Collateral ligaments of metacarpophalangeal joints

In human anatomy, the radial (RCL) and ulnar (UCL) collateral ligaments of the metacarpophalangeal joints (MCP) of the hand are the primary stabilisers of the MCP joints. A collateral ligament flanks each MCP joint - one on either side. Each attaches proximally at the head of the metacarpal bone, and distally at the base of the phalynx. Each extends obliquely in a palmar direction from its proximal attachment to its distal attachment. The collateral ligaments allow spreading our the fingers with an open hand but not with the hand closed into a fist.