The impact of hydrogel lens settling on the thickness of the tears and contact lens

PURPOSE. To investigate the effect of contact lens insertion on the thickness on the prelens tear film (PLTF), the contact lens, and the postlens tear film (PoLTF). METHODS. Twelve contact lens wearers (mean age, 32.7 years; four males) inserted etafilcon A hydrogel lenses (power: -2.00 D, base curve: 8.3 mm) in both eyes immediately before testing. Previously described interference techniques, based on oscillations in reflectance spectra, were used to measure the thickness of the PLTF, contact lens, and PoLTF. The thickness of the layers is derived from the frequency of the oscillations. Spectra were captured 1 minute after lens insertion and every minute thereafter for 30 minutes. Least squares regression fits were used to determine the relation between thickness of each layer and time. RESULTS. The combined data from all subjects for PLTF thickness were fit with an exponential decay plus a constant thickness; the initial thickness was 4.5 micrometers, the time constant was 7.1 minutes (P < 0.001), and final thickness was 2.5 micrometers. The apparent thickness of the contact lens declined linearly at an average rate of 0.051 micrometers/minute (P < 0.001). The PoLTF thickness remained constant at 2.5 micrometers (P = 0.46). CONCLUSIONS. For most subjects, the PLTF thinned significantly over the course of the first 30 minutes of lens wear. The apparent thinning of the contact lens may be caused by a real thinning of the lens, but also may have a contribution from improved centration over the 30-minute period. The PoLTF remained relatively stable during this period.