Femtosecond laser photodisruptive effects on the posterior human corneal stroma investigated with atomic force microscopy

Atomic force microscopy for the evaluation of corneal surface roughness after femtosecond laser flap creation and excimer ablation

INTRODUCTION

It is well known that the femtosecond laser lamellar cut induces some degree of surface roughness. Nevertheless, as in femtosecond laser-assisted LASIK (FS-LASIK), an excimer LASIK ablation is performed, and the post-ablation stromal bed should show some degree of smoothening. We decided to compare, using atomic force microscopy (AFM), the roughness of the corneal stromal bed, after a femtosecond lasers device flap was created with or without an excimer myopic ablation.

METHODS

Using 6 freshly enucleated porcine eyes, we created in every eye a flap using a femtosecond laser. Additionally, in 3 eyes, an excimer laser ablation to correct-3 diopters (D) was made. AFM imaging of the remaining corneal stroma was performed. Ten different square areas of 20 μm x 20 μm at the central area of the stroma of each corneal sample were studied. The roughness parameters used were the root-mean-square deviation from a perfectly flat surface.

RESULTS

The RMS deviation was 360 ± 120 nm in femtosecond laser only, and 110 ± 20 nm in those cases where excimer is also involved (p < 0.0001).

CONCLUSIONS

Our results show that the roughness of the surface treated with excimer is clearly lower than in the group with no excimer ablation; thus, the application of laser excimer after a flap created by femtosecond laser seems to soften the nano-irregularities created by this technique.

The creation of a corneal incision with a femtosecond laser

BACKGROUND: The cutting action of a femtosecond laser (FS-laser) on the corneal tissue is performed using a photodestructive effect. FS-laser technology is used in many different areas of corneal surgery, and studying this effect is an actual issue. AIM: To evaluate the surface of the corneal incision created by a FS-laser (experimental study). MATERIALS AND METHODS: 20 porcine eyes were divided into two groups (FS-laser and keratome). The corneal tissue surface in the area of contact between two lips of the incision was visualized by electron microscopy. RESULTS: In all cases, we received a full-thickness incisions through the cornea. Images obtained by electron microscopy showed regular surface, no signs of thermal or mechanical damage to the corneal structure. CONCLUSIONS: The FS-laser is used as a microscopic scalpel in surgery. The precision cutting process is performed by mechanical forces generated by photodestruction by expanding cavitation bubbles.

Surface quality and endothelial cell viability after femtosecond laser-assisted donor lenticule preparation for endothelial keratoplasty - An in-vitro study

Purpose

To compare surface quality and endothelial cell viability of descemet stripping automated endothelial keratoplasty (DSAEK) donor lenticules prepared with femtosecond laser (FSL) or microkeratome (MK).

Methods

Experimental ex-vivo evaluation of 15 DSAEK donor lenticules prepared from optical quality donor corneas using 200 KHz FSL (9 eyes) or MK (6 eyes). Surface quality and smoothness of the cut were assessed using atomic force microscopy and endothelial cell viability was assessed using transmission electron microscopy.

Results

Mean lenticule thickness was 121.89 ± 17.13 μm in FSL group and 112.67 ± 5.89 μm in MK group (P = 0.33). Average roughness of stromal surface (RMSavg) [FSL- 30.51 ± 4.55 nm, MK-22.37 ± 1.83 nm; P = 0.02] and root mean square roughness (RMSrough) [FSL-31.39 ± 5.75 nm, MK-23.08 ± 0.40 nm; P = 0.012] was significantly more in FSL group. Increased granular and linear irregularities were observed in the FSL group. Endothelial cell disruption was more in FSL group (FSL- 29.49 ± 6.91% MK-13.28 ± 3.62%; P < 0.001) with decreased mean nucleus length (FSL-5.56 ± 0.17 μm, MK-7.52 ± 0.65 μm; P < 0.001).

Conclusion

Automated MKs are still the standard of care for donor lenticule preparation and MK-assisted donor lenticules have smoother surface with less endothelial cell disruption than FSL. Further research is mandatory before FSL platforms can be considered a viable alternative to the MK.

[Femtosecond lasers for cataract and refractive lens surgery]

The femtosecond laser technology continues its progress in ophthalmology and now also plays an increasingly important role in lens surgery. Using this technology it is possible to create corneal sections and to perform capsulorhexis and lens fragmentation. There are currently four instruments on the market with more to follow. The individual benefits and also critical issues are presented and the results of current studies are discussed.

Quality of corneal lamellar cuts quantified using atomic force microscopy

PURPOSE

To quantify the cut quality of lamellar dissections made with the femtosecond laser using atomic force microscopy (AFM).

SETTING

Bascom Palmer Eye Institute, University of Miami Miller School of Medicine, Miami, Florida, USA.

DESIGN

Experimental study.

METHODS

Experiments were performed on 3 pairs of human cadaver eyes. The cornea was thinned to physiologic levels by placing the globe, cornea side down, in 25% dextran for 24 hours. The eyes were reinflated to normal pressures by injecting a balanced salt solution into the vitreous cavity. The eyes were placed in a holder, the epithelium was removed, and the eyes were cut with a Visumax femtosecond laser. The energy level was 180 nJ for the right eye and 340 nJ for the left eye of each pair. The cut depths were 200 μm, 300 μm, and 400 μm, with the cut depth maintained for both eyes of each pair. A 12.0 mm trephination was then performed. The anterior portion of the lamellar surface was placed in a balanced salt solution and imaged with AFM. As a control, the posterior surface was placed in 2% formalin and imaged with environmental scanning electron microscopy (SEM). Four quantitative parameters (root-mean-square deviation, average deviation, skewness, kurtosis) were calculated from the AFM images.

RESULTS

From AFM, the 300 μm low-energy cuts were the smoothest. Similar results were seen qualitatively in the environmental SEM images.

CONCLUSION

Atomic force microscopy provided quantitative information on the quality of lamellar dissections made using a femtosecond laser, which is useful in optimizing patient outcomes in refractive and lamellar keratoplasty surgeries.