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Page 2 of 17 Buncke. Plast Aesthet Res 2022;9:38 https://dx.doi.org/10.20517/2347-9264.2022.08

INTRODUCTION
Hand and wrist injuries occur in 6.6% of emergency room visits in the United States, costing $48.6 billion
[2]
[1]
annually . Peripheral nerve injuries occur in 2.5% of trauma patients , with the average number of
peripheral nerve procedures at 558,862 annually . The most frequently injured nerves treated within
[3]
hospitals include ICD-9-CM 955.6, upper extremity digital nerve; ICD-9-CM 955.2, ulnar nerve; ICD-9-CM
[4]
955.3, radial nerve; and ICD-9-CM 953.4, the brachial plexus . Peripheral nerve injuries have
socioeconomic costs, direct patient costs, and can affect patient quality of life.
The notable socioeconomic costs for the patient include missed work/school due to regular physician
appointments, procedures, and hospital visits. These appointments and procedures can result in significant
direct costs to the patient, which can be compounded by the loss of wages due to missed work .
[5]
Additionally, patient quality of life can be impacted by disrupted sleep patterns, social life, extremity
function, personal life, professional activities, and mood . Notably, 64% of patients with peripheral nerve
[6]
injuries have missed at least one month of work or school, and 24% of patients with nerve injuries have
[5]
missed at least 12 months of work or school . These significant impacts on patients’ economic standing and
quality of life highlight the importance of continuing to improve outcomes in the treatment of peripheral
nerve injuries.

When a peripheral nerve is injured, the resulting injury may lead to varying disruptions in the peripheral
nerve anatomy. These varying injury severities result in different functional impacts, which are related to
the anatomical structure of peripheral nerves. The peripheral nerve is composed of several layers of
connective and functional tissues that support the electrical impulse propagation and the structure of the
nerves.

Peripheral nerves extend from the spinal cord and are comprised of both sensory (afferent) and motor
(efferent) nerve fibers . These nerve fibers, called axons, are either myelinated or unmyelinated [Figure 1] .
[7]
[7]
Unmyelinated axons are ensheathed individually or in small groups within Schwann cells . The Schwann
[7]
cells are in contact with only a small section of the axon, which requires several Schwann cells aligned
[7]
consecutively to cover the length of the axon . Myelinated axons have a similar appearance to
unmyelinated axons, as they are surrounded by Schwann cells; however, the Schwann cells have deposited a
compacted layer of cytoplasm and cell membrane called the myelin sheath . The myelin sheath serves to
[7]
insulate the axons and improves nerve impulse conduction between the nodes of Ranvier, which are areas
[7]
where there are natural interruptions in the myelin sheath . Injuries isolated within the myelin sheath are
classified as Seddon’s neurapraxia or Sunderland’s Type 1 [Table 1], which often recovers spontaneously .
[8,9]
Individual nerve fibers, both myelinated and unmyelinated, are bound together by connective tissues called
the endoneurium, perineurium, and epineurium .
[7]
Each nerve fiber is surrounded by the endoneurium, which is a loose collagenous connective tissue layer .
[7]
Bundles of endoneurium are contained within fascicles, which are surrounded by a connective tissue layer
called the perineurium . The perineurium consists of uniformly organized flattened laminae of fibroblasts
[7]
[7]
with alternating sheets of collagen . The outermost layer of the nerve, the epineurium, is composed of
irregularly arranged collagenous tissue that provides elasticity and absorption of mechanical forces [7,10] .
Peripheral nerve injuries resulting in a complete discontinuity in the peripheral nerve axon and
surrounding connective tissue layers are classified as Seddon neurotmesis and Sunderland type 5.

Subsequent to nerve injury, the nerve proximal to the injury undergoes traumatic degeneration up to the
first node of Ranvier, while the nerve distal to the injury undergoes Wallerian degeneration [Figure 2A] .
[11]