Reanimation of recent palsy — Where to find the motor input

Quantitative and qualitative motor sources and why none suffices alone, the surgical anatomy of the masseteric nerve with its millimetric landmarks, masseteric-to-facial anastomosis and the logic of triple innervation.

Content intended for healthcare professionals. It does not replace clinical assessment and is not written as patient information.

Key points

Motor sources fall into two families with opposite logics: quantitative ones give many axons but an input alien to facial mimicry; qualitative ones give the right input but few axons.
No source does everything: the hypoglossal gives resting tone but little movement, the masseteric gives powerful movement and smile but little tone, and the contralateral facial gives quality of movement but little power and no tone.
Hence the idea of adding three inputs rather than choosing one: the logic of triple innervation, which aims to cover tone, power and spontaneity at once.
The masseteric nerve has become the workhorse for three concrete reasons: 1,542 ± 291 axons, a main trunk of 1.8-2 mm and enough length to reach the facial branches.
Reanimation does not end with the nerve: tarsorrhaphy, upper lid lipofilling and fascia lata suspension are part of the same plan, not extras.

Two families of motor sources

When the facial nerve can no longer carry the impulse, it must be sought elsewhere, and the options are ordered by what they contribute. Quantitative sources are nerves foreign to facial mimicry that supply a high axonal load: they are powerful but do not give the natural input, because each contraction is triggered by a gesture other than the one intended. Qualitative sources are the facial nerve itself — ipsilateral where a usable stump remains, contralateral where it does not — and they alone carry the natural input for mimicry, including the emotional smile. Therein lies the underlying problem: emotion is not transmitted by the same nerves as voluntary movement. Social and spontaneous smiles are neurologically distinct, and a reanimation giving only voluntary movement leaves the patient with a face that moves on command but not on feeling.

The catalogue of motor sources

Source	Type and historical origin	What it gives and what it does not
Hypoglossal	Quantitative. Described in the early twentieth century, the oldest transfer still in use	It gives resting tone, which is what holds the face when the patient does nothing. In exchange, poor movement and the cost to the tongue
Masseteric	Quantitative. Proposed in the late 1970s and consolidated in current practice since 2012	Powerful movement, especially for the smile, and rapid reinnervation. In exchange, little resting tone
Spinal accessory	Quantitative. The oldest described, in the late nineteenth century	Good axonal load, but the cost to trapezius and shoulder has progressively reduced its use in favour of the masseteric
Deep temporal and others	Quantitative. Deep temporal since the mid-1970s; phrenic, glossopharyngeal, C4 and C7 in small series	Salvage options when the above are unavailable. Each with its own specific morbidity
Ipsilateral facial	Qualitative. Direct repair or grafting dates from the 1920s	It is always the first option when a usable proximal stump exists: the right input by the right route
Contralateral facial (cross-face)	Qualitative. Described in the early 1970s, via a nerve graft crossing the face	The only source of emotional, spontaneous smiling. In exchange, little power and no resting tone, and slow reinnervation taking months to arrive

The table that sums up the decision

Set out in three columns, the choice becomes transparent. Hypoglossal: resting tone, at the cost of poor movement. Masseteric: powerful movement, above all the smile, at the cost of little resting tone. Facial: quality of movement, at the cost of little power and no resting tone if the source is contralateral. Read this way the three are complementary rather than alternative, and the question stops being which to choose and becomes which to combine and in what order. That is precisely the reasoning from which triple innervation arises.

Surgical anatomy of the masseteric nerve

It exits the skull base through the foramen ovale and runs over the lateral pterygoid muscle before reaching the masseter on its deep surface.
It branches before entering the muscle, with a variable pattern: one branch in 25 %, two in 47 % and three in 25 %. But there is always a main trunk of 1.8 to 2 mm diameter, and that is the one to find.
The landmarks that locate it: 42 mm anterior to the tragus, 12 mm below the zygomatic arch, 10 mm deep to the masseteric fascia, at 112° to the zygomatic arch.
The axonal count is 1,542 ± 291 axons, and its length suffices for direct anastomosis to facial branches: direct anastomosis onto the cervicofacial trunk without an interposed graft has even been described.

Masseteric-to-facial anastomosis, step by step

ApproachAn extended preauricular incision with a cervical extension, in the same design as a parotid approach. The great auricular nerve is identified and preserved, as it may serve as a graft if needed
Expose the facial nerveThe facial trunk is located at the stylomastoid foramen and followed to the bifurcation, identifying the branch to be reinnervated according to the functional goal
Find the masseteric nerveThe masseteric fascia is raised and the muscle dissected in the window defined by the landmarks: 42 mm anterior to the tragus and 12 mm below the arch, going about 10 mm deep. Intraoperative stimulation confirms the identification
Divide and rotateThe masseteric nerve is divided as distally as possible, to gain length, and rotated backwards and upwards to the chosen facial branch. That rotation is what allows an interposed graft to be avoided in many cases
NeurorrhaphyEpineural suture under the microscope with 9/0 or 10/0 monofilament, without tension: tension at a neurorrhaphy is the most predictable cause of failure

Triple innervation

If each source contributes something different and none contributes everything, the reasonable conclusion is not to choose. Triple innervation means supplying two quantitative sources and one qualitative simultaneously to the same mimetic musculature: masseteric and hypoglossal as motors of power and tone, and a cross-face graft from the contralateral facial nerve as the source of emotional smiling. The technical contribution that makes it possible is end-to-side neurorrhaphy, which allows inputs to be added to a nerve without sacrificing it: the masseteric is anastomosed to the distal stump, and the other two sources are added laterally without interrupting what is already built. In the original series of twenty-four patients the approach was proposed as a way to improve quantity and quality of reanimation at the same time in recent palsies. It is worth being honest about what remains unsettled: whether adding the qualitative source contributes enough to justify the complexity is still debated, since not all authors obtain the same benefit from cross-face grafting.

The details that accompany the nerve

Lateral tarsorrhaphy to protect the cornea while reinnervation arrives. It is a bridge, not a definitive solution, and should be explained as such.
Upper lid lipofilling, of the order of 2 ml equivalent to 1.6 g: it adds weight and volume to the lid and improves gravity-assisted closure, with the advantage over a gold weight of being autologous tissue.
Fascia lata to suspend the lower two thirds of the face: it provides from day one the resting symmetry that reinnervation will take months to give, and unloads tension from the future movement.
The approach is often two-staged: the first for the nerve work, and the second, months later, to enhance cross-face efficacy with additional end-to-side neurorrhaphies and to add ancillary surgery — more lipofilling, fascia re-suspension, selective neurotomies of the depressor labii.

Immediate reanimation in oncological surgery

There is one situation in which the time window is not a problem but an opportunity: radical parotidectomy where it is known in advance that the facial nerve will be sacrificed. There, nerve reconstruction is done in the same operation, with no denervation period, which is the ideal scenario. Described options include direct masseteric-facial neurorrhaphy and interposition of a nerve graft to reconstruct the distal branch tree. The organisational consequence is clear and worth stating: facial reanimation is planned before the resection, not after, and that requires the resecting and reconstructing teams to speak at the same tumour board.

Choosing the motor source

Is there a usable proximal facial stump?

YesDirect repair or graft on the facial nerve itself. It is the only option restoring the right input by the right route, and nothing improves on it.

NoMove to the next point.

What is chiefly lacking in this face?

Smile and powerMasseteric: high axonal load, rapid reinnervation and powerful movement. It is the default quantitative source in current practice.

Tone and resting symmetryHypoglossal, accepting the cost to the tongue, or fascia lata suspension if no further nerve morbidity is wanted.

Spontaneity on emotionCross-face graft from the contralateral facial nerve: the only source that provides it. Slow and weak, so it rarely goes alone.

All threeTriple innervation: masseteric and hypoglossal plus cross-face, added with end-to-side neurorrhaphies. More complex, and with the relative efficacy of the qualitative component still debated.

Diagram of triple innervation: masseteric and hypoglossal as quantitative sources on the distal stump, with a cross-face graft from the contralateral facial nerve added end-to-side.

Locating the masseteric nerve on the masseter: 42 mm anterior to the tragus, 12 mm below the zygomatic arch and 10 mm deep to the fascia.

References

Related specialty: Facial Paralysis

All surgical notes