Predictors Of Visual Outcomes Following Intracorneal Ring Segment Implantation For Paracentral Keratoconus

Intrastromal corneal ring segment (ICRS) implantation is a minimally invasive surgical treatment used in keratoconus, improving corneal geometry and visual function. Nonetheless, postoperative outcomes remain variable and somewhat unpredictable. Previous evidence suggests that visual benefit is strongly influenced by baseline visual acuity. This study aimed to identify preoperative clinical, tomographic, and aberrometric predictors of visual improvement after femtosecond laser–assisted ICRS implantation in paracentral keratoconus (croissant and duck phenotypes).

Single center, observational, retrospective study conducted at the Department of Ophthalmology of Centro Hospitalar Universitário de S. João, a tertiary referral center in Porto, Portugal.

45 patients with paracentral keratoconus treated with femtosecond laser–assisted Keraring-type (Mediphacos) ICRS implantation (FEMTO LDV Z8, Ziemer Ophthalmic Systems) and ≥1 month of follow-up were included. Variation in corrected distance visual acuity (CDVA) was analyzed both as a continuous and dichotomic outcome (gain of ≥2 lines vs. <2 lines). Preoperative visual, tomographic, and aberrometric parameters, and the postoperative variation in these parameters were assessed using both regression and group-comparison approaches, given a quasi-complete separation when baseline CDVA was included in regression models. Continuous variables were compared between groups using the Mann–Whitney U test and correlated with ΔCDVA using Pearson’s r.

Overall, CDVA improved from 0.50±0.29 to 0.21±0.09 logMAR units (p<0.001) and spherical equivalent decreased from -4.75±3.33 to -1.15±2.32 D (p<0.001). In multivariable linear regression, worse baseline CDVA was independently associated with greater visual improvement (p<0.001), explaining 90.5% of outcome variability (R²=0.905). Eyes with higher baseline coma (r=-0.342; p=0.021) and baseline CDVA (r=-0.948; p=<0.001) presented greater visual improvement. In non-parametric analyses, eyes gaining ≥2 CDVA lines had worse preoperative CDVA (p<0.001). Eyes with higher preoperative (p=0.071) or decrease (p=0.059) in anterior astigmatism showed a non-significant trend for greater visual improvement.

Baseline visual acuity is the main determinant of visual improvement after femtosecond laser–assisted ICRS implantation in paracentral keratoconus. Patients with higher preoperative coma, preoperative anterior astigmatism, and a higher decrease in anterior corneal astigmatism may present greater visual improvement.  This finding is biologically plausible, as keratoconus irregular astigmatism has a significant impact in patients’ visual quality and acuity. Limited sample size may explain the lack of statistical significance of some of the assessed parameters.