Rigid Gas Permeable Contact Lens Fitting Using New Software in Keratoconic Eyes

To determine the repeatability of the back optic zone radius (BOZR) of rigid gas permeable (GP) contact lens (CL) proposed by new software for fitting in healthy and keratoconus eyes and to compare with the diagnostic CL fitting method.

Three consecutive corneal topographies (Oculus-Keratograph) were performed and analyzed with APEX new software CL fitting (Hecht-Contactlinsen, Germany) in 40 healthy and 40 keratoconus eyes fitted with GP using conventional diagnostic method. The coefficient of variation (CV) of the BOZR suggested by APEX was calculated. The BOZR of both fitting methods (software versus diagnostic) were compared maintaining the same lens diameter.

BOZR proposed by APEX showed good repeatability in healthy (CV = 0.32%) and keratoconus eyes (CV = 0.51%). APEX proposed flatter BOZR than the diagnostic method in healthy (7.91 ± 0.24 and 7.84 ± 0.26 mm, p < 0.01) and keratoconus eyes (7.34 ± 0.38 and 7.23 ± 0.37 mm, p < 0.01). A strong linear correlation in healthy (BOZR_Diagnostic_Method = (BOZR_APEX × 1.06) - 0.53; p < 0.01, R = 0.969) and keratoconus eyes (BOZR_Diagnostic_Method = (BOZR_APEX × 0.88) + 0.77; p < 0.01, R = 0.825) was found. A detailed analysis showed a similar trend in different keratoconus stages (Amsler-Krumeich classification); stage 1: 7.42 ± 0.30 and 7.40 ± 0.25 mm, BOZR_Diagnostic_Method = (BOZR_APEX × 0.81) + 1.38, R = 0.973; stage 2: 7.30 ± 0.44 and 7.23 ± 0.38 mm, BOZR_Diagnostic_Method = (BOZR_APEX × 0.84) + 1.07, R = 0.929; and stage 3: 7.33 ± 0.39 and 7.11 ± 0.40 mm, BOZR_Diagnostic_Method = (BOZR_APEX × 0.93) + 0.28, R = 0.831. Applying these regression formulas, the BOZR difference could be reduced in healthy (-0.01 ± 0.05 mm) and keratoconus eyes (-0.01 ± 0.14 mm) for each keratoconus stage (0.01 ± 0.04, 0.03 ± 0.10, and 0.02 ± 0.16 mm in stages 1, 2, and 3, respectively).

APEX software provides repeatable BOZR in healthy and keratoconus eyes, but it tends to propose flatter BOZR than the diagnostic method. APEX BOZR should be improved with new equations and helping with the GP fitting procedure.