The Wrist & Hand - Bones

What is the wrist joint?

The wrist joint (also known as the radiocarpal joint) is a synovial joint in the upper limb, marking the area of transition between the forearm and the hand. 

• Ulna

  The distal end of the ulna is much smaller in diameter than the proximal end.

  • It is mostly unremarkable, terminating in a rounded head, with distal projection – the ulnar styloid process.

  • The head articulates with the ulnar notch of the radius to form the distal radio-ulnar joint.

  • The distal end of the ulna also articulates with some carpal bones of the wrist, and is a point of attachment for ligaments stabilising the wrist.

• Radius

  In the distal region, the shaft of the radius expands to form a rectangular end.

  • The lateral side projects distally as the styloid process.

  • In the medial surface, there is a concavity, called the ulnar notch, which articulates with the head of ulna, forming the distal radioulnar joint.

  • The distal surface of the radius has two facets, for articulation with the scaphoid and lunate  carpal bones — these make up the radiocarpal joints.

(For the purpose of this article about the wrist and hand, we will not be discussing the anatomy of the proximal part or shaft of the ulna nor radius, as it will be covered in the article about the elbow)

Understanding the bones of the hand

The bones of the hand provide support and flexibility to the soft tissues. 

They can be divided into three categories:

• Carpal bones (Proximal)

  A set of eight irregularly shaped bones. These are located in the wrist area.

• Metacarpals

  There are five metacarpals, each one related to a digit

• Phalanges (Distal)

  The bones of the fingers. Each finger has three phalanges, except for the thumb, which has two.

What are the carpal bones?

There are eight carpal bones in total, divided into two rows of four.

Going from lateral to medial, the bones are as follows:

• Proximal — Scaphoid, Lunate, Triquetrum, Pisiform

• Distal — Trapezium, Trapezoid, Capitate, Hamate

An acronym to remember the order of the bones in the wrist is “Some Lovers Try Positions That They Can’t Handle”

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

Collectively, the carpal bones form an arch, in the coronal plane

The distal row of carpal bones articulate with the metacarpal bones of the hand

What are the metacarpal bones?

The metacarpal bones articulate proximally with the carpals, and distally with the proximal phalanges. 

Each metacarpal consists of a base, shaft and a head. The medial and lateral surfaces of the metacarpals are concave, allowing attachment of the interossei muscles.

They are numbered, and each associated with a digit:

1. Metacarpal I – Thumb.

2. Metacarpal II – Index finger.

3. Metacarpal III – Middle finger.

4. Metacarpal IV – Ring finger.

5. Metacarpal V – Little finger.

What are the phalanges?

The phalanges are the bones of the fingers. The thumb has a proximal and distal phalanx, while the rest of the digits have proximal, middle and distal phalanges.

Where are the bursae?

The wrist joint is surrounded by two main individual bursae. 

A bursa is a fluid-filled sack that serves to reduce friction in the joint. 

The two major bursae of the wrist are the radial bursa and the ulnar bursa. 

• Radial bursa — reduces friction on the tendons that run from the forearm through the wrist and into the hand, and serves the thumb. This bursa extends to where the wrist creases.

• Ulnar bursa — reduces friction on the tendons that run from the forearm through the wrist and into the hand, and serves the index, middle, and ring fingers.

Accessory ossicles of the wrist and hand are often visualised on plain film radiographs or related cross-sectional imaging. Over 20 of these bones have been described, but some are more common than others.