Ординатура / Офтальмология / Английские материалы / Keratoconus Surgery and Cross-linking_Pinelli, Leccisotti_2008

Detecting Corneal Ectasia

FIGURES 1-7A AND B: Comparison of the Humphrey map (A) with the Pentacam map (B) in the same eye of a keratoconus patient. With very steep corneas, the slit-based topography is able to track the surface, while Placido mires can merge with the loss of data. The data is compared with the same scales using VolPro software (Sarver and Associates, Carbondale, IL) (Topography exams courtesy of Renato Ambrósio, MD, PhD).

in the keratoconus eyes. Moreover, subsets of corneal ectasia have been shown to produce unique wavefront profiles. Pepose and Applegate demonstrated that patients with pellucid marginal degeneration could be differentiated from keratoconus based on wavefront data.24 The patients with pellucid marginal degeneration were noted to possess higher amounts of peripheral aberrations (especially trefoil), whereas the keratoconus patients tended to show higher degrees of coma as in the Maeda study. An example of wavefront analysis of a keratoconus cornea is shown in

Figure 1-8.

Corneal Hysteresis

To date, the bulk of our biomechanical corneal knowledge arises from the measurement of its geometrical aspects such as topography and pachymetry. When attempting to diagnose and treat a poorly understood corneal progressive thinning disease like keratoconus, any information regarding the biomechanical properties of the cornea would be welcome. The Reichert Ocular Response Analyzer (ORA; Reichert, Buffalo, NY) provides measurement of Corneal Hysteresis (CH) and the Corneal Resistance Factor (CRF) which are the result of viscous damping in the corneal tissue. The ORA utilizes a rapid pulse of air, and an advanced electro-optical system to record the displacement or deflection of the corneal surface before, during, and after the perturbation. The signal obtained during this process is shown in Figure 1-9. The response measured by the ORA is a complex combination of corneal and possibly other ocular biomechanical properties.

Importantly, the ORA provides a repeatable, Goldmann tonometer-correlated intraocular pressure (IOP) measurement. The additional parameters of Corneal Hysteresis and the Corneal Resistance Factor are being studied to determine whether they can be utilized for the detection of keratoconus. Early studies suggest a relationship between CH and the presence of keratoconus, although there is a great deal of overlap between normal corneas and those with keratoconus.25-27 It is hoped that CH

and CRF might be useful as diagnostic tools for determining who might be at risk for developing 9 post-refractive ectasia.

Detecting Corneal Ectasia

It should be noted that interpretation of CH and CRF is confounded by the fact that the entire globe and lamina cribosa are involved in the response to an air pulse delivered to the corneal surface. Hence, the characteristics of the response may not reflect only corneal biomechanics, but include those of other ocular components as well. Further, to date, no study has produced evidence that the early changes in the cornea associated with the forme fruste or mild stages of keratoconus can be detected with this approach.

In conjunction with CH and CRF, the other signal waveform characteristics may also yield clues to the presence of altered biomechanics as occurs in keratoconus. Signal analysis of a normal eye shows great symmetry and height between peak 1 and peak 2 and a smoother, less erratic waveform (Figure 1-9). The signal waveform peaks of a keratoconic eye appears altered and CH and CRF can be lower than normal (Figure 1-10). Current research is focusing on further analysis of the waveform characteristics to see if additional biomechanical properties can be gleaned, particularly from the earliest onset of pathology. Success may yield another test that can be used to assess risk factors associated with refractive surgery.

A B

FIGURES 1-10A AND B: Magellan corneal topography (A) of a keratoconus patient and the scan from the ORA (B). Note the reduction in amplitude of the second peak, and additional ‘noise’ on the raw signal.

Conclusion

Please cite figure 2-2 in the text and clear query on page 17

Keratoconus Surgery and Cross-linking

Introduction

The reduced mechanical stability of the cornea in keratoconus for example may be increased by photooxidative cross-linking of the corneal collagen.

Keratoconus is a corneal degeneration which often leads to a severe visual impairment due to an increasing corneal irregularity associated with a worsening of the optical abilities. The biomechanical characteristics of the cornea result from the collagen scaffold, the collagen compound and their bonding with the collagen fibrils. Even though the total amount of collagen lamellas is not significantly lower in keratoconic corneas7 the stability is lowered by factor 0.7.1 A decreased thickness16 in addition to a modified configuration of the stromal collagen lamellas5, 13, 15 are seen. Furthermore in keratoconic corneas twice as much hydroxyproline can be dissolved by pepsin.1 Hydroxyproline stabilizes in a great amount the collagen’s triplehelix. Moreover enzymatical alterations have been observed with an increased expression of lysosomal and proteolytic enzymes.8, 16, 18, 35 as well as a decreased concentration of protease inhibitors.8, 9 These facts indicate a disturbed cross-linking within or/and among the collagen molecules.

By photooxidative collagen cross-linking with riboflavin and ultraviolet (UV)-light additional covalent bindings between the collagen molecules can be achieved which consequently stabilize the collagen scaffold.

Photooxidative Collagen Cross-linking with Riboflavin and Ultraviolet (UV)-light

With the photooxidative collagen cross-linking with riboflavin and ultraviolet (UV)-light we are able to treat a limited area within a short time.

Riboflavin is a vitamin (vitamin B2), nontoxic and available as a drug. It has two important functions: the absorption of the UV-irradiation and as photosensitizer the generation of reactive oxygen species (singlet oxygen). In combination with UV-light, riboflavin creates free radicals which induce new chemical bonds. A significant increase in mechanical stiffness can be achieved through the cross-linking.

Because of the Riboflavin´s molecular weight (376 g/mol) an intact epithelium would be a diffusion barrier. Therefore only after removal of the epithelium the riboflavin can penetrate the stroma. The diffusion process of 0.1% riboflavin in the stroma can theoretically be described by the time-dependent one-dimensional diffusion equation assuming a diffusion coefficient of D = 6.5 × 107 cm2/s. The intrastromal distribution of riboflavin is shown in Figure 2-1 varying in cornea´s depth and time. After 30 minutes in a 400 µm-thick cornea the concentration at the endothelium reaches a level where the absorption of the UV-light is high enough to protect endothelium and intraocular structures sufficiently according to the Lambert-Beer´s-law. If the stromal thickness is smaller than 400 µm, a hypoosmolalic riboflavin solution has to be used to swell the corneal thickness to 400 µm.

Riboflavin has two absorption maxima: 365 nm and 430 nm. According to the higher energy a greater cross-linking effect can be achieved when irradiated with 365 nm.24 Therefore UVA-light of 370 nm wavelength is used for the irradiation. In this way approximately 90% of the UV-light is absorbed in a 400 µm thick deepithelialized cornea and thus there is no risk for endothelium, lens and retina to be damaged.

From investigations on the biomechanical effects with an irradiance of 3 mW/cm² an optimal 14 irradiation time of 30 minutes was found.2, 20 A significant increase in biomechanical stiffness starts

after 15 minutes. An irradiation longer than 45 minutes shows no further stiffening.

Technique of Collagen Cross-linking with Riboflavin and UVA-light

FIGURE 2-1: Intrastromal distribution of 0.1% Riboflavin in the corneal stroma at different times after riboflavin application at the surface.

Safety for Endothelium and Lens

According to industrial safety guidelines for an unprotected eye a daily UVA-irradiation of 1 mW/ cm2 without a photosensitizer is permitted.23 35% of this are absorbed in the cornea and the endothelium is strained with 0.65 mW/cm2. For a 30 minutes irradiation without photosensitizer in vivo experiments showed a damage threshold for the endothelium of 4 mW/cm2 .30 By cross-linking with riboflavin and UV-light in a 400 µm cornea 94% of the UV-irradiation is absorbed and the endothelium gets only 0.18 mW/cm2.26 This value is below the determined damage threshold30 and below the value from the industrial safety guidelines. Therefore there is no risk for lens and retina.

Another reason for its safety lies in the small distance between the irradiation source and eye and the irradiation´s divergency. Thus no UV-light is focussed on the retina.

Effect and Evidence of Cross-linking

Photochemical induced cross-linking cannot be visualized directly by staining or other microscopical techniques. However cross-linking causes modification of numerous physicochemical qualities of the stromal collagen which are indirect evidences for cross-linking. Below a few of these changes are pointed out.

INCREASE IN STIFFNESS AND BENDING STIFFNESS, INCREASE IN ELASTICITY MODULUS

The cross-linked cornea is stiffer by factor 1.8 than normal cornea. With it the reduced stability in a keratoconic cornea is compensated.10, 24, 29 The stiffening effect is stronger in corneas with a higher amount of collagen respectively in elder corneas.2, 29

RAISING OF SHRINKING TEMPERATURE

The cornea´s shrinking temperature is raised from 63°C to 70°C. The shrinking temperature correlates positively with the degree of cross-linking.27

Keratoconus Surgery and Cross-linking

During the 30 minutes of irradiation, drops of Riboflavin solution have to be applied to the cornea every 4-5 minutes. In this way, the necessary concentration of the riboflavin is sustained and desiccation of the cornea is avoided.

After Treatment

After cross-linking, antibiotic eye ointment and vitamin A eye ointment should be applied. Furthermore, analgesics can be prescribed. The ointments should be applied until complete reepithelialization of the cornea is achieved. If necessary, artificial tears and topical steroids can be prescribed.

After cross-linking the corneal curvature changes further by cellulary procedures. Therefore adjustment of new glasses or contact lenses should not be too soon. Apoptosis of keratocytes in the anterior stroma is seen after cross-linking.4, 11, 31 6 months after treatment no keratocytes are seen in the anterior stroma. Over the course of time new keratocytes move in from the limbus. This process is completed by approximately 4 months after cross-linking. This repopulation mechanism leads to a decrease of corneal curvature by approximately 2 diopters. The immigrated keratocytes respectively myofibroblasts´ contraction forces may be responsible.3, 6, 14, 17, 34

Due to the apoptosis and the repopulation of keratocytes the cornea of nearly all patients shows a slight haze of the anterior stroma within the first weeks. This haze can be seen in about 70% of the cornea´s depth and can be called as demarcation line between the mid and the posterior stroma.21 The haze is certainly the reason for augmented glare. This phenomenon disappears normally 3 to 4 months after cross-linking. Sometimes though it can be seen still years after treatment.

Summary

By increasing the biomechanical stability of the cornea by riboflavin and UVA-induced collagen cross-linking it seems to be possible to stop the progression of keratoconus. Collagen cross-linking is a practical out-patient service, minimally invasive, cost-effective treatment with minimal strain for the persons treated. The knowledge and the observance of the biophysical basics of the photooxidative corneal collagen cross-linking account for a safe application of this therapeutical method.

References

1.Andreassen TT, Simonsen AH, Oxlund H. Biomechanical properties of keratoconus and normal corneas. Exp Eye Res 1980; 31:435-41.

2.Ahearne M, Yang Y, R´Then KY, et al. Non-destructive mechanical chracterization of UVA/riboflavin crosslinked collagen hydrogels. Br J Ophthalmol 2008 (in print).

3.Asaga H, Kikuchi S, Yoshizato K. Collagen gel contraction by firboblasts requires cellular fibronectin but no plasma fibronectin. Exp Cell Res 1991; 193:167-74.

4.Caporossi A, Baiocchi S, Mazzotta C, Traversi C, Caporossi T. Parasurgical therapy for keratoconus by riboflavinultraviolet type A rays induced cross-linking of corneal collagen: preliminary refractive results in an Italian study. J Cataract Refract Surg 2006;32:837-45.

5.Daxer A, Fratzl P. Collagen fibril orientation in the human corneal stroma and its implication in keratokonus. Invest Ophthalmol Vis Sci 1997; 38:121-9.

6.Guidry C, Grinnell F. Contraction of hydrated collagen gels by fibroblasts: evidence for two mechanisms by which