AREA 4: Vision-correction system (glasses): selection and evaluation Topic 13: Lens-selection strategies Topic 14: Fitting protocol and evaluation Now that we have studied how to fit single-vision and multifocal lenses (areas 2 and 3), we have enough information to be able to choose the most suitable type of lens for each patient. In topic 14, we will take a look at the different factors that should be analysed in order to guarantee a quality fitting.

Topic 13: Lens-selection strategies The following factors should all be analysed when deciding on the most suitable type of lens for each patient: Refraction Material Surface form Treatment Before deciding which type of lens will suit a patient best, it is essential to talk to the patient and study their prescription details in order to gather the information you need to take a proper decision. A patient's prescription contains objective information that will help you to choose the most suitable lens material and surface form. Other decisions, however, related to lifestyle needs and personal preferences can be taken in conjunction with the patient. This is why it is essential to compile a good patient history to find out what type of glasses (if any) they had before, what they liked and didn't like about these glasses, what they need glasses for, and if they have any particular cosmetic or price concerns.

Refraction Single-vision Multifocal Myopia Hyperopia Astigmatism Whether you choose single-vision or multifocal lenses will depend on the patient's prescription and lifestyle needs. Myopia Hyperopia Astigmatism Strabismus or phoria Single-vision There is a wide choice of single-vision and multifocal (bifocal or progressive) lenses to suit different refractions. Although the patient's prescription will largely determine the choice, it is important to take the patient's opinions and preferences into account. Single-vision lenses are indicated for patients who require correction for just one viewing distance. In the case of presbyopic patients, although progressive lenses are the most effective option, the final decision must lie with the patient as they might prefer single-vision or bifocal lenses for reasons related to lifestyle or occupation needs, price concerns, etc. (We have already looked at multifocal lenses in detail in area 3). Multifocal

Refraction Typical problems associated with single-vision lenses used to correct myopia: Thick edges Reflections Power rings Weight There are several solutions for overcoming the typical problems associated with using high-powered minus lenses (listed on slide). Thick edges are generally reduced by increasing the refractive index of the lens material. One particularly interesting solution for patients who require low and medium powers is to use a new type of organic lens that offers medium refractive indices. Anti-reflective coatings can eliminate most unwanted reflections and power rings (these are the rather unsightly rings visible to others that are so characteristic of thick lenses). One of the best ways of eliminating reflections is to use as small a frame as possible, as this eliminates the thick edges that cause these rings and reflections. Further improvements can be made by ensuring that the pupils are as near as possible to the geometric frame centres. These cosmetic improvements have an added advantage in that they also make the glasses considerably lighter.

Refraction Typical problems associated with single-vision lenses used to correct hyperopia: Thick centre Weight Reduced visual field The use of aspherical surfaces can reduce centre thickness considerably and improve the optical quality of the lens. Aspherical surfaces also improve visual field perception by eliminating peripheral aberrations. The use of organic materials can reduce weight considerably. The best way to reduce both thickness and weight, however, is to use as small a diameter as possible. Increasing the refractive index can also reduce the thickness at the centre of the lens, although this can also make the glasses heavier than desired.

Refraction Typical problems associated with single-vision lenses used to correct astigmatism: Big differences in thicknesses around the edge of the lens Similar problems to those described for myopia and hyperopia Use the same lens-selection criteria described previously for plus or minus lenses. The cosmetic appeal of astigmatic lenses can be affected by the varying thicknesses around the edges of the glasses, particularly when rimless frames are used.

Refraction: prisms Typical problems associated with single-vision lenses featuring induced prisms used to correct phoria or strabismus: Thicker-than-normal edges in the direction of the base The optical centre in a single-vision lens containing a prism does not coincide with the centre of the patient's pupil. These lenses tend to be incompatible with rimless frames or frames that have very thin eyepieces in sections. Aspherical lenses should not be decentred.

Refraction Typical problems associated with multifocal lenses: A change in attitude is required to adapt to this system Reduced near-vision field Image jump Although patients generally adjust to multifocal lenses relatively quickly and easily, they require a little help to do so. The two most common types of multifocal lenses are bifocals and progressives. The choice of one system or the other depends on several aspects including the patient's' job, degree of motivation, and budget.

Material Organic Mineral Polycarbonate Lightweight High impact resistance Mineral Thinner High scratch resistance Polycarbonate High safety The glass used in ophthalmic optics, generally known as mineral glass, is made from the fusion of inorganic oxides such as silica, an essential component in this type of glass. The most common types of mineral glass are crown glass (n=1.523), flint glass (n=1.700), and dense flint glass (n=1.80), although there are many other types of glass with medium and high refractive indices. Mineral lenses have hard surfaces, are fragile (meaning they break easily), and are used to achieve thin designs. Lenses can also be made of plastic. The material used to make these lenses, also known as organic lenses, is the product of chain polymerisation and mainly contains carbon, hydrogen, and oxygen. The most common materials are CR-39 (diallyl diglycol carbonate) (n=1.498) and polycarbonate (n=1.585). A wide range of organic materials with higher refractive indices (1.500, 1.523, 1.6) are currently being developed. Plastic lenses are best known for their lightweight construction, and are generally 40% less dense than mineral lenses. Another major advantage is their resistance to impact, although this is offset by their low scratch resistance. Although polycarbonate is a plastic, it tends to be placed in a category of its own. It has enjoyed tremendous success since its introduction to the glasses market because it offers much greater impact resistance than other lens materials and is therefore a popular choice for rimless frames and goggles.

Surface form Optical surfaces Spherical Aspherical Myodisc Lenticular Spherical optical surfaces: Most of the surfaces used in ophthalmic optics are generated by rotating an arc of circumference around an axis of revolution that passes through the centre of curvature of the arc. These surfaces are the easiest to study as all their meridians are of equal length. Aspherical surfaces, in contrast, are generated by rotating curves belonging to a family of conics. They are formed when a cone is intersected through its different planes, giving rise to parabolas, ellipses, and hyperbolas. Aspherical lens surfaces permit the use of flatter curves that successfully merge high image quality and cosmetic appeal. This is impossible with spherical designs. Aspherical designs are normally used in high-powered plus lenses to achieve flatter primary surfaces. This, in turn, reduces lens thickness and volume and improves comfort and cosmetic appeal for the wearer. Myodisc or minus lenticular designs. These designs are normally used in high-powered minus lenses. They resolve the problem of weight but have a very small visual field in the centre of the lens. The same occurs with lenticular lenses, the equivalent for high-powered plus lenses. Neither myodisc nor lenticular designs are cosmetically appealing, which is why they are gradually being replaced by aspherical designs and other solutions such as high refractive indices, precalibrated lenses, etc.

Surface form Aspherical Spherical design This slide shows the difference between a spherical and aspherical surface.

Surface treatments Surface treatments provide added value by enhancing the existing properties of a lens or equipping it with new properties. Unnecessary surface treatments are a waste of money and can sometimes even interfere with performance. Lens surface treatments simply improve the existing properties of lenses by increasing their resistance to impact and soiling, by making them more transparent, or by providing them with a special tint to improve their fashion appeal. A range of different surface coatings and treatments are available (see next slide).

Surface treatments Anti-reflective Water-repellent Scratch-resistant Heat-tempered UV Tint Anti-reflective coatings eliminate unwanted reflections and increase transmission, creating a strong clear-glass effect and a greater sensation of wearer comfort. They are created by vacuum-applying a series of metallic oxide coatings on the surface of the lens. Water-repellent coatings repel water from the surface of the lens thanks to a thin surface film that reduces surface tension. Scratch-resistant coatings are mainly used on organic and polycarbonate lenses to make their surfaces tougher. Mineral lenses are strengthened in a heat-tempering process that consists of subjecting the glass to high temperatures in a furnace and then cooling it abruptly using cold air. This makes the lens so hard that it is virtually impossible to break. Most ophthalmic lenses have built-in UV protection, which consists of a filter that prevents harmful ultraviolet rays from the sun from reaching the wearer's eyes. Lenses can be tinted, or coloured, in a variety of ways. One of the most common methods of tinting organic lenses is to bathe them in a tint colour for a given time at a high temperature (around 90º). Mineral lenses, in contrast, are normally tinted with a vacuum-applied surface coating (in a process similar to that used for anti-reflective coatings).