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Kinematics of Ring Finger Carpometacarpal Joint in “Jersey Finger”
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ABSTRACT
The ring finger is shorter than the middle finger in extension and longer in flexion. This is pronounced when the proximal interphalangeal joint is fully flexed and the distal interphalangeal joint is fully extended. As the player strongly grabs the jersey (or pants) of an opposing runner, the ring finger is the first to be hooked and last to be released from the jersey. The ring finger tip bears the full brunt of body pull, causing the flexor digitorum profundus tendon to avulse from the distal phalanx. Seventy-five percent of avulsions affect the ring finger.
Why the fourth finger is longer in flexion has not been previously explained. Kinematics of the carpometacarpal joints may explain why the ring finger is longer in flexion than extension. This study correlates the linear displacement of the ring finger tip in the palm in relation to the angular displacement of the ring carpometacarpal joint.
One hundred normal hands of 25 male and 25 female volunteers were studied by 2 independent study groups. The position of the ring finger tip was marked on the palm with the proximal interphalangeal joint in full flexion and the distal interphalangeal joint in neutral. The distance between the tips of the ring and middle fingers was noted. The ring finger metacarpal head was dorsally pushed with a mallet-shaped device. Maximal force was applied, and the position of the ring finger tip was noted.
Because the carpometacarpal joints of the ring (and small) fingers are mobile and the carpometacarpal joints of the middle (and index) fingers are fixed, the ring finger carpometacarpal joint flexes when a fist is made while the carpometacarpal joints of the adjacent middle finger remain stable. In fist position, if the ring finger carpometacarpal joint is manually extended by applying dorsally directed pressure on the metacarpal head in the palm, the ring finger appears shorter than the middle finger. The relation between angular displacement of the ring carpometacarpal joint and linear displacement of the ring finger tip is always positive. Linear displacement at the ring finger tip occurs directly in proportion to flexion at the carpometacarpal joint of the ring metacarpal. Therefore, the greater the flexion at the carpometacarpal joint, the longer the ring finger is in flexion.