The thumb is the shortest and thickest digit in the human hand. The thumb’s anatomy and function differ slightly from the other fingers, so some people may not consider it a finger. However, as one of the five terminal members of the hand, most medical guides refer to the thumb as a finger.

Also known as the pollex or digitus primus manus, the thumb is anatomically different from the hand’s other four digits. These differences allow the thumb to move and function differently from the rest of the fingers.

Humans and other primates have the distinct ability to use their hands with greater prehension and dexterity. As the thumb is opposable, it allows people to position it against other digits of the hand to manipulate or hold an object. Some evidence suggests that this ability makes the thumb responsible for roughly 40% of the hand functions.

This article discusses the function and anatomy of the thumb and common conditions that may affect this digit.

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The thumb, and the rest of the fingers, are appendages or digits of the hand. While the thumb is distinctly different from the other four fingers, many medical professionals consider it a finger due to differences in size, bones, joints, and function. This reflects the Latin name digitus primus manus, which translates to “first digit of the hand”.

However, some people may prefer to say they have 10 digits, or eight fingers and two thumbs, instead 10 fingers.

As with the rest of the hand, the thumb consists of many different structures that allow for movement and dexterity.

Each finger has three bones, or phalanges, except for the thumb, which instead has two phalanges. The first bone — the distal phalange — extends from the tip of the thumb to the knuckle. The second bone — proximal phalange — then extends from the knuckle to the base of the thumb. The thumb then connects to the hand bone, or thumb metacarpal.

As a result of only having two phalanges, the thumb only has one joint, while the other fingers have two joints. The interphalangeal joint (IP) is present at the tip of the thumb, just before the fingernail starts, and is similar to the distal interphalangeal joint of the other four fingers. This joint allows a person to bend the tip of their thumb.

There are also two more joints where the thumb joins the hand, which aids movement. The metacarpophalangeal (MP) joint is where the metacarpal meets the phalanges. This joint allows a person to bend and extend the thumb. The carpometacarpal (CMC) joint is where the metacarpal meets the carpal, or wrist, bones. This joint allows people to the thumb away from and towards the hand.

Nine muscles contribute to the movement of the thumb. Experts can divide these into two groups: the extrinsic muscles in the forearm and intrinsic muscles in the hand.

The four extrinsic muscles include:

  • abductor pollicis longus
  • extensor pollicis brevis
  • extensor pollicis longus
  • flexor pollicis longus

The five intrinsic muscles, also known as the thenar muscle group, consists of:

  • abductor pollicis brevis
  • flexor pollicis brevis
  • opponens pollicis
  • first dorsal interosseous
  • adductor pollicis

The thumb’s primary function is to either work with or against the other fingers to manipulate objects and perform actions such as pinching or grasping.

With the rest of the hand’s digits, the thumb plays a crucial role in performing coordinated hand movements for precise tool use. Its unique position and proportional length allow the thumb to firmly contact the other fingers and other objects. The opposable nature of the thumb and the thenar muscles provide the dexterity and strength to perform these actions.

There are two main types of grips: power and precision grips. Power grips require using the whole hand for higher stability and power on larger objects. The ability of the thumb to position opposite the fingers allows humans to hold and move heavy objects in a controlled way, such as when lifting weights at the gym.

A precision grip only requires the fingertip to apply force between the fingertips to smaller objects. The thumb works opposite to one or more fingertips to allow the fingers to grip objects. This type of grip is important for moving small and delicate objects, such as when writing with a pen.

Many people use their thumbs every day to perform a myriad of functions. When injured, people may experience thumb pain, which can make performing day-to-day activities difficult. Common conditions that may affect the thumb include:

Arthritis

Thumb arthritis, also called basal thumb arthritis, is the second most common arthritis in the hand. It affects the CMC joint, the unique saddle joint of the thumb.

The cause is often due to cartilage wear and tear as a person ages. However, conditions that cause joints to degenerate, such as lupus and rheumatoid arthritis, can also result in arthritis in the CMC joint.

DeQuervain’s tenosynovitis (DQT)

De Quervain’s tenosynovitis is a painful condition often resulting from overuse. A person with DQT usually experiences pain at the side of the wrist near the base of the thumb.

Pain can worsen when a person raises the thumb while tilting the wrist, such as when opening a jar, texting, or wringing a towel.

DQT is a prevalent hand and wrist injury. It often occurs during and after pregnancy. This is why the condition is also commonly known as mother’s thumb.

Learn more about exercises for DQT here.

Carpal tunnel syndrome

Carpal tunnel syndrome occurs when pressure pinches on the median nerve in the carpal tunnel. This nerve gives sensation to the palmar side of the thumb enable the thumb to flex and oppose.

In milder cases, a person may feel pins and needles and mild pain in the wrist and thumb. In severe cases, the thumb muscles may weaken and shrink.

Skier’s thumb

Skier’s thumb results from a sprain or tear in the thumb’s ulnar collateral ligament (UCL). A person often gets this injury from falling from an outstretched hand, such as when falling on the ski slopes with the hand strapped to a ski pole. Improper healing may cause pain and weakness during pinching activities.

Trigger finger

Trigger finger is a condition where a person’s finger locks when they try to straighten or bend it. While it can affect any finger, most people experience the condition in the ring finger, little finger, or thumb.

If a person requires treatment, it may involve avoiding activities and taking medication. In more severe cases, it may be necessary for a person to receive surgery.

The thumb is one of five digits on the hand. Many people consider the thumb to be a finger, despite its differences from the other four digits. It is an essential part of the hand that enables people to perform powerful, precise, and coordinated hand movements. The thumb is a unique digit responsible for manipulating objects and using tools.

As many people often use their thumbs, they can be prone to injury. As such, this can impact a person’s ability to perform day-to-day tasks. Therefore, people should contact a doctor if they experience discomfort in their thumb for diagnosis and treatment.