Causes / typical injury mechanism: The typical patient with an extensor tendon injury is male, with extension deficit following penetrating trauma to the upper extremity, most commonly in the dominant hand. Extensor tendon lacerations are most commonly caused by sharp objects, followed by saw accidents. Saw and crush injuries are more commonly associated with fractures, which are seen in approximately 55% of cases.

Classic history and presentation: Extensor tendon injuries of the hand are classified according to the injured zone of the tendon. The odd-numbered zones overlie joints, with zone 1 overlying the distal interphalangeal (DIP) joint, zone 3 overlying the proximal interphalangeal (PIP) joint, zone 5 overlying the metacarpophalangeal (MCP) joint, and zone 7 overlying the wrist. Even-numbered zones represent the regions between joints.

Presentation and mechanism of injuries vary with zone:

• The most common mechanism of zone 1 extensor tendon injury, also called mallet finger injury, is rapid passive flexion of the DIP joint.
• Zone 2 injuries are located between the DIP and PIP joints. Commonly injured components of the extensor tendon mechanism in this region are the conjoined lateral bands and the triangular ligament. Injuries in this region are typically caused by a laceration or a crush mechanism.
• Zone 3 injuries are located at the PIP joint. Patients may present with a boutonnière deformity, consisting of hyperextension of the DIP joint and impaired extension at the PIP joint, although this deformity can take as long as 10-14 days after the injury to develop.
• Zone 4 injuries are located between the PIP and MCP joints. The typical mechanism is a laceration.
• Zone 5 injuries are located at the MCP joint. The most common cause is a bite from another individual, referred to as a "fight bite." Closed injuries are rare and may only present with rupture of the sagittal band.
• Zone 6 injuries are located between the MCP joint and the wrist.
• Zone 7 injuries are located at the wrist and commonly involve the extensor retinaculum.
• Zone 8 and 9 injuries are located at the distal and proximal forearm, respectively.
• Thumb extensor tendon injuries can affect either the extensor pollicis longus (EPL) tendon or the extensor pollicis brevis (EPB) tendon. EPL tendon injury is more common and can cause a deficit in interphalangeal joint extension.

Prevalence: Extensor tendon of hand injury has an estimated incidence of 14 per 100 000.

• Age – The mean age is reported to be 34.2 years.
• Sex / gender – These injuries most commonly affect male patients.