Anatomy of the Wrist
The wrist is a very complex area of the body. It is made up of many bones, joints, muscles, nerves, ligaments, tendons and blood vessels. In perfect health the wrist is very movable and dexterous. It can move in many directions but is also very strong and can support the body’s weight as well as grasp and hold heavy objects. Without such a detailed and capable area of the body most day to day functions would be impossible.
Bones form the foundation of the wrist and hand, because there are many we rarely think of the bones individually and how they function. Starting in the forearm we have two long bones, the radius and ulna. The radius is located on the thumb side of the arm and the ulna is located on the pinky side of the arm and is the longer of the two. At the end of both the radius and the ulna there is a process or extension of the bone to which an important ligament connects from the forearm to the wrist. This process is called the radial styloid and the ulnar styloid. On the radius the radial styloid is on the thumb side of the bone and on the ulna the ulnar styloid is on the pinky side of the bone. From the forearm we move into the wrist. Because the wrist is so dexterous there are many little bones that comprise it. This group of bones are referred to as carpal bones. Within this group there are two rows of bones. The row closest to the radius and ulna is called the proximal row and the row following is called the distal row. The proximal row has three carpal bones, starting at the thumb side of the hand they are: scaphoid, lunate and triquetrum. This is where the wrist creases when you bend it. The distal row is made up of 5 carpal bones, starting at the thumb side they are: trapezium, trapezoid, capitate, hamate and pisiform. This group of carpal bones connects the bones of the forearm to the bones of the hand, metacarpals and the bones that make up the fingers, phalanges.
Each bone that connects with another bone intersects to form a joint, which allows for diverse range and support. The end of each bone that forms a joint is covered in articular cartilage. This cartilage is rubbery and allows for a smooth slippery environment for the joints. It can be up to 1/4 inch thick in areas that require more support and thinner in areas that don’t support much weight. Besides providing a nice smooth surface for the bones articular cartilage functions as shock absorption as well.
Ligaments support the bones, they hold them in place to ensure the joints function properly. A ligament always connects bones to bones. There are two main ligaments that support the sides of the wrist. The first is the radial collateral ligaments (RCL) and as the name suggests it starts on the radius side of the forearm at the radial styloid mentioned earlier and then connects to the carpal bone, the scaphoid located just below the thumb. The second ligament is the ulnar collateral ligament (UCL) and has two parts. The first part starts on the ulnar styloid and connects to the Pisiform, the carpal bone located in the distal row on the pinky side of the wrist and then onto the transverse carpal ligament. The transverse carpal ligament is a strong fibrous band that crosses over the palm side of the carpal bones. It is this ligament that the tendons (muscles) and nerves (sensation and motion) pass under. The second part of the UCL connects to the Triquetrum, located in the proximal row of the carpal bones. It is the RCL that prevents the wrist from bending too far to the pinky side of the hand and the UCL that prevents the wrist from bending too far to the thumb side of the hand. The UCL also stabilizes the triangular fibro cartilage complex (TFCC), a small disc of cartilage between the ulna and two carpal bones, the lunate and the triquetrum that provides support, increases range of motion and cushions the wrist.
Tendons connect muscle to bone for motion. The extension and flexsion we have in the hand and wrist are from tendons that originate as muscles in the forearm. In the forearm there are as many as 20 muscles. Generally there are two groups of tendons. One group runs on the palm side of the wrist. This group is called the flexor tendons and is responsible for flexing and curling the fingers, thumb and hand. These tendons run underneath the transverse carpal ligament mentioned earlier. It is this ligament that prevents the flexor tendons from bowing out when you curl your fingers, thumb or wrist. The other group of tendons travel on the backside of the hand and are called the extensor tendons. These extensor tendons help to straighten your fingers and tumb as well as lift your palm up. The extensor tendons run through compartments that are lined with tenosynovium fluid which keep the tendons lubricated so they glide easily in their compartments with minimal friction.
There are three main nerves that cross the wrist. Keeping in thyme, they are the radial nerve, the median nerve and the ulnar nerve. Nerves function to send signals from the brain to muscles for motor action as well as sensory information from receptors back to the brain. All three nerves of the wrist and hand originate in the shoulder. From the elbow, the radial nerve crosses the forearm to connect with the back of wrist on the thumb side of the hand. The nerve then branches through to the thumb and the first knuckle of the pointer and middle finger. The radial nerve controls the triceps, wrist and finger extensors. The median and ulnar nerve both travel from the outside of the upper arm down to the inner arm, the median nerve then travels to the middle of the forearm and into the palm side of the wrist. It gives sensation to the thumb, pointer, middle and half of the ring finger. Along with this the median nerve sends a nerve branch into the thumb to control a muscle called the thenar muscles. These muscles give your thumb the ability to cross over the palm and touch the tips of each finger, referred to as opposition. Other muscles controlled by the median nerve include the wrist and finger flexors. The ulnar nerve also travels down the inner arm but passes through a separate tunnel called the Guyon’s canal. This tunnel is formed by carpal bones, the pisiform and the hamate. Once through Guyon’s canal the ulnar nerve supplies sensation to the palm side of the pinky and half of the ring finger. The muscles that the ulnar nerve control are that of the wrist and finger flexors as well as the small muscles of the hand that coordinate fine movements like drawing the thumb to the palm of the hand.
Finally we come to the blood vessels which supply the wrist, hand and fingers with oxygenated blood. There are two large arteries that cross the wrist into the hand. The radial and ulnar artaries are two branches of the brachial artery, which is the major blood vessel of the upper arm. The brachial artery runs from the shoulder down to the elbow where it branches into the radial and ulnar arteries, which travel down the forearm and into the hand. The radial artary runs along the underside of the forearm and crosses the wrist on the thumb side of the hand. This is the location and the artary we use to take someones pulse. The Ulnar artary travels alongside the Ulnar nerve down the forearm and through Guyon’s canal to the palm of the hand on the pinky side. Once the radial and ulnar nerve are in the palm of the hand there are many other small artaries and veins that branch off them to provide blood to the fingers and thumb but the radial and ulnar artary finally come to meet each other in the centre of the palm. This is called the superficial palmar arch.
As you can see there is much going on within the wrist. Most of us may take for granted the capacity to which the wrist functions throughout our day. We may not appreciate the functionality until we have complications or an injury and are then unable to use the wrist like we have in the past. It can be very useful to discover the area in detail to have an understanding of the mechanics of the wrist and then move forward in healing with this awareness. The complexity of the anatomy should be a beautiful example that the body is capable of many things and with the right application of awareness, exercises and stretches is able to heal itself. I have learned from experience that healing is possible and if you have had and injury or have pain in the wrist area please look at the accompanying video for suggestions of exercises you can practice to stretch and strengthen your wrist.