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RGP contact lens fitting in keratoconus using FITSCAN technology
Contact Lens & Anterior Eye 36 (2013) 126–129
Contents lists available at SciVerse ScienceDirect
Contact Lens & Anterior Eye journal homepage: www.elsevier.com/locate/clae
RGP contact lens fitting in keratoconus using FITSCAN technology Preeji S. Mandathara a,b,∗ , Mariya Fatima b , Sobia Taureen b , Srikanth Dumpati a , Mohd. Hasnat Ali a , Varsha Rathi a a b
L V Prasad Eye Institute, Hyderabad, India Bausch & Lomb School of Optometry, Kismathpur, Hyderabad, India
a r t i c l e
i n f o
Article history: Received 19 July 2012 Received in revised form 5 November 2012 Accepted 5 December 2012 Keywords: Keratoconus FITSCAN Contact lens fitting
a b s t r a c t Purpose: To assess and compare the base curve (BC) of rigid gas permeable contact lens (RGP) that were calculated by FITSCAN using corneal topography (Orbscan IIz) and the diagnostic contact lens fitting method in keratoconus eyes. Materials and methods: A prospective comparative study of 85 keratoconus eyes was conducted. Two masked observers calculated the contact lens parameters of RGP lens by diagnostic fitting method and using FITSCAN technology. The base curves calculated by two methods were compared using Wilcoxon signed rank test and agreement between two methods were analysed using Bland–Altman plot. Results: Eighty-five eyes from 55 keratoconus patients were included in the study. The mean age was 17.63 ± 2.78 (range: 12–23) years and among them 46 were males. The keratoconus was graded into mild, moderate and severe based on average keratometry values. The base curve calculated by the FITSCAN is on average 0.22 mm higher than that calculated by diagnostic method (P value <0.0001, 95% CI = 0.155, 0.245, Wilcoxon signed rank test) and the bias between the two methods was found to be 2.7% (Bland–Altman plot), indicating systematic bias between the two modalities. By single linear regression analysis, the base curve of RGP contact lens could calculated by using the formula, base curve (BC) = (FITSCAN calculated BC × 0.86563) + 0.78738. Conclusion: Our study showed that selecting the BC of the initial trial lens 0.22 mm steeper than the FITSCAN calculated base curve, may help to reduce the complexity of RGP contact lens fitting in keratoconus. © 2012 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.
1. Introduction Keratoconus is a non-inflammatory progressive ectatic condition of the cornea characterised by corneal ectasia and thinning, which results in irregular astigmatism and decrease in vision [1]. The irregular astigmatism often necessitates the use of rigid gas permeable (RGP) lenses to improve visual acuity in these patients [2–4]. RGP contact lens fitting in keratoconus is challenging due to altered corneal topography [3,5]. A trial lens (diagnostic) fitting approach is often necessary to achieve an acceptable contact lens fit in keratoconus patients and often multiple trials are required [6,7]. The corneal topography is an important tool in the management of keratoconus and it guide in the parameter selection of the initial trial lens [8–15]. FITSCAN is a contact lens fitting software in built in the OrbscanTM II z (Bausch & Lomb Surgical, Rochester, NY). By setting
∗ Corresponding author at: L V Prasad Eye Institute, Hyderabad, India. Tel.: +91 9849732605. E-mail address: [email protected] (P.S. Mandathara).
the software to individual preferences, the user can constrain the initial lens selection to match the corneal features as closely as possible [16]. The software calculates and displays a simulated fluorescein pattern of the resulting lens as well as the videokeratography map that is useful for comparing elevation or curvatures maps with the simulated contact lens design which will help in reducing the number of trials to finalise the contact lens parameters in keratoconus and thereby reducing the chair time in contact lens fitting in keratoconus. The role of FITSCAN in RGP contact lens fitting had been reported earlier [9,17]. Bhatoa et al. compared FITSCAN calculated RGP contact lens parameters with patients habitual contact lens parameters in keratoconus patients and found poor to moderate agreement between the FITSCAN calculated and patient habitual contact lens parameters. But the sample size in their study was small and majority of them were moderate to advanced keratoconus which could have affected the result of the study as the accuracy of corneal topography reduces with disease severity due to increased irregularity and presence of corneal scarring [18]. Hence the purpose of our study is to compare the contact lens parameters that were calculated by topography guided contact lens fitting software,
1367-0484/$ – see front matter © 2012 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.clae.2012.12.002
P.S. Mandathara et al. / Contact Lens & Anterior Eye 36 (2013) 126–129
127
Fig. 1. Display image of FITSCAN and static fitting of RGP lens in keratoconus eye.
FITSCAN, with the parameters that were calculated by the diagnostic contact lens fitting method in keratoconus eyes without corneal scarring.
2. Materials and methods A prospective comparative study of keratoconus patients who were referred for contact lens trial at Bausch & Lomb contact lens centre, L V Prasad Eye Institute, Hyderabad, India was conducted. To achieve a power of 80% at ˛ level of significance at 0.05, a sample size of 85 was calculated. All patients had undergone a comprehensive ocular examination prior to the entry into the study and a written consent was obtained from all the study participants. The study was undertaken after the approval from the institutional review board and all the procedures were according to the Tenets of declaration of Helsinki.
2.4. FITSCAN The examiner 1 who was masked to the base curve of the contact lens that was obtained from the diagnostic fitting method, calculated the base curve by using FITSCAN software. The demographic data and topographic data of the patient were imported from Orbscan topographer to FITSCAN and the software calculated and displayed the dual map that consisted of the calculated contact lens parameters and simulated fluorescein pattern (Fig. 1). The examiner modified the lens parameters and recalculated the simulated fluorescein pattern to achieve an acceptable lens fit. 2.5. Statistical analysis The base curves calculated by the two methods were compared using Wilcoxon signed rank test and the agreement between two methods were analysed using Bland–Altman plot.
2.1. Inclusion criteria
3. Results
Patients who were diagnosed to have Keratoconus based on clinical as well as corneal topography findings [19–21] and were referred for RGP lens trial.
Eighty-five eyes of 55 keratoconus patients were included in the study. The mean age was 17.63 ± 2.78 (range: 12–23) years and among them 46 were males. The keratoconus was graded into mild (average Sim K < 45D), moderate (average Sim K 45–52D) and severe (average Sim K > 52D) based on the average Sim K values (Table 1). The base curve calculated by the FITSCAN is (7.35 ± 0.41 mm) on average 0.22 mm flatter than that was calculated by diagnostic method (7.18 ± 0.40 mm) (Fig. 2, P value <0.0001, 95% CI = 0.155, 0.245, Wilcoxon signed rank test). The bias between the two methods was found to be 2.7%, indicating systematic bias between the two modalities (Fig. 3). From the single linear regression analysis, the base curve of the RGP contact lens can be calculated by using the formula (Fig. 4),
2.2. Exclusion criteria Patients who had undergone any surgery or trauma, any ocular or systemic pathology that is contra indicated for contact lens wear and keratoconus associated with corneal scar were excluded from the study. Corneal topography was obtained for all patients prior to contact lens fitting in the normal mode using Orbscan II by examiner 1.
2.3. Contact lens fitting procedure RGP contact lens trial was performed by examiner 2. The base curve of the initial trial lens was selected based on average Sim K value on corneal topography map. After adaptation period of 20 min, the dynamic and static fit was evaluated by the examiner 2. The aim was to achieve a well centred lens that exhibit adequate movement with blink and provide a fluorescein pattern of 3 point touch [5] and adequate edge clearance in the periphery. Trials were repeated until an acceptable dynamic and static fit achieved.
Base curve in mm(BC) = (FITSCAN calculated BC(mm) × 0.86563) + 0.78738.
4. Discussion In keratoconus management, RGP lenses play an important role in improving the visual acuity. But as the severity of keratoconus increases the corneal apex become steeper and contact lens fitting
128
P.S. Mandathara et al. / Contact Lens & Anterior Eye 36 (2013) 126–129
Table 1 The simulated keratometry reading and base curve of contact lens calculated by FITSCAN and diagnostic method in all grades of keratoconus. Mild (N = 50) Average Sim K max K1 (mm) ± SD Average Sim K min K2 (mm) ± SD Average FITSCAN base curve (mm) ± SD Average Diagnostic base curve (mm) ± SD
± ± ± ±
Moderate (N = 27)
0.31 0.33 0.28 0.30
6.47 7.50 7.19 7.03
Advance (N = 8)
0.29 0.22 0.29 0.35
0.21 0.21 0.38 0.45
Diagnostic
6.5
7.0
7.5
8.0 7.5 7.0 6.0
6.0
Linear Regression Line Loess Regression Line
6.5
Diagnostic
Fitscan
becomes more challenging. Ultimately contact lens fitting in keratoconus becomes a complex procedure that necessitates longer chair time and multiple diagnostic fitting trials. There are reports on the selection of contact lens base curve based on corneal topography indices [9–13,22] and mostly the parameter selection is based on clinician’s experience. The FITSCAN RGP fitting software utilises the anterior elevation data that represents the actual representation of the anterior corneal surface, to produce the fluorescein maps and hence thought to be a better tool in the selection of the contact lens parameters in keratoconus. In a previous study, Bhatoa et al. found a poor to moderate agreement between the FITSCAN calculated contact lens parameters and patient’s habitual contact
Bland-Altman difference plot
LAL = - 2
= -0.22
0.0
7.0
Fig. 4. Scatter plot between FITSCAN and diagnostic measurements, with fitted linear regression line and loess regression line.
lens parameters in moderate to severe keratoconus [17]. In our study 59% of cases were mild keratoconus and we could find a systematic bias between the FITSCAN and diagnostic contact lens fitting method in keratoconus. The FITSCAN predicted values were found to be 0.22 mm flatter than the diagnostic fit values. The FITSCAN by default calculates the lens diameter as 8.8 mm but in the diagnostic fit the selection of the diameter depends on the morphology and location of the cone [9] and on lens thickness and lid position. But the clinician can alter the diameter to achieve a better fluorescein pattern. The diameter calculated by the diagnostic method in our series ranged between 8.7 mm and 9.7 mm. We have maintained the same diameter calculated by the diagnostic method in FITSCAN in order to compare the base curves calculated by the two methods.
References
-0.5
6.5
8.0
The result of this study shows that the FITSCAN help in selecting the base curve of the RGP contact lens in keratoconus contact lens fitting. Selecting the initial trial lens base curve 0.22 mm steeper than the FITSCAN calculated base curve may reduce the number of trials required and thereby reduce the practitioner’s chair time and patient discomfort associated with multiple trials when we fit mild to moderate keratoconus eyes with small diameter RGP lenses.
1.0
= 0.64
7.5
5. Conclusion
0.5
UAL = + 2
7.0
Fitscan
Fig. 2. Box plot showing relationship between the base curves calculated by FITSCAN and diagnostic method.
Difference
± ± ± ±
6.24 6.98 6.77 6.63
6.5
Base Curve (mm)
± ± ± ±
8.0
6.98 7.60 7.59 7.35
7.5
8.0
Mean Fig. 3. Bland–Altman plots illustrating the degree of bias between the base curves calculated by two methods. X-Axis denotes mean values of base curve calculated by diagnostic method and FITSCAN in mm. Y-Axis denotes difference between diagnostic fit and FITSCAN base curves(mm). The mean difference is 0.22 mm and the limits of agreements are −0.22 and 0.64 mm.
[1] Krachmer JH, Feder RS, Belin MW. Keratoconus and related non-inflammatory corneal thinning disorders. Survey of Ophthalmology 1984;28:293–322. [2] Lass JH, Lemach RG, Park SB. Clinical management of keratoconus: a multicenter analysis. Ophthalmology 1990;97:433–5. [3] Lim N, Vogt U. Characteristics and functional outcomes of 130 patients with keratoconus attending a specialist contact lens clinic. Eye and Contact Lens 2002;16:54–9. [4] Woodward E. The role of rigid contact lenses in the management of keratoconus. Journal of British Contact Lens Association 1991;14:211–7. [5] Leung KK. RGP fitting philosophies for keratoconus. Clinical and Experimental Optometry 1999;82(6):230–5. [6] Zhou AJ, Kitamura K, Weissman BA. Contact lens care in keratoconus. Contact Lens and Anterior Eye 2003;26(4):171–4.
P.S. Mandathara et al. / Contact Lens & Anterior Eye 36 (2013) 126–129 [7] Edrington TB, Szczotka LB, Barr JT, Achtenberg JF, Burger DS, Janoff AM, et al. Rigid contact lens fitting relationships in keratoconus, Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study Group. Optometry and Vision Science 1999;76(10):692–9. [8] Szczotka LB, Thomas J. Comparison of axial and instantaneous videokeratographic data in keratoconus and utility in contact lens curvature prediction. CLAO Journal 1998;24(1):22–8. [9] Sorbara L, Dalton K. The use of video-keratoscopy in predicting contact lens parameters for keratoconic fitting. Contact Lens and Anterior Eye 2010;33(3):112–8. [10] Bufidis T, Konstas AG, Mamtziou E. The role of computerized corneal topography in rigid gas permeable contact lens fitting. CLAO Journal 1998;24(4): 206–9. [11] Donshik PC, Reisner DS, Luistro AE. The use of computerized videokeratography as an aid in fitting rigid gas permeable contact lenses. Transactions of the American Ophthalmological Society 1996;94:135–43, discussion 43–5. PMCID: 1312092. [12] Nejabat M, Khalili MR, Dehghani C. Cone location and correction of keratoconus with rigid gas-permeable contact lenses. Contact Lens and Anterior Eye 2012;35(1):17–21. [13] Rabinowitz YS, Garbus JJ, Garbus C, McDonnell PJ. Contact lens selection for keratoconus using a computer-assisted videophotokeratoscope. CLAO Journal 1991;17(2):88–93.
129
[14] Mandathara Sudharman P, Rathi V, Dumapati S. Rose K lenses for keratoconus—an Indian experience. Eye and Contact Lens 2010;36(4):220–2. [15] Nosch DS, Ong GL, Mavrikakis I, Morris J. The application of a computerised videokeratography (CVK) based contact lens fitting software programme on irregularly shaped corneal surfaces. Contact Lens and Anterior Eye 2007;30(4): 239–48. [16] Mannis JM, Zadnik K, Ghanem CC. Contact lenses in ophthalmic practice, vol. 48. Springer; 2004. [17] Bhatoa NS, Hau S, Ehrlich DP. A comparison of a topography-based rigid gas permeable contact lens design with a conventionally fitted lens in patients with keratoconus. Contact Lens and Anterior Eye 2010;33(3):128–35. [18] McMahon TT, Anderson RJ, Joslin CE, Rosas GA. Precision of three topography instruments in keratoconus subjects. Optometry and Vision Science 2001;78(8):599–604. [19] Holladay JT. Keratoconus detection using corneal topography. Journal of Refractive Surgery 2009;25(10 Suppl.):S958–62. [20] Rabinowitz YS, McDonnell PJ. Computer-assisted corneal topography in keratoconus. Refractive and Corneal Surgery 1989;5(6):400–8. [21] Liu Z, Zhang M, Chen J, Luo L, Chen L, Gong X, et al. Corneal topography and thickness in keratoconus. Zhonghua Yan Ke Za Zhi 2002;38(12):740–3. [22] van der Worp E, de Brabander J, Lubberman B, Marin G, Hendrikse F. Optimising RGP lens fitting in normal eyes using 3D topographic data. Contact Lens and Anterior Eye 2002;25(2):95–9.
Contents lists available at SciVerse ScienceDirect
Contact Lens & Anterior Eye journal homepage: www.elsevier.com/locate/clae
RGP contact lens fitting in keratoconus using FITSCAN technology Preeji S. Mandathara a,b,∗ , Mariya Fatima b , Sobia Taureen b , Srikanth Dumpati a , Mohd. Hasnat Ali a , Varsha Rathi a a b
L V Prasad Eye Institute, Hyderabad, India Bausch & Lomb School of Optometry, Kismathpur, Hyderabad, India
a r t i c l e
i n f o
Article history: Received 19 July 2012 Received in revised form 5 November 2012 Accepted 5 December 2012 Keywords: Keratoconus FITSCAN Contact lens fitting
a b s t r a c t Purpose: To assess and compare the base curve (BC) of rigid gas permeable contact lens (RGP) that were calculated by FITSCAN using corneal topography (Orbscan IIz) and the diagnostic contact lens fitting method in keratoconus eyes. Materials and methods: A prospective comparative study of 85 keratoconus eyes was conducted. Two masked observers calculated the contact lens parameters of RGP lens by diagnostic fitting method and using FITSCAN technology. The base curves calculated by two methods were compared using Wilcoxon signed rank test and agreement between two methods were analysed using Bland–Altman plot. Results: Eighty-five eyes from 55 keratoconus patients were included in the study. The mean age was 17.63 ± 2.78 (range: 12–23) years and among them 46 were males. The keratoconus was graded into mild, moderate and severe based on average keratometry values. The base curve calculated by the FITSCAN is on average 0.22 mm higher than that calculated by diagnostic method (P value <0.0001, 95% CI = 0.155, 0.245, Wilcoxon signed rank test) and the bias between the two methods was found to be 2.7% (Bland–Altman plot), indicating systematic bias between the two modalities. By single linear regression analysis, the base curve of RGP contact lens could calculated by using the formula, base curve (BC) = (FITSCAN calculated BC × 0.86563) + 0.78738. Conclusion: Our study showed that selecting the BC of the initial trial lens 0.22 mm steeper than the FITSCAN calculated base curve, may help to reduce the complexity of RGP contact lens fitting in keratoconus. © 2012 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.
1. Introduction Keratoconus is a non-inflammatory progressive ectatic condition of the cornea characterised by corneal ectasia and thinning, which results in irregular astigmatism and decrease in vision [1]. The irregular astigmatism often necessitates the use of rigid gas permeable (RGP) lenses to improve visual acuity in these patients [2–4]. RGP contact lens fitting in keratoconus is challenging due to altered corneal topography [3,5]. A trial lens (diagnostic) fitting approach is often necessary to achieve an acceptable contact lens fit in keratoconus patients and often multiple trials are required [6,7]. The corneal topography is an important tool in the management of keratoconus and it guide in the parameter selection of the initial trial lens [8–15]. FITSCAN is a contact lens fitting software in built in the OrbscanTM II z (Bausch & Lomb Surgical, Rochester, NY). By setting
∗ Corresponding author at: L V Prasad Eye Institute, Hyderabad, India. Tel.: +91 9849732605. E-mail address: [email protected] (P.S. Mandathara).
the software to individual preferences, the user can constrain the initial lens selection to match the corneal features as closely as possible [16]. The software calculates and displays a simulated fluorescein pattern of the resulting lens as well as the videokeratography map that is useful for comparing elevation or curvatures maps with the simulated contact lens design which will help in reducing the number of trials to finalise the contact lens parameters in keratoconus and thereby reducing the chair time in contact lens fitting in keratoconus. The role of FITSCAN in RGP contact lens fitting had been reported earlier [9,17]. Bhatoa et al. compared FITSCAN calculated RGP contact lens parameters with patients habitual contact lens parameters in keratoconus patients and found poor to moderate agreement between the FITSCAN calculated and patient habitual contact lens parameters. But the sample size in their study was small and majority of them were moderate to advanced keratoconus which could have affected the result of the study as the accuracy of corneal topography reduces with disease severity due to increased irregularity and presence of corneal scarring [18]. Hence the purpose of our study is to compare the contact lens parameters that were calculated by topography guided contact lens fitting software,
1367-0484/$ – see front matter © 2012 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.clae.2012.12.002
P.S. Mandathara et al. / Contact Lens & Anterior Eye 36 (2013) 126–129
127
Fig. 1. Display image of FITSCAN and static fitting of RGP lens in keratoconus eye.
FITSCAN, with the parameters that were calculated by the diagnostic contact lens fitting method in keratoconus eyes without corneal scarring.
2. Materials and methods A prospective comparative study of keratoconus patients who were referred for contact lens trial at Bausch & Lomb contact lens centre, L V Prasad Eye Institute, Hyderabad, India was conducted. To achieve a power of 80% at ˛ level of significance at 0.05, a sample size of 85 was calculated. All patients had undergone a comprehensive ocular examination prior to the entry into the study and a written consent was obtained from all the study participants. The study was undertaken after the approval from the institutional review board and all the procedures were according to the Tenets of declaration of Helsinki.
2.4. FITSCAN The examiner 1 who was masked to the base curve of the contact lens that was obtained from the diagnostic fitting method, calculated the base curve by using FITSCAN software. The demographic data and topographic data of the patient were imported from Orbscan topographer to FITSCAN and the software calculated and displayed the dual map that consisted of the calculated contact lens parameters and simulated fluorescein pattern (Fig. 1). The examiner modified the lens parameters and recalculated the simulated fluorescein pattern to achieve an acceptable lens fit. 2.5. Statistical analysis The base curves calculated by the two methods were compared using Wilcoxon signed rank test and the agreement between two methods were analysed using Bland–Altman plot.
2.1. Inclusion criteria
3. Results
Patients who were diagnosed to have Keratoconus based on clinical as well as corneal topography findings [19–21] and were referred for RGP lens trial.
Eighty-five eyes of 55 keratoconus patients were included in the study. The mean age was 17.63 ± 2.78 (range: 12–23) years and among them 46 were males. The keratoconus was graded into mild (average Sim K < 45D), moderate (average Sim K 45–52D) and severe (average Sim K > 52D) based on the average Sim K values (Table 1). The base curve calculated by the FITSCAN is (7.35 ± 0.41 mm) on average 0.22 mm flatter than that was calculated by diagnostic method (7.18 ± 0.40 mm) (Fig. 2, P value <0.0001, 95% CI = 0.155, 0.245, Wilcoxon signed rank test). The bias between the two methods was found to be 2.7%, indicating systematic bias between the two modalities (Fig. 3). From the single linear regression analysis, the base curve of the RGP contact lens can be calculated by using the formula (Fig. 4),
2.2. Exclusion criteria Patients who had undergone any surgery or trauma, any ocular or systemic pathology that is contra indicated for contact lens wear and keratoconus associated with corneal scar were excluded from the study. Corneal topography was obtained for all patients prior to contact lens fitting in the normal mode using Orbscan II by examiner 1.
2.3. Contact lens fitting procedure RGP contact lens trial was performed by examiner 2. The base curve of the initial trial lens was selected based on average Sim K value on corneal topography map. After adaptation period of 20 min, the dynamic and static fit was evaluated by the examiner 2. The aim was to achieve a well centred lens that exhibit adequate movement with blink and provide a fluorescein pattern of 3 point touch [5] and adequate edge clearance in the periphery. Trials were repeated until an acceptable dynamic and static fit achieved.
Base curve in mm(BC) = (FITSCAN calculated BC(mm) × 0.86563) + 0.78738.
4. Discussion In keratoconus management, RGP lenses play an important role in improving the visual acuity. But as the severity of keratoconus increases the corneal apex become steeper and contact lens fitting
128
P.S. Mandathara et al. / Contact Lens & Anterior Eye 36 (2013) 126–129
Table 1 The simulated keratometry reading and base curve of contact lens calculated by FITSCAN and diagnostic method in all grades of keratoconus. Mild (N = 50) Average Sim K max K1 (mm) ± SD Average Sim K min K2 (mm) ± SD Average FITSCAN base curve (mm) ± SD Average Diagnostic base curve (mm) ± SD
± ± ± ±
Moderate (N = 27)
0.31 0.33 0.28 0.30
6.47 7.50 7.19 7.03
Advance (N = 8)
0.29 0.22 0.29 0.35
0.21 0.21 0.38 0.45
Diagnostic
6.5
7.0
7.5
8.0 7.5 7.0 6.0
6.0
Linear Regression Line Loess Regression Line
6.5
Diagnostic
Fitscan
becomes more challenging. Ultimately contact lens fitting in keratoconus becomes a complex procedure that necessitates longer chair time and multiple diagnostic fitting trials. There are reports on the selection of contact lens base curve based on corneal topography indices [9–13,22] and mostly the parameter selection is based on clinician’s experience. The FITSCAN RGP fitting software utilises the anterior elevation data that represents the actual representation of the anterior corneal surface, to produce the fluorescein maps and hence thought to be a better tool in the selection of the contact lens parameters in keratoconus. In a previous study, Bhatoa et al. found a poor to moderate agreement between the FITSCAN calculated contact lens parameters and patient’s habitual contact
Bland-Altman difference plot
LAL = - 2
= -0.22
0.0
7.0
Fig. 4. Scatter plot between FITSCAN and diagnostic measurements, with fitted linear regression line and loess regression line.
lens parameters in moderate to severe keratoconus [17]. In our study 59% of cases were mild keratoconus and we could find a systematic bias between the FITSCAN and diagnostic contact lens fitting method in keratoconus. The FITSCAN predicted values were found to be 0.22 mm flatter than the diagnostic fit values. The FITSCAN by default calculates the lens diameter as 8.8 mm but in the diagnostic fit the selection of the diameter depends on the morphology and location of the cone [9] and on lens thickness and lid position. But the clinician can alter the diameter to achieve a better fluorescein pattern. The diameter calculated by the diagnostic method in our series ranged between 8.7 mm and 9.7 mm. We have maintained the same diameter calculated by the diagnostic method in FITSCAN in order to compare the base curves calculated by the two methods.
References
-0.5
6.5
8.0
The result of this study shows that the FITSCAN help in selecting the base curve of the RGP contact lens in keratoconus contact lens fitting. Selecting the initial trial lens base curve 0.22 mm steeper than the FITSCAN calculated base curve may reduce the number of trials required and thereby reduce the practitioner’s chair time and patient discomfort associated with multiple trials when we fit mild to moderate keratoconus eyes with small diameter RGP lenses.
1.0
= 0.64
7.5
5. Conclusion
0.5
UAL = + 2
7.0
Fitscan
Fig. 2. Box plot showing relationship between the base curves calculated by FITSCAN and diagnostic method.
Difference
± ± ± ±
6.24 6.98 6.77 6.63
6.5
Base Curve (mm)
± ± ± ±
8.0
6.98 7.60 7.59 7.35
7.5
8.0
Mean Fig. 3. Bland–Altman plots illustrating the degree of bias between the base curves calculated by two methods. X-Axis denotes mean values of base curve calculated by diagnostic method and FITSCAN in mm. Y-Axis denotes difference between diagnostic fit and FITSCAN base curves(mm). The mean difference is 0.22 mm and the limits of agreements are −0.22 and 0.64 mm.
[1] Krachmer JH, Feder RS, Belin MW. Keratoconus and related non-inflammatory corneal thinning disorders. Survey of Ophthalmology 1984;28:293–322. [2] Lass JH, Lemach RG, Park SB. Clinical management of keratoconus: a multicenter analysis. Ophthalmology 1990;97:433–5. [3] Lim N, Vogt U. Characteristics and functional outcomes of 130 patients with keratoconus attending a specialist contact lens clinic. Eye and Contact Lens 2002;16:54–9. [4] Woodward E. The role of rigid contact lenses in the management of keratoconus. Journal of British Contact Lens Association 1991;14:211–7. [5] Leung KK. RGP fitting philosophies for keratoconus. Clinical and Experimental Optometry 1999;82(6):230–5. [6] Zhou AJ, Kitamura K, Weissman BA. Contact lens care in keratoconus. Contact Lens and Anterior Eye 2003;26(4):171–4.
P.S. Mandathara et al. / Contact Lens & Anterior Eye 36 (2013) 126–129 [7] Edrington TB, Szczotka LB, Barr JT, Achtenberg JF, Burger DS, Janoff AM, et al. Rigid contact lens fitting relationships in keratoconus, Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study Group. Optometry and Vision Science 1999;76(10):692–9. [8] Szczotka LB, Thomas J. Comparison of axial and instantaneous videokeratographic data in keratoconus and utility in contact lens curvature prediction. CLAO Journal 1998;24(1):22–8. [9] Sorbara L, Dalton K. The use of video-keratoscopy in predicting contact lens parameters for keratoconic fitting. Contact Lens and Anterior Eye 2010;33(3):112–8. [10] Bufidis T, Konstas AG, Mamtziou E. The role of computerized corneal topography in rigid gas permeable contact lens fitting. CLAO Journal 1998;24(4): 206–9. [11] Donshik PC, Reisner DS, Luistro AE. The use of computerized videokeratography as an aid in fitting rigid gas permeable contact lenses. Transactions of the American Ophthalmological Society 1996;94:135–43, discussion 43–5. PMCID: 1312092. [12] Nejabat M, Khalili MR, Dehghani C. Cone location and correction of keratoconus with rigid gas-permeable contact lenses. Contact Lens and Anterior Eye 2012;35(1):17–21. [13] Rabinowitz YS, Garbus JJ, Garbus C, McDonnell PJ. Contact lens selection for keratoconus using a computer-assisted videophotokeratoscope. CLAO Journal 1991;17(2):88–93.
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