Soft Contact Lens Fitting

Introduction

Soft lenses were introduced initially in the early 1970s and now are the most prescribed lens type worldwide, used by 80–90% of contact lens patients ( ). Over the past 45 years there have been numerous improvements in materials and manufacturing techniques. Soft lenses now can be worn successfully on both a daily and overnight basis and are typically replaced on a frequent basis at intervals ranging from 1 day to 1 month ( ). The last 15 years have seen the most significant change in soft lens materials, with the introduction of silicone hydrogel lenses that transmit increased levels of oxygen to the cornea.

This chapter will review the development and properties of soft contact lens materials, their classification, indications and contraindications for fitting and the fitting process required to ensure that such lenses perform in an optimal fashion.

Historical Overview

For a review of the development of soft lenses, readers are referred to Chapters 1 and 2 and to Section 8 , History, available at: https://expertconsult.inkling.com/ .

Terminology

The terminology used to describe the wearing schedule or replacement period of soft lenses can be confusing ( , ). The following sections aim to clarify this.

Soft contact lens wearing modality

Soft contact lenses can be worn according to a number of different wearing modalities.

Daily Wear

Lenses worn on a daily wear basis are worn during waking hours, usually for periods of 8–16 hours. On removal they are either cleaned and disinfected in preparation for the next wearing period or, in the case of single-use daily disposable lenses, discarded.

Flexible Wear

Lenses worn on a flexible wear basis are typically worn on a daily wear basis, with occasional, infrequent overnight use. When removed, they should either be cleaned and disinfected or discarded.

Extended Wear (for Extended and Continuous-Wear Lenses, See Chapter 12 )

Extended-wear lenses are worn constantly for up to 7 consecutive days and nights. When removed, they should either be cleaned and disinfected (reusable extended wear) or discarded (disposable extended wear).

Historically, reusable extended-wear lenses were used in therapeutic applications or for patients who used their lenses for aphakia or other abnormally high refractive errors, with cosmetic lens wearers using their lenses on a disposable extended-wear basis. However, modern silicone hydrogel lenses are often worn for two or four consecutive 1-week periods of extended wear and then discarded, making them reusable extended-wear lenses.

Continuous Wear

For early continuous wear, see Section 8 , History, available at: https://expertconsult.inkling.com/ .

Since the introduction of silicone hydrogel materials, continuous wear has once again become an option ( , ). In this modality, lenses are worn on a 24-hour basis for periods of up to 30 consecutive days and nights. Lenses may be removed before this time, but they must be cleaned and disinfected before reinsertion. After 1 month of wear, with or without removal, the lenses are discarded. Studies have indicated that the use of silicone hydrogel lenses has not reduced the incidence of microbial keratitis and that the rates are very similar to those seen with hydrogel lenses, even if the severity may be slightly reduced ( , , ).

Soft contact lens replacement schedule

Frequently replacing lenses diminishes lens spoilage and complications and is considered to offer greater safety for contact lens wear, on both a daily and overnight basis ( , , , , , , , , , , , , , , ). Lenses are now frequently classified according to their replacement schedule ( , ).

Conventional Replacement

With a conventional replacement schedule, lenses are typically replaced every 6–18 months. This form of replacement is now rare ( ) and typically occurs only for highly specialised lenses that are custom-made.

Planned Replacement

The term ‘planned replacement’ typically describes lenses that are changed at intervals ranging from 1 week to 6 months.

Disposable Lenses

The term ‘disposable’ should be reserved for lenses which are discarded and replaced after a single wearing period, with no cleaning or disinfecting ever taking place ( ). For daily disposable lenses, this is a 1-day wearing period, and for lenses worn on an extended- or continuous-wear regimen, time frames range from 7–30 days ( ). The term is, however, frequently synonymous with lenses which are removed for regular cleaning and disinfection and replaced after a period of 1–4 weeks ( ).

Soft Contact Lens Properties and Materials (see also Chapter 2 )

Oxygen permeability and transmissibility

Oxygen permeability is an inherent property of a material and is independent of lens thickness. It is expressed as the ‘Dk’ value, where D represents the diffusion coefficient and k the solubility of oxygen ( , , ). From a clinical perspective, oxygen transport to the cornea depends on both the Dk of the material and the lens thickness (t), with thinner lenses allowing more oxygen to reach the cornea ( ). The term Dk/t describes the ‘oxygen transmissibility’ of a lens and gives a quantitative indication of the amount of oxygen that a lens-wearing eye will receive through the lens. It is more clinically useful than Dk, which gives no indication of the effect of lens thickness or design.

Ideally, hydrogel lenses would have both a high Dk, as the oxygen is transported in the water, and a thin centre thickness. Such lenses are, however, impractical because they rapidly dehydrate, resulting in significant corneal staining ( , ). Thin high-water-content lenses are also difficult to manufacture, so in practice, lens thickness is greater, which limits the Dk/t clinically obtainable ( ).

Table 10.1 details typical Dk/t values available for conventional hydrogel materials and commonly encountered rigid gas permeable (RGP) materials, at centre thicknesses normally found for −3.00 D lenses. It is clear that conventional hydrogel materials have relatively low Dk/t compared with RGP materials.

Table 10.1

‘Typical’ Examples of Dk/t for Conventional Lens Materials

Hydrogels	Dk/t	Rigid Gas Permeable	Dk/t
Low water content (38%)	15	Silicone-acrylates	27
Mid water content (55%)	27	Fluorosilicone acrylates	60
High water content (70%)	35	Fluoropolymers	130

Materials

Hydrogel Materials

All lens materials are polymers made up of repeating chains of monomers arranged in patterns, with cross-linking between the polymer chains ( ). These afford strength and further govern the characteristics of the materials. The lens materials have water contents ranging from 24–85% ( ). The monomer most commonly employed in conventional contact lenses is poly-2-hydroxyethyl methacrylate (polyHEMA).

PolyHEMA is:

▪
easily fabricated into contact lenses
▪
relatively cheap
▪
highly flexible
▪
dimensionally stable to changes in pH and temperature
▪
very successful as a contact lens material.

The principal disadvantage of polyHEMA is that it relies on water to transport oxygen across the material. Water has a limited ability to dissolve and transport oxygen, having an approximate oxygen permeability of 80 Dk units ( ).

To increase the Dk of a conventional hydrogel material beyond that of polyHEMA, it is necessary to incorporate monomers that will bind more water into the polymer ( , , ). These higher-water-content materials typically use polyHEMA or methyl methacrylate in conjunction with more hydrophilic monomers such as N-vinyl pyrrolidone or methacrylic acid (MA) ( , ). The constituent monomers determine the various physical and chemical properties of the material, with MA-containing materials having a significant degree of negative surface charge.

Silicone Rubber

Another method of increasing Dk is to incorporate silicone into the polymer ( , ). Silicone-rubber-based flexible contact lenses have been used for therapeutic and paediatric applications for many years (see Chapters 2 and 24 ) ( , , , , ). Although the lenses offer exceptional oxygen transmission and durability, they do have limitations for clinical use:

▪
Fluid is unable to flow through these materials, leading to possible lens binding to the ocular surface ( ).
▪
Lens surfaces are hydrophobic, resulting in marked lipid and mucus deposition ( , ).

Silicone hydrogel materials combine silicone rubber with conventional hydrogel monomers ( ). The silicone component provides high oxygen permeability, while the hydrogel monomers facilitate flexibility, wettability and fluid transport, which aids lens movement.

An additional problem with silicone-based materials is their decreased wettability and increased lipid interaction. To overcome this, a variety of methodologies were adopted to enhance the wettability of the surfaces ( , , , , , , , , , ).

A wide variety of spherical silicone hydrogel lens materials are currently available, in both reusable ( Table 10.2 ) and daily disposable options ( Table 10.3 ). Table 10.2 also indicates that reusable silicone hydrogels may be ordered in custom designs, permitting a large variety of back optic zone radii (BOZR, also referred to as the base curve), diameters and prescriptions.

Table 10.2

Silicone Hydrogel Reusable Lens Materials

CW, continuous wear; DW, daily wear; EW, extended wear; N, nights of wear.

Table 10.3

Silicone Hydrogel Daily Disposable Lens Materials

Water content and thickness

Soft lenses are frequently categorised according to their water content:

▪
20–44% – ‘low’ water content
▪
45–59% – ‘mid’ water content
▪
greater than 60% – ‘high’ water content.

PolyHEMA has a water content of 38% and is obtainable in varying thicknesses; thinner lenses give increased transmissibility (Dk/t) and improved corneal physiology. Hypoxic complications with low-water-content lenses resulted in increasing numbers of mid- and high-water-content lenses being fitted, since increasing the water content is a more efficient means of improving corneal oxygenation than reducing lens thickness ( , , ).

In silicone hydrogel materials, the oxygen is transmitted mainly through the silicone component of the lens material, resulting in a dramatic increase in oxygen permeability with very low-water-content materials ( , ). These lenses have an inverse relationship between water content and Dk, with the highest Dk values corresponding with the lowest water contents ( ).

Manufacturing methods (see also Chapter 29 )

Soft lenses may be manufactured using several different techniques, including lathing, spin-casting, dry-moulding or the more recent techniques of wet or ‘stabilised soft’ moulding ( , , ). Moulding methods are ideal for mass production of lenses, and a number of manufacturers have streamlined their moulding technologies to reduce production costs and allow more frequent replacement of lenses ( ). Lathing is better suited to the production of individual lenses and specialised designs, but with modern automated lathes, it is increasingly successful in mass production. The majority of silicone hydrogel lenses are moulded, but some latheable custom-made options are also available.

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here