Contact Lens Standards

Introduction to Standards

A standard is a technical specification or other document which is available to the public. Such standards are drawn up with the cooperation and consensus or general approval of all interested parties affected by it. Standards should be based on the consolidated results of science, technology and experience and aimed at the promotion of optimum community benefits and approved by a body recognised on the national, regional or international level.

In the case of contact lens practice and manufacture, there are three categories of standards which are commonly referred to:

• 1.

  terminology – a collection of definitions which will enable us all to speak the same language

• 2.

  product specifications – a list of the tolerances that are applicable to the dimensions of the products we are dealing with

• 3.

  test methods – a series of methods which are (preferably) widely available, well understood and which if used according to carefully drawn protocols will credibly determine the dimensions of the products we are dealing with.

There are other categories of standards which are principally to provide information, but as far as commerce in the field of contact lenses is concerned the three categories above are all we are interested in.

Test Methods and Their Validation

The methods used to determine lens dimensions should preferably be generally available. However, this may not always be possible in a field where new technologies are emerging.

In order to validate a test method, it is subjected to a ring test in which representative commercial samples are measured in a number of centres by independent operators using different instruments. Sometimes, the ‘instrument’ may be better described as an ‘apparatus’ as, for example, in the case of determination of oxygen permeability for contact lens materials. This approach is described as an interlaboratory test conducted under conditions of reproducibility . There is an International Standard (ISO 5725) which describes statistical procedures that can be applied to the outcome of such ring tests to provide a statement of the ‘believability’ or ‘credibility’ of the method. For example, it may be interesting to know how credible a single measurement of the power of a soft lens is if measured on a focimeter in air. Alternatively, it might be interesting to know how many measurements would be necessary in order to be reasonably confident (‘reasonably confident’ usually means 95% confident) that the result was credible at, say, the 0.25 dioptre level. A ring test establishes these and other parameters of a particular method. Because of this, it has been agreed at ISO level that all contact lens test methods which are included in the standardisation process will be assessed by means of an international ring test.

Even with this approach, in the case of contact lenses it is not always possible to have standards as well-founded as we would like. The field of contact lenses is a rapidly changing one, with new products, materials and measurement technologies arriving continuously. In such circumstances, it is frequently necessary to make a judgement between the desirability of only standardising established technologies and the pressure of the regulated marketplace, where standards are used to help implement national or international directives that are intended to maintain and improve levels of health and safety by promoting fair competition. It is essential to publish standards which will benefit the manufacturing industry and the clinical profession in its widest sense while monitoring the performance of new and developing technologies so as to end up with standards which will stand the test of time.

Terminology and Symbols

The current ISO (also a European and British standard) on contact lens terminology is BS EN ISO 18369–1:2017, Ophthalmic optics – Contact lenses – Part 1: Vocabulary, classification system and recommendations for labelling specifications.

Table 30.1 gives the key terms as defined in this standard, while the symbols as applied to a spherical lens are shown in Fig. 30.1 .

In an attempt to show realistic lens dimensions, two scale drawings of typical contact lenses are included. Fig. 30.2 shows a typical multicurve (in this case four-curve) rigid lens, drawn to scale. This is a fair representation of the majority of corneal rigid lenses which are currently supplied.

Fig. 30.3 shows a typical minus-powered soft lens. This is also a fair representation of the overwhelming majority of soft lenses, both hydrogel and silicone hydrogel. Most current commercial high-volume lenses will have a back surface composed of either one or two coaxially arranged spherical radii. The front surface frequently has three coaxially arranged radii, with the first peripheral radius used to provide a relatively thick zone in the mid-periphery which is intended to improve lens handling. In cases in which the lens is designed to have aberration-controlled optics, the front central optic zone will have an aspheric geometry which is intended to give the lens an approximately uniform power profile. If aberration-controlled optics are not incorporated, the arrangement of the lens geometry will be as shown in Fig. 30.3 .

Test methods for the physico-chemical properties of contact lens materials, including the determination of oxygen permeability, are specified in BS EN ISO 18369–4:2017, Ophthalmic optics – Contact lenses – Part 4: Physico-chemical properties of contact lens materials.