Upper Extremity Axon Counts and Clinical Implications for Motor Nerve Transfer

BACKGROUND:Nerve transfers are planned based on the following parameterslocation; number of branches and axon count matching of the donor and recipient nerves. We have previously defined the former two in upper limb muscles. Axon counts in literature are obtained from various sources, using different methods of histomorphometry. This study describes the axon counts of the same primary motor nerve branches from our previous study using a uniform method of manual histomorphometry and completes our blueprint of upper limb neuromuscular anatomy for reconstructive surgery. METHODS:The distal ends of the primary nerve branches of twenty-three upper limb muscles were harvested from ten fresh frozen cadaveric upper limbs. Manual quantitative histomorphometry was performed by two independent investigators and the average reported. RESULTS:The primary nerve branches of the arm muscles (range 882 – 1835) had higher average axon counts compared to those of the forearm muscles (267 – 883). In the forearm, wrist flexor (659 – 746) and extensor (543 – 745) nerve branches had axons counts that were similar to potential donors, e.g. supinator (602), pronator teres (625), flexor digitorum superficialis (883) and flexor digitorum profundus (832). CONCLUSIONS:Apart from describing the axon counts of the upper limb, we have found that the forearm axon counts are very comparable. This insight, when combined with information on the location and number of primary nerve branches, will empower surgeons to tailor bespoke nerve transfers for every clinical situation.