Toric lens orientation and visual acuity in non-standard conditions

To evaluate and compare the effect of gravity and gaze direction on toric lens orientation and visual acuity (VA).

This was a 14 subject, randomised, unmasked, non-dispensing study, relating to the effect of gaze direction and posture on toric lens orientation and VA. Four lens types were assessed: Acuvue Oasys for Astigmatism (AOfA), Purevision Toric (PVT), Air Optix for Astigmatism (AOT) and Proclear Toric (PCT). In the first part of the study, subjects were positioned on their side and once lenses had settled, VA was measured and photographs taken of the lens orientation position. In the second part, the subjects were positioned at a slit-lamp and video-recordings taken as they changed from the primary gaze position to the eight cardinal directions of gaze.

In Part 1, all lenses rotated as a result of change in posture and head position. With subjects in a recumbent position mean rotation ranged from 11.0 degrees with AOfA to 29.1 degrees with PCT. The consequent mean reduction in VA ranged from 0.05logMAR for AOfA to 0.15logMAR for PVT and was significantly worse with PVT and PCT compared with AOfA (P<0.05). In Part 2, lenses tended to show inferior-nasal rotation on upgaze and inferior-nasal rotation on downgaze. The AOfA lenses showed less rotation on inferior-nasal version than each of the other designs (P<0.005). The AOT lenses showed significantly less rotation on superior-temporal version than PVT (P=0.01).

Toric soft contact lens stability in extreme versions and postural positions can affect orientation and VA.