Photorefractive Ablation Research Articles

Биоптический подход к эксимерлазерной хирургии при сочетанной патологии роговицы и хрусталика

Purpose. To consider a biooptical approach to excimer laser surgery for combined pathology of the cornea and lens. Material and methods. The work is based on 20-year clinical observations of the results of excimer laser refractive, opto-reconstructive and medical surgery in combined pathology of the cornea and lens based on a biooptical approach (more than 1500 operations). Particular emphasis was placed on the cycloplegic functional test and the use of stromal saturation with 0.25% chilled to +5–7 °C isotonic riboflavin solution during photoablation. The studies used modern diagnostic devices OPD-scan, RTVue XR100 and TMS-5 and excimer ophthalmic lasers from various companies. Results. In case of combined refractive pathology of the cornea and lens, the cycloplegic test made it possible to more accurately determine the degree of ametropia, its accommodative component with compensation of corneal astigmatism, and the presence of lens astigmatism. In all cases, the determination of the volume of photorefractive ablation was calculated taking into account the contribution of the cornea and lens to the formation of ametropia and aberrations of the optical system of the eye. The diameter of the optical and transition zone during photorefractive ablation was calculated depending on the state of the pupil diameter in photopic and mesopic conditions. Obtained in the course of studies on the OPD-Scan device, objective information on the difference between the pupil width in mesopic and photopic conditions made it possible to individualize the sizes of the optical and transition zones during photorefractive ablation, to avoid the «Halo» and «Gler» effects. Photoablation with saturation of tecorneal stroma with riboflavin reduced the response of aseptic inflammatory and regenerative reactions, the severity and duration of the corneal syndrome, and provided earlier stabilization of optical refractive indices. Conclusion. When performing excimer laser refractive, opto-reconstructive and therapeutic operations with combined pathology of the cornea and lens, a biooptical approach is required, taking into account the leading role of the cornea and lens in the formation of the optical system of the eye and visual acuity. Keywords: biooptics, photoablation, excimer laser corneal surgery

Кросслинкинг роговицы излучением эксимерного лазера на аргон-фторе

Purpose. To substantiate the expediency of using ablative and subablative energy densities of an argon-fluorine excimer laser for corneal crosslinking. Material and methods. In the period from 1 month to 6 years, the results of more than 500 photorefractive and phototherapeutic operations with saturation of the cornea with riboflavin and the effect of crosslinking were analyzed. To activate riboflavin, secondary radiation was used, induced by the use of ablative and subablative energy densities of an argon-fluorine excimer laser. For an objective assessment of the effect of excimer laser crosslinking, we used the techniques of computed keratotopography, aberrometry, optical coherence tomography, and densitometry. Results. Clinical observations have shown that in most cases of photorefractive surgery of the cornea for the prevention of keratoectasias, it is sufficient to carry out ablation after saturation of the corneal stroma with 0.25% isotonic riboflavin solution for 3-10 minutes, depending on the degree of ametropia and the volume of tissue removed. This provided photoprotective protection of deeper stromal structures from the negative effects of ablation-induced secondary radiation. At the same time, upon completion of ablation, secondary radiation initiated the effect of crosslinking in the adjacent layers, which did not violate the elastic properties of the cornea. In a number of cases, the cross-linking effect was enhanced by additional exposure to energy densities below the ablation threshold and the formation of a Bowman-like membrane structure on the ablation surface. For keratoconus and corneal pathology, without and in combination with keratoectasia, the technology of therapeutic cross-linking was used. This technology provided for the saturation of the stroma with 0.1% or 0.25% isotonic riboflavin solution and its activation by subablative radiation energy densities of an argon-fluorine excimer laser. During therapeutic crosslinking, according to OCT and densitometry, all the classic signs of traditional corneal crosslinking were revealed. Conclusion. The radiation of an argon-fluorine excimer laser with the use of ablative pulse energy densities can be recommended for prophylactic crosslinking in all cases of corneal thinning during photorefractive surgeries, while the use of energy densities below the ablation threshold allows for therapeutic crosslinking in keratoconus and corneal ectasias of various etiology. Key words: excimer laser crosslinking, cornea, keratoconus, photorefractive ablation, riboflavin.

Optical Coherence Tomography and Densitometry in Assessing the Effect of Corneal Collagen Cross-Linking Upon Photorefractive Ablation with Riboflavin

Optical Coherence Tomography and Densitometry in Assessing the Effect of Corneal Collagen Cross-Linking Upon Photorefractive Ablation with Riboflavin

Features of inflammatory and regenerative reaction to excimer laser refractive keratectomy with photoprotection

Aim. To study the features of inflammatory and regenerative reaction to excimer laser refractive keratectomy with photoprotection.Methods. Clinical cases of 560 excimer laser refractive keratectomies with photoprotection in 280 patients with myopia and myopic astigmatism of different severity. The observation period varied from 1 month to 2.5 years.Results. The study showed that intensity of aseptic inflammation and the type of regenerative response are influenced by the amount of evaporated corneal tissue, which correlates with the severity of myopia. Moreover, in case of equal severity of myopia and amount of evaporated tissue inflammatory and regenerative response was less prominent if excimer laser ablation was performed under the superficial corneal flap. In this case epithelial stromal corneal flap allowed reducing the negative effect of ultraviolet exposure and interaction between proinflammatory epithelial and stromal cytokines. At photorefractive ablation of corneal stroma saturated with 0.25% isotonic riboflavin solution aseptic inflammatory and regenerative response, intensity and duration of corneal syndrome decreased. Due to such technique of photorefractive keratectomy stromal layers saturated with riboflavin absorbed the secondary ultraviolet induced by ablation. Clinical observation demonstrated the need for additional photoprotection by glasses from external unltraviolet in all types of photorefractive surgeries on cornea up to full recovery of visual and optometric parameters. Use of photoprotection during excimer laser refractive ablation allowed achieving early stabilization of vision and refraction, decreasing the duration of postoperative steroidal and nonsteroidal anti-inflammatory drugs intake by one third in average.Conclusion. Local photoprotection with riboflavin during excimer laser refractive keratectomy and photoprotection by glasses from external ultraviolet reduce the risk of oxidative stress in the cornea, postoperative aseptic inflammatory and regenerative reactions, promote stabilization of visual, optic refractive parameters and improve their predictability.

Laser-induced corneal cross-linking upon photorefractive ablation with riboflavin.

AimTo estimate the biomechanical effect of the laser-induced cross-linking resulting from photorefractive ablation of the cornea with riboflavin.MethodsExcimer laser ablation studies were performed ex vivo (32 eyes of 16 rabbits) by phototherapeutic keratectomy (PTK) and in vivo (24 eyes of 12 rabbits) by transepithelial photorefractive keratectomy (TransPRK), with and without riboflavin saturation of the stroma. Then, we performed corneal optical coherence tomography on 36 eyes of 18 patients with varying degrees of myopia at different times after the TransPRK was performed with riboflavin saturation of the stroma.ResultsBiomechanical testing of corneal samples saturated with riboflavin revealed cross-linking effect accompanied by the increase in tensile strength and maximum strength. PTK showed increase in tensile strength from 5.1±1.4 to 7.2±1.6 MPa (P=0.001), while Trans-PRK showed increase in tensile strength from 8.8±0.9 to 12.8±1.3 MPa (P=0.0004). Maximum strength increased from 8.7±2.5 to 12.0±2.8 N (P=0.005) in PTK and from 12.8±1.6 to 18.3±1.2 N (P=0.0004) in TransPRK. Clinical optical coherence tomography studies of the biomicroscopic transparent cornea at different times after TransPRK showed increased density in the surface layers of the stroma and membrane-like structure beneath the epithelium.ConclusionPhotorefractive ablation of the preliminary corneal stroma saturation with riboflavin causes the effect of laser-induced cross-linking, which is attended with an increase in corneal tensile strength, maximum strength, increased density in the surface layers of the stroma, and formation of a membrane-like structure beneath the epithelium after TransPRK.

Riboflavin photoprotection with cross-linking effect in photorefractive ablation of the cornea

Photorefractive ablation is inevitably accompanied by oxidative stress of the cornea and weakening of its biomechanical and photoprotective properties. To validate the expediency of riboflavin use in photorefractive ablation for photoprotection of the cornea and cross-linking. The effects of riboflavin use in photorefractive ablation was first studied in a series of in vitro and in vivo experiments performed on 56 eyes of 28 rabbits, and then on 232 eyes of 142 patients with different degrees of myopia. Biomechanical testing of corneal samples was performed with Zwick/RoellВZ 2.5/TN1S tensile-testing machine. Transepithelial photorefractive keratectomy (TransPRK) and femtosecond laser-assisted in situ keratomileusis (Femto-LASIK) were performed on Wavelight-Allegretto200, MEL-80, and WaveLight-EX500 excimer laser systems and also VisuMax and WaveLight-FS200 femtosecond lasers. For preliminary examinations, an appropriate set of diagnostic tools was used. In vivo experiments have proved that, in the absence of conservative therapy, riboflavin is able to produce both photoprotective and cross-linking effects to the cornea. Corneal syndrome was thus reduced and re-epithelialization after TransPRK accelerated. Biomechanical testing of corneal samples revealed an increase in tolerated load from 12.9±1.4 N to 18.3±1.2 N (p=0.0002) and tensile strength from 8.6±1.7 MPa to 12.4±1.7 MPa (p=0.007). Clinical studies conducted in a group of patients with mild to moderate myopia have also confirmed the photoprotective effect of riboflavin at months 1-12 after TransPRK. There were no significant differences in uncorrected visual acuity (ranged from 0.80±0.16 to 0.85±0.15) and corrected visual acuity at baseline (0.83±0.14). Evaluation of the optical and refractive effect achieved after Femto-LASIK with riboflavin photoprotection in the fellow eye has shown that this technique is not inferior to the traditional one as to its refractive accuracy, but provides better maintenance of biomechanical and photoprotective properties of the cornea. Photorefractive ablation of the cornea with preliminary saturation of the stroma with riboflavin solution ensures its photoprotection and provides an additional cross-linking effect.

PMMA Model of Steep Central Islands Induced by Excimer Laser Photorefractive Keratectomy

We designed a polymethyl methacrylate (PMMA) model of refractive steep central islands (CIs) induced by PRK. A standardized photorefractive ablation procedure was performed using seven different excimer lasers on test PMMA specimens including 70 flat plates and 98 convex contact lenses. The resulting surface was analyzed by high-resolution confocal microscopy and computerized videokeratoscopy using both TMS-1 and CAS-2000 systems. A total of 50 (54.9%) CIs were observed using computerized videokeratoscopy. The rate of occurrence of CIs was significantly reduced by shock wave absorption (P = 0.0001), aspiration of fumes (P = 0.0044), and smaller diameter ablation (P = 0.0296). The diameter of the CIs was significantly increased for broad-beam mode ablation (P = 0.016) and for larger ablation zones, (P = 0.042). The refractive power of CIs was significantly increased in the absence of a shock wave absorption system (P = 0.001). Only 20 (40%) of the CIs detected by the TMS-1 device were identified on CAS-2000 at a 0.5 diopter (D) scale resolution level. Shock wave induced deformation and subsequent dynamic alteration of convection forces applied to emitted particles may be the primary mechanism underlying the formation of CIs after PRK, regardless of any biological response of the ablated tissue. Reported rates and characteristics of CIs may largely depend upon the specific design of videokeratoscopes.

Laser-induced Fluorescence During Photorefractive Keratectomy: A Method for Controlling Epithelial Removal

Laser-induced fluorescence is generated during ablation of corneal tissue with the argon-fluoride 193-nm excimer laser. To investigate possible changes in laser-induced fluorescence spectra emitted during the transition between epithelium and stroma, we developed a system using an intensified charge-coupled device to achieve fast per-pulse temporal resolution of laser-induced fluorescence. Freshly enucleated human cadaver eyes were subjected to 193-nm excimer laser keratectomy. During the procedure, laser-induced fluorescence was measured using an intensified charge-coupled device. Changes in laser-induced fluorescence were detected and used to control epithelial removal. Depth of ablation was determined histologically. The 193-nm excimer laser pulses induced both visible and ultraviolet fluorescence from corneal epithelium and stroma. In each layer two peaks predominated, one at 405 nm and the other at 346 nm. There was a rapid threefold reduction in the 346-nm ultraviolet peak at the transition from epithelium to stroma. By monitoring changes in laser-induced fluorescence at the epithelial-stromal interface, the clinician may be able to control corneal epithelial removal more precisely and reproducibly before performing photorefractive ablation.

Correction of Irregular Astigmatism with the Excimer Laser

Correction of irregular astigmatism has not been possible using available keratorefractive technology. The authors used a topographic map as a guide and created a custom excimer ablation program, designed to create a more regular surface. The program consisted of a combination of phototherapeutic and photorefractive ablation patterns. The amount of tissue to be removed was calculated on the basis of the diameter and steepness of the irregular areas of the corneal surface. A more regular surface, as evidenced by topographic analysis, reduced astigmatism, and improved uncorrected visual acuity, was produced. Using the corneal topographical map as a guide, excimer laser ablation can be used to create a more regular optical surface with improved visual function.