Carpometacarpal joints

Location: In the hand, between the distal row of carpal bones and the bases of the five metacarpals; the first (thumb) joint is at the base of the thenar eminence, between the trapezium and the first metacarpal.

Bones taking part

• Trapezium (with 1st metacarpal; also touches the 2nd)
• Trapezoid (with 2nd metacarpal)
• Capitate (mainly with 3rd metacarpal)
• Hamate (with 4th and 5th metacarpals)
• Pisiform (attachment of the pisometacarpal ligament only)
• First metacarpal (base)
• Second metacarpal (base)
• Third metacarpal (base)
• Fourth metacarpal (base)
• Fifth metacarpal (base)

Articulating surfaces

• First CMC: the saddle-shaped distal surface of the trapezium with the reciprocally saddle-shaped base of the first metacarpal
• Second CMC: base of the second metacarpal mainly with the trapezoid, also touching the trapezium and capitate
• Third CMC: base of the third metacarpal with the capitate
• Fourth CMC: base of the fourth metacarpal with the capitate and hamate
• Fifth CMC: base of the fifth metacarpal with the hamate

Structural

All are synovial joints: the first is saddle type; the second to fifth are plane type.

Functional

Diarthroses. The first is freely movable; the second and third are almost immobile; the fourth and especially the fifth allow slight movement.

Synovial subtype

First CMC: saddle (sellar). Second to fifth CMC: plane.

Axes

First CMC: biaxial (saddle), with an element of rotation that contributes to opposition. Second to fifth: plane joints, gliding only (non-axial).

Capsule & cavity

The first CMC joint has its own separate, loose capsule, which is what allows its wide range of movement. The second to fifth CMC joints are enclosed in a common capsule, and their joint cavity is usually shared with the intercarpal and intermetacarpal joints.

• The first CMC joint gives the thumb its independence and lets it oppose the fingers: the basis of precision and power grips.
• The almost immobile second and third CMC joints give the hand a stable central column.
• Movement at the fourth and fifth CMC joints lets the palm cup around objects and strengthens grip.

Stability vs mobility: Opposite trade-offs within one group: the first CMC joint is mobile with a loose capsule, and relies on surrounding muscles and ligaments for stability, so it is prone to wear. The second and third CMC joints are tightly bound by dorsal, palmar and interosseous ligaments and are very stable, with little movement; stability decreases and mobility increases towards the fifth.

• Osteoarthritis of the first CMC joint (base-of-thumb arthritis) is common, especially in older women, causing pain on pinching and gripping.
• Bennett fracture: an intra-articular fracture of the base of the first metacarpal with subluxation of the first CMC joint.
• Loss of opposition (for example from median nerve injury affecting the thenar muscles) severely impairs hand function.

• Distinguish the first CMC (saddle, own capsule, highly mobile) from the second to fifth (plane, common cavity, little movement).
• Name the articulating bones of the first CMC: trapezium and base of the first metacarpal.
• State that opposition belongs to the first CMC joint and describe it as abduction, flexion and medial rotation.
• Explain why thumb flexion/extension occur in the plane of the palm and abduction/adduction at right angles to it.
• Mobility increases from the second to the fifth CMC joints; the second and third are almost fixed.

• Calling all CMC joints saddle joints. Only the first is saddle; the rest are plane.
• Placing opposition at the MCP joint of the thumb.
• Saying the first CMC joint shares the common carpal joint cavity; it has its own capsule.
• Describing thumb abduction as movement in the plane of the palm; that is extension.
• Saying the fifth CMC is immobile; it is the most mobile of the second to fifth.