The wrist and hand form a continuum with the upper extremities and allow the body to reach out to perform a vast number of functions exclusive to primates and humans. The wrist acts as a hinge that articulates the forearm with the hand, and the hand with fingers. Together wrist, hand and fingers are responsible for fine movements and grasping. The abundance of bones, ligaments, muscles and tendons permits the versatility of these activities. Any pathology to the wrist and hands severely impacts on our daily life and work, often leading to temporary or permanent disability.
Bones of the forearm and wrist
In the proximal region, the radius and ulna articulate with the humerus bone of the upper arm to form the elbow joint. In the distal end, the radius and ulna form the wrist joint and allow the flexion and rotation of the forearm, by turning the palm of the hand up (supination) and down (pronation). In this movement the radius slides around the ulna, which remains in its position. The wrist is a complex joint system connecting the forearm with the hand. It is formed by the distal edges of the arm bones, the radius (largest forearm bone) the ulna and the carpal bones, which are:
Proximal wrist - scaphoid (naviculare), lunate, triquetrum and pisiform.
Distal wrist - hamate, capitate, trapezium and trapezoid.
The carpal bones connect with the proximal area of five metacarpal bones of the hand. The surface of the articulating portion of the arm, and carpal and metacarpal bones is covered by cartilage tissue to absorb friction during the movements of the joints.
Bones of the hand
The hand is a sophisticated part of the body equipped with 29 bones, 29 joints, 123 ligaments, 34 muscles, 48 nerves and 30 arteries that allow for the versatility of its functions. The front of the hand or palmar side is opposed to the back or dorsal side. Some bones are shared by the hand and the wrist. On the hand side the carpal bones connect with 5 metacarpal bones that are the long tubular bones of the palm. Metacarpals connect with each finger bones or phalanges, of which there are 3 in each finger and 2 in the thumb.
The complexity of the wrist joints is supported by a large number of ligaments that keep together the large number of bones forming the carpus and the carpus with the radius and ulna as well as the carpus with the metacarpal bones of the hand. The main joints of the wrist are:
The Triangular FibroCartilage Complex (TFCC) is formed by the triangular fibrocartilage discus (TFC), the Radio-Ulnar Ligaments (RUL’s) and the Ulno-Carpal Ligaments (UCL’s). The TFCC consists of collagenous fibres (ligaments) implanted in a fibrocartilage matrix that extends to the extremity of the radius and ulna to achieve a concave shape. The TFCC allows the distal radius to slide around the ulna during supination and pronation.
The Distal Radio Ulnar Joint (DRUJ) is the joint formed by the distal extremity of the radius connecting to the distal ulna forming a a cushion between the carpus and the ulna. For the rotation of the forearm, the DRUJ is complemented at the elbow by the Proximal Radio Ulnar Joint (PRUJ). A deeper layer of intracarpal ligaments (between the carpal bones) keeps these bones joined together to function as a unit.
The movements of the hand are permitted by distinct joints:
Metacarpo-phalangeal joints (MCP joints) are located between the carpal bones and the phalanges. They form the knuckles and are visible when bending and straightening the fingers and the thumb.
Inter-phalangeal joints (IP joints) are two in each finger and connect the three phalanges with one another with the exception of the thumb that has only two phalanges and one IP joint.
The proximal interphalangeal joint (PIP Joint) is closer to the MCP joint, whereas the distal IP joint (DIP joint) is further away, towards the tip of the finger. The thumb has only one IP joint. These joints are used when the fingers are bent in a fist or extended with an open hand. At each joint the bone ends are covered with articular cartilage, which absorbs shock and creates a smooth surface to facilitate the movement of the bones.
Ligaments are strong bands of elastic tissue that connect bones to bones and play an important role in stabilising the function of the numerous joints of the hand and wrist where bones articulate with one another.
The wrist is capable of three degrees of movements: (1) flexing and extending, (2) pronating and supinating and (3) deviating ulnarly or radially. A complex network of ligaments on the dorsal and palmar side of the wrist allow for this detailed range of movement.
The Distal Radio Ulnar Joint (DRUJ) provides a cushion between the carpus and the ulna. For the rotation of the forearm, the DRUJ is complemented at the elbow by the Proximal Radio Ulnar Joint (PRUJ). A deeper layer of intracarpal ligaments (between the carpal bones) keeps these bones joined together to function as a unit.
The collateral ligaments of the hand are located on each side of all the fingers and thumb joints to limit the lateral movement of the fingers.
The volar plate is the strongest ligament surrounding only the middle inter-phalangeal joints. It prevents the hyperextension of the fingers.
Muscles of the wrist and hand
Several muscles control the wide range of movements such as flexion/extension and supination/ pronation of the wrist, hand and fingers. Most muscles of the hand originate at the elbow, run along the arm and cross the wrist to reach the hand. Some allow the wrist to bend and straighten, while others control the grip/hold function of the thumb and fingers, as well as fine finger movements.
The muscles of the fingers and thumb begin at the carpal bones and are used during the grip. Other muscles (adductor pollicis brevis, opponens pollicis and flexors of the little finger) are used when the tip of the thumb and little finger touch each other, “thumb opposition”.
The intrinsic muscles are found inside the hand and are divided in four groups:
Thenar (controlling thumb movements)
Hypothenar (insert at proximal phalanx of the little finger)
Lumbrical (flexion/extension of metacarpo-phalangeal and interphalangeal joints)
Interosseous muscles (abduct ion/ adduction of fingers).
Flexor digitorum profundus
The flexor digitorum profundus is a deep muscle of the forearm that is used to flex the distal phalanges of the long four fingers (index/middle/ring/little finger). It is defined as an extrinsic muscle since it is located in the forearm but its action results in movements of the fingers.
Flexor digitorum superficialis
The Flexor digitorum superficialis is a muscle of the anterior compartment of the forearm, which flexes the middle and proximal phalanges of the long four fingers (index/ middle/ring/little finger). It is an extrinsic muscle, since it is located in the forearm but its action results in movements of the fingers.
The tendons of the wrist and hand are strong bands of fibres that connect the muscles of these regions to the bones. They are used to transfer the energy from the muscles to allow the movements of the joint.
Flexor tendons - on the palmar side, mediate the flexion of the fingers and wrist. They are mostly innervated by the median nerve with the exception of the flexors at the ulnar side, and the small and ring finger, which are innervated by the ulnar nerve.
Extensor tendons - on the dorsal side of the hand mediate the extension or straightening of the fingers. These tendons originate from the muscles of the forearm and when they reach the fingers take the name of extensor hood. This structure covers the finger’s top and continues to surround the finger's lateral side up to the middle and distal phalanges.
The palmar fascia consists of a thin and flat membrane of connective tissue, similar to tendons, located under the skin of the palm of the hand. It comprises a larger triangular portion in the centre of the palm merging into the tendon palmaris longus at the wrist, which is an extension of the palmaris longus muscle. Distally, it reaches the superficial transverse metacarpal ligament at the base of the long fingers. Its function is to keep together the tendons of the hands and prevent excessive extension of the finger joints. A pathological contracture of the palmar fascia can markedly reduce the movement of the hand and fingers (e.g. Dupuytren's Contracture).
Arteries of the wrist and hands
The radial artery is the largest artery of the hand. It travels into the deep palm across the front/lateral side of the wrist, close to the thumb. The radial artery is felt when the pulse is taken at the wrist.
The ulnar artery runs parallel and to the opposite wrist side of the radial artery. It passes through Guyon's canal together with the ulnar nerve. The ulnar and radial arteries merge at the deep palm forming the deep palmar arch, which supplies the thumb and radial half of the index finger. The radial artery branches into the radial artery of index finger and the princeps pollicis.
On the dorsal hand, the superficialis branch of the radial artery merges with the ulnar artery to form the basal metacarpal arch and the dorsal radiocarpal arch.
The dorsal arteries run through the back of the wrist to supply the dorsal side of the hand, fingers and thumb. The common digital arteries arise from the superficial palmar arch and divide into multiple branches to supply the intrinsic muscles and skin. The common digital arteries bifurcate into the proper digital arteries.
Veins of the wrist and hand
The veins of the hand form a complex network and are divided into superficial and deep veins. The superficial veins are easily recognisable through the skin of the dorsal hand. The deep veins of the hand follow the same path and take the same names of the arteries. The location of the veins on the dorsal side can vary from person to person.
Dorsal veins: The dorsal digital veins of the hand run along the later and medial side of each finger. They communicate with one another with anastomoses at the dorsal venous arches. The metacarpal veins drain into the dorsal venous plexus
Palmar veins: The deep palmar arch is located in the deeper the hand, almost parallel to its superficial counterpart. It receives blood drained from the palmar metacarpal veins (not shown).
The cephalic vein crosses the snuffbox below the thumb to reach the forearm and drain into the brachial veins, then into the axillary vein and further upstream to the subclavian vein. The basilic vein joins the brachial vein that becomes the axillary vein.
The wrist and hand are innervated by the three nerves that arise from the brachial plexus and divide into fine branches as they descend along the extremities. They are the median, ulnar and radial nerves; each one mediating distinct sensory and motor functions.
The median nerve controls the following motor functions: forearm pronation, thumb palmar abduction and thumb/index/long finger flexion.
Motor Branches: these are the median nerve branches and the corresponding muscles they innervate.
Proximal extrinsic motor branches (median nerve)
1. Pronator teres
2. Flexor carpi radialis
3. Palmaris longus
4. Flexor digitorum superficialis
Proximal extrinsic motor branches (anterior interosseous nerve)
5. Flexor digitorum profundus (to index and long finger)
6. Flexor pollicis longus
7. Pronator quadratus
Distal intrinsic motor branches (thenar branch)
1. Abductor pollicis brevis
2. Flexor pollicis brevis (superficial head)
3. Opponens pollicis
Distal intrinsic motor branches
(common digital branches of the median nerve)
4. first lumbrical (to index and long fingers)
5. second lumbrical (to index and long fingers)
Sensory Branches of the median nerve
Common and proper digital nerves innervate the volar thumb index, long, and radial half of the ring finger. These nerves provide sensation to specific areas of the hand - the pinch surfaces of the thumb, index and third fingers. Pathological compression of the median nerve at the wrist is known for causing the carpal tunnel syndrome.
Palmar branch (to radial base of palm)
Consists of articular branches to the elbow and wrist joints (via distal anterior interosseous nerve).
The radial nerve contributes to motor functions of the elbow, wrist, finger and thumb.
Motor branches: These are the muscles innervated by the radial nerve and its branches in addition to those shown in the image.
Triceps brachii / anconeus
Extensor digitorum communis
Extensor indicis proprius
Brachialis (contribution; though main supply to brachialis is the musculo-cutaneous nerve)
Sensory branches of the radial nerve
The radial nerve sensory component, the superficial radial sensory nerve (or radial sensory branch), is the frequent cause of neuropathic pain following injury.
These are the sensory branches of the radial nerve:
Posterior brachial cutaneous nerve/inferior lateral brachial cutaneous nerve
Posterior antebrachial cutaneous nerve
Radial sensory nerve
The ulnar nerve contributes to the following motor functions: fine hand movements, coordination of finger motion and pinch strength, flexion of the small and ring fingers.
Motor branches: These are the muscles innervated by the ulnar nerve and its branches in addition to those shown in the image.
Proximal extrinsic motor branches (ulnar nerve)
Flexor carpi ulnaris
Flexor digitorum profundus (ulnar half to small, ring +/- long finger)
Distal intrinsic motor branch (superficial ulnar nerve branch)
Distal intrinsic motor branches (deep ulnar nerve branch)
Abductor digiti minimi
Flexor digiti minimi
Opponens digiti minimi
3rd and 4th lumbricals (to small & ring fingers)
Palmar and dorsal interosseous muscles
Sensory branches: Ulnar nerve mediates parts of the sensation of the palm and of the small finger and the ulnar border of the fourth finger.
The ulnar nerve is enclosed in the Guyon canal, where nerve entrapment can occur. The nerve forms:
Common and proper digital nerves (superficial branch) (to volar small and ulnar half of ring finger)
Articular branches to the elbow joint and carpal and metacarpo-phalangeal joints.