Effects of rigid contact lens prism ballasting on oxygen access to the cornea under static and dynamic conditions

The effects of rigid contact lens material [polymethyl methyacrylate (PMMA) and itabisfluorofocon A] and prism ballasting (0, 1.5, 2.25, and 3 delta) on tear pump efficiency were studied by measuring corneal oxygen uptake rates on the right eyes of six human subjects under three conditions: (1) normal open eye; (2) after 5 min of static (without blinking) wear of the contact lens; and (3) after 5 min of dynamic (with blinking once every 5 s) wear of the same lens. As expected, corneal oxygen uptake rates (i.e., demand for oxygen) measured with the itabisfluorofocon A material were significantly lower (p < 0.0001) than those measured with PMMA under both static and dynamic conditions. Under static conditions, no significant differences were found across materials among the corneal oxygen uptake rates associated with the four amounts of prism ballasting (p = 0.0514). However, under dynamic conditions, significant differences (p < 0.0001) were found across materials among corneal oxygen uptake rates associated with the four amounts of prism ballasting, with the lower amounts of prism ballasting being associated with lower corneal oxygen uptake rates. The changes in the measured corneal oxygen uptake rates from static to dynamic conditions, relative to those measured for the normal open eye, served as an index of tear pump efficiency. There were no significant differences in these changes for the prism amounts studied; however, static condition data were significantly higher than dynamic condition data for the 0 delta and 1.5 delta lenses only, whereas greater amounts of prism ballasting resulted in no reduction in oxygen uptake under dynamic conditions. In addition, significantly greater differences between static and dynamic condition data were found for the PMMA material than for itabisfluorofocon A.