Understanding Sleep Apnea

Overview

Analysis of the role of passive mechanical deformation in the human pharynx to upper airway collapse is fundamental to understanding how airway patency is maintained. Finite Element Analysis (FEA) was used in this study to examine deformation using a detailed 3D anatomical model that was created in Simpleware software from CT scans. Results showed good agreement with in vivo testing and the literature, providing a starting point for more comprehensive simulations of human upper airway collapse and obstructive sleep apnea therapy.

Characteristics:

Image data of a human upper airway acquired from CT
36 different anatomical structures reconstructed in Simpleware ScanIP
FE mesh generated in Simpleware FE with inner airway pressure boundary condition applied
Simulations of airway deformations carried out in ANSYS Workbench

Thanks to:

University of Alberta (Departments of Mechanical Engineering and Radiology and Diagnostic Imaging) N. Carrigy, J. Carey, A. Martin, W. Finlay, M. Noga
Zephyr Sleep Technologies Inc.: J. Remmers, A. Zareian, Z. Topor, J. Grosse
University of Calgary (Departments of Physiology and Pharmacology and Cell Biology and Anatomy): J. Remmers, Z. Topor

References:

Carrigy, N.B. et al., 2016. Simulation of muscle and adipose tissue deformation in the passive human pharynx. Comput Methods Biomech Biomed Engin.

Image Processing and Mesh Generation

CT scan data of a 79-year-old male with a normal airway was obtained using a multi-slice scanner (Phillips GEMINI TF 16 PET/CT, The Netherlands).

Simpleware ScanIP was used for 3D reconstruction and analysis of the DICOM files. A binarized median filter was applied to the entire image set to reduce noise; a noise curvature flow filter was applied to encourage intra-region smoothing and to inhibit inter-region smoothing, removing noise and enhancing greyscale boundaries. 36 individual anatomical structures of tissues, bones, cartilage, ligaments, muscles and membranes, were segmented. A recursive Gaussian smoothing was applied to the segmented structures to further reduce noise levels and attenuate sharp edges. The inferior side of the model was cropped to near the vocal folds to reduce computational FEA demands.

Renderings of the 3D model for FEA. Each number refers to a specific structure, with colors also differentiating anatomical structures

Simpleware FE was used to create a mesh composed of quadratic tetrahedron elements. Localized mesh refinement was carried out and additional mesh improvements made prior to simulation. The multi-part mesh featured shared nodes at contact interfaces, with an inner airway pressure boundary condition applied to node sets defined on the airway wall, and node sets defined at the interfaces between the cubic domain boundary and the model; fixed boundary conditions were also applied here.

Simulation Results

The completed mesh and node sets were imported in .cdb format to ANSYS Workbench for simulation. Cross-sectional area was analyzed in ImageJ, and mesh convergence demonstrated. The properties of passive pharyngeal tissue were estimated under the restriction of small deformation, with linear elasticity as a reasonable assumption.

Cross-sectional area at the oropharynx and velopharynx were analyzed over varying airway pressure to determine area versus pressure slope near atmospheric (zero) pressure, with slopes comparing well to published in vivo experimental data for relaxed, anesthetized normal subjects. The results provide insights that help inform further image-based modelling research and new medical device design for oral pressure therapy devices.

Sagittal section views of total and directional local deformation in a 3D CT-based FEA model of a human pharynx. (A) Total Deformation; (B) Lateral Directional Deformation; (C) Anterior-Posterior Directional Deformation; and (D) Vertical Directional Deformation