Retrospective Analysis of Changes in the Anterior Corneal Surface After Q Value Guided LASIK and LASEK in High Myopic Astigmatism for 3 Years

Hui Huang; Jianguo Yang; Huijing Bao; Shaorong Chen; Beibei Xia; Jun Zou

Disclosures

BMC Ophthalmol. 2012;12(15) 

In This Article

Discussion

The Q value, which reflects corneal asphericity, is negative for most eyes and not related to the degree of myopia.[3] However, conventional surgery could make the cornea undergo a pathological topographical change, from its initially prolate shape (Q < 0) with a steeper central area and flat peripheral area to an oblate shape (Q > 0) with a flat center and steep periphery.[4–6] Q value guided surgery aimed to minimize changes of the corneal anterior surface asphericity in order to reduce the spherical aberration, which impacts mostly on visual quality.

Most of previous papers about Q value guided ablation were focused on short or medium-term study, correcting low myopia or medium myopia, or comparing aspheric and conventional spheric ablation. Then what about correcting high myopic astigmatism? How the parameters of the anterior corneal surface changed following Q value guided LASIK and LASEK?In the present study, Q value guided LASIK and LASEK showed good efficacy, safety, and predictability in correcting high myopic astigmatism. There were no statistically significant differences between the two groups in the residual refractive errors. Q value guided ablations demonstrated a high safety profile, with no eyes losing lines of CDVA.

After 3 years follow-up, we found the Q value of the anterior surface was inevitably shifted from negative to positive. Consequently, cornea became oblate which means it was relatively flatter in central cornea. The reason might be the differences between ablation algorithm and actual ablation or the setting of Q value target was still not suitable in the present study. For the Q value guided ablation, there were still some controversies over what the ideal target Q should be.[7] Aspheric ablation patterns, such as wavefront optimized algorithm in our excimer laser platform. The target Q was set -0.20 in wavefront optimized algorithm, without the need for customized treatment in every patient.[8] In previous studies, the wavefront-optimized algorithm of the Allegretto Wave Eye-Q 400-Hz excimer laser platform showed good visual and refractive results.[9,10] Anera et al.[11] Compared wavefront optimized (standard) and Q-optimized (F-CAT) algorithms on Strehl ratio and visual discrimination capacity after LASIK. In their study, the target Q in F-CAT algorithms was set -0.5 and optical and visual deterioration were greater after standard ablation. Except -0.20, other values of target Q was also reported. Koller et al.[12] considered that Q-factor customized ablation aiming for a Q-target of -0.40 was as effective as wavefront-guided ablation in correcting myopic astigmatism. In our study, target Q value was set according to the mean of Q1 and Q2, which were calculated by eccentricity of two main meridians within 30 degrees, fully considered each individual's corneal asphericity before surgery. Since we did not carry out controlled trials with conventional ablation in this study, whether the F-CAT algorithm was effective in reducing the Q value shift after the surgery had not been completely verified. In addition, the corneal biomechanical response and the healing response might be the important factors,[13] especially in high myopic astigmatism.

When correcting high myopia, how did the procedure impact the corneal astigmatism? We found Q value ablation could reduce corneal astigmatism with statistically significant differences between preoperative and postoperative values. The aspheric LASIK procedure led to more operative astigmatism compared to aspheric LASEK. In the LASIK procedure, we had to make a corneal flap to expose the stromal bed. The stroma flap procedure was probably the reason that additional astigmatism occurred. Therefore, the effects of correcting astigmatism were not satisfactory.

Furthermore, refractive surgery inevitably changed the regularity of the cornea. We could find that all the indices which reflected corneal regularity increased dramatically. These indices included ISV, IVA and IHA. They all reflected the regularity of the cornea following surgery after 3 years. There were no statistically significant differences between aspheric LASIK and aspheric LASEK. Either type of surgery increased the corneal irregularity.

In the present study, we analyzed high order aberrations of the anterior corneal surface, not including the posterior corneal surface and lens aberrations or other internal aberrations. We think that the changes of the anterior corneal surface could more accurately reflect the effects of LASIK and LASEK refractive procedures. Previous study has reported that changes of the anterior corneal surface could reflect on the optical quality of the visual system.[14] In high order aberrations, we focused on the observations of spherical aberration and coma, the two main HOAs that mostly impact visual quality, not including the total HOA RMS(Root Mean Square). As is commonly accepted, the same total HOAs may have different individual Zernike terms with different visual performances.[15]

As shown in this study, both aspheric LASIK and aspheric LASEK significantly increased spherical aberration and coma at a 6 mm pupil parameter mode compared with preoperative values. The LASIK group exhibited significantly smaller changes in the spherical aberration (Z40) than the LASEK group. We think that was due to a greater induction of HOAs after a LASEK epithelial flap compared to a LASIK stromal flap because of differing cytokine regulation. It is proposed that these differences are primarily due to the different methods of operation. A LASEK epithelial flap and a LASIK stromal flap have different biomechanical changes and postoperative healing responses, which may cause these differences. An epithelial flap has greater postoperative wound remodeling, which may alter HOAs in a different manner. Buzzonetti et al.[16] reported similar results. They compared corneal aberrations after LASIK and LASEK procedures for myopia. Coma-like and spherical-like aberrations increased in both groups, but spherical-like aberrations were greater after LASEK than LASIK over a 3 mm pupil.

According to correlation and regression analysis results, the main impact factors of spherical aberration after surgery were optic zone size, changes in Q value, group and changed refraction . As we all know, spherical aberration after surgery was closely related to the optic zone size of the excimer laser ablation. Setting a larger optical zone size produced less spherical aberration. Furthermore, the changes in Q value had a positive correlation with spherical aberration after surgeries. The difference between the two groups regarding the influence of Z4 0 was stated above. The R2 of this model was 0.644 and therefore we think this model had enough strength to explain the relation between spherical aberration and its impact factors.

Another important aberration that influenced visual quality after surgery was coma. It was widely believed that eccentric ablation was the main reason for coma aberration formation.[17] Our results showed the coma of the anterior corneal surface increased following surgery at year 3 in the two groups, with no difference between them. The main impact factors of coma were gender, optic zone size and MRSE prior to surgery. Female subjects had more coma of the anterior corneal surface than males. It was worth noting that there were more females than males in our study. The unbalanced sex ratio might cause a bias in the statistical test. In addition, according to the linear model, the greater the spherical equivalent before surgery and the smaller the optic zone size of excimer laser ablation were, the more risk for coma after surgery.

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