(038) Optimizing Innervation in Radial Forearm Phalloplasty: Determination of Ideal Nerve Coaptations via Donor:Recipient Axon Ratios

Abstract

Abstract Introduction There is currently no standardized approach to innervation in radial forearm phalloplasty. This includes both the number and the combination of nerve coaptations that are performed. It is also under-recognized that the nerve coaptations executed in phalloplasty are equivalent to sensory nerve transfers. It is well documented that a key component of a successful nerve transfer is a sufficient donor:recipient (D:R) axon ratio. Evidence suggests a minimum ratio of greater than 0.3:1 and an optimal ratio of greater than or equal to 0.7:1 to best optimize nerve regeneration and outcomes. Axon counts and donor:recipient axon ratios have not yet been performed for the sensory nerves used in phalloplasty surgery. Objective The purpose of this study was to obtain axon counts for the donor and recipient nerves used in radial forearm phalloplasty in order to calculate donor:recipient axon ratios for each possible sensory nerve transfer. This information will inform which nerve coaptations are most ideal in radial forearm phalloplasty based on both axon counts and donor:recipient axon ratios. Methods Following IRB approval and informed consent, five transmasculine patients who underwent gender-affirming phalloplasty at our institution had nerve specimens (5 mm) taken from five different sensory nerves that are routinely transected during surgery – the lateral antebrachial cutaneous (LABC), medial antebrachial cutaneous (MABC), posterior antebrachial cutaneous (PABC), ilioinguinal, and dorsal nerve of the clitoris. Histomorphometric evaluation was performed to determine mean axon counts for each nerve respectively. Donor:recipient axon ratios were then calculated for each possible combination of donor and recipient nerves. Results Mean axon counts for the recipient nerves were 6957 (LABC), 1866 (MABC), 1712 (PABC). Mean axon counts for the donor nerves were 2301 (ilioinguinal), and 5141 (dorsal nerve of clitoris). Calculated D:R axon ratios were as follows: clitoral:LABC 0.739, clitoral:MABC 2.754, clitoral:PABC 3.002, ilioinguinal:LABC 0.331, ilioinguinal:MABC 1.233, and ilioinguinal:PABC 1.344. Conclusions Based on both mean axon counts and D:R ratios, the clitoral nerve is the more powerful donor nerve available in phalloplasty and should be considered for use as a primary donor. All D:R ratios exceed 0.3:1. However the best matches based on D:R ratios and clinical size match appear to be clitorial:LABC and ilioingiinal:MABC or ilioinuinal:PABC. If using MABC or PABC to coapt into the clitoral nerve, there should be consideration for using only part of the clitoral nerve to prevent unnecessary levels of clitoral denervation or neuroma formation given significant excess of donor axons available compared to recipient pathways for regeneration. The ilioinguinal nerve is likely under-powered to consistently re-innervate the LABC. In general, most coaptations are highly favorable for functional levels of nerve regeneration. This also suggests additional factors beyond successful nerve regeneration may contribute to optimal sensory outcomes following phalloplasty surgery. Disclosure No