Femtosecond Lasers in Cornea & Refractive Surgery

Postmortal epithelial changes of donor corneas impair applicability of a refractive ultraviolet femtosecond laser

This study evaluates the corneal applicability of a refractive ultraviolet femtosecond laser in postmortal human donor eyes and ex vivo porcine eyes. Refractive lenticule extraction and flap creation were attempted in 10 human donor eyes and 80 ex vivo porcine eyes with and without abrasion of the corneal epithelium. The postmortem interval ranged from 6 to 35 h in the human samples and was set to 4, 24, and 48 h for the porcine specimens. Nine human eyes and 60 porcine eyes were treated with an ultraviolet femtosecond laser. The rest was treated with an infrared laser. Optical coherence tomography and scanning electron microscopy were used to demonstrate success or failure of the procedures. Ultraviolet laser-assisted refractive surgery attempts without prior abrasion of the corneal epithelium were only successful at 6 h p.m. in the human eyes and at 4 and 24 h in the porcine eyes. Upon epithelial abrasion, refractive surgery was always successful with the ultraviolet laser. The infrared laser always performed successfully with and without prior epithelial abrasion. Thus, postmortal changes in the corneal epithelium impair the ability of refractive ultraviolet femtosecond lasers to create stromal cuts. This progresses with time but does not affect infrared femtosecond lasers.

Theranostic-Guided UV-A Light Corneal Wavefront Photo-Reshaping for Presbyopia Correction: A Preclinical Study

This study investigated the effect of a theranostic-guided UV-A light corneal photo-reshaping technique on corneal elevation and wavefront aberration (WA) in human donor eyes. A specialized platform, combining UV-A light with corneal iontophoresis for controlled, patterned, riboflavin delivery, was used for both distribution assessment and concentration-driven photopolymerization of corneal proteins. In all cases, a consistent riboflavin concentration gradient, with lower levels in the central prepupillary zone, was recorded. Corneal topography and WA measurements showed significant corneal steepening and smooth wavefront shaping, respectively, with a delay in the central 2.0 mm of the WA and advancement in the surrounding zone, as well as a 50% reduction in corneal spherical aberration over a 5.0 mm pupil size. Notably, the corneal optical quality, measured via modulation transfer function (MTF), remained stable. This incision-free approach demonstrated the potential to extend focal range without compromising distance vision, presenting a new solution for presbyopia correction.

Multiphoton scaling of femtosecond laser-induced refractive index change (LIRIC) in hydrogels and rabbit cornea

To find optimal conditions for performing laser induced refractive index change (LIRIC) in living eyes with both safety and efficacy, we investigated multiphoton excitation scaling of this procedure in hydrogel and excised corneal tissue. Three distinct wavelength modalities were examined: high-repetition-rate (HRR) and low-repetition-rate (LRR) 405 nm systems, as well as 800 nm and 1035 nm systems, whose LIRIC-inducing properties are described for the first time. Of all the systems, LRR 405 nm-LIRIC was able to produce the highest phase shifts at the lowest average laser powers. Relative merits and drawbacks to each modality are discussed as they relate to future efforts towards LIRIC-based refractive error correction in humans.

Long-term observation of V4c implantable collamer lenses implantation for moderate to extreme high myopia correction: five years follow-up

BACKGROUND

This study aims to assess the long term effectiveness, safety, predictability and stability of V4c implantable collamer lenses (ICL) for correction of moderate to extreme high myopia.

METHODS

We reviewed 125 eyes from 64 patients who implanted V4c ICL at the Refractive Surgery Center of West China Hospital in Chengdu, China, between May 2015 and January 2017. The median spherical equivalent was -11.50 D (interquartile range [IQR]: -13.00 to -9.00 D). We followed up with the patients over five years and evaluated several parameters, including uncorrected visual acuity (UDVA), corrected visual acuity (CDVA), axial length, refractive error, endothelial cell density (ECD), intraocular pressure (IOP), white-to-white distance (WTW), and vault. We performed a correlation analysis to explore the potential impacts on vault following implantation.

RESULTS

The median safety index (postoperative CDVA/preoperative CDVA) during the last follow-up was 1.00 (interquartile range [IQR]: 1.00-1.20), and the efficacy indices (postoperative UDVA/preoperative CDVA) were 1.20 (IQR: 1.00-1.25), 1.20 (IQR: 1.00-1.33), and 0.8 (IQR: 0.65-1.00) at postoperative 1 week, 1 month, and 5 years, respectively. At the five-year mark, 16% of the eyes were within ±0.50 D of expected correction, and 73% were within ±2.00 D. No significant difference in ECD was observed between pre-operative and post-operative measurements. Compared to baseline, we observed a significant increase in IOP at the one-week follow-up, which decreased significantly at the one-month visit. Furthermore, we identified ICL size and spherical equivalent (SE) as independent variables in a multiple linear regression model that accurately predicted the five-year vault after surgery.

CONCLUSION

In conclusion, V4c ICL implantation is an effective and safe treatment for moderate to extreme high myopia with good predictability and stability over the long-term.

[Functional outcomes and corneal condition in cavitation complications of femtosecond LASIK]

The use of femtosecond (FS) laser in LASIK procedures for myopia correction can lead to the formation of an opaque bubble layer (OBL) in stromal layers of different densities and in various areas of the cornea.

PURPOSE

This study evaluated the functional outcomes and the condition of the cornea in eyes with and without OBL in randomized groups.

MATERIAL AND METHODS

The study included 93 patients (186 eyes) with varying degrees of myopia (from 2.25 to 7.5 D): the main group consisted of 137 eyes with OBL, the control group - 49 eyes without OBL. The average age of patients was 30±5 and 32±4 years, respectively, with spherical equivalent refractive errors of -4.9 D and -4.3 D, astigmatism up to 1.75 D and 1.25 D, respectively. The assessment included uncorrected visual acuity (UCVA), best corrected visual acuity (BCVA), clinical refraction, lower-order aberrations (RMS LOA), higher-order aberrations (RMS HOA), spherical aberration (Z40; Pentacam HR Oculus), confocal microscopy of all corneal layers, and tear film assessment (Medmont; TFSQ; TFBUT; TFSQ area). All patients underwent FS-LASIK [Technolas VICTUS; corneal flap thickness of 110-120 μm (optical zone 6.0 mm), pulse energy >1 mJ, frequency 80 kHz]. Follow-up visits were conducted at day 7, and 1, 3, and 6 months.

RESULTS

No statistically significant differences were observed between the groups in terms of functional outcomes and optical aberrations. However, the OBL group showed significantly higher TFSQ values and more pronounced keratocyte changes visualized in the deep stromal layers (under the OBL) at the depth of 130-160 μm. The extent of keratocyte alterations decreased over time, from 118.3±5.1 mm/mm² on day 7 to 71.1±4.8 mm/mm² at 6 months, as observed via confocal microscopy.

CONCLUSION

While there was no difference in functional outcomes for myopia correction between eyes with and without OBL, the presence of OBL was associated with a higher extent of destructive changes in keratocytes, necessitating a longer healing period.

Effect of corneal flap thickness on opaque bubble layer formation in Visumax FS-LASIK using GEE analysis

Introduction

This study aimed to investigate two types of corneal flap thickness on opaque bubble layer (OBL) formation in Visumax femtosecond laser-assisted stromal for situ keratomileusis (FS-LASIK).

Methods

This retrospective study analyzed 203 eyes of 103 patients (32 men and 71 women) who underwent Visumax FS-LASIK between January 2020 and June 2020, and according to corneal flap thickness, they were divided into the 100-μm group (64 eyes) and the 110-μm group (139 eyes). Anterior-segment examination revealed no abnormal findings. Preoperatively, intraocular pressure (IOP), central corneal thickness (CCT), residual stromal thickness (RST), spherical power, cylindrical power, flat keratometry (K1), steep keratometry (K2), and biomechanical parameters including deformation amplitude (DA) ratio, Integrated Radius, stiffness parameter at first applanation (SP-A1), and Ambrosio relational thickness to the horizontal profile (ARTh) were evaluated. Primary outcomes were the incidence of OBL formation in the two groups compared by the Chi-square test and the correlation between the incidence of OBL and the above preoperative data by Spearman's Rho test. Secondary outcomes were the comparisons corrected by the generalized estimating equation (GEE) model.

Results

The incidence of OBL formation in the 100-μm group was 59.4 %, which was higher than that in the 110-μm group (23.0 %) with a significant difference (χ2 = 25.635, P < 0.001). The thinner corneal flap thickness (r = -0.355, P < 0.001) and higher spherical power (r = -0.142, P < 0.05) correlated with OBL formation. Higher K1 (r = 0.217, P < 0.01) and K2 (r = 0.198, P < 0.01) also correlated with OBL formation. The results of the GEE correction analysis showed higher rates of OBL formation in the 100-μm group (odds ratio [OR] = 4.704, 95 % CI 1.681-13.161, P < 0.01).

Conclusions

OBL was more likely to occur with the 100-μm corneal flap than with the 110-μm corneal flap in Visumax FS-LASIK. The risk of OBL formation in the 100-μm group was 4.704 times higher than that in the 110-μm group.

Femtosecond Laser-Assisted Ophthalmic Surgery: From Laser Fundamentals to Clinical Applications

Femtosecond laser (FSL) technology has created an evolution in ophthalmic surgery in the last few decades. With the advantage of high precision, accuracy, and safety, FSLs have helped surgeons overcome surgical limits in refractive surgery, corneal surgery, and cataract surgery. They also open new avenues in ophthalmic areas that are not yet explored. This review focuses on the fundamentals of FSLs, the advantages in interaction between FSLs and tissues, and typical clinical applications of FSLs in ophthalmology. With the rapid progress that has been made in the state of the art research on FSL technologies, their applications in ophthalmic surgery may soon undergo a booming development.

Bioengineered Human Stromal Lenticule for Recombinant Human Nerve Growth Factor Release: A Potential Biocompatible Ocular Drug Delivery System

Small incision lenticule extraction (SMILE), is a surgical procedure for the myopia correction, during which a corneal stromal lenticule is extracted. Given that we have previously demonstrated how this discarded tissue could be repurposed as a bio-scaffold for stromal engineering, this study aimed to explore its use as an ocular drug delivery system of active molecules, using neurotrophic factor Nerve Growth Factor (NGF). We employed human stromal lenticules directly collected from healthy donors undergoing SMILE. Following a sodium dodecylsulfate (SDS) treatment, decellularized lenticules were incubated with a suspension of polylactic-co-glycolic-acid (PLGA) microparticles (MPs) loaded with recombinant human NGF (rhNGF-MPs). Fluorescent MPs (Fluo-MPs) were used as control. Data demonstrated the feasibility to engineer decellularized lenticules with PLGA-MPs which remain incorporated both on the lenticules surface and in its stromal. Following their production, the in vitro release kinetic showed a sustained release for up to 1 month of rhNGF from MPs loaded to the lenticule. Interestingly, rhNGF was rapidly released in the first 24 h, but it was sustained up to the end of the experiment (1 month), with preservation of rhNGF activity (around 80%). Our results indicated that decellularized human stromal lenticules could represent a biocompatible, non-immunogenic natural scaffold potential useful for ocular drug delivery. Therefore, combining the advantages of tissue engineering and pharmaceutical approaches, this in vitro proof-of-concept study suggests the feasibility to use this scaffold to allow target release of rhNGF in vivo or other pharmaceutically active molecules that have potential to treat ocular diseases.

Analytical optimization of the cutting efficiency for generic cavitation bubbles

A theoretical method to determine the optimum laser parameters for maximizing the cutting efficiency for different materials (in particular human cornea) is proposed. The model is simple and reduced to laser beam characteristics and cavitation properties. The model further provides a method to convert energy fluctuations during the cutting process to equivalent deviations in the cavitation bubbles. The proposed model can be used for calibration, verification and validation purposes of laser systems used for cutting processes at relatively low cost and may improve the quality of the results.