The median nerve may be cut cleanly across at the wrist or palm by sharp objects or by falls through windows.

The ulnar nerve can also be damaged in this way but it lies deeper than the median nerve and is protected by the tendon of flexor carpi ulnaris.

The synovial sheaths of the thumb and little fingers extend proximally through the carpal tunnel. The three central digits (index, middle and ring) have a separate flexor sheath in the fingers. There is also a sheath in the palm which extends proximal to the wrist (Fig. 13.2).

Management of flexor injuries in the fingers depends on the tendons involved and the site of injury (Fig. 13.3). The first step is to decide which tendon has been damaged.

Profundus and superficialis action can be distinguished by asking the patient to flex the distal phalanx with the middle phalanx held still (see Fig. 2.26). Only flexor profundus will do this because superficialis does not extend beyond the middle phalanx (Fig. 13.4).

To assess superficialis, hold all the fingers down except the one that is to be tested and ask the patient to flex that finger. If the finger flexes at the proximal interphalangeal (p.i.p.) joint, superficialis is intact. Test this on your own hand.

Eleven flexor tendons (flexor digitorum superficialis (4), flexor digitorum profundus (4), flexor pollicis longus, flexor carpi ulnaris and flexor carpi radialis) cross the volar aspect of the wrist (Fig. 13.5). If all these are divided, there will be 22 cut tendon ends. If the median nerve is divided as well, there will be 24 structures, which must be carefully identified, and if each pair is joined there will be 12 suture lines very close together. However carefully repaired, the tendons and nerves may stick together and form a solid mass which restricts movement at the wrist.

Injuries in the forearm lie outside any sheath and can be accurately repaired more easily than elsewhere.

If the wound is untidy and dirty it must be debrided and all dead tissue removed. If the wound is untidy but clean it is sometimes better to excise the tendon and replace it with a Silastic rod, which can itself be replaced with a graft when the wound has healed.

If the wound is contaminated, a clean and well- healed wound must be obtained before definitive treatment is undertaken.

The hand should be mobilized actively and passively as soon as pain and swelling permit.

Because the extensor tendons only have a synovial sheath where they cross the wrist, the problems encountered in repairing flexor tendons in the fingers do not arise. The tendons are easily identified, repair is straightforward and the fingers can be mobilized after 3 or 4 weeks.

If the tendons are divided on the dorsum of the hand they cannot contract for more than a few millimetres because they are restricted by linking fibrous bands. Even without repair some extensor function will eventually return (Fig. 13.6).

Violent flexion injuries to, or lacerations across the back of, the d.i.p. joint can avulse or divide the insertion of the extensor digitorum longus at the base of the distal phalanx (Fig. 13.7 and Fig. 13.8).

Untreated, the lesion causes the distal phalanx to droop and leaves a ‘mallet’ finger deformity. The condition is inconvenient but function improves without treatment and it is exceptional for the patient to be seriously troubled by the injury 12 months later.

The central slip of the extensor expansion can be detached from its insertion at the base of the middle phalanx by a cut or by violent muscle contraction. This allows the two lateral slips to fall sideways and the p.i.p. joint to protrude between the two, which produces a characteristic deformity and may impair function (Fig. 13.9).