ISTANBUL NOMOGRAM FOR THE LENGTH OF ALLOGRAFT INTRACORNEAL RING SEGMENT

To investigate the accuracy of Istanbul nomogram for allogenic intracorneal ring segment implantation for keratoconus and investigate the changes in visual acuity, spherical equivalent(SE), K1, K2 and Kmax and their relationship with cone location and length of the implanted ring segment.

Department of Ophthalmology, Istanbul Medipol University, Istanbul, Türkiye.

Our study includes 60 eyes of 60 patients with a 5-month follow-up. Patients were treated according to Istanbul nomogram. In this nomogram, we created corneal tunnels with a femtosecond laser with 4 × 7.5 mm diameters and a depth of 200 μm. We implanted ready-to-use sterile allogenic corneal ring segments (KeraNatural^TM, Lions VisionGift, Portland, OR, USA) to the cone location. We detected the cone location according to the highest point of the anterior elevation map. Uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), spherical equivalent (SE), and keratometry values were noted preoperative and postoperatively and compared with each other. In addition, we have used an online protractor to determine the angle of the cone and the implanted allograft ring.

In our study, we found statistically significant improvement in UCVA, CDVA, SE, and corneal tomography values (K1, K2, and Kmax) (P<0.001 for all measurements). Another important finding was a statistically significant correlation between angle of the cone and length of (angle of) the ring segment (P=0.007). However, there was no significant correlation between length of the allograft ring segment and visual acuity (P=0.069). In contrast, postoperative UCVA and CDVA had a negative correlation with the angle of the cone(P<0.01).

Sterile allogenic ring segments are safe and effective for keratoconus treatment. We showed that length of the allogenic ring segment doesn't effect any of the measurements. As expected, patients with larger cones had worse postoperative results.