The Bottleneck Theory: Accounting for Clinical and Experimental Variability in the Outcome of Functional Muscle Transfer

Comparison of Outcomes of Facial Reanimation Between the Use of Cross-facial Nerve Graft and the Masseteric Nerve as the Donor Nerve for Reinnervation of Gracilis Muscle Flap Transfer

BACKGROUND

In patients with facial paralysis, the free functional gracilis muscle transfer is preferred for facial reanimation. The choice of an adequate motor nerve to innervate the transplanted gracilis muscle is one of the procedure's key components. We present a comparative study between cross-facial nerve graft (CFNG) and masseteric nerve as donor nerves for reinnervated gracilis flap transfer in patients with complete facial paralysis.

MATERLALS AND METHODS

Retrospective analysis was performed on all patients with complete facial paralysis who had a free functional gracilis muscle transfer for facial reanimation between January 2014 and December 2021. Only those who received gracilis transfer reinnervated by either CFNG or masseteric nerve were included in this study. The smile excursion and lip angle were measured for evaluating the outcomes postoperatively.

RESULTS

The inclusion criteria were met by a total of 21 free functional gracilis muscle transfers, of which 11 were innervated by CFNG and 10 by the masseteric nerve. Both surgical procedures resulted in a highly considerable smile excursion of the reanimated side and postoperative improvement of static or dynamic lip angle. Masseteric nerve coaptation led to greater smile excursion and more significant improvement of dynamic lip angle than CFNG.

CONCLUSIONS

For patients who have complete facial paralysis, face reanimation can be successfully accomplished by free gracilis transfer reinnervated by the CFNG or the masseteric nerve. In particular, the masseteric nerve is a reliable choice for dynamic smile reanimation.

Axonal Regeneration Through Autologous Grafts: Does the Axonal Load Influence Regeneration?

INTRODUCTION

Two-stage free functional muscle transfers for long-standing facial palsy can yield unpredictable results. Earlier studies have demonstrated incomplete regeneration across neurorrhaphies in native nerve and higher donor axonal counts correlating with improved outcomes but axonal count in nerve grafts have not been as thoroughly reviewed. To investigate the impact of varying axonal counts in autologous grafts on functional outcomes of repair.

MATERIALS AND METHODS

Animals were allocated into three groups: Direct Nerve Repair (DNR, n = 50), Small Nerve Graft (SNG, n = 50), and Large Nerve Graft (LNG, n = 50). All grafts were inset into the Posterior Auricular Nerve with ear movement recovery (EMR) monitored as functional outcome. At various postoperative weeks (POWs), excised specimens were imaged with electron microscopy. Axonal counts were measured proximal to, distal (DAC) to, and within grafts. Total Success Ratio (TSR) was calculated.

RESULTS

In DNR, DAC was significantly lower than proximal axonal counts at all POWs, with maximum TSR of 80%. TSR for LNG and SNG were significantly lower at all POWs when compared to DNR, with maximums of 56% and 38%, respectively. LNG had a significantly larger DAC than SNG at POW12 and beyond. A direct relationship was present between DAC and EMR for all values.

CONCLUSIONS

Higher native axonal count of autologous nerve grafts resulted in higher percentage of regeneration across neurorrhaphies.

Convergent end-to-end neurorrhaphy: An alternative technique for dual innervation of the gastrocnemius muscle in rats

INTRODUCTION

Muscle contraction generated by electrical impulses simultaneously originating from two different neural sources may be an interesting treatment alternative for long term facial palsy. An experimental model was designed to compare single and dual innervation of the gastrocnemius muscle (GM) in rats.

METHODS

Fifty adult Wistar rats underwent transection of their right peroneal nerve and were divided into five groups (n = 10): control (C), tibial nerve section (TS), tibial nerve primary end-to-end neurorrhaphy (PEE), tibial nerve primary repair associated with end-to-side peroneal-to-tibial nerve transfer (PRES), and tibial nerve repair by convergent end-to-end (CEE) neurorrhaphy between the proximal stumps of the tibial and peroneal nerves to the distal stump of the tibial nerve. The outcomes were assessed 12 weeks after the experiment by walking track, electromyography, GM mass index, and histomorphometric analysis of the distal tibial nerve.

RESULTS

The functional recovery of the PRES (-33.77 ± 24.13) and CEE (-42.15 ± 31.14) groups was greater (P < 0.003) than the PEE group (-80.26 ± 17.20). The CEE group (18.35 ± 7.84) showed greater amplitude (P = 0.006) than the PEE group (8.2 ± 4.64). There was no difference in the muscle mass index among the reinnervation groups (P > 0.705). Histologic analysis revealed greater (P < 0.002) axonal density in the CEE group (126.70 ± 15.01) compared to PEE (99.70 ± 12.82) and PRES (92.00 ± 19.17) groups.

CONCLUSIONS

The dual innervation techniques showed earlier and greater functional recovery of the GM than did the single innervation technique. The CEE group showed a 40% higher number of regenerated axons in the distal tibial nerve stump.

Facial Nerve Axonal Analysis and Anatomical Localization in Donor Nerve: Optimizing Axonal Load for Cross-Facial Nerve Grafting in Facial Reanimation

BACKGROUND

Donor nerve axonal count over 900 in two-stage reconstructions using cross-facial nerve grafts is possibly associated with improved outcomes in facial reanimation. Facial nerve axonal analysis was performed to determine the ideal location for optimizing axonal load. Correlation of axonal number, branch diameter, and age was also assessed.

METHODS

Twenty-eight fresh unpreserved cadaveric hemifaces were dissected exposing the extracranial facial nerve branches. Axonal counts at 2-cm intervals from the pes anserinus along branches inserting into the zygomaticus major muscle were taken, noting position relative to the zygomatic arch, posterior ramus border, lateral border of the zygomaticus muscle, and anterior parotid gland border. Nerves were fixed, sectioned, and stained with SMI-31 antineurofilament stain for digital axonal analysis.

RESULTS

All specimens had one or more intraparotid zygomatic branches with over 900 axons, and 96 percent had an extraparotid branch with over 900 axons. The likelihood that a zygomatic branch would have over 900 axons at its last intraparotid point (mean, 6 mm posterior to the parotid border) was 92 percent (range, 67 to 100 percent) in contrast to 61 percent (range, 25 to 100 percent) when sampled at the first extraparotid point (mean, 14 mm anterior to the parotid border). Nerve cross-sectional area was positively correlated to its axonal count (R° = 78 percent; p < 0.0001), with nerve diameter over 0.6 mm predicting over 900 axons. Age did not correlate with axonal counts.

CONCLUSIONS

Branches with adequate axonal load were found in all specimens. The likelihood of adequate branch selection improved from 61 percent to 92 percent with short retrograde intraparotid dissection. Nerve diameter correlated with axonal load.

The degree of facial movement following microvascular muscle transfer in pediatric facial reanimation depends on donor motor nerve axonal density

BACKGROUND

Free functional muscle transfer to the face is a standard of facial animation. The contralateral facial nerve, via a cross-face nerve graft, provides spontaneous innervation for the transferred muscle, but is not universally available and has additional shortcomings. The motor nerve to the masseter provides an alternative innervation source. In this study, the authors compared donor nerve histomorphometry and clinical outcomes in a single patient population undergoing free muscle transfer to the face.

METHODS

Pediatric patients undergoing dynamic facial (re-)animation with intraoperative nerve biopsies and gracilis transfer to the face powered by either the contralateral facial nerve via a cross-face nerve graft or the motor nerve to the masseter were reviewed over a 7-year period. Myelinated nerve counts were assessed histomorphometrically, and functional outcomes were evaluated with the Scaled Measurement of Improvement in Lip Excursion software.

RESULTS

From 2004 to 2011, 91 facial (re-)animation procedures satisfied study inclusion criteria. Average myelinated fiber counts were 6757 per mm2 in the donor facial nerve branch, 1647 per mm in the downstream cross-face nerve graft at the second stage, and 5289 per mm in the masseteric nerve. Reconstructions with either innervation source resulted in improvements in oral commissure excursion and smile symmetry, with the greatest amounts of oral commissure excursion noted in the masseteric nerve group.

CONCLUSIONS

Facial (re-)animation procedures with use of the cross-face nerve graft or masseteric nerve are effective and result in symmetric smiles. The masseteric nerve provides a more robust innervation source and results in greater commissure excursion.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Therapeutic, III.

Use of the interposed nerve graft to reduce differences in functional muscle transfer outcome arising from variability in motor input

Regulation of functional muscle transfer outcome by means of an interposed nerve graft: The gold standard treatment of chronic unilateral facial palsy is a functional muscle transfer (FMT) reinnervated by a branch of the contralateral facial nerve via a cross-facial nerve graft, but clinical outcome varies from underactivity to overactivity of the transferred muscle. Work in an animal model has established that the magnitude of donor motor input received by heterotopically transferred muscle correlates with reinnervation and force production, the corollary being that standardising donor motor input could standardise outcome. We hypothesise that a 'low-capacity' interposed nerve graft could deliver a fixed motor input to an FMT despite variable motor input, thereby standardising force production and clinical outcome. To test this hypothesis, we compared reinnervation and force production by FMTs reinnervated with different-sized motor inputs via high- and low-capacity interposed nerve grafts. Force production was measured in vivo after recovery of transplanted muscle, and reinnervation quantified by both nerve morphometry and intramuscular immunohistochemical staining for protein gene product 9.5. Both reinnervation and force production data suggest that outcome correlates with donor motor nerve magnitude only when a high-capacity interposed nerve graft is used, this correlation being lost when using a low-capacity interposed nerve graft. The implications for human facial reanimation surgery are discussed.