Contact Lens and Anterior Eye, Vol. 22, No. 4, pp. 127-135, 1999 © 1999 British Contact Lens Association Printed in Great Britain PROCESS CAPABILITY MEASUREMENT OF FREQUENT REPLACEMENT SPHERICAL SOFT CONTACT LENSES G. Young*, Y. Lewis'~, S. Coleman~ and C Hunt~ (Received 16th July 1999; in revised form 27th August 1999) Abstract-- Frequent replacement soft contact lenses must have good reproducibility in order to avoid clinical problems arising from manufacturing variations in lens parameters. This study assessed the actual and potential process capability of eight frequent replacement lenses (monthly or 2-weekly replacement) in relation to back vertex power, back optic zone radius, total diameter and centre thickness. Twenty-four lenses in three powers (- 1.00, -3.00, - 6.00 D) were measured for eight frequent replacement lenses: Acuvue, Acuvue 2, Biomedics 55, Encore, Focus, Optima FW, Softens 66, Surevue. In general, the lenses showed adequate reproducibility in respect of those parameters likely to affect lens fit, namely back optic zone radius, total diameter and centre thickness. The process capability results showed some variation in reproducibility of BVP between lens types and between lens powers. Process capability was found to be a useful measure of contact lens reproducibility. KEY WORDS: soft contact lenses; reproducibility;lens measurement; process capability Introduction I t is impracticable for practitioners to verify the specification of all soft contact lenses prior to dispensing supplies to patients. Nevertheless, practi- tioners have a duty of care to ensure that lenses provided are suitable for their intended purpose. With frequent replacement lenses, practitioners must satisfy themselves that a given product shows sufficient reproducibility to minimise the risk of clinical problems from variations in lens parameters. 1'~ Small differences in lens power, for instance, may result in a noticeable reduction in visual acuity) Product consistency is a common concern in production engineering and quality assurance. The competence of a manufacturing process to produce consistent output is referred to as process capability. A number of indices of process capability are convention- ally used in quality control and provide a useful method for assessing the competence of a given process and for comparing the reproducibility of different processes. Although soft contact lens reproducibility has been widely discussed in the literature, to our knowledge, these concepts have not previously been reported in relation to contact lenses. Potential process capability (Cp) is calculated as the ratio of the product tolerance range and six times the standard deviation (6a) of the measured output. A limitation of Cp is that it does not take into account of whether the process is centred within the tolerance limits. Actual process capability (Cpk), on the other hand, takes into account the ability of the process to meet specifications within the tolerance range? 's It uses the ratio of the range of the mean output to the nearest tolerance limit to the spread of output (30). Figure 1 illustrates the relationship between Cp and Cpk. *MPhil FCOptom DCLP FAAO, Visioncare Research Ltd tMSc, Vistakon Inc ,BSc Visioncare Research Ltd. A Cpk value of one is a reasonable target and means that 99.7% of the process output meets the specification. A Cpk of zero indicates that the process mean lies on one of the tolerance limits. A minus Cpk value indicates that the process mean lies outside of the specification range and a Cpk of minus one or less indicates that the entire output is outside of tolerance. A comparison of Cp and Cpk indicates whether the process is centred or off- centre. If the two values coincide, the process mean is centered within the specification limits. The purpose of this study was to evaluate process capability in order to determine how closely eight frequent replacement (monthly or 2-weekly replace- ment) lenses adhered to their labelled parameters. Methods This was an in vitro study to measure the key parameters of a range of frequent replacement soft contact lenses. The measurements were compare d with the labelled parameters and further analysed to deter- mine process capability. The parameters measured were back vertex power (BVP), optical quality, back optic zone radius (BOZR), total diameter (TD) and centre thickness (tc). The potential and actual process capabil- ities were calculated for eight different lenses in three sphere powers (-1.00 D, -3.00 D and -6.00 D). Pilot Study In order to determine the number of repeat measure- ments required for each parameter, a pilot study was undertaken involving three lens types in three powers. Each lens was measured 10 times by the same operator on separate occasions. The required number of measure- ments (N) was calculated using the following formula: N = ((2.58 × Sr)/MT) 2 Where: Sr=Repeatability, MT=measurement tolerance (1 product tolerance). The exercise indicated that only one measurement was required for back optic zone radius, total diameter 127