Since their launch in 1995, daily disposable lenses have seemed to be the most logical way to wear contact lenses. With considerable ease of use, portability and no requirement for a care system, many thought we would see the decline of planned replacement lenses and an increase in daily disposability.
Ten years after the launch Morgan1 reported that 15 per cent of new soft lens wearers worldwide were fitted with daily disposables. But the proportion of new fits varied widely; the
The reason for this slow initial growth has been attributed to several factors, including affordability. Practitioners may tend to pre-judge their patients’ financial position and ability to afford daily disposables, thus denying them the opportunity to experience the advantages of this modality. Attention has also been focused on the clinical performance of these lenses, their physical properties and behaviour on the eye.
Hydrogels and hypoxic effects
The first daily disposables were nearly all manufactured from conventional hydrogel materials. While these materials are oxygen permeable enough for limited daily wear, there was a question mark over whether they could support a full day’s wear.
Harvitt and Bonanno2 suggested a minimum oxygen transmissibility (Dk/t) of 35 x 10-9 units was needed to avoid corneal stromal oedema in daily wear. This value was well above that of most daily disposables on the market at that time.
Consideration must also be given to the fact that most manufacturers quote Dk/t for the centre thickness of a -3.00D lens. In a minus powered lens the Dk/t drops increasingly towards the periphery due to increased thickness at the edge.
Clinically this can manifest itself as corneal swelling; Kaluzny3 demonstrated that, after six weeks’ use, in an eye wearing a standard -3.00D etafilcon A lens the peripheral cornea was swollen by 3.23 per cent. Bearing in mind that overnight physiological swelling is in the order of 3 per cent, if a lens such as a -3.00D etafilcon A is worn all day, the cornea never gets the chance to fully de-swell.
The chronic effects of hypoxia can have other clinical consequences, such as limbal hyperaemia. Papas4 concluded that reduced oxygen concentration at the ocular surface induced more blood flow in limbal vessels and resulted in redness around the limbus the longer patients wore their lenses into the evening.
The freedom to wear contact lenses when they wanted to and the convenience of no solutions are factors that made the daily disposable modality attractive to wearers, but the goal of all-day wear still seemed elusive.
Era of silicone hydrogels
The original silicone hydrogel (SiH) lenses – PureVision (balafilcon A, Bausch+Lomb) and Focus Night & Day (lotrafilcon A, CIBA Vision) – were introduced in 1999, primarily for the continuous wear market. In fact both lenses had
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