Biomechanics of the Eye

Biomechanics is the study of the mechanical interaction of solids and/or fluids with internal and external forces in the context of biology. It has long been a mainstay in the cardiovascular and orthopedic fields, where it has been used to analyze and predict the mechanical and biological mechanisms underlying bone fractures, hard and soft tissue remodeling, and blood flow through arterial stents and aneurysms. Biomechanical techniques are critical in optimizing cardiac and orthopedic implant designs for maximum clinical efficacy and life.

Ocular biomechanics has been primarily focused on diseases of the cornea, trabecular meshwork, sclera, and optic nerve head, with more limited application in the vitreous, lens, and iris. It has provided insight into disease processes and surgical outcomes in various eye disorders. For example, in glaucoma, ocular biomechanics has been used to analyze and predict the importance of the sclera in determining the biomechanics of the lamina cribrosa, the site of axonal damage in glaucoma. In the cornea, the science of biomechanics has been used to better understand the structural basis of corneal ectatic diseases and to develop biomechanically mediated treatments for keratoconus that have already resulted in a global reduction in the number of corneal transplants required for this disease.

Biomechanical engineers use cutting-edge engineering-based computational and experimental techniques to investigate the interaction of ocular tissues with their surroundings, as well as the forces that are common in the eye: intraocular pressure, tensile and torsional muscle tractions, blood flow and vascular pressures, external traumatic forces, cerebrospinal fluid pressure, and tissue growth pressures. The tools bioengineers use include finite element modeling, a computational technique to split complex geometries into small regularly shaped elements, for which loading, mechanical stress (force distribution), and mechanical strain (local deformation) are calculated individually. The results of each of these simple elemental responses are then added up, or superposed, into the overall response of the structure. Measures of tissue deformation under load can now be obtained with imaging techniques, such as ultrasound biomicroscopy, optical coherence tomography, Scheimpflug tomography, infrared corneal reflection monitoring, and magnetic resonance imaging, and these observations can be used to generate various approximations of biomechanical properties and validate computational biomechanics simulations. Many of these measurement technologies are being developed (or are already available) for in vivo and clinical applications.

The structural geometries in the human body are much more complex than typical engineered structures, such as bridges and airplane wings. Biological tissue stiffness is inherently complex in that it changes with orientation (anisotropy), the rate of loading (viscoelasticity), and the level of stretch or compression (hyperelasticity). Computational models require accurate representations of tissue geometry, loading and constraints on the modeled structure, and compliance or stiffness of the tissue constituents. Whereas certain elements of the ocular anatomy such as the cornea are very accessible to measurement, measurements are more difficult to obtain in very small structures and more posterior ocular components. Important factors such as fluid pressures or blood flow are nearly impossible to measure using current technology. When accurate representations of the model inputs are unavailable, simple representative geometries coupled with simplifying assumptions on the loading and tissue material properties can still be used to construct models that reveal fundamental relationships regarding the responses of tissues to load.

The eye boasts one of nature’s most exquisite relationships between structure and function. Ocular function is a complex product of the eye’s constitutive elements, their mechanical properties, and a host of biological processes responsible for normal function, immunological defense, repair, and disease. This book introduces the eye as a biomechanical entity and surveys emerging efforts to apply biomechanical principles to understanding mechanisms of ocular disease, enhancing diagnosis, and optimizing treatment.

J. Crawford Downs, William J. Dupps Jr., Cynthia J. Roberts

Foreword

Introduction

About the editors

List of contributors

Cornea

1. Basics of biomechanics
Jesper Hjortdal

2. Corneal stroma: collagen ultrastructure and orientation in health and disease
Keith M. Meek, Sally Hayes

3. Acoustic radiation force elastic microscopy and corneal structural correlation
Eric Mikula, Donald J. Brown, Moritz Winkler, Elena Koudouna, Tibor Juhasz, James V. Jester

4. Cellular micromechanics of corneal stroma: keratocyte and extracellular matrix interactions
W. Matthew Petroll, Miguel Miron Mendoza

5. The electrochemical basis of corneal hydration, swelling, and transparency
Peter Pinsky, Xi Cheng

6. Material properties of the human cornea: anisotropy
Ashkan Eliasy, Zhou Dong, Harald Studer, Craig Boote, Ahmed Elsheikh

7. Inflation testing of the cornea
Thao D. Nguyen, Jun Liu

Non-destructive corneal biomechanical measurement

8. Optical coherence tomography principles and elastography
Matthew R. Ford, Vinicius De Stefano, William J. Dupps Jr.

9. Optical coherence elastography for ocular biomechanics
Manmohan Singh, Michael D. Twa, Kirill V. Larin

10. Electronic speckle pattern interferometry and lateral shearing interferometry
Abby Wilson, John Marshall

11. Brillouin microscopy
Giuliano Scarcelli, Seok Hyun Yun

12. Deformation response to an air puff: clinical methods
Katie Hallahan, William J. Dupps Jr., Cynthia J. Roberts

13. Factors contributing to air-puff derived corneal responses
Riccardo Vinciguerra, Renato Ambrósio Jr., Simone Donati, Claudio Azzolini, Paolo Vinciguerra

Biomechanics of corneal disease and therapeutics

14. Biomechanics in ectasia detection: ORA and Corvis ST
Renato Ambrósio Jr., Fernando Faria Correia, Bernardo T. Lopes, Rui Carneiro Freitas, Isaac Ramos, Marcella Q. Salomão, Allan Luz

15. Mechanisms of collagen crosslinking and implications on biomechanics
Eberhard Spoerl, David C. Paik

16. Crosslinking kinetics and alternative techniques
Michael Mrochen, Rebecca McQuaid, Nicole Lemanski, Bojan Pajic

17. Computational modeling of corneal refractive surgery, ectasia, and corneal crosslinking
Ibrahim Seven, Vinicius Silbiger De Stefano, William J. Dupps Jr.

18. Comparative biomechanics of intrastromal lenticule extraction and LASIK
Dan Z. Reinstein, Timothy J. Archer, Ibrahim Seven, Cynthia Roberts, William J Dupps Jr.

Trabecular meshwork biomechanics

19. Iris biomechanics
Anup Dev Pant, Rouzbeh Amini

Accommodation, presbyopia, and lens biomechanics

20. Accommodation and presbyopia
Matthew Reilly

21. Biomechanics of the lens and its role in accommodation
Noël M. Ziebarth, Vivian M. Sueiras, Vincent T. Moy, Fabrice Manns, Jean-Marie Parel

Vitreous and vitreoretinal disease

22. Biomechanics of the vitreous humor
Rodolfo Repetto, Jennifer H. Tweedy

Optic nerve head, lamina cribrosa, and sclera

23. Introduction to posterior pole biomechanics
J. Crawford Downs, Vicky Nguyen

24. Intraocular pressure
Daniel C. Turner, Jessica V. Jasien, J. Crawford Downs

25. Collagen anisotropy in scleral mechanics
Neeraj Vij Jr., Jonathan Vande Geest

26. The dynamic response of the corneoscleral shell
Jun Liu, Keyton L. Clayson, Elias R. Pavlatos

27. Scleral remodeling in myopia
Rafael Grytz

28. The connective tissue phenotype of glaucomatous cupping in the monkey eye
Hongli Yang, Juan Reynaud, Howard Lockwood, Galen Williams, Christy Hardin, Luke Reyes, Cheri Stowell, Stuart K. Gardiner, Claude F. Burgoyne

29. Cellular mechanisms of lamina cribrosa remodeling in glaucoma
Reinold K. Goetz, Deborah Wallace, Tabitha Goetz, Colm O’Brien

30. Optic nerve head biomechanics in health, aging, and disease
J. Crawford Downs

31. Cerebrospinal fluid pressure and the translaminar pressure gradient in optic nerve head biomechanics
Julia Raykin, Brian C. Samuels, Andrew J. Feola, C. Ross Ethier

32. Parametric analysis to identify biomechanical risk factors: taking control of population diversity and experiment variability
Andrew P. Voorhees, Yi Hua, Ian A. Sigal

33. In-vivo characterization of optic nerve head biomechanics for improved glaucoma management
Xiaofei Wang, Meghna R. Beotra, Liang Zhang, Michaël J.A. Girard Index