Image: 3D representation of the wrist bones.
Understanding the complexity of the anatomy is important for the diagnosis of wrist pain and wrist injuries. We would like to give laypeople and those interested in medicine an insight into this.
The bones in the wrist:
8 carpal bones, 5 metacarpal bones and the two forearm bones
Which are the 8 carpal bones on the wrist?
As it is not easy to memorize all the carpal bones straight away, there is a well-known mnemonic that every medical student knows:
“A little ship sailed in the moonlight around the triangle and the pea leg. Polygon large and polygon small, the head must be on the hook.” What does this actually mean?
Scaphoid bone = Os scaphoideum (little boat): Looks like a boat.- Moon bone = Os lunatum (moonlight): Looks like a 1/3 moon in the lateral X-ray.
- Triangular bone = Os triquetrum (triangle)
- Pea bone = Os pisiforme: Similar in shape and size to a pea.
- Large polygonal bone = Os trapezium (large polygon)
- Small polygonal bone = Os trapezoideum (small polygon)
- Head bone = Os capitatum (head)
- Hook bone = Os hamatum (hook): Has an extension that actually looks like a hook.
5 metacarpal bones and 2 forearm bones
The metacarpal bones (= Os metacarpale, plural Ossa metacarpalia) together with the carpal bones form the part of the wrist furthest from the body. The strongest metacarpal bone is that of the thumb (9). The others (10-13) are narrower but longer.
The part of the wrist closest to the body is formed by the two forearm bones. The radius (14) makes up approx. 2/3 of the wrist area, the ulna (15) approx. 1/3.
The Discus triangularis
The discus triangularis is an approximately triangular cartilage plate that is attached to the wrist on the little finger side by ligaments. Similar to the meniscus in the knee joint, it serves to improve ‘articulation’, i.e. better joint gliding behavior. The triangular disc is located in the joint space between the ulna (15) and the triangular bone (3). It is not visible in the X-ray image. Injuries to the triangular disc can be diagnosed by MRI (magnetic resonance imaging) or arthroscopy.
Wrist straps
The number of ligaments on the wrist is high. The carpal bones are attached to each other with a complicated network of ligaments on both the flexor and extensor sides. The scapholunate l igament, which stretches between the scaphoid bone (1) and the lunate bone (2), is a very important ligament that can often tear in the event of injury, sometimes with serious consequences.
There are also the collateral ligaments on the wrist. The ulnar collateral ligament extends from the ulna (15) to the 5th metacarpal bone (13). The radial collateral ligament extends from the radius to the large polygonal bone (5).
The metacarpal bones (9-13) are also connected to each other and to the carpal bones at the base (interosseous metacarpal ligament and carpomatacarpal ligament). The same applies to the radius (15) and ulna (14).
Which tendons are found on the wrist?
The image shows a cross-section of the wrist (MRI image). The following structures are clearly visible:
Carpal tunnel with flexor tendons (black spots) and the transverse wrist ligament (light stripe). A constriction in the carpal tunnel, with the following symptoms, is the cause of carpal tunnel syndrome. The bones visible here on the wrist are the hook bone (8) with the small hook, the head bone (7), the small polygonal bone (6) and the first metacarpal bone (9) on the thumb. Also clearly visible: the muscles of the ball of the thumb (16) and the muscles of the ball of the little finger (17), between which the transverse wrist ligament (carpal ligament) is stretched.
There are a large number of tendons on the wrist. Some only extend to the wrist, others to the fingers.
The tendons of the long fingers consist of a superficial and deep flexor tendon. The finger extensor tendons are partly connected to each other via cross-connections (Connexus intertendinei).
The 3 main nerves
The main nerves of the hand are the ulnar nerve (= ulnar nerve), radial nerve (= radial nerve) and median nerve (= median nerve).
The ulnar nerve, which runs along the wrist on the flexor and little finger side, can be seen in the picture on the right (2 yellow dots). Here it is already divided into its superficial and deep branches. The radial nerve runs on the thumb side and divides into several branches before it reaches the wrist, which supply the extensor side of the hand on the thumb side. Only two branches are shown on the left (2 yellow dots), which run to the thumb. The median nerve is the largest nerve (large yellow spot in the middle) and runs under the transverse wrist ligament in the carpal tunnel.
The following images show the areas of supply with skin sensation from left to right: median nerve, ulnar nerve and radial nerve.
