Changes in central corneal thickness and refractive error after thin-flap laser in situ keratomileusis in Chinese eyes

Corneal higher-order aberrations in different types of irregular cornea

OBJETIVE

To assess differences in corneal higher-order aberrations (i.e., HOAs) between six different types of irregular corneas (i.e, keratoconus (KC), keratoglobus (KG), pellucid marginal degeneration (PMD), laser-assisted in situ keratomileusis (LASIK), penetrating keratoplasty (PK), and intrastromal corneal ring segments (ICRS)).

METHODS

Corneal aberration data from the 3rd and 4th order together with secondary vertical and horizontal coma, and secondary spherical aberration were retrospectively retrieved from a corneal tomographer and compared between irregular cornea types and for 3-, 5-, and 7-mm pupil diameters.

RESULTS

Significant differences were observed in vertical coma, oblique trefoil, secondary oblique astigmatism, primary spherical, and secondary spherical between irregular cornea types across all three pupil sizes (all p≤0.025). Vertical coma consistently exhibited the greatest magnitude of HOA across all irregular cornea types and pupil diameters. For the larger pupil diameters (5 and 7 mm), the 3rd order aberrations (mainly coma-like), the 4th order aberrations (mainly spherical-like), and total HOA RMS were significantly lower in the LASIK group compared to all other irregular cornea types (all p≤0.003). Additionally, at pupil sizes of 5 and 7 mm, the 3rd, 4th, coma-like, spherical-like, and total HOA RMS were significantly lower in KC compared to the ICRS and PK groups (all p≤0.01). Furthermore, the 4th, spherical-like, and total HOA RMS were significantly lower in KC compared to KG at pupil diameters of 3 and 7 mm, 5 and 7 mm, and 7 mm, respectively (all p≤0.03).

CONCLUSIONS

Vertical coma was the most elevated HOA across all irregular cornea types and pupil diameters. Corneal aberrations were lower in LASIK and KC, and higher in KG and PK.

Comparison of central corneal thickness treated with small incision lenticule extraction, femtosecond laser-assisted in situ keratomileusis, or laser-assisted subepithelial keratomileusis for myopia

To compare the central corneal thickness (CCT) treated with small incision lenticule extraction (SMILE), femtosecond laser-assisted in situ keratomileusis (FS-LASIK), or laser-assisted subepithelial keratomileusis (LASEK) for myopia correction. This was a retrospective case series study. Patients who had completed 1-year follow-up after receiving SMILE, FS-LASIK, or LASEK at our hospital from January 2019 to July 2021 were included. Pentacam pachymetry was performed to measure the CCT. The predicted CCT reduction was obtained through laser platform. The measured CCT reduction was defined as the difference between the preoperative and postoperative CCT using Pentacam pachymetry. There were 100 eyes treated by SMILE, 100 eyes by FS-LASIK, and 100 eyes by LASEK. There was a significant difference in predicted CCT reduction among the three surgeries (P < 0.001), but no significant difference in measured CCT reduction postoperatively (PGroup = 0.373). At 1 year postoperatively, the CCT reduction was overestimated by 17.85 ± 5.36 µm in the SMILE group, underestimated by 4.31 ± 7.08 µm in the FS-LASIK group, and underestimated by 7.60 ± 8.28 µm in the LASEK group (PGroup < 0.001, PTime < 0.001). In the FS-LASIK group, the difference between predicted and measured CCT reduction was not related to the predicted CCT reduction (P = 0.095). The laser platform may overestimate the CCT reduction for SMILE and underestimate it for FS-LASIK and LASEK. FS-LASIK has a much higher level of accuracy in CCT reduction, which is not influenced by refractive correction.

Actual anterior-posterior corneal radius ratio in eyes with prior myopic laser vision correction according to axial length

We retrospectively evaluate the actual anterior-posterior (AP) corneal radius ratio in eyes with previous laser correction for myopia (M-LVC) according to axial length (AL) using biometry data exported from swept-source optical coherence tomography between January 2018 and October 2021 in a tertiary hospital (1018 eyes with a history of M-LVC and 19,841 control eyes). The AP ratio was significantly higher in the LVC group than in the control group. Further, it was significantly positively correlated with AL in the LVC group. We also investigated the impact of the AP ratio, AL and keratometry (K) on the absolute prediction error (APE) in 39 eyes that underwent cataract surgery after M-LVC. In linear regression analyses, there were significant correlations between APE and AL/TK, while APE and AP ratio had no correlation. The APE was significantly lower in the Barrett True-K with total keratometry (Barrett True-TK) than in the Haigis-L formula on eyes with AL above 26 mm and K between 38 and 40 D. In conclusion, in eyes with previous M-LVC, AP ratio increases with AL. The Barrett True-K or Barrett True-TK formulas are recommended rather than Haigis-L formula in M-LVC eyes with AL above 26 mm and K between 38 and 40D.

Non-contact specular microscopy with Topcon instruments to assess central corneal thickness of healthy human eyes - A 20 year review

BACKGROUND

The purpose of this review was to evaluate the consistency of central corneal thickness (CCT) values reported with use of Topcon SP-2000 P and SP-3000 P non-contact specular microscopes since their introduction in 1999 with the two microscopes having been commonly used in a wide range of studies.

METHODS

As a primary resource, PubMed was used to search for peer-reviewed articles in any language that included CCT values obtained with non-contact specular microscopy reported for humans with nominally healthy corneas. Relevant articles were obtained and any cited publications also checked.

RESULTS

A total of 76 articles were identified which reported CCT on different small-to-moderate sized groups of individuals, published between 1999 and 2019. From these, an overall group mean CCT value of 0.525 ± 0.013 mm (median 0.525 mm) can be calculated. An estimated 95 % confidence interval (CI, based on 1.96 SD) would be between 0.500 and 0.550 mm. For the two Topcon models, the group mean ± SD values were 0.529 ± 0.013 mm and 0.517 ± 010 mm respectively. An assessment of the CCT data sets in relation to the reported average age indicated no statistically significant effect (p = 0.289, r = -0.129). Very similar average CCT values were also encountered in 4 other reports where these microscopes were used in large-scale population studies as well as in 2 other reports using the newer Topcon SP-1 P model.

CONCLUSIONS

The Topcon stand-alone non-contact specular microscopes have yielded consistent and predictable corneal thickness measures over many years.

Comparison of 9.0 and 9.2 mm Flap Diameter Options of Femtosecond Laser In-Situ Keratomileusis for Hypermetropia and Hypermetropic Astigmatism

Aim

To compare the postoperative one-year outcomes of asphericity (Q) and high order aberration (HOA) values of 9.0 and 9.2 mm diameter flap groups in hypermetropia and hypermetropic astigmatism subjects who underwent femtosecond laser in-situ keratomileusis (LASIK).

Materials and Methods

The study included 68 eyes of 34 patients. A femtosecond laser platform (Allegrato Wave, Wavelight AG, Erlangen, Germany) was used for flap cutting. Corneal stroma was ablated using Wavelight EX500 with wavefront-optimized profile (WaveLight GmbH, Erlangen, Germany). 9.0 mm flap diameter was randomly chosen for one eye, and 9.2 mm flap diameter was chosen for the fellow eye. Two eyes of the patients who used two different flap diameters were enrolled into two different groups. Corneal stroma was ablated using Wavelight EX500 with wavefront-optimized profile (WaveLight GmbH, Erlangen, Germany). Postoperative one-year outcomes of Q and HOA values of 9.0 and 9.2 mm diameter flap groups were compared statistically.

Results

The preoperative manifest refraction spherical equivalents of the 9.0 and 9.2 mm diameter flap groups were 1.86 ± 1.81 D and 1.69 ± 1.99 D (p=0.754). No intraoperative or postoperative complications were observed. At postoperative one-year, Q values were 0.98 ± 0.13 D and 0.91 ± 0.15 D (p=0.029). HOAs including horizontal and vertical coma, horizontal and vertical trefoil, spherical aberration, and second order vertical coma were not significantly different (p > 0.05 for all). Total HOA values were 1.62 ± 0.14 and 1.40 ± 0.16, in the 9.0 and 9.2 mm diameter flap groups, respectively (p < 0.001).

Conclusion

Both the 9.0 and 9.2 mm diameter flap options in femtosecond LASIK are equally safe and effective. Many of the HOA values are similar in both options, and better results were provided in terms of total HOA and Q values with the 9.2 mm diameter flap option. This study was registered with trial registration number 118-011.

Shielding effect of the smoke plume by the ablation of excimer lasers

Background

Shielding and scattering effect of the smoke plume column ejected from the laser ablated material is a well-known phenomenon. Debris evacuation system of the excimer laser equipment removes these particles, but insufficient air flow can result in undesired refractive outcomes of the treatment. The aim of this study was to reveal the effect of the air flow speed on the actual ablation depth.

Methods

SCWIND AMARIS 500E flying spot excimer laser was tested in this study. A 150 μm phototherapeutic keratectomy (PTK) profile with 8 mm diameter was applied to the surface of polymethyl methacrylate (PMMA) plates. The velocity of the air flow was changed with adjustable air aspiration system. Ablation depth was measured with highly-precise contact micrometer.

Results

The prediction model was statistically significant, F(1,8) = 552.85, p < 0.001, and accounted for approximately 98.7% of variance of ablation (R² = 0.987, R²_adj = 0.986). Lower air flow speed resulted in a weaker ablation capability of the excimer laser.

Conclusion

Air flow generated by the aspiration equipment is a key factor for the predictable outcomes of refractive treatment. Therefore, manufacturer inbuilt debris removal system should be regularly checked and maintained to ensure proper clinical and predictable refractive results.

Prospective Cohort Comparison of Visual Acuity and Contrast Sensitivity Between Femto Laser In Situ Keratomileusis and Orthokeratology for Low-to-Moderate Myopia

PURPOSE

To compare the visual acuity and contrast sensitivity (CS) after femto laser in situ keratomileusis (fLASIK) and orthokeratology (OK) in patients with low-to-moderate myopia.

METHODS

A total of 68 patients were enrolled in this prospective cohort study. Of these, 40 eyes from 40 patients with low-to-moderate myopia underwent fLASIK and 28 eyes from 28 patients underwent OK. In addition to uncorrected distance visual acuity and corrected distance visual acuity, photopic and mesopic CS (with CSV-1000E) were tested before treatment and at 1, 3, and 6 months after treatment.

RESULTS

The preoperative refraction and CS were not significantly different between the groups (P>0.05). Postoperative visual acuity and refraction were similar (P>0.05). The photopic CS of the fLASIK group was better than that of the OK group (P<0.01). The fLASIK mesopic CS was better than the OK at 3 c/d (P=0.023). In the fLASIK group, only the photopic CS of postoperative 12 c/d decreased at 1 month and 3 months (P<0.01) and 18 c/d decreased at 1 month (P=0.002); whereas in the OK group, all the postoperative photopic CS values decreased at 1 month (all P<0.01) and 3 months (P<0.05) and increased to the normal level at 6 months (P>0.05). While as to the mesopic CS, in the fLASIK group, the 3 c/d, 6 c/d, and 18 c/d decreased at 1 month (P<0.05) and in the OK group, the 3 c/d decreased at 1 month and 3 months (P<0.05) and the 6 c/d decreased at 3 months (P=0.041), and the 12 c/d and 18 c/d decreased at 1 month (P<0.01).

CONCLUSIONS

These results show that fLASIK provides better visual quality than OK for patients with low-to-moderate myopia.

Corneal Thickness in Highlanders

Patyal, Sagarika, Amit Arora, Arun Yadav, and Vijay K. Sharma. Corneal thickness in highlanders. High Alt Med Biol. 18:56-60, 2017.

BACKGROUND

Corneal thickness is an important parameter with diagnostic and therapeutic implications. Various studies have highlighted increase in corneal thickness in lowlanders on ascending to high altitude. However, there are no studies in the published literature pertaining to corneal thickness of the highlanders who are inhabitants of such altitudes. Hence, study was carried out with objective to determine the corneal thickness of highlanders living at heights of more than 11,000 feet and compare it with corneal thickness of lowlanders.

MATERIALS AND METHODS

The highlander participants of the study consisted of inhabitants of Ladakh region of India at an altitude of 11,000 feet or more and lowlander participants consisted of inhabitants at an altitude of 1500 feet. A total of 254 highlanders and 212 lowlanders participated. A mean of 25 measurements of central corneal thickness (CCT) of every participant was obtained for each eye using ultrasonic pachymeter.

RESULTS

The mean age of the participants was 41.8 (15.9) and 47.7 (17.7) years among lowlanders and highlanders, respectively. The highlanders had 11.95 μm lower mean CCT reading compared to lowlanders after adjusting for age and sex (p value <0.001). There was no statistically significant difference in mean CCT readings of right eye and left eye in either lowlanders or highlanders. Age also had a significant effect after adjustment for location and gender (p = 0.001). CCT decreased by 0.31 μm with every year increase in the age. Gender had no statistically significant effect.

CONCLUSION

This study found statistically significant difference in CCT measurements between highlanders and lowlanders. The thinner corneas of highlanders may have a bearing on diagnosis and treatment of glaucoma, refractive surgery, contact lens fitting implantation of Intacs, and astigmatic keratectomy done on such patients. The study also opens the scope of further research in the area.