The important structures of the wrist can be divided into
several categories. These include
- bones and joints
- ligaments and tendons
- muscles
- nerves
- blood vessels
Bones and Joints
There are 15 bones that form connections from the end of
the forearm to the hand. The wrist itself contains eight
small bones, called carpal bones. These bones are grouped in two rows
across the wrist. The proximal row is where the wrist creases when you
bend it. Beginning with the thumb-side of the wrist, the
proximal row of carpal bones is made up of the
scaphoid, lunate, and triquetrum. The
second row of carpal bones, called the distal row, meets the proximal row a little
further toward the fingers. The distal row is made up of the
trapezium, trapezoid, capitate,
hamate, and pisiform bones.
The proximal row of carpal bones connects the two bones
of the forearm, the radius and the ulna, to
the bones of the hand. The bones of the hand are called the
metacarpal bones. These are the long bones that lie
within the palm of the hand. The metacarpals attach to the
phalanges, which are the bones in the fingers and
thumb.
One reason that the wrist is so complicated is because
every small carpal bone forms a joint with the bone next to
it. This means that what we call the wrist joint is actually
made up of many small joints.
Articular cartilage is the material that covers
the ends of the bones of any joint. Articular cartilage can
be up to one-quarter of an inch thick in the large,
weight-bearing joints. It is thinner in joints such as the
wrist that don't support a lot of weight. Articular
cartilage is white, shiny, and has a rubbery consistency. It
is slippery, which allows the joint surfaces to slide
against one another without causing any damage.
The function of articular cartilage is to absorb shock
and provide an extremely smooth surface to make motion
easier. We have articular cartilage essentially everywhere
that two bony surfaces move against one another, or
articulate. In the wrist, articular cartilage covers
the sides of all the carpals and the ends of the bones that
connect from the forearm to the fingers.
Ligaments and
Tendons
Ligaments are soft tissue structures that connect
bones to bones. The ligaments around a joint usually combine
to form a joint capsule. A joint capsule is a watertight sac
that surrounds a joint and contains lubricating fluid called
synovial fluid. In the wrist, the eight carpal bones
are surrounded and supported by a joint capsule.
Two important ligaments support the sides of the wrist.
These are the collateral ligaments. There are two collateral
ligaments that connect the forearm to the wrist, one on each
side of the wrist.
As its name suggests, the ulnar collateral
ligament (UCL) is on the ulnar side of the wrist. It
crosses the ulnar edge (the side away from the thumb) of the
wrist. It starts at the ulnar styloid, the small bump
on the edge of the wrist (on the side away from the thumb)
where the ulna meets the wrist joint. There are two parts to
the cord-shaped UCL. One part connects to the pisiform (one
of the small carpal bones) and to the transverse carpal
ligament, a thick band of tissue that crosses in front
of the wrist. The other goes to the triquetrum (a small
carpal bone near the ulnar side of the wrist). The UCL adds
support to a small disc of cartilage where the ulna meets
the wrist. This structure is called the triangular
fibrocartilage complex (TFCC) and is discussed in more
detail below. The UCL stabilizes the TFCC and keeps the
wrist from bending too far to the side (toward the
thumb).
The radial collateral ligament (RCL) is on the
thumb side of the wrist. It starts on the outer edge of the
radius on a small bump called the radial styloid. It
connects to the side of the scaphoid, the carpal bone below
the thumb. The RCL prevents the wrist from bending too far
to the side (away from the thumb).
Just as there are many bones that form the wrist, there
are many ligaments that connect to and support these bones.
Injury or problems that cause these ligaments to stretch or
tear can eventually lead to arthritis in the wrist.
At the wrist, the end of the ulna bone of the forearm
articulates with two carpal bones, the lunate and the
triquetrum. A unique structure mentioned earlier, the triangular fibrocartilage complex (TFCC), sits between
the ulna and these two carpal bones. The TFCC is a small
cartilage pad that cushions this part of the wrist joint. It
also improves the range of motion and gliding action within
the wrist joint.
There are several important tendons that cross the wrist. Tendons connect
muscles to bone. The tendons that cross the wrist begin as
muscles that start in the forearm. Those that cross the palm
side of the wrist are the flexor tendons. They curl
the fingers and thumb, and they bend the wrist. The flexor
tendons run beneath the transverse carpal ligament
(mentioned earlier). This structure lies on the palm side of
the wrist. This band of tissue keeps the flexor tendons from
bowing outward when you curl your fingers, thumb, or
wrist. The tendons that travel over the back of the wrist,
the extensor tendons, run through a series of
tunnels, called compartments. These compartments are
lined with a slick substance called tenosynovium,
which prevents friction as the extensor tendons glide inside
their compartment.
Muscles
The main muscles that are important at the wrist have
been mentioned above in the discussion about tendons. These
muscles generally start further up in the forearm. The
tendons of these muscles cross the wrist. They control the
actions of the fingers, thumb, and wrist.
Nerves
All of the nerves that travel to the hand cross the
wrist. Three main nerves 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 goes to 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. The median nerve gives
sensation to the palm sides of 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 through the wrist 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 and fingers. Other arteries
travel across the back of the wrist to supply the back of
the hand and fingers.