Abstract
PurposeWe morphometrically analyzed human facial muscles, and evaluated the Yanagihara facial nerve grading system using our data.MethodsWe used 15 types of human facial muscle, 2 types of masticatory muscle and 2 types of skeletal muscle. The materials were obtained from 11 Japanese male cadavers aged 43–86 years. We counted the muscle fibers and measured the transverse area of the muscle fibers (TAMF), and then calculated the number of muscle fibers (NMF) per mm2 and the average TAMF.ResultsWe found a significant correlation between average TAMF and NMF (r = − 0.70; p < 0.01). We classified facial muscles into three types based on the correlational results. Type A had a low average TAMF and high NMF. Type C had a high average TAMF and low NMF. Masticatory and skeletal muscles were characterized as Type C. Type B was intermediate between Types A and C.ConclusionsPathological changes in the facial muscles in facial nerve palsy seem to vary according to the type of facial muscle, because each facial muscle has a unique fiber-type composition. As the nine discrete facial expressive states evaluated in the Yanagihara system involve all three facial muscle types of our classification, the Yanagihara system is an outstanding system for grading facial nerve palsy in terms of the facial muscle morphology.
Highlights
Several grading systems for the assessment of facial nerve function using gross or regional scales have been proposed
As muscles are innervated by nerves, it is possible that sex differences in neurodegeneration influence sexual dimorphism of the muscle morphology
Researchers have studied the morphology of many kinds of skeletal muscle by calculating the muscle fiber number and area—the same method used in this study
Summary
Several grading systems for the assessment of facial nerve function using gross or regional scales have been proposed. The Yanagihara facial nerve grading system, reported by Yanagihara [1], was developed in Japan as a representative regional scale, and was standardized in Japan and in some other countries for grading facial function. The Yanagihara system measures 10 separate aspects of different facial. The facial muscles differ from one another in morphological aspects. Researchers have reported that clinicians must consider the diversity of facial muscles when planning treatments for facial paralysis [2, 3]. We morphometrically analyzed human facial muscles, and evaluated the Yanagihara facial nerve grading system using our data
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