Effect of Corneal, Scleral and  Lamina Cribrosa Elasticity, and  Intraocular Pressure on Optic  Nerve Damages

Match Wai Lun Ko

Effect of Corneal, Scleral and Lamina Cribrosa Elasticity, and Intraocular Pressure on Optic Nerve Damages

Research Article | Open Access | Volume 3 | Issue 1

Article DOI : https://doi.org/10.47739/2333-6447/1024

Match Wai Lun Ko^1*

^1. Department of Mechanical Engineering, School of Engineering, Nazarbayev University, Kazakhstan

+ Show More - Show Less

Corresponding Authors

Match W. L. Ko, Department of Mechanical Engineering, School of Engineering, Nazarbayev University, 53 Kabanbay Batyr Ave. Astana, 010000, Kazakhstan, Tel: 7-7172-709190

Citation

Ko MWL (2015) Effect of Corneal, Scleral and Lamina Cribrosa Elasticity, and Intraocular Pressure on Optic Nerve Damages. JSM Ophthalmol 3(1): 1024

Abstract

Objectives: The aim of this study is to characterize the effect of corneal, scleral and lamina cribrosa (LC) elasticity, and intraocular pressure (IOP) on the optic nerve damages.

Methods: The effect of corneal, scleral and LC elasticity, and IOP on the shear stress in lamina cribrosa were modeled using computational finite element analysis. The nonlinear biomechanical properties of the corneal, scleral and LC were adapted from the literature and were varied to examine the effects of tissue biomechanical properties on the LC. The IOP were varied from 10 to 50 mmHg.

Results and discussion: The results showed that the shear stresses in LC increase with the corneal (Ec ), scleral (Es ) and LC (ELC) modulus, and IOP. Vision loss is determined by the Classical Tresca shear failure criterion. Nerve is classified as damage and vision is classified as lost when the shear stress in an optic nerve fiber along the thickness direction exceeds the Tresca critical damage shear stress. The results showed that nerve damage increases with the elastic moduli and IOP. The percentage nerve damage at 25 mmHg showed a logarithmic increase from 12.5% to 45.7% with the corneal elastic modulus increase from 0.17 MPa to 1.43MPa.The eyes with higher ocular elastic moduli (Ec , Es and ELC) are more sensitive to nerve damages from elevated IOP. The higher magnitude of ocular elastic moduli themselves does not introduce optic nerve damage but it would amplify the effect of IOP induced nerve damages by amplifying the shear stress in the LC.

Conclusion: The finding of optic nerve damage increases with ocular elastic moduli implies that, the clinical general screening guidance for glaucoma development risk assessment based on the IOP is not enough. The parameter of ocular elasticity should also include in the diagnosis stage. The corneal elasticity can be clinically assessed by various available medical devices like, Ocular Response Analyzer (ORA) and Corneal Visualization with Scheimpflug Technology (Corvis ST), while the clinical measurement of scleral and LC elasticity are currently impossible due to the lack of instruments. The corneal elasticity may be used as an independent parameter or to form a combined parameter with IOP, for the risk assessment of glaucoma development and progression.

INTRODUCTION

The eyes are one of our most important sensory organs and the eyes are specialized for the conversion of light into electrochemical signals. Ocular biomechanics play an important role in a significant number of ophthalmic pathologies. Corneal biomechanics is closely related to keratoconus [1] and glaucoma [2], scleral and lamina cribrosa (LC) biomechanics are a contributor to myopia [3] and glaucoma [4]. Material elasticity is a measure of how a material deforms in response to an external stress and literature regarding ex vivo study has shown that the ocular tissue exhibits non linear stress strain behavior such that the elastic angent modulus increases with increasing stress/intraocular pressure [5-11]. The stress strain curve for a nonlinear elastic material is nonlinear and tangent modulus, a measure of the instantaneous rate of change at a specific stress on the nonlinear stress strain curve, should be used instead of the Young’s modulus as a property descriptor of the nonlinear elastic behavior over different stresses (pressures) [12]. The nonlinear behavior of ocular tissue to stress requires a nonlinear structural model that can capture the stiffening induced by the stress at high strains. Optic neuropathy in glaucoma can cause visual field loss and blindness [13,14]. Intraocular pressure (IOP) is one of the major risk factor of glaucoma and used as a screening parameter for diagnosis in clinics. High IOP can cause glaucomatous damage to the optic nerves in the LC [15-18]. From experiments on human cadaver eyes, it is found that elevated IOP can lead to bowing of the LC and the axonal bundles of the optic nerve in the LC are sheared and disrupted under high IOP [15]. From the literature, computational models have been used to understand the biomechanical linkage between IOP and glaucoma [19-24]. Out of a number of structural and geometrical parameters selected, the scleral stiffness is identified to have the greatest influence on the optic nerve head (ONH) and the strains in the LC [20]. These results indicated that the biomechanical property of the sclera is a crucial factor that can affect the LC and that may be a reason that glaucoma patients were found to have higher ocular rigidity [25]. However, all the study on the biomechanical linkage between IOP and glaucoma focus on the sclera and ONH, none of them consider the influence of the corneal biomechanics and geometries into the modeling and analysis. Johnson et al. shows that the corneal elastic properties may serve a buffering mechanism to the micro volumetric change in the eye from the IOP [2]. The greater the corneal elasticity, the more protection to the eye from IOP surges. The greater elasticity may provide a larger buffer to sustain the fluctuation and evaluation of IOP [2]. Central corneal thickness, one of the corneal geometrical factors, has been shown to play a role in the interpretation of IOP and has also been suggested as a glaucoma risk factor [26]. Even though the cornea is far always from the primary site of glaucoma, lamina cribrosa (LC) of the sclera, the corneal biomechanics is still contributed to the ocular rigidity as a whole to protect the eye from the momentary IOP surges. However, a detail understanding of the change in corneal biomechanics to the risk of glaucoma development and progression remains to be determined. In this work, finite element modeling is used to quantify IOP-induced shear stresses within the LC. The aim of this study is to characterize the effect between the corneal, scleral and LC elasticity, and intraocular pressure on the optic nerve damages.

MATERIALS AND METHODS

Finite element (FE) modeling method was used to examine the influence of corneal, scleral and lamina cribrosa elasticity on the shear stress in LC. A 3-dimensional eyeball model (Figure 1) was built in a computer aided design (CAD) software (Solid works 2010, Das sault Systèmes Solid works Corp., MA, USA), and the model was then transferred to a FEA software (ANSYS 12.0.1, ANSYS, Inc., PA, USA) for computation. The model was adapted from Leung et al. [19] and the core structural dimensions of the model were adapted from Signal’s study [20,21]. The thickness of the scleral, retina ,LC and pia mater, internal radius of globe, LC anterior surface central deflection and the dimensions and shape of pre laminar neural tissue were adapted from Signal’s study [20]. The adipose tissue was assumed to be axisymmetric about the central axis of the LC and was set to cover 140 degrees of the globe. The thickness and elastic modulus of the adipose tissues was set to 4.6 mm and 0.047 MPa [27-29]. The corneal geometries were set to mean population value of 550μm (central corneal thickness) [30], 7.8 mm (corneal radius of curvature) [30,31] and 11 mm (corneal diameter) [32], respectively. The optic disc diameter and cup to disc ratio were set to 1.8 mm and 0.45, respectively [33]. The blind spot is around 15 degrees nasally from the fovea [34], so the angle between the central axis of cornea and that of the ONH region was set to 165 degrees. The coverage angle of pre laminar neural tissue was set to 80 degrees [20]. The elastic modulus of pia mater, pre and post laminar neural tissue were set to 3 MPa, 0.03 MPa and 0.03 MPa [20], respectively. The Poisson’s ratio of all ocular tissues was set to 0.49 since it is practically incompressible [20,35].The ocular geometries and biomaterial properties used in the FE model were shown in (Table 1).

The IOP exerted on the inner surface of pre laminar neural tissue and the sclera by the vitreous body, and the inner surface of cornea by the aqueous humor were simulated by applying normal pressure loads onto the inner surfaces of eye. The IOP was varied from 10 to 50 mmHg with 5 mmHg interval. Coarse meshing of the structure was auto generated by the FEA software. The mesh in the ONH region was manually refined until the outputs have <0.5% differences even when the mesh density was doubled. The numerical accuracy was comparable to Leung’s and Signal’s study[19,20]. The nonlinear biomechanical properties of the corneal, scleral and LC were adapted from Woo’s study [7] (Figure 2). To examine the effects of tissue biomechanical properties on the LC, the corneal, scleral and LC elastic moduli were varied under different IOPs (Table 2). The corneal elastic modulus (Ec ) was varied from 0.17 to 1.43 MPa, the scleral elastic modulus (Es ) was varied from 1.84 to 3.72 MPa and the elastic modulus of the lamina cribrosa (ELC) was varied from 0.12 to 0.67MPa, respectively.

RESULTS AND DISCUSSION

The distribution of shear stresses in LC from at 25mmHg is shown in (Figure 3). Glaucomatous vision loss from the periphery and progresses toward the center [34,36] and the optic nerve damage are shown to be sheared [15,37]. The distribution of shear stresses in LC from the computational model showed that the local shear stresses were highest at the peripheral anterior surface and lowest in the central anterior surface as shown in (Figure 3), which is consistent with the literature [19,34,36] and the shear stress based model is able to describe the general observation of glaucomatous vision loss

The magnitude of shear stresses in LC was increased with the elastic modulus of ocular tissue as shown in (Figure 3). Since the corneal, scleral and LC elasticity varied together as shown in (Table 1), corneal elastic modulus was taken to represent the variation of the set of ocular elastic modulus. The effect of corneal tangent modulus on the maximum shear stresses in LC is shown in (Figure 4). The plots showed that the maximum shear stresses in LC (τmax) increase with the increase of corneal elastic modulus and IOP. The τmax increases with corneal elastic modulus even at the same IOP and τmax is more sensitive to the change in IOP than the change in corneal elastic modulus. The maximum shear stress in LC is not only dependent on IOP, but also the corneal elastic modulus. Vision loss is determined by classical Tresca shear failure criterion (maximum shear stress failure criterion) [19, 38-41]. Experimental evident showed that the optic nerves were damaged by shear [15] and the Tresca shear failure criterion was shown to be capable to reasonably predict the nerve damage behavior. The vision of a particular optic nerve is lost when the corresponding optic nerve is damaged. Nerve is classified as damaged and vision is classified as lost when the shear stress in an optic nerve fiber along the thickness direction (τa ) exceeds the Tresca critical damage shear stress (τc ), i.e., τa ≥τc . The maximum shear stresses along the thickness direction in the LC (τmax) were determined from the model. The nerve damage criterion is implemented as follow,

Ocular hypertension refers to a situation that the IOP is greater than 21 mmHg and no optic nerve damage and vision loss are present. The nerve damage in normal eye is negligible statistically, so a τc of 0.0035 MPa is taken so that the τmax (0.0032 MPa) in normal eyes (Ec = 0.17 MPa, Es = 1.84 MPa and ELC = 0.12 MPa) under IOP of 20mmHg does not exceed τc. Using the criterion, the optic nerve damages for eyes under different ocular elasticity configurations and IOPs were estimated and are shown in (Figure 5). The results showed that the nerve damage increases with the corneal tangent modulus and IOP. The nerve damage increases with Ec even at the same IOP. Although the nerve damage is more sensitive to the change in IOP than the change in Ec , the percentage nerve damage at 25 mmHg IOP increased logarithmically from 12.5% to 45.7% with the corneal tangent modulus increased from 0.17 MPa to 1.43MPa.The eyes with higher ocular elastic moduli (Ec , Es and ELC) are more sensitive to nerve damages from elevated IOP. The higher ocular moduli itself does not introduce optic nerve damage but it could amplify the effect of the IOP induced nerve damages by amplifying the shear stresses in LC. The ocular elasticity is known to be stiffened with age [42-46] and that’s mean the older population with stiffened ocular elasticity would be more susceptible to vision loss due to the amplified shear stresses in LC. Elderly are expected to have higher risk to glaucoma development and progression and this population should be monitored more frequently than people with normal ocular elasticity. The susceptibility of an eye to glaucomatous damage is affected not only by the IOP, but also the ocular biomechanical properties. In order to identify the people at high risk of glaucoma development and progression, clinical diagnostic methods can be developed based on the characterization of the ocular tissue elasticity in vivo. Since the sclera and LC are embedded in the eye and there is no available method for the in vivo characterization of the scleral and LC elasticity, the characterization and monitoring of the corneal elastic modulus is the only solution. Several in vivo corneal measurements were developed, including the opto mechanical testing device [47], mechanical indentation testing device [8,48], air jet indentation testing device with optical coherence tomography [49,50], high frequency ultrasound elasticity microscope [51,52], novel scanning acoustic microscopy [53], confocal Brillouin microscopy [54], Ocular Response Analyzer (ORA; Reichert, Corp., Buffalo, NY) [55] and Corneal Visualization with Scheimpflug Technology (Corvis ST; Oculus Inc., Wetzlar, Germany) [56,57]. With the utilization of available corneal elasticity measurement methodologies, further clinical studies can then be performed to examine the potential use of corneal elasticity in glaucoma risk assessment and disease treatment management. The current study agreed with the current understanding that the optic nerve head biomechanics and optic nerve damage are strongly dependent on the scleral biomechanical properties [4,20-24]. The current study used a whole eye model to simulate the LC behavior under different ocular tissue elasticity and IOPs, added the linkage between the corneal biomechanics and the optic nerve damage and proposed the corneal elasticity as an indirect measurement of glaucoma risk. In the current study, we considered only the biomechanical properties variation of the ocular tissue to the optic nerve damage but ignore the other glaucomatous optic neuropathy. The ocular structure was also greatly simplified in order to simplify the modeling and analysis. In the current ocular model, only core load bearing structure was considered. The ocular tissue properties are generally viscoelastic and the ocular response are time dependent. However, in the current model, we followed the literature using elastic modeling approach for analysis [21,22]. Refinement of the model can be incorporated with ocular tissue data from ex vivo animal and clinical human data, and nonlinear viscoelastic modeling.

Table 1: Summary of the ocular geometries and biomaterial properties used in the eyeball FE model.

Parameters	Unit	Value	References
Internal radius of the globe	mm	12.0	[20]
Scleral thickness of the globe	mm	0.8	[20]
Scleral thickness closed to	mm	0.4	[20]
LC central thickness	mm	0.3	[20]
Retinal thickness	mm	0.2	[20]
Pia mater thickness	mm	0.06	[20]
Central corneal thickness	mm	550	[30]
Corneal radius of curvature	mm	7.8	[30, 31]
Corneal diameter	mm	11	[32]
LC anterior surface diameter	mm	1.9	[20]
LC anterior surface central deflection	mm	0.10	[20]
Cup depth	mm	0.33	[20]
Peripapillary rim height	mm	0.3	[20]
Optical disc diameter	mm	1.8	[33]
Optic nerve angle	degree	80	[20]
Canal wall angle to the horizontal	degree	60	[20]
Cup to disc ratio	----	0.45	[33]
Poisson ratio of all materials	----	0.49	[20, 35]
Elastic modulus of adipose tissue	MPa	0.047	[27-29]
Elastic modulus of pia mater	MPa	3	[20]
Elastic modulus of prelaminar neural tissue	MPa	0.03	[20]
Elastic modulus of postlaminar neural tissue	MPa	0.03	[20]

Table 2: The configuration set of corneal, scleral and LC elastic modulus.

	Elastic modulus (MPa)
Set	Cornea	Sclera	LC
1	0.17	1.84	0.12
2	0.31	2.05	0.21
3	0.45	2.26	0.27
4	0.59	2.47	0.32
5	0.73	2.68	0.38
6	0.87	2.88	0.44
7	1.01	3.09	0.50
8	1.15	3.30	0.55
9	1.29	3.51	0.61
10	1.43	3.72	0.67

CONCLUSION

This finding implies that, the clinical general screening guidance for the risk assessment of glaucoma based on the IOP is not enough. The parameter of corneal, scleral and LC elasticity should include also in the diagnosis stage. The corneal elasticity can be clinically assessed by various commercially available medical devices, while the clinical measurement of scleral and LC elasticity are currently impossible due to the lack of instruments. The corneal elasticity maybe used as an independent parameter or to form a combined parameter with IOP for the risk assessment of glaucoma development and progression.