Posterior Corneal Topographic Changes after Retreatment LASIK Alka Rani, MD,1 Balasubramanya R. Murthy, MD,1 Namrata Sharma, MD,1 Jeewan S. Titiyal, MD,1 Rasik B. Vajpayee, MBBS, MS,1 Ravindra M. Pandey, PhD,2 Rajvir Singh, MSc2 Purpose: To evaluate posterior corneal topographic changes after retreatment after myopic laser in situ keratomileusis (repeat LASIK). Design: Retrospective nonrandomized comparative self-controlled trial. Participants: Thirty-three eyes of 23 patients who underwent repeat LASIK for residual myopia. Intervention: Retreatments were performed. Slit-scanning corneal topography was performed before and at 1, 3, and 6 months after repeat LASIK. Main Outcome Measures: Posterior corneal topographic changes before and after repeat LASIK were correlated with central corneal pachymetry (preoperatively, before repeat LASIK, after repeat LASIK), residual bed thickness (RBT) and ablation depth (after primary laser in situ keratomileusis (LASIK) and repeat LASIK). On the basis of the amount of posterior corneal elevation after repeat LASIK, the eyes were divided into two groups: group 1 (⬎66 m) and group 2 (ⱕ66 m). Student’s t test/Mann–Whitney rank-sum test was used to determine the significant difference of mean level of each variable between the two groups. Results: After primary LASIK, an increase in posterior corneal elevation had significant positive correlation with attempted correction (P ⫽ 0.02), ablation depth (P ⫽ 0.008), and significant negative correlation with preoperative central pachymetry (P ⫽ 0.0003), RBT (P ⫽ 0.0003), and postoperative central pachymetry (P ⫽ 0.00008). After repeat LASIK, the mean increase in posterior corneal elevation had significant negative correlation with preoperative central pachymetry (P ⫽ 0.03). However, its correlation with the ablation depth (P ⫽ 0.43) during repeat LASIK and RBT after repeat LASIK (P ⫽ 0.11) was statistically insignificant. On multiple linear regression analysis, the attempted correction (P ⬍ 0.01) and RBT after primary LASIK (P ⬍ 0.001) were two independent significant determinants of an increase in posterior corneal elevation after primary LASIK. However, for increase in posterior corneal elevation after repeat LASIK, preoperative central pachymetry (P ⬍ 0.01) and posterior corneal elevation increase after primary LASIK (P ⬍ 0.05) were the two significant determinants. Compared with group 2, group 1 had significantly high values of posterior corneal elevation both after primary LASIK (P ⫽ 0.0037) and after repeat LASIK (P ⫽ 0.0000). This group also had significantly low values of central pachymetry preoperatively (P ⫽ 0.0003) and after primary LASIK (P ⫽ 0.0001) and repeat LASIK (P ⫽ 0.0001) surgeries. The mean RBT after primary LASIK (P ⫽ 0.0006) and after repeat LASIK (P ⫽ 0.001) was also lower in group 1. Conclusions: Posterior corneal elevation increases after repeat LASIK. Eyes with an increase in posterior corneal elevation after primary LASIK and with thinner cornea are more predisposed. Ophthalmology 2002;109: 1991–1995 © 2002 by the American Academy of Ophthalmology, Inc.
Posterior corneal topography after laser in situ keratomileusis (LASIK) is a better index for evaluation of anteroposterior movement of the cornea.1 There has been a growing concern regarding the factors that affect the posterior corneal surface after LASIK.1–3 The posterior corneal elevation Originally received: October 8, 2001. Accepted: March 22, 2002. Manuscript no. 210849. 1 Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi, India. 2 Department of Biostatistics, All India Institute of Medical Sciences, New Delhi, India. The authors have no proprietary interest in the development or marketing of this or any competing instrument or piece of equipment. Reprint requests to Rasik B. Vajpayee, MBBS, MS, Rajendra Prasad Centre for Ophthalmic Sciences, All India Institute of Medical Sciences, New Delhi 110029 India. E-mail: [email protected]
© 2002 by the American Academy of Ophthalmology, Inc. Published by Elsevier Science Inc.
after LASIK may be attributed to the amount of ablation or residual bed thickness (RBT).1–3 Most of these studies show that the progressive biomechanical deformation of the cornea is greater when the RBT is less than 250 m.1,3 This retrospective case series study was undertaken to evaluate the posterior corneal topographic changes after retreatment after LASIK (repeat LASIK) and to evaluate the factors relevant to this phenomenon.
Subjects and Methods Thirty-three eyes of 23 consecutive subjects (10 bilateral and 13 unilateral), who underwent repeat LASIK for residual myopia from December 1998 to December 2000, were included in the study and were evaluated on a chart review retrospectively. In bilateral cases (10 of 23, 43.5%) both eyes of each patient were studied and analyzed separately. All primary and repeat LASIK ISSN 0161-6420/02/$–see front matter PII S0161-6420(02)01238-1
Ophthalmology Volume 109, Number 11, November 2002 surgeries were performed by experienced surgeons at the Rajendra Prasad Centre for Ophthalmic Sciences, New Delhi, a tertiary eye care center. There were 11 males and 12 females, and their age ranged from 18 to 31 years (23 ⫾ 3.7 years, mean ⫾ standard deviation). The preoperative spherical equivalent refraction (SEQ) before primary LASIK was ⫺9.15 ⫾ 3.5 diopters (D) (range,⫺2.37 to ⫺15.0 D). Before surgery, no eyes showed topographic evidence of keratoconus or suspected keratoconus. Inclusion criteria for primary LASIK at our center were age ⬎18 years, stable refraction for at least 1 year, and a best-corrected visual acuity (BCVA) of 6/9 or better. Patients with astigmatism of more than 1 D, keratoconus, active ocular inflammatory disease, ocular surface disease, and previous ocular surgery were excluded. Patients with hard contact lenses discontinued their lenses for 4 weeks and those with soft contact lenses for 2 weeks before LASIK surgery. Patient selection criteria for LASIK retreatment were residual SEQ equal to or greater than ⫺0.75 D at 3 months after primary LASIK procedures, normal anterior segment, and normal peripheral retina. In addition, the cornea had to be thick enough so that the remaining total corneal thickness after retreatment was at least 380 m (200 m of RBT). Informed written consent was obtained from all the patients after they received a detailed description of repeat LASIK-induced complications, including increased risk of posterior corneal elevation. In the eyes treated at our center up to December 2000 (also the eyes included in this study), calculations were performed to have a minimum RBT of 200 m. However, as a part of our new protocol, a minimum RBT of 250 m is followed. The preoperative SEQ refraction before the repeat LASIK surgery was ⫺0.87 to ⫺6.50 D (⫺2.5 ⫾ 1.5 D), and pachymetry was 400 to 496 m (442.06 ⫾ 25.4 m). Before and after retreatment, patients underwent a comprehensive ocular examination that included slit-lamp biomicroscopy, uncorrected visual acuity (UCVA), and BCVA using Snellen’s visual acuity charts, manifest and cycloplegic refraction (under 1% tropicamide), direct and indirect ophthalmoscopy, anterior and posterior corneal topography, and pachymetry with Orbscan slitscanning corneal topography/pachymetry system (Orbtek Inc, Salt Lake City, UT). In the Orbscan examination, 40 calibrated beams were projected onto the eyes at a 45° angle, 20 slits to the left and 20 slits to the right. Eye-tracking software was used to reduce the data error resulting from eye movement. The elevation of the anterior and posterior corneal surfaces against their best-fit spheres (BFS) was calculated and color coded. Topographic change in the posterior corneal surface was determined by comparing the preoperative and postoperative corneal surface elevation maps, and the highest reading of the relative elevation was noted in the 6-mm central zone. Preoperatively, 2 drops of 0.3% ciprofloxacin and 0.5% proparacaine hydrochloride were instilled before surgery 15 minutes apart. All primary LASIK procedures were performed with the patient under topical anesthesia (0.5% proparacaine) using the Chiron Technolas 217C (Chiron Technolas GmBh, Dornach, Germany) LASIK machine. Simultaneous surgery was done in both eyes, right followed by left. A superior hinged flap was raised with a Hansatome microkeratome (Chiron Vision, Munchen, Germany) using 180 or 160 m microkeratome heads, so that the RBT was ⬎ 200 m. One blade was used for each patient (i.e., one blade was used for two eyes of the same patient). The intraoperative evaluation included measurement of the flap thickness using the Corneo-Gage Plus (Sonogage Inc, Cleveland, OH) ultrasonic pachymeter. The central corneal thickness was measured just before initiating the surgery and then after lifting the stromal flap before laser ablation. The mean optic zone diameter was 5.36 ⫾ 0.44 mm. After stromal ablation, the posterior surface of the flap was irrigated with balanced salt solution (BSS: Alcon, Forth
Worth, TX), and the flap was repositioned. An adherence time of 3 minutes was observed. One drop of 0.3% ciprofloxacin eyedrops was instilled at the completion of the procedure. Postoperatively, TobraDex eyedrops (0.3% tobramycin, 0.1% dexamethasone sodium phosphate, 0.01% benzalkonium chloride) were administered four times a day for 2 weeks and artificial tear supplements for 4 weeks. Patients were asked to follow-up at 1 month, 3 months, 6 months, and 1 year after LASIK or frequently, if required. Letters were written to the patients who did not return for the follow-up, which was as per routine protocol followed at our center. Each follow-up examination included slit-lamp biomicroscopy, UCVA, BCVA, manifest and cycloplegic refraction (under 1% tropicamide), anterior and posterior corneal topography, and pachymetry using Orbscan. The mean interval between primary LASIK and repeat LASIK was 6.48 ⫾ 2.6 months (range, 3–11 months). Retreatment was performed by relifting the flap and ablating the stromal bed. No enhancements were performed within 6 weeks after primary LASIK. Before surgery, 0.5% proparacaine eyedrops were instilled. A universal eye speculum was applied to separate the lids. The edge of the previous flap was identified on slit-lamp examination, and a Sinskey hook was used to separate the flap under sterile conditions. The patient was then taken to the operating room. After draping, peripheral corneal marking was performed with gentian violet. A flat spatula was used to dissect the flap after lifting the flap with McPherson forceps. The spatula was passed along the flap edge circumferentially, and the interface was dissected. The superior hinged flap was lifted gently with McPherson forceps and placed against the sclera. The stromal bed was then reablated using a Chiron Technolas 217C (Chiron Technolas GmBh, Dornach, Germany) excimer machine. There is no specific retreatment nomogram in the software of this machine. In this series, no second microkeratome cut was necessary, because all flaps could be relifted. Single-zone ablation 5.47 ⫾ 0.49 mm in diameter (range, 5.0 – 6.0 mm) was used depending on the intended correction and the preoperative central corneal thickness. The mean attempted central ablation depth during primary LASIK was 128.75 ⫾ 32.6 m (range, 65–198 m) and during repeat LASIK it was 39.8 ⫾ 12 m (range, 18 –72 m). After ablation, the flap was replaced into its original position, and the interface was irrigated copiously with balanced salt solution and dried with a Merocel sponge (Surgical Spears; Merocel Corp., Mystic, CT). An adherence time of 3 minutes was observed. The postoperative regimen was identical to that after the primary LASIK procedure. No eyes lost any spectacle-corrected visual acuity. There was no complication associated with repeat LASIK. RBT was calculated by deducting from the preoperative pachymetry actual flap thickness (measured intraoperatively) and attempted ablation depth. All patients completed 6 months’ follow-up after repeat LASIK.
Statistical Analysis Data were recorded on a predesigned performa and managed on an Excel spreadsheet. All the entries were checked for any error. For UCVA, SEQ, pachymetry, and anterior and posterior corneal elevation, the changes during the preoperative period after primary LASIK, and after repeat LASIK were compared using repeated measures analysis of variance (ANOVA) followed by Bonferroni multiple comparison, if required. To find out the determinants of increase in posterior corneal elevation after primary LASIK (PEV1) and repeat LASIK (PEV2), Pearson’s correlation coefficient followed by stepwise multiple linear regression analysis was done. For this analysis, we considered variables showing significant correlation of up to P ⬍ 0.02. If two variables showed strong
Alka et al 䡠 Posterior Corneal Topographic Changes after LASIK Retreatment Table 1. Refractive and Topographic Changes (Mean ⫾ Standard Deviation) after Primary LASIK and Repeat LASIK
After Primary LASIK
Uncorrected visual acuity Spherical equivalent refraction (diopters) Anterior corneal elevation (m) Posterior corneal elevation (m) Pachymetry (m)
0.03 ⫾ 0.03 ⫺8.8 ⫾ 3.4 11.1 ⫾ 5.0 20.1 ⫾ 7.3 542.5 ⫾ 20.8
0.3 ⫾ 0.1 ⫺2.5 ⫾ 1.5 ⫺19.9 ⫾ 10.5 51.1 ⫾ 15.4 442 ⫾ 25.4
After Repeat LASIK
Repeated Measures Analysis of Variance
0.8 ⫾ 0.2 ⫺0.3 ⫾ 0.5 ⫺25.1 ⫾ 9.6 58.9 ⫾ 17.2 401.1 ⫾ 16.5
F ⫽ 26.01, P ⬍ 0.001 F ⫽ 193.8, P ⬍ 0.001 F ⫽ 295.5, P ⬍ 0.001 F ⫽ 144.7, P ⬍ 0.001 F ⫽ 1500.1, P ⬍ 0.001
* For all the variables change from preoperative to after primary LASIK and repeat LASIK was statistically significant (P ⬍ 0.001).
multicollinearity in the stepwise regression analysis, we considered the one showing higher correlation with PEV1/PEV2. Student’s t test/Mann–Whitney rank-sum test, as appropriate, was used to compare the mean values of variables in two groups. For data analysis, we used STATA 7.0 (intercooled version) and SPSS 8.0 statistical software (SPSS Inc., Chicago, Illinois). All the tests used were two tailed. In this study, P ⬍ 0.05 has been considered statistically significant.
Results Mean UCVA before primary LASIK was 0.03 ⫾ 0.03 and after repeat LASIK was 0.8 ⫾ 0.21 at 6 months follow-up (P ⬍ 0.001). BCVA was maintained in all the eyes. Mean SEQ before primary LASIK was ⫺9.15 ⫾ 3.5 D (range, ⫺2.37 to ⫺15.0 D), and before repeat LASIK it was ⫺2.5 ⫾ 1.5 D (range, ⫺0.9 to ⫺6.5 D). This significantly improved to ⫺0.3 ⫾ 0.5D (range, 0.5 to ⫺2.0 D) after repeat LASIK at 6 months follow-up (P ⬍ 0.001). Orbscan pachymetry readings of the central cornea before primary LASIK was 542.5 ⫾ 20.8 m (range, 508 –588 m) and before repeat LASIK was 442 ⫾ 25.4 m (range, 400 – 496 m). After repeat LASIK, the pachymetry decreased to 401.1 ⫾ 16.5 m (range, 382– 438 m) (P ⬍ 0.001). The mean RBT after primary LASIK and repeat LASIK were 285.7 ⫾ 30.2 m and 244.8 ⫾ 26.4 m, respectively. The mean elevation of the anterior corneal surface above the BFS before primary LASIK, repeat LASIK, and after repeat LASIK were 11.1 ⫾ 5.0 m, ⫺19.9 ⫾ 10.5 m, and ⫺25.1 ⫾ 9.6 m, respectively. The difference between values before and after repeat LASIK was statistically significant (P ⬍ 0.001). The mean elevation of the posterior corneal surface above the BFS before primary LASIK was 20.1 ⫾ 7.3 m, before repeat LASIK 51.1 ⫾ 15.4 m, and after repeat LASIK at 6 months follow-up was 58.9 ⫾ 17.2 m. The difference between values before and after repeat LASIK were statistically significant (P ⬍ 0.001) (Table 1). After primary LASIK, an increase in the posterior corneal elevation had a significant correlation with attempted correction (r ⫽ 0.4, P ⫽ 0.02), preoperative central pachymetry (r ⫽ ⫺0.6, P ⫽ 0.0003), ablation depth (r ⫽ 0.4, P ⫽ 0.008), RBT (r ⫽ ⫺0.6, P ⫽ 0.0003), and postoperative central pachymetry (r ⫽ ⫺0.6, P ⫽ 0.00008). After repeat LASIK, the mean increase in posterior corneal elevation correlated with preoperative central pachymetry (r ⫽ ⫺0.4, P ⫽ 0.03). However, its correlation with the secondary ablation depth (P ⫽ 0.43), final RBT (P ⫽ 0.11), and PEV1 (P ⫽ 0.20) was not statistically significant. On multivariate linear regression analysis (Table 2), it was found that attempted correction and RBT after primary LASIK were the two independent significant determinants of an increase in posterior corneal elevation after primary LASIK (PEV1). These two variables alone account for 51% (R2) of total variability in
PEV1. An increase in attempted correction by 1 D would correspond to an increase in the PEV1 of 1.64 m (P ⬍ 0.001). For RBT after primary LASIK, an increase of 1 m would correspond to a decrease in PEV1 of 0.27 m (P ⬍ 0.001). For an increase in posterior corneal elevation after repeat LASIK (PEV2), preoperative pachymetry and PEV1 were the two independent significant determinants, and they account for 29% of total variability in PEV2. An increase in preoperative pachymetry by 1 m would correspond to a decrease in PEV2 of 0.32 m. For PEV1, an increase of 1 m corresponds to increase in PEV2 of 0.38 m. To evaluate an increase in posterior corneal elevation after repeat LASIK, an arbitrary value of more than one standard deviation away from the mean of pre-repeat LASIK posterior corneal elevation was taken as the cutoff limit (66 m). On this basis, 33 eyes that underwent repeat LASIK were divided into two following groups: Group 1 (n ⫽ 8) included eyes with relative posterior corneal elevation after repeat LASIK of ⬎ 66 m. Group 2 (n ⫽ 25) included eyes with relative posterior corneal elevation after repeat LASIK of ⱕ 66 m. Group 1 had significantly high values of posterior corneal elevation both after primary LASIK (P ⫽ 0.0037) and after repeat LASIK (P ⫽ 0.0000) compared with group 2. They also had significantly low values of central pachymetry preoperatively (P ⫽ 0.0003) and after primary LASIK (P ⫽ 0.0001) and repeat LASIK (P ⫽ 0.0001) surgeries. The mean RBT after primary LASIK (P ⫽ 0.0006) and after repeat LASIK (P ⫽ 0.001) was also lower in this group compared with group 2 (Table 3).
Discussion Iatrogenic corneal ectasia is a topic that has been discussed ever since the introduction of keratorefractive surgery.4,5 Table 2. Determinants of Increase in Posterior Corneal Elevation after Primary LASIK and after Repeat LASIK Regression Equation PEV1 ⫽ 94.69 ⫹ 1.64 *(attempted correction) ⫺ 0.27* (RBT after primary LASIK) PEV2 ⫽‡ 193.0 ⫺ 0.32†(preoperative pachymetry) ⫹ 0.38 (PEV1)
*† P ⬍ 0.001. P ⬍ 0.01. ‡ P ⬍ 0.05. LASIK ⫽ laser in situ keratomileusis; PEV1 ⫽ posterior corneal elevation after primary LASIK; PEV2 ⫽ posterior corneal elevation after repeat LASIK; RBT ⫽ residual bed thickness.
Ophthalmology Volume 109, Number 11, November 2002 Table 3. Distribution of Variables (Mean ⫾ Standard Devation) in the Two Groups Variables Spherical equivalent refraction (diopters) Preoperative After primary LASIK After repeat LASIK Pachymetry (m) Preoperative After primary LASIK After repeat LASIK Posterior corneal elevation (m) Preoperative After primary LASIK After repeat LASIK Residual bed thickness (m) After primary LASIK After repeat LASIK Ablation depth (m) Primary LASIK Repeat LASIK
Group 1 (n ⴝ 8)
Group 2 (n ⴝ 25)
⫺9.6 ⫾ 3.0 ⫺2.1 ⫾ 1.5 ⫺0.3 ⫾ 0.4
⫺8.9 ⫾ 3.6 ⫺2.6 ⫾ 1.5 ⫺0.3 ⫾ 0.5
0.5424 0.2153 0.5565
518.5 ⫾ 12.9 413.7 ⫾ 8.4 382.7 ⫾ 2.0
550.2 ⫾ 16.5 451.1 ⫾ 22.0 407 ⫾ 14.6
0.0003 0.0001 0.0001
22.5 ⫾ 6.5 66.5 ⫾ 14.4 82.7 ⫾ 10.7
19.5 ⫾ 7.5 46.1 ⫾ 12.3 51.3 ⫾ 10.5
0.2747 0.0037 0.0000
255.6 ⫾ 18.9 217.2 ⫾ 18.2
295.4 ⫾ 26.7 253.6 ⫾ 22.4
145.1 ⫾ 25.7 38.4 ⫾ 10.3
123.5 ⫾ 33.3 40.3 ⫾ 12.9
Group 1, Posterior corneal elevation ⬎66 m after repeat LASIK; group 2, Posterior corneal elevation ⱕ66 m after repeat LASIK. LASIK ⫽ laser in situ keratomileusis.
The posterior corneal surface topographic changes after primary LASIK have been well studied.1,3 There are an increasing number of reports on enhancement procedures being performed for residual myopia, which is effective and predictable.6,7 At present, to the best of our knowledge, there are no studies regarding the posterior corneal surface topographic changes after repeat LASIK. The reported factors that determine the forward shift of the posterior corneal surface after primary LASIK include preoperative pachymetry,2 intraocular pressure,2 ablation depth,2 and RBT.1,3 We have used Orbscan slit-scanning system to evaluate the corneal topography. However, the data accumulated by Orbscan may be limited by factors such as the accuracy of the system, which is ⫾ 20 m; the measurement noise that leads to both a positive and negative difference in the height of the posterior corneal surface; and the necessity of aligning the posterior surface before and after surgery, which may be a source of artifactual ectasia.8,9 Because there are many concerns about the proper posterior alignment of the posterior corneal surface while comparing changes of height against BFS in the difference map,8 we calculated the difference in the posterior corneal surface elevation by subtracting preoperative from postoperative corneal elevation. In our study, the mean increase in relative posterior corneal elevation after primary LASIK correlated significantly with the attempted correction and the postoperative pachymetry, in addition to the factors that have been conclusively proven, such as preoperative central pachymetry, RBT, and amount of ablation.1–3 The attempted correction determines the attempted ablation depth and hence the correlation. The linear correlation between the postoperative corneal thickness and posterior corneal elevation may be explained on the basis of the fact that after the flap heals
over the vaulted surface, it is ultimately the final corneal thickness that will determine the entire corneal structure and the subsequent shift. The mean increase in relative posterior corneal elevation after repeat LASIK, correlated with the preoperative pachymetry, suggesting that thinner corneas are at a greater risk of increased posterior corneal elevation after repeat LASIK. However, it did not correlate with final RBT and ablation depth. We found that the increase in posterior corneal elevation after primary LASIK was the important determinant factor for the increase in posterior corneal elevation after repeat LASIK. The eyes with posterior corneal elevation more than one standard deviation away from mean (⬎66 m) after repeat LASIK (n ⫽ 8) significantly differed from those with less than 66 m (n ⫽ 25). Those eyes had thinner corneas at all points of time, thinner RBT, and significantly increased posterior corneal elevation after primary treatment and repeat LASIK, although the posterior corneal elevation before primary LASIK in these two groups was comparable. After primary LASIK, the increase in posterior corneal elevation may be attributed to the instantaneous biomechanical change in which the intraocular pressure pushes against the back surface of a surgically induced, structurally compromised cornea so that it gets pushed forward. After repeat LASIK, the flap is relifted so that the biomechanics of an already healed flap are disturbed. The same intraocular pressure now pushes against a thinner and reablated cornea (with a thinner RBT), and hence there is an increase in posterior corneal elevation after repeat LASIK. Although the significance of RBT in primary LASIK has been reasonably established,1–3 its role after repeat LASIK needs to be determined. The forward shift of the posterior corneal surface after repeat LASIK may be dictated by factors other than those that govern the posterior corneal surface after primary LASIK. After repeat LASIK, RBT and ablation depth may not be the only determinants of a mean increase in the posterior corneal elevation unlike after primary LASIK. We recommend that the relative posterior corneal elevation after primary LASIK should be taken into consideration apart from the corneal pachymetry before a decision about an enhancement ablation is made. The nomograms used for LASIK are calculated in terms of anterior corneal surface change and do not make the assumption of the contribution of the change in the posterior surface. Furthermore, those nomograms are designed primarily for normal corneas, and modification in the nomograms is needed for ablated corneas. In addition, nomograms for enhancement after LASIK need to be refined with respect to the posterior corneal surface changes after primary LASIK. Although no clinically apparent iatrogenic keratectasia occurred in any of our cases, long-term progression may occur, and a longer follow-up on a greater number of subjects may be required to evaluate this.
References 1. Wang Z, Chan J, Yang B. Posterior corneal surface topographic
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case of forme fruste keratoconus. J Cataract Refract Surg 1998; 24:1007–9. Pe´ rez-Santonja JJ, Ayala MJ, Sakla HF, et al. Retreatment after laser in situ keratomileusis. Ophthalmology 1999;106: 21– 8. Zadok D, Maskaleris G, Garcia V, et al. Outcomes of retreatment after laser in situ keratomileusis. Ophthalmology 1999; 106:2391– 4. Maloney RK. Discussion. Ophthalmology 1999;106:409 –10. Hernandez-Quintela E, Samapunphong S, Khan BF, et al. Posterior corneal surface changes after refractive surgery. Ophthalmology 2001;108:1415–22.
Model Eye, c. 1890. This Italian model eye has a wooden base with a turned mahogany and pine stand as well as metal fittings and various glass lenses which allowed a student to examine the retina. An egg-shaped wooden piece on top has a round metal tube at one end and interchangeable lenses at the other. On top is affixed a small metal label: “Oficine Galileo Firenze No. 105161.” Published through the courtesy of the Alcon Laboratories Museum of Ophthalmology, The Sherman Collection, Fort Worth, Texas.