How reliable is labelled power of cast molded soft contact lenses?

Purpose: To compare measured against labelled power across the power range of most commonly prescribed single vision soft contact lenses. Methods: The back vertex powers of 6 lenses each in all available powers between -6.00 D and +4.00 D of lens types: Alcon, Dailies AquaComfort Plus; J&J, 1-Day Acuvue Moist; J&J, Acuvue Oasys 1 Day; CooperVision, Proclear 1 day; CooperVision, Biofinity; B+L, SofLens daily disposable and B+L, PureVision2 were measured using the Nimo TR1504 with the 'thick lens' option and 5 mm aperture in accordance with ISO 18369-3. The required input parameters of center thickness and material refractive index were measured using the Rehder ET-3 thickness gauge and Index Instruments CLR12-70 instruments, respectively. All measurements were performed with lenses soaked in standard phosphate buffered saline solution at 20 °C. Results: Measured power variability within each lens type and power was generally tight (appr. 0.05 D standard deviation, SD) with the exception of Biofinity (-3.25 D has 0.18 D SD). Most low powered lenses (-2.00 D to +2.00 D) fell within the ISO tolerance of ±0.25 D. 4 out of the 7 lens types exceeded ISO tolerance, too much minus power for the high minus lenses and too much plus power for the higher plus lenses. For the worst case, 1 Day Acuvue Moist lenses, the power error increased by 0.13 D for each D of nominal power, while the AquaComfort, PureVision2 and Biofinity lenses stayed within ISO tolerance for the full +4.00 D to -6.00 D range. Averaged power variance from the nominal 0.25 D step size between neighboring powers ranged between 0.04 D (AquaComfort) to 0.14 D (Proclear) and was randomly distributed for all lens types. Conclusions: Some of the lens types/powers, particularly those with high SD, appear to have a bimodal power distribution within the 6 measured lenses. While most lenses in the low power range comply with the ISO standards, high minus lenses in particular can deviate from labelled power by up to -0.75 D. Of particular concern for practitioners and patients is the high power variability between neighboring powers, making it difficult to fit the correct lens power and maintain good visual performance when re-ordering lenses.