Hello,

Mallet finger or baseball finger deformity is a condition which is a fixed flexion deformity of the distal interphalangeal joint of a finger. It can be caused either by dissimilation of the extensor tendon, or avulsion fracture of its insertion.

The dissimilation of the insertion of one of the extensor tendons into the distal phalanx can occur if the distal phalanx is forcibly flexed  by a blow in the tip of the finger when the extensor tendon is taut.

Due to this their is loss of last 20° active extension.