Facial reanimation with neurovascular free muscle transfer is the gold standard treatment for long-established facial paralysis (Dong et al., 2018; van Veen et al., 2018; Oh et al., 2019; Bianchi et al., 2020). Among various muscle transfer techniques, the single-stage dual innervation method uses the masseteric nerve on the paralyzed side, which provides faster reinnervation and minimizes denervation atrophy of the transferred muscle before regeneration through the relatively long facial nerve on the healthy side is completed to recover a full spontaneous smile.

Although numerous dual innervation surgical techniques have been reported and showed marked improvement in outcomes, limitations still exist. The transfer of dual innervated latissimus dorsi (LD) muscle flap using only the thoracodorsal nerve (TDN) requires extensive nerve dissection proximally to obtain a sufficient length to reach the contralateral side of the face; this can be technically difficult and even causes transient brachial plexus palsy in rare cases (Kurita et al., 2007; Biglioli et al., 2009; Watanabe et al., 2009). In addition, neuromuscular neurotization by indirect contact or an end-to-side anastomosis is inevitable for reinnervation through the masseteric nerve, which is known to be inferior to end-to-end anastomosis (Jaeger et al., 2011). Furthermore, techniques using dual neural input sources have been introduced, such as the use of the descending and transverse branches of the TDN, or the TDN and long thoracic nerve. However, these methods require extra time and effort to dissect two different nerves and to harvest two segmental flaps, which often results in an excessively bulky shape (Leckenby et al., 2014; Okazaki et al., 2015; Matsumine et al., 2019).

The problems related to the previous applications of dual innervation, which used only the proximal nerve stump from the donor muscle for reinnervation of the transplanted muscle, were addressed by introducing the single-stage LD transfer with the use of proximal and distal ends of TDN as nerve sources. The aim of the study was to analyze the efficacy of retrograde reinnervation of the transplanted LD flap by the distal stump of TDN.