406.3 - Employing Spherical Harmonics in Assessing the 3D Shape Variation and Evolutionary Potential of Human Sphenoidal Sinuses

Katharine Ryan (University of Tennessee), Benjamin Auerbach (University of Tennessee), Adam Sylvester (Johns Hopkins University), Lauren Butaric (Des Moines University)

The evolutionary potential and shape variation of the human sphenoidal sinus are unknown. Moreover, a consensus about the function or evolutionary forces that influence variation of paranasal sinuses among humans remains unestablished. One initial approach to address this lies in determining the covariance of sinus shape with surrounding morphology. The sphenoidal sinus is centrally located within the basicranium at the intersection of the three cranial fossae, and is adjacent to the synchondroses where primary cranial base angulation occurs. The angulation of the cranial base and its relative orientation with the face is a distinctive, derived trait in Homo sapiens and is important to our understanding of the evolution of our species. Yet, the shape of the sphenoidal sinus is highly variable and cannot be quantified with traditional landmarking methods. Originally developed in gravitational physics, spherical harmonics has been successfully leveraged in biological morphological studies to quantify and assess 3D shape variation of complex structures. This research will use spherical harmonics to for the first time quantify the 3D shape of the sphenoidal sinuses and assess their covariation with the sphenoid body. By demonstrating the utility of spherical harmonics for human morphometric research, we are opening up new ways for investigating biological and evolutionary questions of human anatomical features that to date have been difficult to evaluate.

Here we preliminarily analyze the covariance of sphenoid sinus and basicranial shape. The sphenoidal sinuses of 120 human crania were segmented (Avizo) from CT scans. We quantified the three-dimensional shape variation of sphenoidal sinuses using spherical harmonic analyses (SHARM, Matlab), producing coefficients comparable to traditional landmarks placed virtually on the basicranial surface. Landmarks were chosen that capture the general shape of the sphenoid body. Two-Block Partial Least Squares (2B-PLS) analyses revealed no significant covariation (p=0.59) between the shape of the sphenoidal sinus and its surrounding bony architecture. This indicates the establishment of sinus shape occurring independently of the surrounding bone. This is contrary to previous research, which has suggested that paranasal sinuses opportunistically pneumatize within the available space of their respective cranial bones. Future research will need to investigate this relationship further with additional sets of landmarks representing different developmental regions and functional matrices that interface with the sphenoid body. This research is also foundational to investigate to what extent mutual constraint among these traits may exist in the ability to respond to natural selection, i.e. evolvability.