Metacarpophalangeal joints

Location: At the knuckles, between the heads of the five metacarpals and the bases of the proximal phalanges; the knuckles you see when you make a fist are the metacarpal heads.

Bones taking part

• First metacarpal (head)
• Second metacarpal (head)
• Third metacarpal (head)
• Fourth metacarpal (head)
• Fifth metacarpal (head)
• Proximal phalanx of the thumb (base)
• Proximal phalanx of the index finger (base)
• Proximal phalanx of the middle finger (base)
• Proximal phalanx of the ring finger (base)
• Proximal phalanx of the little finger (base)

Articulating surfaces

• The rounded (convex) head of each metacarpal
• The shallow concave base of the corresponding proximal phalanx
• The articular surface of the metacarpal head extends further onto its palmar side than its dorsal side, which allows more flexion than extension.

Structural

Synovial joints (condylar / condyloid type).

Functional

Diarthroses (freely movable).

Synovial subtype

Condylar (condyloid).

Axes

Biaxial in the fingers: flexion-extension and abduction-adduction, which combine as circumduction. The thumb MCP behaves mainly as a uniaxial (hinge-like) joint.

Capsule & cavity

Each joint has its own fibrous capsule, attached around the margins of the articular surfaces. It is strengthened on each side by a collateral ligament and in front by the palmar ligament (palmar plate). On the back, the capsule is covered and reinforced by the extensor expansion (dorsal digital expansion) of the extensor tendons.

• Positions the fingers to grip objects of different sizes.
• Spreads and closes the fingers (abduction and adduction) for grasping and fine work.
• Together with the interphalangeal joints, curls the fingers into a fist.

Stability vs mobility: Moderately stable. The collateral ligaments give side-to-side stability, which is greatest in flexion when they are taut; the palmar plates prevent hyperextension; the deep transverse metacarpal ligaments bind the finger joints together; and the extensor expansion covers the back. The trade-off is that spreading the fingers is possible only in extension.

• When the hand must be immobilised, the MCP joints are usually splinted in flexion so the collateral ligaments are kept at full length and do not shorten and stiffen the joint.
• Injury to the ulnar collateral ligament of the thumb's MCP joint ('gamekeeper's' or 'skier's' thumb) weakens pinch grip.
• Rheumatoid arthritis commonly affects the MCP joints and can produce ulnar deviation of the fingers.
• A fracture of the neck of the fifth metacarpal ('boxer's fracture'), from punching, lies just proximal to the MCP joint.

• Classify them as condylar (condyloid) synovial joints, biaxial; do not call them hinge or ball-and-socket joints.
• State the reference line for finger abduction and adduction: the axial line through the middle finger.
• Explain why abduction is impossible with the fingers flexed (collateral ligaments taut in flexion).
• Contrast the thumb MCP: mainly flexion and extension, sesamoid bones, no deep transverse metacarpal ligament.
• Name the muscles: interossei and lumbricals flex the MCP joints; dorsal interossei abduct, palmar interossei adduct.

• Calling the MCP joints hinge joints. The finger MCPs are condylar and biaxial; the interphalangeal joints are the hinges.
• Using the midline of the body as the reference for finger abduction; it is the axial line through the middle finger.
• Placing opposition at the thumb MCP joint.
• Saying the collateral ligaments are tight in extension; they are tight in flexion.
• Including the thumb in the deep transverse metacarpal ligament.