Contact lenses can be broadly categorised according to their frequency of use and frequency of replacement. Daily disposable lenses, which are worn during the day and discarded at the end of the day, are single-use, daily replacement lenses; this form of lens wear is discussed in Chapter 19 . All other lens types are reusable, meaning that lenses that are removed from the eye, cleaned and stored in a lens case for a period of time (typically overnight), and then reinserted into the eye the following day (or at a later date if lenses have not been worn for a while).
Reusable lenses are sub-classified according to their frequency of disposal. Unplanned replacement refers to the wearing of lenses indefinitely, until they can no longer be worn due to discomfort, poor vision or an adverse ocular reaction. Such lenses are not prescribed today.
Planned replacement refers to lenses that are intended to be discarded after a specified period of time – daily (i.e. daily disposable lenses), 1 or 2 weekly, or 1, 3, 6 or 12 monthly. It is intended that lenses are discarded after the specified period of time has elapsed (from the time of the first instance of lens wear), regardless of how many times the lenses were worn during the specified period. The term ‘frequent’ or ‘regular’ replacement is generally used to refer to lenses replaced monthly or more frequently (i.e. including daily or 1- or 2-weekly replacement).
This chapter will primarily consider reusable, planned-replacement lenses. To fully understand the clinical rationale for the prescribing of such lenses, it is necessary to consider the relevant issues in an historical context. This is because the clinical rationale for the ubiquitous prescribing of reusable, planned-replacement lenses has evolved over half a century, often learned through bitter experience with early-generation soft lenses.
Global Prescribing Trends of Contact Lenses According to Replacement Frequency
The concept of regular lens replacement began to be introduced in the mid-1980s ( ) ( Fig. 20.1 ). Prior to this, lenses were worn on an unplanned basis. The acceptance of frequent lens replacement by practitioners and patients has been one of the most significant changes in the contact lens market since soft lenses became widely available in the early 1970s.
Fig. 20.2 shows the mean worldwide trends in the extent of fitting soft contact lenses of different replacement frequencies between 2000 and 2020. This graph shows data derived from 347,801 fits in 71 nations. It can be seen that the prescribing of lenses intended to be replaced at periods greater than 1 month (‘Other’ category) has decreased sharply since 2014 and represented only 2% of all fits in 2020 ( ).
Over the same 20 year period, lenses designed for 1- to 2-weekly replacements have ranged between 7% and 20% of all soft lenses fitted, and lenses for monthly replacement have ranged between 35% and 55% of all soft lenses fitted. The fitting of daily disposable lenses has gradually increased from around 20% in the early 2000s to nearly 50% in 2020.
For the year 2020, the average worldwide rate of prescribing lenses of different replacement frequency is as follows: daily – 47%; monthly – 35%; 1–2 weekly – 16%; 3–6 monthly – 1%; annually – 1%; and unplanned replacement – 0% ( ).
Discussion of reusable, planned-replacement lenses in this chapter will be confined to lenses intended to be used on a daily-wear basis (i.e. lenses that are not worn overnight). The specific issue of extended-wear lenses is dealt with in Chapter 23 .
Reasons Why Contact Lenses Need to Be Replaced Regularly
For practitioners or patients new to contact lenses, who are aware that virtually all soft lenses today are discarded after 1 month or sooner, it would not be unreasonable to ask why lenses need to be so regularly replaced. The following discussion provides an historically based rationale for regular lens replacement.
Avoidance of Long-Term Adverse Changes in Contact Lenses
Soft contact lenses have a reactive surface and a low modulus of elasticity, rendering them more susceptible to damage. Furthermore, any damage to the surface or edge of the lens cannot be repaired because a hydrated soft lens cannot be repolished. Although these factors would seem to indicate the desirability of regular soft lens replacement, the high production cost of soft lenses throughout the 1970s and much of the 1980s precluded such, so strategies were devised to extend lens life for as long as possible.
The norm during this period was to prescribe soft lenses in the same way that rigid lenses were being prescribed; that is, for the patient to keep using the same pair of lenses until they became damaged or lost, were too uncomfortable to wear, resulted in a noticeable deterioration of vision, or induced ocular pathology that was either self-diagnosed or detected during an eye examination. Some patients were known to use the same pair of soft lenses for up to 7 years. Indeed, it was even possible to take out insurance policies against the risk of lens loss or damage.
The rationale for the regular replacement of soft contact lenses is simple: cleaner lenses should produce fewer adverse ocular effects. A significant proportion of clinical problems relating to the wear of soft contact lenses can be attributed to deposition on the lens surface with tear-derived substances. Contact lens deposits result in reduced acuity, comfort and wettability and increased inflammatory complications such as papillary conjunctivitis and acute red eye ( ). All soft contact lenses suffer gradual spoilation from the environment and tear film components over time. Daily cleaning can slow this rate of deposition but not prevent its occurrence.
Lens deterioration over time manifests in a variety of ways and is attributable to many factors. Lenses become deposited, irreversibly lose water, suffer surface damage and can become contaminated during storage. Each of these factors will be considered in turn.
An early experiment conducted by , around the time regular contact lens replacement was being introduced, found that the extent of deposition on the lenses of patients who had worn the same lenses for many years (i.e. lenses were never replaced) increases over time ( Fig. 20.3 ).
Numerous factors, many of which are interactive, are involved in the formation of deposits on the front or back surface of contact lenses. These factors include lens wear modality (daily or continuous wear), the bulk chemical composition of the lens, lens water content, the physicochemical nature of the lens surface (such as ionicity), the chemical composition of lens maintenance solutions, the adequacy of lens maintenance procedures (a measure of patient compliance), hand contamination, proximity to environmental pollutants and intrinsic properties of the tears of the patient ( ).
The most common tear-derived components of lens deposits are proteins ( ), which cannot be detected under normal conditions. A heavy deposition of protein can manifest as a general haze on the lens surface ( Fig. 20.4 ), and extensive lipid formation can appear as a clear smear or smudge on the lens surface.
Visible soft lens deposits take months or years to form, and are thus were only encountered previously in patients who wore lenses on a nonplanned replacement basis. Thus such deposits are rarely seen today. The most common form of long-term deposition that was visible to the eye was derived from the tear film and was known as ‘jelly bumps’ or ‘mulberry deposits’ ( Fig. 20.5 ), which consisted of various layered combinations of mucus, lipid, protein and sometimes calcium. Barnacle-like calcium carbonate deposits ( Fig. 20.6 ), which were also derived from the tear film, projected anteriorly and were a source of discomfort. Iron deposits ( Fig. 20.7 ), which contaminated the lens from exogenous sources, appeared as small red-orange spots or rings and formed when iron particles became embedded in the lens and oxidized to form ferrous salts. These were often seen in patients who frequently commuted on trains or trams, as there was a high probability of fine iron particles – which were thrown into the air as the vehicle moved along the steel tracks – coming to rest on the lens surface.
It is clear that proteins and lipids from the tears can deposit on soft lenses within minutes of insertion ( ); however, such deposits are thought to be innocuous over periods of less than 1 month. Deposition of certain lipids may even improve the surface properties of some lenses resulting in enhanced comfort ( ). Lipid is easily removed with surfactant cleaning. A small amount of protein deposition may also be beneficial to the eye, as long as it does not become denatured, because the protein forms a natural biocompatible lens coating ( ). Although these rapidly forming deposits cannot be seen and do not generally compromise vision or comfort, they can reduce lens surface wettability ( ).
Long-term protein deposition can be problematic because, in time, it can become denatured and thus no longer ‘recognized’ by the eye, leading to an adverse immunological reaction ( ). Lens surface protein can also absorb, and concentrate, preservatives and other active ingredients in contact lens care solutions, which may be released back into the eye in noxious concentrations, leading to toxic reactions. The physical presence of excess deposits can also cause direct mechanical insult to the anterior eye.
Soft lenses can also become discoloured over extended time periods (many months or years). The cause may be intrinsic or extrinsic. High levels of melanin can lead to a brown discoloration. Nicotine can become absorbed into the lenses of patients who smoke or spend time in a smoky environment, leading to an orange-brown discoloration. Exposure to mercury can lead to a black or grey discoloration. Extreme lens discoloration can be cosmetically unsightly to an onlooker.
Irreversible Water Loss
noted a significant lens-ageing effect whereby the preinsertion lens water content decreased significantly over a 28-day cycle for four hydrogel lens types evaluated ( Fig. 20.8 ). This ageing process is different from the well-known phenomenon of lens dehydration over the course of a number of hours throughout a day. Although this irreversible water loss was monitored for only 28 days, the trend clearly indicated that water loss would continue well beyond this timeframe, albeit at a progressively slower rate.
It is clear that a combination of physical and/or physiological factors caused a reduction in water content of the hydrogel lenses examined by . It follows that some change to the lens appears to have caused a progressive reduction in water uptake by the lens each night during storage, in what amounts to a ‘lens-ageing’ effect. The most likely explanation for this ageing effect is that lens spoilation acts either to displace water from the lens or to alter the nature of the lens material in such a way that less water is absorbed. Some significant intersubject differences in lens dehydration were observed; the range of daily dehydration for the group of six subjects investigated by was 1.7%–5.9% for all lenses for all days. These differences may relate to intersubject differences in ocular physiology.
One of the most important clinical ramifications of this lens-ageing phenomenon is that there is an associated loss of oxygen performance with dehydration of hydrogel lenses ( ). Thus the corneas of patients wearing hydrogel lenses on a nonreplacement basis will be more prone to hypoxic complications over time.
Surface Damage and Crazing
All soft lenses are manufactured with a shelf-life, which primarily indicates how long the lens can be guaranteed to be sterile. In addition, there is the possibility of natural polymer degradation over time, whereby clinically relevant changes could be noticed after about five years from the time of manufacture.
It is self-evident that physical trauma can lead to a variety of lens defects. If a defect is obvious – such as a large piece of the lens breaking off – then the patient will typically notice this and discard the lens. If such a defect is not noticed, discomfort on insertion will normally alert the wearer to this problem. However, small defects may not be noticed, which is potentially problematic because such defects can compromise ocular integrity at a subclinical level ( ).
Contamination During Lens Storage
Patients may suspend lens wear for extended periods of time, for reasons such as not wearing lenses when unwell or when travelling. Also, for a variety of lifestyle reasons, some patients wear lenses only very occasionally. The potential for contamination of the lens and storage case during such periods is potentially problematic ( ). In particular, some contact lens storage solutions are inefficacious at killing fungi, which have a propensity for invading the lens matrix and destroying lenses in storage ( Fig. 20.9 ) ( ).
In the past, stringent measures needed to be enforced upon patients to ensure preservation of lenses during long-term storage, such as regular cleaning and disinfection and the use of storage solutions known to be highly efficacious at killing all forms of microorganisms. However, long-term lens storage is no longer an issue because regular planned lens replacement requires lenses to be discarded 1 month after first being worn, regardless of the number of times the lenses have been worn during that month. If lens wear is suspended for more than a few days, the previous lenses should be discarded and new lenses used when lens wear resumes.
Avoidance of Long-Term Adverse Changes in the Anterior Eye
Prior to the introduction of regular lens replacement, primary indicators of long-term lens degradation included symptoms of discomfort and reduced vision, and signs associated with adverse ocular reactions. By ensuring that soft lenses are replaced at a regular predetermined interval, one of the most enduring medical management axioms – that of prevention being better than cure – is brought to bear. Early studies conducted during the transition of the market to regular lens replacement found that patients who replaced lenses regularly reported fewer symptoms and exhibited fewer physiological changes in most instances ( ), compared with patients who did not replace lenses regularly.
As indicated above, with early-generation nonregularly replaced soft lenses, numerous factors led to increasing discomfort over time; these included the existence of microscopic lens defects, physical trauma and/or immunological reaction due to lens deposition, and progressive hypoxic effects due to lens ageing. In a retrospective study of nearly 2000 2-weekly and nonplanned replacement daily-wear patients, found a higher rate of symptoms in the nonplanned replacement group.
surveyed the symptoms experienced by 104 patients wearing hydroxyethyl methacrylate (HEMA) lenses that were not being replaced on a planned basis. They found a clear association between the symptom of dryness and age of the lenses. Of those patients whose lenses were older than 6 months, 31% often experienced dryness, whereas only 12% of patients whose lenses were less than 6 months old experienced this symptom ( Fig. 20.10 ).
The loss of vision associated with deposit accumulation on HEMA contact lenses was assessed in an early study by in 51 patients presenting consecutively to a large clinic. Both high- and low-contrast visual acuity decreased with increased deposition and with lens age. As a general rule, unacceptable vision loss and deposit formation occurred after 12 months or 4000 hours of lens wear ( Fig. 20.11 ).