Ординатура / Офтальмология / Английские материалы / LASIK and Beyond LASIK Wavefront Analysis and Customized Ablation_Boyd_2001

Introduction

After the introduction of Keratomileusis by Barraquer, there have been great developments in refractive surgery.1 Pallikaris et al started excimer laser in situ keratomileusis (LASIK) after lifting a flap.1 At present LASIK has become the first choice for eye surgeons for the treatment of high myopia. While LASIK surgeons performed corrections over 20 D, it is now limited to much lower levels of myopia to preserve the integrity of the cornea and quality of postoperative vision.

While there is a great tendency for greater amounts of residual myopia after LASIK, for higher levels of myopia, often the degree of residual refraction is unpredictable. Surgeons have a responsibility to inform the patients, who want 100 % success from LASIK, that the objective of LASIK is to eliminate their dependency on glasses and contact lenses. The percentage of correction that is obtainable depends on several factors such as diopteric power of the patient, and a combination of different ametropias (myopia and astigmatism, hypermetropia and astigmatism).2

As the myopia increases, the amount of residual myopia and subsequent need for enhancement procedures increase as well.2 It may be necessary and useful to do an enhancement in order to refine the results obtained after the first surgery. As LASIK surgery is performed within the stroma, sparing the epithelium and Bowman’s membrane, it allows the surgeon a better adjustability to do a second ablation to correct any regression. It is a better procedure than RK and PRK over previous LASIK treatment because of its less complications like irregular astigmatism, tissue melting and overcorrection.2

There are two options for performing the reablation, lifting a flap with blunt dissection or performing a second cut usually six months after primary LASIK when the process of healing is almost complete.

Procedure

LASIK was performed in 1000 eyes within 20 months between January1997 and August 1998 at our center. 50 eyes (5 %) of 29 patients (17 male and 12 female) had undergone retreatment with secondary LASIK. 21 patients had undergone the procedure bilaterally. All the eyes had a residual myopia more than or equal to –1.0 D spherical equivalent.

The primary LASIK was performed with an Automated Corneal Shaper (ACS) microkeratome (Bausch and Lomb) and Chiron Technolas Keracor 217 excimer laser. (Bausch and Lomb) Fluence was 130mJ/cm2, 10 Hz repetition rate, diameter 7.8 to 8.2 mm and multi zone algorithm with optical zone 4-6 mm. Retreatment with LASIK was done after 5.84 ± 3.24 (SD) months. Previous intraocular surgery, any posterior segment pathology, active inflammations and infections, corneal scarring, pachymetry value less than 410 m, keratoconus, intraocular pressure more than 21 mmHg, narrow palpebral fissure and Schirmer’s test less than 5 mm were the exclusion criteria.

Routine preoperative UCVA and BCVA in decimal equivalent of Snellen’s visual acuity chart, cycloplegic refraction, estimation of palpebral fissure, anterior segment evaluation with slitlamp biomicroscope, Schirmer’s test, intraocular pressure with applanation tonometry, corneal topography

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Chapter 15

(Tomey, TMS 2.1) pachymetry, corneal diameter and detailed fundus examination by indirect ophthalmoscopy were done. As a safety measure eyes having pachymetry less than 410 m (250 m of stromal bed and 160 m of corneal flap) were excluded.

If the patient came to us before 6 months of the primary LASIK we would lift up the same flap. If the patient came to us after 6 months we would recut the cornea. We found the results of both the techniques good with not much difference between the two groups.

The same automated corneal shaper (ACS) microkeratome (Chiron vision) and Chiron Technolas Keracor 217 excimer laser, with the same algorithms was used. All the surgeries were performed under topical anesthesia using 4% lidocaine. Before starting the procedure all the instruments were checked . ACS microkeratome was test run on the base plate before each individual procedure. The eye was cleaned with Betadine‚ 5% solution. A speculum was used to keep the palpebral fissure wide open and eyelashes out of the field. The entrance pupil was marked with gentian violet tip marker. A reference mark was made on the cornea. The suction ring was centered on the corneal marking and activated. The Intraocular pressure was confirmed with the presurgical tonometer to be more than 65 mm Hg. In 10 eyes the primary flap was lifted with a blunt dissection using a spatula. In 40 eyes the second cut was made with the ACS microkeratome. The microkeratome was adjusted on the suction ring. It was moved forward with the forward footswitch untill it stopped at the permanent stopper to prevent a free cap. The microkeratome was then moved back with the reverse footswitch and was removed.

Once the ablation was completed, the tissue and both sides of the flap were cleaned with balanced salt solution and with a Merocel sponge. After checking that there were no foreign particles in interface, the flap was reposited back into original position. The corneal reference markers were checked whether they were in apposition. The suction ring was removed. After a couple of minutes the flap was checked whether it had stuck or not. Then the speculum was taken out carefully.

The patients were examined on the slit lamp half an hour after surgery and sent home. The patients were put on topical antibiotic tobramycin 3%,

topical steroid dexamethasone 0.1 with a tapered dose and artificial tears for one month. The patients were reviewed on the first postoperative day, after one week, one month, six months and one year after surgery. At each follow up uncorrected and best spectacle corrected visual acuity, cycloplegic refraction, anterior segment evaluation, Intraocular pressure, corneal topography and detailed fundus examination were done.

Results

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416.96 ± 27.05 (SD) m. 31 (62%) eyes became emmetropic. 13 (26%) eyes were within ± 1 D of emmetropia. 5 (10 %) eyes were within ± 2 D of emmetropia. One (2 %.) eye had surgery induced irregular astigmatism of –3.5-D cylinders. At the last follow-up uncorrected visual acuity improved from 0.22 ± 0.15 (SD) to 0.56 ± 0.22 (SD) (p<0.05) after

the secondary procedure. 36 (72 %) eyes had UCVA more than 0.5, 12 (24 %) eyes had UCVA between 0.25 to 0.5. (Table –2). BCVA improved from 0.59 ± 0.19 (SD ) to 0.66 ± 0.22 (SD) at last followup. (Table- 3). In 17 eyes BCVA improved by 1 line on Snellen’s visual acuity chart. One eye lost one line of BCVA due to decentered ablation. . Post

Figure 15-2: Corneal Topography-After Lasik

Figure 15-1: Relasik Corneal Topography-Before Lasik

RELASIK corneal topography showed uniform central ablation in 49 eyes. (Figure 15-1 shows corneal topograph before LASIK. Figure 15-2 shows the corneal topograph after LASIK and Figure 15-3 shows the topograph of the same eye after RELASIK.) Only one eye showed decentered ablation.

Discussion

LASIK has become increasingly popular among the eye surgeons to treat refractive errors like high myopias. Residual myopia is common in majority of the refractive procedures. The degree of correction obtained after surgery depends on several factors. Amount of residual power appears to be more common in young patients than adults. 4. Causes of undercorrection, which can be avoided, are improper data entry, poor calibration of laser, discrepancy be-

tween manifest and cycloplegic refraction. We tried

Figure 15-3: Corneal TopographyAfter Relasik

to avoid them by meticulous cross-checking of data entry, careful preoperative cycloplegic refraction algorithm adjustments as necessary. 13

Several studies show that a higher rate of residual myopic refraction after LASIK for high myopia in comparison to moderate myopia. In one

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study A.M Bas et al divided patients into 4 groups depending on the diopteric power before LASIK . 1) –3.0 To –6.0 D 2) –6.25 to –10 D 3)

–10.25 to –15 D 4) –15.25 to –25.5 D . After LASIK group 4 had shown residual myopia > -1.0 D (-1.49

± 1.54 (SD).5 M.C Knortz et al had shown in eyes with myopia > -12 D, accuracy and patient satisfaction was sufficiently poor. In 28 % eyes residual power was > -1 D. 6,7.Perez et al observed similar results .8

Other possible mechanisms are proliferation of keratocytes in corneal stroma, epithelial hyperplasia and central corneal thickness associated with central corneal steepening. 2,9,12 As LASIK surgery was performed intrastromally, it allows the surgeon to do a second ablation to correct any undercorrection or any defect, which have occurred during the first surgery. One can do RK or PRK as an enhancement procedure after LASIK, but it has several disadvantages like the possibility of overcorrection, irregular astigmatism, tissue melting and limitation in correcting high myopia.2

Postoperative refraction in high myopia is supposed to be settled after six months.2 In 10 eyes where we did RELASIK within 3 months of the primary procedure, we could reopen the corneal flap by blunt dissection using a spatula. In 40 eyes we did the secondary procedure after six months. At this time as the healing was almost complete, we could not reopen the flap by blunt dissection. So we used the second cut for RELASIK using the same algorithms but used the residual myopia as a subjective refraction . We limited our ablation zone between 4-6 mm to prevent night glare problems due to smaller zone and unnecessary vertex ablation and overcorrection due to larger zone ablations. 4 As a safety we excluded the patients having pachymetry less than 410 m (250 m-stromal bed and 160 m of corneal flap) from the study. 2,3

The Mean spherical equivalent improved from -4.3 ± 1.83D (SD) to –0.45 ± 0.68(SD) at the last follow-up. (p< 0.005). The UCVA improved from 0.22 ± 0.15 (SD) to 0.53 ± 0.22 (SD) at the last follow-up. (p<0.05). 17 eyes gained one line of BSCVA. Only one eye lost one line of BSCVA due to decentered ablation .No sight threatening complications had occurred.

11)J.J. Perez&ndash, Santonja, M.S. Ayala and H.F. Sakla et al; Retreatment after laser in situ keratomileusis, Ophthalmology, January 1999.

12)Lohman C.P, Guell JL; Regression after LASIK for treatment of Myopia, Role of corneal epithelium, Semin Ophthalmology, 1998 June 13: 2. 79-82

13)Howard Gimbel ,E.E. Permo; LASIK the Technique, Refractive Surgery, Jaypee,25, 254-265.

14)Guell J.L, Lohman C.P, Melecaze FA and Junger J et al; Intraepithelial Photorefractive Keratectomy for regression after LASER in situ Keratomileusis, J.C. Ref Surg, 1999 May 25: 5, 670-4.

T. Agarwal, M.D.

Dr. Agarwal’s Eye Hospital,

19 Cathedral Road,

Chennai (Madras)- 600 086, India

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Introduction

Over the years great attempts have been made to permanently correct refractive errors like high myopia. Surgical procedures that have been used in attempt to correct high myopia include radial keratotomy (RK), keratomileusis, keratophakia, freeze myopic keratomileusis, Excimer laser keratectomy (PRK) and Laser in situ keratomileusis (LASIK)1,2.Among these, radial keratotomy (RK) has been the most popular5. In this procedure, radial incisions flex the peripheral cornea, leading to a compensatory flattening of central cornea. The amount of central flattening that can be achieved with RK, however, is limited5. Although various algorithms were used, radial keratotomy was found to be associated with frequent failures in achieving emmetropia, with surgical results often shifting towards hyperopia2. Because of this possible shift, surgeons tend to undercorrect myopia. Patients therefore frequently need correction with spectacles or contact lenses. Residual myopia after RK, whether intentional or unexpected, may result when RK surgery did not entirely correct a patient’s myopia because of a large optical zone, few incisions, or shallow incisions. If necessary, a later redeepening procedure, which extends existing RK incisions and places additional incisions, may be required to correct the residual error5,15.

Although excimer laser PRK is effective, predictable, and safe in treating myopia, undercorrections persist in some highly myopic eyes even after one or more repeated procedures due to the limitations of available techniques or the unique tissue response of each eye. As LASIK surgery is

performed within the stroma, sparing the epithelium and Bowman’s membrane, it allows the surgeon better adjustability to do ablation to correct any regression.

We conducted a retrospective analysis of 20 eyes treated with LASIK for residual myopia. Ten eyes had formerly been treated with RK, and ten had been treated with PRK. The five post RK patients were all male; there were three males and two females in the post PRK group.

Lasik After RK and PRK

Ten post RK eyes of five patients (all male) and ten post PRK eyes of five patients (three males and two females), which were subjected to LASIK procedure to correct residual myopia, were retrospectively analyzed. The average age (in years) of the patients among the post RK group was 25.6 ± 6.88 SD, and among the post PRK group it was 33.67 ± 5.77 SD. The mean interval between the primary procedure and LASIK was 24.30 ± 0.75 (SD), with a range of 14 to 36 months. The mean interval in the post RK group was 22 ± 1.07 (SD), with a range of 20 to 24 months in the post PRK group.

Patients were treated with the LASIK procedure only if there was a stable residual refractive power > - 1.25 D for at least 1 year, if they did not want to wear spectacles, and if did not tolerate contact lenses. A full informed consent was obtained from all the patients before the procedure.

Exclusion criteria included any active corneal pathology or inflammation, corneal scars, monocular patients, keratoconus and thin corneas, raised Intraocular pressure (more than 21 mm Hg), the pres-

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Chapter 16

ence of active collagen vascular disease, pachymetry less than 420 mm, narrow palpebral fissure, or Schirmer ‘s test less than 5mm.

Routine preoperative tests included UCVA and BCVA according to Snellen’s visual acuity chart, cycloplegic refraction, and slit-lamp biomicroscopic anterior segment examination that assessed the palpebral aperture. Measures were also taken of the corneal diameter, pupil diameter, corneal sensitivity, Schirmer’s test, and intraocular pressure. Corneal topography (Tomey TMS 2.1) and pachymetry were also done. A complete retinal evaluation ruled out any retinal tears or pathology. Patients were instructed to remove soft contact lenses at least 2 weeks before and hard and semi-soft contact lenses at least 4 weeks before evaluation.

LASIK was performed using the Chiron Technolas Keracor 217 Excimer laser (Bausch & Lomb) with the Automated Corneal Shaper microkeratome (Bausch & Lomb). Fluency was 130 j/cm2, with a 10 Hz repetition rate, diameter of 7.8mm to 8.2mm, and multi-zone algorithm with optical zone from 4mm-6mm. All the surgeries were performed under topical anesthesia using 4 % lidocaine. Fluence was checked before every procedure by verifying the homogeneity and symmetry of the pulses according to optimal values of 65 ± 1 shots. Before the procedure was begun, all the instruments were checked. The ACS microkeratome was test-run on the base plate before each individual procedure. The autotracking mechanism was used .A speculum was inserted to keep the palpebral fissure wide open and eyelashes out of field. The entrance pupil was marked with a Gentian violet tip marker. A reference mark was made on the cornea. The suction ring was centered on the corneal marking and activated. The intraocular pressure was confirmed with the presurgical tonometer to be more than 65 mm Hg. The microkeratome was adjusted on the suction ring and moved forward with the forward footswitch till it stopped at the permanent stopper to prevent a free cap. The microkeratome was moved back with a reverse footswitch and removed.The lamellar corneal flap was fashioned with a nasal hinge and carefully lifted with a blunt instrument and reflected on its hinge. The surgeon carefully performed the laser ablation to ensure accurate centration. Once the ablation was completed, tissue and both the sides of

sual acuity chart. One eye lost a single line of BCVA. The mean pachymetry changed from 556 ± 30 (SD) to 426 ± 20 (SD) after LASIK. Postoperative corneal topography showed centered ablation in all the eyes. (Fig. 16-1 shows a corneal topograph after RK. Fig. 16-2 is a corneal topograph of the same eye after LASIK).

PRK Group

At the last follow-up the mean spherical equivalent was –0.4 ± 0.5 (SD), compared to -3.38 ± 1.3(SD) (P<0.005) before LASIK. Five eyes (50 %) became emmetropic, five eyes (50%) were within

±1 D of emmetropia, and one eye(10%) was within

±2 D of emmetropia. The mean UCVA improved from 0.25 ± 0.12 (SD), to 0.87 ± 0.22 (SD)(P<0.05). The UCVA was ≥ 0.5 in eight eyes (80 %) (Table 2). The mean BCVA improved from 0.88 ± 0.19(SD), to 0.9 ± 0.16 (SD)(P<0.5). In three eyes the BCVA improved by 1 line on Snellen’s visual acuity chart. No eye lost a line of BCVA. The mean pachymetry changed from 552 ± 36 (SD), to 452 ± 27 (SD) after LASIK. Postoperative corneal topography showed

Figure 16-1: Preoperative topograph of a case in which RK was Done Previously

Figure 16-2: Postoperative topograph of the same case in which Lasik was done after RK.

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Figure 16-3: Preoperative topograph of a case in which PRK (photorefractive keratectomy) was done.

centered ablation in all the eyes. (Figure 16-3 shows a corneal topograph after PRK. Figure 16-4 is a corneal topograph of the same eye after LASIK). No complications that jeopardize sight like free flap, corneal ectasia, or any retinal complication occurred. Two eyes in the RK group required repositioning of the flap within a few hours after surgery due to wrinkles and irregular opposition of the flap to the stromal bed. There was no statistically significant difference in corneal haze before and after LASIK.

Discussion of Lasik After RK and PRK

The main adverse effects of laser surgery are the loss of BCVA and regression. With LASIK the corneal epithelium as well as Bowman’s membrane are preserved. Therefore, the distinct wound healing that occurs after PRK is not present with this procedure. For this reason, there is less regression after LASIK than after PRK, and the potential for corneal haze or scar formation should be of less concern.

Figure 16-4: Postoperative topograph of the same case in which Lasik was done after PRK

We did LASIK using the same algorithms but considered residual myopia as a subjective refraction. We limited our ablation zone between 4mm6mm to prevent night glare that can result from unnecessary vertex ablation and overcorrections due to a large zone ablation4. As a safety measure we excluded patients having pachymetry less than 410 m (250 m stromal bed and 160mm stromal flap). In our study there was significant reduction in mean spherical equivalent from –6.05 ±1.98 (SD) to –1.19 ± 0.71(SD) in the RK group, and from –3.38 ±1.3 (SD) to –0.4 ± 0.5 (SD) D in the PRK group. We compared our study with the study conducted by Ozdamar et al., who did LASIK after PRK for residual myopia ranging from 1.5 to 12.5 D. The mean spherical equivalent was changed from –5.96 ± 3.06D (SD) to 1.19± 0.77 D (SD). Better statistical results were obtained in the PRK group than the RK group, which may be due to the higher residual myopia in the RK group. The percentage of patients achieving a UCVA of 20/40 (0.5) or more was similar in both the groups after LASIK. No significant complications were reported.

At the last follow-up, the mean spherical equivalent reduced from -6.05 ± 1.98 (SD) to

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