Epithelial and stromal remodelling following femtosecond laser-assisted stromal lenticule addition keratoplasty (SLAK) for keratoconus

Long-term results of allogenic corneal lenticule of hyperopic SMILE for post-LASIK ectasia

Post-laser-assisted in situ keratomileusis (LASIK) corneal ectasia is a severe complication of corneal refractive surgery, and cryopreserved lenticules from hyperopic small incision lenticule extraction (SMILE) may offer a promising treatment though their long-term safety and efficacy are still under investigation. In this prospective case series, six eyes from six patients with post-LASIK ectasia received lenticules (mean cryopreserved time: 63 days). The procedure involved lifting the corneal flap, implanting the lenticule, and repositioning the flap. Over a follow-up period of at least one year, uncorrected distance visual acuity (UDVA) improved from 1.52 ± 0.40 preoperatively to 0.74 ± 0.28 LogMAR. Two eyes gained one line of corrected distance visual acuity (CDVA), three gained two lines, and one gained over three lines. Spherical equivalents decreased from -14.67 ± 2.36 D to -8.75 ± 4.03 D (p = 0.02). Mean anterior K and total corneal refractive power decreased (p < 0.05). Thinnest corneal thickness increased from 359.2 ± 39.3 μm to 401.7 ± 53.4 μm (p = 0.02). These findings support the potential of cryopreserved lenticules for treating post-LASIK ectasia, though further refinement in refractive predictability is needed.

Ex Vivo Lenticule Customization for Stromal Lenticule Addition Keratoplasty

Purpose

The purpose of this study was to explore the optimal shape of customized lenticules for stromal lenticule addition keratoplasty (SLAK) for off-centered ectasia.

Methods

Two different methods to create ex vivo models of eccentric-keratoconus were investigated. Twelve human corneas were used to create model 1 by a hyperopic photorefractive keratectomy (PRK), and model 2 by masked phototherapeutic keratectomy (PTK) on the anterior corneal surface, whereas both types received myopic ablation of the posterior surface. Keratoconus models underwent a modified femtosecond laser (FSL) flap-cut to create stromal pockets. Sixteen human corneas underwent FSL dissection to obtain four lenticule types: type I (planar) and type II (negative) lenticules were used without modifications, whereas type III (customized-planar), and type IV (customized-negative) lenticules underwent further masked-PRK to obtain an asymmetric bow-tie shape. Topographic, aberrometric analysis, and anterior segment optical coherence tomography (AS-OCT) were performed in all recipient corneas before and after lenticule implantation.

Results

Keratoconus model was successfully reproduced. Tomographic analysis showed a significant inferiorly decentered corneal steepening with coherent stromal thinning. Model 2 reproduced better the curvature of real keratoconus. Lenticules type I implantation induced a homogeneous corneal thickening, type III produced higher thickening in the inferior half of the cornea. Type II determined a maximal peripheral pachymetric increase, with a gradual reduction toward the center, and type IV presented an asymmetric peripheral thickening. Topographic assessment showed a cone apex flattening in all cases, but it was significantly higher in types II and IV. Customized lenticules improved significantly corneal surface regularity regarding types I and II.

Conclusions

The approach of customizing lenticules by increasing their asymmetry and tailoring the re-shaping effects, may improve SLAK outcomes in eccentric keratoconus.

Lenticule addition keratoplasty for the treatment of keratoconus: A systematic review and critical considerations

Keratoconus is a corneal disorder characterized by the progressive thinning and bulging of the cornea. Currently, the major goal of management is to halt its progression, restore normal corneal strength, prevent acute complications, and save vision. Penetrating keratoplasty and deep anterior lamellar keratoplasty as conventional surgical methods for advanced keratoconus are limited by relatively high rates of immune intolerance, slow post-operational recovery, high costs, and shortage of donor corneas. Recently, the development of lenticule addition keratoplasty enables the restoration of corneal thickness simply by implanting a lenticule into the stromal pocket created with the femtosecond laser, which can originate from cadaver corneas or more appealing, be extracted from patients via a small-incision lenticule extraction (SMILE) surgery. As the first systematic review in this field, we critically review publications on lenticule addition keratoplasty and provide our perspectives on its clinical application and the focus of future research.

Corneal Epithelial Thickness Mapping: A Major Review

The corneal epithelium (CE) is the outermost layer of the cornea with constant turnover, relative stability, remarkable plasticity, and compensatory properties to mask alterations in the underlying stroma. The advent of quantitative imaging modalities capable of producing epithelial thickness mapping (ETM) has made it possible to characterize better the different patterns of epithelial remodeling. In this comprehensive synthesis, we reviewed all available data on ETM with different methods, including very high-frequency ultrasound (VHF-US) and spectral-domain optical coherence tomography (SD-OCT) in normal individuals, corneal or systemic diseases, and corneal surgical scenarios. We excluded OCT studies that manually measured the corneal epithelial thickness (CET) (e.g., by digital calipers) or the CE (e.g., by confocal scanning or handheld pachymeters). A comparison of different CET measuring technologies and devices capable of producing thickness maps is provided. Normative data on CET and the possible effects of gender, aging, diurnal changes, refraction, and intraocular pressure are discussed. We also reviewed ETM data in several corneal disorders, including keratoconus, corneal dystrophies, recurrent epithelial erosion, herpes keratitis, keratoplasty, bullous keratopathy, carcinoma in situ, pterygium, and limbal stem cell deficiency. The available data on the potential role of ETM in indicating refractive surgeries, planning the procedure, and assessing postoperative changes are reviewed. Alterations in ETM in systemic and ocular conditions such as eyelid abnormalities and dry eye disease and the effects of contact lenses, topical medications, and cataract surgery on the ETM profile are discussed.

Incisional surface quality of electron-beam irradiated cornea-extracted lenticule for stromal keratophakia: high nJ-energy vs. low nJ-energy femtosecond laser

Introduction

Corneal lenticules can be utilized as an additive material for stromal keratophakia. However, following extraction, they must be reimplanted almost immediately or cryopreserved in lenticule banks. Electron-beam (E-beam) irradiated corneas permit room-temperature storage for up to 2 years, enabling keratophakia to be performed on demand. This study aims to compare the performance of high nano Joule (nJ)-energy (VisuMax) and low nJ-energy (FEMTO LDV) femtosecond laser systems on the thickness consistency and surface quality and collagen morphology of lenticules produced from fresh and E-beamed corneas.

Methods

A total of 24 lenticules with -6.00 dioptre power were cut in fresh human donor corneas and E-beamed corneas with VisuMax and FEMTO LDV. Before extraction, the thickness of the lenticules was measured with anterior segment-optical coherence tomography (AS-OCT). The incisional surface roughness of extracted lenticules was analyzed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). Multiphoton microscopy was then used to assess the surface collagen morphometry.

Results

The E-beamed lenticules that were cut using FEMTO LDV were significantly thicker than the fresh specimens as opposed to those created with VisuMax, which had a similar thickness as the fresh lenticules. On the vertex, they were ∼11% thicker than the fresh lenticules. The surface roughness (Rq) of E-beamed lenticules incised with FEMTO LDV did not differ significantly from the fresh lenticules. This contrasted with the VisuMax-fashioned lenticules, which showed notably smoother surfaces (∼36 and ∼20% lower Rq on anterior and posterior surfaces, respectively) on the E-beamed than the fresh lenticules. The FEMTO LDV induced less cumulative changes to the collagen morphology on the surfaces of both fresh and E-beamed lenticules than the VisuMax.

Conclusion

It has been previously demonstrated that the low nJ-energy FEMTO LDV produced a smoother cutting surface compared to high nJ-energy VisuMax in fresh lenticules. Here, we showed that this effect was also seen in the E-beamed lenticules. In addition, lower laser energy conferred fewer changes to the lenticular surface collagen morphology. The smaller disparity in surface cutting quality and collagen disturbances on the E-beamed lenticules could be beneficial for the early visual recovery of patients who undergo stromal keratophakia.

Allogenic Lenticular Implantation for Correction of Refractive Error and Ectasia: Narrative Review

INTRODUCTION

Intrastromal lenticule implantation is a promising treatment option for corneal pathologies, from refractive error to ectasia. In this narrative review, we intend to feature up-to-date literature supporting the use of lenticular tissue, a compelling method that can be customized for a variety of applications, providing an additional source of donor tissue for treating corneal diseases.

METHODS

We searched databases PubMed, Mendeley, and Scopus last accessed 10 May 2023, for literature on stromal lenticules and narrowed based on relevance. Review articles, animal studies, ex vivo studies, and book chapters were excluded, while assessable and relevant articles published in English were included.

RESULTS

Storage methods from using fresh lenticules to dehydration have proven successful, with cryopreservation maintaining structure and cellular viability for up to 10 years. Successful use of lenticules for treatment of numerous pathologies including corneal ectasias, hyperopia, and presbyopia with additional insight into the treatment of corneal ulcers and perforations are highlighted in this narrative review.

CONCLUSION

Lenticular implantation is an innovative and advantageous treatment for various ocular pathologies, offering increased bioavailability, flexibility, and customization for patients. They can treat previously untreatable diseases and serve as a replacement for synthetic implants, with promising outcomes worldwide. Lenticular implantation has the potential to become a leading approach in ophthalmologic surgery. Further studies should aim to provide evidentiary support for a standardization of lenticule banking.

Meniscus-Shaped Stromal Lenticule Addition Keratoplasty for Corneal Regularization and Thickening in Advanced Keratoconus

PURPOSE

The aim of this study was to investigate the outcome of the meniscus-shaped stromal lenticule addition keratoplasty (MS-SLAK) in corneal regularization and thickness.

METHODS

Patients waiting for deep anterior lamellar keratoplasty for advanced keratoconus with an intolerance to contact lenses (CLs) underwent the MS-SLAK procedure by FSL 80 kHz ablation (VICTUS, Technolas Perfect Vision, DE). Customized positive meniscus-shaped stromal lenticules were obtained and implanted. Examination was performed at baseline and at 3-, 6-, and 12-month follow-up and included corrected distance visual acuity both with spectacles and CLs (spectacle CDVA and CL-CDVA), manifest refraction spherical equivalent, slit-lamp examination, anterior segment optical coherence tomography, corneal topography, and in vivo confocal microscopy.

RESULTS

Fifteen patients completed the study. Statistical increases in corneal thickness values were found from the first follow-up ( P < 0.001). Improvement in the Surface Asymmetry Index ( P = 0.04), Symmetry Index ( P = 0.02), spherical aberration ( P < 0.001), coma ( P = 0.18), high-order aberration ( P = 0.37), and anterior asphericity index (Q) ( P = 0.31) were found at 12 months. At the 12-month follow-up, no improvement were found in spectacle CDVA ( P = 0.23); however, all patients reported CL wearing tolerance recovery, and significant improvement in CL-CDVA ( P = 0.002) was found. The confocal microscopy at 12 months showed a significant increase in keratocyte density within the lenticule and absence of fibrotic reactions in both anterior and posterior interfaces.

CONCLUSIONS

MS-SLAK seems to be effective in regularizing the corneal surface as showed by the significant improvement in topographic symmetry indices, coma, and high-order aberration. The corneal regularization is also confirmed by the results in anterior Q and the recovery of the CL wearing tolerance.

Management of keratoconus: an updated review

Keratoconus is the most common corneal ectatic disorder. It is characterized by progressive corneal thinning with resultant irregular astigmatism and myopia. Its prevalence has been estimated at 1:375 to 1:2,000 people globally, with a considerably higher rate in the younger populations. Over the past two decades, there was a paradigm shift in the management of keratoconus. The treatment has expanded significantly from conservative management (e.g., spectacles and contact lenses wear) and penetrating keratoplasty to many other therapeutic and refractive modalities, including corneal cross-linking (with various protocols/techniques), combined CXL-keratorefractive surgeries, intracorneal ring segments, anterior lamellar keratoplasty, and more recently, Bowman's layer transplantation, stromal keratophakia, and stromal regeneration. Several recent large genome-wide association studies (GWAS) have identified important genetic mutations relevant to keratoconus, facilitating the development of potential gene therapy targeting keratoconus and halting the disease progression. In addition, attempts have been made to leverage the power of artificial intelligence-assisted algorithms in enabling earlier detection and progression prediction in keratoconus. In this review, we provide a comprehensive overview of the current and emerging treatment of keratoconus and propose a treatment algorithm for systematically guiding the management of this common clinical entity.

Update of Research Progress on Small Incision Lenticule Extraction (SMILE) Lenticule Reuse

The SMILE lenticule is a complete corneal stroma that is removed from SMILE surgery. Since the increasing number of SMILE surgeries, a large number of SMILE lenticules have been produced, so the reuse and preservation of the stromal lens has become a research hotspot. Due to the rapid development of the preservation and clinical reuse of SMILE lenticules, there have been many related studies in recent years, so we updated it on this basis. We searched PubMed, Web of Science, Embase, Elsevier Science, CNKI, WANFANG Data and other databases for all articles published on the preservation and clinical reuse of SMILE lenticules, screened useful articles, selected relevant articles published in the last five years as the main body for summary, and then reached a conclusion. The existing preservation methods of SMILE lenticule include Moist chamber storage at low temperature, cryopreservation technique dehydrating agent and corneal storage medium, which have their own advantages and disadvantages. Presently, smile lenticules can be used for the treatment of corneal ulcers and perforations, corneal tissue defects, hyperopia, presbyopia and keratectasia, which have been proven to be relatively effective and safe. More research on smile lenticule reuse needs to be carried out to confirm its long-term efficacy.

Changes in the posterior corneal surface after femtosecond laser-assisted lenticule intrastromal keratoplasty (LIKE) performed into a pocket (SMI-LIKE) or under a flap (FS-LIKE)

BACKGROUND

To compare the changes in posterior corneal surface after small-incision lenticule intrastromal keratoplasty (SMI-LIKE) and femtosecond laser-assisted lenticule intrastromal keratoplasty (FS-LIKE) for hyperopia correction.

METHODS

In this prospective comparative randomized study, 23 eyes with hyperopia were recruited. Eyes were categorized into two groups-SMI-LIKE group (11 eyes) and FS-LIKE group (12 eyes). Lenticules from myopia small incision lenticule extraction were implanted into a pocket (SMI-LIKE group) or at a depth of 100 µm under a flap (FS-LIKE group). Posterior corneal elevations in the center, mid-periphery, and periphery, as well as mean keratometry of the posterior corneal surface (Kmb) were measured using a Pentacam over a three-month follow-up.

RESULTS

All surgeries were completed successfully and no complications occurred. At one day postoperatively, there was a slight backward change with SMI-LIKE and a forward change with FS-LIKE in the central region of the posterior corneal elevation. Conversely, the peripheral area showed forward displacement in SMI-LIKE and an apparent backward change in FS-LIKE. The mid-peripheral regions manifested a backward change after the procedure throughout the entire follow-up in both groups. Kmb exhibited flattening at one month postoperatively and subsequently returned to its original level at three months after SMI-LIKE while in FS-LIKE, Kmb steepened after lenticule implantation with a significant change noted at one day postoperatively (P = 0.001).

CONCLUSIONS

Posterior corneal surface after SMI-LIKE and FS-LIKE exhibited different change patterns in various corneal regions, with the most prominent change occurring at one day postoperatively during the three-month follow-up.

TRIAL REGISTRATION

Chinese Clinical Trial Registry: ChiCTR-ONC-16008300. Registered on Apr 18th, 2016.  http://www.chictr.org.cn/edit.aspx?pid=14090&htm=4.

Possible use of corneal lenticule in surgery of corneal diseases (literature review)

In this review, we analyzed the domestic and foreign literature on the use of corneal lenticula obtained by keratorefractive surgery using the SMILE technology (SMall Incision Lenticula Extraction). Research is being actively carried out on the use of a lenticular tissue for refractive purposes: for the correction of hyperopia (LIKE – Lenticular Intrastromal Keratoplasty), for the correction of presbyopia (PEARL – PrEsbyopic Allogenic Refractive Lenticule). A significant amount of works are devoted to the use of lenticular tissue for the treatment of keratectasias of various origins. For example, a number of authors for the treatment of keratoconus suggest implantation of a lenticule into the recipient’s corneal pocket formed by a femtolaser (SLAK – Stromal lenticule addition keratoplasty). Clinical cases of combined treatment are described: implantation of a lenticule and corneal intrastromal segments for the treatment of corneal pellucid degeneration. A large number of works are devoted to the use of lenticules for tectonic coverage of ulcerative defects, marginal thinning in Mooren’s ulcer. Several clinical cases of the use of a corneal lenticule to cover a deep corneal defect in recurrent pterygium are described. This review also included articles on the storage and decellularization of corneal lenticules. The analyzed articles show a wide area of application of the corneal lenticule; however, more research is required in each of the areas of application, and it is also necessary to solve the problem of procurement and storage of lenticular tissue.

Understanding Corneal Epithelial Thickness Mapping

Corneal epithelial thickness (CET) and the regional variations in response to changes in corneal architecture and biomechanics have recently drawn the interest of corneal surgeons. Corneal epithelium possesses the tremendous capability of remodeling and changing its thickness. This remodeling of corneal epithelium takes place in response to underlying stromal irregularities which can result from a variety of corneal disorders including corneal ectasia. Measurement of CET can reveal the underlying stromal abnormalities and supplement in early diagnosis of corneal disorders especially corneal ectasia which has been one of the leading challenges in planning corneal refractive surgery. A significant number of patients ends up in ectasia after refractive surgery and the most common cause of this complication is the presence of preoperative subclinical keratoconus. Furthermore, postoperative complications of corneal refractive surgery are partly masked by epithelial remodeling and make the diagnosis and management difficult and extremely challenging. This leads not only to unpredictable visual and refractive outcome but also the need of multiple interventions to treat these complications. Although corneal tomography is considered as gold standard in the detection and diagnosis of corneal ectasia, a small number of subclinical cases may still go undetected. In this review, we have highlighted the underlying mechanism of epithelial remodeling, the devices and imaging modalities used to measure CET, and application of epithelial mapping in the diagnosis and management of various corneal disorders.

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

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

Effect of corneal stromal lenticule customization on neurite distribution and excitatory property

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Refractive SMILE-derived stromal lenticules are useful in various tissue-engineering approach for therapeutics, of which they are required to be customized before implantation.

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Excimer laser-mediated reshaping, riboflavin-UVA-induced collagen crosslinking and chemical decellularization significantly removed lenticule neurites, but the residual neurites retained excitatory response.

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Reinnervation occurred in the decellularized lenticules, indicating a potential of nerve regeneration.

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Stromal lenticules, as a unique collagen-rich biomaterial with high transparency, refractivity and mechanically robust, together with the ability of neurite regeneration, could hold a potential for various ophthalmic applications.

Introduction

Refractive stromal lenticules from Small Incision Lenticule Extraction (SMILE), though usually discarded, hold a potential for various ophthalmic applications, including refractive correction, stromal volume expansion, and biomechanical strengthening of the cornea.

Objectives

To investigate the effect of lenticule customization on lenticule neurite length profile and the excitatory response (calcium signaling) and the potential of reinnervation.

Methods

Human and porcine stromal lenticules were treated by (1) excimer laser reshaping, (2) ultraviolet A-riboflavin crosslinking (CXL), and (3) decellularization by sodium dodecyl sulfate (SDS), respectively. The overall neurite scaffold immuno-positive to TuJ1 (neuron-specific class III β-tubulin) expression and population of active neurite fragments with calcium response revealed by L-glutamate-induced Fluo-4-acetoxymethyl ester reaction were captured by wide-field laser-scanning confocal microscopy, followed by z-stack image construction. The NeuronJ plugin was used to measure neurite lengths for TuJ1 (NL-TuJ1) and calcium signal (NL-Ca). Reinnervation of lenticules was examined by the ex vivo grafting of chick dorsal root ganglia (DRG) to the decellularized human lenticules. Differences between groups and controls were analyzed with ANOVA and Mann-Whitney U test.

Results

The customization methods significantly eliminated neurites inside the lenticules. NL-TuJ1 was significantly reduced by 84% after excimer laser reshaping, 54% after CXL, and 96% after decellularization. The neurite remnants from reshaping and CXL exhibited calcium signaling, indicative of residual excitatory response. Re-innervation occurred in the decellularized lenticules upon stimulation of the grafted chick embryo DRG with nerve growth factor (NGF 2.5S).

Conclusion

All of the lenticule customization procedures reduced lenticule neurites, but the residual neurites still showed excitatory potential. Even though these neurite remnants seemed minimal, they could be advantageous to reinnervation with axon growth and guidance after lenticule reimplantation for refractive and volume restoration of the cornea.