The metacarpophalangeal (MCP) joints are located where the hand’s fingers and thumb meet the palm. They are commonly known as large knuckles.
In medical terminology, the MCP joints are synovial joints located between the metacarpals and proximal phalanges. The metacarpals are the 5 bones located between the wrist and the fingers, including the thumb. The phalanges are the bones of the fingers, thumbs, and toes.
The MCP joints are condyloid joints that allow for movement back and forth as well as side-to-side (fanning the fingers). The MCP joint in the thumb allows for less side-to-side motion than the MCP joints in the fingers. (The thumb’s circular range of motion is mostly enabled by the joint at the base of the hand, called the carpometacarpal joint.)
Arthritis in the MCP joints
The MCP joints can be affected by arthritis, particularly rheumatoid arthritis. Osteoarthritis can develop in the MCP joints, though it is more likely to occur the fingers’ middle (PIP) and top knuckles (DIP).1 MCP joints can develop post-traumatic arthritis due to a past injury.
An arthritic MCP joint may:
- Feel painful and/or stiff
- Be tender
- Look swollen
- Have less range of motion
Arthritic MCP joints can make it more difficult to grip or pinch items. For example, opening a jar or holding and turning a key may be challenging. Pain and weakness related to MCP arthritis may also cause a person to drop items.
Severe arthritis in the MCP joints can cause a joint deformity called ulnar deviation, also knowns as ulnar drift. This deformity occurs when the large knuckles are so damaged that the fingers begin to dislocate and drift sideways.