S ince its introduction, refractive surgery with an excimer laser has been a proven method to correct refractive errors.15 In eyes where the cornea has a regular surface, conventional excimer laser photorefractive keratectomy (PRK) or laser in situ keratomileusis (LASIK) can provide good results. If, however, the cornea shows an irregular surface shape, custom-tailored, topography-based ablation, which has been adapted to the corneal irregularity, should provide better results.
This treatment aims at obtaining the best corrected visual acuity that can be attained by wearing hard contact lenses. It requires an excimer laser with spot scanning technology in which a small laser spot- by delivering a multitude of single shots fired in diverse positions- is used to create the desired ablation profile. The laser spot is optionally programmable, thus any profile can be created. A videokeratography system that provides an elevation map at high resolution is necessary, and specific software is required to create the program for the spot scanner from the elevation map and the intended topography of cornea.
We chose PRK as the surgical method because with LASIK, disturbed healing of the flap (eg, plication or other surface irregularities) may be an additional source of error in the assessment of videokeratographic images.
PATIENTS AND METHODS
As a main criterion for inclusion in our study, the best corrected visual acuity obtained with hard contact lenses had to be better than the best spectaclecorrected visual acuity. This was necessary because only then could we determine if topographycontrolled PRK would be superior to the standard treatment; success of topography-controlled PRK could be measured directly.
From December 1997 to April 1998, 10 eyes of 10 patients (5 men, 5 women) were treated. The mean age was 46.50 ± 20.07 years (range, 28 to 96 yr).
Visual Acuity and Manifest Refraction Before and 6 Months After Topography-controlled PRK in 10 Eyes
One eye was treated for irregular astigmatism after penetrating keratoplasty (eye #8); 2 eyes for corneal irregularity after corneal scarring (1 eye due to keratitis in childhood, #3, and 1 eye after epidemic keratoconjunctivitis, #7); 1 eye for irregular astigmatism in an eccentric keratoconus (#1); 4 eyes needed treatment after a previous unsuccessful excimer laser treatment (2 eyes for decentration [#4, #6] and 2 eyes for corneal irregularities [#2, #51) and 2 eyes for asymmetrical idiopathic astigmatism (eyes #9, #10).
All patients were fully informed of the experimental nature of the study.
It was the object of this study to achieve emmetropia with simultaneous improvement in spectacle-corrected visual acuity (Table).
Videokeratography, slit-lamp microscopy, and ophthalmoscopy were performed at all examinations. Uncorrected visual acuity, spectacle-corrected visual acuity, and visual acuity with hard contact lenses were examined preoperatively. Four weeks and 6 months postoperatively, only uncorrected and spectacle-corrected visual acuity were tested.
PRK was performed using the MEL-70 system from Aesculap Méditée. This laser is a spot scanner (1.8 mm spot size, with Gaussian profile). The surgical procedure was identical to the method commonly used in PRK today. Circular abrasion was done mechanically using a hockey knife. The diameter corresponded to the diameter intended for the treatment area and was subject to correction- between 5 and 7 mm for all eyes. Centration was not based on the center of the entrance pupil, but instead on the visual axis because the preoperatively produced videokeratograph was also oriented to the visual axis, and the subsequently performed ablation based on the videokeratograph. It is often complicated to find the real optical axis.6 To do so, we performed the following procedure: The patient was asked to look into a coaxial light (green diode); we marked the resulting corneal light reflex, when seen in the center of the pupil. If the reflex was apart from the center, we marked exactly in between the reflex and the center of the entrance pupil.
Throughout the operation, the eyeball was fixed with a small suction ring attached to the sclera with little negative pressure. An active eye-tracking system oriented to the attached suction ring and not to the patient's pupil monitored the constancy of centration. The suction ring additionally contained an evacuation facility that evacuated any gases generated during ablation to avoid inhomogeneous beam attenuation.
The topography-based excimer laser ablation was based on videokeratographic images taken with the Orbscan ? from Orbtek (Salt Lake City, UT).
Reliable topographic images are of particular importance for topography-controlled laser ablation. These images were always taken by the same person. Five images of every eye were taken; at least three of these images had to be similar. From these images, one was chosen as the "initial master."
Further handling of this topographic information is demonstrated for one patient (eye #9) in Figure 1.
Figure 1. a) Preoperative keratometric map of asymmetric myopic astigmatism (eye #9). Manifest refraction: -5.50 -2.25 x 176°; spectacle-corrected visual acuity, 20/50. Six months after topography-controlled PRK, manifest refraction: +1 .00 D; spectacle-corrected visual acuity, 20/40. b) Postoperative clinically achieved keratometric image, 6 months postoperatively. Homogeneous flattening of the central cornea (blue color), c) Elevation map derived from the keratometric map of Fig 1a. Colors indicate deviations from a perfect spherical surface. Yellowish brown = elevation; Blue = depression; Green = nominal surface, d) Elevation map modified according to the target K-value.The oval yellowish brown-red elevation is the area that represents the corneal area to be ablated by the laser. The diameter of the treatment zone has been chosen by the software, e) Elevation map for a desired treatment diameter of 6 mm. This image was obtained by a radial shift of the target spherical surface in the direction of the optical axis (Z-shifting).f) Elevation map; simulation shows the desired topography after ablation of the yellowish brown élévation. Nominal color in the center is green.
First, a keratometric image (Fig la) was chosen. This has been used in PRK for several years as an essential source of additional information. Here, it shows asymmetric myopic astigmatism. This representation, however, is not very useful for the planned calculation of the ablation pattern. For this calculation, an elevation map (Fig Ic) is required from which the operator can read where and how much tissue shall be ablated. The colors indicate deviations from a perfect spherical surface (yellowish brown = elevation, blue = depression, green = nominal surface). Relatively high areas are not elevations-they are just above the sphere. Relatively low values are not depressions- they are just below the sphere. The software automatically selects the keratometric value of this perfect sphere as a reference surface (best-fit sphere). This image, however, is almost insignificant, as the keratometric value that is optimal for the patient's eye not only results from topography, but also from refraction measurement. In the end, the definition of this target keratometric value is the most important decision the physician must take with this method.
With normal ametropia this value can be obtained as follows: From the keratometric image (Fig la), one must take the average keratometric value from the inner-most 3-mm zone and subtract the intended refractive correction from this value (in case of astigmatism, the spherical equivalent). With larger irregularities or asymmetries of topography, only the refractive measurement with hard contact lenses will deliver a reliable target keratometric value.
In our example image, the keratometric image shows an average keratometric value of 43.25 D. For a spectacle refraction of -5.50 - 2.25 ? 180°, a target keratometric value of 36.75 D results for the elevation map (Fig Id, slightly oval yellowish brown elevation to be ablated by the laser).
Here, however, the diameter of the treatment zone was chosen accidentally by the computer software; in this example it would be too small . The program must perform a so-called Z-shift (radial displacement of the target sphere in the direction of the optical axis until the diameter of the treatment zone meets the request of the physician (here, about 6 mm in Fig Ie).
Software can convert the thus completed image information into a control program for the MEL 70 excimer laser, which will then ablate exactly this central colored elevation to the nominal green color. Now, one can switch over again from this target state in the elevation map (Fig If) to the keratometrie image. Figure lb shows the real postoperative keratometric image that represents the clinical result and thus must be compared with the initial image (Fig la).
Figure 2. Change in lines of spectacle-corrected visual acuity 6 months after topography-controlled PRK. All eyes, except for eye #8. have gained lines of spectacle-corrected visual acuity.
Of the 10 eyes treated, all had 4 weeks and 6 months follow-up. The table shows patient characteristics and preoperative and postoperative measurements.
Six months after PRK there was improvement in uncorrected visual acuity of 2 or more lines in 5 eyes and no change in 5 eyes. Spectacle-corrected visual acuity was improved in 2 eyes by 2 to 3 lines and in 9 eyes by 1 to 3 lines; 1 eye was unchanged (#8) (Fig 2).
Preoperatively, mean sphere was +1.92 D (range, -5.50 to +7.00 D) and mean cylinder was -1.95 D (range, 0 to -6.00 D). Four weeks after surgery, mean sphere was +0.88 D (range, -1.00 to +1.75 D) and mean cylinder was -0.08 D (range, 0 to -6.00 D). Six months postoperatively, mean sphere was +0.57 D (range, -1.00 to +1.75 D) and mean cylinder was -0.30 D (range, 0 to -1.00 D) (Fig 2).
The following eyes in our test series represent examples of the four major indications.
Case 1: Eye #10 (female); asymmetric myopic astigmatism- A 36-year-old woman had asymmetric astigmatism in both eyes. Her manifest refraction in the right eye was -1.00 -6.00 ? 15° and in the left eye (eye #10), -0.75 -6.00 ? 170°. We treated the left eye first and those results are presented here. The videokeratograph shows asymmetric astigmatism with the steeper meridian in the axis of 80° (Fig 3). The lower semimeridian was more steepened than the upper one. The keratometric map is shown in Figure 3 (top left).
Figure 3. Top left: Preoperative videokeratograph (TMS) of eye #10 with asymmetric myopic astigmatism. The lower semimeridian (axis 270°) is steeper than the upper meridian (axis 90°). Manifest refraction: -0.75 -6.00 x 170°; spectacle-corrected visual acuity 20/63. Middle left: Preoperative elevation map of eye # 10 according to best-fit sphere algorithm. Bottom left: Preoperative elevation map of eye #10 according to target keratometric value; yellowish brown band shows corneal area that must be ablated in its central corneal portion to attain the nominal green color in the center. Top right: Keratometric map of eye #10, 6 months after topography-controlled PRK. The ablation zone (blue color) is well centered and symmetrical. Manifest refraction, piano; spectacle-corrected visual acuity, 20/40. Middle right: Postoperative elevation map according to best-fit sphere algorithm. Bottom right: Postoperative elevation map of eye #1 0 according to target keratometric value. Six months after surgery the central corneal area has the nominal green color.
Figure 4. Top left: Preoperative videokeratograph (TMS) of eye #7 with central corneal irregularity after epidemic kerato-conjunctivitis. Manifest refraction: cyl -1.25 x 160°; spectacle-corrected visual acuity, 20/25. Middle left: Preoperative elevation map of eye #7 according to best-fit sphere algorithm. Bottom left: Preoperative elevation map of eye #7 according to target keratometric value; yellowish brown area in the center of the cornea must be ablated to attain the nominal green color. Top right: Keratometric map of eye #7, 6 months postoperatively. The initial irregularity in the center of the cornea has been eliminated. Manifest refraction: +1.00 D; spectacle-corrected visual acuity, 20/20. Middle right: Postoperative elevation map according to best-fit sphere algorithm. Bottom right: Postoperative elevation map of eye #7 according to target keratometric value. Six months postoperatively, the central corneal area now shows the nominal green color, The remaining paracentral small yellow spot indicates a slight undercorrection.
Figure 5. Top left: Videokeratograph (TMS) of eye #4 after myopic PRK that resulted in a decentered ablation zone nasally and interiorly. Manifest refraction: +2.00 -1.25 x 170°; spectacle-corrected visual acuity, 20/25. Middle left: Elevation map of eye #4 before topographycontrolled PRK according to best-fit sphere algorithm. Bottom left: Elevation map of eye #4 before topography-controlled PRK according to target keratometric value; yellowish brown area in the center of the cornea indicates the zone to be ablated to attain the nominal green color in the center (spherical surface). Top right: Keratometric map of eye #4, 6 months after topography-controlled PRK, shows a well centered newly created ablation zone. Manifest refraction: +1.75 -0.50 ? 48°; spectacle-corrected visual acuity, 20/20. Middle right: Postoperative elevation map according to best-fit sphere algorithm. Bottom right: Six-month postoperative elevation map of eye #4 according to target keratometric value. Central corneal area now shows the nominal green color. The two remaining paracentral yellow spots indicate a slight undercorrection.
Figure 6. Top left: Videokeratograph (TMS) of eye #6 after hyperopic PRK that resulted in a decentered ablation zone laterally. Manifest refraction, piano; spectacle-corrected visual acuity, 20/25. Middle left: Elevation map of eye #6 before topography-controlled PRK according to bestfit sphere algorithm. Bottom left: Elevation map of eye #6 before topography-controlled PRK according to target keratometric value; yellowish brown area in the corneal center shows the laterally decentered steepened corneal zone. Top right: Keratometric map of eye #6, 6 months after topography-controlled PRK shows a well-centered homogeneous steepening zone. Manifest refraction, piano; spectacle-corrected visual acuity, 20/20. Middle right: Postoperative elevation map according to best-fit sphere algorithm. Bottom right: Postoperative elevation map of eye #6 according to target keratometric value. Six months after topography-controlled PRK, the yellowish brown color indicates the newly created, well-centered steepening zone.
Uncorrected visual acuity in the left eye was 20/400; spectacle-corrected visual acuity was 20/63. Visual acuity with a hard contact lens was 20/32. On March 9, 1998, the left eye had topography-controlled PRK. The intended correction was -0.75 - 6.00 ? 170°. After 3 days, re-epithelialization was complete.
Four weeks later the manifest refraction was piano. Uncorrected and spectacle-corrected visual acuity were 20/40. Six months later, visual acuity had not changed. The videokeratograph 6 months postoperatively is shown in Figure 3 (top right). The ablation zone (blue color) is well centered and symmetrical.
Case 2: Eye #7 (male); treatment of major corneal irregularity following epidemic keratoconjunctivitis- In 1994, a 45-year-old patient got epidemic keratoconjunctivitis in his right eye. Uncorrected visual acuity, due to scar formation, decreased to 20/40; spectacle-corrected visual acuity was 20/25. Visual acuity with a hard contact lens was 20/20. Manifest refraction was cyl -1.25 ? 160°. The videokeratograph is shown in Figure 4. The keratometric image (Fig 4, top left) shows a central irregularity. On March 20, 1998, his eye was treated with topography-controlled PRK. After 3 days, re-epithelialization was complete. Four weeks later, uncorrected visual acuity was 20/50 and spectaclecorrected visual acuity was 20/40. Manifest refraction was +1.75 D. Six months postoperatively, uncorrected visual acuity improved to 20/25; spectaclecorrected visual acuity to 20/20.
Preoperatively, the patient complained of monocular diplopia in his right (operated) eye. Six months postoperatively, the diplopia had disappeared. The videokeratograph 6 months postoperatively (Fig 4, top right) shows a homogeneous spherical surface in the corneal center.
Case 3: Eye #4 (male); treatment of a decentered treatment area following myopic PRK- This 52-year-old patient had PRK for myopia on his right eye in January 1997. Postoperatively he reported monocular metamorphopsia on the right eye. The keratometric map of the videokeratograph demonstrated a decentered ablation zone (Fig 5, top left). The patient had a loss of spectacle-corrected visual acuity ranging from 20/20 to 20/25. The manifest refraction was +2.00 - 1.25 x 170°. Topographycontrolled PRK was done on March 9, 1998. Six months later, uncorrected visual acuity was 20/40; spectacle-corrected visual acuity was 20/20. Manifest refraction was +1.75 -0.50 x 48° and the patient no longer had metamorphopsia. The preoperative elevation map shows that the yellow area must be removed to obtain a spherical surface (Fig 5, bottom left). The postoperative elevation map shows that the central cornea has a spherical surface, the yellow spots indicate a slight undercorrection (Fig 5, bottom right). The keratometric map (Fig 5, top right) shows good centration of the ablation zone after 6 months.
Case 4: Eye #6 (male); correction of a decentered ablation zone after hyperopic PRK-In September 1997, this 30-year-old patient was treated with hyperopic PRK for hyperopic astigmatism in his left eye. Spherical equivalent refraction was + 4.00 -2.00 ? 174°; spectacle-corrected visual acuity was 20/20. Postoperatively, the refraction was piano, but the patient complained of double contours in his left eye; uncorrected visual acuity was 20/25; spectaclecorrected visual acuity was 20/25. The postoperative videokeratograph shows a slight decentration of the ablation zone (Fig 6, top left). For this reason, topography-controlled PRK was performed on March 20, 1998, Six months postoperatively, uncorrected and spectacle-corrected visual acuity were 20/20 and the patient no longer reported double contours. Manifest refraction was piano. The keratometric image (Fig 6, top right) shows a well-centered ablation zone.
Six months after topography-controlled PRK, spherical refraction was reduced from an average +1.92 D to an average +0.57 D; cylinder was reduced from an average -1.95 D to on average -0.30 D. Eight of ten eyes (80%) were within ±1.00 D of emmetropia.
If one compares the 1-month and 6-month postoperative data, regression of the initially obtained correction corresponded approximately to that of standard PRK.
The decisive factor for assessment of topographycontrolled ablation should be postoperative spectacle-corrected visual acuity, which improved by 2 or more lines in 2 eyes and by 1 to 3 lines in 9 eyes, 6 months postoperatively. Six months after surgery, preoperative visual acuity was achieved with hard contact lens in 8 of 10 eyes. Eyes #8 and #10 did not achieve their preoperative visual acuity with hard contact lens; spectacle-corrected visual acuity in one eye was achieved, and in the other eye, it surpassed the preoperative level. These less than optimal results in two eyes can be explained by the fact that both eyes developed central reticular haze 3 to 4 months postoperatively.
At present, topography-controlled PRK can improve spectacle-corrected visual acuity; 9 of 10 eyes at 6 months achieved a spectacle-corrected visual acuity within 1 line of preoperative hard contact lens visual acuity. In this respect, our results contradict other authors7, who did not achieve any postoperative improvement in average spectaclecorrected visual acuity after topography-assisted LASIK in 21 eyes.
Limitations of this technique exist. For two reasons, eyes with irregularities or small size are not suitable for this technique. The level of resolution of presently available videokeratographic instruments is limited8 and interpolation typically presents a smoother surface than is the case. The spot size of the laser beam is a restriction in treatment. Eyes with irregularities (small elevation and depression profiles) should be treated using well-known techniques of PTK with foreign substances as filling material.9,10
Irregularities with steep elevation and depression profiles are somewhat flattened through the leveling effect of the epithelium and therefore also appear to be lower on the topographic image and thus smoother than actually present in the stromal surface. In this situation, our method tends toward undercorrection. Gentle irregularities extending over larger areas represent the most suitable symptom for this method.
Ten treated eyes is a comparatively small study, from which it is not possible to draw conclusions about the efficacy of topography-controlled PRK. There are, however, first indications that in eyes with certain corneal irregularities, this method can correct refractive errors and at the same time improve spectacle-corrected visual acuity.
This technique is still at an early stage of development. The surgeon depends solely on the topographic images, their precision, and reproducibility. The technique will find wider acceptance only if proof can be furnished that this method provides real improvement over standard methods of PRK and LASIK.
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Visual Acuity and Manifest Refraction Before and 6 Months After Topography-controlled PRK in 10 Eyes