Using femtosecond laser to create customized corneal flaps for patients with low and moderate refractive error differing in corneal thickness

This study is designed to evaluate the visual outcomes, accuracy, and predictability of corneal flaps with different thicknesses created by 60-kHz femtosecond laser according to different corneal thicknesses in the patients with low and moderate refractive error. A total of 182 eyes were divided according to the central corneal thickness (470 μm-499 μm in Group A, 500 μm-549 μm in Group B, and 550 μm-599 μm in Group C) and underwent femtosecond laser-assisted LASIK for a target corneal flap thickness (100 μm for Group A, 110 μm for Group B, and 120 μm for Group C). Uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), and refractive status were examined. The flap thickness of each eye was measured by anterior segment optical coherence tomography (AS-OCT) on 30 points at 1-month follow-up to assess the accuracy and predictability. Postoperatively, at least 75% of eyes had a UDVA of 20/16 or better, less than 2% of eyes lost one line, over 30% of eyes gained one or more lines in CDVA, at least 95% of eyes had astigmatism of less than 0.25 D, all eyes achieved a correction within ± 1.00 D from the target spherical equivalent refraction. The visual and refractive outcomes did not differ significantly in all groups (P >0.05). The mean flap thickness was 100.36 ± 4.32 μm (range: 95-113 μm) in Group A, 111.64 ± 3.62 μm (range: 108-125 μm) in Group B, and 122.32 ± 2.88 μm (range: 112-128 μm) in Group C. The difference at each measured point among the three groups was significant (P < 0.05). The accuracy and predictability were satisfactory in all three groups. In conclusion, this customized treatment yielded satisfactory clinical outcomes with accurate and predictable flap thickness for patients with low and moderate refractive error.

Advances in Refractive Surgery: May 2013 to June 2014

PURPOSE

The field of refractive surgery is changing quickly in the areas of screening, treatment, and postoperative management. The objective of this study was to review advances in the field of refractive surgery as reported in the peer-reviewed literature during the previous year.

DESIGN

This was a literature review.

METHODS

In this article, we highlight the progression in the field from May 2013 to the end of June 2014. This article is organized as preoperative, intraoperative, and postoperative considerations.

RESULTS

The growing use of newer modalities of treatment, such as small-incision lenticule extraction (SmILE) and phakic intraocular lenses, and their safety and efficacy add to the armamentarium in treating refractive error. This past year has seen many studies, which show these procedures to be safe and effective. Also, structurally, SmILE may result in a more biomechanically stable cornea, so that we may be able to safely treat higher refractive errors.

CONCLUSIONS

While laser-assisted in situ keratomileusis is the predominant surgical procedure, photorefractive keratectomy is still an appropriate choice in certain scenarios (higher risk for ectasia postrefractive surgery and patient choice) with safe, predictable results, and patient satisfaction. Many of the new developments in laser-assisted in situ keratomileusis have focused on flap creation with femtosecond lasers. Histopathology and in vivo optical coherence tomography studies have shown that the stromal bed is smoother, the side cuts are vertical, and the flap has predictable thickness. New surgical modalities, such as femtosecond lenticule extraction and SmILE, are increasingly being used.

[SMILE: refractive lenticule extraction for myopic correction]

PURPOSE

It is currently possible to obtain refractive correction without using an excimer laser; the technique is called ReLEx(®) (Refractive Lenticule extraction) and uses the femtosecond laser VisuMax(®) (Carl Zeiss Meditec, Jena, Germany). We present initial clinical experience with small-incision (4mm) lenticule extraction (SMILE) for the treatment of moderate to high myopia via a retrospective study of 106 myopic and astigmatic eyes.

MATERIALS AND METHODS

This retrospective study of 106 eyes measures postoperative visual acuity to determine the safety, efficacy and predictability of the technique. For SMILE, an intrastromal lenticule is cut with the VisuMax(®) femtosecond laser; it is then dissected and detached from the anterior and posterior stromal planes and finally removed through a 4mm incision. This procedure does not require the creation of a flap. The study is based on 106 eyes of 54 patients who underwent the surgery for moderate to high myopia, with or without associated astigmatism. Patient selection was the same as for LASIK.

RESULTS

The average age of the patients was 33 ± 9 (20-54). The mean preoperative spherical equivalent was -6.22 ± 1.6 (-3; -9.75 D). The mean preoperative sphere was -5.88 D ± 1.52 (-3; -9.75 D) with a mean cylinder of -0.68 D ± 0.58 (0.00; -2.25). Postoperatively, 95% of patients had distance vision greater than or equal to 8/10. No loss of BCVA was noted. The mean postoperative spherical equivalent was -0.16 ± 0.35 D (-1.38; +0.88) with a mean sphere of 0.00 ± 0.34 D (-1,25; +1) and a mean cylinder of -0.31 ± 0.37 D (0; -1.5). One eye was converted to PRK because the incision site was covered by conjunctiva during the last 5 seconds of the laser phase.

CONCLUSION

SMILE is a predictable and safe technique for the surgical correction of moderate to high myopia.

Effects of laser in situ keratomileusis (LASIK) on corneal biomechanical measurements with the Corvis ST tonometer

Purpose

This study was initiated to evaluate biomechanical changes using the Corvis ST tonometer (CST) on the cornea after laser in situ keratomileusis (LASIK).

Setting

University Medical Center Hamburg-Eppendorf, Germany, and Care Vision Refractive Centers, Germany.

Design

Retrospective cohort study.

Methods

This retrospective study included 37 eyes of 37 refractive patients. All CST measurements were performed 1 day before surgery and at the 1-month follow-up examination. The LASIK procedure included mechanical flap preparation using a Moria SBK microkeratome and an Allegretto excimer laser platform.

Results

Statistically significant differences were observed for mean first applanation length, mean first and second deflection lengths, mean first and second deflection amplitudes, radius of curvature, and peak distance. Significant positive correlations were found between the change (Δ) of radius of curvature and manifest refraction spherical equivalent (MRSE), ablation depth, and Δintraocular pressure as well as between AD and ΔHC-time. Each diopter of myopic correction in MRSE resulted in an increase in Δradius of curvature of 0.2 mm.

Conclusion

Several CST parameters were statistically significantly altered by LASIK, thereby indicating that flap creation, ablation, or both, significantly change the ability of the cornea to absorb or dissipate energy.

Translating ocular biomechanics into clinical practice: current state and future prospects

Biomechanics is the study of the relationship between forces and function in living organisms and is thought to play a critical role in a significant number of ophthalmic disorders. This is not surprising, as the eye is a pressure vessel that requires a delicate balance of forces to maintain its homeostasis. Over the past few decades, basic science research in ophthalmology mostly confirmed that ocular biomechanics could explain in part the mechanisms involved in almost all major ophthalmic disorders such as optic nerve head neuropathies, angle closure, ametropia, presbyopia, cataract, corneal pathologies, retinal detachment and macular degeneration. Translational biomechanics in ophthalmology, however, is still in its infancy. It is believed that its use could make significant advances in diagnosis and treatment. Several translational biomechanics strategies are already emerging, such as corneal stiffening for the treatment of keratoconus, and more are likely to follow. This review aims to cultivate the idea that biomechanics plays a major role in ophthalmology and that the clinical translation, lead by collaborative teams of clinicians and biomedical engineers, will benefit our patients. Specifically, recent advances and future prospects in corneal, iris, trabecular meshwork, crystalline lens, scleral and lamina cribrosa biomechanics are discussed.

Comparison of biomechanical effects of small-incision lenticule extraction and laser in situ keratomileusis: finite-element analysis

PURPOSE

To theoretically compare the corneal stress distribution of laser in situ keratomileusis (LASIK) with the stress distribution of small-incision lenticule extraction.

SETTING

Cleveland Clinic Cole Institute, Cleveland, and The Ohio State University, Columbus, Ohio, USA.

DESIGN

Computational modeling study.

METHODS

A finite-element anisotropic collagen fiber-dependent model of myopic surgery using patient-specific corneal geometry was constructed for LASIK, small-incision lenticule extraction, and a geometry analog model with unaltered material properties from preoperative but with postoperative geometry including thickness. Surgical parameters, magnitude of myopic correction, LASIK flap thickness, and lenticule depth in small-incision lenticule extraction were varied. Two sets of models, 1 with uniform and 1 with depth-dependent material properties, were constructed.

RESULTS

Stress distribution between small-incision lenticule extraction simulations and the geometry analog model were similar. In contrast, LASIK consistently reduced stress in the flap and increased stress in the residual stromal bed (RSB) compared with the geometry analog model. An increase in flap thickness or lenticule depth resulted in a greater increase in RSB stress in the LASIK model than in the small-incision lenticule extraction model.

CONCLUSIONS

Small-incision lenticule extraction may present less biomechanical risk to the residual bed of susceptible corneas than comparable corrections involving LASIK flaps. Deeper corrections in the stroma may be possible in small-incision lenticule extraction without added risk for ectasia.

FINANCIAL DISCLOSURES

Proprietary or commercial disclosures are listed after the references.

Noncontact depth-resolved micro-scale optical coherence elastography of the cornea

High-resolution elastographic assessment of the cornea can greatly assist clinical diagnosis and treatment of various ocular diseases. Here, we report on the first noncontact depth-resolved micro-scale optical coherence elastography of the cornea achieved using shear wave imaging optical coherence tomography (SWI-OCT) combined with the spectral analysis of the corneal Lamb wave propagation. This imaging method relies on a focused air-puff device to load the cornea with highly-localized low-pressure short-duration air stream and applies phase-resolved OCT detection to capture the low-amplitude deformation with nano-scale sensitivity. The SWI-OCT system is used here to image the corneal Lamb wave propagation with the frame rate the same as the OCT A-line acquisition speed. Based on the spectral analysis of the corneal temporal deformation profiles, the phase velocity of the Lamb wave is obtained at different depths for the major frequency components, which shows the depthwise distribution of the corneal stiffness related to its structural features. Our pilot experiments on ex vivo rabbit eyes demonstrate the feasibility of this method in depth-resolved micro-scale elastography of the cornea. The assessment of the Lamb wave dispersion is also presented, suggesting the potential for the quantitative measurement of corneal viscoelasticity.

Small incision lenticule extraction (SMILE) history, fundamentals of a new refractive surgery technique and clinical outcomes

This review summarizes the current status of the small incision lenticule extraction (SMILE) procedure. Following the early work by Sekundo et al. and Shah et al., SMILE has become increasingly popular. The accuracy of the creation of the lenticule with the VisuMax femtosecond laser (Carl Zeiss Meditec) has been verified using very high-frequency (VHF) digital ultrasound and optical coherence tomography (OCT). Visual and refractive outcomes have been shown to be similar to those achieved with laser in situ keratomileusis (LASIK), notably in a large population reported by Hjortdal, Vestergaard et al. Safety in terms of the change in corrected distance visual acuity (CDVA) has also been shown to be similar to LASIK. It was expected that there would be less postoperative dry eye after SMILE compared to LASIK because the anterior stroma is disturbed only by the small incision, meaning that the anterior corneal nerves should be less affected. A number of studies have demonstrated a lower reduction and faster recovery of corneal sensation after SMILE than LASIK. Some studies have also used confocal microscopy to demonstrate a lower decrease in subbasal nerve fiber density after SMILE than LASIK. The potential biomechanical advantages of SMILE have been modeled by Reinstein et al. based on the non-linearity of tensile strength through the stroma. Studies have reported a similar change in Ocular Response Analyzer (Reichert) parameters after SMILE and LASIK, however, these have previously been shown to be unreliable as a representation of corneal biomechanics. Retreatment options after SMILE are discussed. Tissue addition applications of the SMILE procedure are also discussed including the potential for cryo-preservation of the lenticule for later reimplantation (Mohamed-Noriega, Angunawela, Lim et al.), and a new procedure referred to as endokeratophakia in which a myopic SMILE lenticule is implanted into a hyperopic patient (Pradhan et al.). Finally, studies reporting microdistortions in Bowman's layer and corneal wound healing responses are also described.

Association between the percent tissue altered and post-laser in situ keratomileusis ectasia in eyes with normal preoperative topography

PURPOSE

To investigate the association of a novel metric, percent tissue altered, with the occurrence of ectasia after laser in situ keratomileusis (LASIK) in eyes with normal corneal topography and to compare this metric with other recognized risk factors.

DESIGN

Retrospective case-control study.

METHODS

The study included 30 eyes from 16 patients with bilateral normal preoperative Placido-based corneal topography that developed ectasia after LASIK (ectasia group) and 174 eyes from 88 consecutive patients with uncomplicated LASIK and at least 3 years of postoperative follow-up. The following metrics were evaluated: age, preoperative central corneal thickness, residual stromal bed, Ectasia Risk Score System scores, and percent tissue altered, derived from [PTA = (FT + AD)/CCT], where FT = flap thickness, AD = ablation depth, and CCT = preoperative central corneal thickness.

RESULTS

In the ectasia group, percent tissue altered ≥40 was the most prevalent factor (97%), followed by age <30 years (63%), residual stromal bed ≤300 μm (57%), and ectasia risk score ≥ 3 (43%) (P < .001 for all). Percent tissue altered ≥ 40 had the highest odds ratio (223), followed by residual stromal bed ≤ 300 μm (74) and ectasia risk score ≥ 4 (8). Stepwise logistic regression revealed percent tissue altered ≥ 40 as the single most significant independent variable (P < .0001).

CONCLUSIONS

Percent tissue altered at the time of LASIK was significantly associated with the development of ectasia in eyes with normal preoperative topography and was a more robust indicator of risk than all other variables in this patient population.

BAC-EDTA transepithelial riboflavin-UVA crosslinking has greater biomechanical stiffening effect than standard epithelium-off in rabbit corneas

Studies suggest that standard corneal collagen crosslinking (CXL) is a safe and effective treatment to stiffen the cornea for keratoconus and other ectatic corneal disorders. The purpose of the present study was to compare the biomechanical effects of transepithelial benzalkonium chloride-EDTA (BAC-EDTA) riboflavin-UVA crosslinking to standard epithelium-off riboflavin-UVA crosslinking in a rabbit model. Corneal stiffness was quantified using optical coherence elastography at two months after treatment. The mean lateral-to-axial displacement ratio for the BAC-EDTA transepithelial CXL group was lower (greater stiffness) [0.062 ± 0.042, mean ± SD] than epithelium-off CXL (mean ± SD: 0.065 ± 0.045) or untreated control eyes (0.069 ± 0.044). Using ANOVA with Tukey correction, a statistically significant difference was found between the BAC-EDTA transepithelial CXL group and standard epithelium-off CXL (p = 0.0019) or the untreated control (p < 0.0001) groups. A graph of the probability density functions for biomechanical stiffness also showed a greater shift in stiffening in the BAC-EDTA transepithelial CXL group than the standard epithelium-off CXL or untreated control group. These results demonstrated that the biomechanical stiffening effect produced by BAC-EDTA transepithelial CXL was greater than that produced by standard epithelium-off CXL in a rabbit model.

Serial biomechanical comparison of edematous, normal, and collagen crosslinked human donor corneas using optical coherence elastography

PURPOSE

To noninvasively evaluate the effects of corneal hydration and collagen crosslinking (CXL) on the mechanical behavior of the cornea.

SETTING

Cleveland Clinic Cole Eye Institute, Cleveland, Ohio, USA.

DESIGN

Experimental study.

METHODS

An optical coherence elastography (OCE) technique was used to measure the displacement behavior of 5 pairs of debrided human donor globes in 3 serial states as follows: edematous, normal thickness, and after riboflavin-ultraviolet-A-mediated CXL. During micromotor-controlled axial displacements with a curved goniolens at physiologic intraocular pressure (IOP), serial optical coherence tomography scans were obtained to allow high-resolution intrastromal speckle tracking and displacement measurements over the central 4.0 mm of the cornea.

RESULTS

With no imposed increase in IOP, the mean lateral to imposed axial displacement ratios were 0.035 μm/μm ± 0.037 (SD) in edematous corneas, 0.021 ± 0.02 μm/μm in normal thickness corneas, and 0.014 ± 0.009 μm/μm in post-CXL corneas. The differences were statistically significant (P<.05, analysis of variance) and indicated a 40% increase in lateral stromal resistance with deturgescence and a further 33% mean increase in relative stiffness with CXL.

CONCLUSIONS

Serial perturbations of the corneal hydration state and CXL had significant effects on corneal biomechanical behavior. With an axially applied stress from a nonapplanating contact lens, displacements along the direction of the collagen lamellae were 2 orders of magnitude lower than axial deformations. These experiments show the ability of OCE to quantify clinically relevant mechanical property differences under physiologic conditions.

FINANCIAL DISCLOSURES

Proprietary or commercial disclosures are listed after the references.

Biomechanics of corneal ectasia and biomechanical treatments

Many algorithms exist for the topographic/tomographic detection of corneas at risk for post-refractive surgery ectasia. It is proposed that the reason for the difficulty in finding a universal screening tool based on corneal morphologic features is that curvature, elevation, and pachymetric changes are all secondary signs of keratoconus and post-refractive surgery ectasia and that the primary abnormality is in the biomechanical properties. It is further proposed that the biomechanical modification is focal in nature, rather than a uniform generalized weakening, and that the focal reduction in elastic modulus precipitates a cycle of biomechanical decompensation that is driven by asymmetry in the biomechanical properties. This initiates a repeating cycle of increased strain, stress redistribution, and subsequent focal steepening and thinning. Various interventions are described in terms of how this cycle of biomechanical decompensation is interrupted, such as intrastromal corneal ring segments, which redistribute the corneal stress, and collagen crosslinking, which modifies the basic structural properties.

FINANCIAL DISCLOSURES

Proprietary or commercial disclosures are listed after the references.

Patterned corneal collagen crosslinking for astigmatism: computational modeling study

PURPOSE

To test the hypothesis that spatially selective corneal stromal stiffening can alter corneal astigmatism and assess the effects of treatment orientation, pattern, and material model complexity in computational models using patient-specific geometries.

SETTING

Cornea and Refractive Surgery Service, Academic Eye Institute, Cleveland, Ohio, USA.

DESIGN

Computational modeling study.

METHODS

Three-dimensional corneal geometries from 10 patients with corneal astigmatism were exported from a clinical tomography system (Pentacam). Corneoscleral finite element models of each eye were generated. Four candidate treatment patterns were simulated, and the effects of treatment orientation and magnitude of stiffening on anterior curvature and aberrations were studied. The effect of material model complexity on simulated outcomes was also assessed.

RESULTS

Pretreatment anterior corneal astigmatism ranged from 1.22 to 3.92 diopters (D) in a series that included regular and irregular astigmatic patterns. All simulated treatment patterns oriented on the flat axis resulted in mean reductions in corneal astigmatism and depended on the pattern geometry. The linear bow-tie pattern produced a greater mean reduction in astigmatism (1.08 D ± 0.13 [SD]; range 0.74 to 1.23 D) than other patterns tested under an assumed 2-times increase in corneal stiffness, and it had a nonlinear relationship to the degree of stiffening. The mean astigmatic effect did not change significantly with a fiber- or depth-dependent model, but it did affect the coupling ratio.

CONCLUSIONS

In silico simulations based on patient-specific geometries suggest that clinically significant reductions in astigmatism are possible with patterned collagen crosslinking. Effect magnitude was dependent on patient-specific geometry, effective stiffening pattern, and treatment orientation.

FINANCIAL DISCLOSURES

Proprietary or commercial disclosures are listed after the references.

Changes in custom biomechanical variables after femtosecond laser in situ keratomileusis and photorefractive keratectomy for myopia

PURPOSE

To analyze changes in new biomechanical descriptors with myopic femtosecond laser-assisted laser in situ keratomileusis (LASIK), compare them with the biomechanical response after photorefractive keratectomy (PRK) with similar levels of myopic ablation, and evaluate correlations between changes in custom variables and biomechanically relevant variables.

SETTING

Cleveland Clinic, Cleveland, Ohio, USA.

DESIGN

Cohort study.

METHODS

Custom biomechanical variables from the Optical Response Analyzer were assessed preoperatively and 1 and 3 months postoperatively. Differences between preoperative values and postoperative values were determined. Intraindividual change (preoperative value minus postoperative value) was calculated and compared with changes after PRK. The correlation of the change in each custom biomechanical variable with the preoperative central corneal thickness, residual stromal bed tissue ablated, and percentage of tissue depth altered was also studied.

RESULTS

The study enrolled 156 eyes of 156 consecutive patients. Fifteen variables changed significantly after femtosecond myopic LASIK and were stable postoperatively because no significant difference was shown between 1-month values and 3-month values. Comparison of the changes in biomechanical variables between LASIK and PRK eyes showed no significant differences. Surgical changes in several custom biomechanical variables correlated with the percentage of tissue depth altered.

CONCLUSIONS

The results provide the first reference values for a more comprehensive panel of indicators of the biomechanical response to myopic LASIK and PRK. Changes in custom variables reflected a consistent decrease in corneal biomechanical resistance to deformation after myopic femtosecond LASIK and PRK. For comparable attempted corrections, biomechanical changes were comparable between femtosecond laser-assisted LASIK and PRK.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosures are listed after the references.

Distribution of Young's modulus in porcine corneas after riboflavin/UVA-induced collagen cross-linking as measured by atomic force microscopy

Riboflavin/UVA-induced corneal collagen cross-linking has become an effective clinical application to treat keratoconus and other ectatic disorders of the cornea. Its beneficial effects are attributed to a marked stiffening of the unphysiologically weak stroma. Previous studies located the stiffening effect predominantly within the anterior cornea. In this study, we present an atomic force microscopy-derived analysis of the depth-dependent distribution of the Young's modulus with a depth resolution of 5 µm in 8 cross-linked porcine corneas and 8 contralateral controls. Sagittal cryosections were fabricated from every specimen and subjected to force mapping. The mean stromal depth of the zone with effective cross-linking was found to be 219±67 µm. Within this cross-linked zone, the mean Young's modulus declined from 49±18 kPa at the corneal surface to 46±17 kPa, 33±11 kPa, 17±5 kPa, 10±4 kPa and 10±4 kPa at stromal depth intervals of 0–50 µm, 50–100 µm, 100–150 µm, 150–200 µm and 200–250 µm, respectively. This corresponded to a stiffening by a factor of 8.1 (corneal surface), 7.6 (0–50 µm), 5.4 (50–100 µm), 3.0 (100–150 µm), 1.6 (150–200 µm), and 1.5 (200–250 µm), when compared to the Young's modulus of the posterior 100 µm. The mean Young's modulus within the cross-linked zone was 20±8 kPa (2.9-fold stiffening), while it was 11±4 kPa (1.7-fold stiffening) for the entire stroma. Both values were significantly distinct from the mean Young's modulus obtained from the posterior 100 µm of the cross-linked corneas and from the contralateral controls. In conclusion, we were able to specify the depth-dependent distribution of the stiffening effect elicited by standard collagen cross-linking in porcine corneas. Apart from determining the depth of the zone with effective corneal cross-linking, we also developed a method that allows for atomic force microscopy-based measurements of gradients of Young's modulus in soft tissues in general.

Anterior and posterior corneal stroma elasticity assessed using nanoindentation

Corneal biomechanics is an essential parameter for developing diagnostic and treatment methods of corneal-related diseases. It is widely accepted that corneal mechanical strength stems from the stroma's collagenous composition. However, more comprehensive insight into the mechanical properties within the stroma is needed to improve current corneal diagnostic and treatment techniques. The purpose of this study was to perform elasticity characterization of anterior and posterior stromal regions of human corneas using atomic force microscopy (AFM). Nine pairs of human whole globes were placed in 20% Dextran solution, cornea side down, to restore the corneal thickness to physiological levels (400-600 μm). The epithelium and Bowman's membrane were removed from all eyes. Anterior stromal AFM elasticity testing was then performed on left (OS) eyes. Additional stroma was removed from right (OD) eyes to allow posterior stromal measurements at a depth of 50% of the original thickness. All experiments were performed with corneas submerged in 15% Dextran to maintain corneal hydration. The results of the study showed that the Young's modulus of elasticity of the anterior stroma (average: 281 ± 214 kPa; range: 59-764 kPa) was significantly higher than that of the posterior stroma (average: 89.5 ± 46.1 kPa; range: 29-179 kPa) (p = 0.014). In addition, a linear relationship was found between the posterior stromal elasticity and anterior stromal elasticity (p = 0.0428). On average, the elasticity of the posterior stroma is 39.3% of the anterior stroma. In summary, there appears to be an elasticity gradient within the corneal stroma, which should be considered in the design and development of corneal diagnostic and treatment methods to enhance efficacy.

Effects of collagen cross-linking on the interlamellar cohesive strength of porcine cornea

PURPOSE

To determine the interlamellar cohesive strength (ICS) of porcine cornea before and after collagen cross-linking using riboflavin and ultraviolet A (UVA) irradiation.

METHODS

Corneal flaps, 130 μm thick, were created with a microkeratome. Cross-linking was accomplished with 0.1% riboflavin and UVA irradiation for 30 minutes (3 mW/cm, 5.4 J/cm) at a distance of 10 mm. Two experiments were performed. (1) The maximal ICS between corneal flap and the stroma was determined and histological examination was performed. (2) The mean stromal ICS was measured. Untreated eyes served as controls in all experiments.

RESULTS

The mean maximum flap-stroma ICS was 0.088 ± 0.046 N/mm in the experimental group and 0.012 ± 0.004 N/mm in the control group (P = 0.009). In experiment 2, incomplete (2 mm long) corneal flaps were used and the mean stromal ICS was 0.750 ± 0.077 N/mm in the experimental group and 0.338 ± 0.046 N/mm in the control group (P < 0.0001). Histological examination showed an irregular ragged separation of the surface in the stromal margin of the cornea with cross-linking, but a smooth separation surface in the cornea without cross-linking.

CONCLUSIONS

Collagen cross-linking activated by riboflavin/UVA increases ICS in porcine corneas.

Characterization of corneal tangent modulus in vivo

PURPOSE

Intraocular pressure (IOP) measured using Goldmann Applanation Tonometry (GAT) changes with individual's corneal properties, but the method to measure the in vivo corneal material properties to account for individual variation in GAT IOP is not available. In this study, a new method to measure the IOP-dependent corneal tangent modulus in vivo is developed to address this research gap.

METHODS

Instrumented indentation and analysis were developed to measure the corneal tangent modulus. The validity of the method and procedure was verified using model silicone eye pressurized to different IOP. In addition, 15 porcine eyes and 3 rabbit eyes were tested using the corneal indentation at different set intraocular pressure and different indentation rates.

RESULTS

The results from silicone eye showed that the measured tangent modulus is in good agreement with the standard silicone rubber modulus. The results on the porcine eyes and rabbit eyes showed that the method can be used to measure corneal tangent modulus in vivo in the human range of intraocular pressure from 10 to 40 mmHg.

CONCLUSIONS

An indentation method to measure the corneal tangent modulus in vivo was developed, and the IOP dependence of the corneal tangent modulus was characterized. The developed indentation method provides a new means to measure the in vivo corneal tangent modulus to account for individual and pressure variations in measurement of intraocular pressure.

Effect of timolol on refractive outcomes in eyes with myopic regression after laser in situ keratomileusis: a prospective randomized clinical trial

PURPOSE

To compare the effects of timolol on refractive outcomes in eyes with myopic regression after laser in situ keratomileusis (LASIK) with a control-matched group.

DESIGN

Prospective, randomized, parallel-controlled, double-masked clinical trial. A computer-generated randomization list based on random block permutation (length 4 to 8) was used for treatment allocation.

METHODS

setting: Basir Eye Center, Tehran, Iran.

PATIENT POPULATION

Of 124 eyes with myopic regression after LASIK using Technolas 217-Z, 45 eyes in each group were analyzed.

INTERVENTION

Patients were randomly assigned into either Group 1, who received timolol 0.5% eye drops, or Group 2, who received artificial tears for 6 months.

MAIN OUTCOME MEASURE

Spherical equivalent (SE) at 6 months posttreatment.

RESULTS

In Group 1, SE improved from -1.48 ± 0.99 diopter (D) before treatment to -0.88 ± 0.91 D and -0.86 ± 0.93 D 6 months after treatment and 6 months after timolol discontinuation, respectively (P < .001). In Group 2, it was -1.57 ± 0.67 D, -1.83 ± 0.76 D, and -1.91 ± 0.70 D, respectively (P < .001). SE was significantly better in Group 1 6 months after treatment and 6 months after discontinuation of treatment (P < .001 for both comparisons). There was a 0.26 D decrease in SE improvement every 4 months after the surgery in the Group 1 (P < .001).

CONCLUSIONS

Timolol application is effective for the treatment of myopic regression after LASIK compared with control group. Its effects last for at least 6 months after its discontinuation.

In vivo Brillouin optical microscopy of the human eye

We report the first Brillouin measurement of the human eye in vivo. We constructed a Brillouin optical scanner safe for human use by employing continuous-wave laser light at 780 nm at a low power of 0.7 mW. With a single scan along the optic axis of the eye, the axial profile of Brillouin frequency shift was obtained with a pixel acquisition time of 0.4 s and axial resolution of about 60 μm, showing the depth-dependent biomechanical properties in the cornea and lens.

Corneal biomechanics and biomaterials

The ability to clearly observe one's environment in the visible spectrum provides a tremendous evolutionary advantage in most of the world's habitats. The complex optical processing system that has evolved in higher vertebrate animals gathers, focuses, detects, transduces, and interprets incoming visible light. The cornea resides at the front end of this imaging system, where it provides a clear optical aperture, substantial refractive power, and the structural stability required to protect the fragile intraocular components. Nature has resolved these simultaneous design requirements through an exceedingly clever manipulation of common extracellular-matrix structural materials (e.g., collagen and proteoglycans). In this review, we (a) examine the biophysical and optical roles of the cornea, (b) discuss increasingly popular approaches to altering its natural refractive properties with an emphasis on biomechanics, and (c) investigate the fast-rising science of corneal replacement via synthetic biomaterials. We close by considering relevant open problems that would benefit from the increased attention of bioengineers.

Brillouin optical microscopy for corneal biomechanics

PURPOSE

The mechanical properties of corneal tissue are linked to prevalent ocular diseases and therapeutic procedures. Brillouin microscopy is a novel optical technology that enables three-dimensional mechanical imaging. In this study, the feasibility of this noncontact technique was tested for in situ quantitative assessment of the biomechanical properties of the cornea.

METHODS

Brillouin light-scattering involves a spectral shift proportional to the longitudinal modulus of elasticity of the tissue. A 532-nm single-frequency laser and a custom-developed ultrahigh-resolution spectrometer were used to measure the Brillouin frequency. Confocal scanning was used to perform Brillouin elasticity imaging of the corneas of whole bovine eyes. The longitudinal modulus of the bovine corneas was compared before and after riboflavin corneal collagen photo-cross-linking. The Brillouin measurements were then compared with conventional stress-strain mechanical test results.

RESULTS

High-resolution Brillouin images of the cornea were obtained, revealing a striking depth-dependent variation of the elastic modulus across the cornea. Along the central axis, the Brillouin frequency shift varied gradually from 8.2 GHz in the epithelium to 7.5 GHz near the endothelium. The coefficients of the down slope were measured to be approximately 1.09, 0.32, and 2.94 GHz/mm in the anterior, posterior, and innermost stroma, respectively. On riboflavin collagen cross-linking, marked changes in the axial Brillouin profiles (P < 0.001) were noted before and after cross-linking.

CONCLUSIONS

Brillouin imaging can assess the biomechanical properties of cornea in situ with high spatial resolution. This novel technique has the potential for use in clinical diagnostics and treatment monitoring.

[Comparison of Lasik with femtosecond laser versus Lasik with mechanical microkeratome: predictability of flap depth, corneal biomechanical effects and optical aberrations]

OBJECTIVES

To compare the predictability of flap thickness, high-order optic aberrations (HOAs), and biomechanical properties of cornea between patients treated by Lasik with mechanical microkeratome versus patients treated by FemtoLasik.

SETTING

Department of ophthalmology, Pellegrin University Hospital, Bordeaux, France.

PATIENTS AND METHODS

We conducted a retrospective study on 53 myopic patients who underwent Lasik with either mechanical microkeratome (MK group) or femtosecond laser (FS group). Refraction, central corneal thickness, high-order optic aberrations (HOAs), corneal hysteresis (CH), and corneal resistance factor (CRF), were analysed pre- and postoperatively in both groups. The central corneal thickness was measured with OCT-Visante(®) (Carl-Zeiss, Meditec), biomechanical parameters with ORA(®) (Reichert), and optical aberrations with the Wave Scan(®) (AMO) aberrometer.

RESULTS

We included 44 eyes of 22 patients in the MK group and 62 eyes of 31 patients in the FS group. Preoperatively, the mean best-corrected visual acuity was 0.95 in both groups. In the MK group, the flap was significantly thicker than expected (162/130 μm), but in the FS group, there was no significant difference (117/120 μm). The biomechanical properties of the cornea were lower in both groups independently of the flap cutting technique. The HOAs increased after Lasik and were not influenced by the flap cutting technique.

CONCLUSION

Neither mechanical microkeratome, nor femtosecond laser for flap creation, increases HOAs and the biomechanical changes of the cornea, according to ORA(®), significantly after Lasik.

Method for optical coherence elastography of the cornea

The material properties of the cornea are important determinants of corneal shape and refractive power. Corneal ectatic diseases, such as keratoconus, are characterized by material property abnormalities, are associated with progressive thinning and distortion of the cornea, and represent a leading indication for corneal transplantation. We describe a corneal elastography technique based on optical coherence tomography (OCT) imaging, in which displacement of intracorneal optical features is tracked with a 2-D cross-correlation algorithm as a step toward nondestructive estimation of local and directional corneal material properties. Phantom experiments are performed to measure the effects of image noise and out-of-plane displacement on effectiveness of displacement tracking and demonstrated accuracy within the tolerance of a micromechanical translation stage. Tissue experiments demonstrate the ability to produce 2-D maps of heterogeneous intracorneal displacement with OCT. The ability of a nondestructive optical method to assess tissue under in situ mechanical conditions with physiologic-range stress levels provides a framework for in vivo quantification of 3-D corneal elastic and viscoelastic resistance, including analogs of shear deformation and Poisson's ratio that may be relevant in the early diagnosis of corneal ectatic disease.

Persistent subepithelial haze in thin-flap LASIK

PURPOSE

To report persistent subepithelial haze in two patients following femtosecond LASIK associated with creation of a thin flap.

METHODS

Subepithelial haze was assessed by slit-lamp photography, high-resolution Scheimpflug imaging, and corneal confocal microscopy.

RESULTS

Two patients showed distinct subepithelial haze and reduced corrected distance visual acuity at 3 months after LASIK with a think-flap generated by a femtosecond laser. The extent of haze was documented, and the haze was treated topically with steroids up to 12 weeks. The haze was localized approximately 20 to 40 microm below Bowman's layer and dissolved slowly during the 2 months of treatment. At 6 months after surgery, uncorrected visual acuity was 20/20.

CONCLUSIONS

Subepithelial haze formation represents a new potential complication in the thin-flap LASIK.

Influence of flap thickness on visual and refractive outcomes after laser in situ keratomileusis performed with a mechanical keratome

PURPOSE

To study the effect of flap thickness on visual acuity and refractive outcomes after laser in situ keratomileusis (LASIK) using 2 blade types with a mechanical microkeratome.

SETTING

Emory Vision, Atlanta, Georgia, USA.

METHODS

This retrospective analysis was of LASIK cases performed between January 2005 and June 2006 using an Amadeus I microkeratome and an ML7090 CLB blade (blade A) or a Surepass blade (blade B). Outcomes analyzed included flap thickness, uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), manifest refraction spherical equivalent (MRSE), the enhancement rate, and surgical complications 3 months postoperatively.

RESULTS

Two hundred sixty-three eyes of 153 patients were analyzed; blade A was used in 158 eyes and blade B, in 105 eyes. The mean flap thickness was significantly thinner with blade A than with blade B (107 microm +/- 12 [SD] versus 130 +/- 20 microm) (P<.0001). There was no overall correlation with either blade between flap thickness and UDVA, CDVA, or MRSE (all r<0.2). At 3 months, there was no statistically significant difference in UDVA, CDVA, or MRSE between the 2 blade groups at 3 months (all P > .10), and there was no difference in the complication rates.

CONCLUSION

Flap thickness did not affect visual or refractive outcomes with a mechanical microkeratome with either blade type.

Thresholds for interface haze formation after thin-flap femtosecond laser in situ keratomileusis for myopia

PURPOSE

To evaluate the risk factors for interface haze formation after thin-flap femtosecond laser--laser in situ keratomileusis (LASIK).

DESIGN

Prospective case series.

METHODS

One hundred and ninety-nine consecutive eyes that underwent femtosecond laser LASIK with a LADAR 4000 excimer laser (Alcon Laboratories, Fort Worth, Texas, USA), were analyzed from January 1 to April 30, 2008. Treated eyes were divided into 2 groups according to desired flap thickness: 90 microm (106 eyes), and 100 to 110 microm (93 eyes). Cycloplegic refraction, spherical equivalent (SE), uncorrected visual acuity, best spectacle-corrected visual acuity (BSCVA), depth of ablation, central corneal thickness, flap thickness (optical coherence tomography), Scheimpflug images (Pentacam; Oculus Inc, Lynnwood, Washington, USA), and light scattering were assessed in all eyes that developed haze postoperatively.

RESULTS

Haze was observed in 32 eyes treated for moderate to high myopia (SE -6.23 +/- 1.67 diopters and depth of ablation 94.6 +/- 22.9 microm) at 3 months postoperatively. Ultra-thin IntraLASIK flaps (<or=90 microm) and younger age were strongly associated with risk of postoperative haze (P = .003 and P = .01, respectively). SE, depth of ablation, and self-reported history of dry eyes were not independently associated with an increased risk of corneal haze. Patients who developed any degree of haze had significantly higher logarithmic value of scatter (log S) values (mean difference =0.24 units, P < .0005) compared with those who did not develop haze. The mean postoperative BSCVA was good in all eyes with haze (logarithm of the minimal angle of resolution 0.05 +/- 0.04).

CONCLUSION

Interface haze is associated with an ultra-thin femtosecond laser flap setting of 90 microm and younger age among eyes following LASIK for myopia.

Validation of the Ectasia Risk Score System for preoperative laser in situ keratomileusis screening

PURPOSE

To validate the Ectasia Risk Score System for identifying patients at high risk for developing ectasia after laser in situ keratomileusis (LASIK).

DESIGN

Retrospective case-control study.

METHODS

Fifty eyes that developed ectasia and 50 control eyes with normal postoperative courses after LASIK were analyzed and compared using the previously described Ectasia Risk Score System, which assigns points in a weighted fashion to the following variables: topographic pattern, predicted residual stromal bed (RSB) thickness, age, preoperative corneal thickness (CT), and manifest refraction spherical equivalent (MRSE).

RESULTS

In this series, 46 (92%) eyes with ectasia were correctly classified as being at high risk for the development of ectasia, while three (6%) controls were incorrectly classified as being at high risk for ectasia (P < 1 x 10(-10)). Significantly more eyes were classified as high risk by the ectasia risk score than by traditional screening parameters relying on abnormal topography or RSB thickness less than 250 micro (92% vs 50%; P < .00001). There was no difference in the sensitivity or specificity of the Ectasia Risk Score System in the population from which it was derived and this independent population of ectasia cases and controls.

CONCLUSIONS

The Ectasia Risk Score System is a valid and effective method for detecting eyes at risk for ectasia after LASIK and represents a significant improvement over previously utilized screening strategies.