Transepithelial photorefractive keratectomy

The name transepithelial photorefractive keratectomy is often shortened to transFRK or TLK (transepithelial laser vision correction). Also, the procedure is known as "contactless laser vision correction." In English literature, Trans-PRK ^[2] , T-PRK (Transepithelial Photorefractive Keratectomy) ^[3] , CTEN (Customized Trans-Epithelial No-touch) ^[4] and StreamLight (in Alcon WaveLight EX 500 excimer laser) are widely used

• Absolute painlessness during the operation ^[5] ;
• The ability to perform does not depend on the anatomy of the orbit of the eye ^[6] ;
• The mechanical properties of the eye and cornea remain almost unchanged ^{[7] [8]} (transFRC and PRK are recommended for patients whose work involves a high probability of injury - the technique does not add additional risks even in extreme situations), the epithelial layer is restored predictably ^{[9] [10 ]} ;
• There is practically no risk of keratectasia ^[11] ;
• There is no risk of serious complications, including no risk of heyz ^[12] ;
• Suitable for patients with thin corneas ^[7] ;
• Suitable for patients with a very high degree of myopia (myopia): from −1 to −10 diopters or more (determined by the thickness of the cornea) ^[13] ;
• Suitable for patients with hyperopia from +1 to +6 diopters ^[14] ;
• Suitable for patients with astigmatism up to 5-6 diopters ^[15] ;
• Predictability (accuracy) of the method is within ± 0.25 D ^[16] ;

Advantages of transepithelial PRK compared to PRK

• Neither alcohol solution is needed to remove the epithelium, there is no need for surgical instruments - this is a non-contact technology;
• Compliance with the ablation zone and the erosion zone (it distinguishes transFRC from all types of excimer laser vision correction, which is typical only for single-step transFRC ^[1] );
• Reducing corneal injury ^[17] ;
• The patient experiences less discomfort during and after the procedure ^[18] ;
• transFRC (one-step) is faster than PRK ^[18] ;
• Acceleration of healing ^[19] (after healing, visual acuity of 4-5 lines) and full postoperative recovery up to 2-3 weeks ^[20] .

Disadvantages of transepithelial PRK

• Discomfort after surgery can last up to two days. It is reduced by wearing therapeutic contact lenses;
• Other shortcomings characteristic of two-stage transFRK, such as the need for a large surgeon’s experience to determine the completion of epithelium ablation and the transition to the refractive part of ablation, the above-mentioned complications associated with overheating of the cornea, low predictability due to the lack of consideration for the difference in the epithelium ablation coefficient and stroma and energy loss due to the radius of curvature of the cornea, as well as the risk of drying of the surface of the cornea during the transition between the stages of correction, in a single-stage concept transFRK overcomes ^{[21] [22]} .

The transFRC technique is a development of standard PRK ^[23] using an excimer corneal laser (first monkeys ^[24] and rabbits ^[25] , and then humans) ^[26] . Russian experts declare priority in this matter ^[27] , but scientific publications in authoritative sources, confirming this primacy at the international level, are not presented.

Two-and one-step transFRK approaches

There are two-and one-stage approaches to the transepithelial photorefraction keratectomy. The differences between the approaches are associated with the need to solve two problems: removal of the epithelium in the refractive zone of the cornea and a change in the refraction of the cornea. These tasks are solved in different settings, different software and different doctors in different ways. Historically, the first was a two-stage approach, then, due to a number of shortcomings ^[28], it was replaced by a one-stage one.

TransFRC Two-Step Approach

The method was first described in 1998 ^[29] , first in laboratory tests on rabbits, and then in humans, it was possible to show less pronounced apoptosis of keratocytes using transFRK ^[30] . Technically, the operation could be performed by a laser with a scanning slit or a full-aperture laser ^[31] . In both cases, the two-step transFRC approach consisted of:

• The first stage is phototherapeutic karatectomy (FTC);
• The second stage is photorefractive karatectomy (PRK).

The two-stage approach assumed a consistent overlap of ablation profiles: during the first stage, the cornea area was de-capitalized, and during the second, the refractive part of the operation. Responsibility for the moment of termination of the first stage and the launch of the second fell on the ophthalmosurgeon, who had to visually track the passage of the epithelium by changing fluorescence, turn off one software and run another ^[32] . This gave rise to a number of inevitable approach gaps and errors in the results.

Disadvantages of a two-step transFRK

1. Difficulties in loading and setting up two types of software in one operation and a long duration of operation due to two stages and time spent on reconfiguration,
2. Personally, the surgeon should visually track the moment when the entire epithelium was completely removed and the stroma began,
3. Not taken into account the difference in the thickness of the epithelium in the center and on the periphery of the cornea,
4. The loss of energy from the center of the operation zone to the periphery cannot be taken into account when using a full-aperture laser, therefore, the optical zone is reduced ^[33] .

TransFRC One-Step Approach

An attempt to solve these shortcomings was proposed in the one-stage concept of transFRK with the reverse sequence of imposing ablation profiles. One moment is achieved through software that allows you to simultaneously monitor eye movements in five degrees of freedom ^[34] , monitor the surface temperature of the cornea and prevent it from overheating (Intelligent Thermal Effect Control) ^{[35] [36]} , automatically adjust the flow rate (Automatic Fluence Level Adjustment ^[37] ), take into account the change in the thickness of the epithelial layer in different parts of the cornea (online pachymetry security system) ^[38] .

The last point should be highlighted, since the thickness of the corneal epithelium in its optical center in 70% of people is 55 μm, and 65 μm at the periphery (8 mm from the center of the cornea) ^[39] . These parameters should be taken into account when performing transepithelial photorefractive keratectomy, and earlier the experience of the doctor performing the laser correction was responsible for this accounting. That is, he had to “by eye” fix the passage of the epithelial layer (the nature of the luminescence of the stromal tissue in the illumination rays during laser exposure differs from the luminescence of the epithelium and the bowman membrane), and only in the place of its smallest thickness - in other places the epithelium remained, respectively , on the cornea with the old approach of transFRK, not the whole refraction profile fell.

Changing the sequence of imposition of ablation profiles (first the refractive profile, and then the epithelial one) ensures that the refractive part completely hits the cornea.

These changes became possible only after the invention of the “floating” or “scanning spot” technology, in which a narrow beam of an excimer laser ^[40] acts alternately on different points of the cornea (according to the program).

Modern systems, such as Schwind Cam in installations of the Schwind Amaris 500E type ^[41] , point scan the surface of the cornea ^[37] , carry out the maximum number of measurements of individual parameters of the cornea ^[37] , and not only general indicators, take into account the difference in the ablation coefficient between the epithelium and the cornea and the energy loss of the beam due to the change in the curvature of the cornea from the center to the periphery (the radius of curvature of the cornea). Modern systems require adequate energy compensation ^[37] .

After a complete diagnosis and software analysis, the system managed by the ophthalmologic refractionist in one stage produces exact (resolution - 0.25 μm) ^[37] correction of refraction (lasts a few seconds) ^[37] and then evaporates the epithelial layer above the operating area ^{[ 37]} .

Features of a one-step transFRK make it less dependent on the incorrect determination of the thickness of the epithelial layer. If the epithelial layer is thinner than predicted, then the refractive part of the ablation due to the addition of the epithelial unclaimed becomes somewhat deeper, however, the radius of curvature, the diameter of the ablation and the refractive result do not change ^[42] . If the epithelial layer is thicker than intended, then there is no undercorrection - the correction is complete, but in a slightly reduced diameter of the optical zone. Accordingly, this can be avoided by slightly increasing the planned optical zone with small degrees of myopia, laying into its dimensions a similar error ^[43] .

Benefits of one-step transFRK

1. Only one stage (much faster than two-stage; reduced dehydration of the cornea ^[44] );
2. The program takes into account the change in the thickness of the corneal epithelium from the center to the periphery; ^[one]
3. The removed epithelium is exactly the same diameter as the ablation diameter. In this way, damage to the cornea is reduced; ^[21]
4. Acceleration of healing. ^[22]