ARTICLE Long-term stability of the posterior cornea after laser in situ keratomileusis Joseph B. Ciolino, MD, Stephen S. Khachikian, MD, Michael J. Cortese, OD, Michael W. Belin, MD PURPOSE: To study long-term changes in posterior corneal elevation after laser in situ keratomileusis (LASIK) using Scheimpflug topography (Pentacam, Oculus, Inc.) in eyes 1 year after LASIK. SETTING: Department of Ophthalmology, Albany Medical Center, and a private practice, Albany, New York, USA. METHODS: One hundred two myopic eyes of 2 consecutive patients presenting for their 1-year follow-up were prospectively evaluated using the Pentacam to determine elevation changes to the posterior corneal surface between preoperative and 1-year postoperative measurements. Changes in posterior elevation were performed by comparing the best-fit sphere preoperatively and postoperatively with a fixed reference sphere determined by the central 9. mm preoperative cornea. Statistical and graphical analyses were performed. RESULTS: One hundred two post-lasik eyes (mean correction 4.33 diopters; mean ablation depth 68.7 mm; mean estimated residual bed thickness 327 mm) had a mean posterior displacement of.47 mm G 3.48 (SD) (range 1. to C7 mm). The mean follow-up period was 13.6 months (range 8.8 to 19.3 months). CONCLUSIONS: In this population, no patient had significant forward protrusion of the posterior corneal surface a mean of 14 months after LASIK. The posterior cornea in post-lasik myopic eyes was very stable. Contrary to results in previous studies, progressive changes to the posterior corneal surface did not routinely occur after LASIK performed within established parameters. J Cataract Refract Surg 7; 33:1366 137 Q 7 ASCRS and ESCRS Forward protrusion or displacement of the posterior corneal surface is found topographically in eyes with ectasia after laser in situ keratomileusis (LASIK). Studies found that changes to the posterior corneal surface commonly occur after LASIK. 1 The reported forward protrusion of the posterior corneal surface has characteristics similar to those of keratectasia and was Accepted for publication April, 7. From the Albany Medical College (Ciolino, Khachikian, Belin) and a private practice (Belin, Cortese), Albany, New York, USA. No author has a financial or proprietary interest in any material or method mentioned. Presented at the ASCRS Symposium on Cataract, IOL and Refractive Surgery, San Diego, California, USA, April May 7. Corresponding author: Joseph B. Ciolino, MD, 361 State Street, Apartment 3 Front, Albany, New York 1221, USA. believed to represent a subclinical form of post-lasik ectasia. 6 It was thought that these corneal changes represented subclinical ectasia in otherwise normal post- LASIK eyes. The previous topographical studies used the Orbscan (Bausch & Lomb), whose accuracy in assessing the posterior corneal surface has been subsequently questioned. 7 9 An earlier report 1 examined acute changes and found no significant change in the posterior corneal surface within the first month after LASIK. Although posterior corneal elevation changes consistent with progressive ectasia have been noted as early as 4 to 6 weeks after LASIK, post-lasik ectasia is typically diagnosed an average of 13 months after surgery. 6 This study examined changes to the posterior corneal surface in eyes a mean of 14 months after LASIK. PATIENTS AND METHODS The Pentacam (Oculus, Inc.) was used to image the corneas in 2 consecutive patients presenting for their 1-year post- LASIK follow-up. One hundred two post-lasik myopic 1366 Q 7 ASCRS and ESCRS Published by Elsevier Inc. 886-33/7/$dsee front matter doi:1.116/j.jcrs.7.4.16
LONG-TERM STABILITY OF POSTERIOR CORNEA AFTER LASIK 1367 eyes were prospectively evaluated after institutional review board approval was obtained. Only post-lasik patients with myopic eyes were included in the study because most reports of posterior corneal surface are of changes after LASIK in these individuals. 11,12 There were no exclusion criteria. All patients provided informed consent. All patients were healthy individuals who met previously published standard criteria for refractive surgery. 1 In general, no LASIK procedures were performed if the preoperative thinnest pachymetry reading by the Pentacam was less than mm. The minimum residual bed allowance was set at 27 mm, although 1 patient in the study had a computed residual thickness of 263 mm. Corrections were limited to a maximum spherical correction in negative cylinder form of approximately 1. diopters (D). All LASIK procedures were performed with the Visx S4 IR laser and Moria microkeratome (both LSK reusable and disposable) by cornea fellowship trained ophthalmologists on the faculty at Albany Medical Center. All patients were imaged with the Pentacam (software version 1.9) preoperatively and when presenting for the 1-year follow-up visit. Patients were asked to blink twice and then look at the fixation device. Image acquisition involved a 1-second scan of rotational Scheimpflug images. Acceptable maps had at least 1. mm of corneal coverage with no extrapolated data in the central 9. mm zone. When the patient blinked during the scan or other artifacts were introduced, the Pentacam flagged that scan as not acceptable. Scans not meeting acceptable criteria were repeated, but most patients required only 1 image. The thinnest corneal thickness measurement in the central 4. mm was recorded as the thinnest central corneal thickness (CCT) reading. Residual bed thickness (RBT) was estimated using the thinnest CCT reading and subtracting the non-nomogram adjusted ablation depth and nominal flap thickness of 16 mm for the 13 mm head or 13 mm for the 1 mm head (Moria LSK). The flap thickness was not measured intraoperatively in each eye but was based on 2 standard deviations above the norm determined by previously recorded intraoperative ultrasound flap thickness measurements at the institution. Given that there may have been additional outliers, a minimum RBT above mm was also used. Changes in the central posterior surface were determined by subtracting the postoperative elevation data from the preoperative elevation data based on the maximal difference in the central 4. mm zone (difference map). The reference bestfit sphere (BFS) was determined by the central 9. mm zone of the preoperative cornea (ie, the BFS for both preoperative and postoperative maps were identical and determined by the preoperative data). The difference in elevation was determined to be the displacement of the posterior corneal surface. An ectatic change (forward protrusion of the posterior corneal surface) would result in a negative number. A graph of this difference versus spherical equivalent (SE) correction, central ablation depth, thinnest CCT, and estimated RBT was viewed. RESULTS One hundred two eyes were studied with the Pentacam preoperatively and postoperatively at the 1-year follow-up appointment. The postoperative imaging took place a mean of 13.6 months G 2.3 (SD) (range 9 to 19 months) after LASIK. The ratio of right to left eyes was equal. The mean age of the 31 men and 21 women in the study was 37.7 G 9.6 years (range 22 to 6 years). Two patients had LASIK performed in 1 eye only. The mean SE correction was 4.33 G 1.87 D (range.7 to 1. D) (Table 1). Table 1 also shows the thinnest preoperative CCT measurement, ablation depth, and RBT. Posterior Corneal Surface The posterior corneal surface measurements in the 12 eyes were very stable 1 year after LASIK. The mean posterior corneal displacement was.47 G 4.8 mm (range C7 to 1 mm). No post-lasik eye had significant posterior corneal ectatic changes. Two eyes had forward posterior corneal displacement of 1 mm, which was the greatest change in the study population. The posterior corneal displacement was plotted in 4 scatterplot graphs against the preoperative CCT, estimated RBT, ablation depth, and refractive SE (Figures 1, 2, 3, and 4, respectively). The graphs do not show significant negative outlying points along the y-axis (posterior corneal displacement) that would be consistent with postoperative forward posterior corneal shift (corneal ectasia). DISCUSSION This study found that the posterior cornea was stable 1 year after LASIK. The mean posterior corneal displacement in 12 post-lasik eyes was less than 3 mm; the mean was.47 mm. The negative displacement represents a forward shift in the posterior corneal surface. In a previous study that used the Pentacam 1 month after LASIK, 1 the mean displacement was 2.64 G 4.9 mm (range C12. to 14. mm). Because Table 1. Spherical equivalent, thinnest preoperative CCT measurement, ablation depth, and RBT. Value SE (D) CCT (um) Ablation Depth (mm) RBT (mm) Mean 4.33 G 1.87 2 G 24 69 G 23 327 G Median 4. 66 327 Range.7 to 1.12 493 to 614 11 to 1 263 to 39 CCT Z central corneal thickness; RBT Z etimated residual bed thickness; SE Z spherical equivalent J CATARACT REFRACT SURG - VOL 33, AUGUST 7
1368 LONG-TERM STABILITY OF POSTERIOR CORNEA AFTER LASIK Posterior Corneal 1 - -1 - - 4 4 6 6 7 Pre-operative Central Corneal Thickness 1 - -1 - - 3 3 4 Estimated Residual Bed Thickness Figure 1. Posterior corneal displacement versus preoperative CCT measurement. Figure 2. Posterior corneal displacement versus RBT. 1 - -1 - - 4 6 8 1 1 14 Ablation Depth 1 - -1 - - -12-1 -8-6 -4-2 Pre-operative Spherical Equivalent Refraction Figure 3. Posterior corneal displacement versus ablation depth. posterior ectasia typically presents a mean of 13 months after LASIK, this study s longer follow-up (mean 14 months) provides more powerful evidence of the stability of the posterior cornea post LASIK. 6 Figure 4. Posterior corneal displacement versus preoperative SE refraction The exact displacement significant for ectatic change is not known; however, the posterior corneal displacement found in this study was significantly less than the mean displacement reported with the Orbscan Table 2. Results of studies evaluating the change in the posterior corneal elevation post LASIK. Posterior Corneal Forward (mm) Study*/Year Published Number of Eyes Instrument Post LASIK Follow up (Mo) Mean G SD Range Wang /1999 32 Orbscan I 32.1 G 1. to Baek 1 /1 196 Orbscan I 1 4.9 G 24.8 to 118 Lee 13 /3 363 Orbscan II 3 46.7 21 to 117 Miyata 3 /4 164 Orbscan II 1 46.4 G 27.9 C8 to 132 Twa 4 / 1124 Orbscan I 6 11.1 G 9.4 2 to 3 Ciolino 1 /6 121 Pentacam 1 2.64 G 4.9 C12 to 14 Present/ 7 12 Pentacam 14.47 G 4.8 C7 to 1 LASIK Z laser in situ keratomileusis *First author Unknown J CATARACT REFRACT SURG - VOL 33, AUGUST 7
LONG-TERM STABILITY OF POSTERIOR CORNEA AFTER LASIK 1369 4 Microns - -4-6 -8-1 -1 Wang 1999 4 6 8 1 Baek 1 Lee 3 Miyata 4 Twa Ciolino 6* Previous Studies (Average and Range) Post-LASIK Eyes (Pentacam) 1-14 Figure. Posterior corneal displacement and the mean and range in previously published studies of the posterior corneal changes after LASIK. ( 11. to 46.7 mm). 1,3,13 Table 2 shows the results of studies of the change in posterior corneal elevation after LASIK. Figure plots the posterior corneal displacement found in this study and shows the mean and range of the posterior corneal displacement from previous studies of post-lasik eyes. Many earlier investigations report that the Orbscan underestimates CCT in post-lasik eyes when compared with ultrasound pachymetry measurements. 14 18 The pachymetry measured by Orbscan is a function of the difference between the anterior and posterior corneal elevations. An erroneous measurement of the anterior corneal elevation, the posterior elevation, or both can result in an incorrect corneal thickness reading. Because the anterior cornea is easier to assess, the likely source of error is the posterior cornea. Topographically, a falsely thin CCT measurement can appear as a forward protrusion of the posterior corneal surface. This artifact generated by the Orbscan in post-lasik eyes may be responsible for the finding in earlier studies that all corneas develop subclinical ectatic changes after LASIK. 7,19 It is also possible that when these studies were performed, LASIK may have been performed in eyes with an increased risk for developing ectasia. In our study, the 12 eyes of consecutive patients presenting for the 1-year follow-up visit showed little change in the posterior corneal elevations. Previous reports that subclinical ectasia commonly occurs in normal post-lasik corneas may have been erroneous secondary to limitations in measuring the posterior surface and a failure to maintain a common reference surface to compare measurements. This study used the Pentacam s rotating Scheimpflug camera, which has a common central point in all sections, which allows better 3-dimensional reconstruction and reduces the effect of eye movement. This study with the Pentacam topographer found that the posterior cornea was stable a mean of 14 months post LASIK. REFERENCES 1. Baek TM, Lee KH, Kagaya F, et al. Factors affecting the forward shift of posterior corneal surface after laser in situ keratomileusis. Ophthalmology 1; 18:317 3 2. Cheng ACK, Tang E, Lam DSC. Residual bed thickness and corneal forward shift after laser in situ keratomileusis [letter]. J Cataract Refract Surg 4; 3:21; reply by K Miyata, 21 22 3. Miyata K, Tokunaga T, Nakahara M, et al. Residual bed thickness and corneal forward shift after laser in situ keratomileusis. J Cataract Refract Surg 4; 3:167 172 4. Twa MD, Roberts C, Mahmoud AM, Chang JS Jr. Response of the posterior corneal surface to laser in situ keratomileusis for myopia. J Cataract Refract Surg ; 31:61 71. Wang Z, Chen J, Yang B. Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology 1999; 16:46 49; discussion by RK Maloney, 49 41 6. Twa MD, Nichols JJ, Joslin CE, et al. Characteristics of corneal ectasia after LASIK for myopia. Cornea 4; 23:447 47 7. Nawa Y, Masuda K, Ueda T, et al. Evaluation of apparent ectasia of the posterior surface of the cornea after keratorefractive surgery. J Cataract Refract Surg ; 31:71 73 8. Cairns G, McGhee CNJ. Orbscan computerized topography: attributes, applications, and limitations. J Cataract Refract Surg ; 31: 2 9. Cairns G, Ormonde SE, Gray T, et al. Assessing the accuracy of Orbscan II post-lasik: apparent keratectasia is paradoxically associated with anterior chamber depth reduction in successful procedures. Clin Exp Ophthalmol ; 33:147 2 1. Ciolino JB, Belin MW. Changes in the posterior cornea after laser in situ keratomileusis and photorefractive keratectomy. J Cataract Refract Surg 6; 32:1426 1431 11. Seitz B, Torres F, Langenbucher A, et al. Posterior corneal curvature changes after myopic laser in situ keratomileusis. Ophthalmology 1; 18:666 672; discussion by ED Donnenfeld, 673 12. Naroo SA, Charman WN. Changes in posterior corneal curvature after photorefractive keratectomy. J Cataract Refract Surg ; 26:872 878 13. Lee D-H, Seo S, Jeong KW, et al. Early spatial changes in the posterior corneal surface after laser in situ keratomileusis. J Cataract Refract Surg 3; 29:778 784 J CATARACT REFRACT SURG - VOL 33, AUGUST 7
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