Base curve measurements of soft contact lenses

Efforts to fit and duplicate soft contact lenses depend to a certain extent upon the practitioner's ability to determine the base curve (posterior apical radius) of the lenses accurately. In the study, three devices were used to measure the base curve of seven different manufacturers' soft lenses. Each manufacturer provided six to ten -3.00-D lenses in factory-sealed vials. Each vial was masked and coded so that measurements could be taken in a masked fashion. A different investigator used each of the different devices. Each investigator took 10 base curve measurements on each lens in a random fashion. The lens vials were not unmasked until the end of the study, at which time the data were analyzed and the results compared. Not only did we determine the reliability of the three devices, but we also tried to determine the reliability of the labeled posterior apical radius of the lenses. The Hydrovue Analyzer had the best reliability (sigma = +/- 0.09 mm), followed by the Neitz Softmeter (sigma = +/- 0.13) and the Rehder gauge (sigma = 0.30). With the latter two devices, the variability increased as lens center thickness decreased. Spin-cast lenses showed better lens label repeatability than lathe-cut lenses.