A structural analysis of the facial muscle modiolus: using a three-dimensional reconstruction system

The modiolus is a complex anatomical region where multiple facial muscles converge to coordinate movements of the lips, cheeks, and oral fissure. Although its functional importance is well recognized, its intricate anatomy has been difficult to clarify using conventional methods alone. Six formalin-fixed human cadavers (nine hemifacial regions) without deformities were examined. Dissections were performed under 2.6× loupe magnification from the deep periosteal side to preserve the spatial relationships of the facial muscles forming the modiolus. Based on these findings, one well-preserved specimen was selected for detailed three-dimensional (3D) reconstruction. A tissue block containing the upper central modiolus was serially sectioned at 5 μm thickness, stained with hematoxylin and eosin, digitally scanned, and reconstructed using 3D volume-rendering software, enabling selective visualization of individual muscles and their interrelationships. Loupe magnification dissection consistently identified the orbicularis oris, buccinator, zygomaticus major, levator anguli oris, and depressor anguli oris as core components of the modiolus. The orbicularis oris displayed a consistent three-layered structure; however, its detailed internal connections with other muscles could not be clearly identified at the muscle fiber level while preserving tissue relationships. Three-dimensional reconstruction demonstrated that muscle bundles of the labial part of the orbicularis oris and the zygomaticus major crossed obliquely and interwove tightly within the modiolus. Although in this study limited to a single specimen, this combined approach provides valuable insight into minute and intricate structures such as the modiolus, which is difficult to elucidate using conventional techniques alone.