Zona de influencia del proyecto

The cornea is the principal image-forming component of the eye so has the same

properties as an optical lens i.e. a smooth surface, geometric curvature, transparency and a refractive index different from the surrounding media (air/tears anteriorly; aqueous posteriorly). The refractive indices of air (1.000), tears (1.336), cornea (1.376), and aqueous (1.336) are well known as is the central keratometric cornea radius (7.0 – 8.8mm). Thus for optometric purposes, the total power of the air, tear, cornea, aqueous system can be expressed as a single dioptric power of +38 to +48 dioptres (2/3 of the refracting power of the eye).

Corneal visible light transmission (Figure 2.5) is essentially that of water (aqueous) and ranges from approximately 98% at 700nm to 80% at 400nm (Farrell, McCally et al. 1973). Ultraviolet <310nm is strongly absorbed by the stroma, but there are additional peaks of transmission in the infra red that are not present for water/aqueous humour.

a b

Figure 2.5 Light transmittance of (a) cornea, (b) aqueous humour

from (Boettner and Wolter 1962)

Light transmission through the corneal stroma requires minimal absorption and minimal scatter. Light absorbent structures such as pigment or blood are normally absent from the cornea. Light scatter results from local fluctuations in refractive index which is the difference in refractive index of the stromal collagen fibrils (n = 1.41) and cells

(keratocytes), and the optically homogenous (Hart and Farrell 1969) ground substance (n = 1.36) (Leonard and Meek 1997). Keratocytes are scarce, flattened in the corneal plane and have cytoplasm that contains specific proteins (crystallins) which matched it to the refractive index to the surrounding matrix (Jester 2008). The cornea would be opaque if each collagen fibril acted as an independent scatterer. Maurice (Maurice 1957) hypothesised that fibrils did not act in this way but that each fibril formed part of

an hexagonal crystalline lattice with spacings of less that λ/2 for visible light. In this environment only the zero-order Bragg condition is satisfied so scattered waves interfere destructively in all directions except that of the incident light resulting in transparency. The crystalline lattice theory has been questioned as short-range order extends only to about 120nm (Sayers, Koch et al. 1982). Subsequent theoretical models do not require perfect order for transparency (Cox, Farrell et al. 1970; Benedek 1971). An additional model (Twerskyt 1975) in which fibrils were composites of an inner core with an outer coat that matched the interfibrillar matrix is supported experimentally (Fratzl and Daxer 1993). More recently the theory of photonic band structure and photonic crystals (Vukusic and Sambles 2003) have been applied to models of corneal and scleral light transmission (Ameen, Bishop et al. 1998). Whilst these models require a high degree of organization, they are thought to be relevant to the explanation of corneal transparency (Meek 2009). All theoretical explanations of corneal transparency have assumed monochromatic light and as yet cannot explain the corneal light

transmission curve.

Although the mechanisms of corneal transparency remains incompletely understood, the necessary conditions for minimal stromal scatter are uniformly small diameter of its refractile (n = 1.41) collagen fibrils (c. 30nm) which are closely spaced (c. 55nm) in an optically homogenous matrix (n = 1.36). This contrasts with the sclera which comprises less orderly arranged collagen fibrils with diameters ranging from 25nm to 480nm.

2.4.1 Birefringence

Birefringence (§ 3.4.3, §15.3.2) is the ability of some transparent materials to

light ray transmitted through a birefringent material is therefore subject to two

orthogonal refractive indices depending on the atomic/molecular/ structural symmetry of the material in the particular direction of transmission.

The types of birefringence are:

1) Crystalline (intrinsic): birefringence resulting from asymmetries of molecular binding forces within a crystalline material.

2) Form: birefringence resulting from the assembly of parallel and uniformly thin cylindrical fibrils embedded in an homogenous ground substance of different refractive index (Bour 1991; Born and Wolf 2005). Form birefringence therefore arises from symmetries/asymmetries at a supramolecular level and is independent of intrinsic crystalline birefringence.

3) Induced: molecular/structural alignment with resulting birefringence may be induced in both isotropic and anisotropic materials by externally applied forces. Birefringence may be generated or altered in elastic anisotropic materials (elastic birefringence), or in isotropic materials (stress-induced or stress birefringence) by mechanical force. Birefringence may also be induced by electric (Pockels effect) and magnetic (Faraday effect) forces. A particular variant of electric- field induced birefringence is the alignment of liquid crystal molecules now ubiquitously used, together with sheet polarizers, in electronic displays. Stress- induced birefringence (‘photoelasticity’) is used in engineering analysis.

The cornea is birefringent and this has clinical and structural implications. The physiological significance of corneal birefringence, if any, is unclear. The conditions necessary for corneal birefringence are those for corneal transparency with additional

structural/molecular constraints particularly of the stroma. Other ocular structures are also birefringent although the cornea is the principle birefringent, and hence retarding, element (Bour 1991;Blokland and Verhelst 1987; Bueno and Jaronski 2001). Within the retina, the nerve fibre layer (Blokland 1985) and Henle’s layer of the macula (Brink and van Blokland 1988) are significantly birefringent and act as intraocular retarders. The crystalline lens (Weale 1979; Brink 1991; Bueno and Cambell 2001), tears, aqueous and vitreous do not contribute significantly to the total ocular retardation.

Total corneal birefringence is the sum of form and intrinsic components (Maurice 1957; Maurice 1984) throughout the thickness (500 – 600um) of the multilaminar corneal stroma. Form birefringence contributes 2/3 and intrinsic (collagen fibril) birefringence contributes 1/3 of total corneal birefringence. The structural basis for corneal

birefringence is closely linked with that of corneal transparency so any theory of one must be compatible with the other.

Birefringence manifests itself as optical retardation (§3.2, Eq. 3.1). The central cornea behaves as a simple retarder with retardance (see §15.3.2) ranging from between 0 and 0.25 λ (Naylor and Stanworth 1954; Bone 1980; Shute 1974). A study of 73 subjects reported 80% of retardations between 40 and 140nm (0.03λ – 0.12λ) with slow axis orientated 10° – 20° nasally downward (Knighton and Huang 2002). Other studies (Weinreb, Bowd et al. 2002; Zhou and Weinreb 2002) at near infra-red wavelengths (e.g. 780 nm) show similar orientations of the slow axis, and retardances ranging from 0.01 to 0.16 λ (median 0.05 λ). There is no significant difference between adults and

children (central corneal retardation 10 – 77nm: slow axis -11° to 71° nasally downwards) (Irsch and Shah 2012).

Central cornea retardation is interpreted as representing a preferred orientation of collagen within central corneal regions (Shute 1974). Birefringence away from the central cornea has a pattern thought to be analogous to some crystalline materials and will be described in detail in Chapter. 4 et seq.

Some authors have assumed that corneal birefringence is due to intrinsic mechanical forces (e.g. Mountford 1982; Ichihashi, Khin et al. 1995; Volkov, Malyshev et al. 1990). Whilst birefringence may be induced in the cornea by externally applied mechanical forces (Nyquist 1968; Misson and Stevens 1990), stress birefringence (strictly speaking, this is elastic birefringence) is of negligible importance in the normal cornea as are the other forms of induced birefringence (Maurice 1957).

As with many authors before him, Maurice hypothesised that birefringence might be a useful tool in determining the structural and biomechanical properties of the cornea (Maurice 1988): a hypothesis that will be explored in this study.