Thumb

The thumb is on the lateral (outer) side of the hand and lies farthest to the outside when you look at your hand with the palm facing up. The other fingers have three phalanges, but the thumb has two. Below them, a metacarpal bone connects to the wrist bones. The thumb’s most distinctive feature is opposition. This movement brings the tip of the thumb into contact with the tips of the other fingers, allowing you to pick up a needle, fasten a button, and turn a bottle cap. Without the thumb, most fine hand tasks would become difficult.

At the base of the thumb is the First carpometacarpal joint, where the metacarpal meets the trapezium. It is called a saddle joint because its joint surfaces are shaped like a saddle. Unlike a hinge-like joint that bends in only one direction, it allows movement in several directions: flexion, extension, abduction, adduction, and opposition.

In addition to this joint, the thumb has a metacarpophalangeal joint and an interphalangeal joint (the joint between the phalanges). Because several joints each contribute a little movement, the tip of the thumb can reach the tips of the other fingers or the edge of the palm.

When you straighten your thumb and extend it backward, you can see a hollow near the wrist. This is called the anatomical snuffbox. The tendons that extend the thumb stand out on either side, creating this hollow space.

The muscles that move the thumb are located in two areas.

• The thenar eminence on the palm — This is the fleshy mound at the base of the thumb. It contains a group of short muscles that abduct, flex, and oppose the thumb.
• The forearm — Muscles originating in the Forearm, below the elbow, extend to the thumb through long tendons to extend, abduct, and flex it.

Most of the thenar muscles are supplied by the Median nerve, while the muscle that adducts the thumb is supplied by another nerve. Because the median nerve passes through a passage at the front of the wrist, Carpal tunnel syndrome can sometimes cause reduced sensation on the thumb side or weakness in the thenar muscles. For the general structure of tendons and muscles, see Muscles and tendons.

Thumb abduction is the movement of the thumb away from the palm or index finger, and the thumb abductors are muscles that help produce this movement. There are two muscles in different locations, rather than just one.

• Abductor pollicis longus — This muscle begins in the forearm, and its tendon attaches to the metacarpal at the base of the thumb. The tendon can be felt on the lateral side of the wrist, and the muscle works when you move the thumb out to the side.
• Abductor pollicis brevis — This is a short muscle in the thenar eminence of the palm. It is used to move the thumb forward, away from the palm, when wrapping your hand around an object.

Because their names are similar, these muscles can easily be confused with the muscle that abducts the big toe. Checking whether the name refers to the hand or foot, and whether it includes “longus” or “brevis,” helps clarify which structure is meant. Movements that spread the thumb outward, such as opening a bottle cap or holding a bag strap, are typical examples of when these muscles are used.

• Prolonged smartphone use or frequently lifting a child can sometimes leave the base of the thumb and the lateral side of the wrist feeling sore. Some people also feel a pulling sensation around the tendons on the lateral side of the wrist when straightening the thumb and moving it out to the side (see De Quervain tenosynovitis).
• The joint at the base of the thumb may feel stiff, or you may feel an ache when turning a bottle cap (see Thumb carpometacarpal osteoarthritis).
• Tingling or reduced sensation at the tip of the thumb can also be related to the nerve passage at the wrist.

Even when discomfort is in the same thumb, the structures involved differ depending on its location and the movement that brings it on. Noting where and when it feels uncomfortable can help you describe what you are experiencing.

• Thumb stretching — Use one hand to gently press the thumb toward the palm, then release it. Spread the thumb wide out to the side and hold for about 10 seconds. Move gently within a pain-free range.
• Thenar massage — Use the thumb of the opposite hand to slowly massage the thenar eminence in circles.
• Change how you use your hands — Alternate hands when using your smartphone, and use a stand when you would otherwise hold it in one hand for a long time. When picking up something heavy, support it with your whole hand rather than using only your thumb.
• Take breaks — After repeating the same movement for a long time, lightly shake out your hands and gently circle your wrists.