In keratoconus cases, near-normal corneal biomechanics and surface regularity are very important factors for the expected postoperative visual acuity. Patients with highly abnormal corneas have poor BCVA and thus poor expected postoperative UCVA. Different treatment modalities in keratoconus aim to stabilize the case and get the highest possible visual acuity values, ICRS as one of these modalities act as an add tissue in the corneal deep stroma in the mid-periphery forming tension forces on the stromal collagen to regain the normal corneal shape and regularity as much as possible and it was successful as evidenced by different studies with improvement in spherical equivalent, cylindrical refraction and so in visual acuity.[7, 11, 12] In this study for the ICRS group, the mean improvements in spherical equivalent, cylindrical refraction, UCVA and BCVA were 3.18 D, 2.4 D, 0.25 (log MAR) and 0.24 (log MAR) respectively which is comparable and even slightly better than other studies.[11, 12]
Prisant et al. [13] in 2019, reported an improvement (within 3 months) in spherical refraction, cylindrical refraction, UCVA and BCVA of 0.84D, 2.21D, 0.4 (log MAR) 0.1 (log MAR) respectively. In our study, BCVA showed a higher improvement which may be related to the stability and high values of the sphero-cylindical correction while regarding UCVA, our study showed less improvement of 0.25 log MAR which may be related to the preoperative high errors.
On the other side this study regarding ICL implantation variability in the subjective refraction (on which the TICL power and axis of insertion is based on) in keratoconic eyes makes the accuracy in such patients is less and refractive surprises are more than in normal non-keratoconic eyes.[14]
In this study, efficacy index was 1.08 and safety index was 1.19 and so, our results are comparable to the published studies on the outcomes of ICL implantation in keratoconus [15, 16, 17, 18, 19]
Hashemian et al. [20] showed better efficacy and safety indices of 1.345 and 1.56 respectively at 1 year postoperative. These differences could be explained by the different preoperative data reported by Hashemian et al. which indicates milder cases because in our study the preoperative spherical equivalent and cylindrical ranges were from − 11.75 D to -17.75 D and from 1.00 D to 6.00 D respectively, while in that study the preoperative spherical equivalent and cylindrical ranges were from − 1.50D to -10.25 D and from 0.50 D to 6.5 D. Alfonso et al. [21] implant ICL in mild keratoconus and had a nice refractive outcomes with very satisfactory efficacy index (1.07) and safety index (1.16) which is comparable to our results as mentioned before. Alio et al. [22] demonstrated an efficacy index of 0.88 and safety index of 1.21 for patients who received an ICL for keratoconus. Antonios et al. [18] reported an overall efficacy index of 1.04 and a safety index of 0.72 after ICL implantation 2 years postoperative.
In 2018, Ramin et al. [23] compared visual, refractive and aberration measurements of ICRS versus TICL Lens Implantation in mild and moderate cases of keratoconus and their preoperative data regarding log-Mar UCVA, log-Mar BCVA, spherical equivalent (D) and astigmatism (D) were 0.65, 0.37, -5.70 and 6.25 respectively and one year postoperative data for these measurements were 0.39, 0.35, -3.67 and 4.1 respectively for the ICRS group compared with the TICL group with preoperative data for the same measurements respectively 0.82, 0.15, -8.57 and 4.83 and one year postoperative data of 0.13, 0.04, -0.97 and 0.97. With comparison of these data with ours we found lower preoperative UCVA and BCVA of 1.15 and 0.57 for ICRS group compared with 1.19 and 0.33 for TICL group respectively which is explained in ICRS group by the preoperative mean spherical equivalent which was higher (-8.2) and as usual keratoconic eyes data are unexpected specially preoperative due to presence of another factors that is not totally related to the refraction which are the aberrations. Our postoperative data (12 months) regarding logMar UCVA, logMar BCVA, spherical equivalent (D) and astigmatism (D) for ICRS group were 0.9, 0.33, -5.02 and 3.24 respectively and for TICL group were 0.31, 0.27, -0.61 and 0.51. These differences in visual acuity (preoperative and postoperative) may be due to the better preoperative case data as that mild keratoconic changes contributes in less aberrations and better postoperative results. Both studies in general prove the significant superiority of TICL over ICRS in the UCVA, which was expected by the expected spherical equivalent correction but the main critical difference was that in our study there is no significant difference between both groups in the BCVA which is the main treatment goal of cases of keratoconus which may be explained by the preoperative difference in values of UCVA with comparable refraction specially for TICL group which may indicate more advanced keratoconus in our study.
Limitations of this study were its relatively short follow up period and low number of patients. Also, for future studies, it is better to use more comprehensive examinations, such as assessment of high ordered aberrations to obtain better results.