Functional Anatomy of the Wrist

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Wrist Structure

The wrist structure include three joints and nine bones:

• Distal ulna and distal radius make up the distal radioulnar joint
• The scaphoid, the lunate, and the triquetrum, which are proximal carpal bones and the radius are part of the radiocarpal joint
• The trapezium, trapezoid, capitate, and hamate bones are the distal carpal bones. Together with the proximal carpal bones they make up the midcarpal joint.
• The pisiform is the fourth bone located in the proximal row. It does not however articulate as part of the radiocarpal joint, as it sits on top of the triquetrum.

Distal ulna: includes the ulnar head and the styloid process.

• Ulnar head has two articular surfaces: lateral and inferior
  • The lateral surface which is convex articulates with the ulnar notch of the distal radius.
  • The inferior surface articulates with an articular disc (the triangular fibrocartilage).
  • Due to its articulation with the disc, the head of distal ulna is separated from the carpal bones therefore does not have a direct contribution to the wrist joint
• Styloid process: short and rounded. Can be palpated at the dorsomedial aspect of the wrist.

Distal radius: contains four articular surfaces and dorsal tubercle, known as Lister’s tubercle

• Articular surfaces
  • Concaved anterior surface
  • The medial surface with its concaved ulnar notch for holding the head of the ulna
  • The lateral surface with its radial styloid process
  • The inferior surface with two facets articulating with two carpal bones: the scaphoid and lunate
• Dorsal tubercle located on the posterior surface

The scaphoid (navicular): belongs to the proximal raw of the carpal bones. Located the most laterally. Its palmar surface contains the tubercle which is the attachment point for the muscles and the floor for the anatomical snuffbox.

The lunate: part of the proximal raw of the carpal bones. It articulates with the scaphoid on the lateral, and with the triquetrum on the medial side.

The triquetrum: other names include triquetral bone, triangular bone or cuneiform bone. It is triangular and pyramidal shaped bone located on the medial side of the wrist. It contains multiple articulation surfaces: lateral for the lunate, anterior for the pisiform, and the distal for the hamate.

The trapezium: located in the distal raw of the carpal bones, on its radial side. It has fours articulations for the first metacarpal, second metacarpal, scaphoid and trapezoid bones. This bone is part of the mechanism responsible for a wide mobility of human hand where the articulation between the first metacarpal and the trapezium allows for the opposable thumbs function.

The trapezoid: the lesser multangular bone is another name for this bone. This is the smallest bone of the distal raw of the carpal bones. It gives structure to the palm of the hand

The capitate: the largest and most central carpal bone belonging to the distal raw. It articulates with the bases of the 2nd and 3rd metacarpal bones forming part of the common carpometacarpal joint in the hand. In addition it articulates with the following: scaphoid and lunate on the proximal, trapezoid on the lateral, and the hamate on the medial surfaces.

The hamate: sits on the medial side of the distal row of carpal bones. The hook of hamate is the bony process extending from the palmar surface. This carpal bone forms the medial border of the carpal tunnel.

Bones, Articulations and Kinematics of the Wrist

Wrist mobility depends on the interaction between seven carpal bones and the radius. The ulna and the pisiform are excluded from this motion as the pisiform is a sesamoid bone and the ulna is separated from the carpal bones by the disc. Each of the carpal bones has a separate motion axis, however their movement depends on the carpal alignment with the distal radius. The greatest degree of motion occurs between the radius and the proximal raw of the carpal bones and less motion between the proximal and distal raw of the carpal bones. There is no motion between the distal raw of the carpal bones and the metacarpals.

The wrist biomechanics can be described by the following theories:

• The row-theory when the carpal bones are grouped into proximal and distal raws
• The column-theory when the central column include the lunate, the capitate, and the hamate; the lateral column (described as the mobile column) includes the scaphoid, the trapezium, and the trapezoid; and the medial column ( rotation column), contains the triquetrum and the pisiform.
• The row-column-theory which is the modified column-theory. The trapezium and trapezoid are added to the central column and the pisiform was eliminated from the medial column.
• The ring-model where the wrist has two mobile links: the mobile trapezioscaphoid articulation and the rotatory triquetrohamate joint.
• The link joint-theory with the radius, the proximal raw and the distal raw comprising the individual links.
• The clamp-theory where bones are "clamped" together into masses: the scaphoid bone as one mass, the lunate and triquetrum as the second mass, and the hamate, capitate, and trapezoid as a distal mass.

Bones and Articulations

Bones	Articulations	Characteristics	Key palpation points
Distal ulna Distal radius	Radioulnar joint	Essential joint for forearm rotation When forearm supination and pronation are lost, it significantly affects the function of the upper limb
Distal radius Scaphoid Lunate Triquetrum	Radiocarpal
Trapezium Trapezoid Capitate Hamate Scaphoid Lunate Triquetrum	Midcarpal

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