Defining the Optimal Segment for Neurotization—Axonal Mapping of Masseter Nerve for Facial Reanimation

Masseteric Atrophy Following Masseteric Nerve Transfer: Radiographic Findings of Asymmetry in the Paralyzed Face?

OBJECTIVES

To characterize the effect of facial reanimation using masseteric nerve transfer on the masseter muscle itself, examining whether there is any demonstrable atrophy postoperatively.

METHODS

Electronic medical records of adult patients who underwent facial reanimation using masseteric nerve transfer at our institution over a 15-year period were reviewed. To account for the impact of postoperative radiation, randomly selected patients who underwent radical parotidectomy without nerve transfer and received postoperative radiation served as controls in a 1:1 fashion against those who underwent masseteric nerve transfer with postoperative radiation.

RESULTS

Twenty patients were identified who underwent masseteric nerve transfer and had sufficient pre- and postoperative imaging to assess masseter volume (mean age 58.2, 60% female). Of the four patients who did not receive postoperative radiation, each demonstrated masseteric atrophy on the side of their nerve transfer, with a mean reduction in masseter volume of 20.6%. The remaining 16 patients were included in the case-control analysis accounting for radiation. When compared with controls, those in the study group were found to have a statistically significant difference in atrophy (p = 0.0047) and total volume loss (p = 0.0002). The overall reduction in masseter volume in the study group was significantly higher compared with the control group, at 41.7% and 16.6%, respectively (p = 0.0001).

CONCLUSION

Facial reanimation utilizing masseteric nerve transfer appears to result in atrophy of the denervated masseter when compared with the contralateral muscle. This volume deficit may lead to further facial asymmetry for patients undergoing comprehensive reanimation surgery.

LEVEL OF EVIDENCE

3 Laryngoscope, 134:4514-4520, 2024.

Masseteric Nerve Ultrasound Identification for Dynamic Facial Reanimation Purposes

BACKGROUND

The masseteric nerve is one of the main options to neurotize free muscle flaps in irreversible long-term facial paralysis. Several preoperative skin-marking techniques for the masseteric nerve have been proposed to limit the surgical dissection area, shorten the surgical time, and enable a safer dissection. However, these have shown variability among them, and cannot visualize the nerve preoperatively. The authors designed an observational study to validate a high-frequency ultrasound (HFUS) nerve identification technique.

METHODS

A systematic HFUS examination was designed and performed to visualize the masseteric nerve in 64 hemifaces of healthy volunteers. One-third were randomly selected to undergo an additional HFUS-guided needle electrostimulation to validate the HFUS image.

RESULTS

The masseteric nerve was identified by HFUS in 96.9% of hemifaces (95% CI, 0.89 to >0.99) and showed almost perfect agreement with direct needle stimulation as calculated with Cohen kappa coefficient (0.95; 95% CI, 0.85 to 1.00). The masseteric nerve was found within the masseter muscle, in between the deeper muscle bellies, at 18.3 mm (SD ±2.2) from the skin. Only in 12.9% of cases (95% CI, 0.06 to 0.24) did its course become adjacent to the mandible periosteum. Other important features, such as disposition in relation to the parotid gland, or if the nerve was covered directly by a thick intramuscular aponeurosis, could be well observed by HFUS.

CONCLUSION

HFUS enables masseteric nerve identification and can provide the surgeon with specific information on anatomical relations for each examined individual before surgery.

The masseteric nerve for facial reanimation: Macroscopic and histomorphometric characteristics in 106 human cadavers and comparison of axonal ratio with recipient nerves

Peripheral facial palsy causes severe impairments. Sufficient axonal load is critical for adequate functional outcomes in reanimation procedures. The aim of our study was to attain a better understanding of the anatomy of the masseteric nerve as a donor, in order to optimize neurotization procedures. Biopsies were obtained from 106 hemifaces of fresh frozen human cadavers. Histological cross-sections were fixed, stained with PPD, and digitized. Histomorphometry and a validated software-based axon quantification were conducted. Of the 154 evaluated branches, 74 specimens were of the main trunk (MT), 40 of the anterior branch (AB), and 38 of the descending branch (DB), while two halves of one cadaver featured an additional branch. The MT showed a diameter of 1.4 ± 0.41 mm (n = 74) with 2213 ± 957 axons (n = 55). The AB diameter was 0.9 ± 0.33 mm (n = 40) with 725 ± 714 axons (n = 30). The DB diameter was 1.15 ± 0.34 mm (n = 380) with 1562 ± 926 axons (n = 30). The DB demonstrated a high axonal capacity - valuable for nerve transfers or muscle transplants. Our findings should facilitate a balanced selection of axonal load, and are potentially helpful in achieving more predictable results while preserving masseter muscle function.

Anatomic Analysis of Masseteric-to-zygomatic Nerve Transfer in Rat and Pig Models

Background

Nerve transfer from the masseteric branch of the trigeminal nerve is a widely performed procedure for facial reanimation. Despite achieving powerful muscle force, clinical and aesthetic results leave room for improvement. Preclinical animal models are invaluable to establishing new therapeutic approaches. This anatomical study aimed to establish a masseteric-to-zygomatic nerve transfer model in rats and pigs.

Methods

The masseteric branch of the trigeminal nerve and the zygomatic branch of the facial nerve were dissected in 30 swine and 40 rat hemifaces. Both nerves were mobilized and approximated to achieve an overlap between the nerve ends. Over the course of dissecting both nerves, their anatomy, length, and branching pattern were documented. At the coaptation point, diameters of both nerves were measured, and samples were taken for neuromorphometric analysis.

Results

Anatomic details and landmarks were described. Tension-free coaptation was possible in all rat and pig dissections. In rats, the masseteric branch had an average diameter of 0.36 mm (±0.06), and the zygomatic branch average diameter was 0.46 mm (±0.13). In pigs, the masseteric branch measured 0.52 (±0.16) mm and the zygomatic branch, 0.59 (±0.16) mm. No significant differences were found between the diameters and axon counts of both nerves in pigs. In rats, however, their diameters, axon counts, and fascicular areas were significantly different.

Conclusion

Our study demonstrated the feasibility of direct masseteric-to-zygomatic nerve transfer in rats and pigs and provided general anatomic knowledge of both nerves.

Facial reanimation using free partial latissimus dorsi muscle transfer: Single versus dual innervation method

The aim of the present study was to analyze the consequences of partial free latissimus dorsi muscle flap with nerve splitting technique (Partial LD transfer) for facial reanimation and compare outcomes according to innervation method (singer versus dual innervation). Patients with complete unilateral facial paralysis underwent either the single (ipsilateral masseteric nerve only) or dual (ipsilateral masseteric nerve plus contralateral buccal branch of the facial nerve) nerve innervation method for facial reanimation. An assessment was carried out to compare the outcomes between the single and dual innervation. Total of 21 patients were involved in this study. In the single innervation group, 7 out of 8 patients developed a voluntary smile. However, none were able to achieve a spontaneous smile. On the other hand, 9 out of 13 patients developed a voluntary smile and 3 out of 13 patients achieved a spontaneous smile. The mean increases of smile excursion assessed by Emotrics software and Terzis grades showed no significant differences between two groups. Within the limitations of the study it seems that partial LD transfer approach utilizing the dual innervation method has a positive effect on achieving a spontaneous smile and could be a valuable option for facial reanimation.

Microanatomy of the Frontal Branch of the Facial Nerve: The Role of Nerve Caliber and Axonal Capacity

BACKGROUND

A commonly seen issue in facial palsy patients is brow ptosis caused by paralysis of the frontalis muscle powered by the frontal branch of the facial nerve. Predominantly, static methods are used for correction. Functional restoration concepts include the transfer of the deep temporal branch of the trigeminal nerve and cross-facial nerve grafts. Both techniques can neurotize the original mimic muscles in early cases or power muscle transplants in late cases. Because axonal capacity is particularly important in cross-facial nerve graft procedures, the authors investigated the microanatomical features of the frontal branch to provide the basis for its potential use and to ease intraoperative donor nerve selection.

METHODS

Nerve biopsy specimens from 106 fresh-frozen cadaver facial halves were obtained. Histologic processing and digitalization were followed by nerve morphometric analysis and semiautomated axon quantification.

RESULTS

The frontal branch showed a median of three fascicles (n = 100; range, one to nine fascicles). A mean axonal capacity of 1191 ± 668 axons (range, 186 to 3539 axons; n = 88) and an average cross-sectional diameter of 1.01 ± 0.26 mm (range, 0.43 to 1.74 mm; n = 67) were noted. In the linear regression model, diameter and axonal capacity demonstrated a positive relation (n = 57; r2 = 0.32; p < 0.001). Based on that equation, a nerve measuring 1 mm is expected to carry 1339 axons.

CONCLUSION

The authors' analysis on the microanatomy of the frontal branch could promote clinical use of cross-facial nerve graft procedures in frontalis muscle neurotization and free muscle transplantations.

Free Functional Gracilis Flaps for Facial Reanimation in Elderly Patients

Importance: The free functional gracilis flap (FFGF) is a versatile procedure in reanimating the paralyzed face, yet its application in seniors is limited by perceptions of morbidity and inefficacy. Objective: The study objective was to compare the morbidity and effectiveness of FFGF reanimation among senior and younger patients. Design, Setting, and Participants: A retrospective chart review was performed on 20 consecutive patients aged 60 years and above (seniors) and 35 patients aged 40 years and below (juniors) who underwent FFGF for facial reanimation. Among this group, 16 senior and 22 junior patients with available long-term follow-up data were analyzed for functional outcomes. Main Outcomes and Measures: The length of postoperative stay and postoperative complications were compared with assess immediate results. A second analysis for functional outcomes was assessed by resting and smile facial asymmetry index (FAI), as well as maxillary dental display to compare facial tone and lip excursion. Results: The average age of seniors was 67 ± 5 years and that of juniors was 27 ± 10 years. Mean lengths of postoperative stay were 4 ± 2 versus 3 ± 1 days in seniors versus Juniors, respectively (p = 0.16). There were no intraoperative complications and postoperative complications in one (5%) senior and four (11%) juniors (p = 0.64). There was functional muscle recovery in all cases, with more pronounced correction of both resting (Δ3.0 mm vs. Δ2.4 mm, p = 0.66) and dynamic (Δ5.2 mm vs. Δ4.2 mm, p = 0.37) FAI in seniors than in juniors. Among patients who underwent a multivector FGFF, there was an additional three versus one visualized maxillary teeth (p = 0.03) in seniors versus juniors, respectively. Conclusions and Relevance: The FFGF is effective for facial reanimation among seniors and can be performed with minimal morbidity. Age alone should not preclude the application of the FFGF in seniors with a preference for more dynamic options.

Rapid and Precise Semi-Automatic Axon Quantification in Human Peripheral Nerves

We developed a time-efficient semi-automated axon quantification method using freeware in human cranial nerve sections stained with paraphenylenediamine (PPD). It was used to analyze a total of 1238 facial and masseteric nerve biopsies. The technique was validated by comparing manual and semi-automated quantification of 129 (10.4%) randomly selected biopsies. The software-based method demonstrated a sensitivity of 94% and a specificity of 87%. Semi-automatic axon counting was significantly faster (p < 0.001) than manual counting. It took 1 hour and 47 minutes for all 129 biopsies (averaging 50 sec per biopsy, 0.04 seconds per axon). The counting process is automatic and does not need to be supervised. Manual counting took 21 hours and 6 minutes in total (average 9 minutes and 49 seconds per biopsy, 0.52 seconds per axon). Our method showed a linear correlation to the manual counts (R = 0.944 Spearman rho). Attempts have been made by several research groups to automate axonal load quantification. These methods often require specific hard- and software and are therefore only accessible to a few specialized laboratories. Our semi-automated axon quantification is precise, reliable and time-sparing using publicly available software and should be useful for an effective axon quantification in various human peripheral nerves.