Carpal bones

The carpal bones (Latin: os carpale, pl. ossa carpi) are eight small and irregularly shaped bones located in the wrist area. These bones are arranged into a proximal row and a distal row. The proximal row of the carpal bones articulates with the bones of the forearm, while the distal row articulates with the bases of the five metacarpal bones. 

The proximal row, as viewed from lateral to medial direction on the palmar surface, include the following carpal bones:

  • scaphoid,
  • lunate,
  • triquetral,
  • pisiform.

Four bones forming the distal row of the carpal bones, also viewed from lateral to medial direction, include:

  • trapezium,
  • trapezoid,
  • capitate,
  • hamate.

Proximally, the scaphoid and lunate articulate with the distal end of the radius, forming the wrist joint, also known as the radiocarpal joint. All of the carpal bones in the distal row articulate with the metacarpal bones

Collectively, the carpal bones form an arch in the coronal plane. This arch on its carpal surface is convex proximally and concave distally, while on the palmar surface, it is simply concave. A membranous band, known as the flexor retinaculum, extends between the medial and lateral edges of the arch, forming the carpal tunnel.

Each carpal bone has its unique shape and multiple facets for articulation with several bones. 

Scaphoid bone

The scaphoid bone (Latin: os scaphoideum) is the largest of the carpal bones in the proximal row, and it lies beneath the anatomical snuff box.

It is surrounded by the following bones when viewed from a palmar aspect of the hand:

  • proximally - the radius;
  • distally and laterally - the trapezium;
  • distally and medially - the trapezoid;
  • superomedially - the lunate bone;
  • inferomedially - the capitate bone.

The scaphoid bone has a tubercle, which lies subcutaneously and is easily palpable.

Lunate bone

The lunate bone (Latin: os lunatum) is crescent-shaped and has a large proximal articular surface, which communicates with the radius and its articular disk at the wrist joint.

It relates to the following bones:

  • proximally - the radius;
  • medially - the triquetral bone;
  • laterally - the scaphoid bone;
  • inferiorly - the capitate bone.

Sometimes the lunate bone may come in contact with the hamate bone at its inferomedial angle.

The lunate bone, together with the triquetral bone next to it, create the carpal articular surface proximally, which articulates with the head of the radius.

Triquetral bone

The triquetral bone (also known as triquetrum, pyramidal bone, three-faced bone; Latin: os triquetrum, os pyramidale) is a pyramidal-shaped carpal bone. From a palmar view, its apex is directed distally and medially, while its base is faced laterally. 

It relates to the following bones:

  • distally and medially - the pisiform bone;
  • inferiorly - hamate bone;
  • laterally - the lunate bone.

Pisiform bone

The pisiform bone (Latin: os pisiforme) is a small sesamoid bone and is the most medial carpal bone from a palmar view. It forms the ulnar border of the carpal tunnel.

The dorsal surface of the pisiform bone articulates with the ventral surface of the triquetral bone. It lies within the tendon of the flexor carpi ulnaris and can be palpable.

Trapezium bone

The trapezium bone (or greater multangular bone, Latin: os trapeziumos multangulum majus) is the most lateral carpal bone in the distal row when viewed from the palmar side. 

The trapezium bone communicates with several bones of the hand, including:

  • medially - the trapezoid bone;
  • superiorly - the scaphoid bone;
  • inferolaterally - the first metacarpal bone;
  • inferomedially, it sometimes articulates with the second metacarpal bone.

There is a palpable tubercle on the palmar surface of the trapezium bone. On its medial side, there is a groove that gives attachment to the tendon of the flexor carpi radialis.

Trapezoid bone

The trapezoid bone (or lesser multangular bone, Latin: os trapezoideumos multangulum minus) is the most medial and the smallest carpal bone within the distal row. 

It communicates with the following bones:

  • proximally - the scaphoid bone;
  • laterally - the trapezium bone;
  • medially - the capitate bone;
  • distally - the second metacarpal bone.

Capitate bone

The capitate bone (Latin: os capitatum) is the largest of all the carpal bones. It lies in the distal row and is surrounded by the following bones:

  • proximally - the lunate bone;
  • laterally - the trapezoid bone;
  • medially - the hamate bone;
  • distally - the third metacarpal bone.

Hamate bone

The hamate bone (Latin: os hamatum) is the last and most medial carpal bone in the distal row when the hand is seen from its palmar aspect. 

It is palpable, as it lies subcutaneously and because of the bony notch on its palmar surface that curves laterally, known as the hamulus. The flexor digiti minimi and the pisohamate ligament attach to the hamulus.

The hamate bone communicates with several bones, including:

  • proximally and laterally - the lunate bone;
  • proximally and medially - the triquetral bone;
  • laterally - the capitate bone;
  • distally, it articulates with both the fourth and fifth metacarpal bones.