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Editors Selection IGR 15-2

Basic science: Model ONH

Crawford Downs
Ian Sigal

Comment by Crawford Downs & Ian Sigal on:

14671 Mathematical modeling of the biomechanics of the lamina cribrosa under elevated intraocular pressures, Newson T; El Sheikh A, Journal of Biomechanical Engineering, 2006; 128: 496-504


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Studies of ONH biomechanics are essential for understanding the role of intraocular pressure (IOP) in the development and progression of glaucoma. Since glaucomatous axonal degeneration has been shown to occur at the level of the lamina cribrosa (LC), early studies of ONH biomechanics generally focused on the mechanical effects of IOP on the LC. Recent numerical (e.g., finite element analyses) and mathematical studies have indicated that the mechanical interaction between the LC and the peripapillary sclera are also important, because the mechanical behavior of the sclera determines the boundary conditions imposed upon the LC.

Newson and El-Sheikh (1019) use circular thin plate and membrane theories, coupled with an idealized finite element model, to develop mathematical models of the LC that they use to study the influence of boundary conditions on LC biomechanics. They conclude that the peripapillary sclera exerts considerable influence on LC biomechanics, which confirms the results of previous studies.1-4 Their prediction that LC strain decreases as IOP increases is counterintuitive, and they found that a clamped LC boundary (no peripapillary scleral rotation) resulted in a better match with their finite element model. Neither of these results agree with the studies referenced above, which is likely due to artifacts induced by the numerous simplifying assumptions required by a mathematical approach. Their models did not consider curvature of the anterior LC surface, scleral canal eccentricity, or regional variations in either LC thickness or material properties, which other investigators have suggested are important contributors to LC biomechanics. The authors' intention of developing closed-form mathematical models is laudable, but because they did not perform parametric studies of factors relevant to LC biomechanics, they fail to leverage the method's true potential.

References

  1. Bellezza AJ, Hart RT, Burgoyne CF. The optic nerve head as a biomechanical structure: initial finite element modeling. Invest Opthalmol Vis Sci 2000; 41: 2991-3000.
  2. Edwards ME, Good TA. Use of a mathematical model to estimate stress and strain during elevated pressure induced lamina cribrosa deformation. Curr Eye Res 2001; 23: 215-225.
  3. Sigal IA, Flanagan JG, Tertinegg I, Ethier CR. Finite element modeling of optic nerve head biomechanics. Invest Opthalmol Vis Sci 2004; 45: 4378-4387.
  4. Sigal IA, Flanagan JG, Ethier CR. Factors influencing optic nerve head biomechanics. Invest Opthalmol Vis Sci 2005; 46: 4189-4199.

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