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.
The proximal Row [Lateral to medial}
- Pisiform bones
The Distal Row [Lateral to medial]
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
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
- The scaphoid is boat-shaped
- Tubercle on its lateral side directed laterally, forwards and downwards.
- 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.
- 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
- Has only one oval facet on the proximal part of its dorsal surface for triquetral
- The lateral surface is grooved by the ulnar nerve.
- 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.
- Resembles the shoe of a baby
- The distal articular surface is bigger than the proximal.
- The palmar nonarticular surface is prolonged laterally.
- 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.
- 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.
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 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 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.
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).
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 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 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.
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.
Surfaces and Articulations
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 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 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 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.
- Left Capitate Bone
Surfaces & Articulations
The superior surface is round, smooth, and articulates with the lunate bone.
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 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.
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 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.
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
- 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
Clinical Significance of Carpal Bones
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.
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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