Corneal Shape Changes Research Articles

Transconjunctival Sutureless 25-Gauge Versus 20-Gauge Standard Vitrectomy: Correlation Between Corneal Topography and Ultrasound Biomicroscopy Measurements of Sclerotomy Sites

To determine the correlation between corneal shape changes and ultrasound biomicroscopy (UBM) findings at the sclerotomy sites in conventional 20-gauge (G) pars plana vitrectomy (PPV) and 25-G transconjunctival sutureless vitrectomy (TSV) and to compare the effectiveness of the two surgical methods. Prospective, comparative, observational case series. Sixty consecutive eyes (60 patients) undergoing primary 3-port PPV. Thirty eyes (30 patients, group 20-G) were treated with 20-G standard PPV and 30 eyes (30 patients, group 25-G) with 25-G TSV. We compared healing of the sclerotomy sites in the two groups. We determined the correlation between corneal shape changes (surgically induced astigmatism) measured by video keratography and the durations of scleral healing cicatrization by UBM within each group. UBM examination showed that the 20-G sclerotomy sites took about 8 weeks to heal, measured as complete opposition, whereas healing of the 25-G TSV sclerotomy was quite rapid, with complete scleral opposition in about 4 weeks. Corneal topography analysis showed, during the early postoperative period, a surgically induced steepening of the cornea in both groups (20 G, 3.08 +/- 0.56 diopters and 25 G, 0.805 +/- 0.61 diopters, P < 0.001, Mann-Whitney test), which then decreased gradually, recovering to the preoperative level within two months in group 20 G (P > 0.05) and 1 month in group 25 G (P > 0.05). We found a strong statistical correlation between the mean surgically induced keratometric astigmatism and the mean UBM measures of scleral healing (r = 0.99 for group 20 G and r = 0.97 for group 25 G). After PPV, astigmatic changes are especially significant in the early postoperative period in 20-G group; the 25-G TSV system results in faster reduction of surgically induced keratometric astigmatism because of rapid cicatrization of the sclerotomy sites.

Effects of altered corneal stiffness on native and postoperative LASIK corneal biomechanical behavior: A whole-eye finite element analysis.

To investigate the impact of corneal elasticity on corneal shape changes before and after simulated LASIK with and without consideration of whole-eye biomechanics. A finite element whole-eye model of a human eye was constructed. The cornea was modeled as hyperelastic and incompressible using experimental data representing a range of corneal stiffness. The corneal response to intraocular pressure loading and LASIK for 2.00, 4.00, and 6.00 diopters of spherical myopia was analyzed as a function of corneal stiffness and limbal boundary conditions. Myopic LASIK produced different degrees of central flattening and postoperative ametropia in low-stiffness and high-stiffness corneas. Although a cornea-only model demonstrated maximum stresses and displacements in the central cornea and predicted residual myopia, a whole-eye model with equivalent corneal stiffness predicted greater paracentral displacements and less myopic undercorrection. In a whole-eye model with a stiffer cornea, maximum displacements shifted further toward the limbus, favoring additional mechanically mediated central flattening and refractive overcorrection (hyperopia). In postoperative LASIK models thinned by high myopic corrections, corneal stiffening caused central cornea flattening. Differences in the corneoscleral stiffness relationship affect simulated refractive outcomes after LASIK and may be a source of individual variation in refractive surgery outcomes. A whole-eye model allowing limbal motion illustrates a stiffness-dependent biomechanical balance between central corneal flattening and pre-ectatic weakening of the corneal apex not demonstrated in previous computational models and provides insight into under- and overcorrection in myopic LASIK and the previously unexplained phenomenon of corneal flattening after therapeutic collagen cross-linking for keratoconus.

Orbscan II Anterior Elevation Changes Following Corneal Collagen Cross-linking Treatment for Keratoconus

To analyze anterior corneal elevation changes on Orbscan II following corneal collagen cross-linking (CXL) with riboflavin. This retrospective study included 8 patients (14 eyes) with keratoconus who underwent CXL, with a mean follow-up of 7 months (range: 5 to 10 months). Pre- and postoperative (at last clinic attendance) anterior elevation difference maps were examined for overall patterns of change. On preoperative maps, distances from maximum anterior elevation to pupil center and to topographic geometric center were compared between the two patterns identified. Pre- and postoperative topography, best spectacle-corrected visual acuity (BSCVA), and refraction were also compared between the two patterns. Two patterns of anterior elevation change were visually identified: (1) paracentral steepening, no change, or flattening centrally; and (2) central steepening. The preoperative maps of eyes that manifested pattern 1 had shorter mean distances for maximum anterior elevation to pupil center (1.70 vs. 2.27 mm) and maximum anterior elevation to geometric center (1.45 vs. 1.99 mm) than those that resulted in pattern 2. Mean maximum topographic simulated keratometry decreased (P = .004) and mean irregularity indices at 3 mm (P =.03) and 5 mm (P =.04) were reduced postoperatively in pattern 1 eyes; all increased in pattern 2 eyes. Mean BSCVA improved postoperatively for both patterns. Mean preoperative myopia decreased in pattern 1 eyes by 0.44 diopters (D), whereas it increased for pattern 2 eyes by 1.83 D. Corneal shape change influenced by anisotropy of collagen distribution is a factor in the outcome of CXL treatment for keratoconus.

Corneal shape changes induced by first and second generation silicone hydrogel contact lenses in daily wear

To compare the corneal topographical changes induced by two first and second generation silicone-hydrogel (SiH) contact lenses after 3 months of daily wear (DW). Prospective, consecutive case-series in which patients wore one of 3 different contact lenses (either the first generation SiH Focus Night & Day, the second generation SiH Acuvue Oasys or the monthly disposable Soflens 38 hydrogel lens as control group) on a DW basis for 3 months. Over-refraction, visual acuity, mean keratometry, corneal astigmatism, corneal eccentricity, superficial regularity and superficial asymmetry indices were monitored over the 3-month period. Nineteen eyes of 10 patients completed the study. Seven Focus Night & Day, 7 Acuvue Oasys and 5 Soflens 38 contact lenses were fitted. There were no significant changes between any of the parameters measured at the 3-month visit in any of the SiH groups (non-parametric Wilcoxon test, p>0.05). However, the control group (Soflens 38) showed statistically significant changes regarding mean keratometry, corneal astigmatism and corneal eccentricity (p<0.05). Three patients wearing the first generation SiH showed adverse events of different degree related to their high modulus of elasticity. After 3 months of DW, wearers of first and second generation SiH lenses showed greater corneal stability than hydrogel monthly disposable contact lenses users regarding commonly used topographic corneal shape indices. However, complications related to the mechanical properties of first generation SiH were seen in three patients in the follow-up time.

Postoperative corneal shape changes: Microincision versus small-incision coaxial cataract surgery

To compare changes in corneal topography and in regular and irregular astigmatism after coaxial clear corneal microincision cataract surgery (MICS) and after coaxial small-incision cataract surgery (SICS). Hayashi Eye Hospital, Fukuoka, Japan. Induced corneal astigmatism was determined using vector analysis. The averaged corneal shape changes and degree of irregular astigmatism were examined using videokeratography preoperatively as well as 2 days and 1, 2, 4, and 8 weeks postoperatively. One hundred twenty eyes of 60 patients scheduled for phacoemulsification were evaluated. Sixty eyes had MICS (2.00 mm), and the 60 contralateral eyes had SICS (2.65 mm). The mean induced corneal astigmatism was significantly less in the MICS group than in the SICS group (P <or= .0495) 1 week postoperatively and subsequently. The averaged difference map on videokeratography showed focal corneal flattening corresponding to the incision and coupled steepening around the flattened area 2 days postoperatively in both groups. The flattening and steepening gradually reduced to virtually the preoperative shape by 8 weeks; the changes were significantly less in the MICS group than in the SICS group. Of the irregular astigmatic components, higher-order irregularity was significantly less in the MICS group than in the SICS group up to 2 weeks postoperatively (P <or =.0470). Induced astigmatism, focal wound-related flattening of the peripheral cornea, and corneal surface irregularity were significantly less after coaxial MICS than after SICS.

Management of slipped laser in situ keratomileusis flap following intrastromal corneal ring implantation in post-LASIK ectasia

Two cases of intrastromal corneal ring (Intacs, Addition Technology) implantation to treat post-laser in situ keratomileusis (LASIK) ectasia developed progressive slippage of the flap over the stromal bed, resulting in progressive flattening of the affected meridian. The flattening was thought to have occurred because the poorly healed flap edge could not withstand the change in corneal shape induced by the ring segments. The flap was lifted, debris was removed, and the flap was sutured to the residual stromal bed using interrupted 10-0 nylon sutures passed through two thirds of the cornea. The sutures induced a localized area of scarring that acted as "welding spots" and prevented further flap slippage. Refractive stability after suture removal supports the hypothesis that a sutured flap restores (at least partially) the biomechanical strength of post-LASIK corneas.

Reversibility of Effects of Orthokeratology on Visual Acuity, Refractive Error, Corneal Topography, and Contrast Sensitivity

To investigate the changes in corneal shape and optical performance during and after discontinuation of overnight orthokeratology for correction of myopia. Both eyes of 15 subjects were fitted with overnight reverse-geometry orthokeratology lenses, which were then worn for >4 hr overnight for 52 weeks. Subjects were free of ocular disease and had a corrected visual acuity of > or =1.0. Refractive correction, uncorrected visual acuity, corneal topography, and contrast sensitivity (at 4 spatial frequencies) were measured under photopic conditions. Refractive error (spherical equivalent) and contrast sensitivity were decreased, whereas uncorrected visual acuity, the surface asymmetry index, and the surface regularity index were increased, 1 week after the onset of overnight orthokeratology and remained so during the 52 weeks of treatment. These parameters had largely returned to baseline values by 8 weeks after treatment discontinuation. Overnight orthokeratology improved uncorrected visual acuity and reduced refractive error but increased corneal irregularity and impaired contrast sensitivity. However, these changes in visual function and corneal shape were reversed after discontinuation of orthokeratology lens wear.

LASIK-associated Dry Eye and Neurotrophic Epitheliopathy: Pathophysiology and Strategies for Prevention and Treatment

To review the pathophysiology of LASIK-associated dry eye conditions and provide insights into prophylaxis to decrease the incidence of dry eye after LASIK and to treat the condition when it occurs. A review of the literature was performed on LASIK-associated dry eye and the experience of the authors was summarized. LASIK has a neurotrophic effect on the cornea, along with other changes in corneal shape, that affect tear dynamics causing ocular surface desiccation. Dry eye is one of the most common complications of LASIK surgery. Symptoms of dryness may occur in more than 50% of patients, with other complications such as fluctuating vision, decreased best spectacle-corrected visiual acuity, and severe discomfort occurring in approximately 10% of patients. Preoperative dry eye condition is a major risk factor for more severe dry eye after surgery and should be identified prior to surgery. Optimization with artificial tears, nutrition supplementation, punctal occlusion, and topical cyclosporine A in patients with symptoms or signs of dry eye prior to LASIK decreases the incidence of more bothersome symptoms following surgery. Patients with LASIK-induced neurotrophic epitheliopathy often respond to topical cyclosporine A treatment, which treats the underlying inflammation and may benefit nerve regeneration. LASIK-induced dry eye and neurotrophic epitheliopathy are common complications of LASIK surgery. Optimization of the ocular surface prior to surgery decreases the incidence and severity of postoperative symptoms of the condition.

Changes in the Anterior and Posterior Radii of the Corneal Curvature and Anterior Chamber Depth by Orthokeratology

To investigate the mechanism of the refractive effect of orthokeratology by using measurements of the anterior and posterior radii of the corneal curvature and anterior chamber depth with a Pentacam analysis system. Nine women (18 eyes) with a mean age of 29.6+/-3.8 years and low to moderate myopia (<or=-4.00 diopters [D]) were recruited for a 53-week trial of overnight orthokeratology using RD171K lenses (hexafocon A). After wearing the orthokeratology lenses overnight, subjects were examined during the day. With a Pentacam analysis system, subjects were examined at 2, 4, 8, 12, 24, 36, and 53 weeks for the assessment of the anterior and posterior radii of the corneal curvature and anterior chamber depth. Myopic refractive error significantly reduced during the trial (P<0.001, analysis of variance). The mean refractive error was -2.85+/-0.46 D at baseline and significantly reduced to -0.28+/-0.65 D at 2 weeks (P<0.01, Bonferroni-Dunn post hoc test). At any week, no significant differences were seen in the central posterior radius of the corneal curvature (P=0.55, analysis of variance) or the anterior chamber depth (P=0.69, analysis of variance). The amount of change in the central anterior radius of the corneal curvature significantly correlated with the change in refractive error at 24 weeks (r=0.57, P<0.05, Pearson correlation coefficient). Overnight orthokeratology lens wear alters the anterior corneal shape rather than the posterior radius of the corneal curvature and the anterior chamber depth. This finding suggests that the primary factor in the refractive effect of orthokeratology is change in the anterior corneal shape rather than overall corneal bending.

Determination of the Modulus of Elasticity of the Human Cornea

To determine the material behavior of the human cornea in the form of simple relationships between the modulus of elasticity and intraocular pressure (IOP) and to establish the effect of age on the material behavior. Human corneal specimens with age between 50 and 95 years were tested under inflation conditions to determine their behavior. The corneas were subjected to two extreme load rates to represent dynamic and static loading conditions. The pressure-deformation results were analyzed using shell theory to derive the relationship between the modulus of elasticity and IOP. The corneas demonstrated a nonlinear hyperelastic behavior pattern with an initial low stiffness stage and a final high stiffness stage. Despite the nonlinearity of the pressure deformation results, the relationship between the modulus of elasticity and the applied pressure was almost linear. A considerable increase was noted in the values of the modulus of elasticity associated with both age and load rate. General equations were derived to calculate the values of the secant and tangent moduli of elasticity in terms of IOP for any age greater than 50 years, and these equations are presented in a simple form suitable for use in numerical simulations. Adequate representation of corneal material behavior is essential for the accurate predictive modeling of corneal biomechanics. The material models developed in this work could be implemented in numerical simulations of refractive surgery procedures, corneal shape changes due to contact lens wear, and other applications.

Modeling of lateral magnification changes due to changes in corneal shape or refraction

Background and Purpose Especially after corneal surgery the lateral magnification of the eye providing the retinal image size of an object is a crucial factor influencing visual acuity and binocularity. The purpose of this study is to describe a paraxial computing scheme calculating lateral magnification changes (ratio of the image sizes before and after surgery) due to variation in corneal shape and spectacle refraction. Calculation strategy From the 4 × 4 refraction and translation matrices the system matrix representing the entire ‘optical system eye’ and the pupil matrix describing the sub-system from the spectacle correction to the aperture stop were defined for the state before and after surgery. As the chief ray is assumed to pass through the centre of the aperture stop, the 2 × 2 matrix of the lateral magnification ratio from preoperative to postoperative is described by the 2 × 2 sub-matrices of the respective pupil matrices. The cardinal meridians can be extracted by calculating the eigenvalues and eigenvectors. Working example Vertex distance 14 mm, measured distance between corneal apex and aperture stop 3.6 mm, keratometry 39 D + 6 D/0° to 47 D + 3 D/30° and refraction 3.5 D − 5 − 5 D/5° to −4.0 D − 3.5 D/25° preoperatively to postoperatively. The matrix of magnification ratio from preop to postop yields (0.8960 −0.0085;0.0074 0.9371) and the eigenvalues decomposition provided a 10.7% minified image at 170.1° and a minified image of 6.1% at 78.7°, which both are clinically relevant. Conclusion We presented a straight-forward computer-based strategy for calculation of retinal image size changes using 4 × 4 matrix notation. With this model the meridional changes in lateral magnification from the preoperative to the postoperative stage or between follow-up stages can be estimated from keratometry, refraction, vertex distance and anterior chamber depth, which might be important for binocularity and vision tests in corneal surgery.

Corneal Curvature Stability With Increased Intraocular Pressure

To examine the corneal shape and ocular refraction under conditions of increased intraocular pressure (IOP) and to consider the possible influences of direct digital forces, corneal thickness, limbal and scleral rigidity, and the duration and frequency of IOP loading on the results. Forces applied digitally to the temporal sclera were used to obtain 15 optimized videokeratographic records for two subjects under different levels of IOP loading. Stability of refraction was also examined under the same conditions. Relative to normal IOP, four levels of IOP increments were induced: 43% to 61% (from a normal 18.8 mm Hg to approximately 28.6 mm Hg), 119% to 153% (from a normal 12.2 mm Hg to approximately 28.8 mm Hg), 181% to 213% (from a normal 18.8 mm Hg to approximately 55.8 mm Hg), and 374% to 443% (from a normal 12.2mm Hg to approximately 62.1 mm Hg). Significant changes in response to IOP loading were few in number and of a minor degree. For example, the maximum simulated keratometry change was 0.20 diopters (D), and the maximum change in refractive error recorded was 0.625 D in cylinder. Corneal shape and refraction were found to show only minor evidence of instability under conditions of IOP loading. The small number and minor degree of corneal changes recorded under IOP loading suggest that under these conditions of measurement, corneal stability may be the result of stress stiffening. However, mechanisms for corneal shape changes that depend on limbal and scleral rigidity may also contribute to corneal shape stability.

Corneal Refractive Therapy with Different Lens Materials, Part 2: Effect of Oxygen Transmissibility on Corneal Shape and Optical Characteristics

To compare the effects of two different oxygen transmissible (Dk/t) lenses on corneal shape and optical performance after one night of corneal refractive therapy (CRT(R)) for myopia. Twenty myopic subjects were fit with Menicon Z (MZ) (Dk/t = 90.6, Paragon CRT(R) lenses) on one eye and an Equalens II (EII) CRT lens (Dk/t = 47.2) on the contralateral eye (eye randomized). Corneal topography, refractive error and aberrations were measured before lens insertion (baseline), and the following day after overnight lens wear, on lens removal and 1, 3, 6, 12 h later. Root mean square wavefront errors were measured using 4.5 mm pupils. Averaged over position and time, the horizontal corneal curvature was statistically different between the MZ and EII lens-wearing eyes (p = 0.011). The central cornea flattened similarly (p = 0.886) and the mid-periphery steepened in both eyes (p = 0.061) from baseline. The EII lens-wearing eyes were steeper in the mid-periphery than the MZ eyes immediately after lens removal and at the 1-h visit (p < or = 0.032). Central corneal flattening and mid-peripheral corneal steepening regressed over time (all p < 0.001) but did not recover to baseline by 12 h (all p < 0.004). Myopia was reduced equally by 0.84 +/- 0.83 D for the MZ-lens wearing eyes and 0.84 +/- 0.87 D for the EII eyes (p = 0.969). Coma increased from baseline 1.85X (0.056 +/- 0.081 microm) for the MZ-lens wearing eyes and 1.72X (0.048 +/- 0.084 microm) for the EII eyes (both p < 0.001). Spherical aberration increased from baseline 4.55X (0.101 +/- 0.077 microm) for the MZ-lens wearing eyes and 4.31X (0.085 +/- 0.076 microm) for the EII eyes (both p < 0.001), but there were no differences between the MZ and EII eyes (all p > or = 0.308). Coma and spherical aberration did not return to baseline by 12 h (both p < or = 0.007). After one night of CRT lens wear, changes in corneal shape were slightly different, with more mid-peripheral steepening in the EII eyes compared to the MZ eyes. Change in central corneal curvature and optical performance were similar in both eyes.

Reply : Assessing intraocular lens accommodation

We thank Schachar for his interest in our work, although he misleadingly makes some observations that we would like to clarify further. 1. Schachar has noted from Figure 7 in our article that the camera angle toward the implanted device was not aligned accurately between measurements. He therefore concluded that such misalignment would result in distortion of perspective and questionable value of these measurements. The attached Figure 1 shows graphically the maximal extent of optical distortion that the camera angle might induce. The pin distance from the plate edge is very short (50 μm), as can be assessed from the device picture (Figure 3 in our original article). Even complete transition of the camera from parallel to the plate to near vertical induces around 80 μm of perspective-related error. From the same image that Schachar mentioned in his letter, it is easy to see that the angle of photography that we used was within angle B as appeared in Figure 1 of this reply. The maximal perspective error induced by deviation within angle B range is less then 40 μm. If the total movement of the pin was at the 50 μm range, Schachar's criticism would have been accepted without hesitation. However, the pin movements were in the range of 300 μm to 500 μm; therefore, the effect of such perspective error is minimal for force calculations and insignificant at the conceptual level. The key issue is, therefore, a well-known fact that the significance of a measuring technique and consequential errors must always be related to the results magnitude.Figure 1: Spatial relation between the plate and the pin shows that maximal perspective error is limited to less than 40 μm.Figure 3: Three-dimensional construction of the lenses created in 3 principal conditions (cycloplegic, normal, and cyclospasm). The change in curvature is so large that it supports the principle mechanism of a strong secondary lens formation regardless of the scanning angle.2. Schachar further argues that the changes in corneal shape as appeared in Figure 8 of the article indicate that our ultrasound biomicroscopy (UBM) measurements were not taken from the same accurate position and are therefore responsible for significant artifactitious results. This is just more of the same. Figure 2 of this reply shows an eye 1 week postoperative and the same eye 3 weeks later. The UBM shows the significant change in the corneal architecture but only a minor effect on the flexible lens under similar ciliary muscles stimulation. Our UBM measurements were taken in radial sections to enable computer-assisted 3-dimensional construction of the findings. Because of the monkey's face morphology, it was possible to manipulate the probe only over the central 1.0 mm of the lens. No sharp angulations were possible even when attempted in an effort to demonstrate the lens haptics. Figure 3 of this reply shows the computer-assisted 3-dimensional construction of the lenses from Figure 8 of our article. The magnitude of change in lens curvature is very large; therefore, any cross section through the lens center (vertical or oblique) still strongly supports the concept of operation of the lens, which is the main message of this report.Figure 2: A: Corneal edema 1 week postoperative. B: Same eye 4 weeks postoperative. Although significant changes are documented on the cornea, no effect could be noticed on the lens.The only parameter from the known accommodation mechanism that we use for our design is the unarguably accepted fact of ciliary muscle contraction during the accommodative process. Regardless of the primate crystalline lens accommodative mechanism in the adult eye, the product is few diopters only. Measuring error within such small range of performances might prove critical. However, the magnitude of optical power generated by our design in the eyes of primates is 10 times higher and more. Our concept was only preceded by millions of years of evolution that enables waterfowls' eyes to generate 50 to 70 diopters of accommodation by localized deformation of the center of the natural crystalline lens to a secondary small but powerful lens through the pupil (Figure 4). We therefore see no contradiction in concluding that on the principle level, our design provides wide range of accommodation. Meanwhile, ongoing research and development of this new concept is proceeding to enable better understanding and construction of an efficient accommodating intraocular lens that is safe for human use.Figure 4: A: A waterfowl eye during diving creating a secondary high-power lens. B: Image processing of A imitating UBM imaging of the same eye. C: A UBM image of Nulens intraocular lens in the monkey's eye shows similarity of the mechanism that creates the secondary lens of the bird.

Reversal of laser in situ keratomileusis–induced ectasia with intraocular pressure reduction

A 40 year-old woman had laser in situ keratomileusis for --7.75 --0.75 x 20 in the right eye. Preoperative examinations, including topography, pachymetry, and intraocular pressures (IOPs), were normal, and best spectacle-corrected visual acuity (BSCVA) was 20/20 in each eye. By 4 months postoperatively, the uncorrected visual acuity and BSCVA in the right eye had decreased to 20/40. Corneal topography of that eye was consistent with ectasia. One drop per day of timolol 0.5% (Timoptic XE) was prescribed. Five months postoperatively, the IOP had decreased and BSCVA and topography had improved. At 11 months, BSCVA returned to 20/20 and corneal topography normalized. Topographic difference maps were used to monitor corneal shape changes. In this case, early reduction in IOP completely reversed the ectasia.

Conservative Treatment of Early and Moderate Pellucid Marginal Degeneration: A new refractive approach with intracorneal rings

To evaluate the use of the intracorneal ring (ICR; Intacs, Addition Technology, Inc., Fremont, CA) for the treatment of pellucid marginal degeneration (PMD). Case series. Eight patients with PMD who were intolerant to contact lenses received ICR segments in the superior cornea (0.25 mm thickness) and in the inferior cornea (0.45 mm thickness). The diagnosis was supported by Orbscan II tomography (Bausch & Lomb). Preoperative and postoperative uncorrected visual acuity, best spectacle-corrected visual acuity (BSCVA), refractive astigmatism, and spherical equivalent were assessed. Changes in corneal shape were evaluated by corneal tomographic maps. The minimum follow-up was 12 months (range, 12-42 months; standard deviation [SD], 14.14). Uncorrected visual acuity improved in all 8 eyes (100%). At the last postoperative examination, 6 eyes (75%) had a BCVA > or =20/25; mean postoperative refractive astigmatism was -2.53 (range, -4.50 to -1.25; SD, 1.03). Mean postoperative spherical equivalent was -1.52 (range, -7.25 to +1.87; SD, 3.01). At the third postoperative month, refractive stability was obtained. No patients lost any Snellen lines of BCVA. No intraoperative and postoperative complications occurred. Intracorneal ring implantation is a promising new treatment for patients with early to moderate PMD and who are intolerant of contact lenses.

Corneal Topography in Corneal Refractive Therapy (CRT)

To evaluate visual outcome and corneal shape change during the first month of wearing corneal refractive therapy (CRT) reshaping contact lenses to correct myopia in otherwise healthy eyes. Twenty-nine eyes were fit with Paragon CRT contact lenses, which were worn nightly. Before and after CRT fitting, uncorrected visual acuity was measured, and corneal topography was performed and evaluated with the Holladay Diagnostic Summary EyeSys 2000 Videokeratoscope. To evaluate the visual outcome, refractive yield (achieved/attempted correction) was calculated and accounted for distance or monovision. Uncorrected visual acuity improved in all patients. In 1 month, refractive yield was 96.7%. Corneal uniformity index and predicted corneal acuity index were maintained, and the Q value changed from -0.08 to +0.38 (P < 0.05). CRT lenses are effective in improving visual acuity. They significantly changed the corneal from prolate to oblate while maintaining corneal quality.

Evaluation of intraocular pressure change after laser in situ keratomileusis using the pressure phosphene tonometer

Purpose: To evaluate the applicability of pressure phosphene tonometry in measuring intraocular pressure (IOP) after laser in situ keratomileusis (LASIK). Setting: Baptist Eye Clinic, Kyoto, Japan. Methods: Thirty-six eyes of 22 consecutive patients who had had LASIK for myopia were enrolled in this prospective comparative study. The mean age of the patients was 32.6 years ± 9.0 (SD). The IOP was measured using the pressure phosphene tonometer (FPT), Goldmann applanation tonometry (GAT), and noncontact tonometry (NCT). The patients themselves took measurements with the FPT before and 1 week after surgery. One-way analysis of variance followed by a Bonferroni/Dunn multiple comparison post hoc test was administered to compare the preoperative and postoperative measurements with each tonometer. These data were represented graphically using a method described by Bland and Altman. The relationship between each tonometric change and the central corneal thickness (CCT) or keratometry (K) changes after LASIK were evaluated with by Pearson correlation coefficient. Results: Statistically significant differences were found between preoperative and postoperative measurements with GAT and NCT but not with FPT. Preoperative and postoperative FPT readings conformed to the British Standard for reproducibility of a standard test method for IOP estimation. No significant correlation between FPT changes and CCT or K changes was found. Conclusions: Pressure phosphene tonometry measures IOP through the upper eyelid, and changes in corneal shape do not appear to have an effect on the IOP. The IOP measurements with FPT were constant before and after LASIK.

Analysis of changes in corneal shape and bulbus geometry after retinal detachment surgery.

To investigate changes in corneal shape and anterior segment following retinal detachment surgery. 25 consecutive patients with retinal detachment were enrolled in this study. Computer-assisted videokeratography was performed before and after retinal detachment surgery and changes in the anterior segments of postoperative patients were also noted. In the local buckling group, statistically significant changes were observed in the anterior chamber depth, lens thickness, and vitreous length (distance from posterior surface of lens to the retina) in the postoperative period. However, in the encircling group, statistically meaningful changes were observed in the vitreous length, axial length of the globe, and the central corneal curvature and thickness in the postoperative period. Corneal topography and axial measurements may be useful for evaluating the shape of the cornea after retinal detachment surgery. Because the resultant refractive changes are very important for the visual rehabilitation of the patients.

Increase in corneal asphericity after standard laser in situ keratomileusis for myopia is not inherent to the Munnerlyn algorithm.

Standard refractive surgery for myopia induces a shift in corneal asphericity toward positive values, resulting in an increase of spherical aberration. Analytical studies of changes in theoretical corneal shape after application of standard algorithms have yielded controversial conclusions. This study tries to resolve this controversy and discusses causes of optical degradation after refractive surgery. Computationally, we subtracted from real preoperative corneas the ablation depth given by the Munnerlyn equation and the parabolic approximation of the Munnerlyn equation. We compared the predicted postoperative corneal asphericity (and corneal spherical aberration) with real postoperative corneal asphericities of the same eyes, after laser in situ keratomileusis (LASIK). Corneal asphericity increased after LASIK in real eyes, with an increase proportional to the amount of correction. This increase was not predicted by the computational application of the Munnerlyn algorithm, which predicted a slight decrease of corneal asphericity. The parabolic approximation of the Munnerlyn algorithm produced an increase in corneal asphericity that correlated with the amount of correction, but was less than the clinical findings. Potential causes for increased asphericity (radial changes in laser efficiency, epithelial healing, and biomechanical response) are discussed. These conclusions are important for the design of optimized and customized ablation algorithms, since the theoretical performance of a given ablation algorithm (ie, Munnerlyn algorithm) can differ drastically from real outcomes.