November 2012

 

REFRACTIVE SURGERY

 

Russian researchers practice two-stage correction of high myopia


by A.I. Myagkikh, Ph.D., E.V. Makurin, E.A. Subbotin, M.A. Samoylova

   
diagram

Figure 1. Profile of radial energy density distribution of the laser beam. On the right, initial forms, on the left, diaphragming on threshold of effective ablation. The red line is the wide Gaussian ray. The green line is the sharp super-Gaussian (n=2) ray. Zone of edge effect is marked "d." The scales are provisional

The main factors that influence the kind of refractive surgery you choose are the initial corneal thickness and probability of complications. The cardinal problem of PRK is late post-op haze occurrence.

Early on, researchers described significant positive differences of cornea rehabilitation after trans-PRK was performed.1 Up to this time, no reasons or explanations were given about the differences. Let's try to examine this situation from the viewpoint of the physical differences of excimer laser influence on the cornea.

Excimer laser radiation (193 nm) creates two general effects: the ablation (cold evaporation) of the cornea layer and energy absorption by the cornea material that was not ablated. Beyond doubt, the second effect is negative. It was associated with raising the cornea temperature during the operation and other problems.2 However, direct measurements of cornea temperature during the operation indicated the absence of any serious problems.3 As the rising of the cornea temperature is the final stage of any energy dissipation and absorption processes, we can make a supposition about another manifestation of "pre-ablation" laser energy absorption by the cornea.

Suggested estimation of the negative radiation impact on the cornea is based only on the fact of ablation threshold existingthere is no cornea ablation if the value of energy density is below this "threshold."

Geometry and edge effects

Looking at the laser beam's cross section from the viewpoint of geometry, one can say that the main active ablation zone is located in the center. The areas of potential energy absorbing are the edges of the beam when energy density changes from rating value to zero. As a result of non-ideal focusing and diaphragming, this change takes a non-zero space, named "d" (Figure 1). This distance must be minimized the ray must have a distinguished boundary. Let us suppose the value of "d" and the low of energy density change are the same for all lasers.

The laser with scanning beam has covered and treated the operation zone by scanning elements, which can have different forms and proportions. Let us imagine the case of ablation of a flat, thin (one-pulse) layer with its square S by excimer laser beam having energy density W and the square of scanning element s. Unsophisticated calculations resulted in fact: the square of negative zone (energy absorption zone) is proportional to √(S/s).

There are three ablation modes of laser vision correction: wide beam (full aperture), half-scanning (scanning elements are spots and sleets), and flying spot. Let us propose the operation zone takes about 30 sq. mm, and typical squares of scanning elements are 2-4 sq. mm for the half-scanning machine and 0.4-1.0 sq. mm for the flying spot machine. That square of absorption zone for the half-scanning machine will be 3-4 times greater than for wide beam. The same parameter for the flying spot machine is 6-10 times.

The total energy impact for layer ablation will be the same for all ablation modes. But the absorbed energy will be nearly a product of effective ablation threshold and square of absorption zone. With estimation of geometry parameter d=0.05 mm and diameter of flying spot 0.5 mm, we have the square of absorption zone for every pulse: π *0.05 * 0.5 = 0.0785 sq. mm. If the effective ablation threshold is about 100 mJ/sq. cm, then the value of energy absorption may reach 78.5 J for every laser pulse. This energy seems microscopic, but the laser pulse width is about 40 ns, and it resulted in up to 2 kW of absorbed pulse power for every pulse of scanning laser. Therefore, we must look for the manifestation of effect of absorption laser energy and its potential biochemical consequences in the form of imprints of strong but very short energy impact on the microscale of the corneal surface. For example, in the form of microscopic coagulation seats in the treated cornea surface layer the thickness is about laser wavelength, i.e., 0.2 m.

Using the wide beam stands apart from another ablation modes (trans-PRK method) because of relative minimality of radiation harm and the fact that "negative absorption zone" is located at the edge of the operation zone, or beyond the optical zone. So in the case of trans-PRK, the geometric factor of laser energy absorption should not exert influence on the regeneration processes in the optical zone. This conclusion finds confirmation in serious post-op increasing corneal thickness without regression of refraction.4 We coupled this positive effect of trans-PRK with maximal physical perfection of the ablation process.

The application of effect of corneal thickness growth

The appearance of individual cornea thickness increasing effect is waiting for its own investigator. As far as we know, this effect occurs after PRK.4,5,6 It is very important that in most cases the post-op regeneration of corneal thickness does not entail the regress of refraction. From the viewpoint of physical optics, it is possible only in the case of uniform growth across the whole operation zone.

There is no doubt about the real benefit of revealing effect. Laser correction of high and very high myopia becomes possible.7 It is attained by breakdown of a correction procedure into two stages, each of which is the safest complete action with a predicted refractive effect. At the first stage, 70-80% of initial myopia was removed. Cornea tissue considerably restored its thickness 8-12 months later, but with lower myopia. Thus, the second stage of the operation in respect to its basic parameters becomes similar to the correction of weak (and moderate) myopia, which is a more simple procedure with a good, predictable result.

This technique is patented in Russia.8 It may be directly used on the wide Gaussian beam excimer laser installations. As an example of efficacy of high myopia correction, we will share some results from our research. For review, 443 eyes were taken, whose initial myopia was over 10 D, and for whom the second stage was performed. The number of eyes with initial myopia from 10-14 D was 371, from 14-18 D was 60, and over 18 D there were 12 eyes. Correction was performed stage by stage: 1. Initially, not more than 80% of initial myopia was removed by trans-PRK, which was about 10 D and determined by the initial thickness of the cornea.

2. Standard treatment with corticosteroids within 2 months was performed.

3. As desired by the patient, eyeglasses or contact lens correction was selected upon expiration of 2 months.

4. In 8-12 months, the second stage of correction was performed similar to the first one. Parameters of repeated action were selected with some deviations in comparison with the first stage in accordance with residual myopia, corneal thickness, and, of course, refractive results after the first stage. In every specific case, in the course of PRK the designated number of impulses on the stroma was strictly maintained.

5. Standard treatment with corticosteroids within 2 months was performed.

The efficacy of correction was determined with the use of an efficiency coefficient, which is a ratio of uncorrected visual acuity after operation to maximum corrected visual acuity before operation.9

The results of two-stage high myopia correction are tabulated in Table 1.

Conclusions

1. The less a square of the scanning element of an excimer laser machine, the more a relative part of laser energy is absorbed by corneal tissue. At the same time, the absorption seats are distributed evenly at all surfaces of the operation zone. It may be useful for the forecast of PRK-like operation results for the different installations.

2. In the case of the minimization of negative effects of energy absorption within the operation zone (trans-PRK method), it becomes apparent that there is considerable effect of post-op increasing corneal thickness without refractive regress.

3. Corneal thickness growth is weakly dependent on corrected sphero-equivalent value and the patient's age. The array of corneal thickness growth has a considerable positive trend versus time of examination and negative tendency, versus initial corneal thickness.

4. The time history of corneal thickness growth is realized by weakly increasing early in the post-op period, growth intensifying to 6 months post-op, going top and stabilization at 1 year, and more post-op trans-PRK.

5. The patented method of the two-stage correction of myopia with the use of technology trans-PRK on the installation "Profile 500" is safe and enables us to reach high functional results in initial myopia over 10 D. The trans-PRK application permits us to declare the practical absence of restrictions on initial stage of corrected myopia, including in cases of thin cornea.

References

1. Alexander I. Myagkikh, Ph.D., Eugene V. Makurin, Eugene A. Subbotin. Characteristics of trans-PRK performed by the Profile 500 laser. EyeWorld, June 2012. eyeworld.org/article-characteristics-of-trans-prk-performed-by-the-profile-500-laser.

2. Maldonado-Codina, Carole, Morgan, Philip B., Efron, Nathan. Thermal consequences of photorefractive keratectomy. Cornea (2001), 20(5), pg. 509-515.

3. Duryagina M.N., Chuprov A.D., Zamyrov A.A. et al. The temperature dynamics of the cornea during PRK laser ablation. An Actual Ophthalmology Problem: IV Russian Young Researcher Conference. Collected Science Articles, 2009. www.eyepress.ru/article.aspx?6010.

4. Myagkikh A.I., Makurin E.V., Subbotin E.A., Myagkikh M.A. Organ-Preservation of Eye Cornea in Myopia Correction with Trans-PRK Method. Glaz, 2012, № 3 (85), pg. 34-37.

5. James J. Salz, Perry S. Binder. Is There a "Magic Number" to Reduce the Risk of Ectasia after Laser In Situ Keratomileusis and Photorefractive Keratectomy? American Journal of Ophthalmology, August 2007, 144(2), pg. 284-285.

6. Kasparova E.A. Diagnostics and treatment of precocious keratoconus. Glaz, 2001, № 2, pg. 35-38.

7. Myagkikh A.I., Subbotin E.A., Makurin E.V. High and Extra-High Myopia Correction. Glaz, 2008, № 4 (85), pg. 17-18.

8. RU Patent № 2402306. Priority of invention 13.04.2009.

9. A.I. Myagkikh, Ph.D. A new way to determine refractive operations efficacy. EyeWorld, September 2012. eyeworld.org/article-a-new-way-to-determine-refractive-operations-efficacy.

Editors' note: The authors have no financial interests related to this article.

Contact information

ostoptik@mail.ru

myopia correction table
Table 1. Parameter "corneal thickness" in this table is described in format: "arithmetical mean" plus/minus "standard deviation" for general data array

Russian researchers practice two-stage correction of high myopia Russian researchers practice two-stage correction of high myopia
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