Rigid gas permeable (GP) contact lenses (CL) are the first option in keratoconus patient management.1,2 Keratoconus is a progressive corneal disorder characterized by steepening and thinning of the central and paracentral cornea,1–3 which results in irregular astigmatism and decreased spectacle visual acuity.2
The fitting of GP lenses in keratoconus patients can be considered challenging4 because the altered corneal topography often requires long practitioner time and patient chair time, as well as several diagnostic lenses to achieve a final acceptable GP lens.5–7
Currently, several corneal topographers, such as Orbscan (Bausch&Lomb),8,9 Oculus-Keratograph (Oculus Optikgeräte GmbH),5 EyeSys 2000 (EyeSys Laboratories),10 Atlas (Humphrey),10 TMS1 (Tomey),11 or Medmont-300 (Medmont),12 incorporate different computerized software-based CL fitting methods, which employ corneal topography to determine the optimal GP lens parameters. The objective is to help eye care practitioners and simplify the fitting procedure, especially in patients with irregular corneas, such as keratoconus. The goal is to decrease the number of diagnostic lenses necessary to achieve an acceptable CL fit, reducing the chair time.5,8 Computerized software displays a simulated fluorescein pattern, allowing for assessment of the quality of a CL fit and making changes in the parameters or position of the simulated lens to obtain the correct GP lens.5–9
However, the repeatability of the GP parameters proposed by CL fitting software in normal and keratoconus patients has not been previously reported; the clinical utility of these tools could be affected by the decreased corneal topography repeatability in irregular corneas.13,14 These software programs normally use topographical keratometry and corneal eccentricity5 to calculate a spherical or toric lens for an initial suggestion. Moreover, low to moderate agreement between software calculated CL parameters and the final fitted lens has been reported in keratoconus patients.8,9
APEX is a new CL fitting software developed by Hecht Contactlinsen GmbH (Baden-Württemberg, Germany), and it is available in different Oculus devices (Oculus-Easygraph, Oculus-Keratograph, and Pentacam, Wetzlar, Germany). With this software, the user can define the initial lens selection to match the corneal features as closely as possible. However, the role of APEX in GP lens fitting in healthy and keratoconus patients had not been previously reported.
The aim of this study is to analyze the repeatability of the GP proposed by APEX CL fitting software and compare this back optic zone radius (BOZR) with the BOZR fit by the conventional diagnostic methods in healthy and keratoconus eyes.
MATERIALS AND METHODS
Eighty eyes in 62 patients successfully fitted with GP lens were retrospectively analyzed to compare the BOZR of the final fitted lenses with that proposed by the APEX software. Patients were classified into two groups, healthy eyes and keratoconus eyes. Independent corneal specialists confirmed the diagnoses of keratoconus after a complete eye examination, which included Scheimpflug topographical analysis and biomicroscopy examination. The keratoconus stage was identified using the Amsler-Krumeich classification.14,15
Patients with any active ocular-surface disease, corneal opacities, glaucoma, use of medication that could affect ocular physiology, and a history of any type of ocular surgery were excluded. Eyes with stage 4 keratoconus, according to the Amsler-Krumeich classification, were also excluded from the study to guarantee an optimal quality of corneal topography. Healthy group patients fitted with any type of GP toric design and those with corneal astigmatism higher than 1.50 D were also excluded.
Informed consent was obtained from each subject after the Human Sciences Ethics Committee of the University of Valladolid granted approval of the study. All subjects were treated in accordance with the Declaration of Helsinki.
Contact Lens Fitting Procedure
A rotationally symmetric bi-aspheric GP lens design (BIAS-S GP; Conoptica-Hecht Contactlinsen, Baden-Württemberg, Germany) and spherical pentacurve GP lens design (KAKC GP; Conoptica-Hecht Contactlinsen) were fitted in healthy and keratoconus eyes, respectively. Table 1 summarizes both lens designs used in this study. Four experienced practitioners, belonging to the same eye-care center (IOBA Eye Institute), performed the GP CL fitting; the practitioners work at a tertiary referral clinic that addresses patients with irregular corneas and other eye disorders.
The BOZR of the first trial lens was selected based on the keratometry value. This selected GP CL was inserted into the subject’s eye to evaluate the static and dynamic fit after an adaptation period of approximately 30 minutes. Changes in the GP lens parameters were performed to find an acceptable fit of a well-centered lens with adequate movement with blinking that provided a fluorescein pattern recommended by the manufacturer (three-point touch in the keratoconus and alignment pattern in the healthy group).1,2 Trials were repeated until an acceptable static and dynamic fit was achieved. The parameters (BOZR and lens diameter) of the prescribed GP CLs were analyzed.
APEX (version 220.127.116.11) is a new CL fitting software developed by Hecht Contactlinsen in association with Oculus. APEX proposes a first trial lens according to the values of topographical simulated keratometry readings and corneal eccentricity, and it displays a simulated fluorescein pattern of the specified design of CL to help in the GP lens fitting procedure including the specialty design for keratoconus (KAKC GP lens) and healthy eyes (BIAS S GP lens) included in this study (Fig. 1).
Three consecutive corneal topographies were performed with the Oculus-Keratograph (Patient Data Management Software version 6.02r24 and Examination Software version 1.75r11) in the baseline visit. Topographic data were exported to APEX software to determine and record the BOZR of the CL suggested by APEX (using exactly the same design that was clinically fitted; BIAS-S design in healthy eyes and KAKC design in keratoconus eyes) for each corneal topography. The Oculus-Keratograph is a Placido-based device with 22 rings that evaluate 22,000 points on the anterior corneal surface. The repeatability of Oculus-Keratograph topography in healthy and keratoconus eyes has recently been reported, providing repeatable measurements of the corneal power.14 The same blinded and experienced operators performed all Oculus-Keratograph measurements. The corneal topographer was previously calibrated by the manufacturer. The patients were asked to perform a complete blink just before each measurement to spread an optically smooth tear film over the cornea. The patients moved their chin from the chinrest between scans to eliminate the interdependence of successive measurements.
Statistical analysis was performed using the SPSS 15.0 (SPSS, Chicago, IL, USA) statistical package for Windows. We used the definition of repeatability from the British Standards Institution,16,17 as recommended by Bland and Altman.18 A normal distribution of variables was assessed using the Kolmogorov-Smirnov test (p values >0.05 indicated that the data were normally distributed).
The BOZR of the GP CL (BIAS-S and KAKC GP design) proposed by APEX to each corneal topography conducted in the same visit was calculated and recorded. The intraclass correlation coefficient (ICC; classified as follows: less than 0.75 = poor agreement; 0.75 to less than 0.90 = moderate agreement; 0.90 or greater = high agreement)19 was calculated and the differences between the three BOZR were determined with a repeated measured analysis of variance (RM-ANOVA) (p values <0.05 were considered statistically significant). The coefficient of variation (CV) of repeatability was calculated by dividing the standard deviation by the mean value (normalized standard deviation) and multiplying by 100 to represent the percentage value of the variation [CV = SD/mean × 100 (%)] of the BOZR proposed by APEX software.
As suggested by Bland and Altman, graphs of the differences between pairs of BOZR obtained in the same session were plotted against the average of the means of each pair of values (three data points per subject) to ensure that there was no relationship between the differences and ranges of measurement. The limits of the agreement (LoA) were calculated (mean of the difference ± 1.96 × standard deviations).
BOZR differences between APEX and the diagnostic method were compared with a paired t test. A p value <0.05 was considered statistically significant. The mean value of the BOZR proposed by APEX was used as the final value for comparison with the BOZR prescribed with the diagnostic method. The same lens diameter was maintained to guarantee the comparison between the BOZR of the fitted lens (diagnostic method) and the BOZR calculated by the APEX software.
The arithmetic and absolute mean difference of the BOZR were calculated between the APEX and diagnostic fitting method. The absolute difference was calculated to avoid the effect of positive and negative differences that could affect the mean value. An absolute difference could be clearly represented if the BOZR proposed by the software is close to the final fitted BOZR.
Single linear regression (R2 coefficient) was used to quantify the correlation between the APEX BOZR and final GP lens that was conventionally fitted and to propose an equation for improving the BOZR suggested by the software. A p value <0.05 was considered statistically significant.
Sixty-two patients (35 women, 27 men) were included in the study. The mean age of the total sample was 31.8 ± 10.3 years (range 12 to 55). Forty eyes of 40 patients (28 women, 12 men) comprised the healthy group and 40 eyes of 22 patients (7 women, 15 men) comprised the keratoconus group (Table 2).
Good repeatability was found in the BOZR proposed by APEX in both groups. The CV of the BOZR was 0.32% (95% CI: 0.24 to 0.39%) in the healthy group with ICC of 0.994 (95% CI: 0.991 to 0.997, p = 0.846, RM-ANOVA) and 0.51% (95% CI: 0.38 to 0.63%) in the keratoconus group with ICC of 0.989 (95% CI: 0.982 to 0.994, p = 0.323, RM-ANOVA) (Fig. 2).
The BOZR of GP lens achieved with the diagnostic fitting method and the BOZR proposed by APEX in healthy and keratoconus eyes are summarized in Table 3. The difference between both BOZR (APEX minus diagnostic method) is plotted in Fig. 3.
A strong linear relationship was found in the BOZR between the diagnostic fitting method and APEX in healthy (R2 = 0.969, p < 0.01; BOZR_diagnostic_method (mm) = (BOZR_APEX (mm) × 1.06) − 0.53) and keratoconus eyes (R2 = 0.852, p < 0.01; BOZR_diagnostic_method (mm) = (BOZR_APEX (mm) × 0.88) + 0.77) (Fig. 4).
We analyzed the sample of keratoconus eyes by the severity stage and found a similar trend with a strong linear relationship in stage 1 (R2 = 0.973, p < 0.01; BOZR_diagnostic_method (mm) = (BOZR_APEX (mm) × 0.81) + 1.38), stage 2 (R2 = 0.929, p < 0.01; BOZR_diagnostic_method (mm) = (BOZR_APEX (mm) × 0.84) + 1.07), and stage 3 (R2 = 0.831 p < 0.01; BOZR_diagnostic_method (mm) = (BOZR_APEX (mm) × 0.93) + 0.28) (Fig. 5).
The BOZR of the GP lens calculated by the APEX software could be improved with these new formulas in healthy and keratoconus eyes (Table 4). The absolute differences between the BOZR calculated by APEX software and the BOZR fitted by diagnostic method could be reduced.
Keratoconus patients can be managed with glasses or soft CLs in the early stages. However, as keratoconus progresses, the irregular astigmatism often requires GP lenses that can improve the visual acuity.1,2 Fitting GP lenses in keratoconus often requires several diagnostic lenses and can be considered more difficult than fitting GP in healthy patients.5,8,10
Computerized Placido disk-based videokeratography is the most extensively used technique for keratoconus detection and monitoring the progression of this condition. It is a valuable tool for various patient management approaches, including the fitting of specifically designed CLs.5–8,20 Moreover, different software have been designed to help eye care practitioners in CL parameter selection, proposing a BOZR and lens design and showing a simulation of the fluorescein pattern.
To the best of our knowledge, this is the first report on the repeatability of GP lens selections proposed by any CL fitting software in healthy and keratoconus patients. The BOZR suggested by APEX in healthy (CV = 0.32%) and keratoconus eyes (CV = 0.51%) is repeatable. These results could be explained because Oculus-Keratograph topography provides repeatable measurements of the corneal power (simulated keratometry and maximum corneal power) in healthy (CV ≤ 0.22%) and keratoconus (CV ≤ 0.77%) corneal assessment.14 As a result, a single Oculus-Keratograph topography could be sufficient to fit GP lenses using APEX software.
The fitting software, provided by different topographers, is helpful in fitting CLs, but the first lens proposed by the software usually requires some changes and input from an eye care practitioner before it is considered clinically acceptable, especially in irregular cornea management.5,7–9,12 Our results are in agreement with previous reports; we found statistically significant differences between the BOZR proposed by APEX and the diagnostic method in healthy and keratoconus eyes. Using the software FITSCAN (Orbscan II topography) in a keratoconus sample, Mandathara et al.8 found that the BOZR provided by this software was 0.22 mm flatter (mean difference) than the clinical fit curve. Bhatoa et al.9 stressed the existence of poor to moderate agreement between the BOZR calculated by FITSCAN software and the final BOZR in keratoconus patients. However, the BOZR calculated with APEX showed a smaller difference with the final lens fit (0.14 ± 0.12 mm; absolute difference) in keratoconus eyes, advancing the findings from previous reports. The difference between the BOZR proposed by APEX and the final fit lens could be related to the effect of the eccentricity value. In a recent publication that analyzed the repeatability of the Oculus-Keratograph, the eccentricity value showed lower repeatability in healthy (CV = 5.79%) and keratoconus (CV = 14.53%) eyes,14 and this value is included in the BOZR calculation.5 Moreover, the difference between the simulation of the software and final CL fit may also be related to the repeatability of the other CL fitting software corneal topographers and with the effect of the eyelids in the lens movement or tear film and dynamic fit assessment.9,12
We calculated the difference between two fitting methods in the arithmetic and absolute value to improve the data presentation because the mean value (arithmetic difference) could compensate the positive (BOZR_APEX > BOZR_diagnostic_method) differences with negative (BOZR_APEX < BOZR_diagnostic_method) differences, and the absolute difference could show the difference between the final BOZR and APEX BOZR proposal. In healthy eyes, we found the same arithmetic and absolute difference (0.07 ± 0.05 mm) between two fitting methods; therefore, APEX tends to propose the BOZR of GP at least as flat as the BOZR for the conventional fitting method (positive difference). This bias could be minimized using the equation calculated for healthy eyes in this study [BOZR_diagnostic_method = (BOZR_APEX × 1.06) − 0.53)].
However, in keratoconus eyes, the difference (arithmetic and absolute value) between the BOZR of both fitting methods is higher than in healthy eyes. Generally, APEX suggested flatter BOZR than the conventional fitting method; consequently, the number of trial lens required for fitting the KAKC GP lens using this software may be increased. For this reason, we proposed some equations to improve the suggested BOZR for the total keratoconus sample and for each severity stage.
With these new formulas, the difference between the BOZR that was initially proposed by APEX and the BOZR fitted by diagnostic method (Table 3) in keratoconus eyes could be reduced (Table 4), but advanced keratoconus (stage 3) showed greater differences than the lower disease stage. This could be related to the high corneal irregularity in the advanced keratoconus eyes. This may help practitioners select the BOZR of the GP lens and reduce the number of trials, improving the practitioner’s chair time and reducing the patient’s discomfort associated with multiple trials in the keratoconus GP lens fitting procedure.
Nevertheless, further clinical research is needed to prove the usefulness of these new equations in the correlation between the BOZR proposed by the software and the final BOZR fitted with a larger sample of keratoconus and healthy subjects. Moreover, a comparison of the number of trial lens required to achieve the final BOZR using both methods (diagnostic method versus APEX software) could be interesting in future studies.
The APEX software for fitting GP CL provides repeatable BOZR in both healthy and keratoconus eyes in combination with Oculus-Keratograph topography. This software could be useful in selecting the BOZR of GP lenses, but the software tends to propose a flatter BOZR than conventional CL fitting in healthy and keratoconus eyes. The BOZR suggested by APEX in keratoconus eyes should be improved with new equations to reduce this difference, helping the practitioner in the keratoconus GP lens fitting procedure.
IOBA Eye Institute, University of Valladolid
Paseo de Belen, 17
S. Ortiz-Toquero was supported by Junta Castilla y León (Consejeria de Educación), Program: Estrategia Regional de Investigación Científica, Desarrollo Tecnológico e Innovación 2007-2013 with co-funding from the Social European Fund.
None of the authors has a financial or proprietary interest in any material or method mentioned.
Received: February 24, 2015; accepted October 27, 2015.
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Keywords:© 2016 American Academy of Optometry
keratoconus; rigid gas permeable; contact lens fitting software