Use of silicone hydrogel material for daily wear

1. Introduction 

Silicone hydrogel contact lenses which have been available since 1998 were initially developed to be used for extended wear. The pivotal publications in 1989 had shown beyond doubts that wearing hydrogel contact lenses on an extended wear basis [1], [2] was the major risk factor for contact lens associated microbial keratitis, increasing the risk compared to daily wear by a factor of at least four. Because hydrogel contact lenses were known to supply insufficient oxygen to maintain normal corneal metabolism during closed eyewear [3], the industry carried out, over 20 years, research programmes that lead to the production of silicone hydrogel contact lenses that fulfil the corneal oxygen needs during closed eyewear Dk/t>87×10−9cm/s(mlO2/mlmmHg) [4]. The supply of higher levels of oxygen alone is, however, not sufficient to prevent microbial keratitis during overnight contact lens wear of silicone hydrogel contact lenses [5]; all studies to date [6], [7], [8], [9], [10], [11], including that of Morgan et al. [12], confirm that the use of silicone hydrogel contact lenses produces a significant higher level of microbial keratitis than daily wear hydrogel contact lenses.

The performance and acceptance of contact lenses depends upon many factors; to achieve a high level of both necessitates the maintenance of normal ocular physiology, good mechanical interaction between the contact lens and the ocular surface and the maintenance of a stable tear film at the contact lens front surface [13].

Silicone hydrogel contact lenses were initially developed to deal with the corneal metabolic needs during extended wear. The aim was to maximise oxygen transmissibility, with levels several times those of hydrogel contact lenses (hydrogel Dk/t=15–30×10−9cm/s(mlO2/mlmmHg); PureVision® Dk/t=110×10−9cm/s(mlO2/mlmmHg); Focus NIGHT & DAY™ Dk/t=175×10−9cm/s(mlO2/mlmmHg)). Two drawbacks of first generation silicone hydrogels have, however, been their mechanical properties and inherent wettability. The presence of silicone, necessary to increase the material oxygen permeability, gives to the contact lenses higher elastomeric characteristics (e.g. rubber like properties) and greater rigidity. Whereas, conventional hydrogel materials have a modulus of rigidity ranging between 0.207 and 0.689MPa, the two silicone hydrogel materials, developed for extended wear have significant higher modulus [14]. The clinical implications have been higher incidence of superior epithelial arcuate lesion of mechanical aetiology [15]. The hydrophobic properties of silicone have forced the addition of a surface treatment to render the contact lens surface wettable. The hypothesis that the alliance of the surface treatment and the lower water contact of those materials (balafilcon A 36%, lotrafilcon A 24%) compared to conventional hydrogel materials (38–75%) gives a superior performance in patients complaining of dry eye problems has been put forward. Supporting evidences showing that silicone hydrogels may reduce dryness symptoms are found in the literature [16], [17], [18], [19].

More recently in order to supply high levels of oxygen to the cornea, regardless of the contact lens thickness, the new silicone hydrogel contact lenses have been used increasingly for daily wear. At the same time the designing of the second generation of silicone hydrogel contact lenses, that has included better balancing of the three key physico-chemical properties associated with optimal on eye contact lens performance: oxygen supply, lens rigidity and intrinsic material wetting took place. ACUVUE® ADVANCE™ with HYDRACLEAR™ the first contact lens of that generation made from galyfilcon A material has a Dk/t of 86, that is about three times that of conventional hydrogel contact lenses (Dk/t 15–30), but a similar rigidity (modulus: galyfilcon A=0.448MPa, conventional hydrogel ∼0.276MPa). Further galyfilcon A does not require a surface treatment to achieve on eye wettability, a result of HYDRACLEAR™ proprietary wetting agent incorporation technology.

The performance of contact lenses in addition to being lens type specific, does not remain constant over time. Contact lens surface changes associated with wear lead to changes in performance and the need to replace contact lenses. Typical replacement frequencies in the UK are daily or monthly [20], [21]; in contrast in the USA the most common replacement frequency is two weekly [21], [22]. The recommended replacement frequency for contact lenses made from galyfilcon A material, however, departs from market based regional differences in replacement frequency; the recommended frequency based upon clinical findings is two weekly worldwide.

The aim of the current investigation was to compare the performance of silicone hydrogel contact lenses made from galyfilcon A material and replaced two weekly to that of a leading hydrogel contact lens made from FDA group II alphafilcon A material and replaced both monthly as per the UK replacement scheme and two weekly as per the USA scheme. The two primary performance parameters evaluated were contact lens in vivo wettability and comfort, which are known to be influenced by both material characteristics and lens age [23], [24], [25], [26], [27], [28], [29]. The hypothesis tested was that galyfilcon A intrinsic wetting properties are associated with better on eye wetting and comfort than that produced by alphafilcon A independently of the replacement frequency [30], [31], [32], [33], [34], [35], [36].

2. Methodology 

2.2. End point and procedures 

The primary performance end points of interest were contact lens in vivo wettability and subjective acceptance/symptomatology.

In vivo wettability was evaluated using the Tearscope lighting system attached to the slit lamp biomicroscopic observation system [37]. The system allows the observation of the undisturbed pre-contact lens tear film, without the use of staining agent to both, characterise the structure of the tear film and measure its stability.

Two aspects of the tear film structure were quantified, the pattern of the mixing of the different classes of lipids and the interference fringes found within the aqueous layer [37]. The lipid layer mixing patterns were classified on a 14 point scale collapsed into a three point scale, in increasing order of thickness (Table 1). The thinnest layers are the least desirable as they are the least efficient to prevent evaporation [38] and are associated with the lowest tear film stability [39].

Table 1. Lipid mixing pattern classification

	0=None	Thin layers
	1=Not visible
	2=Lipid break-up
	3=Transient pattern
	4=Open meshwork
	5=Tight meshwork	Average thickness layers
	6=Meshwork and wave
	7=Wave
	8=Wave and amorphous	Thick layers
	9=Amorphous
	10=Wave and colours
	11=Amorphous and colours
	12=Colours	Other
	13=More than two
	14=Other

Interference fringes visible within the aqueous layer were classified on a five point scale in increasing order of thickness (Table 2) [37]. The layers which are too thick to produce interference fringes are considered normal, as they are similar to the preocular tear film; the other layers are thinner than normal.

Table 2. Aqueous interference fringe classification

	1=Absent aqueous layer
	2=Less than 5 fringes
	3=5–10 fringes
	4=More than 10 fringes
	5=Aqueous present and not visible (no. of fringes too large to be visible with white light lighting technique)—NORMAL

The tear film stability was quantified by measuring the non-invasive break-up time (NIBUT). The NIBUT is the time measured in seconds between eye opening and the appearance of the first break within the pre-lens tear film. Three measurements were made and for analytical purposes the median (average response) and minimum (worst response) values are reported.

Subjective acceptance was overall rated in terms of comfort on a continuous 0–50 point scale with descriptors to facilitate rating by the subjects (Table 3). Comfort was recorded independently for insertion (first 30min of wear), daytime (rest of the day up to 8 p.m.) and evening (from 8 p.m. to contact lens removal).

Table 3. Comfort classification scale

In addition to recording the primary end points normal contact patient management was undertaken, this included refraction and vision measurement, contact lens fit evaluation and ocular tissue examination.

3. Results 

A total of 30 subjects were successfully enrolled of whom 24 completed the investigation as per the protocol. The premature discontinuations were due to non-contact lens related causes in five cases and poor comfort with the control contact lens in one. The 24 cohort subjects (9 males and 15 females) with an average age of 38.7±0.5 years were all myopes (best sphere −2.69±0.94D) with low levels of astigmatism (cylinder≤1.00D) and fully representative of the target population. No adverse events either serious or non-serious were reported. All the contact lenses achieved an acceptable fit.

The pre-lens tear film structure was different for the two contact lens types. The differences were intrinsic properties of the contact lens materials as they were in general present at both the dispensing visit after 30min of wear and at the follow-up visits. The prevalence of thin lipid layers (open meshwork or less) was significantly (p=0.029 to <0.001) higher for the control hydrogel contact lens than for the test silicone hydrogel contact lens at all visits (Fig. 1). Whereas, with the test contact lens thin lipid layers were found in approximately half the eyes (45% and 56%), thin layers were present in more than two thirds of cases (70–86%) with the control contact lens. The prevalence of thick aqueous layers (Fig. 2), similar to the aqueous layer of the preocular tear film in the absence of contact lenses, was also significantly (p=0.008 to <0.001) greater with the test contact lens (80% and 88%) than with the control contact lens (35–64%).

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Fig. 1. Prevalence of thin lipid layers at the end of each wearing period.

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Fig. 2. Prevalence of thick aqueous layers at the end of each wearing period.

The stability of the tear film was overall greater for the test contact lens than the control contact lens (Fig. 3). The difference was statistically significant between the two contact lens types at the Day 14 follow-up visit, the test contact lens achieving the greater stability (median NIBUT: test=7.8s, control=5.6s).

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Fig. 3. Tear film stability at the end of each wearing period—median NIBUT (s).

The subjective acceptance demonstrated a typical diurnal variation in comfort for both contact lens types, with a significant decrease in comfort in the evening (Fig. 4). For the intended replacement period (test 2 weeks, control 4 weeks), comfort was significantly better at both insertion (p=0.001) (mean: test=40.1, control=33.6) and daytime (p=0.008) (mean: test=39.2, control=35.4) at the end of the wearing period for the silicone hydrogel test contact lens than the hydrogel control contact lens. In the evening, despite the overall decrease in comfort, a significant difference was recorded between the test contact lens and the control contact lens at the Day 28 visit (p=0.008) (mean: test=35.0, control=30.7 (4 weeks)).

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Fig. 4. Comfort scores at the end of each wearing period independently for three times of day (insertion, daytime, evening).

4. Discussion 

The study demonstrated a difference in wetting characteristics between the test silicone hydrogel contact lenses and the control hydrogel contact lenses. The silicone hydrogel contact lens maintained thicker aqueous and lipid layers than the hydrogel contact lens. It is important to stress that such findings are material specific and not a category difference. Clinical reports have suggested that in the event of dry eye complaints silicone hydrogels are beneficial [16], [17], [18], [19]. The rationale has been that via the combination of the low water content and surface treatment, tear evaporation is reduced with silicone hydrogel contact lenses, hence wettability is better. These comments were made with reference to the first generation of surface treated silicone hydrogel materials (lotrafilcon A water content 24% and balafilcon A water content 36%), but to date controversial supporting evidence has been published. Fonn and Dumbleton [40] in a short term daily wear follow-up evaluation found no difference between silicone hydrogels and conventional hydrogels. The same authors found a decrease in symptoms with silicone hydrogels compared to conventional hydrogel under a continuous wear modality [16]. In the current investigation, galyfilcon A silicone hydrogel material is a non-surface treated mid water content (55%) material; hence the enhanced on eye wettability cannot be attributed to either low water content or surface treatment. The conclusion from findings in this study must be limited to the materials tested and cannot be construed to demonstrate superior on eye wettability for the silicone hydrogel class of materials in general.

The significantly better comfort afforded by the galyfilcon A made contact lenses is a benefit resulting at least in part from the improved wettability. Comfort is dependent upon a complex biotribological dynamic influenced during both the interblink and the blink situations. During the interblink the tear film evaporates and slowly destabilises leading ultimately to the breakup of the tear film and exposure of the contact lens surface and/or exposed conjunctiva to the outside environment. In the normal situation the contact lens and exposed conjunctiva are covered by a continuous tear film during the whole interblink period. In such circumstances the tear film acts as a continuous lubricant between the contact lens front surface/exposed conjunctiva and the palpebral conjunctiva during the blink. This type of lubrication is known as hydrodynamic lubrication and associated with a low coefficient of friction. In the abnormal situation, when the tear film breaks up before a blink a continuous lubricant is no longer present between the palpebral conjunctiva and the contact lens/ocular surface; the lubrication in that case is known as boundary lubrication, and characterised by a much higher (10 times) coefficient of friction [41]. It is thought that this increased friction results in enhanced contact lens awareness and decreased comfort; the concept is well known in the cartilage research [42], [43], [44], [45], [46], [47], [48] and frequently quoted as a model for the tear film [36], [49]. This can in part explain the association between a more stable, better structure tear film and greater comfort with galyfilcon A contact lenses than with alphafilcon A contact lenses.

A secondary aspect of contact lens wear tested in the study was the replacement frequency: 2 weeks for galyfilcon A and both 2 and 4 weeks for alphafilcon A. The current results clearly show that the difference in performance is present after 2 weeks of wear for both contact lens types, hence is material specific and not solely due to a difference in replacement frequency.

5. Conclusion 

Two conclusions can be reached for this investigation:

The current investigation findings support the hypothesis that contact lenses made from galyfilcon A achieve better on eye wettability and acceptance than those made from alphafilcon A during daily wear use.