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Changes in the posterior cornea

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					                                           J CATARACT REFRACT SURG - VOL 32, SEPTEMBER 2006




                                                   ARTICLES
                      Changes in the posterior cornea
                      after laser in situ keratomileusis
                      and photorefractive keratectomy
                                      Joseph B. Ciolino, MD, Michael W. Belin, MD



            PURPOSE: To study the changes in posterior corneal elevation after laser in situ keratomileusis (LASIK)
            and photorefractive keratectomy (PRK) using Scheimpflug topography with the Pentacam anterior
            segment imaging system (Oculus, Inc.).
            SETTING: Department of Ophthalmology, Albany Medical Center, and a private clinical practice,
            Albany, New York, USA.
            METHODS: In this prospective study, 121 consecutive myopic eyes (103 LASIK and 18 PRK) were eval-
            uated preoperatively and postoperatively with the Pentacam to determine elevation changes in the
            posterior corneal surface. Changes in posterior elevation were calculated by comparing the best-fit
            sphere preoperatively and postoperatively to a fixed reference sphere using the central 9.0 mm pre-
            operative cornea. Statistical and graphic analyses were performed.
            RESULTS: The 103 LASIK eyes had a mean correction of À3.76 diopters (D) and a mean ablation depth
            of 62.1 mm. The mean estimated residual bed thickness (RBT) (329 mm) demonstrated a mean posterior
            displacement of 2.64 G 4.95 mm. The 18 PRK eyes had a mean correction of À2.69 D and a mean
            ablation depth of 53.2 mm. The mean estimated RBT (464 mm) had a mean posterior displacement
            of À0.88 G 4.64 mm. The difference in the mean posterior corneal displacement between the LASIK
            and the PRK eyes was not statistically significant (P>.05, Student t test).
            CONCLUSIONS: There was no statistically significant difference in posterior corneal displacement be-
            tween the LASIK and PRK patients. The changes in PRK and LASIK eyes appeared to be within accept-
            able measurement variation. Contrary to previous reports, ectatic changes to the posterior corneal
            surface did not routinely occur after LASIK surgery.
            J Cataract Refract Surg 2006; 32:1426–1431 Q 2006 ASCRS and ESCRS



Topography is the science of describing or representing the            PAR Corneal Topography System (PAR Technology) was
features of a particular object in detail. Attempts to map the         developed to measure anterior elevation topography using
cornea were first described in the 17th century when reflec-           the principles of triangulation.2 Although valuable tools,
tions from the anterior cornea were compared to a set of               Placido-based computerized videokeratography systems
spheres of a known diameter. In the 1800s, the Placido                 and PAR systems are limited to the evaluation of the ante-
disk was developed to qualitatively assess the cornea.                 rior corneal surface only.
Through a hole in the disk, irregularities of reflections                    In 1995, the Orbscan (Bausch & Lomb) was intro-
from alternating light- and dark-ringed targets were noted.            duced. The original Orbscan used slit-scanning technology.
Computer technology facilitated the development of com-                A later model, the Orbscan II (Bausch & Lomb), combined
puterized videokeratography, which uses a Placido device               slit scanning with a Placido disk device. The curvature and
and complex algorithms to measure the slope (and subse-                elevation of the cornea’s anterior and posterior surfaces can
quently derive curvature) of the anterior corneal surface.             be assessed with the Orbscan II. The Pentacam (Oculus,
However, computerized videokeratography has intrinsic                  Inc.) uses a rotating Scheimpflug camera to measure the
limitations in measuring the actual corneal shape.1 The                anterior and posterior corneal surfaces.

Q 2006 ASCRS and ESCRS                                                                                   0886-3350/06/$-see front matter
Published by Elsevier Inc.                                                                               doi:10.1016/j.jcrs.2006.03.037

1426
                                              POSTERIOR CORNEAL CHANGES AFTER LASIK AND PRK



     Previous reports used the Orbscan to measure corneal                  and no procedures (PRK or LASIK) were performed if the thinnest
changes after laser in situ keratomileusis (LASIK). These                  readings were less than 475 mm. The minimum residual bed allow-
studies report that the posterior corneal surface becomes                  ance was 275 mm, although 1 patient in this study had a residual
                                                                           thickness of 263 mm. Corrections were typically limited to a max-
more prolate after LASIK; other studies suggest that sub-                  imum spherical correction in negative cylinder form of 10.00
clinical post-LASIK ectasia is common.3–7 The studies,                     diopters (D) or less.
however, are limited by dependence on Orbscan-generated                         The Visx S3 laser was used for all procedures, and the Moria
post-LASIK data measuring the posterior corneal surface,                   microkeratome was used in all LASIK cases. The PRK patients had
data whose validity have yet to be independently con-                      alcohol-assisted epithelial removal. All surgeries were performed
                                                                           by cornea-trained ophthalmologists on faculty at Albany Medical
firmed. Articles report that the Orbscan underestimates                    Center. All patients provided informed consent.
post-LASIK central corneal thickness.8–11 Pachymetry                            The Pentacam software version 1.09 (Oculus Inc.) was used
data depend on accurate anterior and posterior elevation                   for imaging preoperatively and between 1 month and 2 months
data. Inaccurate pachymetry data are the result of faulty                  postoperatively. Patients were asked to blink twice and then
measurements of the anterior elevation, posterior eleva-                   look at the fixation device. Image acquisition was a 1-second
                                                                           scan of 25 rotational Scheimpflug images through the corneal
tion, or both. Because the general assumption is that the an-              sighting point, the point where the ray of light from the fovea to
terior elevation is easier to measure, the likely source of                the fixation device crossed the cornea. Each scan was separated
error is the posterior measurement.                                        by 7.2 degrees. Acceptable maps had at least 10.0 mm of corneal
     Post-LASIK ectasia is a serious LASIK complication. It                coverage with no extrapolated data in the central 9.0 mm zone.
is characterized by poor visual performance and corneal                    Extrapolated data are indicated by black dots on the color map.
                                                                           Scans not meeting acceptable criteria were repeated, but most
thinning, which may progress to require corneal transplan-                 patients required 1 image.
tation. Because investigators suggest that changes in the                       The thinnest cornea in the central 4.0 mm was recorded as
posterior corneal shape are key to early recognition of cor-               the thinnest central corneal thickness (CCT) reading. Residual
neal ectasia, it is important for clinicians to have an accu-              bed thickness (RBT) was estimated using the thinnest CCT read-
rate method of studying posterior corneal elevation in                     ing and subtracting the non-nomogram-adjusted ablation depth
                                                                           and the nominal flap thickness of 160 mm for the 130 head or
post-LASIK patients.3,12,13                                                130 mm for the 100 head (LSK, Moria).
                                                                                Changes in the central posterior surface were determined by
PATIENTS AND METHODS                                                       subtracting the postoperative elevation data from the preoperative
                                                                           elevation data based on the maximum difference in the central
     The Pentacam was used to evaluate 121 corneas of 62 consec-           4.0 mm zone (difference map). The reference best-fit sphere
utive patients who participated in this prospective study from Jan-        (BFS) was determined by the central 8.0 mm zone of the preoper-
uary to June 2005. Only myopic eyes were evaluated because most            ative cornea; the BFS for preoperative and postoperative maps
reports of posterior corneal surface changes after refractive cor-         were identical and determined by the preoperative data. The dif-
neal procedures are of myopic eyes.3,5 The 103 eyes of the 52              ference in elevation was the displacement of the posterior corneal
LASIK patients and 18 eyes of the 10 photorefractive keratectomy           surface. An ectatic change, the forward protrusion of the posterior
(PRK) patients were examined preoperatively and 4 to 8 weeks               corneal surface, was indicated by a negative number. This differ-
postoperatively. The PRK patients served as a comparative group            ence was viewed graphically versus the spherical equivalent
given that postoperative ectasia after PRK is exceedingly rare, with       (SE) correction, central ablation depth, thinnest CCT, estimated
only 1 case reported in the literature.14 The participants in the          RBT, and the estimated RBT/CCT ratio.
study were healthy individuals who met the standard criteria for
refractive surgery.
     In general, no LASIK procedures were performed when the
preoperative thinnest pachymetry reading was less than 500 mm              RESULTS

                                                                               One hundred twenty-one eyes were examined with the
                                                                           Oculus Pentacam. The ratio of right to left eyes was equal in
Accepted for publication March 25, 2006.
                                                                           the PRK group and 60:61 in the LASIK group. The mean SE
From the Albany Medical College and a private clinical practice,           corrected was greater in the LASIK group, which included
Albany, New York, USA.
                                                                           more myopic eyes (Table 1).
Presented at the Corneal Federated Societies Scientific Session,
Chicago, Illinois, USA, October 2005.
Supported in part by an unrestricted educational grant from the            Table 1. Spherical equivalent corrected.
Sight Society of Northeastern New York and the Albany Lions Eye
Bank, Albany, New York, USA.                                                                                      Correction (D)
Neither author has a financial or proprietary interest in any mate-        Procedure               Mean G SD                       Range
rial or method mentioned.
                                                                           PRK                    À2.68 G 2.25               À5.00 to À0.25
Corresponding author: Joseph B. Ciolino, MD, 361 State Street,             LASIK                  À3.76 G 2.17               À10.12 to À0.75
Apartment 3 Front, Albany, New York 12210, USA. E-mail: ciolinoj@
georgetown.edu.                                                            LASIK Z laser in situ keratomileusis; PRK Z photorefractive keratectomy




                                               J CATARACT REFRACT SURG - VOL 32, SEPTEMBER 2006                                             1427
                                               POSTERIOR CORNEAL CHANGES AFTER LASIK AND PRK



Corneal Pachymetry                                                           Table 3. Posterior surface corneal displacement.

     Central corneal measurements were thicker in the pre-                                                      Displacement (mm)
LASIK eyes than in the pre-PRK eyes. The LASIK group had
                                                                             Procedure              Mean G SD                     Range (mm)
a slightly greater mean central ablation depth. As expected,
the postoperative residual bed was thicker in the PRK                        PRK                    À0.88 G 4.65                C7.00 to À10.00
group. The ratio of estimated RBT to CCT was less in the                     LASIK                  À2.64 G 4.95                C12.00 to À14.00
LASIK group than in the PRK group (Table 2).                                 LASIK Z laser in situ keratomileusis; PRK Z photorefractive keratectomy


Posterior Corneal Surface Displacement
                                                                             DISCUSSION
     Neither group had significant posterior corneal surface
                                                                                  This study did not find a statistically significant differ-
displacement. Of the 103 LASIK eyes, 2 had posterior dis-
                                                                             ence in the forward displacement of the posterior corneal
placement outside 2 standard deviations from the mean dis-
                                                                             surface between post-LASIK eyes and post-PRK eyes. Con-
placement in the LASIK group. These 2 eyes had posterior
                                                                             trary to results in previous reports, the post-LASIK corneas
corneal surface displacements of À14 mm and À12 mm, re-
                                                                             in our study displayed a minimal change in the mean eleva-
spectively, significantly less than the mean post-LASIK
                                                                             tion of the posterior surface. Furthermore, in the 103
posterior displacement (from À20 to À40 mm) reported
                                                                             consecutive LASIK eyes, no cornea had significant for-
in other studies using the Orbscan topographer.
                                                                             ward displacement, an early indication of post-LASIK
     The mean difference in preoperative and postoperative
                                                                             ectasia.6,7,15
corneal surface elevations was small in the LASIK and PRK
                                                                                  Studies using the Orbscan3–7 report a frequent forward
groups. The difference between the LASIK group and PRK
                                                                             shift in the posterior surface elevation of the cornea after
group was not statistically significant (PO  .05, Student t test)
                                                                             LASIK. Baek et al.7 found a mean forward shift of as
(Table 3). The posterior corneal displacement in PRK and
                                                                             much as 40.9 G 24.8 mm in a retrospective review of 196
LASIK eyes was plotted against the preoperative CCT, abla-
                                                                             eyes after LASIK. Cairns et al.15 found a mean forward pro-
tion depth, and estimated RBT in 3 scatterplot graphs. The
                                                                             trusion of approximately 20 mm in 154 post-LASIK eyes.
graphs showed similar variation between the LASIK eyes
                                                                             Our study found a mean forward shift of only 2.64 G
and the PRK eyes (Figures 1 to 3). There were no significant
                                                                             4.95 mm. No eye in this study approached the mean forward
negative outlying points along the y-axis (posterior corneal
                                                                             shift that has been described elsewhere. It is likely that
displacement), consistent with an acute postoperative
                                                                             changes to the posterior corneal surface are more common
forward posterior corneal shift (corneal ectasia). The exact
                                                                             when current acceptable standards are not followed (RBT
displacement significant for ectatic change is unknown.
                                                                             !250 mm). Our study is limited in that no patient had an
     The trend lines in Figure 1 show a relationship be-
                                                                             estimated RBT below 260 mm and the deepest ablation
tween a thinner central cornea and a more negative poste-
                                                                             depth was 125 mm.
rior corneal surface displacement in LASIK eyes. The trend
                                                                                  Although the reported mean time to diagnosis of
line for PRK eyes is fairly flat, and a greater ablation depth
                                                                             post-LASIK ectasia is typically longer than 3 months (mean
resulted in a trend toward a more negative displacement
                                                                             13 months, with 30% of cases diagnosed within the first
(forward protrusion of the posterior corneal surface) in
                                                                             6 months), more acute subclinical ectatic changes have
both PRK and LASIK eyes (Figures 1 and 2). Finally, the
                                                                             been reported. Cairns et al.15 found the mean post-LASIK
trend lines in Figure 3 are relatively flat but have an inverse
                                                                             follow-up was 8.3 G 4.0 weeks, a period similar to our
relationship between the estimated RBT and the posterior
                                                                             4- to 8-week follow-up analysis.
corneal surface displacement.
                                                                                  Early studies8,16,17 found that the original Orbscan
                                                                             overestimated pachymetry in normal corneas by approxi-
Table 2. Mean CCT measurements.                                              mately 30 mm compared to ultrasound (US) pachymetry.
                                  Mean G SD                                  Therefore, the Orbscan manufacturer recommended an
                                                                             8% correction factor, which is closer to US pachymetry.
                    Ablation                                                 The 8% calculated correction, the acoustic factor, is used
Procedure CCT (mm) Depth (mm) RBT (mm)                  CCT/RBT              by the Orbscan II. The 0.92 acoustic factor is applied across
PRK          517 G 52      53 G 22       464 G 52 0.900 G 0.05               the entire cornea. However, a single correction factor across
LASIK        546 G 27      62 G 26       329 G 28 0.601 G 0.04               the entire cornea does not correct the discrepancies be-
CCT Z central corneal thickness; LASIK Z laser in situ keratomileusis;       tween the Orbscan and US pachymetry at all points on
PRK Z photorefractive keratectomy; RBT Z estimated residual bed              the cornea.8,18 The original discrepancy in pachymetry
thickness                                                                    values has been attributed to the compressive force of the



1428                                             J CATARACT REFRACT SURG - VOL 32, SEPTEMBER 2006
                                                    POSTERIOR CORNEAL CHANGES AFTER LASIK AND PRK



                 400          450        500          550           600           650
               15


               10


                5


                0                                                                                  Figure 1. Posterior corneal displacement versus thin-
Displacement




                                                                                                   nest CCT.
                                                                                        LASIK
                -5
                                                                                        PRK

               -10


               -15


               -20


               -25
                                           Thinnest CCT


US tip on the cornea and that the Orbscan measures the tear                       the Orbscan to be 13.0 mm thinner in preoperative eyes,
film and mucus layer as part of the cornea. The compressive                       29.0 mm thinner in post-LASIK eyes, and 79.0 mm thinner
force of the US tip may result in artificially thinner central                    in post-PRK eyes than the measurements with US
cornea US pachymetry values. Conversely, the inclusion of                         pachymetry.
the tear film and the mucus layer as part of the corneal                               Alterations to the optical quality of the cornea in
thickness increases the pachymetry values of the noncon-                          a post-LASIK cornea may influence the pachymetry values
tact instrument.                                                                  obtained in optical pachymetry systems such as the Orb-
     Studies13–15 after the acoustic factor was incorporated                      scan and the Pentacam. Optical pachymetry depends on
found less discrepancy between the Orbscan and US CCT                             scattered light measurements through corneal tissue.19
values in normal corneas. However, the Orbscan continues                          These light rays may be interrupted by corneal haze. Stud-
to underestimate CCT values in post-LASIK eyes compared                           ies have found an inverse relationship between increasing
to US pachymetry.6–11 Using the acoustic factor, Iskander                         degrees of corneal haze and Orbscan II pachymetry
et al.10 found Orbscan pachymetry measurements to be                              measurements.20,21 Additional studies may show whether
thinner than US pachymetry measurements by 18.4 mm                                corneal haze influences the pachymetry values of the Orb-
in preoperative eyes and by 50.1 mm in eyes after refractive                      scan and Pentacam topographers differently. A recent pre-
surgery. Prisant et al.11 found similar results, demonstrating                    sentation suggests the Pentacam may be less influenced

                     0   20         40     60        80       100         120     140
               15


               10


                5


                0                                                                                  Figure 2. Posterior corneal displacement versus abla-
Displacement




                                                                                                   tion depth.
                                                                                        LASIK
                -5
                                                                                        PRK

               -10


               -15


               -20


               -25
                                          Ablation Depth




                                                      J CATARACT REFRACT SURG - VOL 32, SEPTEMBER 2006                                            1429
                                                 POSTERIOR CORNEAL CHANGES AFTER LASIK AND PRK



                 200   250   300       350       400        450       500      550
               15


               10


                5


                0                                                                                 Figure 3. Posterior corneal displacement versus resid-
Displacement




                                                                                                  ual bed thickness.
                                                                                      LASIK
                -5
                                                                                      PRK

               -10


               -15


               -20


               -25
                             Estimated Residual Bed Thickness



by post-LASIK stromal changes (M.W. Belin, MD, ‘‘Evaluat-                       2. Belin MW, Cambier JL, Nabors JR, Ratliff CD. PAR Corneal Topography
ing the Posterior Corneal Surface,’’ presented at the annual                       System (PAR CTS): the clinical application of close-range photogram-
                                                                                   metry. Optom Vis Sci 1995; 72:828–837
meeting of the American Academy of Ophthalmology, Chi-                          3. Seitz B, Torres F, Langenbucher A, et al. Posterior corneal curvature
cago, Illinois, USA, October 2005).                                                changes after myopic laser in situ keratomileusis. Ophthalmology
     The Pentacam Scheimpflug system presents new tech-                            2001; 108:666–672; discussion by ED Donnenfeld, 673
nology that is being studied to demonstrate its clinical                        4. Kamiya K, Oshika T, Amano S, et al. Influence of excimer laser photo-
applications. Barkana et al.22 found the Pentacam has                              refractive keratectomy on the posterior corneal surface. J Cataract Re-
                                                                                   fract Surg 2000; 26:867–871
intraoperator repeatability and interoperator reproducibil-                     5. Naroo SA, Charman WN. Changes in posterior corneal curvature after
ity in measuring CCT. In the study, 2 operators measured                           photorefractive keratectomy. J Cataract Refract Surg 2000; 26:
the CCT in the right eye of 24 healthy subjects who had                            872–878
no ocular history. The coefficient of interoperator repro-                      6. Wang Z, Chen J, Yang B. Posterior corneal surface topographic
ducibility for the Pentacam Scheimpflug system was                                 changes after laser in situ keratomileusis are related to residual cor-
                                                                                   neal bed thickness. Ophthalmology 1999; 106:406–409; discussion
1.10% (intraclass correlation coefficient for interoperator                        by RK Maloney, 409–410
reproducibility was 0.985). Differences in CCT measure-                         7. Baek TM, Lee KH, Kagaya F, et al. Factors affecting the forward shift of
ments between the 2 observers were not statistically signif-                       posterior corneal surface after laser in situ keratomileusis. Ophthal-
icant (P Z .42, Wilcoxon paired-measurement test). The                             mology 2001; 108:317–320
results indicate that CCT measurements are operator inde-                       8. Chakrabarti HS, Craig JP, Brahma A, et al. Comparison of corneal thick-
                                                                                   ness measurements using ultrasound and Orbscan slit-scanning to-
pendent. Barkana et al. also found that CCT values mea-                            pography in normal and post-LASIK eyes. J Cataract Refract Surg
sured by the Pentacam and US pachymetry were similar.                              2001; 27:1823–1828
     We found little change in the posterior corneal eleva-                     9. Giessler S, Duncker GIW. Orbscan pachymetry after LASIK is not reli-
tion after LASIK compared to that after PRK. Reports that                          able [letter]. J Refract Surg 2001; 17:385–387
subclinical post-LASIK ectasia commonly occurs have                            10. Iskander NG, Anderson Penno E, Peters NT, et al. Accuracy of Orbscan
                                                                                   pachymetry measurements and DHG ultrasound pachymetry in pri-
been limited by the accuracy of the Orbscan in post-LASIK                          mary laser in situ keratomileusis and LASIK enhancement procedures.
pachymetry. Additional studies may provide more data on                            J Cataract Refract Surg 2001; 27:681–685
the accuracy of the Pentacam in post-LASIK pachymetry.                         11. Prisant O, Calderon N, Chastang P, et al. Reliability of pachymetric
Our study using the Pentacam topographer does not sug-                             measurements using Orbscan after excimer refractive surgery. Oph-
gest that post-LASIK ectasia does not occur but, rather,                           thalmology 2003; 110:511–515
                                                                               12. Twa MD, Nichols JJ, Joslin CE, et al. Characteristics of corneal ectasia
that its subclinical incidence may be less than previously                         after LASIK for myopia. Cornea 2004; 23:447–457
reported.                                                                      13. Rao SN, Raviv T, Majmudar PA, Epstein RJ. Role of Orbscan II in screen-
                                                                                   ing keratoconus suspects before refractive corneal surgery. Ophthal-
REFERENCES                                                                         mology 2002; 109:1642–1646
                                                                                                      ´
                                                                               14. Parmar D, Claoue C. Keratectasia following excimer laser photorefrac-
                         ˜
    1. Applegate RA, Nunez R, Buettner J, Howland HC. How accurately can           tive keratectomy [letter]. Acta Ophthalmol Scand 2004; 82:102–105
       videokeratographic systems measure surface elevation? Optom Vis         15. Cairns G, Ormonde SE, Gray T, et al. Assessing the accuracy of Orbscan
       Sci 1995; 72:785–792                                                        II post-LASIK: apparent keratectasia is paradoxically associated with




1430                                               J CATARACT REFRACT SURG - VOL 32, SEPTEMBER 2006
                                                  POSTERIOR CORNEAL CHANGES AFTER LASIK AND PRK



    anterior chamber depth reduction in successful procedures. Clin Exp         19. Cairns G, McGhee CNJ. Orbscan computerized topography: attributes,
    Ophthalmol 2005; 33:147–152                                                     applications, and limitations. J Cataract Refract Surg 2005; 31:205–220
       ´
16. Modis L Jr, Langenbucher A, Seitz B. Scanning-slit and specular micro-      20. Fakhry MA, Artola A, Belda JI, et al. Comparison of corneal pachyme-
    scopic pachymetry in comparison with ultrasonic determination of                try using ultrasound and Orbscan II. J Cataract Refract Surg 2002; 28:
    corneal thickness. Cornea 2001; 20:711–714                                      248–252
17. Yaylali V, Kaufman SC, Thompson HW. Corneal thickness measure-              21. Boscia F, La Tegola MG, Alessio G, Sborgia C. Accuracy of Orbscan op-
    ments with the Orbscan Topography System and ultrasonic pachyme-                tical pachymetry in corneas with haze. J Cataract Refract Surg 2002;
    try. J Cataract Refract Surg 1997; 23:1345–1350                                 28:253–258
18. Gonzalez-Me  ´ijome JM, Cervino A, Yebra-Pimentel E, Parafita MA. Cen-
                                  ˜                                             22. Barkana Y, Gerber Y, Elbaz U, et al. Central corneal thickness measure-
    tral and peripheral corneal thickness measurement with Orbscan II               ment with the Pentacam Scheimpflug system, optical low-coherence
    and topographical ultrasound pachymetry. J Cataract Refract Surg                reflectometry pachymeter, and ultrasound pachymetry. J Cataract
    2003; 29:125–132                                                                Refract Surg 2005; 31:1729–1735




                                                    J CATARACT REFRACT SURG - VOL 32, SEPTEMBER 2006                                                 1431

				
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