Topography Guided (T-cat) Excimer Laser Treatment

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Causes and types of optical defect

Optical defects of the eye are mainly caused by abnormalities in the cornea, the lens, or in the overall length of the eyeball.

When the cornea is not spherical in shape there will typically be an astigmatic optical defect in the eye. Most astigmatism is regular and symmetrical, and this type of astigmatism can be corrected by spectacles, toric contact lenses, or conventional astigmatic laser surgery. However, sometimes corneal astigmatism is irregular and asymmetrical, and this type of optical defect cannot be corrected with spectacles. Irregular astigmatism is typically seen in the condition of keratoconus, and is also often present in variable degree following corneal graft surgery.

Correction of irregular astigmatism

Because irregular astigmatism is almost always due to abnormalities in the shape of the corneal surface, it can usually be corrected by fitting rigid gas-permeable contact lenses. These work by masking the corneal irregularity behind the smooth surface of the contact lens. Alternatively these corneal irregularities can now be permanently corrected by custom excimer laser treatment.

Accurate measurement of corneal surface irregularities can be made with a corneal topography device, and this information can be used to programme an excimer laser to re-profile the cornea.

For many patients who require topography-guided treatment, the amount of irregular astigmatism is the predominant factor in their overall optical defect. In this situation if one attempts to measure the overall optical state of the eye by wave front aberrometry, the result may be unreliable since the deviations in the optical path may be out of the range that the instrument can measure. In addition, when patients undergo topography-guided laser treatment, correction of the irregular astigmatism may result in a substantial change in the corneal shape, which in turn is likely to have a significant effect on the conventional optical power of the cornea and hence the overall optical state of the eye. For both these aforementioned reasons, it is generally not possible to correct both the irregular astigmatism and the overall optical defect of the eye in a single treatment, and the treatment is best broken down into parts. The initial topography-guided treatment uses only the surface topography data to achieve the most symmetrically shaped corneal profile, and then the whole eye is measured again to determine its regular spherical and astigmatic optical components, which can be treated by further laser (if there is enough corneal thickness), or by lens implant surgery.

Treatment of eyes after penetrating or lamellar grafts

Although it is possible to do surface laser ablation (e.g. epi-LASIK / LASEK / PRK) after corneal grafting, it is more usual to do LASIK, with a microkeratome or femtosecond cut flap. This is because the healing is fast er after LASIK and there is less post-operative inflammation. This is an important consideration in patients who have had penetrating grafts or lamellar grafts with living tissue, as further surgery in these eyes carries a small risk of precipitating graft rejection. However, in eyes which have had lamellar grafting with freeze-dried tissue there is no risk of rejection.

For all cases that have had previous grafting, the creation of a flap for LASIK will often result in some change in the pattern of astigmatism, due to release of irregular tension in the graft wound. For this reason it is often preferable to create the flap as a separate operation initially, and then measure the topography a few weeks later, before actually proceeding to the laser treatment.

Treatment of eyes with keratoconus (that have not had grafts)

In the past it has been felt that it was inappropriate to treat corneas with keratoconus by excimer laser, as the laser treatment results in further thinning of the cornea, and possible destabilisation of the corneal structure, which could lead to progressive deterioration of the corneal shape.

The recent introduction of Corneal Collagen Cross-linking with Riboflavin (C3R) and UV light has changed this situation, since it is now possible to stabilise the keratoconus condition with the C3R treatment, and prevent progression of the corneal ectasia. As a result it is now considered possible to use a limited amount of surface ablation to improve corneal symmetry in early cases of keratoconus. Typically this treatment is a topography-guided surface ablation designed to improve the central corneal symmetry without attempting to correct other spherical or regular astigmatic optical defects. The C3R treatment may be applied before, at the same time, or after the laser treatment, depending on factors such as the corneal thickness and degree of ectasia. Any residual spherical or regular astigmatic optical defect can then be treated by contact lens wear or by phakic intra-ocular lens implantation.