Changes of posterior corneal astigmatism and tilt after myopic laser in situ keratomileusis

Spotlight on the Corneal Back Surface Astigmatism: A Review

Recent evidence indicates that the corneal back surface astigmatism (CBSA) contributes to the refractive state of the eye in cataract surgery, especially with the implantation of toric intraocular lenses. But this has been met with some scepticism. A review of key studies performed over the past three decades shows that the mean CBSA power ranges from 0.18(±0.16)D to 1.04(±0.20)D. The clinical assessment of CBSA is problematic. There is poor agreement between the current automated systems for assessment of CBSA and it is assumed that these systems directly measure the CBSA. But CBSA cannot be measured directly in vivo. A historical review of methods used to quantify the curvature of the posterior corneal surface reveals that CBSA estimated by current systems is based on values for corneal front surface astigmatism, corneal refractive index, central corneal thickness, corneal thickness at peripheral locations and the exact distance between the corneal apex and each one of these peripheral locations. Doubts and errors in these values, coupled with the precise details of the algorithm incorporated to estimate CBSA, are the likely sources of the lack of agreement between current systems. These systematic errors cloud the assessment of CBSA. Mean CBSA may be low, but it varies from case to case. There is a clear need for a realistic, practical procedure for clinicians to independently calibrate systems for estimating CBSA. This would help to reduce uncertainty and the discrepancies between instruments designed to measure the same parameter.

The influence of routine uncomplicated phacoemulsification on the orthogonality of the cornea

Purpose:

The aim of this study was to determine the effect of routine uncomplicated phacoemulsification on the orthogonal distribution of mass within the central optical zone of the cornea.

Methods:

Astigmatism at both corneal surfaces was evaluated using Orbscan II (Bausch &and Lomb) before and up to 3 months after routine phacoemulsification (one eye/patient). The data were subjected to vector analysis to estimate the pre-and postoperative total astigmatism of the cornea (TCA).

Results:

Reporting the chief findings in minus cylinder (diopters, DC) over the central 3 mm (A) and 5 mm (B) optical zones. Mean TCA powers (±sd) at pre- and 3-months postop were A) –4.45DC (±2.00) and –5.69DC (±2.69), B) –2.91DC (±2.22) and –2.71DC (±1.60). Change in mean power was significant over 3 mm (P < 0.01, n = 49) but not over 5 mm. Inter-zonal differences were significant (P < 0.01). There was a significant linear relationship between the change in TCA power (y = preoperative-postoperative) and TCA at preoperative stage (x) where, A) y = 0.45x + 3.12 (r = 0.336, n = 49, P = 0.018), B) y = x + 2.65 (r = 0.753, n = 49, P = <0.01). Over the central 3 mm zone only, change (preoperative-postoperative) in axis (°) of TCA (y₁) was significantly associated with TCA axis at preoperative stage (x₁) where y₁ = 1.391x₁-0.008x₁²-0.701 (r = 0.635, n = 49, P < 0.01).

Conclusion:

Changes in TCA power and axis at 3 months postop, determined using Orbscan II, are indicative of orthogonal alterations in the distribution of corneal tissue. Over the central 3 mm zone, the association between y₁ and x₁ shows that a change in TCA axis is more profound when preoperative axis is near 90° i.e., against-the-rule.

Predicted and Measured Changes in Posterior Corneal Astigmatism after Uncomplicated Femtosecond Assisted LASIK (FsLASIK) and Microkeratome LASIK Correction for Myopia and Low Astigmatism

Purpose: To compare predicted and measured changes in astigmatism at the posterior corneal surface (PCS) after FsLASIK or LASIK.Methods: Astigmatism was measured at both corneal surfaces (Pentacam^TM) before and 3 months after unremarkable FFsLASIK (roup 1,n = 100) or LLASIK (roup 2,n = 100) for myopia (-7.25DS to -0.75DS) and low astigmatism (≤1.00DC). Photoablation was achieved using Schwind Amaris750S^TM laser (Aberration Free profile, centered on corneal vertex). Pre-and postop astigmatic data, according to subjective refraction and estimates for the corneal surfaces (over the central 3.2 mm zone), were subjected to vector analysis to calculate surgically induced astigmatism (SIA) by refraction (SIA_R), at the anterior (SIA_Fact) and posterior corneal surfaces (SIA_Bact). The difference vector between SIA_R and SIA_Fact was regarded as the predicted SIA at the PCS (SIA_Best).Results: Reporting key findings. Mean(±sd,95%CI) SIA_Best and SIA_Bact powers in group 1 were -0.52DC(±0.35,-0.56 to -0.45) and -0.11DC(±0.08,-0.13 to -0.10) in group 1, -0.35DC(0.20,-0.39 to -0.32) and -0.08DC(0.07,-0.09 to -0.06) in group 2. Differences between SIA_Best and SIA_Bact were significant for powers but not axes. Significant correlations(p < .01) were revealed between (I) SIA_R and SIA_Fact powers [Group 1, SIA_R = 0.370.SIA_Fact-0.292,r = 0.299. Group 2, SIA_R = 0.484.SIA_Fact-0.394,r = 0.519] but not the axes and (II) ΔC (difference between pre-[x1] and postop measured PCS astigmatic powers) and x1 [Group 1, ΔC = 0.384x1 + 0.119,r = 0.423. Group 2, ΔC = 0.135x1 + 0.047,r = 0.229,p = .022]. There was no correlation between SIA_Best and SIA_Bact powers or axes.Conclusion: The changes in posterior corneal astigmatic powers according to Pentacam measurements are small and do not account for the deficit between SIA_R and SIA_Fact after FsLASIK or LASIK.

Effect of timolol maleate (0.5%) in the management of myopic regression post laser-assisted in-situ keratomileusis: Clinical and topographical outcomes

Purpose

The aim of this study was to analyze the effect of timolol maleate (0.5%) eye drops in the treatment of myopic regression after laser-assisted in-situ keratomileusis (LASIK).

Methods

The study was conducted at a tertiary care eye hospital in north India between April 2017 & March 2018 as a prospective interventional study. Patients who underwent uneventful myopic LASIK with hansatome mechanical keratome and presented with regression were included in the study. Baseline demographic characteristics, time to presentation with regression best-corrected visual acuity (BCVA), refraction, intraocular pressure, central corneal thickness and keratometry were recorded at baseline and at each follow-up visit. The enrolled patients were prescribed timolol maleate (0.5%) eyedrops twice daily. They were followed up every month till 3 months on timolol maleate (0.5%) eyedrops and at 6 months post stopping the treatment.

Results

Twenty-nine eyes of 15 patients were enrolled in the study. Mean pre LASIK spherical equivalent (SE) was - 7.48 ± 2.9 Diopters (Range-3.125 to -11.75 Diopters) and mean regression spherical equivalent was -1.02 ± 1.1 Diopters. There was a decrease in mean SE from presentation (intervention start point) up to 6 months follow-up (-1.34 ± 0.89 to -0.30 ± 0.29 Diopters). While posterior corneal curvature (K1 and K2 Back) changed significantly over treatment period (P = 0.0029, P = 0.0024 respectively), changes in anterior corneal curvature (K1 and K2 Front) were not significant (P = 0.05, P = 0.06 respectively). Central corneal thickness (CCT) and intraocular pressure (IOP) did not change significantly over treatment course.

Conclusion

Timolol maleate (0.5%) eyedrop is an effective modality for the treatment of refractive regression post LASIK circumventing the need for laser re-treatment in such patients. The most probable mechanism is reversal of the anterior bowing of the cornea in response to intraocular pressure changes.

Intraocular Lens Power Calculation in Eyes After Laser In Situ Keratomileusis or Photorefractive Keratectomy for Myopia

Intraocular power calculation is challenging for patients who have previously undergone corneal refractive surgery. The sources of prediction errors for these eyes are well known; however, the numerous formulas and methods available for calculating intraocular lens power in these cases are eloquent testimony to the absence of a definitive solution. This review discusses some of the available methods for improving the accuracy for predicting the refractive outcome for these patients. It focuses mainly on the methods available on the American Society of Cataract and Refractive Surgery (ASCRS) online calculator and provides some practical guidelines for cataract surgeons who encounter these challenging cases.

The Enigmatic Cornea and Intraocular Lens Calculations: The LXXIII Edward Jackson Memorial Lecture

PURPOSE

To review the progress and challenges in obtaining accurate corneal power measurements for intraocular lens (IOL) calculations.

DESIGN

Personal perspective, review of literature, case presentations, and personal data.

METHODS

Through literature review findings, case presentations, and data from the author's center, the types of corneal measurement errors that can occur in IOL calculation are categorized and described, along with discussion of future options to improve accuracy.

RESULTS

Advances in IOL calculation technology and formulas have greatly increased the accuracy of IOL calculations. Recent reports suggest that over 90% of normal eyes implanted with IOLs may achieve accuracy to within 0.5 diopter (D) of the refractive target. Though errors in estimation of corneal power can cause IOL calculation errors in eyes with normal corneas, greater difficulties in measuring corneal power are encountered in eyes with diseased, scarred, and postsurgical corneas. For these corneas, problematic issues are quantifying anterior corneal power and measuring posterior corneal power and astigmatism. Results in these eyes are improving, but 2 examples illustrate current limitations: (1) spherical accuracy within 0.5 D is achieved in only 70% of eyes with post-refractive surgery corneas, and (2) astigmatism accuracy within 0.5 D is achieved in only 80% of eyes implanted with toric IOLs.

CONCLUSION

Corneal power measurements are a major source of error in IOL calculations. New corneal imaging technology and IOL calculation formulas have improved outcomes and hold the promise of ongoing progress.

Scheimpflug imaged corneal changes on anterior and posterior surfaces after collagen cross-linking

AIM

To compare the anterior and posterior corneal parameters before and after collagen cross-linking therapy for keratoconus.

METHODS

Collagen cross-linking was performed in 31 eyes of 31 keratoconus patients (mean age 30.6±8.9y). Prior to treatment and an average 7mo after therapy, Scheimpflug analysis was performed using Pentacam HR. In addition to corneal thickness assessments, corneal radius, elevation, and aberrometric measurements were performed both on anterior and posterior corneal surfaces. Data obtained before and after surgery were statistically analyzed.

RESULTS

In terms of horizontal and vertical corneal radius, and central corneal thickness no deviations were observed an average 7mo after operation. Corneal higher order aberration showed no difference neither on anterior nor on posterior corneal surfaces. During follow-up period, no significant deviation was detected regarding elevation values obtained by measurement in mm units between the 3.0-8.0 mm-zones.

CONCLUSION

Corneal stabilization could be observed in terms of anterior and posterior corneal surfaces, elevation and higher order aberration values 7mo after collagen cross-linking therapy for keratoconus.

Accuracy of corneal astigmatism estimation by neglecting the posterior corneal surface measurement

PURPOSE

To evaluate the accuracy of corneal astigmatism estimation by neglecting the posterior corneal surface measurement.

DESIGN

Prospective, observational study.

METHODS

The right eyes of 493 subjects were measured with a rotating Scheimpflug camera (Pentacam; Oculus, Wetzlar, Germany). The keratometric corneal astigmatism (KA) was obtained by using the anterior corneal surface measurement and the keratometric index (1.3375) while neglecting the posterior corneal surface measurement. The Pentacam-derived total corneal astigmatism (PA) was derived by doubled-angle vector analysis of the astigmatisms on both corneal surfaces.

RESULTS

The mean arithmetic and absolute estimation errors of the KA magnitude for the PA magnitude were -0.06 +/- 0.28 diopters (D) (range, -0.59 to 0.91 D) and 0.24 +/- 0.16 D (range, 0 to 0.91 D), respectively. The mean arithmetic and absolute estimation errors of the KA angle for the PA angle were -0.6 degrees +/- 12.7 degrees (range, -69.9 degrees to 83.4 degrees) and 7.4 degrees +/- 10.3 degrees (range, 0 degrees to 83.4 degrees), respectively. Among all eyes, 142 eyes (28.8%) had either a KA magnitude that differed by > 0.50 D from the PA magnitude or a KA angle that differed by > 10 degrees from the PA angle. For the 282 eyes with a KA magnitude exceeding 1.0 D (that are candidates for intraoperative correction of a preexisting astigmatism during cataract surgery), 29 eyes (10.3%) had either a KA magnitude that differed by > 0.50 D from the PA magnitude or a KA angle that differed by > 10 degrees from the PA angle.

CONCLUSIONS

Neglecting the posterior corneal surface measurement may lead to significant deviation in the corneal astigmatism estimation in a proportion of eyes.

Repeatability and Reproducibility of Posterior Corneal Curvature Measurements by Combined Scanning-Slit and Placido-Disc Topography after LASIK

OBJECTIVE

To assess the repeatability and reproducibility of posterior corneal curvature (PCC) measurements made by combined scanning-slit/Placido-disc topography (Orbscan II) after LASIK.

DESIGN

Experimental instrument validation study.

PARTICIPANTS

We recruited 22 consecutive postmyopic LASIK patients for the repeatability study and another 50 consecutive postmyopic LASIK patients for the reproducibility study.

METHODS

To analyze intrasession repeatability, 1 examiner measured 22 postmyopic LASIK eyes 10 times successively in the shortest time possible. To study intersession reproducibility, the same operator obtained measurements from another 50 eyes with stable refraction in 2 consecutive visits at the same time of the day between 6 and 9 months after myopic LASIK. We explored any association between residual stromal bed thickness and measurement variability.

MAIN OUTCOME MEASURES

Orbscan II scanning-slit PCC data, precision, within-subject coefficient of variation (CV(w)), limits of agreement (LoA), and intraclass correlation coefficient (ICC).

RESULTS

For intrasession repeatability, precision was 0.067 mm (best-fit sphere [BFS]), 0.110 diopters (D; power within 5 mm), 0.158 D (power within 3 mm), and 0.46 (eccentricity). Repeatability was high for PCC BFS and power measurements within 3-mm and 5-mm zones (CV(w) ranged from 0.5%-1.2%) but poor for eccentricity data (CV(w), 31.6%). Correspondingly, ICCs ranged from 0.89 to 0.98 for PCC BFS and power, and the ICC was 0.20 for PCC eccentricity values. For intersession reproducibility, on average, no difference in PCC measurements could be found, indicating that when there is variability, it is due to random factors. The width of the 95% LoA between sessions was clinically acceptable for BFS (0.25 mm) and power (0.4 D [within 5 mm] and 0.6 D [within 3 mm]). Similarly, ICCs indicated good intersession reliability for BFS and power (0.98, 0.96, and 0.85 for BFS, power within 5 mm, and power within 3 mm, respectively) but poor reliability for eccentricity (0.59). Repeatability and reproducibility were unrelated to stromal bed thickness.

CONCLUSIONS

Orbscan II provides reliable post-LASIK PCC data for symmetrical parameters (BFS and power), independent of the residual stromal bed thickness, but is unreliable for measurements that are radially asymmetrical (eccentricity). Orbscan II is useful for monitoring the PCC after LASIK once the early postoperative period is over.

Posterior Corneal Curvature Changes after Undersurface Ablation of the Flap and In-the-Bed LASIK Retreatment

OBJECTIVE

To analyze LASIK retreatment-induced changes in the posterior corneal curvature (PCC) with undersurface ablation of the flap (UAF) and in-the-bed techniques.

DESIGN

Nonrandomized, comparative, interventional study.

PARTICIPANTS

Forty-six eyes with a residual spherical equivalent refraction between -0.37 and -2.75 diopters (D) and astigmatism between 0.0 and -1.25 D were included prospectively. In 23 eyes, the calculated postenhancement flap thickness exceeded 150 microm using micropachymetric optical coherence tomography, whereas with further ablation of the bed, the residual bed thickness (RBT) would have been <250 microm, or <55% of the pre-LASIK central pachymetry. In another 23 eyes, RBT allowed the planned ablation for a calculated post-retreatment RBT exceeding 250 microm, >55% of the pre-LASIK central pachymetry.

INTERVENTION

Eyes with insufficient RBT for further ablation underwent UAF retreatment, whereas those with adequate RBT received conventional in-the-bed LASIK retreatment. Examinations were performed before retreatment and 3 and 6 months postoperatively. No eye was lost to follow-up.

MAIN OUTCOME MEASURES

Micropachymetry, Orbscan II scanning-slit PCC data, and visual acuity (VA).

RESULTS

The groups did not differ in age, intraocular pressure, or retreatment ablation depth, but the UAF eyes had a lower mean pre-retreatment RBT (270.7+/-25.4 microm) than conventional enhancement eyes (353.0+/-41.5 microm) (P = 0.001). Eyes undergoing UAF had no significant change in PCC, whereas eyes undergoing conventional retreatment had an increase in the posterior corneal power within the central 3-mm zone (P = 0.008) 3 months after retreatment. No significant changes occurred thereafter. The amount of change in posterior corneal power within the 3-mm central zone from before to after retreatment differed significantly between the groups (mean difference, 0.135 D; 95% confidence interval, 0.022-0.248 D; P = 0.02). No keratectasia developed clinically, and no retreated eye lost or gained > or =2 lines of best-corrected VA. Six months after retreatment, the efficacy and safety indices for the UAF procedure were 0.96 and 1.01, respectively, and 1 and 1.06 for conventional LASIK enhancement.

CONCLUSION

Undersurface ablation of the flap retreatment appears to have less potential for changing the posterior corneal surface than conventional LASIK enhancement and can help reduce the likelihood of retreatment-induced keratectasia.

Topographic screening of donor eyes for previous refractive surgery

PURPOSE

To determine whether donor eyes had previous refractive surgery using Orbscan (Bausch & Lomb Surgical) corneal topography.

SETTING

Lions Eye Bank of Oregon, Portland, Oregon, USA, and Maisonneuve-Rosemont, Hospital, Montreal, Quebec, Canada.

METHODS

Orbscan corneal topographies of 50 donor eyes from the Lions Eye Bank of Oregon were obtained; 10 eyes had previous refractive surgery (6 laser in situ keratomileusis, 2 photorefractive keratectomy, 2 radial keratotomy) to correct myopia, and 40 had not had surgery. Algorithms based on corneal anterior and posterior elevations and anterior tangential curvature were developed: The difference in curvature (DC) was based on the difference in the mean anterior tangential curvature between central and midperipheral areas; difference in elevation (DE) represented the difference between the anterior and posterior central elevations. Receiver-operating characteristic (ROC) curves for each algorithm were obtained, and sensitivity values at fixed specificities were calculated.

RESULTS

The mean area under the ROC curve, which corresponds to the probability of correctly identifying the presence of a previous refractive surgery, was 0.853 +/- 0.079 (SE) for DC and 0.933 +/- 0.057 for DE. The DC algorithm resulted in a sensitivity of 80% for a specificity of 87.5%, and DE yielded a sensitivity of 90% for a specificity of 92.5%. There was a strong correlation between the value of the DE and DC algorithms and the amount of previous refractive surgery (DC: r = 0.84, P = .008; DE: r = 0.76, P = .028).

CONCLUSION

The results led to a proposed criteria-based system using Orbscan corneal topography to screen eye-bank eyes for previous refractive surgery.

Response of the posterior corneal surface to laser in situ keratomileusis for myopia

PURPOSE

To describe the response of the posterior corneal surface in laser in situ keratomileusis (LASIK) and determine whether residual stromal bed thickness or treatment magnitude is predictive of the posterior corneal surface elevation after uneventful LASIK.

SETTING

A private hospital-based refractive surgery practice, Hong Kong SAR, China.

METHODS

Orbscan I (Bausch & Lomb) videokeratography examinations were performed on 1124 patients before and 6 months after LASIK for myopia (mean -6.81 diopters [D] +/- 2.52 [SD]; range -0.88 to -14.50 D). The best-fit sphere (BFS) over the central 9.0 mm region of the posterior corneal surface before and after treatment was compared. The location and magnitude of the 1.0 mm diameter region of highest elevation above the BFS for the central 4.0 mm diameter zone were calculated before and after treatment and compared using a paired t test. Stepwise regression was used to model the best predictors of the posterior radius of the BFS and the central elevation of the corneal surface above the BFS before and after treatment.

RESULTS

The mean radius of curvature of the posterior surface BFS decreased 0.10 mm after LASIK, from 6.31 to 6.21 mm (P<.001). Elevation above this BFS was increased 10 microm within a 1.00 mm diameter region of interest, and this was correlated with postoperative corneal thickness, inferotemporal decentration of the highest point, residual myopia, and steeper central posterior radius of curvature.

CONCLUSIONS

No eye was diagnosed with corneal ectasia at the time of the 6-month postoperative visit. After LASIK, there was a decreased radius of curvature for the BFS of the posterior corneal surface, with the highest elevation point located paracentrally. These findings are similar to the anterior corneal surface changes observed in corneal ectasia after LASIK but smaller in magnitude.

The Theoretical vs. Measured Laser Resection for Laser in situ Keratomileusis

PURPOSE

To measure the excimer laser resection during and after laser in situ keratomileusis (LASIK) and to determine the rate of stromal ablation and factors predicting its measurement.

METHODS

A retrospective, comparative, interventional case study of 6010 eyes undergoing LASIK surgery was performed. In vivo ultrasonic pachymetry was performed as a measure of the laser resection (1'MLR). The theoretical laser resection generated by one of three separate excimer lasers (Summit Apex Plus, Alcon LadarVision, VISX) was recorded. The change in central comeal thickness measured at enhancement surgery (2'MLR) and the change in the residual stromal thickness prior to the laser ablation at the primary and enhancement procedure (3'MLR) were recorded as separate measures of the laser resection. Paired sample t test and regression analysis was performed to determine the relationships and to develop a model predictive of laser resection for each laser.

RESULTS

For all lasers, 1'MLR produced the highest estimate compared to 3'MLR. Laser-induced stromal desiccation, microkeratome effects, and change in measured flap thickness were the most likely causes for the differences. Laser resection was overestimated in the Alcon theoretical laser resection group, but underestimated in the Summit and VISX groups. The difference between 2'MLR and 3'MLR was due to epithelial hyperplasia, which measured 8.3, 17.8, and 10.8 microm in the Alcon, Summit, and VISX enhancement groups, respectively. Stromal ablation rates were 11.79, 8.26, and 12.71 microm per spherical equivalent diopter laser setting for the Alcon, Summit, and VISX lasers, respectively, when measured with 3'MLR. Multiple pre- and perioperative factors were associated with laser resection.

CONCLUSIONS

To accurately predict residual corneal thickness, the expected laser resection for an attempted refractive change must be known. As the laser resection can be laser-specific, the surgeon needs to establish the mean and range of tissue removal for a given attempted correction.

Orbscan computerized topography: Attributes, applications, and limitations

An extensive electronic search was undertaken in January 2004 to identify all relevant peer-reviewed publications on Orbscan slit-scanning/Placido computerized topography. Ninety-one publications were identified. These address elevation topography and best-fit sphere, accuracy and repeatability of anterior and posterior corneal elevation and keratometric maps, comparison of Orbscan-acquired data and Placido-based computerized videokeratography instruments, pachymetry measurement and correlation with ultrasound, screening eye-bank corneas, detection of keratoconus, identifying corneal ectasia after refractive surgery, and miscellaneous applications. Studies were analyzed and critically compared in relation to attributes, applications, and limitations of Orbscan corneal topography. The review highlights advantages of this technique in assessing the cornea in health and disease and after surgery and identifies specific aspects that require further investigation and clarification.

Differences between Regressive Eyes and Non-Regressive Eyes after LASIK for Myopia in the Time Course of Corneal Changes Assessed with the Orbscan

The aim of this study was to compare regressive eyes with non-regressive eyes after laser in situ keratomileusis (LASIK) for myopia with regard to the time course of biomechanical changes of the cornea and to evaluate the possible roles of these changes in refractive regression following surgery. 15 eyes of 9 patients with refractive regression and 15 eyes of 11 patients with no refractive regression after LASIK for myopia were enrolled in this study. The time courses of central corneal thickness (CCT) and anterior/posterior corneal curvature radiuses (CCR) were measured with Orbscan slit scanning before and 1 week, 1 month and 3 months after LASIK for myopia. The regressive and non-regressive eyes had similar time courses of CCT before and after LASIK, whereas the 2 groups differed in the time courses of corneal shifting movements. In the regression group, the posterior CCR after LASIK was relatively stable (all p > 0.05). By contrast, in the non-regression group, the postoperative posterior CCR at week 1 had a significantly steeper curvature than that at month 3 (p = 0.000). The anterior CCR in the regression group at week 1 had a significantly flatter curvature than that at month 3 (p = 0.002) postoperatively. In contrast, there was no significant change of anterior CCR postoperatively (all p > 0.05) in the non-regression group. Taken together, these data suggest that refractive regression after LASIK might be mainly induced by corneal protrusion rather than central corneal thickening.

Comparison of Four Corneal Pachymetry Techniques in Corneal Refractive Surgery

PURPOSE

To assess the effectiveness of optical and ultrasonic pachymetry when examining corneas of patients before and after laser in situ keratomileusis (LASIK).

METHODS

We conducted a prospective study of 25 patients (50 eyes) who had LASIK for myopia or myopic astigmatism (mean spherical equivalent refraction -4.80 +/- 3.60 D; range -1.75 to -12.00 D). Corneal thickness was measured using 1) conventional ultrasonic pachymetry (20 MHz probe) in the central cornea, 2) ASL confocal microscopy (CMTF analysis), 3) Orbscan II Analyzer, and 4) ultrasound biomicroscopy (UBM) (50 MHz probe) preoperatively and 3 months postoperatively.

RESULTS

Preoperative pachymetry values measured with the Orbscan II Analyzer (551.3 +/- 27.3 microm) were almost identical to those measured with the 20 MHz ultrasonic probe (553.7 +/- 25.7 microm; t-test P = .652). The mean preoperative pachymetry for UBM was 566.7 +/- 38.8 microm and for CMTF was 553.2 +/- 15.5 microm. The optic and ultrasonic system measurements differed in operated eyes; 20 MHz ultrasonic pachymetry yielded an average 478.7 +/- 23.5 microm and the UBM, 482.4 +/- 40.3 microm. These measurements differed from the Orbscan II (Tukey test; P = .023) measurement of 461.1 +/- 37.5 microm but not from the confocal microscopy measurement of 475.2 +/- 41.8 microm. UBM registered greater values that were more variable than those obtained with ultrasonic pachymetry.

CONCLUSIONS

In non-operated eyes, corneal thickness was similar using conventional ultrasound, Orbscan II, or confocal techniques, with ultrasound biomicroscopy giving thicker readings.

Ectasia after laser in situ keratomileusis

Eighty-five cases of post laser in situ keratomileusis ectasia were reviewed and analyzed. Cases of keratoconus or forme fruste keratoconus were eliminated; many remaining case reports lacked key information. The current literature is unable to define a specific residual corneal thickness or a range of preoperative corneal thickness that would put an eye at risk for developing ectasia. The most logical cause for eyes without preexisting pathology to develop ectasia is a postablation stromal thickness that is mechanically unstable; this "minimal" thickness is probably specific to each eye. The preoperative and postoperative corneal thickness, measured flap thickness, and microkeratome and laser parameters used in a given case are required to determine the range of residual corneal thickness that puts the eye at risk for developing ectasia. Other as yet undetermined factors may play a role in the development of this complication.

Penetrating keratoplasty for iatrogenic keratoconus after repeat myopic laser in situ keratomileusis: Histologic findings and literature review

We report a patient with a sufficiently thick cornea (593 microm) and no topographic signs of keratoconus preoperatively who developed iatrogenic keratoconus 2 months after repeat laser in situ keratomileusis (-4.00 -1.00 x 20) performed 5 months after the primary procedure (-10.50 -1.00 x 55). After penetrating keratoplasty, macrophotography showed severe multidirectional "macrostriae" of the stromal bed. On histologic evaluation, excessive thinning of the residual stromal bed to a minimum of 75 microm in the valleys and a maximum of 200 microm at the peaks of the macrostriae were documented. The flap thickness was 225 microm in the center. The thicker-than-intended flap (160 microm) is thought to be the cause of the severe complication of the LASIK procedure.