Reprints:Ken-Kuo Lin, MD, Department of Ophthalmology, Chang Gung Memorial Hospital, 199Tung-Hwa North Road, Taipei, Taiwan, Republic of China.

Dr. Yen-Chun Lin is from the Department of Ophthalmology, Yee Zen General Hospital,Tao-Yuan, Taiwan, Republic of China. Drs. Lee, Wu, Kao, Li, and Ken-Kuo Lin are fromthe Department of Ophthalmology, Chang Gung Memorial Hospital, Taipei, Taiwan,Republic of China.

The authors have stated that they do not have a significant financial interest or otherrelationship with any product manufacturer or provider of services discussed in thisarticle.

O R I G I N A L A R T I C L E

Keratoconus is a corneal degeneration disease withthe presentation of bilateral asymmetric central

or paracentral corneal thinning and protrusion, whichdeteriorates vision.1 In the early stage of the disease,spectacle lenses are capable of correcting the progres-sive myopia and regular astigmatism. As the diseaseprogresses, however, the astigmatism becomes greaterand irregular, and contact lenses must be fitted at thistime. Soft contact lenses play a limited role here,because they can correct only minor astigmatism;therefore, hard contact lenses are the optimal treat-ment, especially rigid gas-permeable (RGP) lenses.2 Tofit the special change of the corneal surface with local-ized protrusion, keratoconic RGP lenses have beendeveloped with 2 or 3 different base curves on eachlens back; the rate of successful fitting with theselenses is estimated to be 80% to 90%. However, the fit-ting of RGP contact lenses is time- and energy-con-suming work compared with the fitting of hydrogellenses, and the use of specially designed lenses on adiseased cornea is even more difficult; proper fittingin this setting may be experience-dependent. Anotherproblem is that RGP keratoconus lenses are not read-ily available everywhere; thus, many contact lenspractitioners are not familiar with them.

In the past 10 years, we have successfully fitted thekeratoconus eyes with nonkeratoconus RGP lenses.

Yen-Chun Lin, MD, Jiahn-Shing Lee, MD, Shiu-Chen Wu, MD, Ling-Yuh Kao, MD, & Chia-Yun Li,MD, & Ken-Kuo Lin, MD

Correction of Keratoconus WithRigid Gas-Permeable Contact Lenses

This study analyzed the feasibility of fitting keratoconus-affected

corneas with nonkeratoconus rigid gas-permeable (RGP) contact

lenses. We retrospectively studied patients with a diagnosis of ker-

atoconus in the past 10 years who were fitted successfully with

nonkeratoconus RGP lenses by the same physician. Results con-

firmed nonkeratoconus RGP lenses as the first-line correction tool.

Additionally, we found a simple equation to speed the choice of the

base curves of the lenses.

A B S T R A C T

ANN OPHTHALMOL. 2003;35(1):19–24 19

ANN OPHTHALMOL. 2003;35(1)20

We summarize our results here and describe ourattempts to find a simple rule for the quickest and eas-iest fitting.

Materials & MethodsWe retrospectively studied the cases at Chang-GungMemorial Hospital in Taipei, Taiwan, that were diag-nosed with keratoconus and in which nonkeratoconusRGP lenses were successfully fitted by the same physi-cian (K.-K.L.) in the past 10 years. The definitions ofsuccessful fitting were: RGP lens-corrected visual acu-ity of 8/20 or better, daily-wearing time of more than 12hours with no major corneal complication, and follow-up of more than 3 months. We reviewed the records forthe following: age, follow-up period, refractive status,keratometric (K) readings/disease severity, topograph-ic indexes, cone position, brand and design of the RGPlens, other corneal abnormalities at initial diagnosis,corneal complications of lens fitting, and spectacle-and RGP lens-corrected visual acuities.

We also tried to find a simple rule for determiningthe base curve of the RGP lenses, which is the mostimportant factor and the most difficult part in lens fit-ting. Pearson correlation coefficient (using the soft-ware program SPSS 9.0 for Windows) was adapted asthe statistic tool to build the connection between the Kreadings, the topographic indexes, and the basecurves of the RGP lenses. If the correlation was con-firmed, simple or multiple linear regression was per-formed. The influence of the brand (design) of thelenses and the position of the cone on the base curvesof the RGP lenses were analyzed as well.

ResultsTwenty-five patients with keratoconus (7 females and18 males) were included in the study. Forty-eight eyes(23 right eyes and 25 left eyes) were successfully fit-ted with nonkeratoconus RGP lenses. The mean age(± SD) at initial diagnosis was 20.44 ± 5.73 years(range, 12 to 33 years old). The mean follow-up timeafter RGP fitting was 37.73 ± 25.27 months (range, 5to 92 months).

The refractive status and the K readings of theseeyes are summarized in Table 1. Topographic indexesare summarized in Table 2. Table 3 lists the distribu-tions of the eyes according to disease severity, definedby steep K readings. The distributions of the eyesaccording to cone position on the topography aregiven in Table 4. In Table 5, the brand (design) of theRGP lenses is listed.

At the time of initial diagnosis, Vogt striae werefound in 17 eyes (35.42%) and Fleischer ring was seenin 22 eyes (45.83%). After RGP lens fitting, cornealstain was noted in 12 eyes (25%) and apical scarringin 3 eyes (6.25%). No other corneal complication waspresent.

The spectacle-corrected visual acuity was 1/20 to20/20 (logMAR 0.32 ± 035; range, 1.3 to 0), and RGP-corrected visual acuity was 5/20 to 20/20 (logMAR, 0.10± 0.11; range, 0.4 to 0). In terms of Snellen acuity,vision improved from 10/20 to 16/20 on average; apaired-samples t test showed a statistically significantdifference of the logMAR before and after RGP fitting(P<.001). The number of eyes with 20/40 or better cor-rected vision increased from 34 (70.83%) with specta-cle lenses to 47 (97.92%) with RGP fitting.

Refractive Status and Keratometric Readings of Eyes in Study

Refraction and K Reading

Autorefraction

Sphere

Cylinder

Subjective refraction

Sphere

Cylinder

K reading

Flat

Steep*

Cylinder

Mean ± SD (D)

–7.46 ± –4.86

–4.82 ± –2.51

–6.66 ± –5.02

–3.36 ± –3.11

45.83 ± 3.82

50.45 ± 5.49

4.62 ± 3.44

T A B L E 1

Minimum (D)

+2.00

–0.50

+3.00

0.00

39.70

41.70

0.27

Maximum (D)

–20.25

–10.00

–20.00

–13.00

56.00

60.00

13.15

No. of Eyes

41

41

45

45

46

46

46

D indicates diopters; K, keratometric.

*The steep keratometric readings in 5 eyes are limited by the upper limit of the keratometry and are repre-sented by the upper limit 60.

ANN OPHTHALMOL. 2003;35(1) 21

Topographic Indexes of Eyes in Study

Topographic Index

Simulated K reading

Flat

Steep

Minimal K reading

Mean power

3-mm zone

5-mm zone

7-mm zone

Mean ± SD (D)

45.79 ± 3.55

51.46 ± 5.63

45.24 ± 3.26

48.21 ± 4.45

46.08 ± 2.51

44.07 ± 1.47

T A B L E 2

Minimum (D)

40.80

42.50

37.00

41.10

42.13

40.75

Maximum (D)

56.20

65.20

53.10

57.77

53.53

46.92

No. of Eyes

46

46

44

44

42

39

D indicates diopters; K, keratometric.

The Pearson correlation coefficients between Kreadings, topographic indexes, and the base curveradius of the RGP lenses are listed in Tables 6 and 7.

The linear regression between the radius of a flat Kreading (Rf, in millimeters) and the radius of the basecurve (BC, in millimeters) of the RGP lenses was:

BC = 4.727 + 0.364 Rf(r = 0.805, r2 = 0.648, P =.000)

The linear regression between the radius of the Kreading (Rf, in millimeters), the steep K reading (Rs,in millimeters), and BC of the RGP lenses was:

BC = 4.769 + 0.288 Rf + 0.07812 Rs(r = 0.815, r2 = 0.665, P = .000)

The factor that may have influenced the base curveof the RGP lens, as analyzed by multiple linear regres-sion, was a steep K reading by forward selection. Bybackward selection, the factors are: a flat K reading, asteep K reading, and the brand (design) of the RGPlens. The cone position was excluded in both situations.

BC = 4.913 + 0.249 Rf + 0.112 Rs –0.119 RGPb – 0.184 RGPa

(r = 0.855, r2 = 0.731, P = 0.000)

In comparison with the Eagle Vision (bi-aspheric)lens, the Boston RXD (mono-aspheric) lens is repre-sented by RGPb, and the Aime (spheric) lens is repre-sented by RGPa. In other words, with the Eagle Visionlens, RGPa = 0 and RGPb = 0, with the Boston RXDlens, RGPa = 0 and RGPb = 1; with the Aime lens,RGPa = 1 and RGPb = 0.

DiscussionKeratoconus is a corneal degeneration disease withprogressive central or paracentral corneal thinningand protrusion. Because the cornea plays a role in

Distribution of Eyes According to Disease Severity(Steep K Readings)

Steep K Reading

<45

45-52

>52

No record

No. of Eyes

7

24

15

2

T A B L E 3

K indicates keratometric.

Distribution of Eyes According toCone Position on Topography

Cone Position*

Central cone

Paracentral cone

No record

No. of Eyes

27

19

2

T A B L E 4

*The eyes in which the steepest keratometric read-ing lies in the 3-mm zone on topography are classi-fied as central cone, and those in which thesteepest keratometric reading lies in the 5-mmzone are classified as paracentral cone.

Brand and Design of RGP Lenses in Study

Brand

Eagle Vision

Boston RXD

Aime

Design

Bi-aspheric

Mono-aspheric

Spheric

No. of Eyes

20

18

10

T A B L E 5

RGP indicates rigid gas-permeable.

ANN OPHTHALMOL. 2003;35(1)22

converging the light onto the retina, keratoconusleads to corneal astigmatism, blurred vision, and dou-ble vision. Spectacle lenses are feasible only at theearly stage of the disease; contact lenses are the majorcorrection tool for keratoconus, and surgery is consid-ered only when the patient is intolerant to the contact

lenses or when contact lenses do not provide satisfac-tory vision.3 The rate of successful lens fitting was 87%in the report by Smiddy et al,4 with 69% of patientsavoiding surgery after 63 months of follow-up. Fur-thermore, as high as 60% of the patients who hadreceived penetrating keratoplasty needed contact

Pearson Correlation Coefficient Between K Readings and Base Curve Radius of RGP Lenses

Base Curve Radius*

Radius of flat K reading

Radius of steep K reading

Radius of mean K reading

AllCases

0.805

0.720

0.799

Central

0.708

0.675

0.736

Paracentral

0.951

0.794

0.914

Cone PositionEagle Vision

0.733

0.691

0.754

Boston RXD

0.922

0.809

0.896

Aime

0.908

0.963

0.970

Lens Brand

T A B L E 6

K indicates keratometric; RGP, rigid gas-permeable.

*All correlations are significant at the .01 level (2-tailed t test).

Pearson Correlation Coefficient Between Topographic Indexes and Base Curve Radius of RGP Lenses

Base Curve Radius*

Radius of flat simulated K

Radius of steep simulated K

Radius of minimal K

Mean radius at 3 mm

Flat radius at 3 mm

Steep radius at 3mm

Mean radius at 5mm

Flat radius at 5 mm

Steep radius at 5 mm

Mean radius at 7 mm

Flat radius at 7 mm

Steep radius at 7 mm

AllCases

0.551*

0.458*

0.389*

0.478*

0.407*

0.443*

0.506*

0.584*

0.203

0.428*

0.399†

0.053

Central

0.304

0.250

0.043

0.159

0.098

0.243

0.210

0.381

0.073

0.028

0.096

–0.059

Paracentral

0.794*

0.630*

0.837*

0.721*

0.690*

0.518†

0.749*

0.652*

0.370

0.776*

0.504†

0.359

Cone PositionEagle Vision

0.570*

0.651*

0.642*

0.618*

0.415

0.666*

0.631*

0.648*

0.580†

0.566†

0.404

0.200

Boston RXD

0.621

0.324

0.099

0.296

0.334

0.168

0.487

0.554

–0.186

0.333

0.348

–0.154

Aime

0.906*

0.964*

0.899*

0.968*

0.943*

0.950*

0.931*

0.707†

0.889*

0.794†

0.416

0.765†

Lens Brand

T A B L E 7

K indicates keratometric; RGP, rigid gas-permeable.

*Correlation is significant at the .01 level (2-tailed t test).†Correlation is significant at the .05 level (2-tailed t test).

ANN OPHTHALMOL. 2003;35(1) 23

lenses for correcting high postoperative astigmatism.4

One group of authors refit the patients in whom con-tact lens failure had been diagnosed; in 2 studies, 80%and 90% of patients could wear the contact lensesmore than 12 hours with better than 20/40 vision.5,6 Wecan conclude that contact lenses play an importantrole in the management of keratoconus, and themethod of fitting is a key to successful fitting.

Soft contact lenses are limited in correcting kerato-conus for 2 reasons. First, the astigmatism in the samemeridian is not symmetrical in keratoconus, and softcontact lenses are not capable of correcting this kindof astigmatism. Second, the soft lenses must be madethick for correction of high or irregular astigmatism,and corneal edema is a common problem in this situ-ation.7 Combined lenses have been developed, aimingat reaching better vision with the hard lenses and atthe same time providing better comfort and centrationwith the soft lenses. Piggyback lenses are a hard lenson a soft one, but their problem of poor oxygen trans-mission prevented them from gaining popularity;also, the hard lens tends to drop out.7,8 Both Saturncontact lenses and Softperm lenses are made of a cen-tral hard lens and a surrounding circle of soft lens;however, this kind of design tends to suction on thecornea with time, leading to corneal edema.7,9 Hardcontact lenses therefore are the major tool in the cor-rection of keratoconus. Rigid gas-permeable lenses,with the advantages of providing better oxygen trans-mission and comfort than do the traditional poly-methyl methacrylate (PMMA) lenses, are especiallyused currently.

The fitting of hard contact lenses is difficult owingto the localized corneal changes of keratoconus—thecentral or paracentral cornea is protruded, and thenoninvolved area has relatively normal curve. Speciallenses to fit keratoconus cases with a central steeperbase curve and a surrounding flatter base curve hasbeen developed, for example, bicurve Soper lenses,tricurve lenses, or even aspheric designs.10,11 However,because these special lenses are not readily availableeverywhere, many contact lens practitioners may notbe familiar with them. Also, the fitting of contact lens-es on a diseased cornea is especially time-consumingand experience-dependent, especially with the spe-cially designed lenses. If we can fit the keratoconiccornea with nonkeratoconus RGP lenses as the first-line correction tool and reserve use of keratoconicRGP lenses for the cases in which nonkeratoconusRGP lenses failed, it may be a simpler and more prac-tical approach.

In this retrospective study, we found that kerato-conus cases, including all disease severities, could becorrected successfully with nonkeratoconus RGPlenses (mean follow-up, more than 3 years). No majorcorneal complication was noted, and the patientsgained 3 lines of Snellen visual acuity on average. Thepercentage of our patients who had better than 20/40visual acuity increased from 70.83% with spectaclecorrection to 97.92% with RGP lenses. This result is

compatible with the results of Zadnik et al12; thepatients who had better than 20/40 visual acuityincreased from 58% to 88% after contact lens fitting in1579 keratoconus cases.

From the results in Tables 6 and 7, we can makeseveral conclusions. First, K readings have better cor-relation with the base curve (BC) of the contact lensesthan any topographic indexes in general, and amongthe K readings the radius (Rf) is most correlative.From linear regression, we obtained the followingequation:

BC = 4.727 + 0.364 Rf

When BC = Rf, Rf = 7.432

The above equation can then be simplified as:

BC = 7.432 + 0.364 × (Rf – 7.432) ≅ 7.4 + 0.364 ×(Rf – 7.4) ≅ 7.4 + (Rf – 7.4)/3

This means when Rf = 7.4 mm, we can simply useRf as the BC for the contact lenses. For other values ofRf, BC would change 0.1 mm with every 0.3-mmchange of Rf from 7.4 mm in the same direction. Forexample, if Rf = 7.7 mm, then BC = 7.5 mm, and if Rf= 7.1mm, then BC = 7.3 mm.

Using this simplified equation to predict the BC ofthe RGP lenses, we calculated the difference betweenthe predicted BC and actually fitted BC in our casesand found it to be –0.03 ± 0.15 (in millimeters; mean ±SD). Because the scale of the BC of the RGP lenses is0.1, this means that in 95% of the eyes, the predictedBC would be within 0.3 (2SD) steeper or flatter thanthe finally fitted BC. This equation also implies thatwhen Rf = 7.4 mm, BC is the same as Rf; when Rf<7.4mm (such as in severe keratoconus cases), BC is flat-ter (bigger) than Rf. When Rf>7.4 mm (such as in mildkeratoconus cases), BC is steeper (smaller) than Rf,and the difference between BC and Rf is larger whenthe Rf is further from 7.4. This is compatible with thereport by Zadnik et al.2 In their series, they tried to fitkeratoconic eyes with the first definite apical clear-ance lens (FDACL). They had analyzed the habitualcontact lenses in the Collaborative Longitudinal Eval-uation of Keratoconus Study and found that corneaswith mild keratoconus (K readings<45 diopters [D])were fitted 1.18 D flatter than the FDACL; moderatekeratoconus corneas (K readings 45 to 52 D), 2.38 Dflatter; and severe keratoconus corneas (K read-ings>52 D), 4.01 D flatter than the FDACL.2

The second conclusion we made from our results isthat the correlation between BC of the contact lensesand anterior curves of the cornea is much higher inthe paracentral cone group than in the central conegroup. Multiple linear regression, however, failed tofind a linear relationship between the cone positionand the BC. This means that it may be easier to fit thekeratoconus cornea by the above simple rule if theircones are in the paracentral position, but we could notdetermine that a paracentral cone should be fitted

ANN OPHTHALMOL. 2003;35(1)24

steeper or flatter than a central cone. This is not hardto imagine, because in the central cone group, the cen-tral cornea is more distorted, and thus the curve ismore difficult to be measured and fitted.

The third conclusion deals with the brand (design)of the lens; there seems to be a tendency that the cor-relation is strongest in pure spheric design, indicatingeasier fitting with this kind of lens. Multiple linearregressions reveal the effect of this factor:

BC = 4.913 + 0.249 Rf + 0.112 Rs – 0.119 RGPb– 0.184 RGPa

The Eagle Vision lens is set to be the referencepoint in this equation. This means that to fit the sameeye, the BC of Boston RXD lenses should be approxi-mately 0.1 mm smaller (steeper), whereas the BC ofAime lenses should be 0.2 mm smaller (steeper) thanthat of Eagle vision lenses.

In conclusion, nonkeratoconus RGP lenses are fea-sible as the first-line correction tool for keratoconus.Our study results showed that eyes of all diseaseseverities could be successfully fitted with nonkerato-conus RGP lenses during an average follow-up ofmore than 3 years, and the visual acuity improved by3 Snellen lines. When choosing the base curves (BC) ofthe RGP lenses, K readings are more valuable thanany other topographic indexes, and among the K read-ings radius, Rf is the most correlative. Their relation-ship can be simplified by the equation: BC = 7.4 + (Rf– 7.4)/3.

In predicting BC with Rf, cone position and thebrand (design) of the RGP lenses are 2 factors thatmay affect the strength of the correlation. In general,if the cone is in the paracentral position and/or if the

RGP lenses are of a spheric design, the Rf (or theequation) would be more predictive of the BC. Howev-er, cone position itself does not imply steeper or flatterBC. In the same eye, Aime (spheric) lenses should befitted approximately 0.2 mm smaller (steeper) andBoston RXD (mono-aspheric) lenses 0.1 mm smaller(steeper) than Eagle Vision (bi-aspheric) lenses.

References1. Rabinowitz YS. Keratoconus. Surv Ophthalmol. 1998;42:297–319.2. Zadnik K, Barr JT, Edrington TB, et al. Baseline findings in the

collaborative longitudinal evaluation of keratoconus (CLEK)study. Invest Ophthalmol Vis Sci. 1998;39:1537–1546.

3. Lass JH, Lembach RG, Park SB, et al. Clinical management ofkeratoconus: a multicenter analysis. Ophthalmology. 1990;97:433–45.

4. Smiddy WE, Hamburg TR, Kracher GP, Stark WJ. Keratoconus:contact lens or keratoplasty? Ophthalmology. 1988;95:487–492.

5. Fowler WC, Belin MW, Chambers WA. Contact lenses in the visualcorrection of keratoconus. J Contact Lens Assoc Ophthalmol. 1988;14:203–206.

6. Belin MW, Fowler WC, Chambers WA. Keratoconus: evaluation ofrecent trends in the surgical and nonsurgical correction of kerato-conus. Ophthalmology. 1988;95:335–339.

7. Phillips AJ, Speedwell L. Contact Lenses. 4th ed. Oxford, England:Butterworth-Heinemann; 1997:693–706.

8. Tsubora K, Mashima Y, Murata H, Yamada M. A piggyback contactlens for the correction of irregular astigmatism in keratoconus.Ophthalmology. 1994;101:134–139.

9. Mannis MJ, Zadnik K. Contact lens fitting in keratoconus. J Con-tact Lens Assoc Ophthalmol. 1989;15:282–289.

10. Krachmer JH, Feder RS, Belin MW. Keratoconus and related non-inflammatory corneal thinning disorders. Surv Ophthalmol. 1984;293–332.

11. Lembach RG, Keates RH. Aspheric silicone lenses for kerato-conus. J Contact Lens Assoc Ophthalmol. 1984;10:323–335.

12. Zadnik K, Barr JT, Gordon MO, Edrington TB. Biomicroscopicsigns and disease severity in keratoconus. Cornea. 1996;15:139–146.