FP838 : Outcome of Primary Topography- Guided Customized Ablation (Contura ®) in Myopic Eyes

Dr. Sumeet Lahane L19136

Dr.Rohit Shetty, Dr. Rushad Shroff

A prospective study to compare visual outcomes between Wavefront Optimized and Topo-guided Ablation Profiles in contralateral eyes with myopia

Abstract

Purpose:

To study and analyze refractive outcomes of Wavefront-optimized (WFO) and Topography-guided ablation (TCAT) profiles using the Allegretto waveexcimer laser platform (EX500 Wavelight, Alcon Inc., Fort Worth, Texas) in the treatment of myopia. 

Material and Methods:

60 eyes of 30 patients who underwent LASIKwere included in this prospective interventional study. WFO ablation was performed in one eye (WFO group) and TCAT in the fellow eye (TCAT group). The WaveLight FS200 femtosecond laser (Alcon Inc., Fort Worth, Texas)was used to create the flap and Allegretto wave excimer laser was used for photoablation. The Pentacam HR (Oculus OptikgerateGmBH, Wetzlar, Germany).and  AllegrettoTopolyzer were used to measure the ocular aberrations. Refractive visual outcomes were also compared.

Results:

The total root mean square (RMS) and RMS lower order aberrations (LOA’s) were significantly better in eyes that underwent TCAT (p<0.05). There was a decrease in individual higher order aberrations (HOA’s) in the TCAT group, with a statistically significant difference in the spherical aberrations (SA) (P = 0.02). The Q value was not significantly different between the WFO (0.6 + 0.1) and (0.51 +0.09) TCAT groups. The postoperative total RMS of HOA’s was lower in the TCAT group (p=0.5). Accuracy, safety and efficacy of treatment were similar in the two groups. There was a more positive change in Q value and SA in the WFO group, however this was not statistically significant.

Conclusion:

TCAT provided better outcomes than WFO, with induction of fewer lower LOA’s and HOA’s in myopic LASIK. 

Introduction:

Since its inception in 1992, conventional laser assisted in situ keratomileusis (LASIK), has proven to be an effective and safe modality for treating myopia and myopic astigmatism.^1,2However, there have been concerns regarding the effect of changes in corneal curvature on the optical characteristics of the eye. Standard ablation profiles flatten the center and cause a relative steepening in the periphery of the cornea leading to a more oblate pattern. An oblate pattern as well as biomechanical changes, stromal remodeling and epithelial hyperplasia may induce unwanted spherical aberrations (SA) after the procedure.^3,4,5

In an attempt to enhance functional outcomes and reduce SA after surgery, various ablation profiles have been designed. Wavefront-optimized (WFO) ablation uses a precalculated spherical aberration treatment to create an aspherical ablation profile.^6,7Wavefront-guided (WFG) ablation uses aberrometers to measure higher order aberrations (HOA’s) following which a customized ablation pattern is used to treat the total HOA’s of the eye.⁸Topography-guided laser ablation (TCAT) attempts to maintain the aspheric shape of the cornea and neutralize corneal irregularities. TCAT has been shown to be effective in highly aberrated and irregular corneas where aberrometry is not reliable.⁹ Studies have shown that this method is safe and effective in the management of irregular corneal astigmatism and myopia secondary to previous corneal surgeries or disease.^9,10However, outcomes in terms of aberrations, changes in asphericity and refractive results have not been reported in detail.¹¹

The aim of the study was to compare the refractive outcomes of WFO and TCAT using the ALLEGRETTO WAVE excimer laser platform (EX500 Wavelight, Alcon Inc., Fort Worth, Texas) in the treatment of myopic patients, with and without astigmatism.¹²

Material & Methods:

Sixty eyes of 30 consecutive patients (14 male and 16 female) undergoing femtosecond assisted LASIK surgery were included in this prospective, interventional, study. The study was conducted in accordance with the tenets of the Declaration of Helsinki and was approved by the institutional research and ethics committee of NarayanaNethralaya Postgraduate Institute of Ophthalmology, India. A written informed consent was obtained from all patients before the procedure.

Study population:

Patients between 18 to 50 years of age with stable myopia for a minimum period of one year (a change of 0.25D or less), a corrected distance visual acuity (CDVA) of 20/25 or better, a spherical equivalent refraction less than −10D and a refractive astigmatism less than -3D were included in the study. Exclusion criteria included a central corneal thickness (CCT) less than 480 micrometer, a calculated residual stromal bed thickness of less than 250 micrometer after the surgery, patients with a history of keratoconus, diabetes, collagen vascular disease, pregnancy, breastfeeding and any prior ocular surgery. Patients with anisometropia of more than 1.00 diopter (D) of spherical or 0.50 D of cylindrical refractive error were excluded.

Study design:

Eligible patients underwent a detailed ocular examination, which included visual acuity and manifest refraction assessment, and fundoscopy. Aberration measurements and corneal topography were performed using the Pentacam (Oculus OptikgerateGmBH, Wetzlar, Germany). Corneal topography data for TCAT and Q value (asphericity) calculation was obtained from the placido based Allegretto Topolyzer (Version 1.59, Alcon Laboratories Inc.)prior to and six months after LASIK.

Study treatments and procedures:

All surgeries were performed by a single experienced surgeon under aseptic precautions and topical anaesthesia after instilling 0.5% proparacaine hydrochloride (Paracain, SunwaysPvt. Ltd., India). One eye of each patient was randomly assigned to theWFOgroup and the fellow eye to the TCAT group. Topography-guided treatment was planned for both eyes prior to surgery and during the surgery one eye was selected for customized ablation using a random digit table. The other eye was treated using the WFO ablation profile.

The WaveLight FS200 femtosecond laser (Alcon Inc., Fort Worth, Texas) was used to create a flap thickness of 110 mm and a flap diameter of 9 mm with a 70-degree angled side cut. The optical diameter was 6 mm.After drying the stromal bed, excimer laser ablation was performed (EX500 Wavelight, Alcon Inc., Fort Worth, Texas). The bed was thoroughly irrigated with saline and the flap repositioned on the stromal bed. In both groups, a tapering dose of topical prednisolone acetate 1% (Allergan Inc, Irvine, CA) 3 times a day for 3 days, 2 times a day for the next 3 days, and 1 time a day for the last 2 days was administered after surgery. A topical antibiotic (Vigamox, Alcon Inc, Fort Worth, TX) drop was administered 4 times a day for 1 week. Lubricating eye drops (Systane, Alcon Inc) wereadministered 6 to 8 times a day for 3 months.

Statistical Analysis:

The Kolmogorov-Smirnov test was used for confirming normality of data. As the data was normally distributed, parametric tests were employed and all continuous variables were represented as mean and standard deviation. Student t –test was used to compare the uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA), spherical and cylindrical error (diopter), mean refractive spherical equivalent (MRSE) and aberrations with the Pentacam before and after the surgical procedure for each eye, and between the surgical groups. Zernike coefficients were analyzed in a zone of 6mm size with a maximum order limited to 8.A two sided p-value < 0.05 was considered as statistically significant.Univariate regression was used to assess the change in Q value and SA with change in the refractive spherical equivalent. All statistical analyses were performed usingMedCalc v15.6.1 (MedCalc Inc., Belgium).

 Results:

.Mean age of study subjects was 27.2 + 5.6 years. The preoperative CDVA,UDVA, spherical and cylindrical refractive error alongwith MRSE were similar in both groups (Table 2). Six months following the procedure there was no significant difference in the parameters mentioned above between the two groups. In the WFO group, 93% of eyes achieved a UDVA of > 20/20 at 6 months. These percentages were not significantly different in the TCAT group (97%, p=0.61). The CDVA remained stable in 83.3% and 90% of the eyes in the WFO and TCAT group, respectively (p=0.7) (Figure 1).The percentage of eyes with a spherical equivalent less than+ 0.5D was 80% in the WFO group and 83% in the TCAT group (p=0.9). The coefficient of determination between the attempted and achieved MRSE was also similar (p=0.89) between the repositioned (R²=0.96) and non-repositioned group (R²=0.97).

As shown in Table 3 the total root mean square (RMS) of the anterior surface of the cornea was significantly higher (p=0.04) in the WFO group (2.04 + 0.72)than the TCAT group (1.73 + 0.61) six months after surgery. The total RMS of the entire cornea was also higher (p=0.04) in the WFO group (1.94 + 0.78) than the TCAT group (1.60 + 0.63). The RMS of lower order aberrations for the anterior surface and entire cornea was significantly greater in the WFO group (p=0.04 and p=0.03 respectively).

The preoperative Q value was -0.33 + 0.12 in the WFO group and -0.35 + 0.13 in the TCAT group. The difference was not statistically significant (p=0.8). Six months after the procedure, the Q value was+0.6 in the WFO group and+0.51 in the TCAT group. The total spherical aberration in the WFO group(0.2 + 0.08) and TCAT group (0.19 + 0.10) was similar preoperatively (p=0.87). The induced positive spherical aberration 6 months after the surgery was significantly lower (p=0.02) in the TCAT group (0.42 + 0.21) than in the WFO group (0.57 + 0.25).

Discussion:

The WFO ablation profile is a propriety algorithm incorporated in the Allegreto Wavelight Excimer laser and has a population averaged spherical aberration correction built into it.⁷ The drawback of WFO ablation is that it does not take into account the unique aberration of each eye and cannot be customized.¹⁵ Studies have also found that the WFO treatment may not work well in eyes with high myopia and astigmatism, which tend to be more irregular and with greater aberrations.^10,11

Jankov et al. have demonstrated the utility of topography-guided treatment in highly aberrated and irregular corneas.⁹ TCAT has been successfully used for the treatment of irregular corneas following trauma, in keratoconus and in eyes with small or decentered optical zones and ectasia following LASIK surgery.^16-18Studies have also demonstrated the role of TCAT in the correction of astigmatism induced by refractive surgery.⁹ A symmetric shape can then be fit under the height map to correct the existing refractive error by creating an appropriate ablation profile.⁹Corneal curvature measurements, unlike wavefront measurements are unaffected by changes in the pupil size. TCAT is therefore unaffected by the pupil centroid shift and can be used to treat irregularities in the corneal periphery.

On studying aberrations, we found that the anterior surface and total RMS values were significantly lower in eyes that were treated using the TCAT ablation profile, which also induced fewer lower order aberrations. The total RMS of higher order aberrations was lower in eyes that had undergone TCAT but this was not found to be statistically significant. To the best of our knowledge this has not been demonstrated before in a contralateral eye study comparing WFO and TCATablation profiles. Furthermore, the value of the induced spherical aberration and coma was significantly lower in eyes that underwent TCAT.WFO ablation has been reported to maintain corneal asphericity and induce less aberrations than conventional LASIK.⁷.

Myopic ablation induces positive spherical aberration.²³In the present study we found greater induction of positive spherical aberration in eyes treated with WFO ablation profile. There was a greater change in spherical aberration with higher amounts of refractive correction, which has also been demonstrated in earlier studies.^13, Furthermore, since topo-guided treatment induces lower amounts of spherical aberration it may be better to use TCAT in patients with higher refractive errors.⁶ Spherical aberrations have been implicated in scotopic vision abnormalities such as glare, halos and starbursts.²⁵

The aspherical shape of the cornea is represented using the Q value, which is a coefficient that describes the rate of change in corneal curvature from center to the periphery.^13,14 It specifies whether the eye is more oblate or more prolate²⁶. We found a positive change in Q value in both the groups with a greater positive change in eyes that were treated using the WFO ablation profile.However, this difference was not statistically significant. We also demonstrated a more positive Q value in eyes that had a higher amount of refractive correction in our population. This may explain poorer visual quality in this subset of patients despite the presence of 20/20 vision. It may be prudent to consider topography-guided treatments in these patients as potentially cause less changes in the Q value.

To conclude, refractive outcome in terms of lines lost or gained, spherical equivalent achieved and percentage of eyes that obtained 20/20 vision was similar in both, WFO and TCAT groups. The present study also found less change in corneal asphericity, lower root mean square values of lower order and higher order aberrations such as coma and spherical aberrations in eyes treated with TCAT. These findings suggest that though both wavefront-optimized and topo-guided regimens are equally accurate, safe and effective in correcting myopia and myopic astigmatism, TCAT has a lesser effect on the corneal shape and aberrations. This may be useful to attain better visual quality, especially in eyes corrected for higher degrees of ammetropia.

References

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2.Mrochen M, Eldine MS, Kaemmerer M, Seiler T, Hütz W. Improvement inphotorefractive corneal laser surgery results using an active eye-trackingsystem. J Cataract Refract Surg. 2001 Jul;27(7):1000-1006.

3.Munnerlyn CR, Koons SJ, Marshall J. Photorefractive keratectomy: a techniquefor laser refractive surgery. J Cataract Refract Surg. 1988 Jan;14(1):46-52.

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5.Wilson SE, Mohan RR, Hong JW, Lee JS, Choi R, Mohan RR. The wound healingresponse after laser in situ keratomileusis and photorefractive keratectomy:elusive control of biological variability and effect on custom laser visioncorrection. Arch Ophthalmol. 2001 Jun;119(6):889-896.

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Table legends:

Table 1:Inclusion and exclusion criteria for study participants

Table 2:Mean with Standard deviation (SD) of preoperative parameters in wavefront-optimized (WFO) and Topography- guided (TCAT) ablation profile.

 Table 3:Mean with Standard deviation (SD) of wavefront-aberrations in wavefront-optimized (WFO) and Topography- guided (TCAT) ablation profile.

 Table 1 – Inclusion and Exclusion criteria

Inclusion criteria 1. Patients should be between 18 to 50 years of age. 2. Patient must have stable myopia for a minimum period of one year (a change of 0.25D or less) as documented by prior clinical records or current spectacle correction. 3. Patient must have a corrected distance visual acuity (CDVA) of 20/25 or better, 4. Patient must have a spherical equivalent refraction less than −10D 5. Patient must have refractive astigmatism less than -3D. Exclusion criteria 1.  Patient must not have a central corneal thickness (CCT) less than 480 micrometer 2.  Patient must not have a calculated residual stromal bed thickness of less than 250 micrometer after the surgery 3.  Patient must not have symptoms or signs of keratoconus, diabetes, collagen vascular disease, pregnancy, breastfeeding and any prior ocular surgery. 4. Patient must not have an interocular difference of more than ?1.00 diopter (D) of spherical or 0.50 D of cylindrical refractive error. 5. Patient must not be on chronic systemic steroids or other medication that can affect wound healing. 6. Patient must not be allergic to primary or alternative medications. 7. Patient must not be using rigid contact lenses for the last three weeks or soft contact lenses for at least 1 week before the preoperative evaluation

 

Table 2: Mean with Standard deviation (SD) of preoperative parameters in Wavefront-optimized (WFO) and Topography- guided (TCAT) ablation profile. 

Variables	Before Surgery			Six months after Surgery
	WFO (n=30)	TCAT (n=30)	p-value	WFO (n=30)	TCAT (n=30)	p-value
Sphere	-4.55 + 2.39	-4.39 + 2.37	0.78	-0.24 + 0.56	-0.16 +0.51	0.52
Cylinder	-1.10 + 0.76	-1.08 + 0.71	0.93	-0.53 + 0.27	-0.51 + 0.23	0.77
MRSE	-5.08 + 2.50	-4.93 + 2.47	0.78	-0.50 + 0.53	-0.41 +0.49	0.45
CDVA	0.019 + 0.07	0.002 + 0.04	0.219	-0.01 + 0.05	-0.01 + 0.04	0.78
UDVA	1.28 + 0.35	1.251 + 0.34	0.662	-0.0 + 0.05	-0.01 + 0.043	0.33

MRSE: Mean refractive spherical equivalent in diopters, CDVA: Corrected distance visual acuity, UDVA: Uncorrected distance visual acuity. n: sample size

* indicates a statistically significant difference between the two groups.

A p-value <0.05 was considered statistical significant.

Table 3: Mean with Standard deviation (SD) of wavefront-aberrations in wavefront-optimized (WFO) and Topography- guided (TCAT) ablation profile.

Variables	Before Surgery			6 months after Surgery
	WF (n=30)	TCAT (n=30)	p value	WF (n=30)	TCAT (n=30)	p value
FRONT WFA RMS TOTAL	1.77 + 0.50	1.75 + 0.50	0.86	2.04 + 0.72	1.73 + 0.61	0.049*
FRONT WFA RMS LOA	1.74 + 0.53	1.73 + 0.55	0.93	1.86 + 0.68	1.55 + 0.58	0.035*
FRONT WFA RMS HOA	0.36 + 0.07	0.37 + 0.08	0.68	0.81 + 0.30	0.7571 + 0.30	0.39
FRONT WFA N2 ASTIGMATISM 45	-0.04 + 0.55	0.32 + 0.37	0.0015*	-0.074 + 0.32	0.03 + 0.28	0.15
FRONT WFA N2 DEFOCUS	0.88 + 0.27	0.82 + 0.29	0.42	0.89 + 0.59	0.71 + 0.56	0.17
FRONT WFA N2 ASTIGMATISM 0	-0.92 + 0.90	-0.87 + 0.95	0.80	-0.61 + 0.44	-0.65 + 0.40	0.66
FRONT WFA N3 COMA 90	-0.08 + 0.16	-0.08 + 0.18	0.85	-0.23 + 0.38	-0.10 + 0.36	0.15
FRONT WFA N3 COMA 0	0.02 + 0.14	0.12 + 0.11	0.002	-0.036 + 0.26	0.12 + 0.21	0.01*
TOTAL WFA RMS TOTAL	1.53 + 0.53	1.46 + 0.55	0.58	1.94 + 0.78	1.60 + 0.63	0.041*
TOTAL WFA RMS LOA	1.47 + 0.51	1.42 + 0.54	0.64	1.73 + 0.75	1.38 + 0.59	0.03*
TOTAL WFA RMS HOA	0.36 + 0.08	0.36 + 0.08	0.89	0.85 + 0.33	0.78 + 0.33	0.37
TOTAL WFA N2 ASTIGMATISM 45	-0.03 + 0.51	0.30 + 0.35	0.002*	-6.78 + 5.20	0.007 + 0.27	0.48
TOTAL WFA N2 DEFOCUS	0.55 + 0.30	0.46 + 0.31	0.18	0.57 + 0.62	0.37 + 0.63	0.14
TOTAL WFA N2 ASTIGMATISM 0	-0.81 + 0.86	-0.75 + 0.90	0.78	-0.39 + 0.46	-0.41 + 0.42	0.81
FRONT WFA N3 COMA 90	-0.09 + 0.16	-0.09 + 0.16	0.95	-0.24 + 0.42	-0.14 + 0.37	0.28
TOTAL WFA N3 COMA 0	0.01 + 0.14	0.087 0.12	0.01*	-5.69 + 4.34	0.11 + 0.23	0.47
FRONT WFA SA	0.24 + 0.06	0.23 + 0.06	0.96	0.58 + 0.23	0.44 + 0.21	0.02*
FRONT TOTAL SA	0.20 + 0.08	0.19 + 0.1	0.87	0.57 + 0.25	0.42 + 0.21	0.02*
Q value	-0.34 + 0.12	-0.35 + 0.13	0.8	0.60 + 0.5	0.51 + 0.4	0.5

WFA RMS: Wavefront aberrations root mean square, LOA: Lower order aberrations, HOA: Higher order aberrations, SA: Spherical aberrations. Q value: Asphericity value n: sample size

* indicates a statistically significant difference between the two groups.

A p-value <0.05 was considered statistical significant.