Research Facial Palsy Prof. Dr. A. Kehrer

We have been conducting extensive basic research into the nervous system of the face on anatomical specimens of body donors based on scientific studies for years. The aim of these studies is to improve surgical methods and clinical applications. In one of the largest series of studies worldwide, the macro- and microanatomy and other important basics of the facial nerve and other donor nerves important for the plastic reconstruction, such as the masticatory nerve (masseteric nerve), could be examined in detail on more than 100 facial halves. As a consistent goal, a comprehensive representation of the axon capacities (number of nerve fibers) of the entire facial nerve branch system in the face was formulated at clinically relevant sections for facial reanimation (reanimation of facial expressions, i.e. potential connection points for nerve transplants and functional muscle transplants). Another relevant donor nerve branch for neurotizing procedures should be the masseteric nerve (Radix motoria nervi trigemini). This objective should be based on a high number of preparations, since in the current international literature feature only very few corresponding studies with a higher number of samples were found. For this purpose, the branches of the facial nerve and the masseteric nerve were prepared in this study using 53 body donors (fresh frozen) and a total of 106 facial halves under four times magnification.  A detailed photo documentation and description of the anatomical connections (number of main and lower branches, diameters, fusions, nerve anastomoses and lower crossings) was carried out in parallel.

About 1200 nerve samples were taken to investigate the number of nerve fibers (axons), fascicle structure, axon density, microscopic diameter determination, and other details of various nerve branches. The nerve fiber capacity sometimes determines how strongly the musculature is addressed for facial expression. The facial nerve shows a highly variable branching pattern with large individual differences between different patients and even the halves of the face within a face. Contrary to the previously known studies, the facial nerve could for the first time be described in a three-dimensional pattern instead of in a two-dimensional form (in one plane) and the exchange of fibers between individual nerve branches could be investigated more closely. In addition, new forms of lower crossings of nerve branches were discovered which showed no fiber exchange.

After fixation and preparation of semi-thin sections, the preparations were stained with a Richardson staining solution and PPD staining (paraphenylenediamine for nerve staining of osmated semi-thin sections). The histological sections were then digitized. This enabled semi-automated computer-based counting of myelinated axons.

A main focus of our research work is also the exact description of the nerve supply of an important muscle called "Zygomaticus major". It is the main actor of the lateral lifting of the corner of the mouth, the strongest laugh/smiling muscle, and thus one of the most important muscles for facial expression. Our research team was able to define a new anatomical definition for the precise description of its nerve supply in relation to surrounding anatomical structures.

The diameter of the nerve branch, the location of the donor nerve, and the nerve fiber capacity are among other things critical for the success of a free functional muscle transplant to give the paralyzed face the desired muscle tension (tonus) and mobility (muscle contractility) again. The scientific results from our research laboratory have already been transferred directly into clinical practice and have significantly improved the functional results for patients in terms of facial expression, symmetry, aesthetics, and facial tone.

In one of our microanatomical studies, the axon capacity of the extratemporal facial nerve main trunk was measured using computer-based semi-automatic quantification. Furthermore, the fascicular structure was assessed and the main trunk diameter measured. 3.82 fascicles (1-9 fascicles) were found in the main trunk (n=97). An average axonal capacity of 6684 ± 1884 axons (range: 2655 to 12457) was determined on 87 halves of the face. Right facial hemispheres showed 6364 ± 1904 axons (n=43), left 6996 ± 1833 axons (n=44) with a p=0.73. Female preparations showed 6247 ± 2230 (n=22), male 6769 ± 1809 nerve fibers (n=40) without statistical difference (p=0.59). The nerve diameter was measured with 1933 ± 424 µm (975 - 3012) (n=82). Possible potential donor nerves for neurotization including their specific axon capacities are discussed in the publication in connection with the results found.

As a donor nerve for neurotizing procedures, but also for free functional muscle transplantation, we often use the masseteric nerve - a branch of the masticatory muscle nerve. Due to its topographical proximity to the main trunk of the facial nerve and the zygomatic and buccal branch systems, neurotizing operations (nerve transfers) with only one coaptation (nerve suture) can be performed instead of two coaptations required in older procedures such as the hypoglossus jump nerve anastomosis. The nerve to the masseter is a strong donor with minimal loss of axon capacity (high degree of motor input to target).