Journal of Refractive Surgery

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Myopic Keratomileusis: Short-Term Results

Carmen Barraquer, MD; Angela M Gutierrez, MD; Alfredo Espinosa

Abstract

ABSTRACT: The first 97 eyes operated with keratomileusis by two surgeons starting to use this technique ore evaluated. The 30- and 90-day postoperative results of this initial experience are reported. Average preoperative myopia was -10.53 D spherical equivalent (range -5.25 to -22.25 D); average refractive correction at 90 days was -10.20 D. Preoperative visual acuities without correction were between 0.01 and 0.15 Snellen scale (20/2000 to 20/133), and 90 days postoperatively they varied between 0.02 and 0.60 Snellen scale (20/1000 to 20/33) with an average of 0.29 (20/69). Average preoperative visual acuity with correction was 0.70 Snellen scale (20/29), and 90 days afterward it was 0.60 Snellen scale (20/33). Preoperative average refractive cylinder was 1.33 D and postoperative was 1.86 D, showing that significant amounts of astigmatism were not induced. [Refractive and Corneal Surgery. 1989; 5:307-313].

Abstract

ABSTRACT: The first 97 eyes operated with keratomileusis by two surgeons starting to use this technique ore evaluated. The 30- and 90-day postoperative results of this initial experience are reported. Average preoperative myopia was -10.53 D spherical equivalent (range -5.25 to -22.25 D); average refractive correction at 90 days was -10.20 D. Preoperative visual acuities without correction were between 0.01 and 0.15 Snellen scale (20/2000 to 20/133), and 90 days postoperatively they varied between 0.02 and 0.60 Snellen scale (20/1000 to 20/33) with an average of 0.29 (20/69). Average preoperative visual acuity with correction was 0.70 Snellen scale (20/29), and 90 days afterward it was 0.60 Snellen scale (20/33). Preoperative average refractive cylinder was 1.33 D and postoperative was 1.86 D, showing that significant amounts of astigmatism were not induced. [Refractive and Corneal Surgery. 1989; 5:307-313].

Keratomileusis is a procedure to correct medium and high myopia, designed by Jose Ignacio Barraquer who published his original results in 1964.1 The purpose of the technique is to modify the anterior surface of the cornea through freeze-carving a resection of a positive corneal lenticule, resulting in a negative lamellar lenticule which, replaced on the cornea, flattens its curvature thus reducing the total refractive power.1,2

MATERIALS AND METHODS

Ninety-seven eyes were evaluated in a group of 100 eyes operated between 1986 and 1987 with a follow-up of 30 days (97 eyes) and 90 days (60 eyes) using the surgical technique described by the author with its latest modifications.36 Three cases were excluded, where the procedure could not be carried out due to complications mentioned below.

The computer program used in this study was the 1987087 program written on the Apple computer by Jose Barraquer and modified on the basis of its results.

The following parameters were taken into account in planning the surgery:

1. Snellen visual acuity with and without correction at 6 meters preoperative and postoperative acuity at 30 and 90 days. The visual gain was analyzed in Snellen lines and decimal recovery, with and without correction compared to preoperative visual acuity.

2. A correlation was sought between initial refractive error and diopters of correction attained, and a linear regression on these two variables was performed. Diopters of correction were calculated as the spherical equivalent in a large number of eyes. In cases of high astigmatism, the sum of sphere and cylinder was used as diopters for correction, in order to obtain a postoperative mixed astigmatism. For all eyes analyzed, the preoperative spherical equivalent refraction was compared with the postoperative at 30 and 90 days.

3. Eyes were divided into three age groups. The change in refraction was correlated with patient age and sex.

4. The corneal radii of curvature were analyzed as follows. Preoperative and postoperative corneal curvature were measured with the Zeiss keratometer and the eyes were divided into two groups: corneas with an average postoperative radius greater and lesser than 7.6 mm. Patient postoperative corneal radius against the ideal radius was calculated by the computer program. Linear regressions were done at 30 and 90 days to compare the change in the radius of corneal curvature (final radius minus initial radius equals delta radius), was analyzed in comparison to the ideal change in radius calculated by the computer program (final radius minus initial radius).

Figure 1: Definition of parameters of the lenticule. EC: the thickness of the central tissue on the lenticule after cryolathing. ED: disc thickness before cryolathing. ZO: optical zone. DD: disc diameter.Figure 2: Recovery of preoperative spectacle corrected visual acuity between 30 and 90 days. Recovery in 7% of preoperative corrected visual acuity shows the change with time after surgery. The recovery has not been completed by 90 days because most of the eyes have not reached the 100% level.TABLE 1

Figure 1: Definition of parameters of the lenticule. EC: the thickness of the central tissue on the lenticule after cryolathing. ED: disc thickness before cryolathing. ZO: optical zone. DD: disc diameter.

Figure 2: Recovery of preoperative spectacle corrected visual acuity between 30 and 90 days. Recovery in 7% of preoperative corrected visual acuity shows the change with time after surgery. The recovery has not been completed by 90 days because most of the eyes have not reached the 100% level.

TABLE 1

5. Other parameters analyzed were: final optical zones (the diameter of resected tissue area where the refractive change takes place); influence of the disc diameter (the diameter of the piano disc of anterior cornea before cryolathing) on the correction obtained; central thickness of the initial disc of tissue (central uncut tissue on the lenticule), program age correction factor, relation between disc thickness vacuum and the plate used for the microkeratome resection (Figure 1).

The statistical analysis used percentages, standard deviation, variance and in some cases linear regression with the LOTUS 1-2-3 program on an IBM-compatible computer.

RESULTS

Table 1 presents a summary of preoperative refractive and functional data and those obtained at 30 and 90 days. Table 2 gives the surgical parameters used. In every analysis we used 97 eyes at 30 days, and 60 eyes at 90 days postoperatively, although some figures present a different number because of overlapping.

Visual Acuity

Average preoperative visual acuity without correction was 0.04 (SD = 0.03) (20/500; SD 20/2000 to 20/285) and with correction 0.70 (SD = 0.25) (20/29; SD 20/21 to 20/40). Postoperative visual acuity at 90 days without correction was 0.29 (SD = 0.17) (20/69; SD 20/166 to 20/43) and with correction 0.60 (SD = 0.20) (20/33; SD 20/25 to 20/50). These patients included some amblyopes and one with Fuchs' dystrophy. The visual acuity without correction improved at 30 and 90 days in all except two eyes (Figure 1). However, the recovery of preoperative best spectacle corrected visual acuity had not been completed by 90 days (Figure 2).

Accuracy of Refractive Correction

The ratio of diopters of correction requested (mean - 11.25 D spherical equivalent) to diopters corrected at 90 days (mean -10.20 D) is 0.81, that is 0.81 D of correction for every diopter requested. The correlation established between the diopters calculated at 12 mm in the program and the correction obtained in spherical equivalent at 90 days was seen to be good, with a correlation coefficient of 0.75 that is significant at any level.

Table

TABLE 2Surgical Parameters for Myopic Keratomileusis in IQU EyesFigure 3: Scattergram shows spherical equivalent refraction (diopters) tor each eye preoperative and at 30 and 90 days. Most eyes show a loss of the early overcorrection between 30 and 90 days.

TABLE 2

Surgical Parameters for Myopic Keratomileusis in IQU Eyes

Figure 3: Scattergram shows spherical equivalent refraction (diopters) tor each eye preoperative and at 30 and 90 days. Most eyes show a loss of the early overcorrection between 30 and 90 days.

Preoperative refractive cylinder was - 1.33 D and the postoperative was - 1.86 D, showing that significant rates of astigmatism are not induced. The refractive results for each eye are presented in Figure 3. A reduction in overcorrection can be noted related to decrease in the lenticule edema due to surgical trauma.

Age

At 90 days, the age group closest to emmetropia was age 31 to 40 and the group showing least correction was patients over 41 (correction 89.9%, Table 3). This could be correlated with an age correction factor, which is 1.2 for the first group and 1.0 for the second. The loss of correction in all age groups between 30 and 90 days resulted from the decrease in lenticule edema.

Table

TABLE 3Average Percent Refractive Correction*Figure 4: Keratometric radius of curvature after myopic keratomileusis at 90 days. Ideal final radius (mm) versus radius obtained (mm) linear regression. Most of the radii attained are below the equivalent line, with a greater difference between the regression line and the equivalent line for greater change.Figure 5: Correlation of change in keratometric radius of curvature and change in spherical equivalent refraction at 90 days postoperative. There is not a direct correlation between the mm of flattening of the radius and the diopters of correction. The case with 23 D obtained has a radius change of 2.15 mm while the case with 20 D has 2.76 mm of change. It does not show overcorrection, only disassociation between keratometric measures and dioptric change.

TABLE 3

Average Percent Refractive Correction*

Figure 4: Keratometric radius of curvature after myopic keratomileusis at 90 days. Ideal final radius (mm) versus radius obtained (mm) linear regression. Most of the radii attained are below the equivalent line, with a greater difference between the regression line and the equivalent line for greater change.

Figure 5: Correlation of change in keratometric radius of curvature and change in spherical equivalent refraction at 90 days postoperative. There is not a direct correlation between the mm of flattening of the radius and the diopters of correction. The case with 23 D obtained has a radius change of 2.15 mm while the case with 20 D has 2.76 mm of change. It does not show overcorrection, only disassociation between keratometric measures and dioptric change.

Sex

No differences in correction were found by sex. At 90 days the average percent correction in males was 101% and in females was 102%.

Keratomefric Raäius of Curvature

Eyes were divided into two groups: corneas with steeper keratometric radii less than 7.6 mm, and corneas with flatter radii equal to or greater than 7.6 mm. In those with radii below 7.6 more undercorrection was observed at 30 days postoperative; for initial radii greater than 7.6, there was a large percentage of overcorrection at 30 days, which decreases progressively by 90 days. These results in myopic keratomileusis run parallel to our findings in radial keratotomy.7

A comparison of the average postoperative radius showed it to be 9.44 mm at 1 month, while at 90 days it was 9.26 mm, losing some of its initial flattening.

A comparison of the final radius of curvature against the ideal radius at 30 and 90 days showed that most of the radii attained are below the equivalent line, with a greater difference between the regression line and the equivalent line, for greater (or flatter) radii. When the ideal radii are smaller (relatively steeper), the radii obtained come closer to them (Figure 4). The lack of precision of commercial keratometers is clearly evident in the correlation between radius obtained versus ideal radius. The greater the corneal flattening, the more inaccurate is the measurement.

Supplementing this finding, the change in the radius of curvature (final radius minus initial radius) was used to indicate the actual change induced by surgery (Figure 5). However, there was a poor correlation between the mm of flattening of the radius and the diopters of refractive correction at 90 days.

We used linear regression to compare the ideal change in radius of curvature with the refractive correction (Figure 6). The correlation was not good for cases of high myopia.

Figure 6: Ideal change in keratometric radius (delta radius) versus refractive correction requested !diopters). This correlation is very close to the equivalent line because it is a theoretical program.Figure 7: Mean spherical equivalent refraction versus diameter of resected disc shows the smaller the diameter, the greater the correction.Figure 8: Central thickness of resected disc versus mean spherical equivalent refractive correction obtained. There was not a direct correlation between the two factors.Figure 9: Diameter of optical zone (ZF) versus spectacle corrected visual acuity. Eyes having final optical zones of 5.5 mm had better visual acuity in the postoperative period than those with smaller optical zones.

Figure 6: Ideal change in keratometric radius (delta radius) versus refractive correction requested !diopters). This correlation is very close to the equivalent line because it is a theoretical program.

Figure 7: Mean spherical equivalent refraction versus diameter of resected disc shows the smaller the diameter, the greater the correction.

Figure 8: Central thickness of resected disc versus mean spherical equivalent refractive correction obtained. There was not a direct correlation between the two factors.

Figure 9: Diameter of optical zone (ZF) versus spectacle corrected visual acuity. Eyes having final optical zones of 5.5 mm had better visual acuity in the postoperative period than those with smaller optical zones.

Surgical Parameters

Disc Diameter

The diameter of the resected disc is determined by the program following a mathematical calculation that correlates diopters of correction to initial radius of curvature. The diameter of the disc is reduced as diopters of correction increases and it is also progressively reduced with steepening initial radius. In the program number 86 to 87, the largest diameter is 7.25 mm.

In our group of patients we used discs of 7.0 mm, 7.1 mm, and 7.25 mm. We correlated the diameter with the amount of correction achieved at 90 days. Discs with a 7.0 mm diameter had the best correction with a spherical equivalent of +1.02 D (Figure 7).

Effect of Age

In order to offset the lower response to refractive surgery in young patients, a correction factor (Fx) for age was introduced in the program based on Jose Barraquer's experience,6 similar to the one used in radial keratotomy programs: O to 30 years, Fx = 1.3; 31 to 40 years, Fx = 1.2; 41 and older; Fx=LO. The group showing the lowest percent correction at 90 days was the youngest (ages 10 to 20), 92.47%. Correction was better for the group between 21 and 30 years of age (112.55%). In general, the age groups showing the best final correction were between the ages of 21 and 40 years (Fx 1.3 and 1.2) (100.04%). Older patients were intentionally undercorrected (88.95%) to offset presbyopia.

Figure 10: Relationship of resected disc thickness to suction ring vacuum pressure with 0.2 mm plate. The higher the negative pressure, the greater the resection thickness.

Figure 10: Relationship of resected disc thickness to suction ring vacuum pressure with 0.2 mm plate. The higher the negative pressure, the greater the resection thickness.

Central Thickness

We evaluated the effect of the remaining central thickness (EC) of the lenticule after the refractive lathing; a thickness between 0.15 and 0.17 mm gave a spherical equivalent of +0.97 D at 90 days. This group had the most correction and its preoperative refractive error was the greatest (average 12.3 D). In the group with the smallest preoperative refractive error of 8.5 D, EC was 0.18 to 0.30 mm and a final refraction of +0.15 D was achieved at 90 days. There was not a direct relation between EC and refractive correction (Figure 8).

Final Optical Zone

The final optical zone (ZF) was divided into two groups: the ideal size of 5.5 mm and less than 5.5 mm. Similar corrections were obtained in the two groups. As a means of attaining more correction, the program reduces the diameter of the final optical zone when disc thickness is inadequate for the correction desired, especially in high myopia. Those eyes having final optical zones of 5.5 mm are shown to coincide with better postoperative corrected visual acuity. There is not a drop in visual acuity between preoperative and postoperative acuities with an optical zone of 5.5 mm; but there is with smaller zones. With smaller optical zones, a minimal decentering of the zone with respect to the visual axis may mean a decrease in visual acuity.

Ratio of Resected Disc Thickness (ED) to Vacuum Pressure

The factors having the most influence on disc thickness achieved with the microkeratome have been described by Barraquer.3 The microkeratome being a high pressure instrument, each surgeon must be familiar with his or her own personal results in order to adjust the variables accordingly. There exists a direct relationship between resection thickness and vacuum pressure. The higher the negative pressure, the greater the thickness. There is also an inverse relationship to the cutting speed of the microkeratome; the greater the speed the less the thickness. Figure 10 shows the instrument's precision with the 0.2 mm plate and the changes in thickness at different vacuum pressure levels in the hands of two different surgeons. In this series the average disc thicknesses were 0.25 mm with 0.2 mm plate, 0.32 mm with 0.25 mm plate, and 0.37 mm with 0.3 mm plate.

Complications

We experienced three complications that prevented completion of the surgical procedure: resection of a disc of insufficient diameter in two eyes and corneal perforation with penetration into the anterior chamber in one eye due to omission in placing the plate. In a few cases, the discs resected were slightly oval and could induce a good amount of astigmatism. Also, some lenticules were found to be off center, producing irregular astigmatism that reduced the recovery of visual acuity.

DISCUSSION

Many previous reports have shown that myopic keratomileusis is a useful technique to correct myopia of over 5 D in individuals of any age.6,813 The results presented by two surgeons in their initial 100 consecutive eyes and short-term follow-up of 97 eyes at 30 days and 60 eyes at 90 days can be compared to those published by other authors with more experience and for similar periods.5,8,11,12

Visual acuity without correction at 90 days was 0.29 (20/69) on the average and with correction, 0.60 (20/33). Postoperative visual recovery was faster in patients having poorer preoperative visual acuity without correction, mainly in cases with high myopia and some degree of amblyopia.6,9 This might be explained by the surgically-induced magnification of the retinal image.

The mean preoperative spherical equivalent was -10.55 D and at 3 months it was +0.19 D. The preoperative cylinder of -1.39 D went to -1.86 D at 90 days, indicating that very little astigmatism was induced by surgery. For each diopter of correction planned, 0.81 D of correction was attained. This figure, although quite close to that requested, makes it necessary to review the different parameters in order to attain more accurate results. Regression analysis of the diopters of correction requested and the correction obtained in spherical equivalent gave a correlation coefficient of 0.75, which is significant and shows that the program used has good predictability.

Corneas having preoperative keratometric radii of curvature flatter than 7.6 mm get a greater correction because of the inaccuracy of conventional keratometers in reading aspheric surfaces. There is no way to establish an exact correlation between millimeters of flattening and refractive change in the patient.

A direct relationship was also found between age and correction attained.6 The optical zones giving the best visual results were those greater than 5.0 mm. Reducing the optical zone results in higher correction but centering is so critical that the visual acuity attained is generally of poorer quality.

There are a number of other parameters analyzed such as disc thickness and its relation to the vacuum pressure used, diameter of resected disc, central thickness, and optical zone, which play a role and influence the final outcome.6

This was a multivariate study with linear analysis. It is important for surgeons practicing this technique to closely follow all indications and heed the recommendations found in reports of other more experienced surgeons. We think these data are close to the results attained in myopia keratomileusis and will improve with long-term follow-up, as has been shown by other authors.5,6

REFERENCES

1. Barraquer JI. Queratomileusis para la correccion de la miopia. Arch Soc Amer Oftal Optom. 1964; 5:27-48.

2. Barraquer JI. Queratomileusis y Queratofaquia. Bogota, Colombia; Litograna Arco: 1980.

3. Barraquer JI. Keratomileusis for myopia and aphakia. Ophthalmology. 1981; 88:701-708.

4. Barraquer JI. Keratomileusis for the correction of myopia. Arch Soc Am Oftal Optom. 1982; 16:221.

5. Barraquer JI. Long term results of myopic keratomileusis-1982. Arch Soc Am Oftal Optom. 1983; 17:137-142.

6. Barraquer JI, Viteri E. Results of myopic keratomileusis. Journal of Refractive Surgery. 1987; 3:98-101.

7. Barraquer JI, Gutierrez AM, Rodriguez E, Espinosa A. Significancia de los parámetros que intervienen en queratotomia radial. Arch SAOO. 1988-1989; 22(l):2l5-233.

8. Maxwell WA, Nordan LT. Myopic keratomileusis: early experience. Journal of Refractive Surgery. 1985; 1:99-103.

9. Nordan LT, Fallor MK Myopic keratomileusis: 74 consecutive non-ambliopic cases with one year follow-up. Journal of Refractive Surgery. 1986; 2:124-128.

10. Polit P. Keratomileusis for miopia: initial experience in Saudi Arabia. Arch SAOO. 1986; 20(4):195-212.

11. Swinger CA, Barraquer JI. Keratophakia and keratomileusis, clinical results. Ophthalmology. 1981; 88:709-715.

12. Swinger CA, Barker BA. Prospective evaluation of myopic keratomileusis. Ophthalmology. 1984; 91:785-792.

13. Tucker DN, Barraquer Jl. Refractive keratoplasty: clinical results in sixty-seven cases. Ann Ophthalmol. 1973; 5:335.

TABLE 2

Surgical Parameters for Myopic Keratomileusis in IQU Eyes

Figure 3: Scattergram shows spherical equivalent refraction (diopters) tor each eye preoperative and at 30 and 90 days. Most eyes show a loss of the early overcorrection between 30 and 90 days.

TABLE 3

Average Percent Refractive Correction*

Figure 4: Keratometric radius of curvature after myopic keratomileusis at 90 days. Ideal final radius (mm) versus radius obtained (mm) linear regression. Most of the radii attained are below the equivalent line, with a greater difference between the regression line and the equivalent line for greater change.

Figure 5: Correlation of change in keratometric radius of curvature and change in spherical equivalent refraction at 90 days postoperative. There is not a direct correlation between the mm of flattening of the radius and the diopters of correction. The case with 23 D obtained has a radius change of 2.15 mm while the case with 20 D has 2.76 mm of change. It does not show overcorrection, only disassociation between keratometric measures and dioptric change.

10.3928/1081-597X-19890901-08

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