A comparison of a topography-based rigid gas permeable contact lens design witha conventionally fitted lens in patients with keratoconus

Nawtej S. Bhatoa *, Scott Hau, Daniel P. Ehrlich

Moorfields Eye Hospital NHS Foundation Trust, London, United Kingdom

Contact Lens & Anterior Eye 33 (2010) 128–135

A R T I C L E I N F O

Keywords:

Keratoconus

Topography

CVK systems

Orbscan

RGP contact lens

A B S T R A C T

Purpose: To determine if a clinically comparable contact lens could be designed for patients with

keratoconus utilising a corneal topographer and its contact lens design software.

Methods: A total of 30 patients with keratoconus were recruited for this randomized, double-masked

prospective study. Corneal topography was performed on each patient using the Orbscan II (Bausch &

Lomb, NY, USA) topographer. The topographers’ Fitscan RGP fitting software was used to derive a tri-

curve rigid contact lens specification. This was manufactured, and then compared by three experienced

clinicians with the patients’ conventionally fitted (habitual) lens. Each aspect of contact lens fitting was

graded, and each grade was weighted for kappa analysis to determine if there was any significant

difference between the Fitscan-derived lens and the patients’ habitual lens.

Results: Percentage agreement levels between the two fitting techniques were between 74% and 100%.

Kappa values varied from below 0 to 0.60, indicating poor to moderate agreement. A slight bias towards

flatter apical fitting in the Fitscan design was found in some cases of advanced keratoconus. There was no

systematic bias for either fitting technique when comparing visual performance and lens comfort. A

higher percentage of Fitscan-derived lenses were deemed suitable to dispense to patients, either directly

or after minor parameter modification.

Conclusion: This study demonstrates that a satisfactory lens design for keratoconic patients can be

produced utilising a topography-based system, when compared to a patients’ conventionally fitted lens.

The potential advantages of this system include simplified lens selection and reduced trial lens usage,

with a reduction in patient discomfort, clinical chair-time and costs.

� 2009 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.

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1. Introduction

Keratoconus is a chronic, progressive non-inflammatoryprimary ectasia of the cornea, characterised by apical thinning,steepening and distortion of the cornea [1]. This results in irregularastigmatism, and scarring with a subsequent reduction in best-corrected spectacle visual acuity (BCSVA) [2].

The incidence of keratoconus varies between 1 in 285 [3] and 1in 50,000 [4].

Diagnosis is made by clinical signs, and the use of topographicalmaps derived from computerised videokeratoscopy (CVK) [5–7].These maps show a typical pattern of an area of increased surfacepower or elevation surrounded by concentric zones of decreasingpower or elevation in the keratoconic eye [8].

* Corresponding author at: 24 Lancaster Gardens, Ealing, London W13 9JY, United

Kingdom. Tel.: +44 020 8566 3952.

E-mail address: Nawtej@Gmail.com (N.S. Bhatoa).

1367-0484/$ – see front matter � 2009 British Contact Lens Association. Published by

doi:10.1016/j.clae.2009.11.004

Clinical management of keratoconus depends on diseaseseverity, with non-surgical alternatives being the primary method.Rigid gas permeable (RGP) contact lenses are the treatment ofchoice, as these achieve far superior best-corrected visual acuities(BCVA) compared to BCSVA [9].

Conventional fitting of RGPs for keratoconus patients can bechallenging, involving a number of diagnostic contact lenses beingused before a suitable final lens can be prescribed [2]. The choice ofdiagnostic lenses is often based on central keratometry readings,practitioner experience, and previously worn lenses.

Most current CVK instruments include a RGP fitting tool in theirsoftware, allowing the clinician to design a RGP lens from thetopographical data. Jani and Szczotka [10] found greater than 80%success rate in fitting RGP lenses on normal corneas utilising thismethod. It has also been shown that topographic information canbe used to predict contact lens curvature for patients withkeratoconus [11]. The Orbscan II Topographer is a CVK which uses acalibrated video and scanning system to measure the x, y and z

location of several thousand points on the anterior and posteriorcorneal surface, and the anterior chamber. The data is used to

Elsevier Ltd. All rights reserved.

Fig. 1. Example of Fitscan screenshot.

Fig. 2. Example of Fitscan screenshot after parameter adjustment.

N.S. Bhatoa et al. / Contact Lens & Anterior Eye 33 (2010) 128–135 129

construct the topographical maps of these surfaces as well aspachymetry and keratometric measures. The elevation mapsrepresent the true topography of the cornea, and define the actuallocation of corneal surface features and irregularity [8]. The FitscanRGP fitting software utilises the anterior elevation data to producethe fluorescein maps, from which the contact lenses used for thestudy were designed. This is achieved by calculating tear layerthickness from central radius measurements, and the lensesderived can then be modified on-screen. These lenses were thencompared with the patients’ conventionally fitted contact lens. Theaim of this study was to determine whether the topographic datawas useful when trying to produce a comparable lens for this groupof patients.

2. Materials and methods

2.1. Study design

This was a prospective, double-masked randomized controlstudy, located at a tertiary referral contact lens clinic in London,U.K. from April 2005 to July 2006.

A total of 30 patients (30 eyes) were recruited for this study. Thecontrol group was the habitual RGP contact lens for the same eye ofthe same 30 patients.

The inclusion criteria for the study were consecutive, asymp-tomatic patients clinically diagnosed with keratoconus, irrespec-tive of the stage of the disease, wearing lenses which exhibited asatisfactory 3-point touch fitting, as determined by one of the 2authors. This method of fitting is widely accepted as a means ofevenly distributing the weight of a RGP lens between the cone andmore peripheral cornea [12]. The subjects’ right eye was used forthe study.

Exclusion criteria were those patients who had a history ofother co-morbidity, amblyopia, previous surgery or trauma.Patients whose topographical plots yielded insufficient usefuldata after 5 attempts were also excluded.

Subjects had their corneal topography mapped within 30 min ofRGP lens removal. The Fitscan RGP fitting software was utilised todesign a tri-curve rigid contact lens based upon the topographicaldata obtained. (It should be noted that the software only permittedthe design of a tri-curve lens, and, although a toric lens design waspossible, only non-toric tri-curve lenses were designed in thisstudy.)

The default settings of the Fitscan program were initiallyuntouched. On-screen, a fluorescein map was displayed showingthe computed contact lens fit over the topographic scan. Greencolour denoted fluorescein, indicating clearance between lens andcornea. The absence of fluorescein, denoted by black, indicatedzero clearance or contact between the 2 surfaces.

The far-right boxes showed parameters for the curves of thecomputed RGP lenses, and the width of each curve in the ‘zones’boxes. These parameters were then altered to obtain a fluoresceinpattern which matched closest on-screen to an ideal three-pointtouch method of fitting.

‘‘OZ flat/steep’’ were adjusted first to give an apical ‘‘glancingtouch’’ appearance, whereby the fluorescein pattern just showedan absence of green under the apex, or steepest point of the cornea.This was fine tuned by making adjustments to the base diameterand other parameters to achieve the desired endpoint. The effect ofany parameter change could be instantly viewed by clicking ‘‘FitLens’’ which re-computed the fluorescein pattern.

The second and peripheral curves were designed to rapidlyflatten, to closely follow the relatively typical peripheral cornealtopography in the keratoconic eye and to create a peripheralcontact zone to aid the even bearing of the lens over the cornea.

The computed appearance of the peripheral curve was alteredby radius and zone width adjustments to give a regular zone ofedge clearance. Altering the peripheral width was an option toincrease or decrease the total diameter. Adjusting the BOZD wouldautomatically alter the total diameter of the computed lens, e.g.reducing the BOZD resulted in a smaller total diameter (wT).

Fig. 1 illustrates the Fitscan software fluorescein fit using thedefault RGP fitting parameters. It shows heavy apical contact andexcessive mid-peripheral pooling and edge clearance, indicating aflat fitting lens.

Fig. 2 shows the computed fluorescein pattern of the samepatient after manual parameter adjustment Fitscan lenses weredesigned on-screen by 2 masked observers (NB and SH).

An independent masked observer recorded the habitual lens’Back Vertex Power (BVP) and BOZR parameters, which was used toderive the Fitscan-derived lens’ BVP, using a conversion formula[13].

The Fitscan-derived lenses were produced in polymethyl-methacrylate material by 1 senior contact lens technician, andverified to BSI tolerances [14]. Follow-up for in vivo grading forboth habitual and Fitscan lenses was made within a 2-week periodfrom topography, to minimize the effect of any progression of the

Fig. 3. Grading sheet.

N.S. Bhatoa et al. / Contact Lens & Anterior Eye 33 (2010) 128–135130

Fig. 5.

N.S. Bhatoa et al. / Contact Lens & Anterior Eye 33 (2010) 128–135 131

subjects’ keratoconus. Values for steepest keratometry (K) readingswere obtained from the Orbscan plot. The degree of keratoconuswas defined as follows:

Mild keratoconus: steepest ‘K’ � 45.00 Dioptres (D); moderate‘K’ 45.00–52.00 D; advanced ‘K’ � 52.00 D [15].

2.2. Grading assessment

Patients were masked and randomized by NB into 2 groups onthe toss of a coin, such that half wore their habitual lenses for thefirst grading assessment, and the remainder wore the Fitscan-derived lenses. Grading for both lens types was done consecutively,in the same session, with each grader assessing each patientwearing one lens type, then the other.

A grading sheet (Fig. 3) was used to quantify each aspect of lensfitting, with each grader using a separate sheet for the Fitscan lensand one for the habitual. The main fitting characteristics of a rigidcontact lens fitting, such as centration and apical contact, werecategorised and sub-divided into an empirical fitting scale by theauthors.

For the in vivo assessments patients were required to wear eachlens for 20 min to stabilise the lens performance. Each grader thenassessed each lens fitting on the slit-lamp bio-microscope, utilisingwhite light and the cobalt blue filter after instillation of fluorescein.Graders were allowed to refer to a pair of photographic referencesheets produced by NB, illustrating a range of grades of RGP fittingsto be used as guidelines if required (although all graders weresenior contact lens clinic staff with between 5 and 15 years offitting experience). Graders were permitted to manipulate thepatients’ eyelids and lens as per normal clinical routine.

Fig. 4 illustrates the habitual lens fitting for database patient no.6. There is light/medium apical contact, the lens is decentredwithin the limbus, edge clearance is acceptable to good, mid-peripheral pooling is acceptable and there is moderate peripheralcontact.

Fig. 5 illustrates the Fitscan lens for the same patient. Thisshows similar fitting characteristics to the habitual lens, and wasgraded very similarly by all 3 graders.

After assessing each lens, the graders were asked: ‘‘would youdispense this lens’’? If the answer was negative, they would thenanswer the next question ‘‘would you dispense this lens after aminor parameter change’’? This would then give a simpleindication as to how suitable the lens was for issuing in termsof fitting, either directly or by minor parameter change.

Standardised visual function assessment consisted of bestsphere-corrected LogMAR acuity under ETDRS conditions [16], andcontrast sensitivity measurement using a Pelli-Robson chart at 1 m

Fig. 4.

with over-refraction [17]. A figure for comfort score was producedby asking the patient to indicate the comfort of the lens at thatpoint, using a scale of 0–100 with zero indicating poor comfort, 100being ideal [18].

2.3. Data analysis

Statistical analysis was performed with STATA 9.0 (STATA Corp.,Texas, USA).

Weighted kappa figures (k) were assigned to every grade toindicate the clinical importance of grader disagreement for thatparticular category. The weights were assigned empirically by theauthors. Kappa statistics were interpreted using the standardranges [19].

Percentage agreement was calculated to assess the degree ofagreement/disagreement between the graders. To ascertain thedegree of bias between the two lenses for LogMAR acuity, Pelli-Robson and comfort scores, Bland–Altman [20] plots wereproduced from the recorded data and paired t-tests were alsoperformed. P-values were calculated to determine if there werestatistically significant differences in the two fitting methods,P � 0.05 being defined as significant.

3. Results

Fig. 6 illustrates how the majority of the study population werefrom the moderate and advanced cases of Keratoconus, accordingto their steepest-K values.

Fig. 6. Study population distribution according to steepest Sim-k values.

Table 1Intra-observer comparison (lowest agreement per category highlighted).

Grader 1 Grader 2 Grader 3

Agreement Kappa Agreement Kappa Agreement Kappa

Apical contact 89.17 0.13333 76.67 0.1268 91.67 0.4755

Deepest mid-pool 100.00 91.67 �0.0563 95.00 0.3662

Superior edge clearance 86.67 0.4366 82.50 0.1732 92.50 0.5455

Temporal edge clearance 82.50 0.1732 74.17 0.1210 85.00 �0.0345

Nasal edge clearance 82.50 �0.1053 76.67 0.0968 85.83 �0.1233

Inferior edge clearance 88.33 0.1954 89.17 0.4507 89.17 0.3730

Peripheral contact zone 95.00 0.6000 92.50 0.5574 95.00 0.3793

Post-blink movement 88.33 0.3269 85.00 0.1176 89.17 0.2883

Centration 86.67 0.5628 84.17 0.4172 81.67 0.4770

N.S. Bhatoa et al. / Contact Lens & Anterior Eye 33 (2010) 128–135132

On review of the topography maps, all patients in this studyexhibited paracentral, or nipple-type cones, with the conesdisplaced infero-temporally.

3.1. Intra-observer comparison

Comparison of the two lens types (see Table 1) showsconsiderable percentage agreement in all fields, with a range of74.17–100.00%. Kappa values vary from below 0 to 0.60 indicatingpoor to moderate agreement.

To investigate possible source for disagreement, it was decidedto review the raw data, and analyse the cross-tabulation of thegradings derived for individual categories. For consistency, and toavoid selection bias, it was appropriate to use the lowestpercentage agreement of the three graders (highlighted inTable 1) for each individual grading category.

3.2. Visual assessments

Bland–Altman plots were produced to illustrate the degree ofbias between the two lens modalities in terms of the visualfunction and comfort score data.

In Fig. 7, the Y-axis represents the difference in LogMAR valuesbetween the two lens modalities i.e. Fitscan – habitual. The X-axis

Fig. 7. Paired t-test and Bland & Altman plot of bias between the two lenses for

LogMAR acuity.

represents average LogMAR acuity. The reference range representsthe mean value �2 standard deviations (denoted by the dotted lineson the plot). The graph shows the mean difference being close to zero.P-value is >0.05 indicating no systematic bias between the twomodalities.

3.3. Dispensing questions

Fig. 8 shows the difference in Pelli-Robson scores between the 2lenses with habitual being subtracted from the Fitscan. Again, themean is close to zero, and as p > 0.05 indicates no systematic biasbetween the two methods.

In Fig. 9 there is a reasonable cluster around the zero differenceline and a slight preference for the habitual lenses. P > 0.05 sothere indicating that there was no systematic bias between the twomethods.

In Table 2, ‘‘dispense after minor modification’’ refers to thepercentage of lenses out of the study population of 30, whichwould have been dispensed after making a minor parameterchange.

For the Fitscan lens, all 3 graders had higher values fordispensing without modifying the fitting parameters as comparedto the habitual lens. Graders 1 and 3 would have dispensed nearlyall lenses, either directly or after a minor parameter change. Grader2 would have dispensed 90% of Fitscan lenses either directly or viaminor change, with the corresponding figure being 73% for thehabitual lens. Some lenses were not graded as suitable to dispense.

Fig. 8. Paired t-test and Bland & Altman plot of bias between the two lenses for Pelli-

Robson score.

Table 2Dispensing the RGP lenses based upon the fitting characteristics.

Grader 1 Grader 2 Grader 3

Dispense without

modification

Dispense after

minor modification

Dispense without

modification

Dispense after

minor modification

Dispense without

modification

Dispense after minor

modification

Fitscan lens 86.67 13.33 76.67 13.33 80.00 20.00

Habitual lens 76.67 23.33 63.33 10.00 76.67 20.00

Fig. 9. Paired t-test and Bland & Altman plot of bias between the two lenses for

comfort score.

N.S. Bhatoa et al. / Contact Lens & Anterior Eye 33 (2010) 128–135 133

4. Discussion

4.1. Study population

The majority of study eyes were of a moderate or advancedgrade of keratoconus, despite recruiting consecutive cases. Thesecases are more likely to be wearing an RGP correction as comparedto the mild grade, where a spectacle correction may be adequate.

4.2. Fitting categories: apical contact

Data review showed that the Fitscan lens had flatter fitting inthe 3 cases of 2-step disagreement (moderate/heavy contactversus ideal). These cases had advanced keratoconus suggestingthat it may be better to aim for slight pooling or clearance whilstutilising the software for this group.

To achieve the desired apical contact, the user relies on anaccurate measure of the height and position of the corneal apex,and this is affected by observer training [21], patient experienceand cooperation [22], as well as short-term variability andrepeatability of the instrument. There is increased short-termvariability for keratoconic subjects compared to normal subjects[23]. Accuracy is further compromised [24] in more advancedcases.

There were 7 cases of habitual lenses graded as having the leastdesirable apical fittings. This may partly be due to the compositionof the study group, where consecutive asymptomatic patientswere chosen rather than those having ideal fitting lenses. Thesemay have been accepted clinically, and provided there were noadverse clinical signs such as increased stromal scarring orsignificant epithelial staining, the patient could be allowed tocontinue with this fitting.

4.3. Deepest mid-peripheral pooling and peripheral contact zone

High percentage agreements here may be due to the relativelynormal mid-peripheral and peripheral topography in patients withparacentral cones. Fitting normal corneas with CVK systems hasbeen shown to be successful in other studies [10,25]. It should alsobe noted that in some categories such as deepest mid-peripheralpooling, graders had a choice of three grades only, therefore therewas a higher level of expected agreement.

4.4. Edge clearance

The habitual lens group performed slightly better for superioredge clearance, although there was still 82% agreement, with only1 case of 2-step disagreement favouring each type. Agreement forinferior edge clearance was 88%, with only a single 2-stepdisagreement which favoured the habitual lens. Edge clearancedepends very much on centration. With the corneal apexdisplaced infero-temporally in the majority of keratoconuspatients [8] there is a potential for RGP lenses to locate inferiorlyon the cornea, with a tight superior edge resulting in cornealindentation, abrasion, hypoxic stress or increased lens awareness.Peripheral curves are flattened to alleviate this but can result inincreased inferior clearance and potential instability on the eye.Hence compromise in fitting is often required, as demonstrated bythe relatively low number of ideal grades for both lens types.Agreement levels for temporal and nasal edge clearance werelower than in other categories, at 74% and 77%. As indicated above,greater importance is placed on improving vertical edgeclearance, and possibly accepting non-ideal horizontal edgeclearance.

4.5. Post-blink movement

In addition to edge clearance, this aspect of fitting dependsconsiderably on eyelid, tear film and lens–cornea interactions. Asthis can only be assessed by clinical examination, it can be arguedthat the Fitscan lenses performed as well as could reasonably beexpected for a static system, with 85% agreement.

4.6. Centration

Lens centration is highly influenced by apex position, eyelid andtear film interaction. The level of agreement of 82% may be as goodas may reasonably be expected for a static system. As the majorityof cones are displaced inferiorly, this may contribute towards abias of inferiorly displaced lenses observed in the study. This isseen in the relatively high number of corresponding grades forboth lens types.

With respect to total diameter, it can be seen that different RGPfitting methodologies successfully utilise varying lens diameters[12]. In the 3-point touch method adopted, primary concern isgiven to equal weight distribution over the cone. Lens diameter canaffect centration, but there was limited control over total diameterin the software. Hence, acceptance or rejection of a keratoconiclens fitting was not dependant on this parameter for the purposesof this study.

N.S. Bhatoa et al. / Contact Lens & Anterior Eye 33 (2010) 128–135134

The authors noted that the software produced lenses whichwere generally smaller in total diameter than those regularlyprescribed in the clinic. This has also been noted in another study[25]. Larger lenses may result in a more compromised fitting insome cases, which may produce a more acceptable fluoresceinpicture. A smaller lens may result in reduced stability, centration,comfort and vision.

4.7. Visual assessment

There was no evidence of systematic preference for eachmethod with regard to LogMAR acuity, Pelli-Robson and comfortscore measures. Mean LogMAR score difference was nearly zero,with the majority of the differences occurring within 0.1 units,which is within the clinical repeatability of the ETDRS charts[26].

Fig. 5 highlights the difference in Pelli-Robson scores betweenthe 2 lens types. There are more outliers on this plot. Cornealscarring may cause increased variability when performing thismeasure.

4.8. Comfort score

For comfort score there was a slight preference for the habituallens type. Patients were more familiar with the fitting of their ownlens. The habitual lenses, being generally larger, would tend tolocate to a greater degree beneath the top eyelid, resulting in areduced lid sensation on blinking [27].

4.9. Contact lens dispensing based upon fitting characteristics

There was a high percentage of cases where the Fitscan lenseswere deemed suitable for dispensing to the patient, either directlyor after minor parameter change (Table 2). As discussed previously,the habitual lens was not necessarily a perfect 3-point touchfitting, the lens being deemed suitable to wear on a clinical basis,after consideration of history, signs and symptoms at the clinicalvisit. This may provide a reason for the lower habitual lenspercentage scores.

4.10. Statistical analysis

A sample size of 30 was used for this study. It should be notedthat a sample size of 50 is recommended for method comparisonstudies as described by Altman [20]. Without a previouscomparable study, the size of any measured affect could not bereferred to. Further study with a greater sample size may beadvisable.

Statistical analysis showed considerable agreement betweenthe 2 fitting methods in most grading categories, ranging from 74%to 92%. Kappa weighted statistics indicated poor to moderateagreement. Analysis of k-statistics can be problematic as itdepends on the prevalence of the fitting characteristic beingmeasured [20]. Due to low sample size and uneven distribution ofthe data, some low k-measures were obtained. These k-values arecomparable to previous studies involving keratoconic patients andother method comparison studies [28,29].

4.11. Study limitations

The percentage of the cornea mapped decreases with increasedcorneal steepness and irregularity [23]. As the software super-imposes the RGP curvature onto the incomplete topographicalplot, some visual extrapolation is required to complete thefluorescein picture. This is particularly relevant for the peripheralcurves, as the data here is often incomplete.

Repeated scans were required for some patients, and some hadunusable scans. Other studies have also highlighted this disadvan-tage [23,25].

There were variable agreements between graders for both lenstypes, corresponding with results from another study [29]. Cornealdistortion results in asymmetric fluorescein patterns due to highdegrees of irregular astigmatism. In addition, variable lenspositioning and stability can result in differences in interpretationbased on clinician experience. In this study, the most experiencedgrader produced the majority (7/9) of lowest-level agreement levels.

The Fitscan system was designed for regular RGP fitting, notspecifically for keratoconic patients. Other CVK systems areavailable which are designed for irregular corneal topography [30].

All Fitscan lenses were produced in PMMA material to reducestudy expenses. Ideally, the Fitscan lenses would have been madeof the same gas permeable material as the patients’ habitual lensfor a more accurate comparison, and would have also allowedwider tint choice for the Fitscan lens. Furthermore, if costspermitted, the patients’ habitual lenses would have been replacedwith new lenses to match the new condition of the Fitscan lenses.As the habitual lenses had been passed as suitable to continue towear clinically prior to inclusion in the study, one can assume theircondition was reasonable, as they would normally have been re-polished or replaced.

5. Conclusion

This study has shown that customised topography-basedcontact lens software allows patients with keratoconus to befitted satisfactorily when compared to a conventional, diagnosticlens based method.

With this system there is the potential to reduce number of triallenses used, therefore improving comfort in the fitting process,with less risk of abrasion, trauma and cross-infection.

As some aspects of fitting can only be accurately assessed byclinical examination, the software can be regarded as a useful toolfor RGP fitting in keratoconus, but not as substitute for clinicalassessment. A well fitting lens also needs to be dispensed to thepatient, and worn over a period of time. It then requires re-evaluation before it can be considered as successful. Further studyregarding this is warranted. A larger study group could also assistin identifying if the technique were more suitable for differentgrades of the condition in terms of corneal steepening andmorphological grouping.

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