Corpus: Thumb

The term thumb refers to the 1st finger located on the radial side of the hand, which, unlike the other fingers, consists of only two phalanges (finger bones).

2.1. Joints

The thumb is the shortest finger. The thumb is essential for gripping due to its ability to move in opposition. The great mobility of the thumb is achieved by the 3 thumb joints:

• Thumb saddle joint (articulatio carpometacarpalis pollicis): anatomically saddle joint, functionally ball and socket joint. Enables abduction/adduction, flexion/extension and opposition (combined adduction/flexion movement with rotation).
• Metacarpophalangeal joint of the thumb (articulatio metacarpophalangealis pollicis): anatomically condylar joint, functionally hinge joint. Enables flexion/extension, minimal abduction/adduction and rotation.
• Thumb end joint (articulatio interphalangealis pollicis): Hinge joint enables flexion/extension.

2.2. Muscles

The thumb can exert counter-pressure on the other fingers and thus close the grip of the hand. The areas of the brain responsible for the motor function and sensitivity of the thumb are more developed than those of the other fingers. This corresponds to a strikingly large number of thumb muscles. These include:

• Deep flexors of the forearm
  • Musculus flexor pollicis longus: flexion and opposition in the saddle joint, flexion in the base and end joints.
• Deep extensors of the forearm:
  • Musculus abductor pollicis longus: abduction and extension in the thumb saddle joint
  • Musculus extensor pollicis brevis: extension in the thumb saddle and metacarpophalangeal joints
  • Musculus extensor pollicis longus: extension in all joints, adduction in the saddle joint
• Thenar muscles:
  • Musculus abductor pollicis brevis: abduction and opposition in the saddle joint, flexion in the base joint
  • Musculus opponens pollicis: opposition, flexion, adduction in the saddle joint
  • Musculus flexor pollicis brevis: flexion, adduction, opposition in the saddle joint, flexion in the base joint
  • Musculus adductor pollicis: adduction and opposition in the saddle joint, flexion in the base joint

Possible malformations of the thumb include the tripartite thumb, the double thumb and brachydactyly type D.

Arthrosis of the thumb joint is called rhizarthrosis. In the absence of a thumb joint or traumatic amputation, pollicisation is an option for reconstructing a functional thumb.