First: Base of the first metacarpal bone, ulnar side.
Second: Length of the second metacarpal bone, ulnar side.
Third: Length of the fourth metacarpal bone, radial side.
Fourth: Length of the fifth metacarpal bone, radial side.
Insertions: Chiefly into extensor expansion of the respective digit, with possible attachment to the base of the proximal phalanx as follows:
First: Ulnar side of the thumb.
Second: Ulnar side of the index finger.
Third: Radial side of the ring finger.
Fourth: Radial side of the little finger.
Action: Adduct the thumb, index, ring and little finger toward the axial line through the third digit. Assist in flexion of the metacarpophalangeal joints and extension of the interphalangeal joints of the three fingers.
Patient: Sitting or supine.
Fixation: In general, stabilization of adjacent digits to give fixation of the digit toward the finger that is moved and to prevent assistance from the digit on the other side.
Test and Traction: Against the middle phalanx:
First: (Figure A) Adduction of the thumb toward the index finger (acting with the adductor pollicis and first dorsal interosseous). Hold the thumb, and pull in the radial direction.
Second: (Figure B) Adduction of the index finger toward the middle finger. Hold the index finger, and pull in the direction of the thumb.
Third: (Figure C) Adduction of the ring finger toward the middle finger. Hold the ring finger, and pull in the direction of the little finger.
Fourth: (Figure D) Adduction of the little finger toward the ring finger. Hold the little finger, and pull in the ul-nar direction.
Weakness: Decreases the ability to adduct the thumb, index, ring and little fingers. Decreases strength in flexion of the metacarpophalangeal joints and extension of the interphalangeal joints of the index, ring and little fingers.
Shortness: Fingers held in adduction. May result from wearing a cast with the fingers in adduction.
LU M BRI CALES
Flexor digitorum profundus tendons
First and second: Radial surface of the flexor profundus tendons of the index and middle fingers, respectively.
Third: Adjacent sides of the flexor profundus tendons of the middle and ring fingers.
Fourth: Adjacent sides of the flexor profundus tendons of the ring and little fingers.
Insertion: Into the radial border of the extensor expansion on the dorsum of the respective digits.
Action: Extend the interphalangeal joints and simultaneously flex the metacarpophalangeal joints of the second through fifth digits. The lumbricales also extend the interphalangeal joints when the metacarpophalangeal joints are extended. As the fingers are extended at all joints, the flexor digito-rum profundus tendons offer a form of passive resistance to this movement. Because the lumbricales are attached to the flexor profundus tendons, they can diminish this resistive tension by contracting and pulling these tendons distally, and this release of tension decreases the contractile force needed by the muscles that extend the finger joints.
First and second: Median, C(6), 7, 8, Tl. Third and fourth: Ulnar, C(7), 8, Tl.
} / Dorsal mterosseus I
Hyperextension of the metacarpophalangeal joints, resulting from weakness of the lumbricales and interossei, prevents normal function of the extensor digitorum in extending the inter-phalangeal joints.
When the examiner offers fixation that normally is afforded by the lumbricales and interossei, a strong extensor digitorum will extend the fingers.
LUMBRICALES AND INTEROSSEI
LUMBRICALES AN D INTEROSSEI
Patient: Sitting or supine.
Fixation: The examiner stabilizes the wrist in slight extension if there is any weakness of the wrist muscles.
Test: Extension of the interphalangeal joints, with simultaneous flexion of the metacarpophalangeal joints.
Pressure: First, against the dorsal surface of the middle and distal phalanges, in the direction of flexion, and second, against the palmar surface of the proximal phalanges, in the direction of extension. Pressure is not illustrated in the photograph, because it is applied in two stages, not simultaneously.
Weakness: Results in claw-hand deformity.
Shortness: Metacarpophalangeal joint flexion with interphalangeal joint extension. (See following page.)
Note: An important function of the lumbricales and interossei is illustrated by the above photograph. With marked weakness or paralysis of these muscles, an individual cannot hold a newspaper or a book upright in one hand. The complaint by the patient that a newspaper could not be held in one hand was a clue to this type of weakness.
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