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Contact Lens & Anterior Eye 35 (2012) 17– 21

Contents lists available at SciVerse ScienceDirect

Contact Lens & Anterior Eye

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one location and correction of keratoconus with rigid gas-permeable contactenses

ahmood Nejabata, Mohammad Reza Khalili b, Cirous Dehghanic,∗

Associate Professor of Ophthalmology, Poostchi Ophthalmology Research Center, Department of Ophthalmology, Shiraz University of Medical Sciences, Shiraz, IranAssistant Professor of Ophthalmology, Poostchi Ophthalmology Research Center, Department of Ophthalmology, Shiraz University of Medical Sciences, Shiraz, IranOptometrist, Poostchi Ophthalmology Research Center, Department of Ophthalmology, Shiraz University of Medical Sciences, Shiraz, Iran

r t i c l e i n f o

eywords:eratoconusigid gas-permeableone locationisual acuityesidual astigmatism

a b s t r a c t

Purpose: To evaluate the influence of cone location and corneal cylinder on RGP corrected visual acuitiesand residual astigmatism in patients with keratoconus.Methods: In this prospective study, 156 eyes from 134 patients were enrolled. Complete ophthalmologicexamination including manifest refraction, Best spectacle visual acuity (BSCVA), slit-lamp biomicroscopywas performed and corneal topography analysis was done. According to the cone location on the topo-graphic map, the patients were divided into central and paracentral cone groups. Trial RGP lenses wereselected based on the flat Sim K readings and a ‘three-point touch’ fitting approach was used. Over contactlens refraction was performed, residual astigmatism (RA) was measured and best-corrected RGP visualacuities (RGPVA) were recorded.Results: The mean age (±SD) was 22.1 ± 5.3 years. 76 eyes (48.6%) had central and 80 eyes (51.4%) hadparacentral cone. Prior to RGP lenses fitting mean (±SD) subjective refraction spherical equivalent (SRSE),subjective refraction astigmatism (SRAST) and BSCVA (logMAR) were −5.04 ± 2.27 D, −3.51 ± 1.68 D and0.34 ± 0.14, respectively. There were statistically significant differences between central and paracentralcone groups in mean values of SRSE, SRAST, flat meridian (Sim K1), steep meridian (Sim K2), mean Kand corneal cylinder (p-values < 0.05). Comparison of BSCVA to RGPVA shows that vision has improved0.3 logMAR by RGP lenses (p < 0.0001). Mean (±SD) RA was −0.72 ± 0.39 D. There were no statisticallysignificant differences between RGPVAs and RAs of central and paracentral cone groups (p = 0.22) and

(p = 0.42), respectively. Pearson’s correlation analysis shows that there is a statistically significant rela-tionship between corneal cylinder and BSCVA and RGPVA, However, the relationship between cornealcylinder and residual astigmatism was not significant.Conclusions: Cone location has no effect on the RGP corrected visual acuities and residual astigmatism inpatients with keratoconus. Corneal cylinder and Sim K values influence RGP-corrected visual acuities butdo not influence residual astigmatism.

Britis

© 2011

. Introduction

Keratoconus is a non-inflammatory disorder characterized byrogressive corneal thinning, steepening, anterior protrusion andctasia of the cornea. These corneal changes may result in irreg-lar astigmatism and corneal scarring, both of which reduce theest-corrected visual acuity (BCVA) of the patient [1–3]. Despitextensive research into the potential aetiology, the exact cause of

eratoconus is not fully understood. Indeed, the aetiology of theisease is multifactorial and it has been putatively associated with

∗ Corresponding author at: Optometry Clinic, Dastgheib Hospital, Shiraz Univer-ity of Medical Sciences, Shiraz, Iran. Tel.: +98 7116288548; fax: +98 7116288548.

E-mail addresses: dehghanic@sums.ac.ir, cirdeh@gmail.com (C. Dehghani).

367-0484/$ – see front matter © 2011 British Contact Lens Association. Published by Elsoi:10.1016/j.clae.2011.08.007

h Contact Lens Association. Published by Elsevier Ltd. All rights reserved.

genetic inheritance, atopic disease, eye rubbing, contact lens wearand Down syndrome [1,2,4–6].

In early stages of the disease, spectacles may improve vision toan acceptable level. As the disease progresses, myopia and irregularastigmatism are advanced and the vision corrected with spectaclesor soft contact lenses is no longer acceptable to the patient. Rigidgas permeable (RGP) contact lenses are the optimal treatment forkeratoconus as they provide good vision by forming a new, regularoptical surface [2,3,7–9].

Computer-assisted videokeratoscopes, which generate color-coded maps and topographical indices, are currently the mostsensitive and sophisticated devices for confirming the diagnosis of

keratoconus [1]. Several numeric summaries of videokeratographicdata exist to facilitate quantitative analysis but there is no gen-erally accepted classification system for keratoconus. One of theproposed classifications is based on the cone apex conical degree

evier Ltd. All rights reserved.

1 s & Anterior Eye 35 (2012) 17– 21

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Table 1Corneal topographic Sim Ks data in all patients.

Minimum (D) Maximum (D) Mean ± SD

Flat meridian (Sim K1) 41.92 56.53 48.14 ± 3.25Steep meridian (Sim K2) 44.23 66.96 53.30 ± 4.70Mean K 43.50 61.75 50.72 ± 3.85Corneal cyl. −0.52 −11.14 −5.17 ± 2.44

D, diopter; SD, standard deviation.

Table 2Refractive status and topographic data for central and paracentral cone groups.

Cone position Mean ± SD

SRSE (D) Central −5.46 ± 2.59Para central −4.64 ± 2.10

SRAST (D) Central −3.85 ± 1.82Para central −3.20 ± 1.50

BSCVA (LogMAR) Central 0.36 ± 0.21Para central 0.31 ± 0.16

Sim K1 (D) Central 49.00 ± 3.56Para central 47.31 ± 2.70

Sim K2 (D) Central 54.69 ± 5.16Para central 51.98 ± 3.80

Mean K (D) Central 51.85 ± 4.24Para central 49.56 ± 3.10

Corneal cylinder (D) Central −5.69 ± 2.59Para central −4.67 ± 2.19

8 M. Nejabat et al. / Contact Len

hich is classified as mild, moderate and advanced [10,11]. Basedn the shape of the cone, keratoconus has been classified as roundr nipple, oval or sagging and globus. The nipple cone is smallernd more centralized; the oval cone is more inferior, whereas thelobus cone is quite large in diameter [12,13]. Based on the loca-ion of the cone, keratoconus has also been classified as central andaracentral cone [10,14].

Due to the irregular shape of the cornea, fitting the keratoconicatient with RGP lenses can be very difficult and time consuming9,11,14,15]. Three widely used fitting techniques in keratoconusre: apical clearance, apical bearing and three-point touch. Thehree point-touch technique is one of the most accepted lens-to-ornea fitting relationship in clinical practice [9,16].

The purpose of this study was to evaluate the influence of coneocation and corneal cylinder on RGP-corrected visual acuities andesidual astigmatism in patients with keratoconus.

. Methods

In this prospective study, one hundred thirty four keratoconusatients (one hundred fifty six eyes) referring to Poostchi Corneallinic, affiliated with Shiraz University of Medical Sciences between006 and 2009 were selected for investigation. The patients werexamined by two experienced clinicians and were diagnosed aseratoconus according to clinical findings (such as corneal thinning,one-like shape deformity, Vogt’s striae, Munson’s sign, Fleischer’sing, Descemet’s folds), refractive findings and corneal topographyata. Current RGP lens wearers, subjects with corneal scars, otherctatic disorders such as pellucid marginal degeneration, ptery-ium and any other ocular abnormalities and systemic diseases thatould interfere with vision or patients who could not be fitted with

ur trial lens set (based on assessment of centration, movement anduorescein pattern) were excluded from the study.

This study was performed in accordance with the Declaration ofelsinki, and approved by our institution’s ethics committee. Writ-

en informed consent was obtained from all the subjects beforenclusion. All the patients underwent complete ophthalmologicxamination including manifest refraction, best spectacle visualcuity (BSCVA), slit-lamp biomicroscopy, and corneal topographynalysis (EyeSys Vision, Houston, TX). Visual acuities were recordeds the smallest Snellen line where at least half of the numbers ofetters were read correctly.

According to the topographic map, cone location was classifieds central (if the highest power was located in central 3 mm) andaracentral (if the highest power was located out of central 3 mm).ased on Mean K reading on corneal topography, the patients werelassified into three stages: (1) K value less than 45 D as mild, (2) Kalue between 45 and 52 D as moderate and (3) K value more than2 D as advanced.

Trial lens was selected based on the flat Sim K readings onorneal topography. The lens fitting was evaluated with the slitamp with white light initially for centration and movement, andhen with cobalt blue illumination with fluorescein for assessmentf the fit.

In our fitting protocol, we attempted to have a ‘three-pointouch’ approach. If a three-point touch was not achievable, apicalearing pattern with a moderate amount of touch on the cornealpex was accepted. Maximum accepted touch at the apex of theone was 2–3 mm. Conflex KE rigid gas permeable lenses (Wohlk-ontact-Linsen GmbH) were used. These are multicurve spherical

enses with a secondary curve 1.5 mm flatter than the base curve

nd aspheric edge shaping. The trial lens parameters ranged asollows: the base curves (BC) ranged from 6.40 to 7.70 mm (in.1 mm steps), the total diameter was 9.50 and optic zone was fixed7.50 mm) and did not vary with the base curve radius.

SRSE, subjective refraction spherical equivalent; SRAST, subjective refraction astig-matism; BSCVA, best spectacle corrected visual acuity; D, diopter.

Over contact lens refraction was performed and residual astig-matism (RA) was measured using autorefractometer (TopconRM8800, Topcon Corporation, Japan). Also; retinoscopic refinementand then subjective refraction were performed. RGP visual acuities(RGPVA) were recorded by using a Snellen chart at 6 M distance.All the statistical analyses were performed using Statistical Pack-age for Social Sciences software version 11.5 (SPSS Inc., Chicago, IL,USA). The results were analyzed using Independent 2 sample t test,paired sample t test, regression analysis and Pearson’s correlationanalysis. The p-value less than 0.05 was considered as statisticallysignificant.

3. Results

One hundred fifty six eyes (82 right eyes and 74 left eyes) from134 patients were enrolled in this study. Seventy-seven (57.5%)patients were male and 57 patients (42.5%) were female. The meanage (±SD) was 22.1 ± 5.3 years. Ages ranged from 11 to 41 years.According to the cone location the patients were divided into cen-tral and paracentral cone groups. 76 eyes (48.6%) had central coneand 80 eyes (51.4%) had paracentral cone.

Based on the Mean K readings, 8 eyes (5%) had mild keratoconus(K value less than 45 D), 95 eyes (61%) had moderate keratoconus(K value between 45 and 52 D) and 53 eyes (34%) had advancedkeratoconus (K value more than 52 D). Prior to contact lens (RGP)fitting, subjective refraction was performed. Mean spherical equiv-alent was −5.04 ± 2.27 D, mean astigmatism was −3.51 ± 1.68 Dand mean BSCVA (logMAR) was 0.34 ± 0.14 (range: 0.05–1).

Table 1 summarizes minimum, maximum, mean and standarddeviation values of the flat meridian, steep meridian, mean K andcorneal cylinder according to the measurements obtained fromthe corneal topography. Table 2 summarizes refractive status andtopographic data obtained from both central and paracentral conegroups.

There were statistically significant differences between the cen-tral cone and paracentral cone groups in values of subjectiverefraction spherical equivalent, subjective refraction astigmatism,flat meridian (Sim K1), steep meridian (Sim K2), mean K and corneal

M. Nejabat et al. / Contact Lens & Anterior Eye 35 (2012) 17– 21 19

Table 3Pearson’s correlation coefficient between Sim Ks and BSCVA, RGPVA and RA.

BSCVA(logMAR)

RGPVA(logMAR)

RA (D)

Sim K1 Two groups 0.477a 0.244a 0.027Central 0.464a 0.302a 0.035Para central 0.455a 0.277a 0.017

Sim K2 Two groups 0.617a 0.341a 0.018Central 0.606a 0.416a 0.008Para central 0.610a 0.386a 0.056

Mean K Two groups 0.578a 0.311a 0.024Central 0.563a 0.380a 0.020Paracentral 0.571a 0.357a 0.042

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Table 4Pearson’s correlation coefficient between corneal cylinder and BSCVA, RGPVA andRA.

Coneposition

BSCVA(logMAR)

RGPVA(logMAR)

RA (D)

Corneal cylinder (D) Two group 0.522a 0.328a 0.033Central 0.569a 0.408a 0.023Para central 0.499a 0.328a 0.076

SCVA, best spectacle corrected visual acuity; RGPVA, RGP corrected visual acuity;A, residual astigmatism, D, diopter.a Correlation is significant at 0.01 level (2-tailed t test).

ylinder (p-values < 0.05). There was no statistically significant dif-erence between groups in values of best spectacle corrected visualcuity (p = 0.075).

RGP corrected visual acuity (logMAR) was 0.036 ± 0.04. Com-arison of BSCVA to RGP corrected visual acuity shows that visionas improved 0.3 logMAR by RGP lenses. In terms of Snellen visualcuity, vision improved from 20/42 to 20/22 on average. The num-er of the eyes with 20/40 or better corrected vision increasedrom 88 (56.4%) with spectacles to 153 (98.1%) with RGP fitting.aired sample t-test showed that there is a statistically signifi-ant difference between BSCVA and RGP corrected visual acuityp < 0.0001). Mean residual astigmatism (±SD) was −0.72 ± 0.39 Drange 0.00 to −2.00). Mean (±SD) RGP-corrected visual acuitiesf central cone and paracentral cone groups were 0.032 ± 0.035nd 0.040 ± 0.044, respectively. There was no statistically signifi-ant difference between RGP corrected visual acuities of the centralone and paracentral cone groups (p = 0.22). Mean (±SD) residualstigmatism of the central cone and paracentral cone groups was0.70 ± 0.38 and −0.75 ± 0.39, respectively. There was no statisti-

ally significant difference between residual astigmatism of centralone and paracentral cone groups (p = 0.42).

In this study, there was also an attempt to find a simple ruleor determining the base curve of this type of RGP lens. The linearegression between the Sim K1 (flat meridian) reading and the baseurve (mm) of the RGP lenses in all the patients (both central andaracentral groups) was:

RGP BC = 10.59 − 0.071 Sim K1(R = 0.819, R2 = 0.671, p < 0.0001)

The linear regression between the Sim K1 (flat meridian) readingnd the base curve (mm) of the RGP lenses in the central cone groupas:

RGP BC = 10.38 − 0.067 Sim K1(R = 0.827, R2 = 0.684, p < 0.0001)

The linear regression between the Sim K1 (flat meridian) readingnd the base curve (mm) of the RGP lenses in the paracentral coneroup was:

RGP BC = 11.255 − 0.086 Sim K1(R = 0.833, R2 = 0.693, p < 0.0001)

The results of Pearson’s correlation coefficients between Sim KsSim K1, Sim K2 and mean K) and BSCVA (LogMAR), RGPVA (Log-

AR) and RA (D) are summarized in Table 3. The results show thatn both the central and paracentral cone groups there is a statisti-ally significant relationship between Sim Ks (Sim K1, Sim K2 andean K) and BSCVA (LogMAR). There is also a statistically signif-

BSCVA, best spectacle corrected visual acuity; RGPVA, RGP corrected visual acuity;RA, residual astigmatism; D, diopter.

a Correlation is significant at 0.01 level (2-tailed t test).

icant relationship between Sim Ks (Sim K1, Sim K2 and mean K)and RGPVA (LogMAR). However, there is no statistically signifi-cant relationship between Sim Ks (Sim K1, Sim K2 and mean K)and residual astigmatism. Table 4 shows the results of Pearson’scorrelation coefficients between corneal cylinder (D) and BSCVA(LogMAR), RGPVA (LogMAR), and RA (D). The results show thatin both the central and paracentral cone groups there is a statis-tically significant relationship between the corneal cylinder andBSCVA (LogMAR). There is also statistically significant relationshipbetween corneal cylinder and RGPVA (LogMAR). However, thereis no statistically significant relationship between corneal cylinderand residual astigmatism.

4. Discussion

Due to the irregular shape of the cornea, fitting the keratoconicpatient can be very difficult and time consuming. As a result, manydifferent lens types and designs have been developed. At present,many designs of contact lenses such as rigid gas permeable (RGP),hybrid designs such as SynergEyes, piggy-back, semi-scleral, andscleral contact lens types are used [1–3,7–10,16–21]. In additionto the advantages and disadvantages that are attributed to each ofkeratoconic lenses, the use of these specially designed lenses is dif-ficult, experience-dependent and time-consuming. Furthermore,these lenses are not readily available everywhere and thus manypractitioners are not familiar with them. So, non-specially designedRGP lens could be considered as the treatment of choice concerningthe optical correction of keratoconus due to its unique character-istics and advantages. Previously in several studies, keratoconuspatients have been corrected successfully with nonkeratoconic RGPlenses [10,11,22]. Our results showed that RGP lenses improvedvisual acuity about 0.3 logMAR compared with BSCVA, In terms ofSnellen’s visual acuity, vision improved from 20/42 (0.34 logMAR)to 20/22 (0.036 logMAR) on average. The number of eyes with 20/40or better corrected vision increased from 88 (56.4%) with spectaclesto153 (98.1%) with RGP fitting. These results are somewhat bet-ter than those of the study performed by Lin et al. on 25 patients(48 eyes) with keratoconus corrected with RGP contact lenses. Intheir study, compared to BSCVA, RGP lenses corrected visual acu-ity 0.22 logMAR. In terms of snellen visual acuity (logMAR), visionimproved from 10/20 (0.32 logMAR) to 16/20 (0.11 logMAR) onaverage. The number of eyes with 20/40 or better corrected visionincreased from 70.83% with spectacles to 97.92% with RGP fitting[10]. In the study of Zadnik et al. on 1579 keratoconus cases, thenumber of eyes with 20/40 or better corrected vision increasedfrom 58% with spectacles to 88% with RGP fitting [22].

Because the central aspherical area of the cone cannot be paral-leled by a spherical back optic zone of the contact lens, three fittingrelationships are possible: these three widely debated philoso-phies are apical clearance, apical bearing and divided support

(three-point-touch) [9]. Although other philosophies such as apicalbearing and apical clearance have advantages, divided support orthree-point-touch is a delicate balance between the apical clear-ance and apical bearing techniques. This method lightly touches

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he apex with peripheral alignment and provides a well-distributedressure between the cone apex and the relatively normal periph-ral cornea. This delicate balance between a steep and a flat-fittingethod reduces the shortcomings of the two techniques. In thisethod, the weight of the lens is distributed over a larger area of

he cornea and minimizes the corneal scarring compared to apicalearing lenses and is easier to achieve than apical clearance method9]. The ‘three-point-touch’ approach is one of the most widelyccepted corneal lens fitting techniques in clinical practice and haseen found to be successful for long-term comfort and increasedearing time [9]. This cornea-lens fitting philosophy technique was

he method of choice in our study. We used RGP lenses with a largeiameter (9.50 mm) in order for the lens to touch the peripheralornea, as this part of the cornea is thicker than the central partnd can facilitate better alignment of the lens.

According to our results, in patients with central cone, meanalues of subjective refraction spherical equivalent, subjectiveefraction astigmatism, flat meridian (Sim K1), steep meridian (Sim2), mean K and corneal cylinder were higher compared withatients with paracentral cone. These finding can be explained onhe basis of the influence of the cone position on the visual axis.

centrally placed cone impinges upon and covers the visual axisnd therefore influence of irregular central cornea results in higheralues of spherical equivalent, astigmatism, flat meridian (Sim K1),teep meridian (Sim K2), mean K and corneal cylinder comparedith patients with paracentral cone. In patients with paracentral

one, the cone location is relatively outside the visual axis and hasess effect on spherical and astigmatism refraction and also on Keadings.

However, there was no difference between the central coneroup and paracentral cone group in RGP corrected visual acuitiesp = 0.22). In the study performed by Zadnik et al. on 6 patients10 eyes) with keratoconus, the eyes were divided according tolacement of the cornea’s apex to central cone (6 eyes) and inferi-rly displaced cone (4 eyes). In their study, regardless of the lensype or base curve, the patients with central cone had poorer visualcuity than those with inferiorly displaced cone [14]. Their studyncluded only 10 eyes but our study on 156 eyes (76 eyes in theentral cone and 80 eyes in the paracentral cone groups) showedhat the position of the cone cannot affect the RGP-corrected visualcuities of patients with keratoconus. The finding that RGP cor-ected visual acuity is independent of the location of cone positionan be explained by the fact that regardless of the corneal topogra-hy, RGP contact lenses form a new, regular optical surface over theurface of the cornea and virtually neutralize the regular and irreg-lar corneal astigmatism. However, measurement of visual acuities

n our study was high-contrast measurements. Low-contrast visualcuity testing might have revealed more differences between theentral and paracentral cone groups.

According to our results, there was no significant differenceetween residual astigmatisms of central cone and paracentralone groups (p = 0.42). This indicates that RGP lenses can equallyorrect different types of cone with different amounts of irregularstigmatism.

Our results showed that, in both the central and paracentralone groups there is a relationship between Sim Ks (Sim K1, Sim2 and mean K) and BSCVA (LogMAR) and RGPVA (LogMAR). Thisnding can be explained by the fact that as the keratoconus pro-resses, Sim K values increase. So, high Sim K values indicate andvanced disease and consequently worse BSCVA (LogMAR) andGPVA (LogMAR).

However, after correction with RGP lenses, there was no signif-

cant relationship between Sim Ks (Sim K1, Sim K2 and mean K)nd residual astigmatism. This is because RGP contact lenses neu-ralize the regular and irregular corneal astigmatism and lenticularstigmatism persists.

[

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nterior Eye 35 (2012) 17– 21

Based on the results, in both the central and paracentral conegroups there was a relationship between corneal cylinder andBSCVA (LogMAR). There was also a significant relationship betweencorneal cylinder and RGPVA (LogMAR). These findings can beexplained by the natural history of the keratoconus. As the diseaseprogresses and the corneal shape deformity increases, regular andirregular astigmatism become greater. So, higher values of cornealcylinder indicate a more severe stage of the disease and conse-quently worse visual acuities. However, as it can be predicted, aftercorrection with RGP lenses and conversion of the surface of thecornea to a spherical shape, there was no significant relationshipbetween corneal cylinder and residual astigmatism.

Finding a simple rule for determining the initial diagnostic RGPlens will reduce chair time in fitting keratoconus patients and canresult in a more efficient contact lens fitting process. In this studywe tried to achieve this purpose. Because the optic zone diameterdirectly determines the sagittal height of the RGP lens, the initialdiagnostic lens formula that we derived from the linear regressionthat was calculated in our patients (RGP BC ≈ 10.6 − 0.07 Sim K1)may be useful in selecting an initial diagnostic lens for this type oflens and other keratoconic GP lenses with similar optic zone sizes.

In conclusion, the results of our study performed on patientswith clinically diagnosed keratoconus clearly demonstrate that RGPlenses, due to their unique advantages, could be considered as thetreatment of choice in the correction of keratoconus. The linearregression that was calculated in our patients (RGP BC ≈ 10.6 − 0.07Sim K1) can introduce a good start point for Conflex KE lenses andother keratoconus RGP lenses with similar optic zone sizes.

According to our results, cone location has no effect on the RGPcorrected visual acuities and residual astigmatism in patients withkeratoconus. Corneal cylinder and Sim K values influence RGP-corrected visual acuities but do not influence residual astigmatism.

Acknowledgements

The authors have no commercial or proprietary interests in thematerials discussed in this manuscript.

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