Two of the main tendons to the thumb pass through a tunnel (or series of pulleys) located on the thumb side of the wrist. Tendons are rope-like structures that attach muscle to bone. Tendons are covered by a slippery thin soft-tissue layer, called synovium. This layer allows the tendons to slide easily through a fibrous tunnel called a sheath.

Any swelling of the tendons and/or thickening of the sheath, results in increased friction and pain with certain thumb and wrist movements.