Delineation of the Innervation of the Face Utilizing Modifications to Sihler's Technique in a Primate Model

Sihler's techcnique can preferentially stain myelinated nerves within whole tissue which is rendered transparent. With transillumination, entire neural networks may be studied in fine detail without the loss of anatomic relationships from microdissection and sectioning. To date, no one has applied this technique to the face, although this is valuable information that would further the understanding of facial animation and sensibility.

The primate model was chosen because of correlation with human anatomy. The authors harvested the facial tissues as one whole composite in eight macaque monkeys. Initial failure with the conventional Sihler technique required modifications with respect to timing and sequence of steps. A method was developed to restore 3D configuration and to improve tissue transillumination by the creation of a silicone mount with an internal light source.

The entire motor and sensory network of the face was clearly seen up to the terminations in the facial muscle and soft tissue. The findings were: 1. The motor and sensory nerves appeared morphologically different with respect to the innervating territory and configuration. i. The motor nerves exit from the parotid and pass from lateral to medial into the facial muscles. The sensory nerves are divided into multiple territories with nerves that pass around the border of adjacent muscles and converge in the center of these areas. ii. Motor nerves run a straight course while sensory nerves are wavy and extensible, allowing them to accommodate to tissue stretch. 2. Although there is a correlation in the general branching pattern, each side of the face appears distinct, with asymmetry of the neural networks and no passage of the nerves across the midline. 3. Motor nerves branch prior to an angular entry into the facial muscles, which are innervated in a segmental fashion. 4. The branching pattern of the motor nerves is most complex in the buccal branch that has the most number of nerve-to-nerve communications, corresponding to the area with the most number of target muscles.

Sihler's technique is a powerful tool that reveals previously unseen intramuscular neural anatomy of the face and retains fine anatomic relationships. This technique can be applied to the study of injury-reinnervation patterns and facial reanimation. In the human face, this technique will provide a much needed roadmap for surgery.