AIM: To investigate the diagnostic value of ocular morphological parameters under different corneal diameters for early keratoconus.

METHODS: A retrospective case-control study. A total of 201 patients(201 eyes)who were treated in our hospital from January 2019 to March 2022 were included. They were divided into 135 cases(135 eyes)in the control group(patients with history of refractive error)and 66 cases(66 eyes)in the subclinical keratoconus group. The Pentacam anterior segment analyzer was used to determine the horizontal central curvature of corneal posterior surface(Kf), posterior vertical central curvature of corneal posterior surface(Ks), average curvature of corneal posterior surface(Km), Posterior I-S ratio, corneal posterior surface height after the thinnest point(PE at the thinnest point), maximum posterior elevation from best fit sphere(MPE from BFS), maximum posterior elevation from best fit toric ellipsoid(MPE from BFTE), posterior asphericity asymmetry index(AAI), thinnest point thickness of the cornea(TCT), central corneal thickness(CCT), depressed corneal thickness(DCT), pachymetric progression index average(PPIavg), Ambrósio relational thickness maximum(ARTmax)and Belin D value. The differences of each parameter between the two groups were analyzed. Receiver operating characteristic(ROC)curves were analyzed to determine the best diagnosis point. The control group was further divided into groups according to the corneal diameter: corneal diameter ≤11.0mm, 11.1mm≤ corneal diameter ≤11.5mm, 11.6mm≤ corneal diameter ≤12.0mm, corneal diameter ≥12.1mm. The differences of each parameter among these groups were compared. Pearson correlation analysis was used to analyze the correlation between corneal diameter and other parameters.

RESULTS: There were significant differences in posterior I-S ratio, PE at the thinnest point, MPE from BFS, MPE from BFTE, posterior AAI, TCT, DCT, PPIavg, ARTmax, Belin D value between the subclinical keratoconus group and the control group(P<0.05). Sensitive index of Pentacam to diagnosis subclinical keratoconus were Belin D value, posterior I-S ratio, PPIavg, posterior AAI and MPE from BFTE(AUC≥0.9). In the control group, there was no significant difference in posterior I-S ratio, MPE from BFTE, posterior AAI, TCT, CCT, and DCT among different corneal diameter groups (P>0.05), and there was no significant correlation with corneal diameter(all P>0.05).

CONCLUSION: The Belin D value, posterior I-S ratio, PPIavg, posterior AAI, MPE from BFTE obtained by Pentacam are sensitive indicators for the diagnosis of early keratoconus, among which posterior I-S ratio, posterior AAI, MPE from BFTE are less affected by corneal diameter. They play an important role in the early diagnosis of keratoconus under different corneal diameters.