- Email: [email protected]
Applanation tonometry in silicone hydrogel contact lens wearers
Contact Lens & Anterior Eye 30 (2007) 267–269 www.elsevier.com/locate/clae
Applanation tonometry in silicone hydrogel contact lens wearers R.J. Allen a,*, A. Dev Borman c, G.M. Saleh b,c a
Essex County Hospital, Colchester, United Kingdom Institute of Ophthalmology, London, United Kingdom c Royal Surrey County Hospital, Guildford, United Kingdom b
Abstract Introduction: Previous studies have investigated intraocular pressure (IOP) measurements through conventional soft (hydrogel) therapeutic contact lenses, and have found that an accurate IOP can be recorded in normal eyes, and in eyes with abnormal anterior segments. The IOP measurement through soft contact lenses may be affected by the water content and centre thickness of the lens. Silicone hydrogel contact lenses are now being used as therapeutic contact lenses due to their high oxygen permeability. The purpose of this study is to investigate if IOP can be accurately measured in a subject wearing a silicone hydrogel contact lens. Methods: In a cohort study, the IOP was measured with a Goldmann applanation tonometer without a contact lens and then repeated with a hydrogel contact lens in situ. Results: The IOP of 20 eyes of 10 volunteers with no ocular pathology was measured. The mean difference (S.D.) found between IOP measurement with (mean 15.55 1.70 mmHg) and without (mean 16.05 1.90 mmHg) contact lens was found to be 0.5 0.89 mmHg. Statistical analysis was performed which revealed a correlation coefficient of 0.89. No significant statistical difference was found between the two groups with paired t-test ( p = 0.19). Conclusion: Accurate measurement of IOP by applanation tonometry can be achieved through a silicone hydrogel contact lens. # 2007 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. Keywords: Intraocular pressure; Tonometry; Contact lens; Therapeutic
1. Introduction Many studies have previously investigated the measurement of intraocular pressure (IOP) through conventional therapeutic soft (hydrogel) contact lenses, and have found that accurate IOP measurements can be recorded in normal eyes, and in eyes with abnormal anterior segments [1–3]. The use of therapeutic contact lenses in clinical practice is widespread. They are used primarily in the treatment of corneal disease to relieve pain, protect the cornea from mechanical trauma, to act as a splint to treat lacerations and small perforations, enhance corneal healing and improve corneal hydration [4]. They may also be used to relieve irregular corneal surfaces, thereby improving visual acuity * Corresponding author at: Ophthalmology Department, Essex County Hospital, Colchester, Essex CO3 3NB, United Kingdom. Tel.: +44 1206 744656; fax: +44 1206 744646. E-mail address: [email protected] (R.J. Allen).
[1]. Invariably, patients using therapeutic contact lenses require accurate IOP measurement. Options for IOP measurement for patients using therapeutic contact lenses include: (i) removal of the contact lens prior to measurement and then using standard techniques such as Goldmann tonometry, Tono-pen or pneumotonometry or (ii) retaining the contact lens in situ and using the above techniques, with or without modifications [1,2,5,6]. For patients wearing therapeutic contact lenses, removal of the lens to measure IOP may lead to adverse consequences, such as impairment of corneal epithelial healing [1]. It is, therefore, desirable to measure IOP in such patients with their contact lens in situ. Alternative techniques to measure the IOP with the contact lens in situ have been investigated. The Tono-pen (Oculab), a hand-held electronic applanation tonometer, is purported to be equivalent in accuracy to Goldmann tonometry, however it consistently overestimated IOP with a soft contact lens in situ [2]. Non-contact tonometry
1367-0484/$ – see front matter # 2007 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.clae.2007.02.008
268
R.J. Allen et al. / Contact Lens & Anterior Eye 30 (2007) 267–269
(pneumotonometry), with contact lens in situ, has also been investigated [1,7], and found to be accurate or comparable to Goldmann applanation tonometry. Many studies have looked at the use of standard Goldmann applanation tonometry, with or without modifications such as using a very small amount of sodium fluorescein, or none at all, to measure IOP in patients with therapeutic hydrogel contact lenses in situ [1,2,5,6,8]. Measurement of IOP via Goldmann tonometry, with a contact lens in situ faces a number of complications. These include permanent staining of the lens with fluorescein sodium, and the influence of the lens’ physical parameters, i.e. lens material properties and centre thickness on the accuracy of the measurement. Silicone hydrogel contact lenses are now being used as therapeutic contact lenses due to their high oxygen permeability [4,9]. The purpose of this study is to investigate if IOP can be accurately measured in subjects wearing a silicone hydrogel contact lens.
2. Materials and methods Ten healthy volunteers with no ocular pathology were recruited for this cohort study, with a total of 20 eyes being measured. The age range of the volunteers was between 21 and 48 years, with a mean of 31.4 (10.3) years, and equal number of male and female subjects. Measurement of IOP was carried out in the following ways:
Statistical analysis was performed using the paired t-test and a correlation coefficient was calculated.
3. Results The IOP of 20 eyes with no ocular pathology was measured consecutively in the absence and then the presence of a silicone hydrogel lens. The results of IOP measurement with and without contact lenses, and the difference obtained between them are illustrated in Fig. 1. Table 1 shows the results of the statistical analysis.
4. Discussion Our study compared the IOP measurements in eyes with no ocular pathology, in the absence and presence of a silicone hydrogel soft contact lens. The aim was to investigate if accurate IOP measurement can be obtained in subjects wearing this type of contact lens (Fig. 2). A 0.50 DS Bausch & Lomb PurevisionTM Silicone Hydrogel contact lens was utilised in each subject, giving identical lens material, water content and central lens thickness. Statistical analysis revealed a mean difference (S.D.) in IOP measurements with and without contact lens to be 0.5 0.889 mmHg. This is within the previously recorded, and accepted, margin of error of 3 mmHg found for inter-observer variability [11].
(i) Goldmann applanation tonometry with proxymetacaine hydrochloride (0.5%) and fluorescein sodium (0.25%) mixed minims without contact lens in situ. (ii) Goldmann applanation tonometry with a 0.5 DS Bausch & Lomb PurevisionTM Silicone Hydrogel contact lens in situ, with proxymetacaine hydrochloride (0.5%) and fluorescein sodium (0.25%) mixed minims. Contact lens specifications are as follows: Bausch & Lomb PurevisionTM, base curve 8.60 mm, diameter 14.00 mm, power 0.50 DS, water content 36%, centre thickness of 0.09 mm at 3.00 DS and a DK/t value of 110. The IOP of both eyes in each of the volunteers was first measured without the contact lens in situ. The same operator (RA), calibrated Goldmann tonometer (Haag streit AT900 M/Q) with supplied doubling prism (Haag Streit) and slit lamp (Haag Streit 900) were used throughout this study as standard. Prior to measurement the tonometer scale was reset to 10, the operator would then measure IOP, with the scale obscured from his vision by a small screen. An independent technician would then read off and record the IOP measurement to reduce the risk of bias. The IOPs were then retested 20–30 min following insertion of the contact lens. The same operator, Goldmann tonometer and slit lamp were used throughout this study as standard.
Fig. 1. Graph of IOP measurement with and without CL in situ. Table 1 Results Mean IOP (S.D.) without contact lens
16.05 1.90 mmHg
Mean IOP (S.D.) with contact lens in situ Mean difference (S.D.) between IOP measurements with and without contact lens
15.55 1.70 mmHg
Correlation coefficient Results of paired t-test
0.89 No statistically significant difference between the two groups ( p = 0.19)
0.50 0.889 mmHg
R.J. Allen et al. / Contact Lens & Anterior Eye 30 (2007) 267–269
269
matrix [14]. Further studies should be carried out in order to determine the extent and/or duration of staining of these lenses while measuring IOP using a method of staining with fluorescein sodium. Our results have shown that accurate measurement of intra-ocular pressure can be achieved through a silicone hydrogel contact lens.
5. Conclusion Fig. 2. Bland and Altman [10] plot showing difference in IOP measurement against mean IOP value.
As stated previously, silicone hydrogel contact lenses are now being used as therapeutic contact lenses [1,4]. It is, therefore, imperative that an accurate method of measuring intra-ocular pressure in these patients, who invariably have ocular pathology, is utilised. It has been shown that measurement of IOP through soft contact lenses can be affected by the water content and centre thickness of the lens (which is influenced by lens design, refractive index of the material and refractive power of the lens) [7,8,12]. The age of the lens is also significant, as lenses that have been worn for more than three months can affect the accuracy of the IOP measurement [1]. Further studies looking into the accuracy with contact lenses of varying powers and thickness will be required to evaluate the use of this technique in refractive contact lens wearers. Previous studies have shown that Goldmann applanation tonometry is the most accurate method of measuring IOP in subjects wearing contact lenses, especially in those with irregular corneal surfaces [1]. However, Goldmann applanation tonometry measurement is not without its own sources of error, such as hypofluorescence of the pre-corneal tear film, accommodation, the valsalva manoeuvre, vertical gaze, repeated tonometry leading to a reduction of IOP, and disease states such as corneal epithelial oedema [13]. Studies have shown that the use of Goldman applanation tonometry without fluorescein underestimates the intraocular pressure by +2 mmHg in subjects both with and without contact lenses in situ [2]. For this reason, in our study, fluorescein with topical anaesthesia was utilised as standard to measure IOP in the subjects. Other studies have investigated various techniques in order to reduce the chances of permanently staining the contact lenses during applanation tonometry. These include the use of a high molecular weight substance (Fluorexon) as a substitute for standard fluorescein dye [6], application of fluorescein to the tonometer tip directly [5], and Goldmann tonometry without the use of fluorescein (or topical anaesthesia) [2]. Anecdotal results from our study showed the contact lenses used displayed staining with fluorescein. This staining is due to the molecular structure of soft contact lenses allowing the fluorescein molecule to enter the lens
Our study agrees with the published literature on conventional hydrogel contact lenses that accurate Goldmann applanation tonometry through a silicone hydrogel contact lens is feasible. This technique will, therefore, allow simple and accurate intraocular pressure measurement without the need to remove the contact lens. This will be valuable to those who require contact lenses for therapeutic use, and will reduce clinical time.
References [1] Scibilia GD, Ehlers WH, Donshik PC. The effects of therapeutic contact lenses on intraocular pressure measurement. CLAO J 1996;22(October (4)):262–5. [2] Lim L, Ng TP, Tan DT. Accurate intraocular pressure measurement in contact lens wearers with normal pressures. CLAO J 1997;23(April (2)):130–3. [3] Panek WC, Boothe WA, Lee DA, Zemplenyi E, Petitt TH. Intraocular pressure measurement with the Tono-Pen through soft contact lenses. Am J Ophthalmol 1990;15(109(1)):62–5. [4] Lim L, Tan DT, Chan WK. Therapeutic use of Bausch & Lomb PureVision contact lenses. CLAO J 2001;27(October (4)):179–85. [5] Au Y-K, Reynolds M. Goldmann tonometry and fluorescein solution: a way to avoid contact lens staining. CLAO J 1991;17(October (4)):256–7. [6] Kreda SH. Applanation tonometry in contact lens practice. J Am Optom Assoc 1987;58(March (3)):208–10. [7] McMonnies CW. Noncontact tonometry through soft contact lenses. Am J Optom Physiol Opt 1986;63(December (12)):948–51. [8] Patel S, Illahi W. Non-contact tonometry over soft contact lenses: effect of contact lens power on the measurement of intra-ocular pressure. Cont Lens Anterior Eye 2004;27(March (1)):33–7. [9] Montero J, Sparholt J, Mely R. Retrospective case series of therapeutic applications of a lotrafilcon A silicone hydrogel soft contact lens. Eye Contact Lens 2003;29(January (Suppl. 1)):S54–6. discussion S57-9, S192-4. [10] Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;8 (1(8476)):307–10. [11] Phelps CD, Phelps GK. Measurement of intraocular pressure: a study of reproducibility. Graefes Arch Clin Exp Ophthalmol 1976;198(January (1)):39–43. [12] Insler MS, Robbins RG. Intraocular pressure by noncontact tonometry with and without soft contact lenses. Arch Ophthalmol 1987;105 (October (10)):1358–9. [13] Whitacre MM, Stein R. Sources of error with use of Goldmann-type tonometers. Surv Ophthalmol 1993;38(July (1)):1–30. [14] Lo´pez-Alemany A, Compan˜ V, Refojo MF. Porous structure of PurevisionTM versus Focus1 Night&DayTM and conventional hydrogel contact lenses. J Biomed Mater Res 2002;63(April):319–25.
Applanation tonometry in silicone hydrogel contact lens wearers R.J. Allen a,*, A. Dev Borman c, G.M. Saleh b,c a
Essex County Hospital, Colchester, United Kingdom Institute of Ophthalmology, London, United Kingdom c Royal Surrey County Hospital, Guildford, United Kingdom b
Abstract Introduction: Previous studies have investigated intraocular pressure (IOP) measurements through conventional soft (hydrogel) therapeutic contact lenses, and have found that an accurate IOP can be recorded in normal eyes, and in eyes with abnormal anterior segments. The IOP measurement through soft contact lenses may be affected by the water content and centre thickness of the lens. Silicone hydrogel contact lenses are now being used as therapeutic contact lenses due to their high oxygen permeability. The purpose of this study is to investigate if IOP can be accurately measured in a subject wearing a silicone hydrogel contact lens. Methods: In a cohort study, the IOP was measured with a Goldmann applanation tonometer without a contact lens and then repeated with a hydrogel contact lens in situ. Results: The IOP of 20 eyes of 10 volunteers with no ocular pathology was measured. The mean difference (S.D.) found between IOP measurement with (mean 15.55 1.70 mmHg) and without (mean 16.05 1.90 mmHg) contact lens was found to be 0.5 0.89 mmHg. Statistical analysis was performed which revealed a correlation coefficient of 0.89. No significant statistical difference was found between the two groups with paired t-test ( p = 0.19). Conclusion: Accurate measurement of IOP by applanation tonometry can be achieved through a silicone hydrogel contact lens. # 2007 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. Keywords: Intraocular pressure; Tonometry; Contact lens; Therapeutic
1. Introduction Many studies have previously investigated the measurement of intraocular pressure (IOP) through conventional therapeutic soft (hydrogel) contact lenses, and have found that accurate IOP measurements can be recorded in normal eyes, and in eyes with abnormal anterior segments [1–3]. The use of therapeutic contact lenses in clinical practice is widespread. They are used primarily in the treatment of corneal disease to relieve pain, protect the cornea from mechanical trauma, to act as a splint to treat lacerations and small perforations, enhance corneal healing and improve corneal hydration [4]. They may also be used to relieve irregular corneal surfaces, thereby improving visual acuity * Corresponding author at: Ophthalmology Department, Essex County Hospital, Colchester, Essex CO3 3NB, United Kingdom. Tel.: +44 1206 744656; fax: +44 1206 744646. E-mail address: [email protected] (R.J. Allen).
[1]. Invariably, patients using therapeutic contact lenses require accurate IOP measurement. Options for IOP measurement for patients using therapeutic contact lenses include: (i) removal of the contact lens prior to measurement and then using standard techniques such as Goldmann tonometry, Tono-pen or pneumotonometry or (ii) retaining the contact lens in situ and using the above techniques, with or without modifications [1,2,5,6]. For patients wearing therapeutic contact lenses, removal of the lens to measure IOP may lead to adverse consequences, such as impairment of corneal epithelial healing [1]. It is, therefore, desirable to measure IOP in such patients with their contact lens in situ. Alternative techniques to measure the IOP with the contact lens in situ have been investigated. The Tono-pen (Oculab), a hand-held electronic applanation tonometer, is purported to be equivalent in accuracy to Goldmann tonometry, however it consistently overestimated IOP with a soft contact lens in situ [2]. Non-contact tonometry
1367-0484/$ – see front matter # 2007 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.clae.2007.02.008
268
R.J. Allen et al. / Contact Lens & Anterior Eye 30 (2007) 267–269
(pneumotonometry), with contact lens in situ, has also been investigated [1,7], and found to be accurate or comparable to Goldmann applanation tonometry. Many studies have looked at the use of standard Goldmann applanation tonometry, with or without modifications such as using a very small amount of sodium fluorescein, or none at all, to measure IOP in patients with therapeutic hydrogel contact lenses in situ [1,2,5,6,8]. Measurement of IOP via Goldmann tonometry, with a contact lens in situ faces a number of complications. These include permanent staining of the lens with fluorescein sodium, and the influence of the lens’ physical parameters, i.e. lens material properties and centre thickness on the accuracy of the measurement. Silicone hydrogel contact lenses are now being used as therapeutic contact lenses due to their high oxygen permeability [4,9]. The purpose of this study is to investigate if IOP can be accurately measured in subjects wearing a silicone hydrogel contact lens.
2. Materials and methods Ten healthy volunteers with no ocular pathology were recruited for this cohort study, with a total of 20 eyes being measured. The age range of the volunteers was between 21 and 48 years, with a mean of 31.4 (10.3) years, and equal number of male and female subjects. Measurement of IOP was carried out in the following ways:
Statistical analysis was performed using the paired t-test and a correlation coefficient was calculated.
3. Results The IOP of 20 eyes with no ocular pathology was measured consecutively in the absence and then the presence of a silicone hydrogel lens. The results of IOP measurement with and without contact lenses, and the difference obtained between them are illustrated in Fig. 1. Table 1 shows the results of the statistical analysis.
4. Discussion Our study compared the IOP measurements in eyes with no ocular pathology, in the absence and presence of a silicone hydrogel soft contact lens. The aim was to investigate if accurate IOP measurement can be obtained in subjects wearing this type of contact lens (Fig. 2). A 0.50 DS Bausch & Lomb PurevisionTM Silicone Hydrogel contact lens was utilised in each subject, giving identical lens material, water content and central lens thickness. Statistical analysis revealed a mean difference (S.D.) in IOP measurements with and without contact lens to be 0.5 0.889 mmHg. This is within the previously recorded, and accepted, margin of error of 3 mmHg found for inter-observer variability [11].
(i) Goldmann applanation tonometry with proxymetacaine hydrochloride (0.5%) and fluorescein sodium (0.25%) mixed minims without contact lens in situ. (ii) Goldmann applanation tonometry with a 0.5 DS Bausch & Lomb PurevisionTM Silicone Hydrogel contact lens in situ, with proxymetacaine hydrochloride (0.5%) and fluorescein sodium (0.25%) mixed minims. Contact lens specifications are as follows: Bausch & Lomb PurevisionTM, base curve 8.60 mm, diameter 14.00 mm, power 0.50 DS, water content 36%, centre thickness of 0.09 mm at 3.00 DS and a DK/t value of 110. The IOP of both eyes in each of the volunteers was first measured without the contact lens in situ. The same operator (RA), calibrated Goldmann tonometer (Haag streit AT900 M/Q) with supplied doubling prism (Haag Streit) and slit lamp (Haag Streit 900) were used throughout this study as standard. Prior to measurement the tonometer scale was reset to 10, the operator would then measure IOP, with the scale obscured from his vision by a small screen. An independent technician would then read off and record the IOP measurement to reduce the risk of bias. The IOPs were then retested 20–30 min following insertion of the contact lens. The same operator, Goldmann tonometer and slit lamp were used throughout this study as standard.
Fig. 1. Graph of IOP measurement with and without CL in situ. Table 1 Results Mean IOP (S.D.) without contact lens
16.05 1.90 mmHg
Mean IOP (S.D.) with contact lens in situ Mean difference (S.D.) between IOP measurements with and without contact lens
15.55 1.70 mmHg
Correlation coefficient Results of paired t-test
0.89 No statistically significant difference between the two groups ( p = 0.19)
0.50 0.889 mmHg
R.J. Allen et al. / Contact Lens & Anterior Eye 30 (2007) 267–269
269
matrix [14]. Further studies should be carried out in order to determine the extent and/or duration of staining of these lenses while measuring IOP using a method of staining with fluorescein sodium. Our results have shown that accurate measurement of intra-ocular pressure can be achieved through a silicone hydrogel contact lens.
5. Conclusion Fig. 2. Bland and Altman [10] plot showing difference in IOP measurement against mean IOP value.
As stated previously, silicone hydrogel contact lenses are now being used as therapeutic contact lenses [1,4]. It is, therefore, imperative that an accurate method of measuring intra-ocular pressure in these patients, who invariably have ocular pathology, is utilised. It has been shown that measurement of IOP through soft contact lenses can be affected by the water content and centre thickness of the lens (which is influenced by lens design, refractive index of the material and refractive power of the lens) [7,8,12]. The age of the lens is also significant, as lenses that have been worn for more than three months can affect the accuracy of the IOP measurement [1]. Further studies looking into the accuracy with contact lenses of varying powers and thickness will be required to evaluate the use of this technique in refractive contact lens wearers. Previous studies have shown that Goldmann applanation tonometry is the most accurate method of measuring IOP in subjects wearing contact lenses, especially in those with irregular corneal surfaces [1]. However, Goldmann applanation tonometry measurement is not without its own sources of error, such as hypofluorescence of the pre-corneal tear film, accommodation, the valsalva manoeuvre, vertical gaze, repeated tonometry leading to a reduction of IOP, and disease states such as corneal epithelial oedema [13]. Studies have shown that the use of Goldman applanation tonometry without fluorescein underestimates the intraocular pressure by +2 mmHg in subjects both with and without contact lenses in situ [2]. For this reason, in our study, fluorescein with topical anaesthesia was utilised as standard to measure IOP in the subjects. Other studies have investigated various techniques in order to reduce the chances of permanently staining the contact lenses during applanation tonometry. These include the use of a high molecular weight substance (Fluorexon) as a substitute for standard fluorescein dye [6], application of fluorescein to the tonometer tip directly [5], and Goldmann tonometry without the use of fluorescein (or topical anaesthesia) [2]. Anecdotal results from our study showed the contact lenses used displayed staining with fluorescein. This staining is due to the molecular structure of soft contact lenses allowing the fluorescein molecule to enter the lens
Our study agrees with the published literature on conventional hydrogel contact lenses that accurate Goldmann applanation tonometry through a silicone hydrogel contact lens is feasible. This technique will, therefore, allow simple and accurate intraocular pressure measurement without the need to remove the contact lens. This will be valuable to those who require contact lenses for therapeutic use, and will reduce clinical time.
References [1] Scibilia GD, Ehlers WH, Donshik PC. The effects of therapeutic contact lenses on intraocular pressure measurement. CLAO J 1996;22(October (4)):262–5. [2] Lim L, Ng TP, Tan DT. Accurate intraocular pressure measurement in contact lens wearers with normal pressures. CLAO J 1997;23(April (2)):130–3. [3] Panek WC, Boothe WA, Lee DA, Zemplenyi E, Petitt TH. Intraocular pressure measurement with the Tono-Pen through soft contact lenses. Am J Ophthalmol 1990;15(109(1)):62–5. [4] Lim L, Tan DT, Chan WK. Therapeutic use of Bausch & Lomb PureVision contact lenses. CLAO J 2001;27(October (4)):179–85. [5] Au Y-K, Reynolds M. Goldmann tonometry and fluorescein solution: a way to avoid contact lens staining. CLAO J 1991;17(October (4)):256–7. [6] Kreda SH. Applanation tonometry in contact lens practice. J Am Optom Assoc 1987;58(March (3)):208–10. [7] McMonnies CW. Noncontact tonometry through soft contact lenses. Am J Optom Physiol Opt 1986;63(December (12)):948–51. [8] Patel S, Illahi W. Non-contact tonometry over soft contact lenses: effect of contact lens power on the measurement of intra-ocular pressure. Cont Lens Anterior Eye 2004;27(March (1)):33–7. [9] Montero J, Sparholt J, Mely R. Retrospective case series of therapeutic applications of a lotrafilcon A silicone hydrogel soft contact lens. Eye Contact Lens 2003;29(January (Suppl. 1)):S54–6. discussion S57-9, S192-4. [10] Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;8 (1(8476)):307–10. [11] Phelps CD, Phelps GK. Measurement of intraocular pressure: a study of reproducibility. Graefes Arch Clin Exp Ophthalmol 1976;198(January (1)):39–43. [12] Insler MS, Robbins RG. Intraocular pressure by noncontact tonometry with and without soft contact lenses. Arch Ophthalmol 1987;105 (October (10)):1358–9. [13] Whitacre MM, Stein R. Sources of error with use of Goldmann-type tonometers. Surv Ophthalmol 1993;38(July (1)):1–30. [14] Lo´pez-Alemany A, Compan˜ V, Refojo MF. Porous structure of PurevisionTM versus Focus1 Night&DayTM and conventional hydrogel contact lenses. J Biomed Mater Res 2002;63(April):319–25.