The extensor tendons are the most common site of tendon injuries because of the superficial nature of the tendons on the dorsum of the hand. A classification system has been developed for assessing injury patterns, repair techniques, and rehabilitation.
ZONE I Involves the area over the distal phalanx and DIP. Injury can occur from blunt or sharp trauma. Complete laceration or rupture of the tendon at this level will result in the DIP joint flexed 40°. This injury after blunt trauma is often referred to as "mallet finger," and it is the most common tendon injury in athletes. This injury has been classified as type I if there is tendon only rupture, type II if there is a small avulsion fracture, and type III if greater than 25 percent of the articular surface is involved. Types I and II can be treated with the DIP joint immobilized in slight hyperextension continuously for 6 to 10 weeks. Some hand surgeons may prefer operative treatment. Controversy exists whether treatment of type III injuries should be conservative or operative. Chronic untreated mallet finger may develop a swan-neck deformity (Fig 26.P..-10). This is caused when the lateral bands are displaced proximally and dorsally, resulting in increased extension forces on the PIP
ZONE II Involves the area over the middle phalanx. Injuries are usually due to laceration. Treatment is similar to zone I injuries.
ZONE III Involves the area over the PIP. The central tendon is the most commonly injured structure. Complete disruption of the central tendon may result in the volar displacement of the lateral bands, causing them to be flexors, along with the unopposed FDP. Additionally, the extensor hood retracts, causing extension of the MP and the DIP joints resulting in the boutonnière deformity ( Fig 260-11J. Closed injuries are treated with the petrosal interphalangeal (PIP) joint immobilized in extension
FIG. 260-11. Boutonnière deformity.
ZONE IV Involves the area over the proximal phalanx. These injuries have clinical findings similar to zone III injuries. Often these injuries are less problematic because the joint is not involved and the tendon at this level is broad and flat.
ZONE V Involves the area over the MP. Open injuries to this area should be considered human bites until proven otherwise. Wounds from human bites should have delayed repair when free from infection. Clean wounds can be repaired primarily.
ZONE VI Involves the area over the dorsum of the hand. Because the tendons in this area are so superficial, even minor-appearing lacerations may be associated with one or more tendon injuries. If the laceration is proximal to the junctura tendineae, the patient may be able to extend the involved MP joint, because weak extensor forces are transmitted to the junctura from adjacent extensor tendons.
ZONE VII Involves the area over the wrist. Repair here can be difficult because of the presence of the extensor retinaculum. This thick, fibrous structure on the dorsum of the wrist contains 12 extensor tendons and 6 retinacular compartments that are lined with synovium.
ZONE VIII Involves the area of the distal forearm. Injuries to this area require a thorough exploration to identify all injured structures. The tendons frequently retract into the forearm and must be retrieved and repaired. As a general principle, lacerations of less than 25 percent don't require repair; 25 to 50 percent need simple suture repair, and greater than 50 percent need repair with a modified Kessler or similar technique. After repairs in zones V through VII, splinting should occur with the wrist in 15° extension, the MP joint in 15° flexion, and the IP in 15° flexion in the involved and adjacent digit.
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