Doi:10.1016/j.jcrs.2007.12.040

Changes in posterior corneal elevation after laser Diego Vicente, Thomas E. Clinch, MD, Paul C. Kang, MD PURPOSE: To evaluate changes in posterior corneal elevation using the Pentacam topographer(Oculus) in patients having laser in situ keratomileusis (LASIK) enhancement.
SETTING: Private practice, Chevy Chase, Maryland, USA.
METHODS: The Pentacam device was used to evaluate the changes in posterior corneal elevationabove the best-fit sphere before LASIK enhancement and after LASIK enhancement in 24 eyes.
The change in posterior corneal elevation in eyes for which pre-primary LASIK data were availablewas also evaluated.
RESULTS: After LASIK enhancement, the mean change in posterior corneal elevation was 5 mm. Themean posterior corneal elevation was 12 G 7 mm before LASIK enhancement and 16 G 6 mm afterenhancement; the difference was statistically significant (P Z .004). In eyes for which pre-primaryLASIK data were available, the mean change in posterior corneal elevation after primary LASIK was2 mm. The mean posterior corneal elevation was 11 G 5 mm before LASIK enhancement and 11 G7 mm after enhancement.
CONCLUSIONS: There was a statistically significant difference in posterior corneal elevationbetween before LASIK enhancement and after LASIK enhancement. However, the change in poste-rior corneal elevation was much smaller than previously reported values and below the sensitivity ofthe Pentacam topographer.
J Cataract Refract Surg 2008; 34:785–788 Q 2008 ASCRS and ESCRS Corneal ectasia can be a serious complication after bed thickness, topographical abnormality such as laser in situ keratomileusis (LASIK) surgery. Because forme fruste keratoconus, and multiple LASIK proce- the treatment options for corneal ectasia are limited, dureIt has been suggested that changes in the for- the best strategy may be its prevention by detecting ward protrusion of the posterior cornea or posterior patients that may be at risk for its development and corneal elevation are key to early detection of ectasia.
Several papers suggest that subclinical ectatic Various risk factors for corneal ectasia have been change in the posterior cornea is relatively common af- reported including high myopia, low residual stromal ter routine LASIThis finding has also been rou-tinely found after LASIK enhancement procedures.However, these studies relied on Orbscan topography(Orbtek) to demonstrate their findings. Although the Accepted for publication December 13, 2007.
Orbscan topographer is a vast improvement over pre- From the Georgetown University School of Medicine (Vicente), vious technology used to image the cornea, it is based Washington, District of Columbia, and a private practice (Clinch, on Placido-disk and slit-scanning beam imaging and uses mathematical calculations to recreate the poste-rior cornea.This strategy can cause measurement No author has a financial or proprietary interest in any material or variability, resulting in falsely positive ectatic readings when comparing preoperative and postoperative to- Presented at the ASCRS Symposium on Cataract, IOL and Refrac- tive Surgery, San Diego, California, USA, May 2007.
Another corneal imaging modality, the Pentacam Corresponding author: Paul C. Kang, MD, 2 Wisconsin Circle, Suite (Oculus, Inc.), uses a rotating Scheimpflug camera to 200, Chevy Chase, Maryland 20815, USA. E-mail: directly image the posterior cornea and allows calculation of posterior corneal elevation without POSTERIOR CORNEAL ELEVATION CHANGES AFTER LASIK ENHANCEMENT mathematical reconstruction.Ciolino and Belinused Table 1. Values before and after primary LASIK (N the Pentacam device to evaluate posterior corneal ele- vation changes after primary LASIK and photorefrac- tive keratectomy and found forward protrusion of theposterior cornea to be rarer than previously reported.
The purpose of this study was to use the Pentacam topographer to evaluate the effect of LASIK enhance- ment on posterior corneal elevation and compare the results with those in previous studies that used Orbs- This study evaluated 24 eyes of 19 consecutive patients hav- ing LASIK enhancement. The primary and enhancement LASIK surgeries were performed at a single institution by 6 Z change; ABL Z ablation depth; CCT Z central corneal thickness; 1 of 2 surgeons (T.E.C., P.C.K). Initial flaps were created PCE Z posterior corneal elevation; RBT Z residual bed thickness; with the IntraLase FS 2.35 laser (AMO) with a superior hinge measuring 110 mm thick and between 8.7 mm and 9.0 mm in diameter. The Visx Star S4 Laser with CustomVue technol-ogy (AMO) was used to perform the vision corrections.
Laser in situ keratomileusis surgeries were performed in change (forward protrusion) of the posterior cornea was the following manner: A wire eyelid speculum was placed between the eyelids. The cornea was marked with a corneal The statistical analysis was done by a paired-sample t test, marker. A Sinskey hook (Katena) was used to score the edge testing the null hypothesis that the Pentacam-generated dif- of the entire perimeter of the flap edge. A Fukasaku LASIK ference in posterior corneal elevation before and after LASIK spatula (Katena) was used to gently separate the flap from the stromal bed. Iris registration was obtained, and pupiltracking was engaged. CustomVue laser treatment was ap-plied. Dry Merocel eye sponges (Medtronic) were used to sweep loose epithelium away from the flap edge. Gentlescraping along the underside of the flap edge was performed The mean age of the 9 men and 10 women in the study to reduce the chance of epithelial ingrowth. Ultrasonic pa- was 42 years (range 18 to 68 years). The mean postop- chymetry with the Advent pachymeter (Accutome) was per- erative follow-up was 11 weeks (range 4 to 22 weeks).
formed at the pupillary center to measure residual bed shows the data for the 10 eyes for which pre– thickness (RBT). The LASIK flap was gently repositionedover the stromal bed and refloated using a balanced salt solu- initial LASIK information was available. There were tion through a 27-gauge spatulated LASIK cannula. A wet no statistically significant differences between the pre- Merocel sponge was used to gently sweep radially along operative mean and postoperative mean values for the flap edge. Alignment of the corneal marks was evaluated the following: SE, CCT, ablation depth, and RBT.
to verify adequate flap positioning. A bandage contact lenswas placed over the corneal flap. A single drop of gatifloxacinophthalmic 0.3% (Zymar) and prednisolone acetate 1% (Pred Table 2. Values after primary LASIK and after repeat LASIK Forte) was instilled. The eyelid speculum was removed.
Each eye was examined preoperatively and postopera- tively to evaluate corneal parameters and visual acuity. The mean spherical equivalent (SE) was calculated preopera-tively and postoperatively. The ablation depth was recorded from the standard operative report of the Visx Star S4 laser.
The central corneal thickness (CCT) was recorded preopera- tively and postoperatively and was represented by the thin- nest corneal measurement within the central 4.0 mm zone of The posterior corneal elevation was measured using the Pentacam in the manner previously described by Ciolino and Belin.The posterior corneal elevation was defined by the maximum forward protrusion of the posterior cornea above the best-fit sphere (BFS) in the central 4.0 mm zone of topography. The preoperative and postoperative BFS were identical, and determined by the central 8.0 mm zoneof the preoperative cornea. The change in posterior corneal 6 Z change; ABL Z ablation depth; CCT Z central corneal thickness; elevation was calculated by subtracting the postoperative LASIK Z laser in situ keratomileusis; PCE Z posterior corneal elevation;RBT elevation data from the preoperative data based on the max- Z residual bed thickness; SE Z spherical equivalent imum difference in the central 4.0 mm zone. An ectatic J CATARACT REFRACT SURG - VOL 34, MAY 2008 POSTERIOR CORNEAL ELEVATION CHANGES AFTER LASIK ENHANCEMENT Table 3. Reported Pentacam and Orbscan measurements of the change in posterior corneal elevation after primary LASIK.
6PCE Z change in posterior corneal elevation; ABL Z ablation depth; RBT Z residual bed thickness; SE Z spherical equivalent*Ectatic changes measured as negative numbers†Ectatic changes measured as positive numbers The difference between the mean posterior corneal represents a much smaller value than studies using elevation before primary LASIK and the mean after primary LASIK was also not statistically significantly One explanation for this observation is the differ- different (P Z .28). The mean change in posterior ence in technology used to measure the cornea. It corneal elevation after primary LASIK was 2 G 4 mm.
has been reported that differences exist between the shows the data before and after LASIK en- Pentacam and Orbscan devices regarding the mea- hancement (N Z 24). The mean posterior corneal eleva- surement of posterior corneal elevation. The Orbs- tion was 12 G 7 mm after primary LASIK and 16 G 6 mm can’s mathematical reconstruction of the posterior after LASIK enhancement. The difference was statisti- cornea may lead it to overestimate the posterior cor- cally significant (P Z .004). The mean change in poste- neal elevation above the Furthermore, Her- rior corneal elevation after LASIK enhancement was na´ndez-Quintela et al.and Maloneysuggest that variability between pre-LASIK and post-LASIK pos-terior corneal elevation Orbscan measurements may be a source of artificially observed ectasia. Therefore,the Pentacam’s ability to directly image the posterior We believe our results represent several significant cornea could be a more accurate representation of the findings. First, we report a minimal change in posterior corneal elevation after primary LASIK. This substanti- In our study, the Pentacam device measured a much ates a previous study using the Pentacbut smaller change in posterior corneal elevation afterLASIK enhancement than the change reported by Table 4. Pentacam versus Orbscan measurements of the change in posterior corneal elevation after LASIK enhancement.
this difference may be attributed to discrepancies in the 2 patient populations, our study had a smaller mean ablation depth, fewer myopic eyes, and largerRBTs. Thus, we believe that the difference in technol- ogy between the Orbscan and Pentacam devices was In addition, although the small change in posterior corneal elevation after LASIK enhancement in our study was statistically significant (P Z .004), it is within the manufacturer’s reported range of error (G5 mm) for the Pentacam and is therefore below the sensitivity of the instrument. Although Rani et al.report that the Orbscan showed a statistically significant change in posterior corneal elevation after LASIK enhancement, the amount of change was well 6 Z change; ABL Z ablation depth; CCT Z central corneal thickness; within the range of error for Orbscan-measured pos- LASIK Z laser in situ keratomileusis; NR Z not recorded; PCE Z poste- terior corneal elevation (G20 mm). As a result, rior corneal elevation; RBT Z residual bed thickness; SE Z sphericalequivalent a change in posterior corneal elevation after LASIK, although statistically significant, may not be clini- J CATARACT REFRACT SURG - VOL 34, MAY 2008 POSTERIOR CORNEAL ELEVATION CHANGES AFTER LASIK ENHANCEMENT We recognize the limitations of our study. Pentacam 4. Baek TM, Lee KH, Kagaya F, et al. Factors affecting the forward imaging is relatively new; thus, there is a limited un- shift of posterior corneal surface after laser in situ keratomileu-sis. Ophthalmology 2001; 108:317–320 derstanding of the significance of Pentacam posterior 5. Cairns G, Ormonde SE, Gray T, et al. Assessing the accuracy of corneal elevation measurements. In addition, the study Orbscan II post-LASIK: apparent keratectasia is paradoxically could be improved with a larger patient population associated with anterior chamber depth reduction in successful followed for a longer period. However, the number procedures. Clin Exp Ophthalmol 2005; 33:147–152 of patients and duration of follow-up in our study 6. Rani A, Murthy BR, Sharma N, et al. Posterior corneal topo- graphic changes after retreatment LASIK. Ophthalmology are similar to those in other published reports.
In conclusion, this study found that change in poste- 7. Cairns G, McGhee CNJ. Orbscan computerized topography: at- rior corneal elevation after LASIK enhancement may tributes, applications, and limitations. J Cataract Refract Surg not be as common or as large as previously reported.
This information may be beneficial as future criteria 8. Kopacz D, Maciejewicz P, Kec¸ik D. [Pentacamdthe new way for anterior eye segment imaging and mapping]. [Polish] Klin Oczna for determining the significance of change in posterior corneal elevation as a risk factor for developing ectasia 9. Ciolino JB, Belin MW. Changes in the posterior cornea after laser in situ keratomileusis and photorefractive keratectomy. J Cata- Using the Pentacam, we found a statistically signif- icant change in posterior corneal elevation after LASIK 10. Quisling S, Sjoberg S, Zimmerman B, et al. Comparison of Pen- tacam and Orbscan IIz on posterior curvature topography mea- enhancement. However, this change was much small- er than previously reported values and below the sen- 11. Herna´ndez-Quintela E, Samapunphong S, Khan BF, et al. Pos- terior corneal surface changes after refractive surgery. Ophthal- 12. Maloney RK. Discussion of paper by Wang Z, Chen J, Yang B.
1. Randleman JB, Russell B, Ward MA, et al. Risk factors and prog- nosis for corneal ectasia after LASIK. Ophthalmology 2003;110:267–275 2. Seitz R, Torres F, Langenbucher A, et al. Posterior corneal cur- vature changes after myopic laser in situ keratomileusis. Oph- thalmology 2001; 108:666–672; discussion by ED Donnenfeld, 3. 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; 106:406–409; J CATARACT REFRACT SURG - VOL 34, MAY 2008

Source: http://www.oculus.de/us/downloads/dyn/oculus/presse/174/changes_post_surface.pdf