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Hand, Tendon Lacerations: Extensors

Author: D Glynn Bolitho, MD, PhD, FACS, FRCSC, FCS(SA), Associate Clinical Professor, Department of Plastic Surgery, University of California at San Diego; Private Practice, LaJolla, California
Contributor Information and Disclosures

Updated: Oct 5, 2006

Introduction

Do not regard extensor tendon lacerations as a "fitting challenge for the neophyte." The general principles and timing of management are similar to those pertaining to flexor tendon injuries. In combined repairs, flexor tendon rehabilitation must take priority. The mechanism of injury generally includes lacerations, crush injuries, avulsions, burns, and deep abrasions. Associated fractures are common with extensor injuries in the digits. In closed injury over the dorsum of the proximal interphalangeal joint, suspect extensor tendon injury. The criterion period for primary repair has been extended from the classic 6 hours to 12 hours if antibiotics are used.

The extensor tendon is extrasynovial except at the wrist level. Tendons are surrounded by paratenon for nourishment, supplying segmental arterial input. Interposition of scar between the tendon ends may result in the extensor tendon unit becoming too long, thus losing the ability to perform a useful function. Splinting extensor tendons even longer than the flexors is important to prevent damage to the repair by the more powerful flexor tendons. Although the extensor action is weaker than that of the flexors, the extensor mechanism is able to accommodate the full range of flexor tendon excursion by distal shift of the extensor expansion during digital flexion.

As in all hand surgery, meticulous handling of the tissues is vital. Scar formation can be demonstrated at every contact point along the tendon.

Relevant Anatomy

The superficial layer of the fascia covering the dorsum of the hand has a delicate, fatty component that contains the dorsal venous channels. A deeper membranous layer is attached to the deep fascia by areolar tissue. This areolar layer balloons with acute injury and is involved in the classic degloving, leaving the underlying paratenon intact. The extensor retinaculum is divided into 6 compartments: (1) abductor pollicis longus, extensor pollicis brevis; (2) extensor carpi radialis longus, extensor carpi radialis brevis; (3) extensor pollicis longus; (4) extensor indicis proprius, extensor digitorum communis; (5) extensor digiti quinti; and (6) extensor carpi ulnaris. The extensor carpi radialis longus and extensor carpi radialis brevis insert onto the bases of the second and third metacarpals, respectively, and the extensor carpi ulnaris inserts onto the base of the fifth metacarpal. Since the extensor carpi radialis brevis lies closest to the central axis of the wrist, it is the purest wrist extensor.

Preservation of the function of this tendon thus is a priority. Extension of the wrist is a secondary function for the digital flexors, with wrist flexion enhancing their action. Extension of the digits is a combination of extrinsic and intrinsic muscle action. The extrinsic extensors are primarily responsible for metacarpophalangeal extension, with extension of the interphalangeal joints an intrinsic function. The extensor digitorum communis to the little finger usually arises from that of the ring finger at the metacarpal level. An inconstant pattern of intertendinous connections (juncturae tendineae) exists at the metacarpal level. These may limit independent metacarpophalangeal extension.

Some extensor digitorum communis attachment to the base of the proximal phalanx or capsule of the metacarpophalangeal joint may be present. The extensor indicis proprius passes through the fourth space in the extensor retinaculum on the ulnar side of the extensor digitorum communis to the index finger to insert into the extensor expansion. The extensor indicis proprius contributes to ulnar deviation of the index finger and to pointing. The extensor digiti quinti travels alone through the fifth compartment, ulnar to the extensor digitorum communis to the little finger, and frequently splits into 2 tendons. The radial division usually is joined to the extensor digitorum communis to the little finger. The interossei and lumbricals are the principal extensors of the interphalangeal joints of the fingers.

Extensor apparatus of the fingers

The extensor expansion or hood covers most of the dorsum of the proximal and middle phalanges. The retinacular system stabilizes the apparatus. The sagittal bands are orientated transversely over the capsule and collateral ligaments of the metacarpophalangeal joints, separating them from the intrinsic muscles. During extension they overlie the metacarpophalangeal joints but move distally in digital flexion. Most of the fibers of the central tendon (slip) insert into the capsule of the proximal interphalangeal joint and the base of the middle phalanx. Distal to the sagittal bands, the intrinsic muscles that make up the lateral bands (interossei, lumbricals) contribute proximal vertical and distal oblique fibers to the sides of the central tendon. Some contribution of the extensor digitorum communis to the lateral bands is present, in addition to the intrinsics.

Distal to the proximal interphalangeal joint, the lateral bands first are separated by a triangular ligament and then fuse to form a conjoined tendon, which inserts into the base of the distal phalanx. The extensor hood is free to slide proximally with metacarpophalangeal extension and distally with metacarpophalangeal flexion. With the hood in the distal position, the interossei contribute to metacarpophalangeal flexion through their vertical fibers, with little effect on the interphalangeal joints. With the hood in the proximal position and the metacarpophalangeal joints fixed in extension, the interossei, through the oblique fibers of the lateral bands, are able to extend the interphalangeal joints.

Conversely, the lumbrical muscles are effective interphalangeal extensors irrespective of the degree of metacarpophalangeal flexion. Similar to the interossei, they provide flexion of the metacarpophalangeal joint via their vertical fibers. The lateral bands normally lie dorsal to the axis of motion of the extended proximal interphalangeal joint and shift volarward with proximal interphalangeal flexion. The triangular ligament prevents the lateral bands from shifting volar to the axis of motion to become flexors of the proximal interphalangeal joint.

The primary function of the extensor digitorum communis is metacarpophalangeal function, but with full metacarpophalangeal extension or metacarpophalangeal extension blocked, the extensor digitorum communis can extend the interphalangeal joints.

The principal components of the retinacular system are the transverse and oblique ligaments (Landsmeer ligament). The transverse fibers originate from the flexor sheath and proximal phalanx on the volar aspect, pass through a window in the Cleland ligament, and insert into the lateral bands and triangular ligament dorsal to the axis of proximal interphalangeal joint rotation. The deeper and more tendinous oblique retinacular ligament arises from the volar proximal interphalangeal joint capsule and proximal two thirds of the middle phalanx and inserts distally into the conjoined tendon.

The extensor pollicis longus traverses the third dorsal compartment of the wrist. It adducts and supinates the first ray and extends the metacarpophalangeal and interphalangeal joints.

The extensor pollicis brevis and abductor pollicis longus are important stabilizers of the first metacarpal base. The extensor pollicis brevis extends the carpometacarpal and metacarpophalangeal joints, inserting into the base of the proximal phalanx. It occasionally inserts into the distal phalanx as well. The anatomy of the thumb metacarpophalangeal joint resembles the proximal interphalangeal joint anatomy of the digits. The adductor and abductor pollicis longus tendons contribute to a dorsal expansion, which has transverse fibers acting like the retinacular system to stabilize the tendons of extensor pollicis brevis and extensor pollicis longus. The intrinsics are able to extend (but not hyperextend) the interphalangeal joint through fibrous interconnections in the dorsal expansion.

Contraindications

The most common injury at the metacarpophalangeal joint level is the "fight bite" due to human tooth injury. In these contaminated injuries, primary repair is contraindicated. Once the acute infection has been treated, the swelling has lessened, and the passive range of motion has been restored, repair can be undertaken.

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