Measurement of the eye wavefront has been used for various reasons, especially in customized corneal ablation^1,2 where it was made popular. It is also very useful in clinical practice to evaluate the optical quality of the eye and irregular astigmatism³ by detecting higher-order aberrations (HOA). Studies have shown that coma and spherical aberration can affect the highresolution imaging of any optical system, including the eye.^4-6
Anterior corneal aberration analysis has been demonstrated to be an effective tool for detecting and grading keratoconus^7-9. This is because in keratoconus there can be a substantial increase in higher-order aberrations, resulting in visual acuity loss that cannot be corrected with conventional spectacles or soft contact lenses.
Several studies have compared keratoconic eyes and their relationship with corneal aberrations. It has been shown that there are increased HOAs, particularly in coma and spherical aberration ^9-11. The mean wavefront error can be quantitatively expressed as the root mean square (RMS). Third order and higher are considered higher-order aberrations (HOA).
Ferrara intrastromal corneal ring segments (ICRS) have been used effectively to treat keratoconus and related keratectasias.^12-15 A recently published study¹⁶ has shown that the Ferrara ICRS, in addition to improving uncorrected and corrected visual acuity, can also improve corneal asphericity, reducing the hyperprolate state usually found in keratoconus. This finding, along with wavefront analysis, is important as a tool for evaluation of quality of vision after ICRS implantation.
The purpose of this study was to evaluate the wavefront corneal changes induced by the implantation of the Ferrara ICRS, and its relationship to visual acuity in keratoconus grade I, II and III using the AmslerKrumeich scale.

In this study we retrospectively reviewed patient records involving the 96 eyes of 92 patients with keratoconus, grade I, II and III detected using the AmslerKrumeich scale. These patients consecutively underwent surgery performed by the same experienced surgeon (RS) using the manual technique. The rings where chosen according to the Ferrara Ring Nomogram,¹⁵ and only non-complicated surgeries were included in these study.
Fifty-nine patients were male (61.4%) and 37 were female (38.6%). The mean age of patients was 29.04 ± 7.43 years old (range: 17 to 47 years-old). The mean follow-up time was 13.6 ± 11.2 [SD] months (range 3 to 60 months). After a complete ophthalmic examination and a thorough discussion of the risks and benefits of the surgery, the patients gave written informed consent. The main indication for ICRS implantation was contact lens intolerance and/or progression of ectasia. Disease progression was defined by: worsening of uncorrected distance visual acuity (UDVA) and corrected distance visual acuity (CDVA), progressive intolerance to contact lens wear and progressive corneal steepening documented by the topography.
Total (corneal and internal) wavefront analysis was performed with the Nidek OPD-Scan II (Optical Path Difference Scanning System; NIDEK Co Ltd, Gamagori, Japan). This machine provides total aberrometric analysis by deconstructing the whole eye into anterior corneal surface, posterior corneal surface, anterior chamber, lens, vitreous and retina. It uses the same principle as dynamic retinoscopy by calculating the time and intensity it takes reflected light from the retina to reach 1440 given points over a 6 mm pupil, comparing to the theoretical time it would take light to illuminate the point in an eye with perfect emmetropia.¹⁷ Previous studies confirmed the precision and repeatability of its results.¹⁸ Statistical analysis included preoperative and postoperative UDVA, CDVA, total aberration, Total Root Mean Square (RMS) at 3 mm (RMS3) and 5 mm (RMS5), Spherical aberration, primary coma, Trefoil, Tetra foil, induced astigmatism (named as high astigmatism by the aberrometer) and Tilt. Statistical analysis was carried out using Excel (2010, Microsoft Corp.). Student�s t test for paired data were used to compare preoperative and postoperative data, and Pearson Test for Correlation between the data.

After analyzing the data on keratoconus grade I, II and III using the Amsler-Krumeich scale, the results where as follows (Table 1, Figures 1, 2, 3 and 4): Keratoconus grade I: The mean UDVA improved from 0.94 ± 0.38 to 0.65 ± 0.38 LogMAR (p < 0.001) and the mean CDVA improved from 0.35 ± 0.17 to 0.19 ± 0.16 LogMAR (p < 0.001). The RMS3 values decreased from 1.92 ± 0.73 to 1.25 ± 0.58 (p < 0.001); the RMS5 decreased from 2.69 ± 1.10 to 1.88 ± 0.81 D (p < 0.001); Total aberrations decreased from 8.21 ± 3.14 to 5.33 ± 2.31 microns (p < 0.001); coma decreased from 1.49 ± 0.73 to 0.98 ± 0.50 microns (p = 0.002); tilt decreased from 4.64 ± 1.88 to 2.33 ± 1.12 microns (p < 0.001); trefoil decreased from 1.05 ± 0.49 to 1.00 ± 0.49 microns (p = 0.73); tetra foil increased from 0.16 ± 0.11 to 0.54 ± 0.21 microns (p < 0.001); spherical aberration increased from 0.21 ± 0.16 to 0.36 ± 0.25 microns (p = 0.01).
Keratoconus grade II: the mean UDVA improved from 1.15 ± 0.35 to 0.67 ± 0.36 LogMAR (p < 0.001) and the mean CDVA improved from 0.50 ± 0.22 to 0.25 ± 0.18 LogMAR (p < 0.001). The RMS3 values decreased from 2.42 ± 1.12 to 1.96 ± 0.88 (p = 0.002); RMS5 decreased from 3.29 ± 1.22 to 2.81 ± 1.47 (p = 0.08); total aberrations decreased from 9.76 ± 3.55 to 7.19 ± 2.56 microns (p < 0.001); coma decreased from 1.90 ± 0.72 to 1.48 ± 0.82 microns (p = 0.008); tilt decreased from 5.42 ± 2.54 to 3.60 ± 1.97 microns (p < 0.001); trefoil increased from 1.14 ± 0.54 to 1.42 ± 0.63 microns (p = 0.02); tetrafoil increased from 0.17 ± 0.09 to 0.60 ± 0.22 microns (p < 0.001); spherical aberration increased from 0.42 ± 0.24 to 0.59 ± 0.30 microns (p = 0.001).
Keratoconus grade III: the mean UDVA improved from 1.28 ± 0.28 to 0.92 ± 0.45 LogMAR (p < 0.001) and the mean CDVA improved from 0.65 ± 0.27 to 0.46 ± 0.25 LogMAR (p < 0.001). The RMS3 values increased from 2.11 ± 1.02 to 2.19 ± 1.20 (p = 0.75); RMS5 decreased from 3.60 ± 1.53 to 3.30 ± 1.82 (p = 0.39); total aberrations decreased from 12.36 ± 3.41 to 8.98 ± 3.31 microns (p < 0.001); coma decreased from 2.11 ± 0.92 to 1.82 ± 1.15 microns (p = 0.10); tilt decreased from 5.37 ± 2.36 to 3.79 ± 2.26 microns (p = 0.001); trefoil increased from 0.98 ± 0.49 to 1.41 ± 0.75 microns (p = 0.01); tetrafoil increased from 0.17 ± 0.11 to 0.62 ± 0.39 microns (p < 0.001); spherical aberration increased from 0.71 ± 0.36 to 0.93 ± 0.50 microns (p = 0.01).

The decrease of vision in patients with keratoconus is not only caused by sphero-cylindrical refractive errors (low order aberrations) due to elevated corneal curvature, but also to a significant extent by high order ones. This is clearly explained by the fact that spectacles in most cases are not able to provide full correction. The predominant defect is the coma aberration, specifically its vertical component^19-23. This was so evident that different studies using different instruments had the same result^19-23 . Ali� and Shabayek²⁴ used this property to modify the classic Amsler-Krumeich classification for keratoconus, the amount of coma aberration being one of the parameters.
In addition, trefoil, tetrafoil and secondary astigmatism are greater in keratoconic eyes^19-23. When the directions of each term were analyzed by Zernike vector analysis, keratoconic eyes tend to have a reverse coma pattern, that is, a prominent vertical coma with an inferior slow pattern.²⁰ In addition, trefoil aberration was the opposite of that of normal eyes.²⁰ The relationship between HOAs and visual function is complicated and not fully understood. Applegate et al.¹⁹ demonstrated that all aberrations are not equivalent, in that different Zernicke polynomial coefficients with the same amount of RMS error result in varying degrees of degradation in high and low contrast acuity. In normal subjects, the RMS wavefront error appears to have a low correlation to visual acuity, in the absence of irregular corneas.
Maeda et al.²⁰ evaluated eyes with mild and moderate keratoconus using a KR-9000 aberrometer (Topcon, Tokyo, Japan). The study was one of the first to look at a population of keratoconus and to identify dominant higher-order aberrations. In a study published by Jafri et al.⁹ in 2007 higher vertical coma and higher RMS values were found in patients with suspected and early keratoconus. Similar results were found by Pantanelli et al.²¹ when studying the wavefront in keratoconus. Mean RMS values of 2.24 ± 1.22 D were found, which are similar to the values in the present study. These values are approximately 5.5 times the 0.41 ± 0.16 D found in normal corneas.
Pinero et al.²⁵, in a study comparing two different types of ICRS (Intacs and Keraring), found that coma-like aberrations tended to decrease after ICRS implantation, and the change occurred more rapidly with the Keraring. They also compared the visual and wavefront outcomes between the manual and the femtosecond laser, and found that both procedures provide similar visual and refractive outcomes.
More limited aberrometric correction was observed in the manual technique. They found that the use of mechanical tunnelization specifically for Intacs implantation in eyes with early to moderate keratoconus has limited the potential aberrometric correction of these implants because the procedure itself generated new aberrations, especially negative primary spherical aberration and primary coma. This trend was not specifically confirmed for Kerarings segments because of the limitations of the study in the sample size for this segment type.
Analyzing the pre and post-operative by AmslerKrumeich keratoconus scale, in grade I, II and III groups, the Ferrara intrastromal corneal rings statistically improved UDVA and CDVA (TABLE 1), providing similar results as showed by a recently published large sample study²⁶.
We showed that the postoperative RMS value is improved in mild cases of keratoconus. RMS at 3mm significantly improved (p < 0.01) in grade I and II but not in grade III (p = 0.75) which slightly increased. RMS at 5mm improved in all groups, but was only statistically significant in grade I (p < 0.001) but not in grade II (p=0.08) and III (p=0.38) (Figure 1). Corneal flattening reduces central keratometry in keratoconus²⁶ reducing the amount of myopia. This can be indirectly evaluated by total corneal aberration that statistically decreased in all groups. (p < 0.01) (Figure 2)
In the present study we found a statistically significant decrease in coma in patients in groups I and II ( p<0.01 ) but not in grade III (p=0.10). As for the spherical aberration the values statistically increased in all groups, in accordance with results also found in another studies^27,28. All other peripheral aberrations statistically increased, except for trefoil in cone I that decreased but had no statistical significance (p=0.73).
The overall improvement in UDVA and CDVA can be related to a decrease in Total aberrations, by the reduction in the myopic component (corneal flattening) and HOAs, especially Coma and Tilt, which are related to the central area of the cornea and therefore have an important role in the quality of vision. The increase in trefoil, tetrafoil and secondary astigmatism can be explained by the increase of peripheral aberration measured in the 6mm area, which includes the area where the ICRS is implanted, but their influence on the visual outcome is less significant than central ones. Spherical aberrations, despite the decrease in the prolateness of the cornea, showed statistically significant increases. This was also found in a study by Pinero et al.²⁵
Our study showed that there was improvement in the quality and quantity of vision in all keratoconus groups. The best clinical outcomes were found in patients with keratoconus grade I and II.
Further studies with larger samples and longer follow-up periods must be undertaken in order to confirm the presented results.

1. Maeda N. Wavefront technology in ophthalmology. Curr Opin Ophthalmol 2001;12:294-299.
2. Seiler T, Dastjerdi MH. Customized corneal ablation. Curr Opin Ophthalmol 2002;13:256-260.
3. Porter J, Guirao A, Cox IG, Williams DR. Monochromatic aberrations of the human eye in a large population. J Opt Soc Am A Opt Image Sci Vis 2001;18:1793-803.
4. Packer M, Fine, IH, Hoffman RS. Wavefront technology in cataract surgery. Curr Opin Ophthalmol 2004;15:56-60.
5. Kuroda T, Fujikado T, Ninomiya S, et al. The effect of aging on ocular light scatter and higher order aberrations. J Refract Surg 2002;18:S598-S602.
6. Donnely WJ III, Pesudovs K, Marsack JD, et al. Quantifying scatter in Shack-Hartmann images to evaluate nuclear cataract. J Refract Surg 2004;20:S515-S522.
7. Mespli� N, L�oni-Mespli� S, Gallois A, Colin J, Touboul D. Detection of keratoconus using wavefront analysis. J Fr Ophtalmol. 2011 Oct;34(8):547-56.
8. Gobbe M, Guillon M. Corneal wavefront aberration measurements to detect keratoconus patients. Cont Lens Anterior Eye. 2005 Jun;28(2):57-66.
9. Jafri B, Li X, Yang H, Rabinowitz YS. Higher order wavefront aberrations and topography in early and suspected keratoconus. J Refract Surg. 2007 Oct;23(8):774-81.
10. Schlegel Z, Lteif Y, Bains HS, Gatinel D. Total, corneal, and internal ocular optical aberrations in patients with keratoconus. J Refract Surg. 2009 Oct;25(10 Suppl):S951-7
11. Ali� JL, Pi�ero DP, Ales�n A, et al. Keratoconus-integrated characterization considering anterior corneal aberrations, internal astigmatism, and corneal biomechanics. J Cataract Refract Surg. 2011 Mar;37(3):552-68.
12. SiganosD,FerraraP,ChatzinikolasK,etal.Ferraraintrastromal corneal rings for the correction of keratoconus. J Cataract Refract Surg 2002; 28:1947-1951.
13. Kwitko S, Severo N. Ferrara intracorneal ring segments for keratoconus. J Cataract Refract Surg 2004; 30:812-820.
14. Ferrara P, Torquetti L. Clinical outcomes after implantation of a new intrastromal ring with a 210-degree of arch. J Cataract Refract Surg 2009; 35:1604�1608.
15. Torquetti L, Ferrara P. Long term follow-up of intrastromal corneal ring segments in keratoconus. J Cataract Refract Surg 2009; 35:1768�1773.
16. Torquetti L, Ferrara P. Corneal asphericity changes after intrastromal corneal ring segments implantation in keratoconus. Journal of Emmetropia 2010; 1:178-180.
17. Rozema JJ, Van Dyck DE, Tassignon MJ. Clinical comparison of 6 aberrometers. Part 1. Technical specifications. J Cataract Refract Surg 2005; 31: 1114-1127.
18. Holzer MP, Goebels, S and Auffarth GU. Precision of NIDEK OPD-Scan measurements. J Refract Surg 2006; 22: S1021-S1023.
19. Applegate RA, Ballentine C, Gross H, et al. Visual acuity as a function of Zernicke mode and level of root mean square error. Optom Vis Sci 2003;80:97-105.
20. Maeda N, Fujikado T, Kuroda T, et al. Wavefront aberrations measured with Hartmann-Schack sensor in patients with keratoconus. Opthalmology 2002;109:1996-2003.
21. Pantanelli S, MacRae S, Jeong TM, Yoon G. Characterizing the wave aberration in eyes with keratoconus or penetrating keratoplasty using a high-dynamic range wavefront sensor. Ophthalmology. 2007 Nov;114(11):2013-21.
22. Chalita MR and Krueger RR. Wavefront aberrations associated with the Ferrara intracorneal ring in a keratoconic eye. J Refract Surg 2004; 20: 823-830.
23. Shah S, Naroo A, Hosking A, et al. NIDEK OPD-Scan analysis of normal, keratoconic, and penetrating keratoplasty eyes. J Refract Surg 2003; 19: S255-S259.
24. Ali� JL and Shabayek MH. Corneal high order aberrations: A method to grade keratoconus. J Refract Surg 2006;22:539-545.
25. Pinero DP, Alio JL, Bassam EK, Pascual I. Corneal aberrometric and refractive performance of 2 intrastromal corneal ring segments models in early and moderate ectatic disease. J Cataract Refract Surg 2010;36:102-109.
26. Ferrara G, Torquetti L, Ferrara P, Merayo-Lloves J. Intrastromal corneal ring segments: visual outcomes from a large case series. Clin Experiment Ophthalmol. 2011 Sep 8. 2
27. Barbero S, Marcos S, Merayo-Lloves J, MorenoBarriuso E. Validation of the estimation of corneal aberrations from videokeratography in keratoconus. J Refract Surg 2002;18:263�70.
28. Kosaki R, Maeda N, Bessho K et al. Magnitude and orientation of Zernike terms in patients with keratoconus. Invest Ophthalmol Vis Sci 2007; 48: 3062�8.