterior) to the proper collateral ligament. It plays a key role in stabilizing the volar plate and enhancing joint stability during extension.

It originates from the volar aspect of the proximal phalanx (or metacarpal head at MCP) and inserts into the volar plate, blending with the fibrous tissues that support the palmar side of the joint. The accessory collateral ligaments become tight in extension and relax in flexion, opposite to the behavior of the proper collateral ligaments.

Synonyms

  • Secondary collateral ligament

  • Volar collateral ligament (older terminology, not preferred)

Location and Structure

  • Located: On the radial and ulnar sides of each MCP and PIP joint

  • Shape: Thin, triangular, and fan-like

  • Origin: Volar aspect of metacarpal head (MCP) or proximal phalanx (PIP)

  • Insertion: Lateral margins of the volar plate

  • Orientation: Lies anterior/volar to the proper collateral ligament

  • Composition: Dense fibrous connective tissue continuous with joint capsule

Relations

  • Dorsally: Proper collateral ligament

  • Volarly: Volar plate and flexor tendon sheath

  • Laterally: Joint capsule and collateral recess

  • Medially/Laterally: Adjacent accessory collateral ligament on opposite side

Function

  • Tightens in extension, stabilizing the joint and supporting the volar plate

  • Prevents hyperextension of the MCP and PIP joints

  • Works with proper collateral ligaments to provide side-to-side joint stability

  • Assists in limiting excessive volar translation of the joint surfaces

  • Provides structural continuity between the capsule and volar plate

MRI Appearance

T1-weighted images:

  • Accessory collateral ligament appears as a low-signal (dark) thin linear structure

  • Distinct from surrounding fat (bright) and muscle (intermediate)

  • Volar plate shows slightly thicker low-signal band merging with ligament

T2-weighted images:

  • Ligament remains dark to intermediate signal

  • Clear contrast with surrounding soft tissues and joint fluid (bright)

  • Well visualized as a fine linear band at joint margins

STIR:

  • Normal ligament appears dark to intermediate

  • High contrast against bright fat-suppressed background

  • Volar plate and proper collateral ligament also show low signal

MRI image

Accessory collateral ligament HAND MRI CORONAL image-img-00000-00000

MRI image

Accessory collateral ligament HAND MRI CORONAL image-img-00000-00000