Carpal Bones

The hand is made up of a set of 27 different bones. These are divided into 3 main groups: Carpal Bones, Metacarpal Bones, and Finger Bones. In this article, we review and summarize the Carpal Bones.

What are the Carpal Bones?

The Carpal Bones are the bones located primarily at the wrist. There are a total of 8 bones divided into 2 rows.

The Carpal Bones form the so-called Carpal Joint, along with the Metacarpal bones to form the Carpometacarpal Joint. Additionally, together with the forearm bones (Ulna and Radius), they form the Wrist Joint (Radiocarpal and Ulnocarpal Joint).

What are the Carpal Bones?

The Carpus is made up of 8 bones which are subdivided into 2 rows. The first row of the Carpus is formed by the bones:

• Scaphoid
• Lunate
• Triquetrum
• Pisiform

While the second row of the Carpal bones is formed by the bones:

• Capitate
• Hamate
• Trapezium
• Trapezoid.

Next, we will review each of these bones.

Interactive Anatomy of Carpal Bones

First Row of Carpal Bones

The first row of the Carpal bones is related to the Radius and Ulna bones. It is composed of 4 bones: Scaphoid, Lunate, Triquetrum, and Pisiform.

Scaphoid Bone

It is the largest and most voluminous of the bones in the first row of the Carpal.

On its anterior surface or palmar surface, there is a small protuberance known as the Tubercle of the Scaphoid (marked with a light blue circle in Image 3). This is also where the Lateral External Ligament attaches.

The Lateral External Ligament attaches to the Tubercle of the Scaphoid. The Scaphoid Bone also has 3 articular surfaces: Superior, Inferior, and Internal.

The Superior or Superoexternal articular surface articulates with the Radius and has a convex shape. The inferior surface articulates with the Trapezium and Trapezoid Bones (which are part of the second row of Carpal Bones), and the internal surface articulates with the Capitate and Lunate Bones.

The Scaphoid Bone has the highest likelihood of injury among the Carpal Bones.

Thus, the Scaphoid Bone articulates with 5 bones:

• Radius
• Trapezium
• Trapezoid
• Capitate
• Lunate

Lunate Bone

The Lunate Bone is located between the Scaphoid Bone and the Triquetrum Bone. It is the second bone in the first row of the Carpus.

The name “Lunate” comes from the fact that one of its concavities is shaped like a crescent moon.

The Lunate Bone has 4 articular surfaces. The superior surface is convex and articulates with the Radius. The inferior articular surface is concave in the shape of a crescent moon (also known as the lunate surface) and articulates with the Capitate Bone and part of the Hook Bone. It also has an external articular surface that articulates with the Scaphoid and an internal surface for the Triquetrum.

Thus, the Lunate Bone articulates with 5 bones:

• Radius
• Scaphoid
• Triquetrum
• Hook Bone
• Capitate Bone

Triquetrum Bone

The Pisiform Bone is located between the Semilunar Bone and the Pisiform Bone. It is named because it has the shape of a pyramid.

The Pyramidal Bone has 4 articular surfaces. The superior surface is irregular and convex and is related to the Triangular Ligament of the distal Radioulnar joint. The inferior surface is concave and articulates with the Hooked Bone. The external articular surface is flat and articulates with the Semilunar Bone. The internal surface articulates with the Pisiform Bone, which can sometimes be flat and other times convex.

Thus, the Pyramidal Bone articulates with 4 bones:

• Ulna
• Pisiform Bone
• Semilunar Bone
• Hooked Bone

Pisiform Bone

The Pisiform Bone is the last of the bones in the first row of the Carpus. It is a prominent bone that stands out from the rest of the bones.

The Pisiform Bone has two ends or portions. The first is a prominence that extends upward and outward. The second portion or end extends in continuity with the rest of the Carpal Bones.

The Pisiform Bone has 4 articular surfaces. On the inner surface, the Internal Lateral Ligament of the Wrist is attached. The outer surface features a longitudinal canal through which the Ulnar Artery and the Ulnar Nerve in its deep branch pass. On its anterior surface, the Abductor Muscle of the Little Finger and the Anterior Ulnar Muscle are attached.

The Pisiform Bone articulates only with the Pyramid Bone.

Second Row of Carpal Bones

The second row of Carpal Bones is related to the Metacarpal Bones. It consists of 4 bones: Capitate, Hamate, Trapezium, and Trapezoid.

Trapezium Bone

The Trapezium Bone is the first bone of the second row of the Carpus and is related to the Scaphoid Bone and the First Metacarpal.

The Trapezium Bone has 3 articular surfaces. The upper surface is triangular and concave, which articulates with the Scaphoid Bone. The lower surface has 2 articulations, the first concave with the First Metacarpal and the second flat with the Second Metacarpal. The inner surface articulates with the Trapezoid Bone.

Thus, the Trapezium Bone articulates with 4 bones:

• Scaphoid
• Trapezoid
• First Metacarpal
• Second Metacarpal

Trapezoid Bone

The Trapezoid Bone is located between 4 Carpal Bones. Therefore, it has 4 articular surfaces. The first one, triangular and concave, is in contact with the Scaphoid Bone. The inferior surface is elongated and convex and articulates with the Second Metacarpal. The external surface is also convex and is related to the Trapezium Bone. Lastly, it has an irregular internal surface that articulates with the Capitate Bone.

Thus, the Trapezoid Bone articulates with 4 bones:

• Scaphoid
• Second Metacarpal
• Trapezium
• Capitate

Capitate

The Capitate Bone is the bone with the largest volume and size among the Carpal Bones (hence its name).

The Capitate Bone is divided into 3 portions:

• Head: It is the upper portion and is round in shape.
• Neck: It is the medial portion and is narrow compared to the other 2 portions.
• Body: It is the lower portion and has the greatest volume.

The Capitate bone has 4 articular surfaces. The superior surface is convex and articulates with the Scaphoid and Lunate bones. The external surface articulates with the Scaphoid and Trapezoid bones. The inferior surface articulates with 3 Metacarpal bones (Second, Third, and Fourth Metacarpal). Lastly, the internal surface is an extensive facet and articulates with the Hook bone.

Thus, the Capitate articulates with 7 bones:

• Scaphoid
• Lunate
• Trapezoid
• Hook bone
• First Metacarpal
• Second Metacarpal
• Third Metacarpal

Hook bone

The Hook bone is the last of the bones in the second row of the carpus. It is a bone that is wider than it is long. It has a pyramidal shape and its base relates to the last 2 Metacarpal bones.

The Hook-shaped bone has 3 articular surfaces. The upper surface is found as a blunt edge facing downward, articulating with the Semilunar Bone and the Pyramidal Bone. The inner surface is oblique and articulates with the Capitate and Semilunar Bones. The lower articulation is concave and relates to the Fourth and Fifth Metacarpal Bones.

The Hook-shaped Bone gets its name because it has a forward protrusion on its lower surface that resembles a hook.

Thus, the Hook-shaped Bone articulates with 5 bones:

• Pyramidal
• Capitate
• Semilunar
• Fourth Metacarpal
• Fifth Metacarpal

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