Intrinsic muscles of the thumb
- Thenar muscles
- Adductor pollicis
- Clinical Kinesiology separately lists the ulnar head of the flexor pollicis brevis. Many anatomists now classify this muscle as the first palmar interosseous.
Three muscles make up the thenar eminence. Innervated by the median nerve, they position the thumb for grasping. Superficial to deep, they are the:
- abductor pollicis brevis
- attaches to the trapezium and to base of the first proximal phalanx; crosses CMC and IP joints.
- also contributes to IP extension because its fibers attach to extensor mechanism
- flexor pollicis brevis
- two heads, both of which attach to base of proximal phalanx. The tendon of the flexor pollicis longus "splits" the two heads:
- ulnar head
- now usually classfied as first palmar interosseus
- attaches proximally to first metacarpal
- attaches distally to promimal phalanx and the extensor mechanism; crosses only first MP joint
- innervated by ulnar nerve
- radial head
- Kendall further divides this muscle into superficial and deep heads
- attaches proximally to trapezium, trapezoid, and capitate
- crosses two joints; flexes CMC and MP joints
- Kendall attributes innervation to both ulnar and median nerves.
- opponens pollicis
- attaches proximally from trapezium and flexor retinaculum
- attaches distally to shaft of first metacarpal
- crosses only CMC joint; its oblique fibers abduct, flex, and rotate the CMC to oppose thumb to other digits.
- Palpate its activity by placing the thumb in lateral prehension position with thumb pad on radial side of second DIP joint. Palpate at base of first metacarpal while performing gentle lateral prehension.
- forms bulk of "web space" between first and second metacarpals
- adducts first MP joint by approximating 1st and 3rd metacarpals
- transverse head:
attaches proximally to shaft of third metacarpal and distally to base of first phalanx
- oblique head:
attaches proximally to capitate, to base of third metacarpal (and to base of 2nd metacarpal, according to Kendall)
attaches distally to base of first phalanx base and to extensor mechanism
- depending on the CMC's initial position, it can flex the MP joint
- can assist with IP extension through its attachment to the thumb's extensor mechanism.
- innervated by the ulnar nerve