It is God’s answer to the evolution of the thumb. And since God has a sense of humor, She or He eventually
thumbed Her or His nose at us by creating osteoarthritis (degenerative joint disease [DJD]). Carpometacarpal (CMC) squaring is an oftenoverlooked deformity of the base of the thumb, typical of DJD of the first CMC joint—a common problem, especially among women. The dominant hand is the one most affected, with onset typically after age 40.
Initially, the thumb appears unremarkable, but as the disease progresses the CMC subluxes dorsally, starting
to protrude proximally to the snuff box (thus creating a CMC squaring, or mushroom deformity). With time,
the CMC becomes even more subluxated and stiff, and as a result the thumb metacarpal joint becomes fixed in adduction (i.e., tucked into the palm). As this happens, the only way for patients to lift the thumb off their
palms (so that they can grasp) is by hyperextending the MCP joint itself. Eventually, this becomes so painful
that the thumb remains frozen into a swan-neck deformity.