The important structures of the hand can be divided into
several categories. These include
- bones and joints
- ligaments and tendons
- muscles
- nerves
- blood vessels
The front, or palm-side, of the hand is referred to as
the palmar side. The back of the hand is called the
dorsal side.
Bones and Joints
There are 27 bones within the wrist and hand. The wrist
itself contains eight small bones, called carpals. The carpals join with the two forearm
bones, the radius and ulna, forming the wrist
joint. Further into the palm, the carpals connect to the
metacarpals. There are five metacarpals forming the
palm of the hand. One metacarpal connects to each finger and
thumb. Small bone shafts called phalanges line up to
form each finger and thumb.
The main knuckle joints are formed by the connections of
the phalanges to the metacarpals. These joints are called
the metacarpophalangeal joints (MCP joints). The MCP
joints work like a hinge when you bend and straighten your
fingers and thumb.
The three phalanges in each finger are separated by two
joints, called interphalangeal joints (IP joints).
The one closest to the MCP joint (knuckle) is called the
proximal IP joint (PIP joint). The joint near the end
of the finger is called the distal IP joint (DIP
joint). The thumb only has one IP joint between the two
thumb phalanges. The IP joints of the digits also work like
hinges when you bend and straighten your fingers and
thumb.
The joints of the hand, fingers, and thumb are covered on
the ends with articular cartilage. This white, shiny material
has a rubbery consistency. The function of articular
cartilage is to absorb shock and provide an extremely smooth
surface to facilitate motion. There is articular cartilage
essentially everywhere that two bony surfaces move against
one another, or articulate.
Ligaments
and Tendons
Ligaments are tough bands of tissue that connect
bones together. Two important structures, called collateral ligaments, are found on either side of
each finger and thumb joint. The function of the collateral
ligaments is to prevent abnormal sideways bending of each
joint.
In the PIP joint (the middle joint between the main
knuckle and the DIP joint), the strongest ligament is the
volar plate. This ligament connects the proximal
phalanx to the middle phalanx on the palm side of the joint.
The ligament tightens as the joint is straightened and keeps
the PIP joint from bending back too far (hyperextending).
Finger deformities can occur when the volar plate loosens
from disease or injury.
The tendons that allow each finger joint to straighten
are called the extensor tendons. The extensor tendons
of the fingers begin as muscles that arise from the backside
of the forearm bones. These muscles travel towards the hand,
where they eventually connect to the extensor tendons before
crossing over the back of the wrist joint. As they travel
into the fingers, the extensor tendons become the extensor hood. The extensor hood flattens out to
cover the top of the finger and sends out branches on each
side that connect to the bones in the middle and end of the
finger.
The place where the extensor tendon attaches to the
middle phalanx is called the central slip. When the extensor muscles contract,
they tug on the extensor tendon and straighten the finger.
Problems occur when the central slip is damaged, as can
happen with a tear.
Muscles
Many of the muscles that control the hand start at the
elbow or forearm. They run down the forearm and cross the
wrist and hand. Some control only the bending or
straightening of the wrist. Others influence motion of the
fingers or thumb. Many of these muscles help position and
hold the wrist and hand while the thumb and fingers grip or
perform fine motor actions.
Most of the small muscles that work the thumb and pinky
finger start on the carpal bones. These muscles connect in ways that allow the hand to
grip and hold. Two muscles allow the thumb to move across
the palm of the hand, an important function called thumb opposition.
The smallest muscles that originate in the wrist and hand
are called the intrinsic muscles. The intrinsic muscles guide the
fine motions of the fingers by getting the fingers
positioned and holding them steady during hand
activities.
Nerves
All of the nerves that travel to the hand and fingers
begin together at the shoulder: the radial nerve, the
median nerve, and the ulnar nerve. These
nerves carry signals from the brain to the muscles that move
the arm, hand, fingers, and thumb. The nerves also carry
signals back to the brain about sensations such as touch,
pain, and temperature.
The radial nerve runs along the thumb-side edge of the
forearm. It wraps around the end of the radius bone toward
the back of the hand. It gives sensation to the back of the
hand from the thumb to the third finger. It also supplies
the back of the thumb and just beyond the main knuckle of
the back surface of the ring and middle fingers.
The median nerve travels through a tunnel within the wrist
called the carpal tunnel. This nerve gives sensation
to the thumb, index finger, long finger, and half of the
ring finger. It also sends a nerve branch to control the
thenar muscles of the thumb. The thenar muscles help
move the thumb and let you touch the pad of the thumb to the
tips each of each finger on the same hand, a motion called
opposition.
The ulnar nerve travels through a separate tunnel, called
Guyon's canal. This tunnel is formed by two carpal
bones, the pisiform and hamate, and the
ligament that connects them. After passing through the
canal, the ulnar nerve branches out to supply feeling to the
little finger and half the ring finger. Branches of this
nerve also supply the small muscles in the palm and the
muscle that pulls the thumb toward the palm.
The nerves that travel to the hand are subject to
problems. Constant bending and straightening of the wrist
and fingers can lead to irritation or pressure on the nerves
within their tunnels and cause problems such as pain,
numbness, and weakness in the hand, fingers, and
thumb.
Blood Vessels
Traveling along with the nerves are the large vessels
that supply the hand with blood. The largest artery is the
radial artery that travels across the front of the
wrist, closest to the thumb. The radial artery is where the
pulse is taken in the wrist. The ulnar artery runs
next to the ulnar nerve through Guyon's canal (mentioned
earlier). The ulnar and radial arteries arch together within
the palm of the hand, supplying the front of the hand,
fingers, and thumb. Other arteries travel across the back of
the wrist to supply the back of the hand, fingers, and
thumb.
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