Optimal parameters to improve the interface quality of the flap bed in femtosecond laser-assisted laser in situ keratomileusis

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.

Early LASIK flap displacement without signs of infection

A 37-year-old woman was referred for refractive surgery evaluation. Poor visual quality in her left eye is her chief concern. The patient had undergone laser in situ keratomileusis (LASIK) in both eyes 3 days previously. Detailed history revealed that the patient underwent surgery to correct low myopia and astigmatism (-2.50 -2.75 × 180 in the right eye and -1.25 -2.75 × 180 in the left eye). The preoperative evaluation corrected distance visual acuity (CDVA) was 20/20 in the right eye and 20/20 in the left eye. The surgery was performed with a mechanical microkeratome and was uneventful. The patient reports that after her vision improved on the first day, she woke up with blurry vision in her left eye on the second day and it remained that way until the third day when she sought medical help. The slitlamp examination showed LASIK flap displacement without signs of infection ( Figure 1JOURNAL/jcrs/04.03/02158034-202212000-00023/figure1/v/2022-12-01T092452Z/r/image-tiff ). The patient was using topical corticosteroids and topical antibiotics every 6 hours. Considering the likely stromal exposure time, what would be the best approach for this case? Does the probable time of displacement of this flap change its behavior? In what way? If you decide to reposition the flap, would it be interesting to use fibrin glue or suture? Considering that there was no major trauma in the patient's report, what is the importance of the LASIK flap having been created by a mechanical microkeratome and not by a femtosecond laser in the displacement of the flap?

Comparison of the optical quality after SMILE and FS-LASIK for high myopia by OQAS and iTrace analyzer: a one-year retrospective study

BACKGROUND

To compare the correction effect and optical quality after small-incision lenticule extraction (SMILE) and femtosecond laser assisted laser in situ keratomileusis (FS-LASIK) for high myopia.

METHODS

51 high myopia eyes after SMILE and 49 high myopia eyes after FS-LASIK were enrolled and divided into two groups retrospectively. The OQAS and iTrace analyzer were used for optical quality inspection. Between the two groups the spherical equivalent (SE), astigmatism, uncorrected distant visual acuity (UDVA), strehl ratio (SR), modulation transfer function cutoff frequency (MTF cutoff), objective scatter index (OSI) and wavefront aberrations were analyzed and compared before surgery and at 1, 6 and 12 months after surgery.

RESULTS

After the operation: (1) SE and astigmatism declined and UDVA increased significantly in both groups, and UDVA was better after SMILE than FS-LASIK. (2) SR and MTF cutoff reduced and OSI increased significantly after SMILE and FS-LASIK. SR and MTF cutoff were significantly higher after SMILE than FS-LASIK. OSI was significantly lower after SMILE than FS-LASIK. (3) The total wavefront aberration, total low-order wavefront aberration, defocus and astigmatism aberration as well as trefoil aberration reduced significantly in both groups. The total high-order wavefront aberration increased significantly after FS-LASIK. The spherical and coma aberration increased significantly in both groups. The total high-order wavefront aberration and coma aberration at 1 month were higher after FS-LASIK than SMILE.

CONCLUSION

The optical quality descended after SMILE and FS-LASIK. SMILE was superior to FS-LASIK at the correction effect and optical quality for high myopia. The combination of OQAS and iTrace analyzer is a valuable complementary measurement in evaluating the optical quality after the refractive surgery.

TRIAL REGISTRATION

This is a retrospective study. This research was approved by the ethics committee of Xiangya Hospital and the IRB approval number is 201612074.

A microscopic study of the corneal stromal lenticules extracted during femtosecond laser-assisted small incision lenticule extraction

The aim of the present study was to investigate the histological and morphological characteristics of corneal stromal lenticules extracted during femtosecond laser-assisted small incision lenticule extraction (SMILE) surgery by light and electron microscopy. A total of 20 human corneal stromal lenticules extracted during SMILE surgery were sent for microscopic examination immediately after surgery. Of these, six were observed under a light microscope and 14 were observed under an electron microscope. The smoothness of the front and rear surface of the lenticules observed under an electron microscope was rated on a scale of 0 to 4 according to unified evaluation criteria and the scores were statistically compared. Under the light microscope, the edge of the cross section of the corneal stromal lenticules was deeply stained and certain burrs and broken collagen fibers were observed. The swollen corneal stromal fibers were distributed irregularly, with a few bubbles of different sizes. Under the electron microscope, the surface of the lenticules ablated using a femtosecond laser was not perfectly smooth and the front surface was smoother than the rear surface. The side edge of the lenticules ablated using a femtosecond laser was not as regular as the edge cut using microscissors. Necrosis and dissolution of collagen fibers were observed near the surface of the lenticules. In conclusion, the surface quality of corneal stromal lenticules ablated using a femtosecond laser was not optimal when observed under a microscope. Increased attention should be paid to the histology and morphology of the corneal surface following laser ablation.

Refractive outcomes following cataract combined with lamellar keratoplasty: femtosecond-DSEK versus microkeratome-DSAEK

PURPOSE

Prediction of postoperative refraction following posterior lamellar keratoplasty is crucial for choosing proper intraocular lens power in combined surgeries. Femtosecond laser-assisted Descemet stripping endothelial keratoplasty (FS-DSEK) creates thin, planar grafts while microkeratome-assisted Descemet's stripping automated endothelial keratoplasty (DSAEK) creates non-planar, concaved grafts. We evaluated whether this fundamental difference affects the refractive outcomes in cataract surgery combined with FS-DSEK compared to cataract surgery combined with microkeratome-assisted DSAEK.

METHODS

A retrospective analysis of 28 patients who underwent FS-DSEK combined with phacoemulsification and intraocular lens (IOL) implantation (group A) compared to 26 patients who underwent microkeratome-assisted DSAEK combined with phacoemulsification and IOL implantation (group B). Pre- and 1-year postoperative best-corrected visual acuity (BCVA), keratometry values, corneal thickness, central-to-peripheral graft thickness ratio (C/P ratio), and target postoperative spherical equivalent (SE) versus actual postoperative SE were analyzed.

RESULTS

Target postoperative SE and actual postoperative SE significantly shifted toward hyperopia in group B, but not in group A. Postoperative hyperopic shifts were 0.14 D and 1.13 D in groups A and B, respectively (P < 0.001). BCVA improved after surgery in both groups, with no significant difference between the groups. Postoperative C/P ratio differed significantly between the groups and was negatively correlated with postoperative hyperopic shift (r =  - 0.616, P < 0.001).

CONCLUSION

Refractive outcomes of cataract surgery combined with FS-DSEK are relatively neutral, whereas those of cataract surgery combined with microkeratome-assisted DSAEK cause significant hyperopic shift. Clinicians should select accordingly an appropriate intraocular lens power when performing these surgeries.

Comparison of visual quality after EVO-ICL implantation and SMILE to select the appropriate surgical method for high myopia

Background

This study sought to compare the visual quality between intraocular collamer lens (EVO-ICL) implantation and small-incision lenticule extraction (SMILE) and determine the appropriate surgical method to treat patients with high myopia (− 6.25 to − 10 D).

Methods

A total of 48 eyes underwent EVO-ICL implantation and another 48 eyes underwent SMILE. The uncorrected distance visual acuity (UDVA), best-corrected distance visual acuity (BCVA) and equivalent spherical degree were compared across the SMILE (− 6.25 to − 10 D) and EVO-ICL (− 6.25 to − 10 D) implantation groups. Preoperative and postoperative visual quality parameters were compared between and within groups.

Results

The OQAS II values (OV 100%) one week and one month after surgery and the modulation transfer function (MTF), OV 20% and OV 9% values one week after surgery in the SMILE group were lower than the respective preoperative values. The objective scatter index (OSI) value increased one week as well as one month after surgery compared with the preoperative values. The MTF cut-off value of the SMILE group was lower than that of the EVO-ICL implantation group three months after surgery.

Conclusions

For patients with high myopia, the postoperative visual quality of EVO- ICL implantation was slightly better than that of SMILE.

Development of a liquid dissection technique for small-incision lenticule extraction: Clinical results and ultrastructural evaluation

PURPOSE

To determine the effect of a liquid dissection technique on clinical outcomes with ultrastructural analysis of the lenticule surface in small-incision lenticule extraction (SMILE).

SETTING

Zhongshan Ophthalmic Center, Guangzhou, China.

DESIGN

Prospective case series.

METHODS

Consecutive patients with myopia scheduled for small-incision lenticule extraction were included. The liquid dissection technique was performed in 1 eye and the traditional dissection technique was performed in the other eye by randomized assignment. Ophthalmic examinations were evaluated preoperatively and at different timepoint follow-ups after small-incision lenticule extraction. Ten human corneal lenticules were analyzed using scanning electron microscopy.

RESULTS

The study comprised 29 patients (58 eyes). Uncorrected distance visual acuity (logarithm of the minimum angle of resolution) postoperative measurements at 2 and 4 hours were significantly better in the liquid dissection group than in the traditional dissection group (P < .001 and P = .001, respectively); however, there were no significant differences between the 2 groups at 1 day, 1 week, and 1 month postoperatively. Compared with the traditional dissection technique, the liquid dissection technique induced significantly fewer corneal aberrations at 2 hours and 1 month after the procedures (P = .031 and P = .016, respectively), the postoperative contrast sensitivity in the liquid dissection group was significantly higher after 1 day (P = .01). The liquid dissection samples showed smoother lenticule surfaces compared with the traditional dissection samples qualitatively and quantitatively (P = .004 and P < .001, respectively).

CONCLUSION

The liquid dissection technique was helpful in facilitating better visual acuity recovery and produced smooth cuts in the early postoperative period.

Energy Setting and Visual Outcomes in SMILE: A Retrospective Cohort Study

PURPOSE

To assess the independent effect of energy setting on postoperative uncorrected distance visual acuity (UDVA) in small incision lenticule extraction (SMILE) and further investigate an optimal energy setting for the 4.5-μm spot-track-distance, which is in wide clinical use.

METHODS

A total of 1,130 eyes were included in a retrospective cohort study from Tianjin Eye Hospital, Tianjin Medical University from April 2015 to July 2016. Energy settings and baseline characteristics were recorded and 3-month UDVA was tested by a nurse blinded to the energy settings used. Multiple regression analysis and generalized estimating equations were used to take into account the correlation between the measurements from two eyes.

RESULTS

The 3-month UDVA (mean ± standard deviation) of 125 to 160 nJ (by 5-nJ increments) was 1.39 ± 0.19, 1.40 ± 0.32, 1.33 ± 0.27, 1.36 ± 0.27, 1.34 ± 0.25, 1.29 ± 0.19, 1.36 ± 0.27, and 1.19 ± 0.22, respectively. Energy was significantly associated with postoperative logMAR UDVA in different models and the regression coefficient (β) was robust (β = 0.01, 95% confidence interval = 0.00 to 0.01). The regression coefficient β (0.01, 95% confidence interval = 0.00 to 0.02, P = .0029) of energy (125 to 150 nJ, by 5-nJ increments) on 4.5-μm spot-track-distance was still associated with the logMAR UDVA when adjusted for sex, age, myopia, astigmatism, mean keratometry, central corneal thickness, preoperative logMAR CDVA, and side spot-track-distance.

CONCLUSIONS

The lower end of the energy studied was associated with a better postoperative UDVA in this population. The spot-track-distance of 4.5 μm with 125 nJ energy was the optimal combination within this range. [J Refract Surg. 2018;34(1):11-16.].

Comparison of four methods of surface roughness assessment of corneal stromal bed after lamellar cutting

Corneal lamellar cutting with a blade or femtosecond laser (FSL) is commonly used during refractive surgery and corneal grafts. Surface roughness of the cutting plane influences postoperative visual acuity but is difficult to assess reliably. For the first time, we compared chromatic confocal microscopy (CCM) with scanning electron microscopy, atomic force microscopy (AFM) and focus-variation microscopy (FVM) to characterize surfaces of variable roughness after FSL cutting. The small area allowed by AFM hinders conclusive roughness analysis, especially with irregular cuts. FVM does not always differentiate between smooth and rough surfaces. Finally, CCM allows analysis of large surfaces and differentiates between surface states.

Lower Laser Energy Levels Lead to Better Visual Recovery After Small-Incision Lenticule Extraction: Prospective Randomized Clinical Trial

PURPOSE

To determine the effect of lowering laser energy on clinical outcomes in small-incision lenticule extraction (SMILE).

DESIGN

Prospective randomized clinical trial.

METHODS

A total of 151 patients (151 eyes) with moderate myopia scheduled for SMILE were included: 58 eyes received SMILE with low energy (100, 105, and 110 nJ; L-SMILE group) and 93 with conventional energy (115-150 nJ; C-SMILE group). Patients received complete ophthalmic examinations preoperatively and over 3 months postoperatively.

RESULTS

Uncorrected distance visual acuity (logMAR UDVA) 1 day and 1 week postoperatively was significantly better in L-SMILE than in C-SMILE (P < .001 and P = .005, respectively). There was no significant difference between the groups at 1 and 3 months. L-SMILE induced significantly fewer corneal aberrations compared with C-SMILE at 1 week and 1 month postoperatively (both P < .01), but there were no significant differences at 3 months. Though there was no difference in logMAR UDVA over the postoperative period between the 100, 105, and 110 nJ subgroups, there was a significant difference in logMAR UDVA on postoperative day 1 between L-SMILE and each subgroup in which an energy level of 115 nJ or higher was used. Furthermore, logMAR UDVA on postoperative day 1 showed a significant correlation with laser energy (r = 0.451, P < .001) and multiple linear regression analysis revealed that energy level was the only independent factor associated with logMAR UDVA on postoperative day 1 (P < .001).

CONCLUSIONS

SMILE using femtosecond energy of less than 115 nJ facilitates better visual acuity with less induction of corneal aberrations in the early postoperative period.

Comparison of self-reported quality of vision outcomes after myopic LASIK with two femtosecond lasers: a prospective, eye-to-eye study

Purpose

To compare self-reported quality of vision (QoV) outcomes after myopic LASIK (laser-assisted in situ keratomileusis) with two femtosecond lasers.

Design

Prospective, randomized, eye-to-eye study.

Methods

Consecutive myopic patients were treated with wavefront-guided LASIK bilaterally. Eyes were randomized according to ocular dominance. The flap of one eye was made with the IntraLase FS 60 kHz femtosecond laser with a conventional 70° side-cut, and the flap of the fellow eye was made with the IntraLase iFS 150 kHz femtosecond laser with an inverted 130° side-cut. Patients completed the validated, Rasch-tested, linear-scaled 30-item QoV questionnaire preoperatively and at Months 1, 3, 6, and 12.

Results

The study enrolled 120 fellow eyes in 60 patients. None of the measured QoV parameters exhibited statistically significant differences between the groups preoperatively or at any postoperative time point.

Conclusion

Creating LASIK flaps with an inverted side-cut using a 150 kHz femtosecond laser and with a conventional 70° side-cut using a 60 kHz femtosecond laser resulted in no significant differences in self-reported QoV assessed by the QoV questionnaire.

Scanning Electronic Microscopy Evaluation of the Roughness of the Stromal Bed After Deep Corneal Cut with the LDV Femtosecond Laser (Z6) (Ziemer) and the ONE Microkeratome (Moria)

PURPOSE

To compare the stromal bed surface quality and the accuracy of dissection depth after deep lamellar cuts using the Leonardo Da Vinci (LDV) femtosecond laser (Z6) and the ONE Microkeratome.

METHODS

Deep lamellar cuts were performed on nine human donor corneoscleral buttons: five with the LDV femtosecond (FS) laser (Z6) (Ziemer) and four with the ONE Microkeratome (MK) (Moria). Corneal thickness was measured with ultrasound pachymetry before and after the dissection. The Stromal bed quality was evaluated using light microscopy (n = 4) and scanning electron microscopy (SEM) (n = 9). The surface roughness on SEM images was graded on the scale of 1 (smoothest) to 5 (roughest) by four observers, blinded to the method used. Particle analysis on the SEM images was performed in order to have an objective measure of smoothness.

RESULTS

The achieved dissection depth using the FS laser was 496.4 ± 46.4 µm when attempting 500 µm and 474 ± 60 µm with the microkeratome when attempting 350 µm. Histological evaluation of the corneoscleral buttons by both light and electron microscopy showed significantly smoother surface using the FS laser compared to the microkeratome. There were fewer and smaller particles observed in the SEM images of FS laser cut buttons (p < 0.001).The average observer based score of anterior surface roughness (50×) was 2.2 for the FS laser and 3.9 for the microkeratome dissections (p < 0.001).

CONCLUSIONS

The LDV femtosecond laser (Z6) platform is capable of creating deep corneal lamellar dissection with smoother surface quality and with more predictable cut depth as compared to the One Microkeratome.

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.

Effect of femtosecond laser setting on visual performance after small-incision lenticule extraction for myopia

AIM

To compare the effect of the two femtosecond laser settings on visual performance after small-incision lenticule extraction (SMILE) for myopia and myopic astigmatism.

METHODS

Forty-four eyes of 22 consecutive patients who underwent SMILE with an energy level of 140 nJ (spot distance 3.0 µm) in one eye, and with an energy level of 170 nJ (spot distance 4.5 µm) in the other eye, the eyes being randomly assigned. Preoperatively, 1 week, and 1 and 3 months postoperatively, the values of the modulation transfer function (MTF) cutoff frequency, Strehl ratio, objective scattering index (OSI) and Optical Quality Analysis System (OQAS) values (OVs) for these eyes were quantitatively assessed using an Optical Quality Analysis System (Visiometrics).

RESULTS

No significant differences were detected between the two groups in the visual and refractive outcomes, or in the MTF cutoff frequency, the Strehl ratio, the OSI, the OV 100%, the OV 20% or the OV 9% at any time point before or after surgery.

CONCLUSIONS

SMILE with energy settings of 140 and 170 nJ was effective for the correction of myopia and myopic astigmatism. It is indicated that the differences in laser setting (140 nJ, spot distance 3.0 µm vs 170 nJ, spot distance 4.5 µm) did not significantly affect the optical quality including the intraocular scattering of eyes undergoing SMILE.

CLINICAL TRIAL NUMBER

The protocol was registered with University Hospital Medical Information Network Clinical Trial Registry (UMIN000016241).

Interface quality of different corneal lamellar-cut depths for femtosecond laser-assisted lamellar anterior keratoplasty

PURPOSE

To evaluate the interface quality of different corneal lamellar-cut depths with the femtosecond laser and determine a feasible range of depth for femtosecond laser-assisted lamellar anterior keratoplasty.

SETTING

Casey Eye Institute, Portland, Oregon, USA.

DESIGN

Experimental study.

METHODS

Full lamellar cuts were made on 20 deepithelialized human cadaver corneas using the femtosecond laser. The cut depth was 17% to 21% (100 μm), 31%, 35%, 38% to 40%, and 45% to 48% of the central stromal thickness. Scanning electron microscopy images of the cap and bed surfaces were subjectively graded for ridge and roughness using a scale of 1 to 5 (1 = best). The graft-host match was evaluated by photography and optical coherence tomography in a simulated procedure.

RESULTS

The ridge score was correlated with the cut depth (P = .0078, R = 0.58) and better correlated with the percentage cut depth (P = .00024, R = 0.73). The shallowest cuts had the fewest ridges (score 1.25). The 31% cut depth produced significantly fewer ridges (score 2.15) than deeper cuts. The roughness score ranged from 2.19 to 3.08 for various depths. A simulated procedure using a 100 μm host cut and a 177 μm (31%) graft had a smooth interface and flush anterior junction using an inverted side-cut design.

CONCLUSIONS

The femtosecond laser produced more ridges in deeper lamellar cuts. A depth setting of 31% stromal thickness might produce adequate surface quality for femtosecond laser-assisted lamellar anterior keratoplasty. The inverted side-cut design produced good edge apposition even when the graft was thicker than the host lamellar-cut depth.

FINANCIAL DISCLOSURE

Proprietary or commercial disclosures are listed after the references.

A comparison of LASIK flap thickness and morphology between the Intralase 60- and 150-kHz femtosecond lasers

PURPOSE

To prospectively compare the achieved thickness and consistency of LASIK flaps created with a 60- and 150-kHz femtosecond laser.

METHODS

One hundred twenty eyes of 60 patients with myopia were treated with LASIK. One eye had flap created by the Intralase FS 60-kHz femtosecond laser (Abbott Medical Optics, Abbott Park, IL) and the fellow eye was treated with the IntraLase iFS 150-kHz femtosecond laser (Abbott Medical Optics). Eyes were randomized according to ocular dominance. Flap morphology and measurements were taken with anterior segment optical coherence tomography (AS-OCT) at the 1-month postoperative visit.

RESULTS

AS-OCT showed similar regular planar morphologies in both groups. The mean thickness of the flaps in the FS 60 group was significantly higher than that of the iFS 150 group (105.4 ± 3.9 μm vs 103.9 ± 4.8 μm, P = .009). The mean deviation from targeted flap thickness was not significantly different between the two groups.

CONCLUSIONS

The Intralase iFS 150-kHz femtosecond laser creates flaps of similar thickness and uniformity to the Intralase FS 60-kHz femtosecond laser.

Analysis of femtosecond laser assisted capsulotomy cutting edges and manual capsulorhexis using environmental scanning electron microscopy

Purpose. To investigate the structure and irregularity of the capsulotomy cutting edges created by two femtosecond (FS) laser platforms in comparison with manual continuous circular capsulorhexis (CCC) using environmental scanning electron microscopy (eSEM). Methods. Ten anterior capsulotomies were obtained using two different FS laser cataract platforms (LenSx, n = 5, and Victus, n = 5). In addition, five manual CCC (n = 5) were obtained using a rhexis forceps. The specimens were imaged by eSEM (FEI Quanta 400, OR, USA). Objective metrics, which included the arithmetic mean deviation of the surface (Sa) and the root-mean-square deviation of the surface (Sq), were used to evaluate the irregularity of both the FS laser capsulotomies and the manual CCC cutting edges. Results. Several microirregularities were shown across the FS laser capsulotomy cutting edges. The edges of manually torn capsules were shown, by comparison of Sa and Sq values, to be smoother (P < 0.05) than the FS laser capsulotomy edges. Conclusions. Work is needed to understand whether the FS laser capsulotomy edge microirregularities, not seen in manual CCC, may act as focal points for the concentration of stress that would increase the risk of capsular tear during phacoemulsification as recently reported in the literature.

Hydrated human corneal stroma revealed by quantitative dynamic atomic force microscopy at nanoscale

The structures and mechanical properties of human tissues are significantly influenced by water. The functionality of the human cornea can be linked to the hydrated collagen fibers. By applying quantitative dynamic atomic force microscopy to investigate morphological and mechanical property variations of corneal stroma under different hydration levels, we found that the collagen fibers in the stromal tissue show the specific periodicities and the stiffness of giga-Pa magnitude at 40% humidity. However, under increasing hydration, the collagen fibers clearly show nanoparticle structures along the fibers with the stiffness in mega-Pa magnitude. By increasing the hydration time, the stroma regains the fiber structure but with larger diameter. The age-dependency in stiffness was further investigated. The interplay of structures and nanomechanical mapping may be applied for the future diagnosis and assessment or even pathologic analysis.

Pre-processing, registration and selection of adaptive optics corrected retinal images

PURPOSE

In this paper, the aim is to demonstrate enhanced processing of sequences of fundus images obtained using a commercial AO flood illumination system. The purpose of the work is to (1) correct for uneven illumination at the retina (2) automatically select the best quality images and (3) precisely register the best images.

METHODS

Adaptive optics corrected retinal images are pre-processed to correct uneven illumination using different methods; subtracting or dividing by the average filtered image, homomorphic filtering and a wavelet based approach. These images are evaluated to measure the image quality using various parameters, including sharpness, variance, power spectrum kurtosis and contrast. We have carried out the registration in two stages; a coarse stage using cross-correlation followed by fine registration using two approaches; parabolic interpolation on the peak of the cross-correlation and maximum-likelihood estimation. The angle of rotation of the images is measured using a combination of peak tracking and Procrustes transformation.

RESULTS

We have found that a wavelet approach (Daubechies 4 wavelet at 6th level decomposition) provides good illumination correction with clear improvement in image sharpness and contrast. The assessment of image quality using a 'Designer metric' works well when compared to visual evaluation, although it is highly correlated with other metrics. In image registration, sub-pixel translation measured using parabolic interpolation on the peak of the cross-correlation function and maximum-likelihood estimation are found to give very similar results (RMS difference 0.047 pixels). We have confirmed that correcting rotation of the images provides a significant improvement, especially at the edges of the image. We observed that selecting the better quality frames (e.g. best 75% images) for image registration gives improved resolution, at the expense of poorer signal-to-noise. The sharpness map of the registered and de-rotated images shows increased sharpness over most of the field of view.

CONCLUSION

Adaptive optics assisted images of the cone photoreceptors can be better pre-processed using a wavelet approach. These images can be assessed for image quality using a 'Designer Metric'. Two-stage image registration including correcting for rotation significantly improves the final image contrast and sharpness.