Characteristics of trans-PRK performed by the Profile 500 laser
by Alexander I. Myagkikh, Ph.D., Eugene V. Makurin, and Eugene A. Subbotin
Figure 1. Trans-PRK haze
Figure 4. Trans-PRK value of correction: Sph 19.0, pre-op corrected distance visual acuity was 0.1. The uncorrected visual acuity 8 months later was 0.2. The patient was a 34-year-old female
Figure 6. Trans-PRK CDVA before surgery
Figure 7. Trans-PRK total result
Table 3. Characteristics of classic PRK and trans-PRK on the Profile 500 Source (all): Alexander I. Myagkikh, Ph.D., Eugene V. Makurin, Eugene A. Subbotin
Approach in Russia reveals another way to perform PRK
In 2000 at the VII Ophthalmologists' Conference of Russia, the fifth-generation Profile 500 excimer laser was introduced. Developed in Russia, Profile 500 uses the technique (also developed in Russia) of photorefractive keratectomy (PRK), which does not require pre-op de-epithelialization of the cornea.1 The impressive results with the Profile 500 (The Center for Physics Instrument-Making in the General Physics Institute, Russian Science Academy, Troitsk, Moscow Region, Russia) were reported in some scientific papers but were not widely discussed. This was likely because complications were not immediately seen after operation of the Profile 500. In addition, after the introduction of the Profile 500 into medical practice, no decision was made about the registration of the official name (trans-PRK) of this variety of PRK. More confusion ensued when results conducted by different researchers were not immediately comparable. We have found many biases against the use of PRK have no solid grounds. To accurately discuss and understand differences in PRK performance technology, the authors propose the following definition of the subject: Trans-PRK involves a working element of action on the cornea that exclusively involves an impulse-modulated excimer laser ray, which has a width equal to the width of the operation zone and regulated Gaussian radial distribution of energy density in the cross-section of the ray.
A physical pattern of action of the Profile 500 on the cornea has been reported elsewhere.2,3 The first clinical observations of some hundred operated eyes also are described elsewhere.4 In this article, we will report on how the new physical principles forming the basis of trans-PRK are expressed in surgical results. These results reflect data from more than 6,000 operations performed in Vladivostok, Russia, by surgeons at Ost-Optic K Co. Ltd., who used the Profile 500 installation.
With trans-PRK, there is no need for the preliminary stage of epithelium removal. The time of full epithelialization with trans-PRK is 1.5-2 days without using any additional means to protect the corneal epithelium. This takes about twice as long with PRK with mechanical or chemical removal of the epithelium. With trans-PRK, blepharospasm is observed for no more than 2 days. Because the patient experiences only moderate pain, it is safe to perform the operation on both eyes in the same day. As there is no mechanical damage to the epithelium, there is a very even margin of operation field that minimizes the probability of the occurrence of a mixed corneal syndrome. This is aided by smoothness of the treated surface of the cornea, which is first, a consequence of the absence of mechanical action on the epithelium and Bowman's membrane, and second, the action from a laser ray does not worsen the properties of the corneal surface because the ray has an equal distribution of energy density. The same factors reduce the risk for the development of superficial corneal haze (notwithstanding haze that arises in 36.8% cases that then resolves without any treatment). In other cases, resolution occurs after a course of therapy is prescribed (Figure 1).
The point of using a wide ray is that every laser pulse contains full information on treatment, and a final refractive effect is determined only by the number of pulses. This brings to the forefront the possibility of performing the operation in stages. A feature of trans-PRK is that the standard diameter of the working zone is 6-8 mm (and more, when necessary). A transitional zone of operation field to the optical visual zone is smooth, which excludes halos and glare. With post-op keratotopograms (Figures 2 and 3), one can vividly see the smoothness of a transitional zone and how the conditions for maintenance and increase of a functional optical zone are created. We would like to specifically note the absence of "central islands." One more consequence of the use of the wide ray is that there is no need for an eye tracking system or any eye-fixing mechanisms. The number of impulses is counted in the hundreds, and the influence of casual minor eye movements (like saccades) for the time of the operation is averaged up to 0.
Gaussian radial distribution of energy density
Corneal stromal ablation provides a refractive effect with the thickness of a removed layer that is proportional to the logarithm of laser ray energy density.5 Thus, every laser impulse that has a Gaussian distribution of energy density removes the "ideal lens" from the surfaceideal from the optical standpoint because its surfaces are paraboloids of rotation. The initial surface of the cornea has such a form (if there is astigmatism stretched over a weak axis). This means that the action from trans-PRK should not cause considerable distortions in the eye, such as high-order aberrations.6 A consequence of this is "natural, normal" vision, even at night and at dusk. A choice of the parameter of width of Gaussian distribution enables the ability to correct both very weak and super high myopia in a single step.
Choosing energy density
It is possible to reach monofocal and multifocal reprofiling on the cornea.1,2 In the event of a definite choice of energy density in the center of the laser ray, the obtained optical profile is non-homogeneous in optical force. This phenomenon is vividly demonstrated by a keratotopogram (Figure 4).
A bright distinctive feature of the obtained optical profile of the cornea is the availability of physiological asphericity (i.e., monotonous growth of myopia toward the periphery of the optical zone). Such growth of myopia reduces a jump of optical force on the border of the ablation zone and ensures a smooth transitional zone. In addition, the patients with a similar optical profile should experience so-called "pseudoaccommodation," which enables them to see better at various distances. Once again, we underline that the effect of multifocal reprofiling is "automatically" achieved (i.e., the ray makes no special actions and mechanical manipulations, just a relevant value of energy density chosen for the laser).
Since 2004, Ost-Optic K Co. Ltd. has used an optimized version of the excimer laser on the cornea as well as a corrected plan of rehabilitation treatment after performing trans-PRK. This enabled the company to considerably increase the number of patients whose vision could be corrected. This included patients with a low initial corrected visual acuity. The total distribution of pre-op myopia for trans-PRK is shown in Figure 5. For example, the number of operated eyes with an initial corrected visual acuity less than 0.5 with the value of initial myopia over 12 D (Figure 6) exceeds 50%. To correctly and vividly determine efficiency (productivity), we can compare post-op uncorrected distance visual acuity (UDVA) against pre-op corrected distance visual acuity (CDVA) as their mathematical ratio of efficiency coefficient Keff.7 Figure 7 shows the efficiency of trans-PRK on a general basis of operation (depending on initial myopia) at Ost-Optic K Co. Ltd. at 1 year post-op.
To correctly evaluate trans-PRK results based on a safety criterion,8 we excluded the following types of patients from the analysis: where the operation was planned to be performed in two stages but the second stage was not performed for various reasons undercorrection was planned according to age;
post-surgical follow-up was less than 2 months; and maximum visual acuity with correction before surgery was less than 0.5. In view of the above we see the following observations as shown in Table 2. A corresponding graph is shown in Figure 8.
The main features and differences of trans-PRK on the Profile 500 versus classic PRK are listed in Table 3.
The basic advantages of the trans-PRK with the use of the Profile 500 in comparison with classic PRK are vividly demonstrated. The large number of surgeries performed (more than 6,000) show the efficiency and safety of trans-PRK. These results also demonstrate the efficiency of laser correction for high and super high myopia.
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2. Kachalina GF. Surgical Ttechnology of transepithelial PRK for myopia with the "Profile-500" eximer laser. Abstract of MD Dissertation. 2000;25.
3. Semyonov AD, Doga AV, Kachalina GF, et al. Photoastigmatic refractive keratectomy with the "Profile-500" for correction of compound myopic astigmatism. Ophthalmosurgery. 2000;4:3-8.
4. Semyonov AD, Doga AV, Kachalina GF, et al. Specific features of clinical course in photoastigmatic refractive keratectomy with the "Profile-500" at different terms postoperatively. Ophthalmosurgery. 2001;1:3-7.
5. Arba-Mosquera S, Hollerbach T. Ablation resolution in laser corneal refractive surgery: The dual fluence concept of the AMARIS Platform. . Advances in Optical Technologies. 2010;1.
6. Kodo O, Kachalina G, et al. Superficial PRK. The Report Theses VIII Russian Ophthalmology Congress. 2005;262-263.
7. Myagkikh AI. Method of determination of refractive operations efficacy. Fyodorov's collected science articles. 2002;246-248.
8. Standardized graphs and terms for refractive surgery results. JRS. 2011;27(1).
Editors' note: The authors have no financial interests related to this article.