Journal of Refractive Surgery

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WAVEFRONT ABLATION 

Clinical Outcomes of CATz Versus OPDCAT

Mihai Pop, MD; Harkaran S Bains

Abstract

ABSTRACT

PURPOSE: To compare refractive outcomes, wavefront outcomes, and corneal asphericity indices (Q values) for patients treated with wavefront-guided or topography-guided custom ablations using the NIDEK Advanced Vision Excimer Laser System (NAVEX).

METHODS: A total of 196 eyes of 98 patients underwent wavefront-guided or topography-guided LASIK. A contralateral study of 28 eyes of 14 patients who underwent customized aspheric treatment zone (CATz) ablation in one eye and optical path difference customized aspheric treatment (OPDCAT) in the fellow eye comprised one part of the study. The second part of the study was a retrospective review of myopic LASIK using CATz in 84 eyes and LASIK using OPDCAT in 84 eyes. In the CATz-treated eyes, the preoperative mean spherical equivalent refraction was -4.06±1.69 diopters (D). In OPDCAT-treated eyes, the preoperative mean spherical equivalent refraction was -3.67±2.17 D.

RESULTS: In the CATz-treated eyes, 95% of eyes achieved ≥20/20 best spectacle-corrected visual acuity (BSCVA) with 15% gaining lines of BSCVA. In the OPDCAT-treated eyes, 94.5% of eyes achieved ≥20/20 vision with 4.5% gaining lines of BSCVA. The difference in lines gained between CATz and OPDCAT in the retrospective component was statistically significant (P<.005). OPDCAT-treated eyes showed a statistically significant worsening in OPD root-mean-square (OPD-RMS), higher order wavefront error, and corneal asphericity (P<.005) compared to preoperative in the retrospective portion of the study. In the contralateral arm of the study, OPD-RMS value was significantly higher (P<.005) in the OPDCAT eyes than the fellow CATz-treated eyes. OPDCAT-treated eyes show a larger change in the Strehl ratio compared to the CATz-treated eyes (P<.005).

CONCLUSIONS: The refractive outcomes were excellent and showed no clinically significant difference between CATz and OPDCAT treatments. The higher induction of aberrations with the OPDCAT ablations may be due to the fact that these treatments are based on Zernike polynomials to drive the ablation. [J Refract Surg. 2005;21(Suppl):S636-S639.]

Abstract

ABSTRACT

PURPOSE: To compare refractive outcomes, wavefront outcomes, and corneal asphericity indices (Q values) for patients treated with wavefront-guided or topography-guided custom ablations using the NIDEK Advanced Vision Excimer Laser System (NAVEX).

METHODS: A total of 196 eyes of 98 patients underwent wavefront-guided or topography-guided LASIK. A contralateral study of 28 eyes of 14 patients who underwent customized aspheric treatment zone (CATz) ablation in one eye and optical path difference customized aspheric treatment (OPDCAT) in the fellow eye comprised one part of the study. The second part of the study was a retrospective review of myopic LASIK using CATz in 84 eyes and LASIK using OPDCAT in 84 eyes. In the CATz-treated eyes, the preoperative mean spherical equivalent refraction was -4.06±1.69 diopters (D). In OPDCAT-treated eyes, the preoperative mean spherical equivalent refraction was -3.67±2.17 D.

RESULTS: In the CATz-treated eyes, 95% of eyes achieved ≥20/20 best spectacle-corrected visual acuity (BSCVA) with 15% gaining lines of BSCVA. In the OPDCAT-treated eyes, 94.5% of eyes achieved ≥20/20 vision with 4.5% gaining lines of BSCVA. The difference in lines gained between CATz and OPDCAT in the retrospective component was statistically significant (P<.005). OPDCAT-treated eyes showed a statistically significant worsening in OPD root-mean-square (OPD-RMS), higher order wavefront error, and corneal asphericity (P<.005) compared to preoperative in the retrospective portion of the study. In the contralateral arm of the study, OPD-RMS value was significantly higher (P<.005) in the OPDCAT eyes than the fellow CATz-treated eyes. OPDCAT-treated eyes show a larger change in the Strehl ratio compared to the CATz-treated eyes (P<.005).

CONCLUSIONS: The refractive outcomes were excellent and showed no clinically significant difference between CATz and OPDCAT treatments. The higher induction of aberrations with the OPDCAT ablations may be due to the fact that these treatments are based on Zernike polynomials to drive the ablation. [J Refract Surg. 2005;21(Suppl):S636-S639.]

Wavefront-guided ablations or custom corneal ablation for the treatment of myopia and myopic astigmatism have been introduced within the past 5 years. Wavefront-guided ablations are steadily increasing in popularity, comprising a significant amount of refractive procedures worldwide. Various definitions of wavefrontguided ablation or corneal ablation exist. Wavefront-guided ablations differ from wavefront-optimized treatments, which refer to laser treatment software that has been designed with certain corrections pre-programmed. With wavefront-optimized treatments, true customized treatments are not performed. Wavefront-guided or custom ablation involves the treatment of the individual aberrations of the eye that are being corrected.1 True wavefront-guided ablation may consist of topography-based treatment that treats the specific aberrations of the anterior corneal surface and refractive error of the whole eye. Alternately, wavefront-guided ablation can also consist of treatment of specific aberrations of the entire eye in addition to the refractive error.

In this study, we compare the accuracy of a topographybased wavefront-guided ablation to wavefront-guided ablation based on entire eye aberrations. The NIDEK Advanced Excimer Laser System (NAVEX) provides a unique opportunity to conduct this study as it contains both treatment options in addition to aspheric and prolate ablations. The customized aspheric treatment zone (CATz) allows wavefront-guided ablation based on corneal topography.2 The optical path difference customized aspheric treatment (OPDCAT) allows wavefrontguided ablation based on the entire aberrations of the eye.

PATIENTS AND METHODS

This study compared the effect of CATz and OPDCAT on wavefront aberrations, corneal shape, and visual acuity in LASIK for myopia with or without astigmatism. The NIDEK NAVEX system (NIDEK, Gamagori, Japan) was used for all treatments. Final Fit ablation planning software was used to plan and simulate the appropriate ablation for each eye. The ablation parameters are transferred to the NIDEK CXII excimer laser via floppy disk or united serial bus drive. The CATz ablation algorithm delivers an aspheric transition zone combined with corneal wavefront ablation within the optical zone. The OPDCAT ablation algorithm uses an aspheric ablation for both optical and transition zones combined with entire eye wavefront treatment in both treatment zones.

Table

TABLE 1Pre- and Postoperative Variables (Mean ± Standard Deviation) of CATz-treated Eyes (N=84) Used for Retrospective AnalysisTABLE 2Pre- and Postoperative Variables (Mean ± Standard Deviation) of OPDCAT-treated Eyes (N =84) Used for Retrospective Analysis

TABLE 1

Pre- and Postoperative Variables (Mean ± Standard Deviation) of CATz-treated Eyes (N=84) Used for Retrospective Analysis

TABLE 2

Pre- and Postoperative Variables (Mean ± Standard Deviation) of OPDCAT-treated Eyes (N =84) Used for Retrospective Analysis

This study comprised two parts: a retrospective review and a prospective, nonrandomized, contralateral study. All patients underwent CATz or OPDCAT ablations with LASIK. Only patients with myopia with or without astigmatism were included in the study. For both components of the study, a total of 196 eyes of 98 patients with a mean age of 35 years were analyzed. In the retrospective review, LASIK using CATz was performed in 84 eyes and LASIK using OPDCAT was performed in 84 eyes. In the CATz-treated eyes, the preoperative mean spherical equivalent refraction was -4.06±1.69 diopters (D) with mean sphere of -3.77±1.70 D and mean cylinder of -0.60±0.51 D. In the OPDCAT-treated eyes, the preoperative mean spherical equivalent refraction was -3.67±2.17 D with mean sphere of -3.52±1.78 D and mean cylinder of -0.70±0.68 D.

In the prospective component of this study, 28 eyes of 14 patients underwent CATz ablation in one eye and OPDCAT in the fellow eye. Prior to surgery and 1 month postoperatively, all eyes underwent a comprehensive baseline examination including measurement of distance uncorrected visual acuity (UCVA) and best spectacle-corrected visual acuity (BSCVA), slit-lamp examination, dilated fundus examination, and NIDEK OPD-Scan analysis of corneal topography and wavefront aberrations. Six parameters of the optical and visual quality of the eye were assessed. These parameters included OPD-rootmean-square (RMS), the magnitude of coma, wavefront higher order error, total spherical aberration, the Strehl ratio, and the corneal asphericity index (Q). We also evaluated the number of eyes with postoperative BSCVA of ≥20/20 and the postoperative gain or loss of BSCVA.

An automated mechanical keratome was used to create all keratectomies. Analysis of variance (ANOVA) of mean pre- and postoperative variables was conducted. Additionally, t testing was used for the comparison between subgroups with Bonferroni correction (0.05/10). Statistical significance was set at P<.005.

RESULTS

The 1-month postoperative results are presented.

RETROSPECTIVE ANALYSIS

CATz. Table 1 shows raw values of the mean variables used in retrospective analysis of CATz-treated patients. CATz treatments manifested no difference between pre- and postoperative RMS values, Strehl ratio, or wavefront error. Postoperatively, a statistically significant increase was noted (P<.005) in both coma (0.08 µm) and corneal asphericity index (0.13). Postoperatively, 95% of eyes achieved ≥20/20 BSCVA with 15% of eyes having a statistically significant (P<.005) gain of 1 line of BSCVA. No eyes lost ≥2 lines of BSCVA.

Table

TABLE 3Change in Study Variables for OPDCAT- and CATz-treated eyes Used in Retrospective AnalysisTABLE 4Postoperative Values (Mean ± Standard Deviation) for Patients Whose One Eye Received OPDCAT Treatment and Fellow Eye Received CATz Treatment (N =14)

TABLE 3

Change in Study Variables for OPDCAT- and CATz-treated eyes Used in Retrospective Analysis

TABLE 4

Postoperative Values (Mean ± Standard Deviation) for Patients Whose One Eye Received OPDCAT Treatment and Fellow Eye Received CATz Treatment (N =14)

OPDCAT. Table 2 shows raw values of the mean variables used in retrospective analysis of OPDCAT-treated patients. OPDCAT treatments had a statistically significant increase in RMS of 0.26 D (P<.005) compared to preoperatively. Coma increased 0.07 pm whereas wavefront error increased by 0.229 pm, both of which were statistically significant (P<.005). Postoperatively, spherical aberration, Strehl ratio, and the corneal asphericity index remained unchanged. Postoperatively, 94.5% of eyes achieved ≥20/20 BSCVA, with 4.5% gaining 1 line of BSCVA. No eyes lost ≥2 lines of BSCVA.

CATz vs OPDCAT. Comparison of CATz and OPDCAT found that the differences in sphere, cylinder, coma, spherical equivalent refraction, Strehl ratio, and BSCVA were not statistically significant (Table 3). OPDCAT-treated eyes showed a statistically significant increase in RMS, higher order wavefront error, and corneal asphericity (Table 3).

PROSPECTIVE ANALYSIS RESULTS

Table 4 shows the mean postoperative variables for CATz and OPDCAT treatments from the contralateral design portion of this study. The difference in spherical aberration between the OPDCAT-treated eyes and fellow CATz-treated eyes was negligible at 0.011 µm. The OPDCAT-treated eyes had an RMS value of 0.67±0.98 D, which was 0.31 D greater than the CATz-treated eyes. This difference in RMS value between OPDCAT and fellow CATz-treated eyes was statistically significant (P<.005). OPDCAT-treated eyes show a larger change in Strehl ratio compared to the CATz-treated eyes. This difference in Strehl ratio between fellow eyes was statistically significant. The spherical equivalent refraction for the OPDCAT-treated eyes was -0.31×0.33 D with residual cylinder of -0.44×0.38 D. The change in corneal asphericity with either ablation algorithm was similar. Best spectacle-corrected visual acuity postoperatively was equivalent for both ablation algorithms. The differences in coma between OPDCAT- and CATz-treated eyes did not attain statistical significance (Table 4).

DISCUSSION

Wavefront-guided ablations have shown promise in the treatment of myopia and myopic astigmatism.35 The ability to choose between topography-guided wavefront treatment or wavefront treatment based on whole eye aberrations is likely to be an option that will be found on most custom ablation platforms in the future. Initially, topography-guided ablations were technically challenging and the refractive outcomes were poor.6 In contrast, the initial results for whole eye wavefront correction were acceptable.7 Theoretically, the results of wavefront correction of the entire eye should provide better visual quality in outcomes than topographybased ablations as the former accounts for all irregularities of the eye. Additionally, by basing the treatment on the cornea alone, the compensation of lenticular and corneal aberrations may become uncoupled, causing a decrease in visual quality and refractive outcomes. This study evaluated the outcomes of topography-guided and wavefront-guided LASIK for primary myopia and myopic astigmatism using the same laser system.

This study revealed a significant difference in optical quality between the CATz- and OPDCAT-treated eyes. The optical quality was worse in OPDCAT treatments as shown by the statistically significant increases in RMS, higher order wavefront error, and Strehl ratio compared to CATz eyes. This study shows that topography-guided ablations (CATz) are slightly better and induce less aberrations than OPDCAT treatments. However, the refractive outcomes were excellent with both treatment algorithms, and no clinically significant difference was noted. The higher induction of aberrations with the OPDCAT ablations may be due to the fact that these treatments are based on Zernike polynomials to drive the ablation. Zernike polynomials are smoothing functions that may not provide adequate resolution compared to the topography-guided ablation algorithms.8

This is the first study to treat one eye with topography-guided and the other eye with whole eye wavefront treatments. The contralateral study design is considered one of the most powerful and statistically sound methods for comparing various treatment modalities. Although the number of eyes was small for the contralateral treatments, we attained statistical significance. One issue with this study is that we present early postoperative results, and corneal wound healing may require more time than conventional myopic treatments. Longer-term follow-up is required to assess this possibility.

When selecting a CATz versus an OPDCAT treatment for a given patient, it is necessary that the surgeon consider the preoperative corneal thickness. Depending on the parameters selected in the Final Fit ablation planning software, tissue removal with the CATz ablation can be 2% to 15% greater compared to an OPDCAT ablation. Theoretically, more tissue removal should increase corneal light scatter, which should reduce the optical quality of the eye for the CATz treatments. However, this is not what we found in this study. We can only explain the improved quality of vision in the CATz group based on the fact that these were not Zernike-based treatments.

Our study reveals small but significant differences in topography-guided and whole eye wavefront-guided treatments, which we believe warrant further investigation.

REFERENCES

1. Ciccone J. LASIK terms "Wavefront-guided" and "Wavefrontoptimized" cause confusion. Eye Surgery Education Council Issues Clarification. ASCRS Communications. December 2004.

2. Kermani O, Schmiedt K, Oberheide U, Gerten G. Early results of Nidek customized aspheric transition zones (CATz) in laser in situ keratomileusis. J Refract Surg. 2003;19:S190-S194.

3. Awwad ST, El-Kateb M, Bowman RW, Cavanagh HD, McCuIley JP. Wavefront-guided laser in situ keratomileusis with the Ale on CustomC ornea and the VISX CustomVue: three -month results. J Refract Surg. 2004;20:S606-S613.

4. Pop M, Payette Y. Correlation of wavefront data and corneal asphericity with contrast sensitivity after laser in situ keratomileusis for myopia. J Refract Surg. 2004;20:S678-S684.

5. Vongthongsri A, Phusitphoykai N, Naripthapan P. Comparison of wavefront-guided customized ablation vs. conventional ablation in laser in situ keratomileusis. J Refract Surg. 200;18:S332-S335.

6. Alio JL, Belda JI, Osman AA, Shalaby AM. Topography-guided laser in situ keratomileusis (TOPOLINK) to correct irregular astigmatism after previous refractive surgery. J Refract Surg. 2003;19:516-527.

7. Mrochen M, Kaemmerer M, Seiler T. Wavefront-guided laser in situ keratomileusis: early results in three eyes. J Refract Surg. 2000;16:116-121.

8. Klyce SD, Karon MD, Smolek MK. Advantages and disadvantages of the Zernike expansion for representing wave aberration of the normal and aberrated eye. J Refract Surg. 2004;20:S537-S541.

TABLE 1

Pre- and Postoperative Variables (Mean ± Standard Deviation) of CATz-treated Eyes (N=84) Used for Retrospective Analysis

TABLE 2

Pre- and Postoperative Variables (Mean ± Standard Deviation) of OPDCAT-treated Eyes (N =84) Used for Retrospective Analysis

TABLE 3

Change in Study Variables for OPDCAT- and CATz-treated eyes Used in Retrospective Analysis

TABLE 4

Postoperative Values (Mean ± Standard Deviation) for Patients Whose One Eye Received OPDCAT Treatment and Fellow Eye Received CATz Treatment (N =14)

10.3928/1081-597X-20050902-16

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