As more fluid needs to be displaced from under steep corneas
than flat, in comparison to the calibration cornea, steep corneas
can overestimate the IOP and flat corneas can lead to an underestimation (see Figure 2).
The disadvantages of piggyback lens systems are: (a) Not all eyes, particularly very steep corneas such as advanced keratoconus and steep grafts, will be able to wear a soft lens without fluting at the edge.
(d) On very steep corneas it may not be possible to get a soft lens cushion to fit the eye without edge fluting or displacement
(27, 41-43) The general consensus appears to be that the IOP will be underestimated slightly in flat corneas and overestimated slightly in steep corneas
, but the effect is probably confined to the extreme limits of "normal" corneal curvature and thus is generally not a significant factor in the average clinical setting.