Corpus: Thumb

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

2.1. Joints

The thumb is the shortest finger, but is crucial for gripping due to its ability to move in opposition to the other fingers. The thumb's extensive mobility is made possible by its three main joints:

• Thumb saddle joint (carpometacarpal joint, articulatio carpometacarpalis pollicis): Anatomically a saddle joint, but functionally acts as a ball-and-socket joint. It allows for abduction/adduction, flexion/extension, and opposition (a combined movement of adduction, flexion, and rotation).
• Metacarpophalangeal joint of the Thumb (articulatio metacarpophalangealis pollicis): Anatomically a condylar joint, functionally it behaves like a hinge joint. It allows for flexion/extension, minimal abduction/adduction, and some rotation.
• Thumb interphalangeal joint (articulatio interphalangealis pollicis): A hinge joint that allows for flexion and extension.

2.2. Muscles

The thumb’s ability to apply counter-pressure against the other fingers enables a strong grip. The areas of the brain responsible for the thumb's motor control and sensitivity are more developed than those for the other fingers, corresponding to the large number of thumb muscles, which include:

• Deep flexors of the forearm:
  • Flexor pollicis longus muscle: Responsible for flexion and opposition at the saddle joint, and flexion at the base and interphalangeal joints.
• Deep extensors of the forearm:
  • Abductor pollicis longus muscle: Abducts and extends the thumb at the saddle joint.
  • Extensor pollicis brevis muscle: Extends the thumb at both the saddle and metacarpophalangeal joints.
  • Extensor pollicis longus muscle: Extends the thumb in all joints and adducts at the saddle joint.
• Thenar muscles:
  • Abductor pollicis brevis muscle: Abducts and opposes at the saddle joint, and flexes at the base joint.
  • Opponens pollicis muscle: Responsible for opposition, flexion, and adduction at the saddle joint.
  • Flexor pollicis brevis muscle: Flexes, adducts, and opposes at the saddle joint, and flexes at the base joint.
  • Adductor pollicis muscle: Adducts and opposes at the saddle joint, and flexes at the base joint.

Possible malformations of the thumb include the triphalangeal 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.