Classification of Techniques in Refractive Surgery

2.4.2 Classification Following the Intention of Change

Barraquer classified the surgical techniques into subtraction, addition, substitution, relaxation, retraction, compression and mixed techniques [Bar89]. This reflects a very systematic approach, independent of existing procedures. However, over and above this useful classification all techniques can be divided according whether their first intentions is to change the biostatics of the cornea or to directly change the shape of the cornea. This additional classification feature seems very useful in the author's point of view, as it reveals the theoretical limitations of the techniques. Any technique which primarily changes the biostatics of the cornea will lower the stability qualities of the eye. The dioptric value of the cornea will be less stable, resulting in daily changes, refractive changes in relation to height or long term refractive shifts. The lower stability quality of the cornea can even have more serious effects, for instance, accidents involving the eye will have greater consequences. Any technique following the thickness law, directly changes the shape of the cornea. If the biostatics of the eyes are not changed due to excess excision of tissue, the stability of the cornea will not be changed. The following diagram shows the classification. At the first stage, techniques are separated by either following the thickness law or not. At the second stage, the classification of Barraquer is applied. Compression has been not been included in the diagram, as it can be seen as a kind of virtual subtraction. Mixed techniques has also been left out, as the classification is used to demonstrate the basic ideas of changing refraction. At the third stage, techniques are separated by more detailed differences. Subtraction is differed into intrastromal and superficial keratomileusis according to the destruction of the Bowman's membrane, techniques of relaxation are divided into tension release, keratotomy and ectasia. Most common of the relaxation techniques are the keratotomies, ectasie has been used to correct hyperopia (Ruiz). As earlier mentioned, new theories of tension release in respect to the Bowman's membrane seem very promising. On the most detailed level, known techniques of each class are listed. Unfortunately, many techniques intended to be of single effect are of multiple effect for their limited instrumentation or because of using them above their limits. Some techniques even change the shape of the cornea in a different way as originally calculated.

Paths and frames of techniques, either not fully understood or not handled well, are marked with little lines. The only paths marked in the diagram are of the keratomileusis techniques, as firstly these keratomileusis techniques follow the law of thickness and secondly, subtraction of tissue is somewhat better handled than addition or replacement.