Arthritis (at the base of the thumb)
This is also known as a dorsal digital ganglion cyst. Usually in middle-aged or older people and associated with wearing out of the end joint of a finger. Pressure from the cyst may cause a furrow in the fingernail.
Occasionally the cyst fluid leaks through the thin overlying skin from time to time. The risk of recurrence after surgery is around 10%, and problems after surgery include infection, stiffness, and pain from the worn out joint.