Changes in posterior corneal elevation after laserin situ keratomileusis enhancement

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

ARTICLE

Corneal ectasia can be a serious complication afterlaser in situ keratomileusis (LASIK) surgery. Becausethe treatment options for corneal ectasia are limited,the best strategy may be its prevention by detectingpatients that may be at risk for its development andthus avoiding surgery.

Various risk factors for corneal ectasia have beenreported including high myopia, low residual stromal

Accepted for publication December 13, 2007.

From the Georgetown University School of Medicine (Vicente),Washington, District of Columbia, and a private practice (Clinch,Kang), Chevy Chase, Maryland, USA.

No author has a financial or proprietary interest in any material ormethod mentioned.

Presented at the ASCRS Symposium on Cataract, IOL and Refrac-tive Surgery, San Diego, California, USA, May 2007.

Corresponding author: Paul C. Kang, MD, 2 Wisconsin Circle, Suite200, Chevy Chase, Maryland 20815, USA. E-mail: pkang3@yahoo.com.

Q 2008 ASCRS and ESCRS

Published by Elsevier Inc.

bed thickness, topographical abnormality such asforme fruste keratoconus, and multiple LASIK proce-dures.1 It has been suggested that changes in the for-ward protrusion of the posterior cornea or posteriorcorneal elevation are key to early detection of ectasia.2

Several papers suggest that subclinical ectaticchange in the posterior cornea is relatively common af-ter routine LASIK.2–5 This finding has also been rou-tinely found after LASIK enhancement procedures.6

However, these studies relied on Orbscan topography(Orbtek) to demonstrate their findings. Although theOrbscan topographer is a vast improvement over pre-vious technology used to image the cornea, it is basedon Placido-disk and slit-scanning beam imaging anduses mathematical calculations to recreate the poste-rior cornea.7 This strategy can cause measurementvariability, resulting in falsely positive ectatic readingswhen comparing preoperative and postoperative to-pographical maps of the posterior cornea.7

Another corneal imaging modality, the Pentacam(Oculus, Inc.), uses a rotating Scheimpflug camera todirectly image the posterior cornea and allowscalculation of posterior corneal elevation without

0886-3350/08/$dsee front matter 785doi:10.1016/j.jcrs.2007.12.040

786 POSTERIOR CORNEAL ELEVATION CHANGES AFTER LASIK ENHANCEMENT

mathematical reconstruction.8 Ciolino and Belin9 usedthe 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 Pentacamtopographer to evaluate the effect of LASIK enhance-ment on posterior corneal elevation and compare theresults with those in previous studies that used Orbs-can topography.

PATIENTS AND METHODS

This study evaluated 24 eyes of 19 consecutive patients hav-ing LASIK enhancement. The primary and enhancementLASIK surgeries were performed at a single institution by1 of 2 surgeons (T.E.C., P.C.K). Initial flaps were createdwith the IntraLase FS 2.35 laser (AMO)with a superior hingemeasuring 110 mm thick and between 8.7 mm and 9.0 mm indiameter. The Visx Star S4 Laser with CustomVue technol-ogy (AMO) was used to perform the vision corrections.

Laser in situ keratomileusis surgeries were performed inthe following manner: A wire eyelid speculum was placedbetween the eyelids. The cornea was marked with a cornealmarker. A Sinskey hook (Katena) was used to score the edgeof the entire perimeter of the flap edge. A Fukasaku LASIKspatula (Katena) was used to gently separate the flap fromthe stromal bed. Iris registration was obtained, and pupiltracking was engaged. CustomVue laser treatment was ap-plied. Dry Merocel eye sponges (Medtronic) were used tosweep loose epithelium away from the flap edge. Gentlescraping along the underside of the flap edgewas performedto reduce the chance of epithelial ingrowth. Ultrasonic pa-chymetry with the Advent pachymeter (Accutome) was per-formed at the pupillary center to measure residual bedthickness (RBT). The LASIK flap was gently repositionedover the stromal bed and refloated using a balanced salt solu-tion through a 27-gauge spatulated LASIK cannula. A wetMerocel sponge was used to gently sweep radially alongthe flap edge. Alignment of the corneal marks was evaluatedto 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% (PredForte) was instilled. The eyelid speculum was removed.

Each eye was examined preoperatively and postopera-tively to evaluate corneal parameters and visual acuity. Themean spherical equivalent (SE) was calculated preopera-tively and postoperatively. The ablation depth was recordedfrom 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 ofthe Pentacam topographer.

The posterior corneal elevation was measured using thePentacam in the manner previously described by Ciolinoand Belin.9 The posterior corneal elevation was defined bythe maximum forward protrusion of the posterior corneaabove the best-fit sphere (BFS) in the central 4.0 mm zoneof topography. The preoperative and postoperative BFSwere identical, and determined by the central 8.0 mm zoneof the preoperative cornea. The change in posterior cornealelevation was calculated by subtracting the postoperativeelevation data from the preoperative data based on the max-imum difference in the central 4.0 mm zone. An ectatic

J CATARACT REFRACT SU

change (forward protrusion) of the posterior cornea wasrecorded as a negative number.

The statistical analysis was done by a paired-sample t test,testing the null hypothesis that the Pentacam-generated dif-ference in posterior corneal elevation before and after LASIKwas zero.

RESULTS

The mean age of the 9 men and 10 women in the studywas 42 years (range 18 to 68 years). The mean postop-erative follow-up was 11 weeks (range 4 to 22 weeks).

Table 1 shows the data for the 10 eyes for which pre–initial LASIK information was available. There wereno statistically significant differences between the pre-operative mean and postoperative mean values forthe following: SE, CCT, ablation depth, and RBT.

Table 1. Values before and after primary LASIK (N Z 10).

Exam Mean G SD (Range)

PreopSE (D) �5.38 G 3 (�10.88 to �2.25)CCT (mm) 561.2 G 43 (611 to 505)PCE (mm) 11 G 5 (4 to 19)*

PostopSE (D) �0.52 G 0.48 (�1.30 to 0.125)ABL (mm) 83 G 30 (46 to 149)RBT (mm) 365 G 45 (319 to 465)CCT (mm) 492 G 36 (457 to 555)PCE (mm) 11 G 7 (3 to 22)*6PCE (mm) �2 G 4 (�6 to 6)

6 Z change; ABL Z ablation depth; CCT Z central corneal thickness;PCE Z posterior corneal elevation; RBT Z residual bed thickness;SE Z spherical equivalent*P Z .28

Table 2. Values after primary LASIK and after repeat LASIK(N Z 24).

After Mean G SD (Range)

Primary LASIKSE (D) �0.37 G 0.73 (�1.375 to 2.00)CCT(mm) 485 G 44 (418 to 555)PCE (mm) 12 G 7 (3 to 29)*

Repeat LASIKSE (D) �0.05 G .06 (�1.125 to 1.875)ABL (mm) 19 G 7 (9 to 28)RBT (mm) 356 G 37 (297 to 453)CCT (mm) 476 G 48 (391 to 573)PCE (mm) 16 G 6 (6 to 33)*6PCE (mm) �5 G 6 (�24 to 9)

6 Z change; ABL Z ablation depth; CCT Z central corneal thickness;LASIK Z laser in situ keratomileusis; PCE Z posterior corneal elevation;RBT Z residual bed thickness; SE Z spherical equivalent*P Z .004

RG - VOL 34, MAY 2008

787POSTERIOR CORNEAL ELEVATION CHANGES AFTER LASIK ENHANCEMENT

Table 3. Reported Pentacam and Orbscan measurements of the change in posterior corneal elevation after primary LASIK.

Pentacam* Orbscan†

Parameter Current Study Ciolino & Belin9 Wang et al.3 Baek et al.4 Cairns et al.5

Eyes (n) 10 103 d d d

SE (D) �5.38 G 3 �3.76 d d d

ABL (mm) 80 G 30 62.1 d d d

RBT (mm) 460 G 48 329 d d d

DPCE (mm) �2 G 4 �2.64 G 4.95 17.2 G 7.2 40.9 G 24.8 16.9 G 24.6

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 cornealelevation before primary LASIK and the mean afterprimary LASIK was also not statistically significantlydifferent (P Z .28). The mean change in posteriorcorneal elevation after primary LASIK was 2 G 4 mm.

Table 2 shows the data before and after LASIK en-hancement (NZ 24). Themeanposterior corneal eleva-tionwas 12G7mmafter primaryLASIKand16G 6mmafter LASIK enhancement. The difference was statisti-cally significant (P Z .004). The mean change in poste-rior corneal elevation after LASIK enhancement was5 G 6 mm.

DISCUSSION

We believe our results represent several significantfindings. First, we report aminimal change in posteriorcorneal elevation after primary LASIK. This substanti-ates a previous study using the Pentacam9 but

Table 4. Pentacam versus Orbscan measurements of the changein posterior corneal elevation after LASIK enhancement.

AfterCurrent Study

(N Z 24)Rani et al.6

(N Z 33)

Primary LASIKSE (D) �0.37 G 0.73 �2.5 G 1.5RBT (mm) 417 G 65 286 G 30CCT (mm) 485 G 44 442 G 25PCE (mm) 12 G 7* 51.1 G 15.4†

LASIK enhancementSE (D) �0.05 G 0.6 0.3 G 0.5ABL (mm) 19 G 7 40 G 12RBT (mm) 356 G 37 245 G 26CCT (mm) 476 G 48 401 G 16.5PCE (mm) 16 G 6* 58.9 G 17.2†

DPCE (mm) �5 G 6 NR

6 Z change; ABL Z ablation depth; CCT Z central corneal thickness;LASIK Z laser in situ keratomileusis; NR Z not recorded; PCE Z poste-rior corneal elevation; RBT Z residual bed thickness; SE Z sphericalequivalent*P Z .004†P Z .00

J CATARACT REFRACT SU

represents a much smaller value than studies usingthe Orbscan (Table 3).3–5

One explanation for this observation is the differ-ence in technology used to measure the cornea. Ithas been reported that differences exist between thePentacam and Orbscan devices regarding the mea-surement of posterior corneal elevation. The Orbs-can’s mathematical reconstruction of the posteriorcornea may lead it to overestimate the posterior cor-neal elevation above the BFS.10 Furthermore, Her-nandez-Quintela et al.11 and Maloney12 suggest thatvariability between pre-LASIK and post-LASIK pos-terior corneal elevation Orbscan measurements maybe a source of artificially observed ectasia. Therefore,the Pentacam’s ability to directly image the posteriorcornea could be a more accurate representation of theposterior corneal topography.10

In our study, the Pentacam device measured amuchsmaller change in posterior corneal elevation afterLASIK enhancement than the change reported byRani et al.6 using the Orbscan (Table 4). Althoughthis difference may be attributed to discrepancies inthe 2 patient populations, our study had a smallermean ablation depth, fewer myopic eyes, and largerRBTs. Thus, we believe that the difference in technol-ogy between the Orbscan and Pentacam devices wasa significant contributing factor.

In addition, although the small change in posteriorcorneal elevation after LASIK enhancement in ourstudy was statistically significant (P Z .004), it iswithin the manufacturer’s reported range of error(G5 mm) for the Pentacam and is therefore belowthe sensitivity of the instrument. Although Raniet al.6 report that the Orbscan showed a statisticallysignificant change in posterior corneal elevation afterLASIK enhancement, the amount of change was wellwithin the range of error for Orbscan-measured pos-terior corneal elevation (G20 mm). As a result,a change in posterior corneal elevation after LASIK,although statistically significant, may not be clini-cally relevant.

RG - VOL 34, MAY 2008

788 POSTERIOR CORNEAL ELEVATION CHANGES AFTER LASIK ENHANCEMENT

We recognize the limitations of our study. Pentacamimaging is relatively new; thus, there is a limited un-derstanding of the significance of Pentacam posteriorcorneal elevationmeasurements. In addition, the studycould be improved with a larger patient populationfollowed for a longer period. However, the numberof patients and duration of follow-up in our studyare similar to those in other published reports.5,9

In conclusion, this study found that change in poste-rior corneal elevation after LASIK enhancement maynot be as common or as large as previously reported.This information may be beneficial as future criteriafor determining the significance of change in posteriorcorneal elevation as a risk factor for developing ectasiais established.

Using the Pentacam, we found a statistically signif-icant change in posterior corneal elevation after LASIKenhancement. However, this change was much small-er than previously reported values and below the sen-sitivity of the Pentacam.

REFERENCES1. 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,

673

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;

discussion by RK Maloney, 409–410

J CATARACT REFRACT S

4. Baek TM, Lee KH, Kagaya F, et al. Factors affecting the forward

shift of posterior corneal surface after laser in situ keratomileu-

sis. Ophthalmology 2001; 108:317–320

5. 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 2005; 33:147–152

6. Rani A, Murthy BR, Sharma N, et al. Posterior corneal topo-

graphic changes after retreatment LASIK. Ophthalmology

2002; 109:1991–1995

7. Cairns G, McGhee CNJ. Orbscan computerized topography: at-

tributes, applications, and limitations. J Cataract Refract Surg

2005; 31:205–220

8. Kopacz D, Maciejewicz P, Kecik D. [Pentacamdthe new way for

anterior eye segment imaging and mapping]. [Polish] Klin Oczna

2005; 107:728–731

9. Ciolino JB, Belin MW. Changes in the posterior cornea after laser

in situ keratomileusis and photorefractive keratectomy. J Cata-

ract Refract Surg 2006; 32:1426–1431

10. Quisling S, Sjoberg S, Zimmerman B, et al. Comparison of Pen-

tacam and Orbscan IIz on posterior curvature topography mea-

surements in keratoconus eyes. Ophthalmology 2006;

113:1629–1632

11. Hernandez-Quintela E, Samapunphong S, Khan BF, et al. Pos-

terior corneal surface changes after refractive surgery. Ophthal-

mology 2001; 108:1415–1422

12. Maloney RK. Discussion of paper by Wang Z, Chen J, Yang B.

Ophthalmology 1999; 106:409–410

First author:Diego Vicente

Georgetown University School of Medi-cine, Washington, District ofColumbia, USA

URG - VOL 34, MAY 2008