Corneal photoablations by Excimer laser cover 98% of operative indications

Myopia surgery has been performed since the 1980s. Reduction in the dioptric strength of the cornea is the most commonly used correction concept. The arrival of the 193nm excimer laser revolutionised myopia surgery during the nineties and confirmed the superiority of the principle of corneal tissue sculpture by photoablation over former incision techniques in terms of efficiency, precision, stability and safety of results. Very precise correction of astigmatism which is frequently combined with myopia, has taken full advantage of this major technological development. In addition to surface photoablation (PRK which stands for Refractive Photokeratectomy) offered for weak or moderate levels of myopia, we now have, secondarily, the Lasik technique. This consists of sculpting the corneal stroma under a superficial flap, prepared surgically. The advantage of Lasik, which reduces the scarring reaction by respecting the corneal epithelium, is to make the post-operative period much more comfortable and rapid, meaning that bilateral surgery can be performed at the same time, as well as extending indications to stronger cases of myopia. “Laser” techniques now represent 98% of surgical indications, with experience going back over fifteen years.

Clinical observation and rapid, consistent progress in terms of biological knowledge of healing conditions and in complementary exploration and technical surgical platforms, have transformed the selection of indications radically over the past twenty years along with the conditions under which this type of surgery is performed, with results constantly improving.

Indications remain limited for phakic intraocular implants

At the end of the 1980s, the technique of phakic intraocular implants was proposed for correcting strong myopia. Twenty-five years after their introduction into the range of refractive surgery techniques, indications for phakic intraocular implants remain limited to cases where Lasik surgery is contraindicated. The proximity of anatomical relations between the implant and neighbouring tissues quickly proved to be a potential source of complications. The most delicate problem for implants in the anterior chamber (Fig. 1), is that of tolerance by the corneal endothelium, the non-renewable single cell corneal layer that covers the posterior side and controls the hydration and therefore the transparency of the cornea. A gradual reduction of this density, exposing it to the risk of oedema, is always possible, even though this has become much rarer with the latest generation of implants. It requires systematic surveillance using the specular microscope in order to decide on explantation in case of continued cell loss. Implants placed in the posterior chamber are very close to the crystalline and ciliary sulcus diameter measurement is still not very precise, leading to random personalised sizing of implants. Early cataract is possible, mainly by contact. Implantation techniques are still limited to very selective indications, decided on after a complementary anatomical and biometric examination, which should be as precise as possible, subject to being able to carry out systematic, regular clinical and paraclinical surveillance.

Fig. 1: "ARTISAN" anterior chamber phakic intraocular implant. Annual surveillance of endothelial cell density in the cornea using a specular microscope is systematic.

Recent progress in corneal photoablation techniques: SBK and personalised protocols.

Current developments in photoablation techniques mainly concern two aspects which are linked to the safety and quality of the result: biomechanical resistance of the cornea and vision quality.

1- Biomechanical resistance of the cornea

It is essential to control the biomechanical resistance of the cornea in order to prevent complications linked to mechanical sagging of the cornea, which could lead to refractive regression and, above all, to secondary corneal ectasia. Risk factors are excessive corneal thinning and, above all, unknown infra-clinical keratoconus.

Systematic screening for keratoconus (Fig. 2) by means of corneal topography is obligatory. Any suspect curve asymmetry, even very small, is an absolute contraindication. Unknown keratoconus is the main cause of corneal ectasia, which is becoming an increasingly rare complication due to better knowledge of topographical analysis.

Fig. 2: Infra-clinical keratoconus screening using corneal topography is obligatory. It represents an absolute surgical contraindication for the prevention of secondary corneal ectasia.

Recent work [1] has enabled us to identify the fact that there exist within the cornea areas of greater resistance thanks to the particular organisation of collagen fibres located in the anterior third of the corneal stroma. Surgical techniques have therefore evolved to concentrate action at this level, avoiding the posterior stroma which is mechanically weaker. It was already known that surface photoablations (PKR, and its variations, Lasek or Epilasik) entailed less risk of the occurrence of a secondary corneal ectasia. The Lasik technique has therefore developed towards what is now commonly known as “SBK” (Sub Bowman Keratomileusis). The thickness of the corneal flap, which varied from an estimated 160 to 180 microns, is now much thinner, at around 100 microns. The latest generation microkeratomes and above all ultra-brief impulse Femtosecond lasers (Fig. 3) enable highly precise (5 microns) creation of these very thin flaps coupled with a high degree of security.

Fig.. 3: The Femtosecond laser is used to dissect a superficial corneal flap, 100 microns thick, precisely and securely, using the Lasik technique.

Finally, current corneal sculpture software offers ablation profiles that are optimised in the transition zones resulting in up to 20% corneal tissue saving. Pachymetry is therefore a defining clinical parameter in limiting indications. It is measured on corneal elevation topography maps and anterior segment OCT structures can also provide extremely precise complementary measurements.

Exclusion of all suspect forms of keratoconus, sculpting in the stronger anterior corneal stroma, reducing corneal thinning during stroma sculpting and, finally, respecting the posterior stroma, whose theoretical minimal residual thickness should be 250 microns, are the biomechanical security standards prevailing in current photoablation techniques.

2- Vision quality

The refractive precision of surgery means that very high percentages of visual acuity of over 8/10 [2] without correction can be achieved. However, this improvement in acuity can be accompanied by deterioration in the quality of night vision (mesopic contrasts, haloes, glare).

Risk factors are a wide scotopic pupil diameter of over 6mm, a diameter treatment zone less than the scotopic pupil diameter, the induction of high-order optic aberrations (HOA), coma and corneal asphericity, proportional to the degree of corrected myopia.

To limit the occurrence and, above all, the impact on daily life of problems with night vision quality, treatment protocols have evolved to counter identified risk factors. Treatment zones with diameters between 7 and 9mm must always be greater than scotopic pupil diameters, and the optical zone must never be less than 6 mm.

The degree of myopic correction authorised is a function of preoperative keratometry, in order to avoid post-operative keratometry that is too low (be very careful in the indication if this is to be below 32 dioptres). Aspherical ablation profiles aim to reduce the induction of positive aspherical aberrations, which lead to haloes. Customised treatments using aberrometry aim to reduce further the induction of high order aberrations (3 and over), mainly coma and asphericity [2, 3]. Discussions continue as to the clinical efficiency of these treatments. However technical tools for exploration (aberrometers) and treatment (lasers and beam delivery systems, eye-trackers, specific software) combine to offer a major contribution in terms of precision and quality of result. Aberrometers are excellent instruments for the overall measurement of refraction, leading to greater precision in terms of defining the correction formula. Biometric recognition of the iris optimises centring and correction of the astigmatism axis combined with myopia in over 90% of cases (Fig. 4a and 4b).

Fig. 4: Appearance of a wavefront with staggered phase (Figure 4a) before and after (Figure 4b) myopia surgery using a customised protocol on aberro metry (post-operatively the wave front is flat without de-phasing on the treatment zone, which objectivizes refractive correction).  

Can myopic presbyopia be operated?

Presbyopia is not a contraindication but it must be taken into account to preserve a certain degree of near vision capacity, without correction. The best compromise in myopics is monovision. Two conditions are required here. The refraction differential between the two eyes must not exceed 1.50 dioptres. A prior test with contact lenses must verify good binocular functional tolerance of the system. An anisometrope is naturally a good candidate for monovision.

Myopia surgery, satisfaction and quality of life

Media coverage of complaints made by dissatisfied patients in the USA has recently alerted the FDA, which has now implemented a supervisory observation board. However, all the studies and results evaluation surveys show very high satisfaction scores of around 98% in our experience [4]. Quality of life surveys carried out amongst patients confirm the very favourable impact of surgical correction [5]. Sources of dissatisfaction are mainly permanent and handicapping vision disorders, that are usually the consequence of an error made with indication. This can be linked to ocular anatomy, excessive degree of myopia correction in view of pre-operative clinical parameters, failure to take into account specific vision requirements linked to socioprofessional activity, or patient psychology.

Failures, of which there are very few in view of the millions of operations performed, are no condemnation of the surgical techniques. They underline, however, the importance of efficient prevention. Aiming for a target of 100% satisfied patients implies ethical practice, selecting indications using multiple clinical, psychological and occupational criteria and expertise in the control of the whole chain of care and treatment from indication through to management of follow-up, including information, choice of the best protocol and finally perfect utilisation of technical tools.