Digital Nerve Injury

Rachel Michael

Roy E. Schneider

Margaret Jain

INTRODUCTION

Pathoanatomy
- Nerve layers (from deep to superficial)¹
  - Endoneurium—Connective tissue that surrounds each axon or nerve fiber
  - Perineurium—Strong layer of connective tissue that surrounds each fascicle and can withstand high pressures
  - Epineurium—Outer layer that covers the entire nerve, allows for nerve gliding and protection
- Median nerve (C5-T1) traverses the forearm and runs through the carpal tunnel. Proximal to the carpal tunnel, the palmar cutaneous branch divides from the median nerve proximal to the wrist and provides sensation to the radial palm. The recurrent motor branch divides from the median nerve at or around the level of the carpal tunnel and supplies most of the thenar muscles. The median nerve divides into the common digital nerves, which subsequently divide into proper digital nerves. Proper digital nerves supply the radial three digits and the radial side of the ring finger. Dorsal sensory nerves branch from the proper digital nerves just proximal to the proximal interphalangeal joint and travel dorsal to supply sensation to the dorsal, distal fingers.
- Ulnar nerve (C7-T1) traverses the forearm deep to the flexor carpi ulnaris muscle belly and tendon. Five centimeters proximal to the wrist crease, the dorsal cutaneous branch splits from the ulnar nerve to provide sensory innervation of the dorsal, ulnar hand, whereas a palmar cutaneous branch provides sensation to the ulnar palm via branching dorsal digital nerves. The ulnar nerve proper continues through Guyon canal, distal to which it divides into a sensory branch and a (deep) motor branch. The sensory branch divides in the midpalm into common and proper digital nerves, which supply sensation
  to the ulnar side of the ring finger and the small finger. The deep motor branch innervates the intrinsic muscles of the hand.
- At the finger level, the digital nerves run volar to the digital arteries.
Mechanism of injury
- Seddon classification
  - I—Neurapraxia¹ (first-degree injury)
    - Temporary loss of nerve conduction without axonal disruption
  - II—Axonotmesis¹
    - Axons are cut though Schwann cells and surrounding nerve layers remain intact.
    - Usually a result of a crush or traction injury
    - Wallerian degeneration occurs distal to injury.
  - III—Neurotmesis¹
    - Complete nerve disruption
    - Requires surgical intervention for recovery
    - Wallerian degeneration occurs distal to injury.
- Injury mechanism will predict severity of injury and recovery
  - Sharp injuries have smaller zones of injury.
  - Crush injuries are associated with severe and diffuse tissue injury.
    - Edema formation within the endoneurium leads to diminished axonal transport/nerve dysfunction.
    - May require resection of larger segment of nerve, resulting in larger nerve deficits, may preclude primary repair
  - Stretching/traction injuries
    - Stretching a nerve 8% of its normal length results in a 50% decrease in its blood flow; 15% stretch results in complete nerve ischemia.²
- Associated injuries
  - Phalangeal fractures/dislocations/amputations
  - Digital artery injury
  - Flexor tendon injuries

FIGURE 37.1 Anatomic course of the median nerve. APB, abductor pollicis brevis; FPB, flexor pollicis brevis; OP, opponens pollicis.

FIGURE 37.2 Anatomic course of the ulnar nerve. ADM, abductor digiti minimi; ADP, adductor pollicis; FDM, flexor digiti minimi; FPB, flexor pollicis brevis; ODM, opponens digiti minimi.