Causes / typical injury mechanism: Sagittal band rupture, sometimes referred to as "flea flicker injury" or "boxer's knuckle," is a rare injury. The long finger is the most commonly injured finger. In amateur fighters, this is more common in the ring finger and small finger due to poor punching technique. In professional fighters, injury often occurs in the index / middle fingers.

The condition arises as a result of trauma or degenerative changes to the sagittal band, which can lead to instability of the extensor tendon at the level of the metacarpophalangeal (MCP) joint. The sagittal bands run on the radial and ulnar sides of the extensor tendon in the fingers and act to prevent subluxation of the extensor digitorum communis (EDC) tendon in the coronal plane at the MCP joint.

The proposed mechanism of injury is forced passive flexion of the finger while the wrist is in flexion and ulnar deviation. The injury can also occur following a fall onto the MCP joint. Spontaneous ruptures as well as congenital dislocations of the EDC tendon have been described.

In patients with rheumatoid arthritis, degenerative changes to the sagittal band can lead to ulnar subluxation of the EDC tendon without a history of trauma.

Classic history and presentation: The condition presents as subluxation of the extensor tendon of the finger at MCP joint. Patients may report a snapping sensation on the digit with extension, or inability to extend the digit out of flexion.

Risk factors: Risk factors include prior diagnosis of rheumatoid arthritis or participation in combative sports such as boxing or martial arts.