Process capability measurement of frequent replacement toric soft contact lenses

Frequent replacement soft contact lenses, and particularly toric soft lenses, require good reproducibility in order to avoid clinical problems arising from manufacturing variations in their parameters. This study assessed the actual and potential process capability of six frequent replacement toric soft lenses in relation to sphere power, cylinder power, cylinder axis, total diameter, back optic zone radius and centre thickness.

In total, 1260 lenses consisting of three sphere powers (plano, -3.00 and -6.00 D) were measured for six lens types. Three cylinder powers and six cylinder axes were assessed for each sphere power, except for one lens type where only two cylinder powers were available.

Significant differences in reproducibility were found between the lens types tested. Process capability and optical quality decreased with increasing sphere power for all lenses. Cylinder power process capability tended to be better than that for sphere power. In general, lenses showed adequate reproducibility with those dimensions likely to affect lens fit, namely total diameter, back optic zone radius and centre thickness.

Process capability was shown to be a useful measure of contact lens reproducibility for comparing toric soft lenses and assessing their potential for use on a frequent replacement basis. The results suggest some need for improvement in manufacturing accuracy and precision.