Persistent subepithelial haze in thin-flap LASIK

Visual outcomes of early enhancement following small incision lenticule extraction versus laser in situ keratomileusis

Purpose

To compare visual outcomes of early enhancement following small incision lenticule extraction (SMILE) versus laser in situ keratomileusis (LASIK).

Methods

Retrospective analysis of eyes (patients operated in the setting of a tertiary eye care hospital between 2014 and 2020) requiring early enhancement (within one year of primary surgery) was conducted. Stability of refractive error, corneal tomography, and anterior segment Optical Coherence Tomography (AS-OCT) for epithelial thickness was performed. The correction post regression was done using photorefractive keratectomy and flap lift in eyes, wherein the primary procedure was SMILE and LASIK, respectively. Pre- and post enhancement corrected and uncorrected distance visual acuity (CDVA and UDVA), mean refractive spherical equivalent (MRSE), and cylinder were analyzed. IBM SPSS statistical software.

Results

In total, 6350 and 8176 eyes post SMILE and LASIK, respectively, were analyzed. Of these, 32 eyes of 26 patients (0.5%) post SMILE and 36 eyes of 32 patients (0.44%) post-LASIK required enhancement. Post enhancement (flap lift in LASIK, and PRK in SMILE group) UDVA was logMAR 0.02 ± 0.05 and 0.09 ± 0.16 (P = 0.009), respectively. There was no significant difference between the refractive sphere (P = 0.33) and MRSE (P = 0.09). In total, 62.5% of the eyes in the SMILE group and 80.5% in the LASIK group had a UDVA of 20/20 or better (P = 0.04).

Conclusion

PRK post SMILE demonstrated comparable results to flap lift post LASIK and is a safe and effective approach for early enhancement post SMILE.

Three-Year Follow-Up of Laser In Situ Keratomileusis Treatments for Myopia: Multi-Center Cohort Study in Korean Population

This multi-center cohort study included 3401 myopic laser in situ keratomileusis (LASIK) procedures conducted in 1756 myopia patients between 2002 and 2005. Pre- and postoperative uncorrected visual acuity (UCVA), best corrected visual acuity (BCVA), and manifest refraction spherical equivalent (SE) were recorded. Factors predicting low postoperative efficacy (defined as a postoperative UCVA < 0.5) were identified using univariate and multivariate logistic regression analysis. Compared with 1 month postoperatively, logMAR UCVA at 3 months postoperatively was significantly decreased (p = 0.002) and that at 2 and 3 years was significantly increased (p < 0.001). LogMAR BCVA at 2 years postoperatively was significantly decreased compared with 1 month postoperatively (p = 0.008). Over the 3-year postoperative period, overall refractive predictability within ±1.00 D and ±0.50 D ranged from 69.0% to 86.2% and from 43.3% to 67.8%, respectively. This also decreased from 1 month to 6 months postoperatively (p < 0.005). Multivariate logistic regression analysis using generalized estimating equations, revealed that higher preoperative SE (odds ratio [OR], 2.58 and 7.23; p < 0.001) and lower preoperative BCVA (OR, 2.44; p = 0.003) were predictive of a low postoperative efficacy. In summary, myopic LASIK can be effective and safe with a high refractive predictability in a Korean population, but myopic regression occurs over time. Higher preoperative SE and lower preoperative BCVA are predictive of a low postoperative efficacy.

Establishing a Porcine Eye Model for Manual Sub-Bowman Layer Photorefractive Keratomileusis

Objective

To establish a porcine eye model for manual sub-Bowman layer photorefractive keratomileusis (SBPRK), which is a reformed surface ablation refractive surgery that results in preserving the corneal Bowman layer (BL).

Methods

The SBPRK group consisted of eleven eyes of 8 healthy pigs with BL flaps by mechanical technique followed by laser ablation. Regarding the remaining 5 eyes, 3 random eyes had transepithelium photorefractive keratectomy (TransPRK) (the TransPRK group), while the other 2 eyes were untreated (the blank control group). All the pigs were followed up for 8 weeks. Slit-lamp biomicroscopy and optical coherence tomography (OCT) were examined before the surgeries and at 1 week, 4 weeks, and 8 weeks after the surgeries.

Results

In a few days after the surgery, 3 eyes of the SBPRK group were excluded from the study because of poor healing of the corneal flaps. At the 1^st postoperative week, one eye had an irregular defect of about 3 mm in the central corneal epithelium area; the cornea of the other 7 eyes had just light edema with intact epithelium just like the cornea of the TransPRK group. At the 4^th week, in the SBPRK group, the cornea was slightly hazy (haze stage 1). While in the TransPRK group, the cornea was hazier (haze stage 2). At the 8^th week, in the SBPRK group, both corneas were almost transparent, and the edges of the BL flaps could not be clearly seen. Meanwhile, in the TransPRK group, the corneal haze became lighter and thinner. OCT showed that, in the SBPRK group, there was high reflection in the BL layer, and it was obvious at 1 week postoperation, decreased at 4 weeks, and calmed down at 8 weeks. However, in the TransPRK group, the high reflection diffused in the anterior corneal stroma at 1 week postoperation, enhanced at 4 weeks, and weakened at 8 weeks.

Conclusions

Preserving the BL while conducting surface refractive surgery may result in less haze than TransPRK. However, further study is still needed, and this technique still requires refining until it becomes a standard clinical procedure.

Comparison of central corneal thickness measurement by scanning slit topography, infrared, and ultrasound pachymetry in normal and post-LASIK eyes

OBJECTIVES

To compare central corneal thickness (CCT) measurements by scanning slit topography (SST), infrared pachymetry (IRP), and ultrasound pachymetry (USP), and their agreement in normal and post-laser in situ keratomileusis (LASIK) eyes.

METHODS

Sixty normal and 35 post-LASIK subjects were recruited. Only one eye from each subject was analyzed. Non-contact pachymetry was performed first, and the order for SST (Orbscan IIz) and IRP (Tonoref III) was randomized for each patient, to be followed by contact USP (Echoscan US-4000). Pearson's correlation, paired t test, and Bland-Altman plots were used to investigate association, difference, and agreement among different instruments respectively.

RESULTS

The measurements obtained with the instruments were highly correlated. Compared to CCT determined by USP (CCT_USP), CCT determined by SST (after correction with acoustic factor) (CCT_SST^C) was thicker by 7 µm in normal eyes (P < 0.001). There was no significant difference between CCT_SST^C and CCT_USP (P = 0.128), but a thickness-dependent deviation in post-LASIK eyes (P = 0.003). The CCT determined by IRP (CCT_IRP) was thicker than CCT_USP in normal (P < 0.001) and post-LASIK eyes (P < 0.001) and demonstrated proportional overestimation with thinner corneas, with less predictable ultrasonic equivalent corneal thickness in normal eyes. Conversely, CCT_IRP significantly underestimated CCT compared to CCT_SST and showed increasing underestimation with thinner corneas in both normal and post-LASIK eyes (both P < 0.001).

CONCLUSION

Central corneal thickness determined by SST, IRP and USP were not interchangeable or interconvertible, probably attributed to difference in methodologies. Compensation with algorithms may improve agreements amongst instruments.

Enhancement Options After Myopic Small-Incision Lenticule Extraction (SMILE): A Review

PURPOSE

To provide an overview of the currently available retreatment methods after myopic small-incision lenticule extraction (SMILE).

DESIGN

Systematic literature review.

METHODS

The PubMed library was searched for articles containing the terms "small-incision lenticule extraction" and "enhancement" or "retreatment". The last search was performed on May 1, 2019.

RESULTS

In contrast to laser in-situ keratomileusis (LASIK), which can be retreated by a flap relift, repeat SMILE retreatment is currently not approved and only seldomly performed. As substitutes, surface ablation, cap-to-flap conversion using the CIRCLE program in the VisuMax platform, and thin-flap LASIK have been recently established. While all options offer safety and efficacy comparable to LASIK retreatments, each has its patient-specific advantages and disadvantages. While surface ablation preserves the flap-free approach of the primary procedure, the aspect of pain and a slow visual recovery might render it less attractive as compared with CIRCLE and thin-flap LASIK which offer quick recovery, however at the price of flap creation. Besides, each retreatment method generates specific tissue responses and has a different impact on corneal biomechanics, which is strongly dependent on the previous SMILE parameters, especially the cap thickness.

CONCLUSIONS

Refractive enhancement after SMILE is currently mostly performed by surface ablation, CIRCLE cap-to-flap conversion or thin-flap LASIK, which all offer safety and efficacy comparable to LASIK retreatments. In this review, a detailed overview over each method, its technical aspects, and specific advantages and disadvantages is given.

Corneal Molecular and Cellular Biology for the Refractive Surgeon: The Critical Role of the Epithelial Basement Membrane

PURPOSE

To provide an overview of the recent advances concerning the corneal molecular and cellular biology processes involved in the wound healing response after excimer laser surface ablation and LASIK surgery.

METHODS

Literature review.

RESULTS

The corneal wound healing response is a complex cascade of events that impacts the predictability and stability of keratorefractive surgical procedures such as photorefractive keratectomy and LASIK. The generation and persistence of corneal myofibroblasts (contractile cells with reduced transparency) arise from the interaction of cytokines and growth factors such as transforming growth factor beta and interleukin 1 produced by epithelial and stromal cells in response to the corneal injury. Myofibroblasts, and the opaque extracellular matrix they secrete into the stroma, disturb the precise distribution and spacing of collagen fibers related to corneal transparency and lead to the development of vision-limiting corneal opacity (haze). The intact epithelial basement membrane has a pivotal role as a structure that regulates corneal epithelial-stromal interactions. Thus, defective regeneration of the epithelial basement membrane after surgery, trauma, or infection leads to the development of stromal haze. The apoptotic process following laser stromal ablation, which is proportional to the level of attempted correction, leads to an early decrease in anterior keratocyte density and the diminished contribution of these non-epithelial cells of components such as perlecan and nidogen-2 required for normal regeneration of the epithelial basement membrane. Haze persists until late repair of the defective epithelial basement membrane.

CONCLUSIONS

Defective regeneration of the epithelial basement membrane has a critical role in determining whether a cornea heals with late haze after photorefractive keratectomy or with scarring at the flap edge in LASIK.

Measuring the Refractive Index of Bovine Corneal Stromal Cells Using Quantitative Phase Imaging

The cornea is the primary refractive lens in the eye and transmits >90% of incident visible light. It has been suggested that the development of postoperative corneal haze could be due to an increase in light scattering from activated corneal stromal cells. Quiescent keratocytes are thought to produce crystallins that match the refractive index of their cytoplasm to the surrounding extracellular material, reducing the amount of light scattering. To test this, we measured the refractive index (RI) of bovine corneal stromal cells, using quantitative phase imaging of live cells in vitro, together with confocal microscopy. The RI of quiescent keratocytes (RI = 1.381 ± 0.004) matched the surrounding matrix, thus supporting the hypothesis that keratocyte cytoplasm does not scatter light in the normal cornea. We also observed that the RI drops after keratocyte activation (RI = 1.365 ± 0.003), leading to a mismatch with the surrounding intercellular matrix. Theoretical scattering models showed that this mismatch would reduce light transmission in the cornea. We conclude that corneal transparency depends on the matching of refractive indices between quiescent keratocytes and the surrounding tissue, and that after surgery or wounding, the resulting RI mismatch between the activated cells and their surrounds significantly contributes to light scattering.

Complications of femtosecond-assisted laser in-situ keratomileusis flaps

Femtosecond-assisted laser in-situ keratomileusis flaps have revolutionized refractive surgery since their introduction. Although these lasers are exceedingly safe, complications still do occur. This review focuses specifically on examining the literature and evidence for flap complications during femtosecond-assisted laser in-situ keratomileusis as well as their management.

Longitudinal comparison of outcomes after sub-Bowman keratomileusis and laser in situ keratomileusis: randomized, double-masked study

PURPOSE

To compare the outcomes of sub-Bowman keratomileusis (100-μm flap) and laser in situ keratomileusis (LASIK) (120-μm flap) using 150-kHz femtosecond laser.

DESIGN

Randomized, double-masked, contralateral clinical trial.

METHODS

One hundred patients (200 eyes) with myopia or myopic astigmatism were included. Postoperative examinations were performed at week 1 and months 1, 3, 6, and 12. Main outcome measures included postoperative uncorrected (UCVA) and best-corrected distance visual acuity (BCVA); manifest refraction spherical equivalent; efficacy and safety indices; corneal thickness; and complications.

RESULTS

The mean age of patients was 33.9 ± 7.9 years. Overall, the preoperative UCVA, BCVA, and manifest refraction spherical equivalent were 1.349 ± 0.332, -0.022 ± 0.033, and -5.81 ± 1.61 diopters, respectively. No significant difference was observed in preoperative (P ≥ .226) or intraoperative parameters (P ≥ .452) between both groups, except residual stromal thickness (P < .001). The UCVA, manifest refraction spherical equivalent, and central corneal thickness stabilized by 1 week, while the thinnest corneal thickness stabilized by 3 months postoperatively. There was no significant difference between both groups for any parameter during all follow-up visits (P ≥ .132) except the 3-month safety index, which was better in the sub-Bowman keratomileusis group (P = .007). Soft opaque bubble layer was noted intraoperatively in 12 cases (7, 100-μm group; 5, 120-μm group; P = .577). No postoperative complications were observed.

CONCLUSIONS

Our study did not find any differences in the visual and refractive outcomes between femtosecond-assisted sub-Bowman keratomileusis and LASIK. Both surgeries resulted in quick visual recovery as early as 1 week postoperatively.

Long-term observation and evaluation of femtosecond laser-assisted thin-flap laser in situ keratomileusis in eyes with thin corneas but normal topography

PURPOSE

To evaluate the long-term outcomes of thin-flap laser in situ keratomileusis (LASIK) in eyes with thin corneas (central corneal thickness [CCT] <500 μm) but normal topography.

SETTING

Private center, Tokyo, Japan.

DESIGN

Retrospective randomized comparative study.

METHOD

The efficacy, safety, predictability, and topography of LASIK were evaluated in eyes with a CCT of less than 500 μm but with normal topography (thin-cornea group) to 6 years postoperatively. The LASIK outcomes in the thin-cornea group were compared with those in eyes with a CCT of 500 μm or greater (control group). Analysis was performed to determine whether there were differences between the groups at the last checkup 3 to 4 years postoperatively.

RESULTS

In the thin-cornea group (291 eyes; 146 patients), no significant differences were observed in LASIK outcomes when eyes were subdivided by the time of final checkup (3, 4, and ≥ 5 years). There was a significant difference in visual and refractive outcomes between 3 months postoperatively and the last checkup in the thin-cornea group and the control group (371 eyes; 193 patients). No significant difference in visual, refractive, or topography outcomes was observed between the 2 groups at the last checkup.

CONCLUSIONS

Laser in situ keratomileusis in eyes with thin corneas was as safe and effective as and showed similar long-term stability in eyes with a CCT of 500 μm or greater. No eye in either group had a postoperative complication.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Prospective contralateral eye study to compare 80- and 120-μm flap LASIK using the VisuMax femtosecond laser

PURPOSE

To compare visual outcomes and flap stability of LASIK with ultrathin 80- and 120-μm flaps created with a VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany) for moderate to high myopia and to evaluate the effect of corneal flap thickness on outcomes.

METHODS

In a prospective contralateral eye study, 36 consecutive patients (72 eyes) underwent bilateral LASIK for myopia ranging from -2.00 to -10.00 diopters using the VisuMax femtosecond laser and MEL-80 excimer laser (Carl Zeiss Meditec). One eye of each patient was randomized to have the 80-μm flap and the other to the 120-μm flap created with 200-kHz VisuMax femtosecond laser. Preoperative and postoperative tests included visual acuity, manifest refraction, contrast sensitivity, and flap thickness measured by anterior segment optical coherence tomography. Main outcomes and complications were checked at postoperative 1 week and 1, 3, and 6 months.

RESULTS

There were no differences in visual outcome, residual refractive error, or contrast sensitivity between groups during follow-up, except for better uncorrected visual acuity at postoperative 1 day in the 120-μm group. Mean standard deviations of measured flap thickness during follow-up ranged from 3.16 to 3.80 μm in both groups. Opaque bubble layer, a unique complication in femtosecond LASIK, was more frequent in the 80-μm group (7 of 36: 19%) than in the 120-μm flap group (3 of 36: 8%) without a statistically significant difference (P = .301) and was related to thicker central cornea and steeper keratometric value, although it did not influence clinical results. Comparison of the intended versus achieved correction showed no significant differences between groups.

CONCLUSIONS

LASIK using the VisuMax femtosecond laser supplied good clinical results and flap reproducibility in both the 80- and 120-μm flap groups. Patients with relatively thin cornea may benefit from 80-μm flap LASIK.

Femtosecond lasers for LASIK flap creation: a report by the American Academy of Ophthalmology

OBJECTIVE

To review the published literature to assess the safety, efficacy, and predictability of femtosecond lasers for the creation of corneal flaps for LASIK; to assess the reported outcomes of LASIK when femtosecond lasers are used to create corneal flaps; and to compare the differences in outcomes between femtosecond lasers and mechanical microkeratomes.

METHODS

Literature searches of the PubMed and Cochrane Library databases were last conducted on October 12, 2011, without language or date limitations. The searches retrieved a total of 636 references. Of these, panel members selected 58 articles that they considered to be of high or medium clinical relevance, and the panel methodologist rated each article according to the strength of evidence. Four studies were rated as level I evidence, 14 studies were rated as level II evidence, and the remaining studies were rated as level III evidence.

RESULTS

The majority of published studies evaluated a single laser platform. Flap reproducibility varied by device and the generation of the device. Standard deviations in flap thicknesses ranged from 4 to 18.4 μm. Visual acuities and complications reported with LASIK flaps created using femtosecond lasers are within Food and Drug Administration safety and efficacy limits. Of all complications, diffuse lamellar keratitis is the most common after surgery but is generally mild and self-limited. Corneal sensation was reported to normalize by 1 year after surgery. Unique complications of femtosecond lasers included transient light-sensitivity syndrome, rainbow glare, opaque bubble layer, epithelial breakthrough of gas bubbles, and gas bubbles within the anterior chamber.

CONCLUSIONS

Available evidence (levels I and II) indicates that femtosecond lasers are efficacious devices for creating LASIK flaps, with accompanying good visual results. Overall, femtosecond lasers were found to be as good as or better than mechanical microkeratomes for creating LASIK flaps. There are unique complications that can occur with femtosecond lasers, and long-term follow-up is needed to evaluate the technology fully.

Corneal haze following disruption of epithelial basement membrane on ultra-high-resolution OCT following femtosecond LASIK

PURPOSE

To analyze in vivo structural properties of a case with interface haze that developed after femtosecond laser-assisted LASIK.

METHODS

Case report.

RESULTS

A 42-year-old man underwent uncomplicated LASIK. Four months after the surgery he developed regression of his refractive error and interface haze in both eyes. Flap thickness was calculated as 73 μm and 81 μm in the right and left eyes, respectively. A novel ultra-high-resolution spectral domain optical coherence tomography (OCT) system was used to demonstrate in vivo morphology of the cornea. On ultra-high-resolution OCT, haze was demonstrated as increased bright white reflectivity of the flap interface. Areas of focal disruption of the basement membrane and Bowman layer corresponded with the most significant areas of interface haze.

CONCLUSIONS

Our results support the hypothesis that haze after femtosecond LASIK may be a result of damage to the basement membrane and Bowman layer of the cornea.

Femtosecond laser in-situ keratomileusis flap configurations

PURPOSE OF REVIEW

The corneal flap laser in-situ keratomileusis (LASIK) is among the most important determinants in the successful outcome of the surgery. Femtosecond lasers have evolved over the last decade to all but replace the mechanical microkeratome as the preferred method to create these flaps. With improvements in femtosecond laser technology, there has been a reduction in the time taken for the cut and the quality of the stromal bed has improved. Improved predictability has led surgeons to explore the possibility of thin flap LASIK.

RECENT FINDINGS

Corneal flaps created with the femtosecond laser have been shown to be more predictable in depth and have a more desirable planar morphology. Corneal flaps created by the femtosecond laser can be customized according to depth, profile, morphology, and side-cut configuration. Changes in the angulation of the side cut, to reduce the incidence of epithelial ingrowth, have not been corroborated by clinical evidence as yet. Thin flap LASIK, also referred to as sub-Bowman's keratomileusis, has the advantage of preserving more stroma and potentially reducing the incidence of corneal ectasia but seems to be associated with an increased incidence of interface haze.

SUMMARY

This review examines the advantages of creating a flap with the femtosecond laser and the various configurations of these flaps. It also explores the advantages of varying the thickness and profile of femtosecond flaps.