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Carpal Bones

The carpal bones are the eight small bones that make up the wrist (or carpus) that connect the hand to the forearm and the carpus is the sole cluster of bones in the wrist between the radius and ulna and the metacarpus. The term “carpus” is derived from the Latin word carpus and the Greek καρπός (karpós), meaning “wrist”. The carpus consists of two rows of bones: proximal and distal. The proximal row, which is the more mobile of the two, articulates with the distal radius and moves in concert with the distal radius and ulna. The scaphoid, lunate, and triquetrum serve as the connecting bones that make up the proximal row. The more rigid distal row—which contains the trapezium, trapezoid, capitate, and hamate serves as a bridge between the proximal row and metacarpal bases. The carpus’ stability is maintained through its bony articulations and intrinsic and extrinsic ligaments.

These 8 bones are named for their unique shapes and size; Scaphoid (boat), Lunate(Crescent), Triquetrum(3-cornered), Pisiform(pea), Trapezuim(Table), Trapezoid(quadrilateral), Capitate(head shaped), and Hamate (hook-shaped).

The proximal row of carpal bones, as seen in a lateral to medial direction on the palmar surface, include:

  • the scaphoid bone
  • the lunate bone
  • the triquetral bone
  • the pisiform bone

The distal row, as seen from the same viewpoint as above, includes:

  • the trapezium bone
  • the trapezoid bone
  • the capitate bone
  • the hamate bone

Details Description of Carpal Bone One by One

Scaphoid Bone

The scaphoid bone(also known as the hand navicular) is one of the largest carpal bones of the proximal and distal row and it lies beneath the anatomical snuff box in the wrist. It is situated between the hand and forearm (also called the lateral or radial side) on the thumb side of the wrist it is surrounded on the proximal side by the radius, on the distal lateral side by the trapezium bone, and on the distal medial side by the trapezoid bone. It forms the radial border of the carpal tunnel and superomedially, it articulates with the lunate bone and anteromedially with the capitate bone. The radial side of the wrist articulates with the radius, lunate, trapezoid, trapezium, and capitate bone covering 80% of the bone in articular cartilage. In the palm of the hand, its tubercle is easily palpable as it sits subcutaneously with a long axis being from above downward, lateralward, forward and the scaphoid bone slowly heals because of the limited circulation to the bone and poor muscle attachment. The blood supply of scaphoid bone is received primarily from lateral and distal branches of the radial artery, which are the branches of the palmar and dorsal artery of the wrist.

scaphoid bone

Bone

The palmar surface of the scaphoid is concave in shape and forms a tubercle easily palpable subcutaneously, giving attachment to the transverse carpal ligament.

The proximal surface is triangular in shape, smooth, convex area, and articulates with the radius and joining carpal bones, such as the lunate, capitate, trapezium, and trapezoid bone.

The lateral surface is a narrow bony surface and gives attachment to the radial collateral ligament.

The medial surface has two facets, one is a flattened semi-lunar facet articulating with the lunate bone, and another one is an inferior concave facet, both of two articulating with the lunate and the head of the capitate bone.

The dorsal surface of the bone is narrow in shape, with a groove running the narrow length of the bone and allowing ligaments to attach, and the surface facing the fingers.

The inferior surface is smooth and convex triangular in shape and divided into two parts by a slight ridge.

The scaphoid tubercle is a blunt, nonarticular hollowed projection for the capitate facet on the lateral edge of the bone. The tubercle is one of the attachment points for the flexor retinaculum, a fibrous band across the wrist joints for stabilization. The facet for the head of the capitate is the large concave shape facet on the proximal side of the scaphoid bone. The crescent-shaped facet for the lunate bone is also on the proximal side of the scaphoid but forms the medial edge of the bone. The radial facet is the single, convex shape facet on the distal end of the scaphoid. The facet for the trapezoid bone runs along the dorsolateral edge of the bone towards the tubercle of the scaphoid. The facet for the trapezium is at the lateral end of the dorsal side of the scaphoid bone.

scaphoid bone

Lunate Bone

The lunate bone is one of the eight carpal short bones a crescent-shaped structure situated in the center of the proximal row of the carpal bones of the hand between the lateral scaphoid and medial triquetrum and works closely with the two forearm bones (lie between the radius and ulna) to help the wrist move. The name of the lunate bone comes from the “crescent-shaped” (Latin: lunatus), from Latin luna (“moon”), semilunar bone, and from the bone’s resemblance to a crescent moon. It sits medially to the scaphoid bone, superior to the capitate bone, and lateral to the triquetral bone, and comes into contact with the hamate bone at its inferior medial angle. The lunate articulates with five bones that are the radius proximally, capitate and hamate distally, scaphoid laterally, and triquetrum medially.

The lunate is stabilized by a medial ligament to the scaphoid and a lateral ligament to the triquetrum. Ligaments between the radius and carpal bone also stabilize the position of the lunate, as does its position in the lunate fossa of the radius.

Surface

  • The proximal surface of the lunate bone is smooth and convex, abuts the radius articulating with the radius.
  • The lateral surface is flat and narrow in shape, with a crescentic facet for articulation with the scaphoid bone.
  • The medial surface possesses a smooth with the triquetral, and quadrilateral facet for articulation with the triquetral.
  • The palmar surface is rough, as is the dorsal surface.
  • The dorsal surface is broad and rounded.
  • The distal surface of the bone is deep and concave in shape and articulates distally with the capitate bone.
  • The lunate also articulates on its distal and medial surface with the hamate bone.

These 8 bones are named for their unique shapes; Scaphoid (boat), Lunate(Crescent), Triquetrum(3-cornered), Pisiform(pea), Trapezuim(Table), Trapezoid(quadrilateral), Capitate(head shaped), and Hamate (hook-shaped).

Capitate Bone

The capitate bone is the largest of all carpal bones both proximally and distally in the human hand. It is surrounded by the lunate bone proximally, the third metacarpal bone distally, and it’s above, a rounded portion or head, which is received into the concavity formed by the scaphoid and lunate bones; a constricted portion called a neck; and below this, the body. It articulates the trapezoid bone laterally and the hamate bone medially with the third metacarpal bone (the middle finger) and forms the third carpometacarpal joint. The capitate lies directly connect to the metacarpal bone of the ring finger on its distal surface, hamate bone on its ulnar surface, trapezoid on its radial surface, lunate and scaphoid proximally.

capitate bone

Surfaces

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

The inferior surface is separated by two ridges among three facets, for articulation/connection with the second, third, and fourth metacarpal bones, and the third one is the largest in shape. The dorsal surface is broad and rough shape. The palmar surface is narrow, rounded in shape, and rough, for the attachment of ligaments and a part of the adductor pollicis muscle.

The lateral surface is connected with the lesser multi-angular by alittle facet at its anterior inferior angle, behind has a rough depression for the attachment of an interosseous ligament. Above this is a deep, rough groove, forming a part of the neck, and serving for the attachment of ligaments; it’s bounded superiorly by a smooth, convex surface, for articulation with the ossification of the scaphoid.

The medial surface articulates with the hamate by a smooth, concave facet, which occupies its posterior and superior parts; it’s a rough front, for the attachment of an interosseous ligament.

The capitate articulates with the metacarpal of the index finger its normal articulation is with the center finger.
The joints of the distal carpal row connect the surfaces of the trapezium, trapezoid, capitate, and hamate bones. These articulations form the trapeziotrapezoid, trapezoideocapitate, and capitohamate joints which are less mobile than those of the proximal carpal row.

capitate bone

Hamate Bone

The hamate is a wedge-shaped, triangular-shaped most medial unciform bone that forms part of the distal carpal row, surrounded by three other carpal bones; the capitate, triquetrum, and lunate bones and articulating with the capitate (radially), triquetrum (proximally), and fifth and fourth metacarpals (distally) and hamate serve as an attachment point for three tendons (opponens digiti minimi, flexor digiti minimi brevis, abductor digiti minimi, and flexor carpi ulnaris) ligaments flexor retinaculum, pisohamate ligament, triquetrohamate ligament, capitohamate ligament.

Hamate Bone

Structure

The hamate is found within the distal row of carpal bones and about the metacarpals of the little finger and ring finger.

Adjacent to the hamate on the ulnar side is lightly above it, which is adjacent on the radial side is the capitate, and proximal is the lunate bone.

Surfaces

The hamate bone has six surfaces the body of the hamate has six surfaces: medial and lateral, proximal and distal, palmar, and dorsal.

  • The superior surface, the apex of the wedge, is narrow, convex, smooth, and articulates with the lunate bone.
  • The inferior surface by concave facets is separated by a ridge that articulates with the fourth and fifth metacarpal bones.
  • The dorsal surface is a triangular shape and rough for ligamentous attachment.
  • The palmar surface presents, at its lower and ulnar side, a curved, hook-like process, the hook-like osseous projection called the hamulus (or hook) of the hamate bone, directed forward,  laterally, and the hamulus curved with a lateral concavity and its tip inclines laterally serves as the attachment point for a number of different muscles and ligaments of the hand and forearm, including the flexor retinaculum.
  • The medial surface articulates with the triangular bone by an oblong facet, cut obliquely from above, downward and medialward.
  • The lateral surface is rough, and articulates with the capitate by its upper and posterior parts, for the attachment of ligaments.

Hamate Bone

Hook

The hook of hamate (Latin: hamulus) is found at the proximal, ulnar side of the hamate bone contributes to the formation of the medial wall of the carpal tunnel and the lateral wall of the ulnar canal (i.e. Guyon’s canal). The hook is a curved, hook-like process that projects 1–2 mm distally, radially, forms the ulnar border of the carpal tunnel, and the radial border for Guyon’s canal. The ulnar nerve hooks around the hamate as it crosses towards the medial side of the hand.

The various structures are attached to it, including ligaments from the pisiform bone, the transverse carpal ligament, and the tendon for flexion of the hand the flexor carpi ulnaris. Its medial surface to the flexor digiti minimi brevis muscle and opponens digiti minimi muscle; its lateral side forms a grooved for the passage of the Flexor tendons into the palm of the hand.

Trapezoid Bone

A trapezoid, also known as a trapezium is the first and most lateral of the distal row of carpal bones is a flat closed shape having 4 straight surfaces or sides, with one pair of parallel sides found within the hand.

The parallel sides of a trapezium are known as the bases, the broad end of the wedge constituting the dorsal, the narrow end the palmar surface; and their non-parallel sides are called legs. A trapezium can also have parallel legs, and parallel sides can be horizontal, vertical, or slanting.

Surfaces

The superior surface, quadrilateral, smooth, and slightly concave, articulates with the scaphoid. The inferior surface articulates with the proximal end of the second metacarpal bone; it is convex from side to side, concave from before backward, and subdivided by an elevated ridge into two unequal facets.

The lateral surface, convex and smooth, articulates with the trapezium. The medial surface is concave and smooth in front, for articulation with the capitate; rough behind, for the attachment of an interosseous ligament.

The dorsal and palmar surfaces are rough for the attachment of ligaments, the former being the larger of the two.

The trapezium bone is bordered medially by the trapezoid bone and superiorly by the scaphoid bone. Inferolaterally, its main articulation is with the first metacarpal bone via a saddle-shaped facet. Inferomedially however, it sometimes also articulates with the second metacarpal bone.

Distally, it forms a stable, relatively immobile joint with the second metacarpal, radially and proximally it forms strong ligaments with the trapezium and the capitate clearly, scaphoid respectively.

However, injury can occur through axial force applied to the second metacarpal base. Subluxations, such as ones caused by delivering a blow, are not uncommon. Direct trauma to the bone can also cause a fracture.

On the palmar aspect of the bone, there is a palpable tubercle and on its medial side runs a groove that holds the tendon of the flexor carpi radialis. it is bound by strong ligaments to the trapezium radially, the capitate clearly, and the scaphoid proximally. Its shape is like a keystone, and it is two times wider dorsally than primarily.
The trapezoid may look very small as compared to the opposite bones from a palmar aspect, however, it’s much wider on its dorsal side. It communicates via its proximal facet with the os scaphoideum, laterally with Trapezium bone, medially with the capitate and its distal facet allows it to articulate with the second metacarpal.The carpal bones are aligned in proximal and distal rows. The proximal carpal row from the radial side consists of the scaphoid, lunate, triquetrum, and pisiform bones. The distal carpal row from the radial side is formed from Trapezium, trapezoid, capitate, and hamate bones. The carpal bones are held together by the interosseous, volar, and dorsal ligaments and a triangular fibrocartilage complex. The dorsal ligaments are weaker than the volar ligaments, making dorsal dislocation more common.

Trapezium Bone

The trapezium bone is one of eight small bones which make up the wrist joint. The trapezium is the last bone in the row of wrist bones, located beneath the thumb joint. The trapezium bone along with the base of the thumb joint forms what is called a carpometacarpal joint, also known as a CMC joint.

Surface

The trapezium bone may be a small oblong bone that plays a task in the stability of the carpal tunnel also because of the stability of the thumb joint. the form and size of this bone and its location below the thumb cause the integrity of the trapezium to deteriorate over time. The trapezium is involved in many small motions over time thanks to how frequently most people use their thumbs.

The superior, or upper, the surface of the trapezium is an irregular shape that’s smooth and touches another carpal bone called the scaphoid. The inferior, or lower, the surface of Trapezium is oval and resembles a saddle, which contributes to the joint being classified as a saddle joint.

The inferior surface touches the primary metacarpal, also referred to as the thumb. The palmar, or front-facing, the surface of Trapezium holds an attachment to the flexor carpi radialis tendon, the transverse carpal ligament, and muscles including opponents pollicis and flexor pollicis Brevis.

The dorsal, or rear-facing, the surface of Trapezium assists in holding these ligaments and tendons in situ. The lateral surface also assists keep these attachments in situ.

One aspect of the medial surface of the Trapezium touches another carpal bone called the trapezoid. The second aspect of the trapezium’s medial surface touches the second metacarpal or the bone of the second finger.

or

The trapezium is an irregular-shaped carpal bone found within the hand. The trapezium is found within the distal row of carpal bones and is directly adjacent to the metacarpal bone of the thumb. On its ulnar surface are found the trapezoid and scaphoid bones.

The superior surface is directed upward and medialward; medially it is smooth, and articulates with the scaphoid; laterally it is rough and continuous with the lateral surface.

The inferior surface is oval, concave from side to side, convex from before backward, so as to form a saddle-shaped surface for articulation with the base of the first metacarpal bone. This saddle-shaped articulation is partially responsible for the thumb’s opposable motion.

The dorsal surface is smooth.

The palmar surface is narrow and rough. At its upper part is a deep groove, running from above obliquely downward and medialward; it transmits the tendon of the Flexor carpi radialis and is bounded laterally by an oblique ridge. This surface gives origin to the Opponents pollicis and to the Abductor and Flexor pollicis brevis; it also affords attachment to the transverse carpal ligament.

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

The medial surface presents two facets; the upper, large and concave, articulates with the trapezoid bone; the lower, small and oval, with the base of the second metacarpal.

Triquetrum Bone

The triquetrum is a pyramidal-shaped, three-faced, formerly cuneiform types bone and one of eight carpal bones that forms part of the medial side of the proximal row of the carpus between the lunate and pisiform bones. It is the 2nd commonly fractured carpal bone and the name triquetrum has come from the Latin word triquetrous which means “three-cornered” sometimes it is also called triquetral bone, triangular bone, or cuneiform bone.  It is on the ulnar side of the hand and makes a connection with the pisiform, hamate, and lunate bones that do not articulate with the ulna.

Structure

Triquetrum Bone Triquetrum Bone

It is a three-faced bone seen within the proximal row of carpal bones situated beneath the pisiform, forming the carpal arch, within which lies in the carpal tunnel and under the pisiform.

To facilitate its palpation in an exam, the hand help to deviate radially on the palmar perspective so that the triquetrum moves out from under the ulnar styloid process. Its apex points disto-medially towards the pisiform bone, which is situated upon its palmar facet. It sits upon the hamate bone, which is in the distal row of carpal bones, and its base faces laterally articulating with the lunate bone.

Surfaces

The superior surface is situated on a medial side, rough, non-articular portion,

The lateral surface is convex in shape and articulates with the triangular articular disk of the wrist joint.

The inferior surface is concavely directed lateralward, sinuously smooth curved shape, for articulation with the hamate.

The dorsal surface is rough for the articulation of ligaments.

The volar surface oval shape facet is situated on its medial part, for articulation with the pisiform with rough ligamentous attachment.

The lateral surface, by a flat, quadrilateral facet on the base of the pyramid shape, for articulation with the lunate.

The medial surface roughened the summit of the pyramid, for the attachment of the ulnar collateral ligament of the wrist.

 

Triquetrum Bone

Pisiform Bone

The pisiform bone is a sesamoid bone spherical shape, pea-shaped bone in the proximal carpal row, located on the anterior plane of the volar and ulnar aspect with a slight distolateral long axis, or anteromedial side of the wrist joint in the proximal row of carpal bones by articulating with the triquetrum where the ulna joins the wrist. The pisiform bone also spelled pisiform (from the Latin piriformis, pea-shaped it develops in a tendon), is a small knobbly, one of eight, and smallest sesamoid bones that are found in the wrist. It forms the ulnar border of the carpal tunnel, one side that acts as a joint for articulating with the triquetral bone, and is enveloped in the flexor carpi ulnaris tendon, and can be easily palpated from the exterior.

It is the only moving structure of the carpal canal. The pisiform bone has four surfaces:

  1. The dorsal surface is smooth, oval, flat, forming the pisotriquetral joint and articulates with the triquetral with this facet approaches the superior, but not the inferior border of the bone.
  2. The palmar surface is rounded, rough, and provides for muscular attachment to the transverse carpal ligament, the flexor carpi ulnaris, and the abductor digiti quinti.
  3. The lateral surface is rough and concave in shape.
  4. The medial surface is rough and usually convex in shape.

The pisiform bone, along with the hamulus of the hamate, forms the medial boundary of the carpal tunnel because the pisiform body acts as one of the four attachments places of the flexor retinaculum. It also works as an attachment site for tendons of the abductor digit minimi and for the flexor carpi ulnaris tendon from where form bone. The pisiform is the only carpal bone in which insertions and attachments for the abductor digiti minimi and the flexor carpi ulnaris muscle. It is universally known that due to the pisiform’s large range of movement along its articulation surface with the triquetral bone (about 1 cm of movement is allowed), contraction of the flexor carpi ulnaris is urgent for the pisiform to remain stable for the abductor digiti minimi to function strongly.

What each carpal bone joints to

Name Proximal/radial
articulations
Lateral/medial
articulations
Distal/metacarpal
articulations
Proximal row
Scaphoid radius capitate, lunate trapezium, trapezoid
Lunate radius, articular disk scaphoid, triquetral capitate, hamate (sometimes)
Triquetrum articular disk lunate, pisiform hamate
Pisiform triquetral
Distal row
Trapezium scaphoid trapezoid first and second metacarpal
Trapezoid scaphoid trapezium, capitate second metacarpal
Capitate scaphoid, lunate trapezoid, hamate third, partly second
and fourth metacarpal
Hamate triquetral, lunate capitate fourth and fifth

References

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