Variation and Covariation of the Shape and Volume of the Sphenoidal Sinus Among Craniofacial Anatomy

Abstract Body : Understanding the shape variation of the human sphenoidal sinus is important to several areas of research. This includes clinical investigation (sinus pathology and safe endoscopic endonasal surgical practice) and paranasal sinus evolution (for which there is still no consensus). Yet, the sphenoidal sinus has high morphological variation, prohibiting its quantification through more commonly used geometric morphometric landmarking methods. The sphenoid body, and thus also the sinus contained within, is located directly at the developmental center of the basicranium in humans, where the three cranial fossae meet at the midline, and adjacent to the three synchondroses which are the sites of cranial base angulation (CBA). It is surrounded by the cavernous sinus (and various vital nerves and blood vessels), endocrine and brain anatomy, and the posterior upper airway.

Despite its prime real estate, there is a paucity of research on the potential anatomical integration of the sphenoidal sinus that could shed light on its interactions with our highly derived craniofacial architecture and how the paranasal sinuses develop. This project investigates how the shape of the sphenoidal sinus covaries with adjacent anatomy using three-dimensional shape data of the sphenoidal sinus quantified for the first time using spherical harmonic shape analyses. Given previous research that suggests paranasal sinuses opportunistically develop in all available space, it is likely that the shape of the sphenoidal sinus is determined by the shape of the sphenoid body in which it forms, and by the anatomy and flexion that determines the shape and orientation of the body itself. However, the shape of the sphenoidal sinus was found to be independent of all these structures, including the body, except for the temporal fossa. Findings suggest that there is no allometric relationship between the sphenoidal sinus shape and the great diversity of sphenoidal sinus volume (relative to sphenoidal body volume), nor significant differences in shape between subsamples of sinuses below and above the average sphenoidal sinus volume. However, when only above-average sized sinuses were tested, a significant covariation was found between the sinus and sphenoid body--though not any other structure. Thus, even in a size-skewed sample, evidence suggests differences in how integrated the sphenoidal sinus is with surrounding bony architecture compared to the maxillary sinus. This research generates new questions about how the sphenoidal sinus relates to craniofacial and neuroanatomy, and calls into question the utility of our current models for paranasal sinus development.