Objective analysis of contact lens fit

To assess the validity and repeatability of objective compared to subjective contact lens fit analysis.

Thirty-five subjects (aged 22.0±3.0 years) wore two different soft contact lens designs. Four lens fit variables: centration, horizontal lag, post-blink movement in up-gaze and push-up recovery speed were assessed subjectively (four observers) and objectively from slit-lamp biomicroscopy captured images and video. The analysis was repeated a week later.

The average of the four experienced observers was compared to objective measures, but centration, movement on blink, lag and push-up recovery speed all varied significantly between them (p<0.001). Horizontal lens centration was on average close to central as assessed both objectively and subjectively (p>0.05). The 95% confidence interval of subjective repeatability was better than objective assessment (±0.128 mm versus ±0.168 mm, p=0.417), but utilised only 78% of the objective range. Vertical centration assessed objectively showed a slight inferior decentration (0.371±0.381 mm) with good inter- and intrasession repeatability (p>0.05). Movement-on-blink was lower estimated subjectively than measured objectively (0.269±0.179 mm versus 0.352±0.355 mm; p=0.035), but had better repeatability (±0.124 mm versus ±0.314 mm 95% confidence interval) unless correcting for the smaller range (47%). Horizontal lag was lower estimated subjectively (0.562±0.259 mm) than measured objectively (0.708±0.374 mm, p<0.001), had poorer repeatability (±0.132 mm versus ±0.089 mm 95% confidence interval) and had a smaller range (63%). Subjective categorisation of push-up speed of recovery showed reasonable differentiation relative to objective measurement (p<0.001).

The objective image analysis allows an accurate, reliable and repeatable assessment of soft contact lens fit characteristics, being a useful tool for research and optimisation of lens fit in clinical practice.