Upper Extremity Axon Counts and Clinical Implications for Motor Nerve Transfer

Area of Science:

• Anatomy
• Histology
• Surgical Planning

Background:

• Nerve transfers require matching donor and recipient nerves by location, branch number, and axon count.
• Previous studies used varied methods for axon counting, leading to inconsistencies.
• This research establishes a uniform method for axon counting in upper limb neuromuscular anatomy.

Purpose of the Study:

• To determine the axon counts of primary motor nerve branches in the upper limb using a standardized manual histomorphometry technique.
• To complete a comprehensive anatomical blueprint for upper limb neuromuscular structures relevant to reconstructive surgery.
• To provide surgeons with detailed axon count data for improved nerve transfer planning.

Main Methods:

• Manual quantitative histomorphometry was performed on distal nerve branches from 23 upper limb muscles in 10 cadavers.
• Axon counts were determined by two independent investigators, with averages reported.
• The study focused on primary motor nerve branches within the arm and forearm.

Main Results:

• Arm muscle nerve branches exhibited higher axon counts (882–1835) than forearm branches (267–883).
• Forearm nerve branches, particularly wrist flexors and extensors, showed comparable axon counts to potential donor nerves.
• Specific forearm donor nerve examples include supinator (n=602), pronator teres (n=625), flexor digitorum superficialis (n=883), and flexor digitorum profundus (n=832).

Conclusions:

• This study provides crucial axon count data for upper limb nerve branches, complementing existing anatomical information.
• Forearm nerve branch axon counts are notably comparable, offering valuable insights for surgical planning.
• The combined data on location, branch number, and axon counts empowers surgeons to design individualized nerve transfers for optimal patient outcomes.