Adults using mobile phones punch out numbers with their thumbs and develop synovitis in their carpometacarpal joints.
This versatility of motion is provided by the saddle-shaped carpometacarpal joint at its base.
The variation in the range of movement at the carpometacarpal joint is not great, but at the other 2 joints it is considerable.
Since muscle control passes from proximal to distal, the 3-bone longitudinal linkage is subject to the posture of the carpometacarpal joint.
In the rheumatoid hand, collapse can start anywhere in the chain, but in the osteoarthritic patient, it starts most often in the carpometacarpal joint, occasionally in the interphalangeal joint, and rarely if ever in the metacarpophalangeal joint.
Several extrinsic muscles cross the carpometacarpal joint, but only the abductor pollicis longus inserts onto the metacarpal.
When the carpometacarpal joint is osteoarthritic and the joint surfaces are worn, the joint becomes unstable.