Six modes of corneal topography for evaluation of ablation zones after small-incision lenticule extraction and femtosecond laser-assisted in situ keratomileusis

Effective optical zone following small incision lenticule extraction: a review

Small incision lenticule extraction (SMILE) is a "flapless" keratorefractive surgery with excellent safety, efficacy, stability, and predictability for myopia correction. A recent global multicenter study also reported good refractive outcomes for hyperopic SMILE. SMILE has shown advantages including improved biomechanical strength, fewer dry eye symptoms, less corneal denervation, and fewer surgery-induced higher-order aberrations over laser in situ keratomileusis (LASIK). However, night vision complaints, including glare, halos, and starbursts, could still occur after SMILE. These symptoms have been proven to be closely related to the effective optical zone (EOZ), which is defined as the achieved area of corneal ablation. A larger postoperative EOZ may indicate better visual quality, making EOZ an important safety parameter for keratorefractive surgeries. As SMILE has gained wider application globally, the EOZ following SMILE has also been increasingly studied in the field of refractive surgery. This review provides an update on topics related to the EOZ after SMILE, including its measurement and influencing factors, aiming to benefit the personalization of the surgical algorithm and ultimately improve the visual quality after the SMILE procedure.

Effective Optical Zone and Centration Following SMILE and FS-LASIK for High Myopia Calculated With a Novel Method

PURPOSE

To compare the effective optical zone (EOZ) and centration in eyes with high myopia after small incision lenticule extraction (SMILE) and femtosecond laser-assisted in situ keratomileusis (FS-LASIK) using a novel method.

METHODS

Forty eyes of 40 consecutive patients with high myopia scheduled for SMILE or FS-LASIK were enrolled in the study. The EOZ, optical zone decentration, and corneal aberrations were analyzed using Scheimpflug imaging. These values were then analyzed and compared between the two procedures 6 months after surgery.

RESULTS

The mean EOZ diameter for SMILE (4.41 ± 0.14 mm) was larger than that for FS-LASIK (4.24 ± 0.28 mm; P = .002), corresponding to reductions of 1.60 ± 0.11 and 1.71 ± 0.21 mm, respectively, compared with the programmed optical zone (POZ) (P = .007). Moreover, the total decentration for SMILE (0.33 ± 0.12 mm) was greater than that for FS-LASIK (0.27 ± 0.15 mm; P = .020). The induction of spherical aberration (SA) was lower with SMILE than with FS-LASIK (P = .007).

CONCLUSIONS

A larger EOZ and less SA were observed after SMILE than after FS-LASIK in eyes with high myopia. [J Refract Surg. 2023;39(11):736-740.].

The influence of corneal ablation patterns on prediction error after cataract surgery in post-myopic-LASIK eyes

Purpose

To evaluate the influence of corneal ablation patterns on the prediction error after cataract surgery in post-myopic-LASIK eyes.

Methods

Eighty-three post-myopic-LASIK eyes of 83 patients that underwent uneventful cataract surgery were retrospectively included. Predicted postoperative spherical equivalence (SE) was calculated for the implanted lens using the Haigis-L and Barrett True-K formula. Prediction error at one month postsurgery was calculated as actual SE minus predicted SE. For each eye, area and decentration of the ablation zone was measured using the tangential curvature map. The associations between prediction errors and corneal ablation patterns were investigated.

Results

The mean prediction error was − 0.83 ± 1.00 D with the Haigis-L formula and − 1.00 ± 0.99 D with the Barrett True-K formula. Prediction error was positively correlated with keratometry (K) value and negatively correlated with ablation zone area using either formula, and negatively correlated with decentration of the ablation zone using the Barrett True-K formula (all P < 0.05). In the K < 37.08 D group, prediction error was negatively correlated with decentration of the ablation zone with both formulas (all P < 0.05). Multivariate analysis showed that with the Haigis-L formula, prediction error was associated with axial length (AL), K value and decentration, and with the Barrett True-K formula, prediction error was associated with AL and decentration (all P < 0.05).

Conclusion

A flatter cornea, larger corneal ablation zone and greater decentration will lead to more myopic prediction error after cataract surgery in post-myopic-LASIK eyes.

Effective Optical Zone Following Small Incision Lenticule Extraction for Myopia Calculated With Two Novel Methods

PURPOSE

To evaluate the effective optical zone (EOZ) following small incision lenticule extraction in myopic eyes using two novel methods and investigate factors influencing postoperative EOZ.

METHODS

In this prospective study, 45 patients (45 eyes) with a mean spherical equivalent of -5.82 ± 1.58 diopters underwent SMILE and were observed during a 6-month follow-up. Postoperative EOZ was calculated using custom software that automatically distinguishes EOZ on the tangential curvature difference map (EOZc) and total corneal refractive power map (EOZp) of the Pentacam HR (Oculus Optikgeräte GmbH). The agreement between the two methods, the difference between postoperative EOZ and programmed optical zone (POZ), and its relationship with parameters including corrected spheres, cylinders, ablation ratio (ablation depth/central cornea thickness), and Q-value change were investigated.

RESULTS

The EOZc area was 20.76 ± 2.43 mm² (diameter: 5.04 ± 0.60 mm) and the EOZp area was 20.22 ± 4.70 mm² (diameter: 5.13 ± 0.30 mm). Both were significantly smaller than POZ (P < .001). Bland-Altman plots showed 4.44% (2/45) points located outside the 95% limits of agreement. EOZc and EOZp reductions were negatively related to corrected cylinders (r = -0.631, P < .001 and r = -0.594, P < .001, respectively). EOZp reduction was positively correlated with corrected spheres (r = 0.336, P = .024). Subgroup analysis revealed significant differences in EOZc and EOZp reduction between low and high astigmatism groups despite myopia degree.

CONCLUSIONS

EOZ after incision lenticule extraction, measured using two novel methods, was smaller than POZ. The reduction of EOZ was negatively correlated with the corrected cylinders. [J Refract Surg. 2022;38(7):414-421.].

Corneal Lenticule Creation Using a New Solid-State Femtosecond Laser Measured by Spectral Domain OCT in a Porcine Eye Model

Purpose

To determine the accuracy and precision of corneal lenticule creation with a new solid-state femtosecond laser in a porcine eye model.

Methods

Corneal lenticule creation was performed using a new solid-state femtosecond laser on 60 porcine eyes with 10 subgroups. Optical coherence tomography images were acquired immediately after laser treatment. Cap thickness (CT), cap diameter (CD), and lenticule thickness (LT) were measured manually by three independent readers. Additionally, CT and LT were measured by an automated algorithm (aLT, aCT).

Results

Measured LT was significantly greater than the intended LT (average difference 14.3 ± 5.6 µm, P < 0.001). aLT was closer but still significantly different from the intended LT (-2.9 ± 5.8 µm, P < 0.001). Measured CT showed no significant difference from the intended CT (2.6 ± 13.3, P = 0.145). aCT was significantly smaller compared to the intended CT (-9.6 ± 13.6, P < 0.001). Measured CD was significantly smaller compared to the intended CD (-0.21 ± 0.20 mm, P < 0.001). All lenticules were cut as planned with no laser-related complications.

Conclusions

This new solid-state femtosecond laser used in our trial provides corneal lenticule creation in a porcine eye model comparable to other established systems. However, measuring those lenticules in the provided setting seems too challenging even when using semiautomated algorithms, which seems to be due to the experimental setting of the trial.

Translational Relevance

This trial shows the precision and repeatability of corneal cuts performed by a new femtosecond laser that could translate to refractive corneal lenticule surgery.