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Appearance of cornea immediately after photorefractive keratectomy. T7he central concentric ring patten represents the reprofiled cornea. The larger, irregular, outer reflection is the edge of the debrided corneal epithelium
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Summary points Excimer laser photorefractive keratectomy is the latest in a long line of surgical treatments for myopia (short sightedness) Around 50 centres (including three research centres) in the United Kingdom offer this treatment and tens of thousands of patients have been treated worldwide Considerable individual variation in corneal wound h...
Context in source publication
Context 1
... An extension of the ability to remove a thin surface layer from the cornea is the removal of more tissue from the central part of the circular photoablated zone than the edge. This is achieved by placing a computer controlled iris diaphragm in the path ofthe beam (fig 2) and results in a flattening of the central cornea and therefore treatment of myopia. The procedure has been termed photorefractive keratectomy and, since the comea is reprofiled directly, it is quite distinct from radial keratotomy. ...
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Purpose:
To compare the 3-month postoperative refractive results and flap thickness homogeneity after correction of myopia using the Intralase femtosecond platform and the Victus dual femtosecond laser platform.
Setting:
Novovision Clinic, Madrid, Spain.
Design:
Prospective case series.
Methods:
Eyes treated with a 60 kHz femtosecond laser (...
Scarring associated with regression of refractive effect can occur after photorefractive keratectomy (PRK) for myopia. The experience of treating these complications is reported.
Eighteen of 285 eyes (6.3%) were retreated with the excimer laser. Age, sex, preoperative primary treatment keratometry, pre-primary treatment, pre-retreatment and post-re...
Background:
To compare the clinical outcomes of transepithelial photorefractive keratectomy (TPRK) with femtosecond laser-assisted in situ keratomileusis (FS-LASIK) for correction of high myopia.
Methods:
In this prospective, non-randomised, cohort study, 85 eyes of 46 patients treated with TPRK and 80 eyes of 42 patients treated with FS-LASIK w...
Background: The postoperative results of excimer laser photorefractive keratectomy (PRK) are encouraging, as in procedures of up to -6.00 diopters more than 80% of eyes are within ±1.00 diopter of their intended correction. However, throughout the dioptric range, some unexpected individual results have been observed with excessive myopic regression...
Background:
Studies suggest that transepithelial photorefractive keratectomy (TransPRK) with the all-surface laser ablation (ASLA)-SCHWIND platform is effective and safe for both low-moderate myopia and high myopia. In most studies, mitomycin-C is administered immediately after surgery to prevent corneal opacification (haze), which is a significan...
Citations
... Photoablation of the stromal bed is typically accomplished via a single-photon absorption mechanism at 193 nm. [1][2][3] This process modifies the cornea's refractive and biomechanical properties and induces additional optical power in the cornea by changing the shape of its anterior surface. However, complications, such as dry eye, 4 stromal haze, 5 diffuse lamellar keratitis, 6 or inflammation 7 can occur from stromal photoablation, epithelial debridement, and/or the creation of the epithelial flap. ...
Blue intratissue refractive index shaping (blue-IRIS) is a method with potential to correct ocular refraction noninvasively in humans. To date, blue-IRIS has only ever been applied to cat corneas and hydrogels. To test the comparability of refractive index change achievable in cat and human tissues, we used blue-IRIS to write identical phase gratings in ex vivo feline and human corneas. Femtosecond pulses (400 nm) were focused ∼300 μm below the epithelial surface of excised cat and human corneas and scanned to write phase gratings with lines ∼1 μm wide, spaced 5 μm apart, using a scan speed of 5 mm/s. Additional cat corneas were used to test writing at 3 and 7 mm/s in order to document speed dependence of the refractive index change magnitude. The first-order diffraction efficiency was immediately measured and used to calculate the refractive index change attained. Our data show that blue-IRIS induces comparable refractive index changes in feline and human corneas, an essential requirement for further developing its use as a clinical vision correction technique. © 2017 Society of Photo-Optical Instrumentation Engineers (SPIE).
... While the epithelium has completely healed within 4 to 5 days after surgery in most cases (53,54), epithelial wound healing has occurred as late as several months after surgery (48) . Studies found that a smooth corneal surface at 1 to 3 months after surgery did not guarantee a smooth surface at 6 months, as the extent of deposition of new corneal tissue was unpredictable (55,56) . ...
This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The United States Government assumes no liability for the contents thereof. ___________ This publication and all Office of Aerospace Medicine technical reports are available in full-text from the Civil Aerospace Medical Institute’s publications Web site: www.faa.gov/library/reports/medical/oamtechreports/index.cfm 2
... [5][6][7][8][9][10] Reduction of tonometric readings has been reported in eyes after photorefractive keratectomy (PRK) [11][12][13][14] and laser in situ keratomileusis (LASIK). [15][16][17] These procedures flatten the anterior corneal surface and reduce CCT proportionally to the extent of myopia, 14,18 providing lower IOPs compared with preoperative ones. ...
... 39 In this regard, a recent report has shown no significant difference between measurements even in patients undergoing PRK and LASIK, and for each degree of treated myopia, even if noncontact tonometry readings were slightly higher. 40 In agreement with previous studies on decreased IOP measurements after PRK and LASIK, [11][12][13][14][15][16][17][18] this study showed a statistically significant IOP decrease after excimer laser treatment at baseline as well as during WDT. However, in our patient sample, this reduction was not constant at the different time intervals during WDT, with increasing reductions toward the peak at the 20-minute time interval, and successive decreasing reductions up to the 60-minute time interval (see Fig. 1), which is when the effect of WDT should end, as suggested by previous studies. ...
To determine whether tonometric readings of increases in intraocular pressure (IOP) during the water-drinking test (WDT) are affected by variations in central corneal thickness (CCT) induced by photorefractive keratectomy (PRK).
Data from 30 randomly selected eyes of 30 patients (18 men and 12 women; mean age, +/- SD: 33.9 +/- 7.6 years) undergoing bilateral PRK for myopia (-6.57 +/- 2.39 D) were obtained. Objective refraction, anterior radius of corneal curvature (R), CCT, and IOP measurements at baseline and at different time intervals after ingestion of 1 L of water within 5 minutes, were performed before and 6 months after PRK. All measured IOPs were recalculated by a correction factor for R and CCT and expressed as corrected intraocular pressure (IOPC) measurements.
The mean R +/- SD was 7.84 +/- 0.20 and 8.76 +/- 0.34 mm, and the mean CCT was 544.83 +/- 19.69 and 453.97 +/- 29.95 microm, before and after PRK, respectively. The mean IOP at baseline was 15.05 +/- 2.78 and 9.83 +/- 2.56 mm Hg, and during WDT was 18.32 +/- 3.42 and 11.42 +/- 3.10 mm Hg at 10 minutes, 18.59 +/- 2.99 and 11.54 +/- 2.54 mm Hg at 20 minutes, 17.80 +/- 2.85 and 10.87 +/- 2.22 mm Hg at 30 minutes, 16.35 +/- 3.02 and 10.26 +/- 2.21 mm Hg at 45 minutes, and 14.90 +/- 2.52 and 9.81 +/- 2.32 mm Hg at 60 minutes, before and after PRK, respectively. The mean IOPC at baseline was 13.64 +/- 2.33 and 13.05 +/- 2.98 mm Hg, and during WDT was 16.61 +/- 2.77 and 15.08 +/- 3.59 mm Hg at 10 minutes, 16.96 +/- 2.69 and 15.33 +/- 2.96 mm Hg at 20 minutes, 16.10 +/- 2.50 and 14.42 +/- 2.60 mm Hg at 30 minutes, 14.92 +/- 2.72 and 13.62 +/- 2.65 mm Hg at 45 minutes, 13.82 +/- 2.27 and 13.05 +/- 2.55 mm Hg at 60 minutes, before and after excimer laser treatment, respectively. Pre- and postoperative IOPs and percentages of IOP increase differed significantly (P < 0.05), in particular at the peak, as did IOPCs but not the percentages of increase in IOPC, apart from the highest values.
Corneal changes after PRK for myopia may induce an uneven underestimate of the IOP increases. The inadequacy of a correction factor to compensate for CCT and R at high IOP levels indicates that other biomechanical factors may play a role when the cornea is subjected to dynamic actual IOP variation. Such increase of the well-known underestimate of IOP after PRK at higher actual IOPs may have significant clinical implications in tonometric assessment of subjects at risk of glaucomatous damage.
... Myopic PRK and LASIK flatten and reduce CCT proportionally to the extent of myopia. 13 Increasing evidence has shown that IOP readings are reduced after PRK and LASIK. 14 -18 Consequently, the reliability of IOP measurements in patients with myopia who have undergone refractive photoablation has been questioned, and even clinically significant IOP increases may be overlooked. ...
To assess whether tonometric measurements of the drop in intraocular pressure (IOP) induced by 0.005% latanoprost are modified after photorefractive keratectomy (PRK).
Data from 24 randomly selected eyes of 24 patients (12 men and 12 women, mean age +/- SD: 31.7 +/- 6.2 years) who were undergoing bilateral PRK for myopia (-6.38 +/- 2.26 D) were obtained. Objective refraction, central corneal thickness (CCT), anterior radius of corneal curvature (R), and IOP measurements at baseline and 24 hours after 1 drop of 0.005% latanoprost, were performed before and 6 months after PRK. All measured IOPs were recalculated by a correction factor for CCT and R and expressed as true IOP (IOPT) measurements.
The mean CCT +/- SD was 544.58 +/- 36.03 and 463.21 +/- 38.59 micro m, and the anterior radius of corneal curvature was 7.73 +/- 0.26 and 8.33 +/- 0.37 mm, before and after PRK, respectively. The mean IOP at baseline was 15.8 +/- 2.92 and 12.23 +/- 2.37 mm Hg, and after latanoprost administration was 12.54 +/- 1.97 and 10.19 +/- 1.47 mm Hg, before and after PRK, respectively. The mean IOPT at baseline was 15.46 +/- 1.08 and 16.18 +/- 2.31 mm Hg, and after latanoprost administration was 11.85 +/- 1.56 and 12.96 +/- 1.71 mm Hg, before and after PRK, respectively. The mean IOP and IOPT reductions after latanoprost administration were, respectively, 3.25 +/- 1.66 and 3.61 +/- 1.7 mm Hg before PRK, and 2.03 +/- 1.42 and 3.22 +/- 1.79 mm Hg after PRK. Pre- and postoperative IOP reduction significantly differed (P < 0.001), but not IOPT.
The effect of hypotensive drugs on IOP readings may be underestimated because of measurement errors due to CCT reduction and R increase after PRK for myopia. Misdiagnosis of reduced pharmacologic efficacy may be avoided if the measured IOP is corrected by a proper nomogram.
... Popular current techniques of refractive surgery are excimer laser photorefractive keratectomy (PRK) (e.g. Munnerlyn et al., 1988; Steiler and McDonell, 1995; Gartry, 1995) and laser in situ keratomileusis (LASIK) (Pallikaris et al., 1990; Pallikaris et al., 1991; Buratto et al., 1993; Gu Èell and Muller, 1996). PRK uses short-wavelength radiation from an excimer laser to ablate the central 4±7 mm diameter of the cornea, thereby changing its curvature and thickness and altering the refractive state of the eye. ...
The aim of this study was to determine whether intraocular pressure (IOP), as measured by Goldmann applanation or non-contact tonometry, shows systematic changes in patients who have undergone photorefractive keratectomy (PRK) or laser in situ keratomileusis (LASIK). IOP was measured by central Goldmann and non-contact tonometry in 54 patients pre and post-PRK, and in 43 patients pre- and post-LASIK. An interval of 12 months was allowed after surgery. Patients were selected to have one of four specific initial values of refractive error (−2.5, −5.0, −7.5 and −10.0 D). Fellow unoperated eyes were used as controls. A paired Student's t-test and a one-way ANOVA test were used for statistical analysis. After PRK and LASIK, a statistically significant decrease (p<0.01) was observed in the IOP of the treated eyes (but not for control eyes; p>0.01). Although the magnitude of the change increased with the attempted refractive correction, this trend was not statistically significant ( >p>0.01). No statistically significant differences were found between the results obtained following the two types of surgery, although the recorded fall in IOP was smaller following LASIK( >p>0.01). The IOP measured after PRK and LASIK for myopia may be reduced because of reduced corneal thickness and curvature and, possibly, tissue softening after natural healing. The presence or absence of Bowman's membrane does not appear to be important in this context. The reduction in measured IOP following refractive surgery, by about 0.5 mmHg/D of myopic correction, needs to be remembered when possible abnormality of IOP in such patients is being considered.
... Popular current techniques of refractive surgery are excimer laser photorefractive keratectomy (PRK) (e.g. Munnerlyn et al., 1988; Steiler and McDonell, 1995; Gartry, 1995) and laser in situ keratomileusis (LASIK) (Pallikaris et al., 1990; Pallikaris et al., 1991; Buratto et al., 1993; Gu Èell and Muller, 1996). PRK uses short-wavelength radiation from an excimer laser to ablate the central 4±7 mm diameter of the cornea, thereby changing its curvature and thickness and altering the refractive state of the eye. ...
The aim of this study was to determine whether intraocular pressure (IOP), as measured by Goldmann applanation or non-contact tonometry, shows systematic changes in patients who have undergone photorefractive keratectomy (PRK) or laser in situ keratomileusis (LASIK). IOP was measured by central Goldmann and non-contact tonometry in 54 patients pre and post-PRK, and in 43 patients pre- and post-LASIK. An interval of 12 months was allowed after surgery. Patients were selected to have one of four specific initial values of refractive error (-2.5, -5.0, -7.5 and -10.0 D). Fellow unoperated eyes were used as controls. A paired Student's t-test and a one-way ANOVA test were used for statistical analysis. After PRK and LASIK, a statistically significant decrease (p < 0.01) was observed in the IOP of the treated eyes (but not for control eyes; p > 0.01). Although the magnitude of the change increased with the attempted refractive correction, this trend was not statistically significant (p > 0.01). No statistically significant differences were found between the results obtained following the two types of surgery, although the recorded fall in IOP was smaller following LASIK (p > 0.01). The IOP measured after PRK and LASIK for myopia may be reduced because of reduced corneal thickness and curvature and, possibly, tissue softening after natural healing. The presence or absence of Bowman's membrane does not appear to be important in this context. The reduction in measured IOP following refractive surgery, by about 0.5 mmHg/D of myopic correction, needs to be remembered when possible abnormality of IOP in such patients is being considered.
... Short laser pulses are used to ablate thin tissue layers in refractive surgery, utilising laser light which penetrates only a few micrometers in tissue. 116 The wavelength of the laser light can be chosen such that the light is absorbed selectively by the target. Selective coagulation of enlarged, disfiguring blood vessels in the skin can be performed by using laser light which is selectively absorbed by haemoglobin. ...
This thesis is concerned with the measurement of the scattering and absorption properties of biological tissues using the technique of Coherent Backscatter (also known as Coherent Enhancement or Weak Localization). The Coherent Backscatter method offers a remote sensing technique that may be superior to conventional alternatives through its applicability to in vivo measurements, its sensitivity at the relatively high values of scattering often found in biological media, the small volumes of tissue that can be interrogated, and its low equipment costs. A Monte Carlo approach is used to accurately evaluate the spatial characteristics of light that is backscattered from biological media and an important prediction is made on the likely depth to which the technique probes. The phenomenon of Coherent Backscatter is introduced in the context of tissue optics and the development of a numerical method based on the Monte Carlo results is reported that predicts the coherent enhancement peaks that are likely to be recorded from biological media. This includes accurate predictions of the effects of scattering anisotropy, refractive index mismatches, and certain illumination geometries that have not, as yet, been achieved exactly by any analytical methods. A neural network has been implemented to provide a very fast and robust method for processing an experimental reading in order to extract the reduced scattering and absorption coefficients. The experimental data presented evaluates the accuracy of the technique and demonstrates its use on a variety of media including an in vivo human hand. The technique is also used to investigate the possible dependence of the optical properties on the polarization state of the probing light and the orientations of fibres in certain media.
The goals of this study are to assess the results of photorefractive keratectomy (PRK) using a new algorithmic correction for the Nidek EC-5000 excimer laser. A modified algorithm was obtained from the manufacturer after data previously presented indicated that the existing algorithm tended to overcorrect the spherical error and undercorrect the astigmatism. In the new algorithm the spherical component was undercorrected as the associated cylinder increased and the cylinder was overtreated by a similar amount. In this study, 482 consecutive patients undergoing PRK & photoastigmatic refractive keratectomy were evaluated. The spherical corrections performed were between 0.00 D and — 10.75 D and the astigmatic corrections between −0.00 D and −3.50 D. Postoperative results at six months of follow-up showed a mean spherical equivalent of −0.05 ± 0.78 diopters, a mean spherical correction of +0.18 ± 0.77 D and mean astigmatism of −0.45 D ± 0.40 D. Vector analysis showed that mean surgically induced astigmatism was +0.29 D ± 1.02 D. When compared with results obtained using the original algorithm these patients achieved improved spherical corrections although their astigmatic results were similar. It is recommended that manufacturer’s algorithms are regularly updated on the basis of the actual patient results.
