The wrist joint is the complex joint formed between the distal ends (furthest from the body) of the Radius and Ulna (two forearm bones) and the carpal bones. It connects the forearm to the hand and allows a good range of motion. Repetitive use does however frequently lead to injuries. To be more specific, the wrist joint is, as a matter of fact, a collection of multiple bones and joints. The bones comprising the wrist include the distal ends of the radius and ulna, 8 carpal bones, and the proximal portions of the 5 metacarpal bones.
Below, we will provide you with detailed information about bones, muscles and joints of the wrist. Even though it is smaller than the knee or the shoulder, for example, it has a very complex structure that combines numerous elements with important significance for the proper movements of your hand. Now, let`s start with the bones.
The Ulna is the larger of the two forearm bones, although it tapers at the wrist end, to become narrower than the Radius at this point. The Radius is positioned on the thumb side of the wrist, and the ulna on the little finger side.
They form the wrist joint with the carpal bones. Altogether there are 8 carpal bone which are arranged in two rows, proximal and distal:
- Lunate -proximal. The lunate bone (semilunar bone) is a carpal bone in the human hand. It is distinguished by its deep concavity and crescentic outline. It is situated in the center of the proximal row carpal bones, which lie between the ulna and radius and the hand. The lunate carpal bone is situated between the lateral scaphoid bone and medial triquetral bone.
- Triquetrum – proximal.The triquetrum is located just distal to the ulna and the triangular fibrocartilage complex (TFCC), and proximal to the base of the hamate. The triquetrum is a small, pyramid-shaped bone that is largely covered in ligaments that connect it to surrounding structures.
- Pisiform – proximal. The pisiform is a sesamoid bone. It is located in the flexor carpi ulnaris (FCU) wrist tendon. It protects this tendon by supporting and bearing its forces as it moves across the triquetrum during wrist movement. The triquetrum is a proximal carpal bone located between the pisiform and lunate bones. The pisiform is located opposite the wrist’s carpal base plate and communicates with the abductor digiti minimi of the hand.
- Capitate – distal. The capitate is a carpal bone located in the most central portion of the wrist. The bones of the wrist are called carpals and the bones of the hand are called metacarpals. The capitate is the largest of the carpal bones. It lies between the trapezoid and hamate, which are also carpal bones. It articulates with the third metacarpal, and a small angle helps it to also junction with the fourth metacarpal. It is convex to work with the scaphoid and lunate, which have concave surfaces.
- Trapezium – distal. The trapezium rests in the row closest to the metacarpals. The scaphoid is located directly behind it, and the trapezoid is located to its side. The trapezium has three contact surfaces. These surfaces provide articulation points with other bones. The superior surface faces an upward and medial direction. It articulates with the scaphoid.
- Trapezoid – distal. The trapezoid bone (lesser multangular bone) is a carpal bone in tetrapods, including humans. It is the smallest bone in the distal row. It may be known by its wedge-shaped form, the broad end of the wedge constituting the dorsal, the narrow end the palmar surface; and by its having four articular facets touching each other, and separated by sharp edges. It is homologous with the “second distal carpal” of reptiles and amphibians.
- Hamate – distal.The hamate is a wedge-shaped carpal bone. It is located on the outside area of the wrist, which is the same side as the pinkie finger. The hamate articulates—meaning it meets or connects with—five other bones.
- Scaphoid. The scaphoid bone is one of the carpal bones in your hand around the area of your wrist. It is the most common carpal bone to break (fracture). A scaphoid fracture is usually caused by a fall on to an outstretched hand.
Besides, ligaments are the other wrist anatomy important elements we need to discuss. Each bone within the wrist is joined to the one next to it by one or more ligaments. As you can imagine, this results in a large number of ligaments! Two of the largest ligaments of the wrist are the medial (ulnar) and lateral (radial) collateral ligaments. The MCL passes from the distal end of the ulnar and crosses the wrist to attach to the triquetrum and the pisiform. The LCL passes from the end of the radius, across the joint to the scaphoid.
Last, but not least, the muscles: Most of the muscles which act on the wrist joint, are situated within the forearm, with only the tendon crossing the joint and inserting on the hand. The muscles on the back of the forearm (dorsal aspect) act to extend the wrist or pull it back as if pulling a ring-pull.