Dually Innervated Free Gracilis Transfer in Smile Reanimation
Joseph R Dusseldorp MS, MBBS, FRACS1,2; Nate Jowett, MD1; Tessa A Hadlock, MD1
1Harvard Medical School, Boston, USA; 2Royal Australasian College of Surgeons and University of Sydney, Sydney, Australia
Background
When two populations of motor axons are directed at a recovering muscle this technique is known as “supercharging”. There are relatively few clinical studies that have assessed the outcomes of this technique or determined the relative inputs of each donor nerve. In facial palsy, selection of donor nerves to re-innervate the free gracilis muscle transfer remains controversial. The cross facial nerve graft (CFNG) provides an ideal source of motor axons from the healthy contralateral side but can lead to unfavourable outcomes especially in older patients. The nerve to masseter (NTM) has several advantages, providing a strong source of axons without the need for an interposition graft. Its downsides are that the patient must learn to smile by activating their masseter muscle and it does not activate spontaneously in the majority of cases. An option is to utilize both the CFNG and NTM in a dually innervated fashion. The purpose of this study was to determine the effectiveness of the dual innervation technique using novel computer vision outcome assessment tool.
Methods
Patients undergoing free gracilis transfer by dual innervation were identified and demographic, operative, and outcomes data was collected prospectively. Standardised clinical photography and videography was reviewed as well as clinician-graded and patient related outcome scales (eFACE and FaCE instrument). Spontaneous smile was assessed using validated humorous videos. Synchronicity was assessed using blinded facial movement detection of healthy side and affected side by two independent observers. A novel computer vision algorithm was employed to detect expression of joy during both voluntary and spontaneous smiling. Comparisons were made between NTM, CFNG and dually innervated gracilis cases.
Results
Dually innervated free gracilis transfer was performed in 26 patients between 2004 and 2016. Mean time of follow-up was 12 months. eFACE and FaCE scale improvements were statistically significant. Results of both voluntary and spontaneous expression of joy in CFNG, NTM and dually innervated gracilis flaps will be presented.
Discussion
Harnessing two sources of motor axons may enable us to optimise the recovery of a denervated muscle. Each motor nerve may be able to control certain segments of muscle or trigger contraction of the whole muscle. The precise interplay between the two populations of motor neurones has yet to be elucidated in animal or clinical models. This large group of dually innervated free muscle transfers gives an insight into this important question.
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