Carpal Bones Anatomy

The wrist is made up of eight carpal bones, which are arranged in two rows. The wrist or carpus is the region between metacarpals and distal ends of radius and ulna.

[Read complete anatomy of the wrist]

The carpus or the wrist functions to facilitate effective positioning of the hand and thus allowing efficient use of the power of the extensors and flexors of the forearm.

Carpal bones move together in different groups in different movements of the wrist. There combined movements increase the freedom of movements at the wrist.

Carpal Bones

There are eight carpal bones in each wrist arranged in two rows – proximal and distal. . Each row contains four bones.

The arrangement of bones in carpal rows from lateral to medial [radial to ulnar or from the side of thumb to little finger] direction is

The proximal Row [Lateral to medial}

• Scaphoid
• Lunate
• Triquetral
• Pisiform bones

The Distal Row [Lateral to medial]

• Trapezium
• Trapezoid
• Capitate
• Hamate

The arrangement of bone is such that both the rows are convex proximally. The proximal row is concave distally to accommodate convexity of the distal row and the distal row is flat distally to articulate with metacarpal heads.

How to Identify Carpal Bones and Their Side Determination

General Points

Following general features help to identify the row of carpal bone and give an idea of which carpal bone it is. The side of the bone could be then determined by specific points.

Each bone has 6 surfaces

• The palmar and dorsal surfaces are non-articular
  • Except for the triquetral and pisiform
• The dorsal nonarticular surface is always larger than the palmar nonarticular surface
  • Except lunate, in which the palmar surface is larger.
• The lateral surfaces of all the bones are articular
  • Except for scaphoid and trapezium [lateral bones]
• The medial surfaces of all the bones are articular
  • Except for the triquetral, pisiform and hamate [medial bone]
• Proximal and distal surfaces are articular
  • Except for pisiform

The side can be finally determined with the help of the specific points.

Specific Features of Different Carpal Bones

Scaphoid

• The scaphoid is boat-shaped
• Tubercle on its lateral side directed laterally, forwards and downwards.

Lunate

• Half moon shape or crescent
• Small semilunar articular surface for the scaphoid is on the lateral side
• Quadrilateral articular surface for the triquetral is on the medial side.

Triquetral

• The shape is pyramidal
• Isolated oval facet on the distal part of the palmar surface for the pisiform
• Continuous and nonarticular medial and dorsal surfaces

Pisiform

• Pea-shaped
• Has only one oval facet on the proximal part of its dorsal surface for triquetral
• The lateral surface is grooved by the ulnar nerve.

Trapezium

• Quadrangular in shape
• Has a crest and a groove anteriorly [palmar surfcae]
  • Groove for the tendon of the flexor carpi radialis
  • The groove is limited laterally by the crest of the trapezium
• Concavoconvex distal articular surface for the base of the first metacarpal bone.

Trapezoid

• Resembles the shoe of a baby
• The distal articular surface is bigger than the proximal.
• The palmar nonarticular surface is prolonged laterally.

Capitate

• The largest carpal bone, with a rounded head
• The dorsomedial angle is the distal-most projection from the body of the capitate.
• It bears a small facet for the 4th metacarpal bone.

Hamate

• The hamate, is wedge-shaped, with a hook near its base.
• The hook projects from the distal part of the palmar surface, and is directed laterally.

Scaphoid Bone

The scaphoid bone is the largest bone of the proximal row of carpal bones and is situated at the radial (radius bone) side of the carpus [see the image].

Skaphe means “a boat,” and eidos means “form” in Greek. The name scaphoid is derived by combining two words.

It is approximately the size and shape of a medium sized cashew. The long axis of the bone is directed from above downward, lateralward, and forward.

Surfaces & Articulations

The superior or proximal surface is smooth, triangularly convex for articulation with the lower end of the radius.

The inferior or distal surface is directed downward, lateralward, and backward. It is also smooth, convex, and triangular like superior surface. A small ridge divides into lateral and medial parts which articulate respectively with the trapezium,  the trapezoid bones of distal row. The trapezium and trapezoid are also called greater and lesser multangular bones.

The dorsal surface bears a narrow, rough groove, which runs the entire length of the bone, and serves for the attachment of ligaments.

The volar surface is concave and has a tubercle. The tubercle is directed forward and gives attachment to the transverse carpal ligament. Occasionally, it also provides origin to a few fibers of the abductor pollicis brevis.

The lateral surface is rough and narrow and gives attachment to the radial collateral ligament of the wrist.

The medial surface has two articular facets. The superior articulates with the lunate bone and the inferior one is for the head of the capitate bone.

Lunate

The lunate bone is a semilunar carpal bone that sits in the center of the proximal row of the carpus (wrist). The shape is similar to the crescent moon. The name derives from the Latin luna which means moon.

Lunate is situated between the lateral scaphoid bone and medial triquetral bone.

The superior surface, convex and smooth, articulates with the radius.

The inferior surface is deeply concave and articulates with the head of the capitate, and, by a long, narrow facet with the hamate.

The dorsal and palmar surfaces are rough, for the attachment of ligaments.

The lateral surface articulates with the scaphoid.

The medial surface is marked by a smooth, quadrilateral facet, for articulation with the triangular bone (triquetral).

Triquetral

The triquetral bone is also called triquetrum bone, pyramidal bone, three-cornered bone, and triangular bone. The name is derived from Latin triquetrus which means “three-cornered.”

It is located in the wrist on the medial side of the proximal row of the carpal bones between the lunate and pisiform bones.

It is on the ulnar side of the hand and articulates with the pisiform, hamate, and lunate bones.

It is the 3rd most commonly fractured bone out of all carpal bones fractures.

The bone can be distinguished by its pyramidal shape and presence of a single oval facet for articulation with the pisiform bone.

The superior surface is visibly divided into a medial, rough, non-articular portion, and a lateral convex articular portion. The lateral part articulates with the triangular articular fibrocartilage of the wrist.

The inferior surface is directed lateralward. It is concave and sinuous. It articulates with the hamate.

The dorsal surface is rough for the attachment of ligaments.

The volar has an oval facet, for articulation with the pisiform on its medial part. The lateral part is rough for ligamentous attachment.

The lateral surface forms the base of the pyramid and is marked by a flat, quadrilateral facet for articulation with the lunate.

The medial surface forms the summit of the pyramid. It is pointed and roughened for the attachment of the ulnar collateral ligament of the wrist.

Pisiform

The pisiform bone (also lentiform bone) is a small, pea-shaped carpal bone in the proximal row of the carpal bones within the flexor carpi ulnaris. It is located where the ulna joins the carpus. It articulates only with the triquetral.

It is considered a sesamoid bone though some authors have contested this..

Its dorsal surface has a smooth, oval facet, for articulation with the triquetral.

The palmar surface is rounded and rough and gives attachment to the transverse carpal ligament, flexor carpi ulnaris, abductor digiti quinti.

The lateral and medial surfaces are also rough, the former being concave, the latter usually convex.

Trapezium

The trapezium bone or greater multangular bone is a carpal bone situated at the distal row on the radial side of the carpus, between the scaphoid and the first metacarpal bone.

Surfaces and Articulations

The inferior surface is oval, concave from side to side and convex from before backward. It has a  saddle-shaped surface for articulation with the base of the first metacarpal bone.

The dorsal surface is rough.

The palmar surface is narrow and rough. It has a groove at its upper part for transmission of the flexor carpi radialis.

The lateral surface is broad and rough, for the attachment of ligaments.

The medial surface presents two facets. The upper is large and concave that articulates with the trapezoid. The lower facet is small and oval for articulation with the base of the second metacarpal.

Trapezoid

The trapezoid bone or lesser multangular bone is the smallest bone in the distal row.

It is wedge-shaped and has four articular facets touching each other, separated by sharp edges.

Surfaces and Articulations

The superior surface is quadrilateral and slightly concave. It is smooth for articulation with the scaphoid.

The inferior surface articulates with the proximal end of the second metacarpal bone.

The dorsal and palmar surfaces are rough for the attachment of ligaments. The dorsal surface is the larger of the two.

The lateral surface articulates with the trapezium.

The medial surface is concave and smooth in front, for articulation with the capitate. The rough part behind provides an attachment of an interosseous ligament.

Capitate

Capitate bone is the largest of the carpal bones. Capitatus in Latin means having a head.

It occupies the distal row in the center of the wrist.

The bone is named so because it has a head or rounded portion which is received into the concavity formed by the scaphoid and lunate bones. A constricted portion distal to head is called neck and beyond that is the body.

Surfaces & Articulations

The superior surface is round, smooth, and articulates with the lunate bone.

The inferior surface is divided by two ridges into three facets, for articulation with the second, third, and fourth metacarpal bones, that for the third being the largest.

The dorsal surface is broad and rough.

The volar surface is narrow, rounded, and rough, for the attachment of ligaments and a part of the adductor pollicis muscle.

The lateral surface articulates with the trapezoid by a small facet at its anterior inferior angle.

The medial surface articulates with the hamate.

Thus, the capitate articulates with seven bones

• Scaphoid and lunate proximally
• Second, third and fourth metacarpal distally
• Trapezoid on the radial side, and hamate on the ulnar side.

Hamate

The hamate bone is also called unciform bone. It is a wedge-shaped bone and has a hook-like process which projects from its volar surface. [From Hamatus – hooked, Latin]

The bone is the medial most of the distal row. The base is distal resting on the 4thand 5th metacarpal bones. The apex is directed upward and lateralward.

The superior or proximal surface is the apex of the wedge. It is narrow, convex, smooth, and articulates with the lunate.

The inferior surface has concave facets separated by a ridge for articulation with the fourth and fifth metacarpal bones.

The dorsal surface is triangular and rough for ligamentous attachment.

The volar surface has a hook-like process on ulnar side, the hamulus, directed forward and lateralward.

The medial surface has an oblong facet downward and medialward for articulation with the triquetral bone.

The lateral surface articulates with the capitate by its upper and posterior part. Rest of the surface is rough for ligament attachments.

Blood Supply of Carpal Bones

Blood supply to the carpal bones can be divided into extraosseous or intraosseous.

Extraosseous

The radial and ulnar arteries and their branches form six transeverse arches around the carpal bones that supply these bones

These transverse arches are

• Dorsal Intercarpal
• Palmar Intercarpal
• Dorsal Radiocarpal
• Palmar Radiocarpal
• Dorsal Basal Metacarpal
• Deep Palmar

Dorsal intercarpal, palmar radiocarpal and deep palmar arches are dominant suppliers.

Scaphoid, pisiform & trapezium have direct branches off the Radial & Ulnar arteries

Intraosseous

The blood supply to most carpal bones enters the distal half, leaving the proximal half at risk. There are three types of blood supplies in carpal bones and varies with bone

• Scaphoid and capitate have only a single  vessels entering one surface.  In these large areas of bone is  dependent on single vessel
• Hamate and trapezoid have more than one sites of vessel entry but lack significant anastomosis
• Trapezium, 80% lunates, triquetrum and pisiform have got numerous anastomoses and rich internal network
• 80% of lunate bones have rich blood supply like trapezium but about 20% have only single vessel like scaphoid.

Vascular Supply of Scaphoid

Scaphoid receives the majority of its blood supply via dorsal vessels at or just distal to the waist area, perfusing proximal pole in a retrograde fashion.

The bone is mainly supplied by branches of radial artery which enter the scaphoid through foramina along its dorsal ridge. It supplies 70-80% of bone, including the entire proximal pole.

Palmar & superficial palmar branches of radial artery enter carpal scaphoid in the region of its distal tubercle perfusing distal 20-30 % of bone, including tuberosity.

Clinical Significance of Carpal Bones

Injuries

Carpal bones can be frequently injured.

The scaphoid bone is commonly injured. As discussed before, the precarious blood supply of the scaphoid makes it vulnerable to osteonecrosis after the trauma.

[Read more about scaphoid fractures]Because of its unusual retrograde vascular supply, the scaphoid has a high risk of nonunion and osteonecrosis after fracture.

Dislocations of the wrist are also common and understanding of the arrangement of the wrist bones become very important for proper diagnosis and treatment.

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