Donor and Recipient Nerve Axon Counts in Gender-affirming Radial Forearm Phalloplasty: Informing Choice of Nerve Coaptations

A key component of success of a nerve transfer is the innervation density, which is directly affected by the donor nerve axonal density and donor-to-recipient (D:R) axon ratio. Optimal D:R axon ratio for a nerve transfer is quoted at 0.7:1 or greater. In phalloplasty surgery, there are currently minimal data available to help inform selection of donor and recipient nerves, including unavailability of axon counts. METHODS: Five transmasculine people who underwent gender-affirming radial forearm phalloplasty had nerve specimens processed with histomorphometric evaluation to determine axon counts and approximate donor-to-recipient axon ratios. RESULTS: Mean axon counts for recipient nerves were 6957 ± 1098 [the lateral antebrachial (LABC)], 1866 ± 590 [medial antebrachial (MABC)], and 1712 ± 121 [posterior antebrachial cutaneous (PABC)]. Mean axon counts for donor nerves were 2301 ± 551 [ilioinguinal (IL)] and 5140 ± 218 [dorsal nerve of the clitoris (DNC)]. D:R axon ratios using mean axon counts were DNC:LABC 0.739 (0.61–1.03), DNC:MABC 2.754 (1.83–5.91), DNC:PABC 3.002 (2.71–3.53), IL:LABC 0.331 (0.24–0.46), IL:MABC 1.233 (0.86–1.17), and IL:PABC 1.344 (0.85–1.82). CONCLUSIONS: The DNC is the more powerful donor nerve with greater than two times the axon count of the IL. The IL nerve may be under-powered to re-innervate the LABC based on an axon ratio consistently less than 0.7:1. All other mean D:R are more than 0.7:1. DNC axon counts may be excessive for re-innervation of the MABC or PABC alone with D:R of more than 2.5:1, potentially increasing risk of neuroma formation at the coaptation site.