Carpometacarpal (CMC) Joints Anatomy

The 2nd through 5th CMC joints are a continuous interconnected synovial lined cavity.
The number of ligaments and articular facets of the 2nd through 5th CMC joints are quite variable.
The 2nd and 3rd CMC joints are extremely stable joints with a limited range of motion because of their cornerstone bony morphology and strong ligaments.
The ligaments include multiple palmar and dorsal carpometacarpal ligaments and two longitudinal interosseous carpometacarpal ligaments.
The two longitudinal interosseous ligaments include first the thickest ligament which runs longitudinally between the 2nd and 3rd metacarpals, the capitate, and the hamate while second radial longitudinal ligament connects the 2nd mwtacarpal to the trapezium. There are also interosseous ligaments between the bases of the metacarpals.
Typically the 2nd metacarpal articulates with the trapezium, the trapezoid, and slightly with the capitate; the 3rd metacarpal articulares the capitate; the 4th metacarpal articulates with the hamate and some with the capitate; and the 5th metacarpal articulates with the hamate.
Viegas et al have shown that the average flexion/extension arc of motion in the 2nd CMC joint averages 11 degrees, in the 3rd CMC joint averages 7 degrees, in the 4th CMC joint averages 20 degrees, and in the 5th CMC joint averages 27 degrees.

Diagrams & Photos

Key Points

The flexion/extension 4th & 5th CMC joints' arc of motion is greater when the two CMC joints are allowed to move together.
The carpal boss is a bony prominence and partial coalition of the 2nd metacarpal base, the 3rd metacarpal base, the trapezoid, and the capitate.
The carpal boss bony prominence are be painful and the EIP and EDC II tendons can click on the prominence during active motion.
The excision of the carpal boss bony prominence can lead to recurrence and the need for CMC joint arthrodesis.