Hand anatomy

• Anatomical terms describing movement
• Bones of the hand
  • Wrist
  • Palm
  • Fingers
• Muscles of the forearm affecting the hand
• Muscles of the hand
• Muscles of the palm
  • Thenar muscles
  • Hypothenar muscles
  • Midpalmer muscles
• Muscles of the thumb
• Muscles of the fingers

The hand consists of the wrist, palm and fingers. Movements of the hand are controlled by muscles in the forearm (extrinsic muscles) as well as muscles within the hand itself (intrinsic muscles).

Anatomical terms describing movement

The movements of the hand (and of other body structures) are usually described in anatomical terms. These include:

• Flexion – A bending movement that decreases the angle between two parts. Eg. You flex your fingers when you clench your fist.
• Extension – A straightening movement that increases the angle between two parts. Eg. Stretching out your fingers. Flexion and extension are opposites of one another.
• Abduction – A motion that pulls a structure away from the middle finger. Eg. Spreading out your fingers.
• Adduction – A motion that brings a structure toward the middle finger. Eg. Closing the fingers together.

Bones of the hand

The skeleton of the hand can be divided up into three sections: the carpus (wrist), metacarpus (palm) and the phalanges (bones of the fingers).

Wrist

The “true wrist” can be found on the proximal part of the hand and consists of eight small bones, known as carpals, which are linked together by ligaments. By this definition, the part of the arm we use to wear a wrist-watch is not actually part of the wrist but is in fact the lower part of the forearm. The carpal bones of the wrist glide against each other, making the carpus quite flexible as a whole. The bones lie in two irregular rows, with four bones in each row. The bones in one row are called the scaphoid, lunate, triquetral and pisiform. The scaphoid and lunate come into contact with the radius (a bone of the forearm), forming the wrist joint. The trapezium, trapezoid, capitates and hamate are the bones making up the other row.

Palm

The palm of the hand is formed by five metacarpal bones that extend from the wrist. These bones are numbered from 1 to 5 from the thumb to the little finger. The bones of the wrist come into contact with the bottoms of the metacarpals. In addition, the metacarpals come into contact with each other on their sides. Their bulging heads come into contact with the bottom bones of the fingers. These heads can be seen as knuckles when the fist is clenched.

The metacarpal associated with the thumb, metacarpal 1, is the shortest and most mobile. A special joint called a saddle joint is present between metacarpal 1 and part of the wrist. This allows the thumb to touch the tips of the fingers, an action known as opposition.

Fingers

The digits of the upper limb, the fingers, are numbered from 1 to 5 beginning with the thumb (also known as the pollex). The phalanges refer to the miniature long bones within the fingers. Except for the thumb, each finger has three phalanges – the distal (the bone on the tip of the finger), middle and proximal (the bone at the base of the finger) phalanx. The thumb has no middle phalanx, giving a total of 14 phalanges in each hand.

Muscles of the forearm affecting the hand

There are some muscles of the forearm that act upon the hand. The bulk of these muscles form the fleshy roundness of the forearm, with tendons extending into the wrist and hand. Most of the tendons pass under the transverse carpal ligament which is located on the underside of the wrist, and the dorsal carpal ligament which is located on the other side. This prevents the tendons from standing up when the muscles contract. The tight space formed between the wrist bones and the transverse carpal ligament is known as the carpal tunnel. The tendons passing through this tunnel are enclosed in sheaths, allowing them to slide back and forth with ease. However, repetitive motion in this region makes it very subject to injury in the form of carpal tunnel syndrome.

Several of the muscles on the forearm originate from the humerus (the upper arm bone). As a result, they cross the elbow joint and hence weakly contribute to the flexion and extension of the elbow. However, this action is relatively insignificant and their actions are focussed on the wrist and fingers.

The muscles of the forearm are divided into two compartments, known as anterior and posterior comparments. These compartments are then further subdivided into two more layers known as the superficial and deep layers. The flexors of the humerus are mainly located within the anterior compartment. A tendon called the palmaris longus passes over the transverse carpal ligament at the lower end of the humerus near the wrist, with other tendons passing beneath it. Palpating the wrist can reveal two prominent tendons belonging to the palmaris longus and the flexor carpi radialis. The flexor carpi radialis is an important muscle clinically, as it is generally used as a landmark for locating the radial artery where a pulse is usually taken.

 

Muscles of the hand

The muscles within the hand itself help to move the fingers and metacarpals, allowing fine control of precise movements such as threading a needle, while more powerful movements are controlled by the muscles in the forearm.

Muscles of the palm

In the palm, the muscles can be divided into three groups:

1. The thenar eminence
2. The hypothenar eminence
3. The midpalm

The thenar eminence and hypothanar eminence refer to the balls of the thumb and little finger respectively. Muscles in the thenar and hypothenar group are nearly mirror images of one another. Each group contains a small flexor, abductor and opponens muscle. Within the midpalm group are muscles called lumbricals and interossei, which help extend the fingers.

Thenar muscles

The thenar muscles are those located in the ball of the thumb. They are:

• Abductor pollicis brevis, which abducts the thumb.
• Flexor pollicis brevis, which flexes the thumb.
• Opponens pollicis. This muscle allows opposition, where the thumb moves to touch the tip of the little finger.
• Adductor pollicis, which allows adduction and helps to oppose the thumb.

Hypothenar muscles

The hypothenar muscles in the ball of the little finger are:

• Abductor digiti minimi, which abducts the little finger.
• Flexor digiti minimi brevis, which flexes the little finger.
• Opponens digiti minimi. This muscle is involved in opposition, bringing the little finger (metacarpal 5) toward the thumb, cupping the hand.

Midpalmer muscles

The midpalmar muscles are:

• Lumbricals, which are four worm-like muscles that flex the fingers at metacarpophalangeal joints and extend them at interphalangeal joints.
• Palmer interossei are three muscles involved in the adduction of fingers.
• Dorsal interossei are four muscles that allow the abduction of fingers.

Muscles of the thumb

The movements of the thumb occur at a joint called the carpometacarpal joint, as well as at two joints involving the phalanx. At the carpometacarpal joint, the movements allowed include:

1. Flexion/extension
2. Abduction/adduction
3. Opposition/reposition

During opposition, the metacarpal bone moves towards the midline of the hand, allowing the thumb’s distal phalanx to come into contact with the distal phalanges of the other fingers. At the carpometacarpal joint, there is opposition by the opponens pollicis and reposition (opposite of opposition) by the flexor pollicis longus and flexor pollicis brevis.

The thumb allows for enormous versatility of the hand. The length of the thumb muscles and the thumb as a whole allow for great flexibility in using the thumb along with other fingers, especially because of its ability for opposition.

Muscles of the fingers

The movements of the remaining fingers of the hand are a result of interplay between several muscle groups. Each finger consists of three joints:

1. The metacarpophalangeal joint (MP), also known as the knuckle joint, between the metacarpals and bottom phalanges
2. The proximal interphalangeal joint (PIP) between the bottom and middle phalanges
3. The distal interphalangeal joint (DIP) between the middle and uppermost phalanges

The MP joint permits front/back movements, circular movement, as well as side-to-side movements of the fingers. The proximal interphalangeal joint (PIP) and distal interphalangeal joint (DIP) only allow front/back movements.

In the MP joint, there are seven muscles between the bones and four other muscles that act as flexors. A seperate muscle called the extensor digitorum acts as the extensor.

In the PIP  joint, two muscles help to flex the fingers. These are called the flexor digitorum superficialis and the flexor digitorum profundus.

The flexor of the DIP joint is the flexor digitorum profundus. The extensors of both the PIP and DIP joints are muscles between the bones and internal muscles of the hand.

References

1. Marieb EN, Hoehn K. Anatomy & physiology. 3rd ed. San Francisco: Pearson Benjamin Cummins 2008.
2. Saladin KS. Anatomy and physiology – the unity of form and function. 3rd ed. New York: McGraw-Hill 2004.
3. Rogers AW. Textbook of Anatomy. Edinburgh; New York: Churchhill Livingstone 1992.