PURPOSE. Preliminary osmolarity measurements of packaging solutions from 7 soft contact lens brands revealed a range from 270 mOsmol/kg (Nelfilcon A) to 413 mOsmol/kg (Etafilcon A). Reports on the osmolarity of human tears range from 312-323 mOsmol/kg. The International Standard ISO 10344 specifies that the saline solution for the testing of contact lenses should be phosphate buffered at a pH of 7.4 +/- 0.1 and have a nominal osmolarity of 310 mOsm/kg. To determine whether soft contact lenses stored in phosphate buffered saline (PBS) outside the osmolarity range stated in ISO 10344 exhibit significant parameter change.
METHOD. Nine lenses (–4.00DS) with same lot numbers were selected from each of the four FDA polymer categories (Type I: Lotrafilcon A (24%), Type II: Alphafilcon A (66%), Type III: Balafilcon A (36%), Type IV: Etafilcon A (58%)). Lenses were randomised and masked from the metrologist. All lenses were systematically soaked in three different PBS solutions with osmolarities of 273, 310 and 409 mOsmol/Kg for a minimum of 48 hours. Solution pH was kept constant at 7.4 +/- 0.1. After each soaking, lens diameter (LD), base curve (BOZR) and back vertex power (BVP) were measured. One way ANOVA was used for an overall difference between groups followed by multiple comparisons (with Bonferroni adjustment) applied to test the difference between pairs of groups.
RESULTS. The osmolarity change from 310 to 409 mOsmol/kg produced the following parameter changes: Type IV LD, 13.84 to 13.59 mm (p<0.01), Type III LD, 13.97 to 13.95 mm (p=0.01), Type IV BOZR, 8.65 to 8.50 mm (p=0.01), Type IV BVP, -4.24 to -4.41 D (p<0.01). Osmolarity change from 310 to 273 mOsmol/kg produced the following parameter changes: Type IV LD, 13.84 to 13.95 mm (p<0.005). Type I and II lenses did not show any change in parameters.
CONCLUSIONS. Contact lenses soaked in PBS with osmolarities departing from ISO 10344 produced statistically significant changes in all lens parameters measured for the Type IV category lens (Etafilcon A) and the LD only for the Type III category lens (Balafilcon A). Since the osmolarity of ISO grade PBS is similar to human tears, these results may have consequences for contact lens fitting.