High-irradiance CXL combined with myopic LASIK: flap and residual stroma biomechanical properties studied ex-vivo

Effect of corneal cross-linking on biomechanical changes following transepithelial photorefractive keratectomy and femtosecond laser-assisted LASIK

Purpose: To evaluate the change in corneal biomechanics in patients with postoperative ectasia risk when combining two common laser vision correction procedures (tPRK and FS-LASIK) with cross-linking (in tPRK Xtra and FS-LASIK Xtra). Methods: The study included 143 eyes of 143 myopic, astigmatic patients that were divided into non-cross-linked refractive surgery groups (non-Xtra groups, tPRK and FS-LASIK) and cross-linked groups (Xtra groups, tPRK Xtra and FS-LASIK Xtra) according to an ectasia risk scoring system. The eyes were subjected to measurements including the stress-strain index (SSI), the stiffness parameter at first applanation (SP-A1), the integrated inverse radius (IIR), the deformation amplitude at apex (DA), and the ratio of deformation amplitude between apex and 2 mm from apex (DARatio2mm). The measurements were taken preoperatively and at 1, 3, and 6 months postoperatively (pos1m, pos3m, and pos6m). Posterior demarcation line depth from the endothelium (PDLD) and from the ablation surface (DLA) were recorded at pos1m. Results: SP-A1 significantly decreased, while IIR, deformation amplitude, and DARatio2mm increased significantly postoperatively in all four groups (p < 0.01)-all denoting stiffness decreases. In the FS-LASIK group, the changes in IIR, DA, and DARatio2mm were 32.7 ± 15.1%, 12.9 ± 7.1%, and 27.2 ± 12.0% respectively, which were significantly higher (p < 0.05) compared to 20.1 ± 12.8%, 6.4 ± 8.2%, and 19.7 ± 10.4% in the FS-LASIK Xtra group. In the tPRK group, the change in IIR was 27.3 ± 15.5%, significantly larger than 16.9 ± 13.4% in the tPRK Xtra group. The changes of SSI were minimal in the tPRK (-1.5 ± 21.7%, p = 1.000), tPRK Xtra (8.4 ± 17.9%, p = 0.053), and FS-LASIK Xtra (5.6 ± 12.7%, p = 0.634) groups, but was significant in the FS-LASIK group (-12.1 ± 7.9%, p < 0.01). After correcting for baseline biomechanical metrics, preoperative bIOP and the change in central corneal thickness (△CCT) from pre to pos6m, the changes in the IIR in both FS-LASIK and tPRK groups, as well as DA, DARatio2mm and SSI in the FS-LASIK group remained statistically greater than their corresponding Xtra groups (all p < 0.05). Most importantly, after correcting for these covariates, the changes in DARatio2mm in the FS-LASIK Xtra became statistically smaller than in the tPRK Xtra (p = 0.017). Conclusion: The statistical analysis results indicate that tPRK Xtra and FS-LASIK Xtra effectively reduced the biomechanical losses caused by refractive surgery (tPRK and FS-LASIK). The decrease in corneal overall stiffness was greater in FS-LASIK than in tPRK, and the biomechanical enhancement of CXL was also higher following LASIK than after tPRK.

Application of Decellularized Porcine Sclera in Repairing Corneal Perforations and Lamellar Injuries

Scleras are mainly used for the treatment of glaucoma, eyelid damage, and scleral ulcers. Given that the sclera and cornea collectively constitute the complete external structure of the eyeball and both have the same tissue and cell origin, we attempted to identify scleral materials to treat lamellar and penetrating corneal injuries. Based on research in our center, antigenic components in decellularized porcine sclera (DPS) were removed using a simplified decellularization method, leaving the collagen structure and active components undamaged. DPS preserved the mechanical properties and did not significantly inhibit the proliferation and replication of human corneal epithelial cells. In vivo, the graft epithelium healed well after lamellar and penetrating scleral grafting, and the graft thickness did not change evidently. DPS can resist suture traction during scleral transplantation and maintain anterior chamber stability until day 28 post-operatively, especially in penetrating repairs. No obvious immune rejection of lamellar or penetrating scleral grafts was found 28 days after DPS transplantation. This study shows that DPS could be used as an alternative material for the emergency repair of corneal perforations and lamellar injuries, representing another application of sclera.

Long-Term Follow-Up of Combined Photorefractive Keratectomy and Corneal Crosslinking in Keratoconus Suspects

Purpose

To present the long-term outcomes of photorefractive keratectomy (PRK) combined with accelerated corneal cross-linking (CXL) for refractive error correction in a series of keratoconus suspects.

Setting

University practice.

Design

Retrospective case series.

Methods

A series of patients with topographic findings suspicious for keratoconus underwent simultaneous PRK and prophylactic accelerated CXL (5 minutes with intensity of 18 mW/cm²) for the correction of their refractive error. The results were recorded for more than 4 years postoperatively.

Results

Ten eyes of 5 patients were included. Mean follow-up was 58.2 months (range from 54 to 62 months). Mean age at presentation was 25 years (range from 22 to 32 years). Mean spherical equivalent (SE) refraction was −2.76 (standard deviation [SD] 0.97D, range from −1.25 to −4.00 diopters [D]), while mean central corneal thickness was 511μm (SD 13μm, range from 485 to 536 μm). At last, follow-up 9 out of 10 eyes had SE refraction within ± 0.50D and all eyes had SE within ± 1.00D. None of the eyes lost any line of corrected distance visual acuity (CDVA), whereas 1 eye gained one line of CDVA. All eyes demonstrated stability of their results during the follow-up period.

Conclusion

Simultaneous PRK followed by prophylactic accelerated CXL (PRK plus) appeared to be a safe and effective option for the correction of the refractive error in this series of keratoconus suspect patients, without compromising corneal stability for up to 5 years postoperatively.

Quasi-Static Optical Coherence Elastography to Characterize Human Corneal Biomechanical Properties

Purpose

Quasi-static optical coherence elastography (OCE) is an emerging technology to investigate corneal biomechanical behavior in situations similar to physiological stress conditions. Herein OCE was applied to evaluate previously inaccessible biomechanical characteristics of human corneal tissue and to study the role of Bowman's layer in corneal biomechanics.

Methods

Human corneal donor buttons (n = 23) were obtained and Descemet's membrane and endothelium were removed. In 11 corneas, Bowman's layer was ablated by a 20 µm stromal excimer laser ablation. Buttons were mounted on an artificial anterior chamber and subjected to a pressure modulation from 10 to 30 mm Hg, and back to 10 mm Hg, in steps of 1 mm Hg. At each step, a spectral-domain optical coherence tomography scan was obtained. Displacements were analyzed by optical flow tracking, and strain over the entire stromal depth was retrieved from the phase gradient of the complex interference signal.

Results

During pressure increase, corneal tissue moved upward (486–585 nm/mm Hg) but did not fully recover (Δ= 2.63 to 8.64 µm) after pressure decrease. Vertical corneal strain distribution was negative in the anterior and positive in the posterior cornea, indicating simultaneous corneal compression and expansion, respectively. Bowman's layer caused minor localized differences in corneal strain distribution.

Conclusions

Corneal strain distribution is more complex than previously assumed, with a fundamental difference in mechanical response between the anterior and posterior stroma. Clinically, OCE technology might be used to monitor the progression of corneal ectatic diseases and to determine the success of corneal cross-linking.

Enzymatic Digestion of Porcine Corneas Cross-linked by Hypo- and Hyperosmolar Formulations of Riboflavin/ultraviolet A or WST11/Near-Infrared Light

Purpose

To assess enzymatic digestion rate after Riboflavin (RF) and Water-Soluble-Taurine (WST11) based corneal cross-linking (CXL), with or without the addition of high molecular weight dextran (RF-D and WST-D).

Methods

Eighty-eight paired porcine corneas were cross-linked by either RF (n = 11) or RF-D (n = 11) and ultraviolet light (UVA), or WST11 (n = 11) or WST-D (n = 11) and near-infrared (NIR) light, or used as paired control (n = 44). Corneal buttons of treated and paired control eyes were placed in a 0.3% collagenase solution. Time to full digestion and remaining dry sample weight after six hours were compared.

Results

A strong treatment effect was seen with all four formulations, as all controls had been fully digested whilst all treated samples were still visible at the experiment's endpoint. After irradiation, central corneal thickness was significantly higher in samples treated with hypo-osmolar formulations, compared to dextran enriched formulations (P < 0.001). Dry sample weight after digestion was nonsignificantly different between corneas treated by the four different formulations (P = 0.102). Average dry sample weight was 1.68 ± 0.6 (n = 10), 2.19 ± 0.50 (n = 8), 1.48 ± 0.76 (n = 11), and 1.54 ± 0.60 (n = 9) mg, for RF, RF-D, WST11, and WST-D treated samples, respectively. Enzymatic resistance was similar for RF and WST based CXL (P = 0.61) and was not affected by the addition of dextran (P = 0.221).

Conclusions

Both RF and WST11 based CXL significantly increases resistance to enzymatic digestion, with similar effect for hypo-osmolar and hyperosmolar (dextran enriched) formulations.

Translational Relevance

Our findings indicate these formulations are interchangeable, paving the way for the development of novel PACK-CXL protocols for thin corneas and deep-seated infections.

Combined photorefractive keratectomy and cross-linking. Pushing the limits

Correction of refractive error through laser-assisted means has soared in popularity in recent years, allowing it to become an increasingly routine surgical procedure. Technique refinement and adjustments resulted in laser-assisted refractive surgery to be combined with treatments such as collagen cross linking (CXL). This has broadened safety parameters and widened the treatment boundaries. Laser correction combined with CXL has been advocated in the treatment of high refractive errors as a safe option for full refractive correction while increasing corneal biomechanical stability. We present a complicated case where a young female patient with a preoperative best-corrected visual acuity (BCVA) of 20/20 in each eye was fully corrected by excimer laser followed by CXL. Factors potentially leading to inflammation, such as ocular surface disease, in addition to laser treatment and CXL, resulted in persistent epithelial defect followed by corneal melt and subsequent thinning. After the treatment, the patient relies on rigid gas-permeable contact lenses, achieving a BCVA of 20/25 and 20/23 in the right eye and left eye, respectively.

Management of progressive keratoconus with partial topography-guided PRK combined with refractive, customized CXL - a novel technique: the enhanced Athens protocol

Purpose

To report a novel application of partial topography-guided photorefractive keratectomy combined with topographically customized, higher fluence, and variable pattern corneal cross-linking applied on the same day of the treatment of keratoconus.

Methods

A topography-guided partial photorefractive keratectomy treatment of maximum 30 µm over the thinnest cone area was applied initially followed by a 7 mm, 50 µm phototherapeutic keratectomy treatment to address epithelial removal. 0.02% Mitomycin C was applied for 20 seconds and then the exposed stroma was soaked with 0.1% riboflavin solution for 5 minutes. The cornea was then treated with a customized, variable-pattern and 20 mW/cm² fluence for a total of 5-10 J, and up to 15 J of energy was delivered with the KXL-II device employing an active tracker. The center of the pattern that received the 15 J was topography-matched with the thinnest area of the cone. Visual acuity, refractive error, cornea clarity, keratometry, topography, pachymetry with a multitude of modalities and endothelial cell density were evaluated over 36 months.

Results

Keratoconus was stabilized in all cases. The severity of keratoconus stage by Amsler- Krumeich criteria improved from an average of 3.2 (1-4) to 1.8 (0-3). Uncorrected distance visual acuity changed from preoperative 20/80 to 20/25 at 6 months. A maximum astigmatic reduction of 7.8 D (5.3-15.6), and a significant cornea surface normalization (an index of height decentration improvement from 0.155 [±0.065] to 0.045 [±0.042]) were achieved by 1 month and remained relatively stable for 36 months postoperatively. Two cases delayed full reepithelialization for up to 9 days.

Conclusion

This paper introduces a novel technique in order to maximize the refractive normalization effect along with ectasia stabilization in young keratoconus patients. This may facilitate the use of less tissue ablation, in comparison to utilizing a homogeneous UV light beam for corneal cross-linking in Athens Protocol cases. It broadens the number of potential candidate cases that would have been limited to employ this technique due to tissue thickness limitations.

Epithelium-on corneal collagen crosslinking for management of advanced keratoconus

PURPOSE

To report the 1-year visual and keratometric results of epithelium-on corneal collagen crosslinking (CXL) for advanced keratoconus (median maximum keratometry [K] ≥58.0 diopters [D]).

SETTING

School of Ophthalmology and Optometry and Eye Hospital of Wenzhou Medical College, Wenzhou, China.

DESIGN

Prospective case series.

METHODS

Patients with bilateral progressive keratoconus had tetracaine-enhanced epithelium-on CXL. The worse eye had CXL, and the fellow eye was not treated. Results were reported 1, 3, 6, and 12 months postoperatively. The outcomes were compared with those in the fellow untreated eyes.

RESULTS

Twenty-one eyes of 21 patients with a median age of 20.4 years (interquartile range [IQR], 9.5 years) were treated. A significant improvement in postoperative uncorrected distance visual acuity was observed at 12 months (P = .002). Postoperative corrected distance visual acuity improved at 6 months and 12 months (P ≤ .009) compared with baseline values. The maximum K decreased by 1.63 D from the median preoperative maximum K of 62.7 D (IQR, 12.9 D) at 12 months (P < .001). The reduction in maximum K was higher after CXL than in untreated eyes at the end of 12 months (P = .001). Correlation analysis between the preoperative maximum K values and the change over 6 to 12 months between different studies showed a significant correlation (r = -0.764, P < .001; Spearman correlation).

CONCLUSIONS

Epithelium-on CXL was an effective treatment for patients with advanced keratoconus. A higher preoperative maximum K value correlated with greater corneal flattening after CXL.

FINANCIAL DISCLOSURE

None of the authors has a financial or proprietary interest in any material or method mentioned.

Comparison of corneal biomechanics after myopic small-incision lenticule extraction compared to LASIK: an ex vivo study

Purpose

To investigate ex vivo potentially different corneal biomechanical properties after small-incision lenticule extraction (SMILE) versus LASIK for myopic correction.

Methods

Thirty human donor corneas were subjected to either myopic SMILE or femtosecond laser-assisted LASIK. Donor corneas were assigned to six investigative groups: Group A, −3.00 D (diopters) SMILE; Group B, −8.00 D SMILE; Group C, −3.00 D LASIK; and Group D, −8.00 D LASIK. Additionally, two control groups were formed: Group E, SMILE and Group F, LASIK. All groups consisted of five corneas, randomly allocated. The corneas in the control groups were subjected to the corresponding femtosecond-laser lamellar cuts but not to tissue removal. Evaluation of biomechanical tensile strength was conducted by biaxial force application. Primary outcome measures were stress at 10% and 15% strain, and Young’s modulus at 10% and 15% strain.

Results

In SMILE, the average relative difference (Δ) of the four outcome measures was −34.46% for −3.00 D correction versus control Group E and −49.34% for −8.00 D correction versus control Group E. In LASIK, average Δ was −24.88% for −3.00 D correction versus control, and −52.73% for −8.00 D correction versus control. All these differences were statistically significant; SMILE compared to LASIK for the same myopic correction appears to result in more biomechanical reduction for −3.00 D corrections by −26%, while a nonstatistically significant difference was noted in −8.00 D corrections.

Conclusion

Both SMILE and LASIK procedures do substantially alter corneal biomechanical properties, and the degree of tensile strength reduction is statistically significantly correlated to the extent of myopic correction. Additionally, SMILE procedure seems to result in more tensile strength reduction in lower myopic corrections compared to LASIK, and similar tensile strength reduction to LASIK in higher myopic corrections when compared to LASIK.

Combined application of prophylactic corneal cross-linking and laser in-situ keratomileusis - a review of literature

Laser in-situ keratomileusis (LASIK) is safe and effective laser refractive procedures in treating refractive errors. However, regression of treatment and iatrogenic keratectasia remain to be a major concern, especially in treating thin cornea with high ametropia. Collagen cross-linking (CXL) is an effective method in stopping keratoconus progression through increasing the biomechanical strength of the cornea. Adjuvant cross-linking to refractive procedures can theoretically help prevent regression and reduce the risk of keratectasia development by increasing the mechanical stability of cornea. During the procedure, riboflavin is directly applied to the corneal stroma, thereby reducing the need of de-epithelialization as in the conventional protocol for keratoconus. Currently, there is still no consensus regarding the indication of CXL during refractive procedure, nor any standardized treatment protocol. This article aims to summarize the current evidence regarding the use of adjuvant CXL in LASIK.

High-intensity corneal collagen crosslinking with riboflavin and UVA in rat cornea

Corneal collagen cross-linking (CXL) halts human corneal ectasias progression by increasing stromal mechanical stiffness. Although some reports describe that this procedure is effective in dealing with some infectious and immunologic corneal thinning diseases, there is a need for more animal models whose corneal thickness more closely resemble those occurring in these patients. To meet this need, we describe here high-intensity protocols that are safe and effective for obtaining CXL in rat corneas. Initially, a range of potentially effective UVA doses were evaluated based on their effectiveness in increasing tissue enzymatic resistance to dissolution. At UVA doses higher than a threshold level of 0.54 J/cm², resistance to enzymatic digestion increased relative to that in non-irradiated corneas. Based on the theoretical threshold CXL dose, a CXL regimen was established in which the UVA tissue irradiance was 9 mW/cm², which was delivered at doses of either 2.16, 2.7 or 3.24 J/cm². Their dose dependent effects were evaluated on ocular surface morphological integrity, keratocyte apoptotic frequency, tissue thickness and endothelial cell layer density. Doses of 2.16 and 2.7 J/cm² transiently decreased normal corneal transparency and increased thickness. These effects were fully reversed after 14 days. In contrast, 3.24 J/cm² had more irreversible side effects. Three days after treatment, apoptotic frequency in the CXL-2.16 group was lower than that at higher doses. Endothelial cell losses remained evident only in the CXL-3.24 group at 42 days posttreatment. Stromal fiber thickening was evident in all the CXL-treated groups. We determined both the threshold UVA dose using the high-intensity CXL procedure and identified an effective dose range that provides optimal CXL with minimal transient side effects in the rat cornea. These results may help to provide insight into how to improve the CXL outcome in patients afflicted with a severe corneal thinning disease.

Collagen cross-linking treatment increases adhesion in mock corneal grafts

PURPOSE

We tested the hypothesis that collagen cross-linking (CXL) could be used to promote adhesion in mock corneal grafts.

METHODS

Donated human corneal tissue underwent epithelial debridement and was cut into sections measuring 4mm×3mm. Paired sections were sutured together with 10-0 vicryl, forming mock corneal grafts. Looped 6-0 sutures were placed at each end to facilitate tension measurement. Mock grafts underwent CXL before being cultured for 2days in Eagle's MEM culture medium. Control mock grafts did not undergo CXL treatment before culture. Tissue was obtained from 4 donors and a maximum of 2 controls and 2 treated grafts was obtained from each donor. Following the culture period, the 10-0 sutures were cut. The mock grafts were mounted on force transducers and were put under increasing tension until eventually the sections were pulled apart.

RESULTS

The mean applied stress required to generate graft failure was calculated for all mock grafts±standard error of the mean. In the control group 0.236±0.09mPa of applied stress was required to cause graft failure, in comparison to 0.691±0.12mPa in the treated group. A paired t-test showed this result to be significant, (p=0.0087).

CONCLUSION

The results of our study are consistent with our hypothesis that CXL treatment could be used to promote early adhesion between separate sections of corneal tissue.

Advances in Refractive Surgery: June 2014 to July 2015

PURPOSE

The objective of this study was to review the advances in the field of refractive surgery as reported in the peer-reviewed literature during the previous year.

DESIGN

A literature review.

METHODS

In this review, we highlight the most pertinent articles in the field from June 2014 to the end of July 2015.

RESULTS

This past year has seen a growing body of research on small-incision lenticule extraction, presbyopic inlays, and phakic intraocular lenses, as more clinicians are adopting these techniques into their armamentarium.

CONCLUSIONS

Laser-assisted in situ keratomileusis and photorefractive keratectomy continue to dominate the keratorefractive literature, as they remain the most frequently performed refractive surgeries.

Corneal Collagen Cross-linking for the Treatment of Progressive Corneal Ectasia: 6-Year Prospective Outcome in a French Population

PURPOSE

To evaluate 6-year results of standardized epithelium-off corneal collagen cross-linking (CXL) for treatment of progressive corneal ectasia.

DESIGN

Prospective, consecutive, interventional case series.

METHODS

Thirty-six eyes of 25 consecutive patients with documented progressive primary or iatrogenic corneal ectasia underwent CXL following the Siena protocol. The main outcome measures included uncorrected (UDVA) and corrected (CDVA) distance visual acuities, biomicroscopy and fundus appearance, topography-derived steep and flat keratometry (Kmax, Kmin), central corneal thickness (CCT), intraocular pressure with Goldmann applanation tonometer (GAT-IOP), and endothelial cell density (ECD), recorded at baseline and months 1, 3, 6, 12, 24, 36, and 72. Bilateral macular optical coherence tomography was performed at the endpoint visit. The mean follow-up was 66 ± 6 months (range, 60-78 months).

RESULTS

At 6 years, CXL stabilized primary and iatrogenic corneal ectasia in 89% of the patients. In bilateral CXL, the progression of the first eye was highly predictive of the fellow eye's outcome. At the endpoint follow-up, the mean outcome variations were: UDVA: -0.08 ± 0.36 logMAR (P = .2); CDVA: -0.14 ± 0.28 logMAR (P = .004); Kmax: +0.11 ± 1.70 diopters (D) (P = .7); Kmin: -0.25 ± 1.25 D (P = .2); CCT: -16.38 ± 37 μm (P = .01); GAT-IOP: +1.0 ± 2.3 mm Hg (P = .01); ECD: +31 ± 400 cells/mm(2) (P = .6); no cases of macular toxicity or severe adverse events were reported.

CONCLUSIONS

At 6 years, CXL maintains long-term results in halting the progression of corneal ectasia, with significant improvement in CDVA and long-term stability of keratometry. Further clinical studies with longer follow-up and larger series would be necessary to definitely confirm these results.