Correlation between epithelial thickness in normal corneas, untreated ectatic corneas, and ectatic corneas previously treated with CXL; is overall epithelial thickness a very early ectasia prognostic factor?

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Keratoconus and keratoectasia:advancements in diagnosis and treatment

Full text: Keratoconus (KC) and iatrogenic keratoectasia are receiving increasing attention, due to the improvements in diagnostic modalities and the availability of therapeutic options, which now include collagen cross-linking, intrastromal implants, intraocular lenses, microwave remodeling, and anterior lamellar keratoplasty. Limitations of surgical treatments of keratoconus are well known. Intrastromal implants, built in various shapes and now implanted more safely through femtosecond-laser-obtained stromal channels, still retain reduced predictability as for the refractive results and do not modify the structure of the diseased cornea. Anterior lamellar keratoplasty, even in its more advanced and technically difficult variant of deep anterior lamellar keratoplasty (DALK), cures the disease by the (almost) complete replacement of the ectatic stroma but, even when a regular and transparent interface is achieved, final refractive errors and higher-order aberrations may severely affect visual rehabilitation. The use of femtosecond laser in DALK to shape the donor and recipient margins has not significantly improved the picture yet. Parasurgical treatments of KC are therefore regarded as a temporary or definitive alternative to surgical interventions. Among the newest ideas, the promising use of microwave to heat and reshape the corneal apex shares the principle with previous modalities of thermal keratoplasty, which were characterized by regression and induction of irregular astigmatism. The long-term validity of microwave reshaping is, therefore, still being investigated. The use of collagen corneal cross-linking (CXL) with riboflavin and ultraviolet (UV) has rapidly expanded in the world and is currently regarded as the only recognized treatment to slow or arrest KC progression, obtaining in some cases a significant improvement of corneal curvature and regularity. However, as most new treatments, CXL is still far from being ideal. Riboflavin for CXL is unreasonably expensive; the treatment is long and tedious and is followed by postoperative pain and slow visual rehabilitation. Complications are not uncommon, including infections and scarring. The indications to the treatment are still debated as for age, KC stage, and corneal thickness. Alternative attempts to reduce the CXL operating time by increasing the irradiation energy or by avoiding epithelial removal have been made, but all deviations from the defined original protocol may reduce the efficacy of treatment, and therefore new treatment protocols are currently further investigated. In this special issue, various and new aspects of CXL are examined, rehabilitation with contact lenses of KC is reviewed, and the features of posterior KC at ultrasound biomicroscopy are evaluated. Patient selection for CXL is not completely codified, and age limits are conventionally established. For example, the Italian National Health Service limits CXL reimbursement for patients between 12 and 40 years, the lower limit being dictated by common sense and the upper limit by the presumption of spontaneous KC stabilization after 40. A. Caporossi and Mazzotta et al., leading experts of CXL, in their original study in this issue, compare KC stabilization, improvement of corneal curvature, visual acuity, and aberrations 48 months after CXL in different age groups, concluding that the highest benefits were obtained in younger eyes. CXL procedure was originally developed to stiffen the keratoconic cornea, but its indications have been recently extended to postrefractive surgery ectasia, to infectious keratitis (due to a powerful antimicrobial action), and to corneal edema, where CXL temporarily reduces the space for fluid accumulation. These new indications of CXL, as well as its physical and chemical background, biomechanical effects, and clinical results, are thoroughly reviewed in the paper by M. Hovakimyan et al., where the real possibilities of transepithelial CXL and of the new approach combining photorefractive keratectomy (PRK) and CXL are discussed. Several reports of infectious keratitis after CXL have recently raised the issue of CXL safety: it would appear that the risk of infection is considerably higher than after PRK. The length of the procedure or the slow epithelialization time could be the reasons for such increased infectious risk. In addition, the peculiar “demarcation” haze, regarded as a demonstration of the cross-linking effect, can sometimes turn into a significant, long-term scar. These complications and others are well reviewed in the paper by S. Dhawan et al. Fortunately, most patients with KC will never need to undergo any surgical or parasurgical procedure. Visual rehabilitation is sometimes possible with the sole help of spectacles, but the reduction of higher-order aberrations is only possible with contact lenses. The extended wear of contact lenses and the difficult adaptation in keratoconic eyes imply a thorough knowledge of various contact lens models available: this is the subject of the article by Ozkurt et al. The paper by B. Rejdak et al. is a case report of a rare, nonprogressive variant of KC, circumscribed posterior keratoconus. The correct diagnosis of this form of ectasia is only possible by modern three-dimensional imaging technique, and in this case ultrasound biomicroscopy and slit scanning topography were used to reveal the protrusion of the posterior corneal surface. In this historical period we are directly witnessing the rise (and fall) of many therapeutic modalities for KC, but we can nevertheless look with optimism at the future of a complex and multiform disease, characterized by individualised treatment and prognosis. We hope that this special issue will contribute to stimulating discussion

Donor cross-linking for keratoplasty: a laboratory evaluation

Purpose This laboratory-based investigation compares the topographic outcomes of conventional penetrating keratoplasty with that of a novel procedure in which donor corneas are cross-linked prior to keratoplasty. Methods Penetrating keratoplasty procedureswith continuous running sutures were carried out in a porcine whole globe model. Sixty eyes were randomly paired as ‘donor’ and ‘host’ tissue before being assigned to one of two groups. In the crosslinked group, donor corneas underwent riboflavin/UVA crosslinking prior to being trephined and sutured to untreated hosts. In the conventional keratoplasty group, both host and donor corneas remained untreated prior to keratoplasty. Topographic and corneal wavefront measurements were performed following surgery, and technical aspects of the procedure evaluated. Results Mean keratometric astigmatism was significantly lower in the cross-linked donor group at 3.67D (SD 1.8 D), vs. 8.43 D (SD 2.4 D) in the conventional keratoplasty group (p<0.005). Mean wavefront astigmatism was also significantly reduced in the cross-linked donor group 4.71 D (SD 2.1) vs. 8.29D (SD 3.6) in the conventional keratoplasty group (p<0.005). Mean RMS higher order aberration was significantly lower in the cross-linked donor group at 1.79 um (SD 0.98), vs. 3.05 um (SD 1.9) in the conventional keratoplasty group (P=0.02). Qualitative analysis revealed less tissue distortion at the graft-host junction in the cross-linked group. Conclusion Cross-linking of donor corneas prior to keratoplasty reduces intraoperative induced astigmatism and aberrations in an animal model. Further studies are indicated to evaluate the implications of this potential modification of keratoplasty surgery

Break in microkeratome oscillating pin during LASIK flap creation

We describe the case of a 40-year-old female myope who presented for bilateral LASIK. Intra-operatively, the microkeratome oscillating pin broke during flap creation resulting in the separation of the disposable blade from the motor. This resulted in an irregular flap with missing pieces. The procedure was abandoned and the macerated partial flap repositioned as best as possible. The patient recovered a BCVA of 6/7.5. The manufacturer has since reported taking corrective measures to prevent this problem in the future. This case is a reminder that despite care and maintenance by user and manufacturer, extreme and rare hardware malfunctions can occur. Furthermore, although potentially sight threatening if managed well these complications can be followed by good recovery of vision.3 page(s

Analysis of the effects of Eye-Tracker performance on the pulse positioning errors during refractive surgery

Purpose: To analyze the effects of Eye-Tracker performance on the pulse positioning errors during refractive surgery. Methods: A comprehensive model, which directly considers eye movements, including saccades, vestibular, optokinetic, vergence, and miniature, as well as, eye-tracker acquisition rate, eye-tracker latency time, scanner positioning time, laser firing rate, and laser trigger delay have been developed. Results: Eye-tracker acquisition rates below 100 Hz correspond to pulse positioning errors above 1.5 mm. Eye-tracker latency times to about 15 ms correspond to pulse positioning errors of up to 3.5 mm. Scanner positioning times to about 9 ms correspond to pulse positioning errors of up to 2 mm. Laser firing rates faster than eye-tracker acquisition rates basically duplicate pulse-positioning errors. Laser trigger delays to about 300 μs have minor to no impact on pulse-positioning errors. Conclusions: The proposed model can be used for comparison of laser systems used for ablation processes. Due to the pseudo-random nature of eye movements, positioning errors of single pulses are much larger than observed decentrations in the clinical settings. There is no single parameter that ‘alone’ minimizes the positioning error. It is the optimal combination of the several parameters that minimizes the error. The results of this analysis are important to understand the limitations of correcting very irregular ablation patterns

Aspheric photorefractive keratectomy for myopia and myopic astigmatism with the SCHWIND AMARIS laser: 2 years postoperative outcomes

Purpose: To evaluate mid-term refractive outcomes and higher order aberrations of aspheric PRK for low, moderate and high myopia and myopic astigmatism with the AMARIS excimer laser system (SCHWIND eye-tech-solutions GmbH, Kleinostheim, Germany). Methods: This prospective longitudinal study evaluated 80 eyes of 40 subjects who underwent aspheric PRK. Manifest refractive spherical equivalent (MRSE) of up to −10.00 diopters (D) at the spectacle plane with cylinder up to 3.25 was treated. Refractive outcomes and corneal wavefront data (6 mm pupil to the 7th Zernike order) were evaluated out to 2 years postoperatively. Statistical significance was indicated by P < 0.05. Results: The mean manifest spherical equivalent refraction (MRSE) was −4.77 ± 2.45 (range, −10.00 D to −0.75 D) preoperatively and −0.12 ± 0.35 D (range, −1.87 D to +0.75 D) postoperatively (P  0.05, both cases). There was a statistical increase in postoperative coma (+0.12 μm) and spherical aberration (+0.14 μm) compared to preoperatively (P < 0.001, both cases). Conclusion: Aspheric PRK provides excellent visual and refractive outcomes with induction in individual corneal aberrations but not overall corneal aberrations

Phacoemulsification and implantation of an accommodating IOL after PRK

PURPOSE: To present a case of phacoemulsification and implantation of an accommodating intraocular lens (IOL) in a patient with cataract formation after previous refractive surgery. METHODS: A 50-year-old man, who initially had photorefractive keratectomy to correct moderate myopia, developed a cataract in one eye. He subsequently underwent phacoemulsification and implantation of a ICU accommodating IOL, as he wished to remain spectacle independent. RESULTS: The patient's distance vision was fully restored. However, accommodative function, which was assessed using subjective and novice objective techniques, was only partially restored. CONCLUSIONS: Although the accommodating IOL fully restored the patient's distance vision, accommodative function was only partially restored