title: Verification of a virtual fields method to extract the mechanical properties of human optic nerve head tissues in vivo.
abstract: We aimed to verify a custom virtual fields method (VFM) to estimate the patient-specific biomechanical properties of human optic nerve head (ONH) tissues, given their full-field deformations induced by intraocular pressure (IOP). To verify the accuracy of VFM, we first generated 'artificial' ONH displacements from predetermined (known) ONH tissue biomechanical properties using finite element analysis. Using such deformations, if we are able to match back the known biomechanical properties, it would indicate that our VFM technique is accurate. The peripapillary sclera was assumed anisotropic hyperelastic, while all other ONH tissues were considered isotropic. The simulated ONH displacements were fed into the VFM algorithm to extract back the biomechanical properties. The robustness of VFM was also tested against rigid body motions and noise added to the simulated displacements. Then, the computational speed of VFM was compared to that of a gold-standard stiffness measurement method (inverse finite element method or IFEM). Finally, as proof of principle, VFM was applied to IOP-induced ONH deformation data (obtained from one subject's eye imaged with OCT), and the biomechanical properties of the prelamina and lamina cribrosa (LC) were extracted. From given ONH displacements, VFM successfully matched back the biomechanical properties of ONH tissues with high accuracy and efficiency. For all parameters, the percentage errors were less than 0.05%. Our method was insensitive to rigid body motions and was also able to recover the material parameters in the presence of noise. VFM was also found 125 times faster than the gold-standard IFEM. Finally, the estimated shear modulus for the prelamina and the LC of the studied subject's eye were 33.7 and 63.5 kPa, respectively. VFM may be capable of measuring the biomechanical properties of ONH tissues with high speed and accuracy. It has potential in identifying patient-specific ONH biomechanical properties in the clinic if combined with optical coherence tomography.
authors.affiliation: NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore, Singapore.|Ophthalmic Engineering and Innovation Laboratory, Department of Biomedical Engineering, National University of Singapore, 4 Engineering Drive 3, Block E4 #04-08, Singapore, 117583, Singapore.
authors.forename: Liang
authors.initials: L
authors.lastname: Zhang
authors.affiliation: Singapore Eye Research Institute, Singapore National Eye Centre, Singapore, Singapore.
authors.forename: Sri Gowtham
authors.initials: SG
authors.lastname: Thakku
authors.affiliation: Ophthalmic Engineering and Innovation Laboratory, Department of Biomedical Engineering, National University of Singapore, 4 Engineering Drive 3, Block E4 #04-08, Singapore, 117583, Singapore.
authors.forename: Meghna R
authors.initials: MR
authors.lastname: Beotra
authors.affiliation: Singapore Eye Research Institute, Singapore National Eye Centre, Singapore, Singapore.|Eye-ACP, Office of Clinical, Academic and Faculty Affairs, Duke-NUS Medical School, Singapore, Singapore.
authors.forename: Mani
authors.initials: M
authors.lastname: Baskaran
authors.affiliation: Singapore Eye Research Institute, Singapore National Eye Centre, Singapore, Singapore.|Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore.
authors.forename: Tin
authors.initials: T
authors.lastname: Aung
authors.affiliation: Tissue Repair Laboratory, Department of Biomedical Engineering, National University of Singapore, Singapore, Singapore.
authors.forename: James C H
authors.initials: JCH
authors.lastname: Goh
authors.affiliation: Singapore Eye Research Institute, Singapore National Eye Centre, Singapore, Singapore.|NIHR Biomedical Research Centre, Moorfields Eye Hospital NHS Foundation Trust and UCL Institute of Ophthalmology, London, UK.|Discipline of Clinical Ophthalmology and Eye Health, University of Sydney, Sydney, NSW, Australia.
authors.forename: Nicholas G
authors.initials: NG
authors.lastname: Strouthidis
authors.affiliation: Ophthalmic Engineering and Innovation Laboratory, Department of Biomedical Engineering, National University of Singapore, 4 Engineering Drive 3, Block E4 #04-08, Singapore, 117583, Singapore. mgirard@nus.edu.sg.|Singapore Eye Research Institute, Singapore National Eye Centre, Singapore, Singapore. mgirard@nus.edu.sg.
authors.forename: Michael J A
authors.initials: MJA
authors.lastname: Girard
mesh_terms: D001696:Biomechanical Phenomena; D001703:Biophysics / Q000379:methods*; D020342:Finite Element Analysis; D006801:Humans; D007429:Intraocular Pressure; D008954:Models, Biological; D009898:Optic Disk / Q000473:pathology / Q000502:physiology*; D013314:Stress, Mechanical
keywords: Glaucoma; Intraocular pressure; Inverse finite element method; Ocular biomechanics; Optic nerve head; Virtual fields method