2.6 Laser In Situ Keratomileusis (LASIK)

The LASIK procedure, being today's most promising refractive surgical technique, is discussed and compared with other refractive techniques.

2.6.1 The Development of LASIK

L. Buratto, Italy and I.G. Pallikaris, Greece, first realised the theoretical advantages in joining the Seiler's PRK with Barraquer's intra stromal keratomileusis: combining the precision of the excimer laser with intra stromal keratomileusis leaving the epithelium and the Bowman's layer untouched.

The procedure was very similar to freeze keratomileusis, but instead of freezing the 300 µm lamellar tissue and turning it on a lathe, they applied Laser treatment from the stromal side. The results of this treatment were very good especially for high myopes of up to -30 D. Pallikaris first applied the LASIK procedure in 1989, he was capping the eye with a microkeratome of about 140µm and applying laser treatment on the stromal bed directly. However, his manual microkeratome was not easy to handle for daily operations. Barraquer's former student L. Ruiz, Colombia and G.O. Waring, US, could overcome this technical problem by using their Automated Corneal Shaper, a microkeratome, originated from the Barraquer microkeratome, constructed for Ruiz's procedure of ectasie and the procedure of in situ keratomileusis. As in ectasie the thickness of the cut is of utmost importance, using this microkeratome for only capping the cornea to allow laser ablation intra stromal, the microkeratome has been rather "abused" for a much easier job than it was originally designed for.

Many western ophthalmologists think that LASIK is a totally new procedure, developed after superficial PRK had been invented. LASIK as intra stromal PRK. However, they forget that intra stromal freeze keratomileusis has been successful for over thirty years. LASIK is keratomileusis with the means of today's laser technology. Conceptually it builds onto a long existing experience. The microkeratome has been improved over this period of time and is nothing but a new invention. Often mentioned difficulties in the application of the microkeratome are rather due to lacking experience of using this device than that LASIK is in the investigational phase. On the contrary superficial PRK has only been done for seven years, and there is no long term experience in respect to the destruction of Bowman's layer. Intrastromal excisions however, have had no side effects for over 30 years.
 

2.6.2 Description

The following description of Laser In Situ Keratomileusis will be conceptual and limited to the surgical process, for detailed and overall process see the sixth chapter.

The surgery is ambulant and with topical anaesthesia. Correcting one eye the patient will not spend more than five minutes in the operation room with an experienced surgeon. After usual surgical preparations, the automated microkeratome is applied, producing a circular segment of 160µm thickness and a diameter of about 8 mm. This corneal flap is moved to the nasal side there the flap remains attached to the cornea. The eye is now centered and laser ablation will be started (fig.4). The flap is moved back without stitches. The patient is strongly advised not to touch or rub his eye. After three months the flap can hardly be removed.

2.6.3 Conceptual Advantages to former techniques

Why should the easy superficial PRK procedure be replaced by a more complex procedure as LASIK? Answering this question is not trivial and in the author's point of view, its benefit is rather for the patient than for ophthalmologists or laser manufactures. The in part higher complexity and the necessity of sterile operation conditions induces centralisation of this medical service. Instead of many local, superficial PRK ophthalmologists like today's contact lens specialists, there will be instead few LASIK centres, less surgeons and less lasers sold. Evaluating the pros' and cons' of LASIK one must take these economic factors into account.

Of the many developed surgical techniques, until LASIK, only two survived a bigger market introduction: RK and superficial PRK. RK was successfully practised during the late seventies until the early 90's and PRK since the early 90's. There have been many discussions, as to whether PRK or RK brought better results. However, both procedures have a similar limited diopter range of application: myopic patients of up to -7 diopters. Above six diopters RK seems too dangerous and PRK too unpredictable. Theoretically speaking, the superficial PRK suffers from the effect of the Bowman's bark function, resulting in haze, pain and slow recuperation of vision. RK suffers from the indirect applied effect of changing of cornea's biostatics, its refractive effect is difficult to control and difficult to calculate without long manual surgical experience.

Somehow, cultural beliefs and the countries' infrastructure decided whether PRK or RK was preferred. For instance, in Germany, -low risk taking, and technocratic- the ophthalmologist community denied Refractive Corneal Surgery, especially the manual RK procedure. However, with PRK, interest is growing. Germany's ophthalmologists' associations in this field assume the word laser rather than the word refractive in its name, limiting themselves to one technology. In America, Refractive Corneal Surgery has been supported, and RK was tolerated, often even accepted, due to the overall good results and quick recuperation of vision. Their association, and representative journals stick to the name Refractive Corneal Surgery.

Replacing one technique by another there must be significant advantages to justify the friction of change. Limitations of RK and PRK are summarised in the following diagram, not for criticism (both have been milestones) but to demonstrate the LASIK progress.

As already mentioned the idea of LASIK is combining the 30 year experience and the advantages of freeze keratomileusis with the precision and ease of laser ablation. Former experience denied any destruction of the Bowman´s membrane without the occurrence of strong side-effects. With the superficial PRK, side-effects of Bowman's membrane destruction were surprisingly low and for low myopia even acceptable. However, it still seems to be of great advantage to avoid the destruction of the Bowman's membrane due to undesired effects of its barkfunction and the difficulty in controlling its refractive influence due to its form shaping corset function [2.2.2]

2.6.4 Results of various LASIK studies

Since the beginning, LASIK expected best results, especially for high myopes. However, formal results seemed to be rare and PRK protagonists started being sceptical about the assumed better results. Nevertheless, during the last year a couple of studies were published. What they all have in common: 1. results above 6 D. are much better than in superficial PRK, (even with a 6 mm zone), 2. instant results, 3. no haze, 4. no pain, 5. no significant change in best corrected vision. [BF93, PDS94, FT95, Kno95, RS95, GM96, HSB+96]

Variation in outcome in the superficial PRK procedure strongly depends on the laser, the diameter of the treated zone and the patient selection. Results in LASIK seem most influenced by the surgeon's experience and skills. Conceptual ablation software for LASIK should be different than that for superficial PRK. Ablation is closer to the retina and the tissue is slightly different. In practice however, these effects seem to be insignificant or even compensate one another. It even looks as if the original spherical ablation software works better for LASIK than for superficial PRK procedure. However, newer designed ablation software for superficial PRK, which does not strictly follow the thickness law, will most probably result in overcorrection when using them for LASIK.

Better software and integrated training programs can shorten the learning curve. However, the necessary OP condition will always be an infrastructural disadvantage.

Although all the theoretical effects, have so far not been proven, the results obtained are excellent. The following diagram will summarise LASIK's characteristics.