Mutual comparative analysis: a new topography-guided custom ablation protocol referencing subjective refraction to modify corneal topographic data

Clinical results of topography-guided laser-assisted in situ keratomileusis using the anterior corneal astigmatism axis and manifest refractive astigmatism axis

PURPOSE

To evaluate the clinical results of primary topography-guided femtosecond laser-assisted in situ keratomileusis (TG-FS-LASIK) using the Pentacam-measured anterior corneal astigmatism axis (ACA) or manifest refractive astigmatism axis (MRA).

METHODS

In this prospective cohort study, all eyes were treated with primary TG-FS-LASIK using the manifest cylinder. Thirty-two right eyes were treated using ACA with axis disparity > 5° in the experimental group, and 32 right eyes were treated using MRA with axis disparity ≤ 5° in the control group. Visual, refractive outcomes, and corneal higher-order aberrations were evaluated. Vector analysis of astigmatism was performed using Alpins method with the ASSORT software.

RESULTS

The mean logMAR UDVA in the experimental group was - 0.12 ± 0.06, - 0.05 ± 0.08, and - 0.08 ± 0.08 at 1-week, 1-month, and 3-month follow-up, whereas - 0.05 ± 0.06, - 0.12 ± 0.06, and - 0.14 ± 0.06 in the control group (p = 0.017, p < 0.001, and p = 0.003). At 3-month follow-up, 79% eyes achieved a UDVA of 20/16 or better, 9% gained one line of corrected distance visual acuity, the mean manifest cylinder was - 0.375 ± 0.254 D, 84% showed a manifest cylinder within ± 0.50 D, the mean magnitude of difference vector was 0.41 ± 0.35 D, the mean absolute angle of error (AE) was 7.36 ± 6.21°, and 41% exhibited an arithmetic AE within ± 5° in the experimental group, whereas 94%, 19%, - 0.203 ± 0.148 D, 100%, 0.21 ± 0.15 D, 3.39 ± 3.68°, and 69% in the control group (all p < 0.05). The postoperative and preoperative corneal spherical aberrations were comparable in both groups (p > 0.05).

CONCLUSION

Primary topography-guided FS-LASIK using Pentacam-measured anterior corneal astigmatism axis exhibited slightly inferior clinical results to that using the manifest refractive astigmatism axis. Both methods achieved comparable postoperative and preoperative corneal spherical aberrations.

Hemidivisional vector planning to reduce and regularize irregular astigmatism by laser treatment

PURPOSE

To demonstrate how hemidivisional vector planning of refractive laser treatments of astigmatism can be used to directly address idiopathic corneal irregular astigmatism that has an asymmetrical, non-orthogonal bow tie topography appearance.

DESIGN

Case study.

METHODS

The cornea is conceptually divided into two hemidivisions along the flat meridian of the corneal topographic astigmatism (CorT), which means that each hemidivision will approximately correspond to one lobe of the asymmetric, non-orthogonal topographic bow tie. An astigmatism reduction treatment can then be planned separately for each hemidivision using the vector planning technique, based on both its two hemidivisional CorT measures and common manifest refractive cylinder. The remaining irregularity is then regularized, and the junctional zone smoothed across the flat meridian. The final intended treatment thus combines hemidivisional astigmatism reduction and regularization of the corneal astigmatism and spherical refractive error in one treatment application. This could be applied to LASIK, PRK, SMILE, and Transepithelial PRK procedures using Designer Cornea^® software.

RESULTS

A theoretical treatment profile is derived from an actual example of a cornea with idiopathic asymmetric non-orthogonal astigmatism. The three steps of the derivation are as follows: (i) astigmatism reduction through the use of the vector planning technique; (ii) regularization, and (iii) smoothing across the hemidivisional midline.

CONCLUSIONS

Hemidivisional vector planning treatments could potentially both reduce and regularize asymmetric non-orthogonal astigmatism. These treatments can be systematically customized to account for qualitative and quantitative differences between the two corneal hemidivisions at the same time as correction of coexistent myopia or hyperopia.

Large Axis Difference Between Topographic Anterior Corneal Astigmatism and Manifest Refractive Astigmatism: Can Topography-Guided LASIK Target the Manifest Axis?

PURPOSE

To investigate whether the degree of astigmatism axis discrepancy between preoperative manifest refractive astigmatism and anterior corneal astigmatism impacts refractive and visual outcomes of primary topography-guided laser in situ keratomileusis (LASIK) targeting the refractive astigmatism, and to provide guidance on treating eyes with very large axis discrepancy.

METHODS

Comparative retrospective analysis of 25,396 consecutive eyes treated with topography-guided LASIK on the manifest refractive astigmatism. Standard outcomes of the 14,534 eyes with small axis discrepancy (SAD) (Δ in axis ⩽ 10°) were compared to the 2,222 eyes with very large axis discrepancy (VLAD) (Δ in axis ⩾ 45°). Pearson correlation coefficient was used to assess relationships between selected variables.

RESULTS

The mean axis discrepancy was 4.47 ± 2.92° in SAD eyes, and 65.0 ± 13.4° in VLAD eyes. An equivalent number of eyes achieved a cumulative postoperative unilateral uncorrected distance visual acuity of 20/20 in both the SAD and VLAD groups (93.02% vs 93.42%; P = .4892). The efficacy index (0.98 ± 0.13 vs 0.98 ± 0.07; P = .3931) and the safety index (1.00 ± 0.11 vs 1.00 ± 0.03; P = .4757) were identical between groups. There was no clinically meaningful correlation between the preoperative axis discrepancy and preoperative total root mean square anterior corneal coma, postoperative refractive astigmatism, defocus equivalent, spherical equivalent, and angle of error, all with weak correlation coefficients (R = -0.02, -0.03, -0.02, 0.01, and 0.05).

CONCLUSIONS

Large preoperative axis discrepancy (45° to 90°) between refractive astigmatism and topography-measured anterior corneal astigmatism does not negatively impact topography-guided LASIK, having identical refractive and visual outcomes compared to eyes with small (0° to 10°) discrepancy. There is no basis to exclude eyes with large axis discrepancy from topography-guided LASIK, and these eyes should be treated on the manifest refractive astigmatism. [J Refract Surg. 2021;37(10):662-673.].