Extensor Tendon Injuries

Unlike flexor tendons, the extensor tendons are not constrained in a sheath, but nevertheless have a complex and intricate anatomy. The dorsal skin being thinner and pliable is more susceptible to trauma. As extensor tendon repairs are perceived to be a lot easier, they end up being delegated to junior surgical trainees.

Although the surgical repair itself may be simple, thorough understanding of extensor tendon dynamics is necessary to optimize hand function after repair. In contrast to flexor tendon injuries some extensor injuries like closed mallet and closed boutonnière-type injuries are best treated conservatively. Some of the extensor tendon injuries have associated joint injuries and require careful debridement and lavage to prevent joint infections. Associated nerve injuries may be present, usually the sensory branches of the radial nerve and occasionally the posterior interosseous nerve, requiring microsurgical repair.

The cross-sectional morphology of extensor tendons varies significantly according to the site of the injury. Thin flat tendons encountered over the dorsum of the fingers require a different suture technique in comparison to the more rounded thicker counterparts encountered over the dorsal wrist joint. Appropriate and postoperative splinting, mobilization, and hand therapy are essential to avoid early tendon ruptures and joint contractures and to minimize adhesions to overlying skin.

Anatomy

Long extensors arise from the lateral epicondyle of the humerus and the dorsal surface of the forearm (radius, ulna, and interosseous membrane). Wrist extensors, which include extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), and extensor carpi ulnaris (ECU) originate from the lateral epicondyle, as do the extensor digitorum communis (EDC) and extensor digiti minimi (EDM).

The dorsal surface of the radius, ulna, and the interosseous membrane give origin to extensor indicis (EI), abductor pollicis longus (APL), extensor pollicis brevis (EPB), extensor pollicis longus (EPL), and extensor indicis proprius (EIP).

Brachioradialis (elbow flexor, forearm mid-pronator with no action on the wrist) and ECRL are innervated by the radial nerve while the remaining extensors are supplied by the posterior interosseous nerve (in addition to the supinator).

Wrist and Hand

At the level of the wrist the extensor tendons pass under a synovial-lined condensation of the deep fascia called the extensor retinaculum ( Fig. 48.1 ). The extensor retinaculum has transverse and vertical fibers which form septae separating six compartments. From radial to ulnar the first dorsal compartment contains the APL and EPB while the second contains ECRL and ECRB. The third compartment contains EPL while the fourth contains EDC and EIP. The fifth compartment contains EDM and the sixth, ECU.

Over the dorsum of the hand, the EDC tendons contain several short oblique interconnecting tendinous slips called juncturae tendinum. These intricate interconnections prevent significant tendon retraction following complete division of a single EDC tendon in zone V/VI (see below). The EIP tendon has no attached juncturae and this can help distinguish it from EDC to index.

At the level of the metacarpal heads the EDC tendons are stabilized by sagittal bands which keep them in close relation to the metacarpophalangeal (MCP) joint while preventing ulnar or radial subluxation. At this level the extensor tendons tend to sublux ulnarly and this renders the radial sagittal band extremely important in preventing such subluxation. The EDC (and EIP) are the primary extensors of MCP joints.

Fingers

Distal to the MCP joints the EDC divides into a central slip and paired lateral slips which are joined by tendons of intrinsic muscles (interossei and lumbricals) to form the lateral band. The central slip inserts into the dorsal aspect of the base of the middle phalanx and is joined by medial slips from the lateral bands ( Figs. 48.2 and 48.3 ). The lateral bands continue and insert into the dorsal aspect of the base of the distal phalanx as the conjoint lateral bands before which they are joined by the oblique retinacular ligament of Landsmeer.

Extension at the proximal interphalangeal (PIP) and distal interphalangeal (DIP) joints is primarily a function of intrinsic muscles (4 lumbricals, 3 palmar, and 4 dorsal interossei). The intrinsic muscles arise in the hand and course distally dorsal to the deep transverse metacarpal ligaments at which point they are volar to the joint axis. Hence these intrinsic muscles act as the primary flexors of the MCP joints. Once the intrinsic tendons join the lateral slip of the EDC, they are dorsal to the axis of PIP and DIP joints, thus becoming the extensors of PIP and DIP joints. Although the dorsal aspect of the base of the middle phalanx is the primary bony insertion of the EDC, the intrinsic muscles extend the PIP joint through medial slips which join the central slip. EDC can act on the PIP joint only while the MCP joint is flexed.

The transverse retinacular ligament extends from the conjoint lateral bands to the flexor sheath at PIP joint level, and acts to prevent dorsal dislocation of the lateral bands. The oblique retinacular ligament (ORL) connects the conjoint lateral band from the dorsum of the distal phalanx to the volar check rein ligaments at the PIP joint. The ORL thus lies volar to the PIP joint and sits dorsal to the axis of the DIP joint, allowing coordination of flexion and extension within those joints. The triangular ligament a fascial band connecting the conjoint lateral bands and the terminal tendon. It prevents volar subluxation of the conjoint lateral bands during PIP joint flexion.

Thumb

APL, EPB, and EPL all play separate roles in thumb extension. APL inserts into the base of the first metacarpal, EPB attaches to the base of the proximal phalanx, while EPL maintains a broad area of contact with the base of the distal phalanx. As it traverses the dorsum of the MCP joint, EPL is anchored on the ulnar side by the aponeurosis of adductor pollicis, whereas radially, abductor pollicis brevis (APB) performs a similar role. Consequently, EPL is held securely in a centralized position as it courses over the metacarpal head. Consequently, proximal stump retraction is found only when EPL division occurs proximal to the MCP joint.

Presentation and Management by Zone

Kleinert and Verdan proposed a division of extensor tendon injuries into eight zones with an additional more proximal ninth zone proposed later by Doyle. The management of extensor tendon injuries is dependent on the zone of the injury. As an aide-memoire, excepting zone IX, odd-numbered zones are located over joints ( Fig. 48.4 ).

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Zone I – DIP joint
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Zone II – middle phalanx
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Zone III – PIP joint
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Zone IV – proximal phalanx
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Zone V – MCP joint
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Zone VI – dorsum of hand
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Zone VII – extensor retinaculum
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Zone VIII – distal forearm
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Zone IX – muscular area, middle and proximal forearm

The anatomical zone of extensor injury in the thumb are as follows:

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Zone TI – IP joint
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Zone TII – proximal phalanx
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Zone TIII – MCP joint
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Zone TIV – metacarpal
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Zone TV – carpometacarpal joint

Zone I: Dorsal Interphalangeal Joint

Zone I extensor tendon injuries result in a mallet finger, a deformity characterized by persistent flexion at the DIP joint.

Open injuries require a repair of zone I and as the tendon itself lacks substance, a variety of methods have been described. The tendon repair can be made more robust by inclusion of dermis. This procedure, called dermotenodesis or tenodermodesis, can be done using interrupted external polypropylene sutures or buried sutures using PDS (polydioxanone dermal suture). The authors prefer to protect open tendon repairs using a transarticular Kirchner wire (K-wire) (1.25 mm). The wire may be removed once the healed wound is robust enough to withstand pressure from splinting.

Closed injuries involving extensor tendon (soft tissue mallet) are best treated nonoperatively in a well-fitting splint which holds the DIP joint in neutral to full hyperextension. The splinting needs to be continuous for approximately 8 weeks and continued for a further 2–4 weeks at night or during strenuous use. Up to 10 degrees lag post treatment may be accepted and usually causes little functional problem. Persistent lag may cause a secondary swan-neck deformity due to a proximal shift in the extensor mechanism. If noted early this can be prevented and treated using an oval-8-type splint which allows full PIP joint flexion but prevents hyperextension. Extensor lag unresponsive to splinting may be improved by central slip Fowler tenotomy, which allows the entire extensor mechanism to shift proximally. This proximal shift of the extensor mechanism corrects the lag caused by the lax terminal extensor tendon. This is best done at least 6 months after injury in order to allow the scar tissue to mature.

Closed extensor tendon avulsion fractures (mallet fractures) can be associated with volar subluxation of the distal interphalangeal joint. Subluxation is frequent in mallet fractures involving more than 50% of the articular surface of the base of the distal phalanx. Mallet fractures with no subluxation can be suitably treated nonoperatively by continuous splinting in full extension or slight hyperextension for 6 weeks followed by mobilization and night splinting for 2–4 weeks. Mallet fractures with subluxation are best treated surgically using a transarticular K-wire alone or in combination with a dorsal blocking wire ( Fig. 48.5 ). Rarely, if the fragment is substantial, other methods like interfragmentary screw, hook plate fixation or interosseous wire fixation can be used.