Extension lag of the patient’s hand
Entrance wound on the forearm of the patient, who was stabbed during an altercation
A man, aged 22 years, presents with a stab wound to the forearm that occurred during an altercation 5 days ago. The knife entered the dorsal and distal one-third forearm on the ulnar side. He is unable to extend his ring, middle, and index fingers and thumb since the injury. He can actively extend the small finger and wrist. Sensation over the dorsum of the wrist and hand is intact. Flexion of all the fingers and wrist is intact. The extension lag deformities and entrance wound are shown above.
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The patient was taken to the operating room for wound exploration one week after the injury. The extensor digitorum communis (EDC) to the index, middle, and ring fingers was found to be lacerated. The extensor pollicis longus (EPL) and extensor indicis proprius (EIP) tendons were also lacerated. The posterior interosseous nerve was found to be intact. The lacerated tendons were repaired with 3-0 prolene suture. The patient was placed in a short arm cast with the wrist and metacarpophalangeal (MCP) joints in extension for 3 weeks postoperatively. Passive range of motion with occupational therapy was initiated at 3 weeks.
The forearm has superficial and deep layers of muscle. The superficial layer, from radial to ulnar, includes the extensor carpi radialis longus (ECRL), extensor carpi radialis brevis (ECRB), EDC, extensor digiti minimi (EDM), and extensor carpi ulnaris (ECU). The deep layer, from radial to ulnar, includes the abductor pollicis longus (APL), extensor pollicis brevis (EPB), EPL, and EIP.
Extensor tendon lacerations are classified by zone of injury. Zone 1 injuries involve the terminal extensor tendon as it attaches to the distal interphalangeal (DIP) of the fingers or the interphalangeal of the thumb. An example of a zone 1 injury is a mallet finger. Zone 2 injuries occur at the level of the middle phalanx of the fingers or proximal phalanx of the thumb. Zone 3 injuries occur over the proximal interphalangeal (PIP) joints of the fingers or the MCP joint of the thumb. A zone 3 injury may result in a boutonnière deformity. A zone 4 injury occurs over the proximal phalanx of the fingers or metacarpal of the thumb. A zone 5 injury occurs over the finger MCP joints or the metacarpal of thumb. Lacerations over the thumb in zones 3 through 5 may cause disruption of the EPL and EPB tendons. Zone 6 injuries occur over the metacarpals, zone 7 over the wrist, zone 8 over the distal forearm, and zone 9 over the extensor muscle bellies proximally.
The patient’s injury involved the extensor digitorum muscle in zone 8 of the forearm. The extensor digitorum muscle or the EDC originates from the lateral epicondyle and divides into four extensor tendons of the hand. The four tendon insertions are to the second through the fifth digits. The EDC tendons divide over the proximal phalanx into one central and two lateral bands. The central band inserts into the base of the middle phalanx while the two lateral bands reunite over the middle phalanx and insert into the base of the distal phalanx. The EDC extends the MCP joints and the interphalangeal joints of the second through fifth digits, and aids in wrist extension.
The patient is able to extend the small finger, which indicates that the EDM is intact. He is also able to extend the wrist, which indicates the extensor carpi radialis is likely intact. The patient’s inability to extend the index finger indicates that the EDC and EIP are lacerated, as both extend the index finger.
Proximal forearm lacerations that cause dysfunction of the EDC, EDM, ECU, APL, EPL, EPB, and EIP may be from an isolated injury to the posterior interosseous nerve (PIN). The PIN innervates the muscles of the finger extensors at the proximal forearm so proximal injury causes distal motor deficits. The PIN does not have cutaneous innervation, so light touch sensation will be intact distally despite the motor deficits. As the PIN tracks distally, the nerve becomes sensory and innervates the dorsal capsule of the wrist. PIN neurectomy at the wrist level may be performed for pain relief in patients with chronic wrist pain who would rather avoid major surgery such as wrist arthrodesis.
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor in chief of the Journal of Orthopedics for Physician Assistants (JOPA).
- Fischer CR, Tang P. Lacerations to zones VIII and IX: It is not just a tendon injury. Adv Orthop. 2011;261681.
- Hatch D. Extensor tendon injuries. Orthobullets Web site. Updated December 15, 2014. orthobullets.com/hand/6028/extensor-tendon-injuries