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ARTICLE Keratometric index in keratoconic eyes before and after intracorneal ring segment implantation Victoria de Rojas, MD, PhD1; Antía Gestoso, MD1; Alejandra Gómez, MD1; Margarita de la Fuente, MD1; María López, MD1; Renata Rodrígues, MD1; Isabel López, MD1; Josefina Pombo, MD1 OBJETIVE: To compare anterior and posterior corneal surface curvature in normal (control) eyes, keratoconic (KC) eyes and keratoconic eyes after intracorneal ring segment (KCICRS) implantation. SETTING: Complexo Hospitalario Universitario, A Coruña (Spain). METHOD: Retrospective study in which 30 eyes with KC before and after ICRS implantation and 30 control eyes were analyzed. The mean radius of the anterior (antR) and posterior (postR) corneal surface and true net power were determined using images from a rotating Scheimpflug camera, from which the antR/postR ratio and the mean keratometric index (KI) were calculated for each group. RESULTS: Mean antR and postR were 7.82 ± 0.31 mm and 6.42 ± 0.37 mm in control eyes, 6.94 ± 0.46 mm and 5.51 ± 0.5 mm in KC eyes, and 7.18 ± 0.5 mm and 5.5 ± 0.49 mm in KCICRS eyes, antR being significantly different between all groups (p = 0.0001) and postR significantly different between control and KC eyes and between control and KCICRS eyes (p = 0.0001). The antR/postR ratio between the three groups was significantly different (p = 0.0001). Mean KI between control, KC and KCICRS eyes was statistically different (p = 0.0001 KC vs. KCICRS, KCICRS vs. control; p = 0.012 KC vs.control). CONCLUSION: Anterior and posterior corneal curvature is significantly greater in KC eyes than in control eyes, with an increase in the antR/postR ratio. After ICRS implantation, the anterior surface is flattened, while there are no significant changes in the posterior surface. These findings suggest that the standard KI is not valid for calculating intraocular lens power in KC or KCICRS eyes. J Emmetropia 2014; 5: 9-13 Calculation of intraocular lens power is more complex after corneal refractive surgery. Two causes of error have been identified: error in calculating real corneal power after surgery and error in calculating the effective position of the lens after surgery. The first of these errors, corneal power calculation, has a double origin. On the one hand, the instruments cannot Submitted: 11/19/2013 Accepted: 01/05/2014 Department of Ophthalmology. Complexo Hospitalario Universitario, A Coruña (Spain). 1 Financial disclosure: The authors do not have any financial interest in any of the products mentioned. Corresponding Author: Victoria de Rojas Silva Urbanización Aldea Nova, 68 15220 Ames, A Coruña, Spain. E-mail: [email protected] © 2014 SECOIR Sociedad Española de Cirugía Ocular Implanto-Refractiva accurately determine the power in the central area modified by surgery, and on the other hand, an error is introduced in the keratometric index when the ratio between the anterior and posterior corneal surface radii is changed. All these errors have been well documented in the setting of laser corneal refractive surgery1,2,3. It was thought for years that surgery by radial keratotomy would be affected by the former errors but not by the latter, as the anterior and posterior corneal surfaces were modified simultaneously. Recently however, Camellin et al.4 showed that after radial keratotomy, both the anterior and the posterior corneal surfaces are flattened, but the second to a greater extent than the first, causing an decrease in the ratio between the anterior and posterior surfaces, with the resulting increase in the keratometric index. Calculating intraocular lens power in KC eyes is complex and has been addressed in very few studies in the literature5,6,7. Even rarer are those dealing with the calculation after ICRS implantation8. As occurred ISSN: 2171-4703 9 10 KERATOMETRIC INDEX IN KERATOCONIC EYES WITH ICRS in radial keratotomy, some authors assume that since the procedure is additive, the change in curvature would be similar in both the anterior and the posterior surfaces8. In this study, we use a design similar to that of Camellin et al.4 in radial keratotomy to evaluate firstly the ratio between the anterior and posterior corneal radii and the keratometric index in a group of patients with keratoconus compared to a control group without keratoconus, and secondly, to evaluate the change in these parameters after ICRS implantation in the group with keratoconic eyes. In this way, the keratometric index was evaluated in keratoconic patients before and after ICRS implantation, with the aim of estimating if such a change would contribute to the error in the calculation of intraocular lens power in these cases. PATIENTS AND METHODS A retrospective study was carried out of patients with keratoconus undergoing ICRS implantation. Patients were identified from the database of the Ophthalmology Department of the Complexo Hospitalario Universitario A Coruña and their clinical records were reviewed to collect the following information: age, sex and time since surgery. Only one eye per patient was included and cases with a follow-up of less than 5 months were excluded. Topographical data were obtained from the examinations carried out using the anterior segment image analyzer with a rotating Scheimpflug camera (Pentacam, Oculus Optikgeräte GmbH, Wetzlar, Germany) before and after surgery in keratoconic eyes. A Pentacam examination was performed in a control group without keratoconus. Subjects with previous or current eye disease which might affect the measurements were excluded and only one eye per participant was included. They were informed about the study aim and signed an informed consent for participation. In view of the retrospective nature of the study, informed consent from the keratoconic patients was not required. Surgical procedure Intracorneal ring segment implantation was carried out with topical anesthesia in all cases, except for two children who required general anesthesia. Tunnels were created manually assisted by suction. Patients with keratoconus with mean corneal power less than or equal to 58 D and pachymetry in the area of the implant of more than 450 microns with a transparent cornea were included. In all cases, Ferrara (AJL) intracorneal ring segments were implanted in the 5-6 mm zone following the nomogram of Alfonso et al.9 The pupil center and the path of the tunnels were marked at the beginning of surgery. An incision was made in the curved axis with a diamond knife calibrated to 70% of the thinnest corneal depth in the tunnel area and the tunnel was created using a curved trephine while the eye was stabilized with a vacuum segment connected to a syringe maintaining suction. After surgery, a contact lens was implanted and ofloxacin 0.3% (Exocin®, Allergan S.A., Madrid, Spain) eye drops and dexamethasone 0.1% (Maxidex®, Alcon Cusi S.A., El Masnou, Barcelona) four times a day for one week, along with preservative-free artificial tears, were prescribed. The contact lens was removed between 2 and 4 days after surgery, if no epithelial defect was observed. Topographical analysis Topographies were obtained using an anterior segment imaging analysis system with a rotary Scheimpflug camera. For study purposes, the following data were recorded in a 3 mm central diameter: mean anterior radius, mean posterior radius and true net power in a 3 mm central diameter. The repeatability and reproducibility of the Pentacam examination was studied in both normal eyes10 and in keratoconic eyes11. Calculation of keratometric index The study measurement was the fictitious keratometric index. When the radii of the anterior and posterior surface of the cornea and pachymetry had been determined, using the Gauss formula, the true total corneal power may be determined; the Pentacam provides this data automatically as true net power. If these data are entered into the fine lens formula for paraxial imagery, the fictitious keratometric index for each eye can be calculated4: Keratometric Index = corneal power × anterior radius + 1. Statistical analysis The data were entered in an Excel table (Microsoft Corp) and the statistical analysis was performed using an SPSS (IBM, NY, USA) package. Data were compared between the control group and the keratoconus group using the Mann-Whitney test, while for the comparison of pre- and post-operative data of keratoconus patients, the Wilcoxon test was used. Differences were considered statistically significant in case of p ≤ 0.05. RESULTS In this study, 30 eyes from 30 patients (20 males and 10 females) with keratoconus undergoing intracorneal ring segment implantation and 30 normal control eyes from 30 patients (7 males and 23 females) with mean ages of 33 ± 14 and 34.16 ± 9.07 years, respectively, were included in this study. Of patients with keratoconus, 24 received 6 mm optical zone ring segment implants, JOURNAL OF EMMETROPIA - VOL 5, JANUARY-MARCH 11 KERATOMETRIC INDEX IN KERATOCONIC EYES WITH ICRS while six received 5 mm optical zone implants. Only one segment was implanted in 19 of the cases and two segments in 11 cases. The mean follow-up of patients with keratoconus was 8 ± 3 months (range 5 to 18 months). The study results are shown in Table 1. Both the anterior radius and the posterior radius are significantly lower in keratoconus patients than in normal subjects, i.e., both the anterior and posterior surfaces were significantly more curved in patients with keratoconus. The ratio between the anterior and the posterior radii, the keratometric index and the corneal power are all significantly higher in patients with keratoconus than in normal subjects. In patients who received intracorneal ring segments, there was a significant reduction in total corneal power with significant flattening of the anterior corneal surface, but not of the posterior surface. A significant increase in the ratio between the anterior and posterior corneal surface and a significant decrease in the keratometric index were observed for both the pre-surgical measurement and in comparison with the index in normal patients. DISCUSSION The keratometric index is a fictitious number used by instruments to convert the curvature of the anterior corneal surface into the dioptric power of the whole cornea (modeled as a single refractive surface). This index, which provides excellent results for intraocular lens power calculation in unoperated eyes, is unrelated to the anatomy or other physical properties of the eye and depends on a fixed ratio between the anterior and posterior corneal curvature4. If the ratio between these is changed by a surgical or pathological process, an error in the index is produced which affects the calculation of the corneal power. The standard keratometric index has been set at 1.3375, which was selected for convenience rather than for optical reasons, since it allowed concordance of the important values (7.5 mm and 45 D)12. Nevertheless, more recent studies based on the posterior corneal curvature have calculated it to be between 1.3273 and 1.329013,14. In line with this data, the mean keratometric index in the control group of our study was 1.3282. This index is not only changed by surgery (refractive laser, radial keratotomy, intracorneal ring segment implantation), but it may also be altered by disease processes, as shown by our study results in the keratoconus patient group, which already before segment implantation have a significantly greater keratometric index than normal patients. Few studies are available on the calculation of intraocular lens power in keratoconus patients, but the error obtained is greater than in normal corneas5,6,7. Our study shows that the index error can contribute to a less predictable calculation in these cases. In our series, both the anterior and the posterior surfaces were significantly curved in patients with keratoconus compared to normal patients, as has been shown in other papers in which it has been observed that curvature changes affect both corneal surfaces15 and are correlated16. In laser corneal refractive surgery in which the anterior corneal curvature is modified without changing the posterior surface, the relation between the anterior and posterior surface radii are changed, so the keratometric index changes, producing the referenced index error2,3. It has recently been shown that corneas Table 1. Results of parameters studied in normal eyes, keratoconic eyes and the same keratoconic eyes after intracorneal ring segment (KCICRS) implantation Control Keratoconus KCICRS p-value n 30 30 30 Mean anterior radius (mm) 7.82 ± 0.31 6.94 ± 0.46 7.18 ± 0.5 > 0.001 Mean posterior radius 6.42 ± 0.37 5.51 ± 0.5 5.50 ± 0.49 C vs KC y C vs KCICRS 0.0001; KC vs KCICRS 0.846 Ratio antR/postR 1.22 ± 0.02 1.26 ± 0.04 1.31 ± 0.06 > 0.001 True net power (D) 41.99 ± 1.43 47.67 ± 2.95 45.15 ± 3.3 > 0.001 Keratometric index 0.0001 KC vs KCICRS 1.3282 ± 0.0074 1.3300 ± 0.0052 1.3228 ± 0.0039 and KCICRS vs C; 0.012 KC vs C n, number; KCICRS, keratoconus treated with ICRS; antR, anterior radius; postR, posterior radius JOURNAL OF EMMETROPIA - VOL 5, JANUARY-MARCH 12 KERATOMETRIC INDEX IN KERATOCONIC EYES WITH ICRS undergoing radial keratotomy show a change in the ratio between the posterior and anterior corneal surface, with greater flattening of the posterior surface compared to the anterior surface, thus producing a different keratometric index from the standard, which may also cause index error in these cases4. In contrast to the assumptions of some authors8, and despite the technique being additive, intracorneal ring segment implantation may also lead to greater flattening of the anterior surface compared to the posterior surface, which does not change significantly, and to a corresponding change in the keratometric index. Thus, lens positioning error, radius error and index error would come into play in the intraocular lens power calculation error, in the same way as in eyes undergoing refractive surgery. In the case of intracorneal ring segment implantation, the results of our study show a reduction in total corneal power due to the corneal flattening of the anterior surface, since the posterior radius of the cornea is not significantly modified. These results do not agree with those of a previous study carried out by Sogütlü et al.17 in which flattening was found on both corneal surfaces, with the anterior surface being affected more than the posterior. In this case, although the study was also carried out with a Pentacam, the measurements used for evaluating the topographical changes in the cornea were different from ours; namely, the anterior and posterior best-fit sphere and the maximum elevation. Moreover, the exact mechanism of action of the intracorneal ring segments is not well defined. It has been suggested that the additive effect on the middle of the corneal lamellae make these increase their trajectory to surround the implant, producing the compensatory effect of shortening the arc, thus causing the flattening. This, however, does not explain the increased curvature in the midpoint of the segment and the flattening that appears at the ends. Other theories, such as the twisting effect produced in the lamellae have been proposed, which could explain a flattening effect in the anterior surface with hardly any changes in the posterior surface18. Olsen has calculated that a change in the keratometric index of 0.001 induces a change of approximately 0.13 D in corneal power12, and in keratometry an error of 1.00 D causes a 1 D error in refraction after intraocular lens implantation19. Accordingly, the differences found in our study, together with the other causes of error already described in these cases can lead to significant errors in the calculation of intraocular lens power. The keratometric index used in the third-generation formulae is the conventional value of 1.3375 or 1.3315, while that used for normal corneas measured with topographs which analyze the posterior surface has been calculated at between 1.3273 and 1.329013,14. The use of a different keratometric index both in normal corneas and in those modified by surgery or eye disease, such as keratoconus, will therefore produce a significant error. For normal eyes, the error produced by the current third-generation formulae has been compensated by the optimization of the constants. Thus, the change in the keratometric index without a new optimization of the constants would lead to errors, both by applying the physiological keratometric index of normal corneas, or other indexes from modified corneas. To correct this type of error, the total corneal power would have to be calculated, using either the Gauss formula or ray-tracing4. The results of this study show that both keratoconic eyes and keratoconic eyes operated with intracorneal ring segments have a different ratio between the anterior and posterior corneal surfaces than normal eyes, and thus, a different keratometric index from the standard value, which may lead to errors in intraocular lens calculation. REFERENCES 1. Aramberri J. Intraocular lens power calculation after corneal refractive surgery: double-K method. J Cataract Refract Surg. 2003; 29:2063-68. 2. Seitz B, Langenbucher A. Intraocular lens calculations status after corneal refractive surgery. Curr Opin Ophthalmol. 2000; 11:35-46. 3. Hoffer KJ. Intraocular lens power calculation after previous laser refractive surgery. J Cataract Refract Surg. 2009; 35:75965. 4. Camellin M, Savini G, Hoffer KJ, Carbonelli M, Barboni P. Scheimpflug camera measurement of anterior and posterior corneal curvature in eyes with previous radial keratotomy. J Refract Surg. 2012; 28:275-79. 5. Thebpatiphat N, Hammersmith KM, Rapuano ChJ, Ayres BD, Cohen EJ. Cataract surgery in keratoconus. Eye & Contact Lens. 2007; 33:244-46. 6. Leccisotti A. Refractive lens exchange in keratoconus. J Cataract Refract Surg. 2006; 32:742-46. 7. Watson MP, Anand S, Bhogal M et al. Cataract surgery outcome in eyes with keratoconus. Br J Ophthalmol. 2013; doi: 10.1136/bjophthalmol-2013-303829. 8. Cezón J. Cirugía de cataratas en pacientes con segmentos intraestromales. In: Lorente R, Mendicute J. Cirugía del Cristalino. LXXXIV Ponencia Oficial de la Sociedad Española de Oftalmología, 2008:961-71. 9. Alfonso JF, Lisa C, Merayo-Lloves J, Fernández-Vega L, MontésMicó R. Intrastromal corneal ring segment implantation in paracentral keratoconus with coincident topographic and coma axis. J Cataract Refract Surg. 2012; 38:1576-82. 10. Aramberri J, Luis Araiz L, Garcia A et al. Dual versus single Scheimpflug camera for anterior segment analysis: Precision and agreement. J Cataract Refract Surg. 2012; 38:1934–49. 11. Szalai E, Berta A, Hassan Z, Módis L. Reliability and repeatability of swept-source Fourier-domain optical coherence tomography and Scheimpflug imaging in keratoconus. JCRS. 2012; 38:485-94. JOURNAL OF EMMETROPIA - VOL 5, JANUARY-MARCH KERATOMETRIC INDEX IN KERATOCONIC EYES WITH ICRS 12. Olsen T. On the calculation of power from curvature of the cornea. Br J Ophthalmol. 1986; 70:152–54. 13. Fam HB, Lim KL. Validity of the keratometric index: Large population-based study. J Cataract Refract Surg. 2007; 33:686–91. 14. Ho JD, Tsai ChY, Tsai RJF, Kuo LL, Tsai IL, Liou ShW. Validity of the keratometric index: Evaluation by the Pentacam rotating Scheimpflug camera. J Cataract Refract Surg. 2008; 34:137–45. 15. 15. Tomidokoro A, Oshika T, Amano S, Higaki S, Maeda N, Miyata K. Changes in anterior and posterior corneal curvatures in keratoconus. Ophthalmology. 2000; 107:1328–32. 16. Piñero DP, Alió JL, Alesón A, Escaf Vergara M, Miranda M. Corneal volume, pachymetry, and correlation of anterior and posterior corneal shape in subclinical and different stages of clinical keratoconus. J Cataract Refract Surg. 2010; 36:814–25. 17. Sögütlü E, Piñero DP, Kubaloglu A, Alió JL, Cinar Y. Elevation Changes of Central Posterior Corneal Surface After Intracorneal Ring Segment Implantation in Keratoconus. Cornea. 2012; 31:387–95. 13 18. Barraquer RI. Segmentos intracorneales y finalidad refractiva. In: Cezón J. Técnicas de moldeado corneal: desde la ortoqueratología hasta el cross-linking. Monografías de la Sociedad Española de Cirugía Ocular Implanto-Refractiva, 2009: 121-37. 19. Shammas HJ. Measuring the corneal power. In: Shammas HJ, ed. Intraocular Lens Power Calculations. Thorofare, NJ, Slack, 2004; 183–88. JOURNAL OF EMMETROPIA - VOL 5, JANUARY-MARCH First author: Victoria de Rojas Silva Complexo Hospitalario Universitario, A Coruña, Spain