5.1
Abductor pollicis brevis
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Surface anatomy : Most superficial layer of thenar muscles and lies most radial side.
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Origin : Transverse carpal ligament and radial carpal bones.
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Insertion : Radial base of proximal phalanx of thumb.
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Function : Abduction thumb at carpometacarpal and metacarpophalangeal joints of the thumb.
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Motor point (MP) : Midpoint between the midpoint of the wrist joint (A) and the lateral part of the thumb metacarpophalangeal joint (B).
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Injection tip : At forearm fully supinated and hand opening position, insert needle to the motor point (remember this is a superficial muscle). Make sure the thumb abduction not flexion or opposition.
5.2
Opponens pollicis
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Surface anatomy : Mostly underlies abductor pollicis brevis.
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Origin : Transverse carpal ligament and trapezium.
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Insertion : Metacarpal bone of thumb.
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Function : Flexion and abduction the thumb at carpometacarpal joint.
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Motor point (MP) : One-third (close to wrist) between midpoint of wrist (A) and the midpoint of the palmar digital crease of the thumb (B) 15 .
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Injection tip : At forearm fully supinated and hand opening position, insert needle to the motor point. Make sure the thumb moves toward to the little finger.
5.3
Flexor pollicis brevis
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Surface anatomy : One of thenar muscles superficially and distally located with two (superficial and deep) parts.
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Origin : Trapezium and flexor retinaculum.
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Insertion : Proximal phalanx of thumb.
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Function : Flex thumb at MP joint.
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Motor point (MP) : Midpoint between the distal connection (A) of the thenar and hypothenar eminence and medial side of the thumb metacarpophalangeal joint (B).
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Injection tip : At forearm fully supinated and hand opening position, insert needle to the motor point (remember this has both superficial and deep layers). Make sure the thumb flexion at metacarpophalangeal joint. If the needle is in the adductor pollicis, the thumb adduction is seen.
5.4
Adductor pollicis
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Surface anatomy : Volar side of space between thumb and index finger
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Origin : Transverse part: third metacarpal. Oblique part: second and third metacarpals, trapezoid, and capitate.
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Insertion : Medial side of proximal phalanx of thumb.
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Function : Adduction thumb at carpometacarpal joint.
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Motor point (MP) : Midpoint of the volar side of ridge (skin fold) between the thumb and index finger.
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Injection tip : At hand opening position, insert needle to the motor point toward the thumb base. Make sure the thumb adduction (thumb moves toward the index finger). If thumb flexion at metacarpophalangeal joint is noted, the needle is not in the adductor pollicis, but in the flexor pollicis brevis.
5.5
Flexor digiti minimi brevis
