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Simplified recording of soft contact lens fit
Contact Lens & Anterior Eye 32 (2009) 37–42
Contents lists available at ScienceDirect
Contact Lens & Anterior Eye journal homepage: www.elsevier.com/locate/clae
Simplified recording of soft contact lens fit James S. Wolffsohn *, Olivia A. Hunt, Amritpreet K. Basra Ophthalmic Research Group, Life and Health Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, UK
A R T I C L E I N F O
A B S T R A C T
Keywords: Soft contact lens Fitting characteristics Lens evaluation Record keeping
Purpose: To determine the critical fitting characteristics of modern soft contact lens fits and from this to devise a simplified recording scheme. Methods: Ten subjects (aged 28.1 7.4 years) wore eight different modern soft contact lenses. Video was captured and analysed of blink (central and up-gaze), excursion lag (up, down, right and left gaze) and pushup movement, centration and coverage. Results: Lens centration was on average close to the corneal centre. Movement on blink was significantly smaller in up-gaze than in primary-gaze (p < 0.001). Lag was greatest in down-gaze and least in up-gaze (p < 0.001). Push-up test recovery speed was 1.32 0.73 mm/s. Overall lens movement was determined best by assessing horizontal lag, movement on blink in up-gaze and push-up recovery speed. Steeper lens base-curves did not have a significant effect on lens fit characteristics. Contact lens material did influence lens fit characteristics, particularly silicone-hydrogels which generally had lower centration and a faster push-up speed of recovery than HEMA lenses (p < 0.05). Conclusion: Lag on vertical gaze, and movement on blink in primary gaze generally provide little extra information on overall lens movement compared to horizontal lag, movement on blink in up-gaze and push-up recovery speed. They can therefore be excluded from a simplified recording scheme. A simplified and comprehensive soft contact lens fit recording system could consist of a cross-hairs indicating the centre of the cornea; a circle to indicate the lens centration; a mark on the relevant position of the circle to indicate any limbal incursion; a grade (B) below for movement with blink in upgaze, a grade (L) to the side for horizontal lag and a grade above (P) for the assessed push-up recovery speed. ß 2009 Published by Elsevier Ltd on behalf of British Contact Lens Association.
1. Introduction The assessment of contact lens fit is critical to contact lens practice. While a gas permeable lens fit can be evaluated by the pattern of fluorescein under the lens, a soft contact lens drapes onto the lens surface and is stained by fluorescein dye. Therefore assessment is limited to lens centration and coverage, movement, surface wettability and subjective comfort. However, to assess all these parameters takes time and the fit of soft lenses trialed in clinical practice are often recorded as ‘‘good’’ or ‘‘poor’’ which is highly subjective and of limited use in future patient aftercare. Therefore, the relative importance of contact lens fit characteristics should be assessed to develop a time efficient, but sufficiently detailed description of lens performance for recording in clinical practice. There is very little published evidence for the proposed damage caused by contact lenses repeatedly crossing the limbal area as assessed by lens centration and coverage. There is a relatively large
* Corresponding author. Tel.: +44 121 204 4140; fax: +44 121 204 4048. E-mail address: [email protected] (J.S. Wolffsohn).
change in surface curvature at the corneo–limbal junction which could result in mechanical damage from lens edge interactions. The end of the corneal avascular area also occurs in the limbal region. Changes in physiology with soft lens wear is known to cause chronic filling of pre-existing capilliaries [1] and this can predispose the cornea to neovascularisation from repeated mechanical insult. Finally, the cornea relies on limbal stem cells for tissue regeneration [2] and therefore the limbal area is of vital importance. Corneal conjunctivalisation has been shown to occur in some long-standing, full day contact lenses wearers wearing high powered lenses, presumed to be the result of stem cell damage [3]. Poor fitting soft lenses have been shown to have a more negative impact on ocular physiology than well fitting lenses. Greater fluorescein staining has been found with both loose and tight fitting lenses and higher levels of bulbar and limbal hyperaemia with loose fitting lenses [4]. It is generally believed that, in addition to providing sufficient oxygen levels at the tear– lens interface, it is necessary to have adequate tear interchange beneath the contact lens to remove trapped debris, inflammatory cells, and other tear components that would otherwise accumulate
1367-0484/$ – see front matter ß 2009 Published by Elsevier Ltd on behalf of British Contact Lens Association. doi:10.1016/j.clae.2008.12.004
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under the lens [5]. The tear layer between the contact lens and cornea also reduce the friction between the surfaces, avoiding significant mechanical interaction. Smaller diameter soft lenses provide substantially better tear mixing than larger lenses, but only by 0.6% between 12.0 and 13.5 mm diameters (1.8% vs 1.2%) [5]. However, this is still substantially less than rigid contact lenses (15–16%) [6]. Tear mixing with differing amounts of lens movement does not seem to have been examined. There is debate over when trial lens fit characteristics should be evaluated. Most studies have shown a decrease in lens movement over the initial 10–15 min post-insertion [7–10]. However, movement on blink increases again during the day, equating to the movement measured 5 min after insertion [7,8]. Models have been developed to account for the lens and anterior eye parameters that influence lens movement [11,12]. Proposed mechanisms of dehydration dependent steepening of the base-curve or osmotic dependent binding from hypotonic lacrimation, that could account for the initial decrease in lens movement after lens insertion, have been disproved [9,13,14]. Instead, post-lens tear film has been shown to be the major determinant of lens movement [15,16], with gradual post-lens tear film expulsion accounting for the initial decrease in lens mobility [17]. A negative pressure has been shown to build up beneath the lens, maximal at the apex, due to the hydrodynamic squeeze of the eyelid [11]. In a lens on cornea ex vivo model, this pressure was greater for steeper, thicker and lower water content contact lenses, but in a non-linear relationship. These pressures last longer than the blink and therefore affect the dynamics of the lens movement together with the lens thickness profile and exerted lid forces. However, clinically at least, HEMA material lens movement does not appear to be greatly influenced by water content or other material properties [18], although it is affected by the method of manufacture fit, with lathe cut HEMA lenses centering lower and spun cast lenses moving less than lathe or cast moulded HEMA lenses [10]. Young [19] has performed one of the few studies to assess all the soft lens fit characteristics (subjective evaluation), which he did retrospectively on data from several clinical studies in comparison to the examiners view of the lens suitability. He noted lenses often show conflicting fitting characteristics when assessed by direct observation and showed that push-up ease (recovery speed was not assessed) had the highest sensitivity/ specificity ratio, with movement on blink in primary gaze, a sensitive indicator of tight but not loose fitting lenses. Centration had poor sensitivity. Horizontal lag showed better sensitivity for loose fitting lenses, whereas vertical lag was better for tight lens fits, although the method of assessment was not described and he concluded the test had limited value. Some studies even advocate ignoring movement on blink over eyelid push-up findings, particularly with the Acuvue Etafilcon A material [20]. Several studies have tried to overcome clinical bias and lack of precision by assessing lens movement on blink from video, but not all define the direction of gaze (primary or up-gaze), and other lens
movements such as lag and push-up recovery speed have not been objectively evaluated [7,10,14,18]. Most have gauged lens centration, but few have assessed lens lag on excursions, especially in more than one position of gaze. Therefore, this study aimed to assess objectively the interrelationship between soft contact lens fit characteristics and to determine how these inform the clinician regarding overall lens movement. This allowed for a simplified method of recording soft lens fit of trialed lenses in clinical practice to be devised. 2. Method Ten habitual contact wearing subjects (average 28.10 7.37 years: 4 male, 6 female) gave informed consent to take part in the study. The study was approved by the Human Sciences Ethical Committee and conformed to the Declaration of Helsinki. All contact lenses used were commercially available and CE marked. Each subject was only included in the study if there was no evidence or history of binocular vision anomalies, or ocular disease including dry eye, or any pathology that would normally contraindicate contact lens wear. None of the subjects were on ocular medication. The subjects, with a range of different corneal curvatures (horizontal meridian 7.82 0.83 mm; vertical meridian 7.77 0.22 mm; difference 0.05 0.11 mm), each wore in their right eye eight different modern soft contact lenses chosen to allow comparison of lens material and base-curve (Table 1). This allowed for a range of contact lens fit parameters that are commonly seen in clinical practice to be observed, with all subjects being fitted with each of the lenses in random order. Each contact lens was marked with a surgical pen (DeRoyal, Tennessee, USA) so that movement on blink in primary-gaze and push-up recovery speed could be observed in patients where the lower lid covered the inferior contact lens. After insertion of the contact lens, 5 min to adapt to the contact lenses was given before assessment as this period of time has been shown to best reflect the contact lens fit measured after 8 hours of wear [7,8]. The patient was asked to look straight ahead (Fig. 1), then blink twice in primary gaze, look up and blink a further two times, look down while the upper lid was raised by the examiner to expose the superior lens edge and to look to the left and right (Fig. 2). The lens was then pushed upwards digitally while the patient viewed in primary gaze so that the lower lens edge was raised to the middle of the cornea if this was possible, before being released (Fig. 3). The same experienced optometrists conducted the routine on all the patients. The whole examination was video recorded through a slit-lamp biomicroscope providing 6 magnification by a JAI C-S2300 digital camera with a resolution of 767 569 pixels. The resulting video was objectively analysed by a second masked observer using a purpose-developed image analysis program (Labview, National Instruments, Austin, Texas). Lens centration was determined from the difference in the horizontal and vertical limbus to lens edge
Table 1 Design parameters of the contact lenses assessed in the study. Manufacturer
Brand
Material
Base-curve (mm)
Total diameter (mm)
Centre thickness (mm)
Water content (%)
Bausch and Lomb
SofLens Daily Disposable Purevision
Hilafilcon B Balafilcon A
8.6 8.3 8.6
14.2 14.2 14.0
0.09 0.09
59 36
3.00 3.00
Johnson and Johnson
Acuvue Advance
Galyfilcon A
14.0
0.07
47
3.00
1 day Acuvue Moist
Etafilcon A
8.3 8.7 8.5 9.0
14.2
0.084
58
3.00
Focus Dailies AquaComfort
Nelfilcon A
13.8
0.10
59
3.00
Ciba
8.6
Power (D)
J.S. Wolffsohn et al. / Contact Lens & Anterior Eye 32 (2009) 37–42
Fig. 1. Example image of lens centration, showing purple marks on lens used to measure movement on blink in primary gaze.
distance on either side of the contact lens. Lag was assessed as the difference between the limbus to lens edge distance in each of the horizontal and vertical positions of gaze compared to the same distances when viewing in primary gaze. Movement on blink was assessed by the change in vertical lens position relative to the cornea from the first video frame following the blink. Finally, pushup recovery speed was calculated from the change in vertical lens position relative to the cornea from the first video frame following the lens release, divided by the number of frames over which the movement occurred, multiplied by 25, to account for the PAL frame refresh speed. In addition, the practitioner subjectively rated the ease of contact lens displacement on push-up as difficult, moderate or easy. All measurements were taken by the same individual.
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Fig. 3. Example image of contact lens push-up displacement.
Repeated analysis of the same image showed a 95% confidence limit of 0.033 mm. Imaging a ruler through the same slit-lamp and camera system determined the calibration as 1 pixel was equivalent to 0.022 mm. The relationship between contact lens fit characteristics were compared by Pearson’s correlation (with Bonferroni correction of significant levels for multiple test). The ability of the contact lens fit characteristics to predict overall lens movement was determined with stepwise multiple regression (step-up method) modelling. The difference between contact lens fit characteristics with lens curvature were compared by t-test (with Bonferroni correction of significant levels for multiple test) and materials were compared with analysis of variance.
Fig. 2. Example images of excursion lag in up, down, left and right gaze.
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3. Results Lens fit characteristics across all lenses and subjects were averaged. Lens centration was 0.06 0.42 mm (superior) vertically and 0.07 0.14 horizontally (temporal) compared to the centre of the cornea. There was no significant difference between vertical and horizontal centration (p = 0.87). Movement on blink was significantly lower (p < 0.001) in up-gaze (0.15 0.20 mm) than in primary-gaze (0.22 0.14 mm). Lag was 0.61 0.38 mm in temporal-gaze, 0.41 0.25 mm in nasal-gaze, 0.21 0.27 mm in up-gaze and 0.70 0.36 mm in down-gaze excursions, each being significantly different from the others (F = 49.33, p < 0.001). The contact lens rarely moved towards the corneal centre on excursion gaze (1.2–3.8%) except for in up-gaze when the lens often drifted downwards (23.8%). Push-up test recovery speed was 1.32 0.73 mm/s, ranging from no movement to 6.0 mm/s. Vertical contact lens centration was correlated with superior and inferior directions of gaze excursion lags, whereas horizontal centration was only correlated with temporal gaze lag (Table 2). Centration was not related to movement on blink or push-up recovery speed as expected. Movement on blink in up-gaze was correlated to movement on blink in primary gaze (p < 0.005), however the latter was also correlated to push-up recovery time and horizontal centration (Table 2). Lag on nasal excursion was related to temporal excursion and inferior excursion as well as push-up recovery speed. Lag on inferior gaze was correlated with nasal and temporal gaze lag as well as vertical centration. Overall lens movement was objectively modelled as being the average percentage difference from the maximum movement for each of the blink, lag and push-up recovery lens fit characteristics. The direction of movement was ignored as any movement is an indicator to lens mobility and is likely to result in tear exchange. The only lens-cornea combination showing excessive movement (6 mm/s recovery push-up test recovery speed) was removed from the analysis, so more movement was always a positive characteristic. Stepwise multiple regression (step-up method) identified that lag on temporal gaze was the strongest factor, accounting for 62.9% of the variance, and a combination of lag on horizontal excursions (nasal and temporal), movement on blink in up-gaze and push-up test recovery accounting for 90.9% of the variance. The remaining factors accounted for only between 2.3 and 3.5% of the variance each (Fig. 4). If the range was divided by three grades for lag on horizontal excursions (nasal <0.35 mm, 0.35–0.70 mm and
Fig. 4. Comparison of overall contact lens movement parameter with movement on blink, excursion lag and push-up recovery speed fit characteristics. Each point represents one of the 10 subjects wearing one of the 8 lenses. Each of the contact lens fit parameters were converted to a percentage of that parameter compared to the range of that parameter experienced in the study to allow the overall contact lens movement parameter to be calculated and for comparison between parameters, n = 79.
>0.70 mm, and temporal <0.66 mm, 0.66–1.32 mm and >1.32 mm), movement on blink in up-gaze (<0.24 mm, 0.24– 0.48 mm and >0.48 mm) and push-up test recovery (<2 mm/s, 2– 4 mm/s and >4 mm/s), 81.7% of the variance in the overall lens movement was still accounted for. Objective push-up recovery speed was not significantly correlated with subjectively assessed ease of contact lens displacement on push-up (r = 0.02, p = 0.87), and if the former was replaced by the latter, the variance accounted for in the overall lens movement, in conjunction with grading of horizontal lag and movement on blink in up-gaze, was reduced to 72.3%. Steeper lens base-curves (Purevision 8.3; Acuvue Advance 8.3; 1 Day Acuvue 8.5) did not have a significant effect compared to flatter base-curves (Purevision 8.6; Acuvue Advance 8.7; 1 Day
Table 2 A comparison of objective lens fit characteristics, n = 10. Centration
Movement on blink
Excursion lag
Horizontal
Up-gaze
Primary-gaze
Nasal-gaze
Temporal-gaze
Superior-gaze
r = 0.097 p = 0.394 r = 0.087 p = 0.444
r = 0.179 p = 0.111 r = 0.497* p < 0.000
r = 0.325* p = 0.003
r = 0.246 p = 0.028 r = 0.063 p = 0.576 r = 0.349* p = 0.002 r = 0.422* p < 0.000
r = 0.177 p = 0.117 r = 0.496* p < 0.000 r = 0.051 p = 0.654 r = 0.217 p = 0.055
r = 0.138 p = 0.224 r = 0.028 p = 0.806 r = 0.244 p = 0.029 r = 0.050 p = 0.662
r = 0.309* p = 0.005 r = 0.274 p = 0.014 r = 0.062 p = 0.585 r = 0.251 p = 0.025
r = 0.400* p < 0.000 r = 0.070 p = 0.539 r = 0.722* p < 0.000
r = 0.014 p = 0.901 r = 0.492* p < 0.000
r = 0.256 p = 0.023
r = 0.084 p = 0.458
r = 0.099 p = 0.382
r = 0.106 p = 0.349
r = 0.290* p = 0.009
r = 0.361* p = 0.001
r = 0.173 p = 0.124
r = 0.169 p = 0.134
Vertical Centration
Movement on blink
Vertical Horizontal
Up-gaze Primary-gaze
Excursion lag
Nasal-gaze Temporal-gaze Superior-gaze Inferior-gaze
Push-up recovery speed *
Inferior-gaze
r = 0.046 p = 0.687
Bonferroni corrected level of significance of p < 0.006.
r = 0.247 p = 0.028
Push-up recovery speed
J.S. Wolffsohn et al. / Contact Lens & Anterior Eye 32 (2009) 37–42
Fig. 5. Contact lens fitting characteristics with lens material, n = 80. Error bars = 1S.D. *Significance at p < 0.05;
Acuvue 9.0) on lens centration. movement or excusion lags (p > 0.05). However, steeper base-curves showed a trend towards small lens lag on down-gaze excursion (0.65 0.33 m vs 0.77 0.39 mm, p = 0.02), but conversely a greater movement on blink in up-gaze (0.17 0.21 mm vs 0.09 0.21 mm, p = 0.007) compared to flatter lenses (Bonferroni correction for multiple t-tests requires a p < 0.0056 level for statistical significance). Comparing lens material affects on contact lens fit, vertical centration was generally higher and less central in the HEMA than the silicone hydrogels, but not with the Focus Dailies lenses (F = 2.67, p < 0.05; Fig. 5). Lag was significant in up (F = 5.94, p = 0.001) and downward (F = 2.94, p < 0.05) excursions with the Focus Dailies more prone to be located higher than the other lens materials. Push-up speed of recovery was significantly greater with the silicone hydrogel than the HEMA material contact lenses (F = 3.34, p < 0.05).
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**
significance at p = 0.001.
was well described by a combination of lag on horizontal excursions (nasal and temporal), movement on blink in up-gaze and push-up test recovery. This was the case even if objective measures were replaced by three point grading. This finding is similar to that of Young [19] on retrospective analysis of subjective records, although push-up had been assessed by ease of displacement rather than recovery speed and movement on blink was only assessed in primary gaze. A comparison of observer rated ease of displacement (three point grade) with objective speed of recovery in this study did not show a significant correlation between these two measures, and the former weakened the predictive ability to assess overall contact lens movement, so support is given to the assessment of push-up speed of recovery rather than ease of pushup displacement. Therefore, to complete the description of lens fit, a grade (+ quick or large; 0 medium; or slow or minimal) should be recorded for B (movement on blink in up-gaze; B+ = >0.5 mm, B0 = 0.25–0.50 mm, B = <0.25 mm), L (horizontal
4. Discussion This study aimed to determine the most important parameters of soft contact lens fit characteristics to promote the appropriate recording of the assessment of lenses, but taking the shortest possible time to maximise compliance. To achieve this, the study is the first to extensively assess contact lens movement objectively. Despite the lack of published evidence for the proposed damage caused by contact lenses repeatedly crossing the limbal area, the change in surface curvature at the corneo–limbal junction, and the physiological importance of the underlying limbal blood vessel arcades and stem cells highlight the importance of recording contact lens centration and coverage. A decentred lens may also compromise the optimal visual performance. A simple way to denote this is by drawing a horizontal and vertical crossing line to indicating the corneal centre, with a circle overlaid to show the relative contact lens position (Fig. 6). Any incursion between the lens edge and limbus can be marked at the appropriate position on the lens circumference (Fig. 6). Minimally mobile contact lenses have been shown to have a negative impact on ocular physiology, causing fluorescein staining, bulbar and limbal hyperaemia [4] and potentially less tear exchange [5]. In this study, a model of overall lens movement
Fig. 6. The proposed simplified method of recording soft lens fit of trialed lenses in clinical practice. The lens schematic depicts a soft contact lens in the right eye of the subject, which is displace slightly up and temporal, with limbal incursions occurring nasally, large movement on blink in up-gaze, medium excursion on horizontal lag and a quick push-up recovery time.
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lag; L+ = >1.0 mm, L0 = 0.5–1.0 mm, L = <0.5 mm) and P (push-up test; P+ = >4 mm/s, P0 = 2–4 mm/s, P = <2 mm/s; Fig. 6). In addition to optimising the recording of soft lens fit, the study confirmed that the relationship between corneal curvature and lens back optic zone radius does not affect lens movement, perhaps counter to expectation and some studies on early hydrogels [21]. However, this has previously been demonstrated by with thinner, higher water contact lens designs [22,23], with the hydrodynamic squeeze effect predicted to be less significant between lenses [11]. The Etafilcon A Acuvue material movement did not correlate to movement on blink in primary-gaze (r = 0.13, p = 0.59) and was inversely linked to blink in up-gaze (r = 0.29, p = 0.21) as has been previously suggested [20]. Silicone-hydrogels were generally more mobile than HEMA contact lenses as would generally be expected from their higher stiffness. Interestingly, the Focus Dailies lenses performed slightly differently to other materials, perhaps due to its geometry, material physical characteristics or because of its polyvinyl alcohol release properties [24]. In conclusion, together with centration and limbal incursions, horizontal lag, movement on blink in up-gaze and push-up recovery speed should be recorded to adequately describe soft contact lens fit. The described simple diagrammatic representation of lens fit can be used to minimise the time to record this information and to standardise the technique between practitioners, enhancing practitioner compliance and the service provided to patients. Conflicts of interest None. Acknowledgment The authors thank Richard Armstrong for his statistical support with the data analysis. References [1] McMonnies CW, Chapman-Davies A, Holden BA. The vascular response to contact lens wear. Am J Optom Physiol Opt 1982;59:795–9.
[2] Barrandon Y. Crossing boundaries: stem cells, holoclones, and the fundamentals of squamous epithelial renewal. Cornea 2007;26:S10–2. [3] Martin R. Corneal conjunctivalisation in long-standing contact lens wearers. Clin Exp Optom 2007;90:26–30. [4] Young G, Coleman S. Poorly fitting soft lenses affect ocular integrity. CLAO J 2001;27:68–74. [5] McNamara NA, Polse KA, Brand RJ, Graham AD, Chan JS, McKenney CD. Tear mixing under a soft lens: effects on lens diameter. Am J Ophthalmol 1999;127:659–65. [6] Kok JHC, Boets EPM, van Best JA, Kijlstra A. Fluorophotometric assessment of tear turnover under rigid contact lenses. Cornea 1992;11:515–7. [7] Schwallie JD, Bauman RE. Fitting characteristics of Dailies daily disposable hydrogel contact lenses. CLAO J 1998;24:102–6. [8] Brennan NA, Lindsay RG, McCraw K, Young L, Bruce AS, Golding TR. Soft lens movement: temporal characteristics. Optom Vis Sci 1994;71:359–63. [9] Golding TR, Harris MG, Smith RC, Brennan NA. Soft lens movement: effects of humidity and hypertonic saline on lens settling. Acta Ophthalmol Scand 1995;32:139–44. [10] Moldonado-Codina C, Efron N. Impact of manufacturing technology and material composition on the clinical performance of hydrogel lenses. Optom Vis Sci 2004;81:442–54. [11] Martin DK, Holden BA. Forces developed beneath hydrogel contact lenses due to squeeze pressure. Phys Med Biol 1986;30:635–49. [12] Bibby MM, Tomlinson A. A model to explain the effect of soft lens design specifications on movement. Am J Optom Physiol Opt 1983;60:287–91. [13] Martin DK, Holden BA. Variations in tear fluid osmolality, chord diameter and movement during wear of high water content hydrogel contact lenses. Int Cont Lens Clin 1983;10:332–42. [14] Pritchard N, Fonn D. Dehydration, lens movement and dryness ratings of hydrogel contact lenses. Ophthalmic Physiol Opt 1995;15:281–6. [15] Little SA, Bruce AS. Postlens tear film morphology, lens movement and symptoms in hydrogel lens wearers. Ophthalmic Physiol Opt 1994;14:65–9. [16] Little SA, Bruce AS. Osmotic determinants of postlens tear film morphology and hydrogel lens movement. Ophthalmic Physiol Opt 1995;15:117–24. [17] Golding TR, Bruce AS, Gaterell LL, Little SA, Macnamara J. Soft lens movement: effect of blink rate on lens settling. Acta Ophthalmol Scand 1995;73:506–11. [18] Tranoudis I, Efron N. In-eye performance of soft contact lenses made from different materials. Cont Lens Anterior Eye 2004;27:133–48. [19] Young G. Evaluation of soft contact lens fitting characteristics. Optom Vis Sci 1996;73:247–54. [20] Walker J, Hay M, Blalock J, Mack C, Knezich G, Henderson TL. Evaluating steeper base curve lens movement. Contact Lens Spectrum 2003;18:44–6. [21] Cedarstaff TH, Tomlinson A, Bibby M. Validation of a model describing soft lens movement as a function of soft lens specification. Am J Optom Physiol Opt 1983;60:292–6. [22] Young G, Holden B, Cooke G. Influence of soft contact lens design on clinical performance. Optom Vis Sci 1993;70:394–403. [23] Bruce AS. Influence of corneal topography on centration and movement of low water content soft contact lenses. Int Cont Lens Clin 1994;21:45–9. [24] Peterson RC, Wolffsohn JS, Nick J, Winterton L, Lally J. Clinical performance of daily disposable soft contact lenses using sustained release technology. Cont Lens Anterior Eye 2006;29:127–34.
Contents lists available at ScienceDirect
Contact Lens & Anterior Eye journal homepage: www.elsevier.com/locate/clae
Simplified recording of soft contact lens fit James S. Wolffsohn *, Olivia A. Hunt, Amritpreet K. Basra Ophthalmic Research Group, Life and Health Sciences, Aston University, Aston Triangle, Birmingham B4 7ET, UK
A R T I C L E I N F O
A B S T R A C T
Keywords: Soft contact lens Fitting characteristics Lens evaluation Record keeping
Purpose: To determine the critical fitting characteristics of modern soft contact lens fits and from this to devise a simplified recording scheme. Methods: Ten subjects (aged 28.1 7.4 years) wore eight different modern soft contact lenses. Video was captured and analysed of blink (central and up-gaze), excursion lag (up, down, right and left gaze) and pushup movement, centration and coverage. Results: Lens centration was on average close to the corneal centre. Movement on blink was significantly smaller in up-gaze than in primary-gaze (p < 0.001). Lag was greatest in down-gaze and least in up-gaze (p < 0.001). Push-up test recovery speed was 1.32 0.73 mm/s. Overall lens movement was determined best by assessing horizontal lag, movement on blink in up-gaze and push-up recovery speed. Steeper lens base-curves did not have a significant effect on lens fit characteristics. Contact lens material did influence lens fit characteristics, particularly silicone-hydrogels which generally had lower centration and a faster push-up speed of recovery than HEMA lenses (p < 0.05). Conclusion: Lag on vertical gaze, and movement on blink in primary gaze generally provide little extra information on overall lens movement compared to horizontal lag, movement on blink in up-gaze and push-up recovery speed. They can therefore be excluded from a simplified recording scheme. A simplified and comprehensive soft contact lens fit recording system could consist of a cross-hairs indicating the centre of the cornea; a circle to indicate the lens centration; a mark on the relevant position of the circle to indicate any limbal incursion; a grade (B) below for movement with blink in upgaze, a grade (L) to the side for horizontal lag and a grade above (P) for the assessed push-up recovery speed. ß 2009 Published by Elsevier Ltd on behalf of British Contact Lens Association.
1. Introduction The assessment of contact lens fit is critical to contact lens practice. While a gas permeable lens fit can be evaluated by the pattern of fluorescein under the lens, a soft contact lens drapes onto the lens surface and is stained by fluorescein dye. Therefore assessment is limited to lens centration and coverage, movement, surface wettability and subjective comfort. However, to assess all these parameters takes time and the fit of soft lenses trialed in clinical practice are often recorded as ‘‘good’’ or ‘‘poor’’ which is highly subjective and of limited use in future patient aftercare. Therefore, the relative importance of contact lens fit characteristics should be assessed to develop a time efficient, but sufficiently detailed description of lens performance for recording in clinical practice. There is very little published evidence for the proposed damage caused by contact lenses repeatedly crossing the limbal area as assessed by lens centration and coverage. There is a relatively large
* Corresponding author. Tel.: +44 121 204 4140; fax: +44 121 204 4048. E-mail address: [email protected] (J.S. Wolffsohn).
change in surface curvature at the corneo–limbal junction which could result in mechanical damage from lens edge interactions. The end of the corneal avascular area also occurs in the limbal region. Changes in physiology with soft lens wear is known to cause chronic filling of pre-existing capilliaries [1] and this can predispose the cornea to neovascularisation from repeated mechanical insult. Finally, the cornea relies on limbal stem cells for tissue regeneration [2] and therefore the limbal area is of vital importance. Corneal conjunctivalisation has been shown to occur in some long-standing, full day contact lenses wearers wearing high powered lenses, presumed to be the result of stem cell damage [3]. Poor fitting soft lenses have been shown to have a more negative impact on ocular physiology than well fitting lenses. Greater fluorescein staining has been found with both loose and tight fitting lenses and higher levels of bulbar and limbal hyperaemia with loose fitting lenses [4]. It is generally believed that, in addition to providing sufficient oxygen levels at the tear– lens interface, it is necessary to have adequate tear interchange beneath the contact lens to remove trapped debris, inflammatory cells, and other tear components that would otherwise accumulate
1367-0484/$ – see front matter ß 2009 Published by Elsevier Ltd on behalf of British Contact Lens Association. doi:10.1016/j.clae.2008.12.004
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under the lens [5]. The tear layer between the contact lens and cornea also reduce the friction between the surfaces, avoiding significant mechanical interaction. Smaller diameter soft lenses provide substantially better tear mixing than larger lenses, but only by 0.6% between 12.0 and 13.5 mm diameters (1.8% vs 1.2%) [5]. However, this is still substantially less than rigid contact lenses (15–16%) [6]. Tear mixing with differing amounts of lens movement does not seem to have been examined. There is debate over when trial lens fit characteristics should be evaluated. Most studies have shown a decrease in lens movement over the initial 10–15 min post-insertion [7–10]. However, movement on blink increases again during the day, equating to the movement measured 5 min after insertion [7,8]. Models have been developed to account for the lens and anterior eye parameters that influence lens movement [11,12]. Proposed mechanisms of dehydration dependent steepening of the base-curve or osmotic dependent binding from hypotonic lacrimation, that could account for the initial decrease in lens movement after lens insertion, have been disproved [9,13,14]. Instead, post-lens tear film has been shown to be the major determinant of lens movement [15,16], with gradual post-lens tear film expulsion accounting for the initial decrease in lens mobility [17]. A negative pressure has been shown to build up beneath the lens, maximal at the apex, due to the hydrodynamic squeeze of the eyelid [11]. In a lens on cornea ex vivo model, this pressure was greater for steeper, thicker and lower water content contact lenses, but in a non-linear relationship. These pressures last longer than the blink and therefore affect the dynamics of the lens movement together with the lens thickness profile and exerted lid forces. However, clinically at least, HEMA material lens movement does not appear to be greatly influenced by water content or other material properties [18], although it is affected by the method of manufacture fit, with lathe cut HEMA lenses centering lower and spun cast lenses moving less than lathe or cast moulded HEMA lenses [10]. Young [19] has performed one of the few studies to assess all the soft lens fit characteristics (subjective evaluation), which he did retrospectively on data from several clinical studies in comparison to the examiners view of the lens suitability. He noted lenses often show conflicting fitting characteristics when assessed by direct observation and showed that push-up ease (recovery speed was not assessed) had the highest sensitivity/ specificity ratio, with movement on blink in primary gaze, a sensitive indicator of tight but not loose fitting lenses. Centration had poor sensitivity. Horizontal lag showed better sensitivity for loose fitting lenses, whereas vertical lag was better for tight lens fits, although the method of assessment was not described and he concluded the test had limited value. Some studies even advocate ignoring movement on blink over eyelid push-up findings, particularly with the Acuvue Etafilcon A material [20]. Several studies have tried to overcome clinical bias and lack of precision by assessing lens movement on blink from video, but not all define the direction of gaze (primary or up-gaze), and other lens
movements such as lag and push-up recovery speed have not been objectively evaluated [7,10,14,18]. Most have gauged lens centration, but few have assessed lens lag on excursions, especially in more than one position of gaze. Therefore, this study aimed to assess objectively the interrelationship between soft contact lens fit characteristics and to determine how these inform the clinician regarding overall lens movement. This allowed for a simplified method of recording soft lens fit of trialed lenses in clinical practice to be devised. 2. Method Ten habitual contact wearing subjects (average 28.10 7.37 years: 4 male, 6 female) gave informed consent to take part in the study. The study was approved by the Human Sciences Ethical Committee and conformed to the Declaration of Helsinki. All contact lenses used were commercially available and CE marked. Each subject was only included in the study if there was no evidence or history of binocular vision anomalies, or ocular disease including dry eye, or any pathology that would normally contraindicate contact lens wear. None of the subjects were on ocular medication. The subjects, with a range of different corneal curvatures (horizontal meridian 7.82 0.83 mm; vertical meridian 7.77 0.22 mm; difference 0.05 0.11 mm), each wore in their right eye eight different modern soft contact lenses chosen to allow comparison of lens material and base-curve (Table 1). This allowed for a range of contact lens fit parameters that are commonly seen in clinical practice to be observed, with all subjects being fitted with each of the lenses in random order. Each contact lens was marked with a surgical pen (DeRoyal, Tennessee, USA) so that movement on blink in primary-gaze and push-up recovery speed could be observed in patients where the lower lid covered the inferior contact lens. After insertion of the contact lens, 5 min to adapt to the contact lenses was given before assessment as this period of time has been shown to best reflect the contact lens fit measured after 8 hours of wear [7,8]. The patient was asked to look straight ahead (Fig. 1), then blink twice in primary gaze, look up and blink a further two times, look down while the upper lid was raised by the examiner to expose the superior lens edge and to look to the left and right (Fig. 2). The lens was then pushed upwards digitally while the patient viewed in primary gaze so that the lower lens edge was raised to the middle of the cornea if this was possible, before being released (Fig. 3). The same experienced optometrists conducted the routine on all the patients. The whole examination was video recorded through a slit-lamp biomicroscope providing 6 magnification by a JAI C-S2300 digital camera with a resolution of 767 569 pixels. The resulting video was objectively analysed by a second masked observer using a purpose-developed image analysis program (Labview, National Instruments, Austin, Texas). Lens centration was determined from the difference in the horizontal and vertical limbus to lens edge
Table 1 Design parameters of the contact lenses assessed in the study. Manufacturer
Brand
Material
Base-curve (mm)
Total diameter (mm)
Centre thickness (mm)
Water content (%)
Bausch and Lomb
SofLens Daily Disposable Purevision
Hilafilcon B Balafilcon A
8.6 8.3 8.6
14.2 14.2 14.0
0.09 0.09
59 36
3.00 3.00
Johnson and Johnson
Acuvue Advance
Galyfilcon A
14.0
0.07
47
3.00
1 day Acuvue Moist
Etafilcon A
8.3 8.7 8.5 9.0
14.2
0.084
58
3.00
Focus Dailies AquaComfort
Nelfilcon A
13.8
0.10
59
3.00
Ciba
8.6
Power (D)
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Fig. 1. Example image of lens centration, showing purple marks on lens used to measure movement on blink in primary gaze.
distance on either side of the contact lens. Lag was assessed as the difference between the limbus to lens edge distance in each of the horizontal and vertical positions of gaze compared to the same distances when viewing in primary gaze. Movement on blink was assessed by the change in vertical lens position relative to the cornea from the first video frame following the blink. Finally, pushup recovery speed was calculated from the change in vertical lens position relative to the cornea from the first video frame following the lens release, divided by the number of frames over which the movement occurred, multiplied by 25, to account for the PAL frame refresh speed. In addition, the practitioner subjectively rated the ease of contact lens displacement on push-up as difficult, moderate or easy. All measurements were taken by the same individual.
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Fig. 3. Example image of contact lens push-up displacement.
Repeated analysis of the same image showed a 95% confidence limit of 0.033 mm. Imaging a ruler through the same slit-lamp and camera system determined the calibration as 1 pixel was equivalent to 0.022 mm. The relationship between contact lens fit characteristics were compared by Pearson’s correlation (with Bonferroni correction of significant levels for multiple test). The ability of the contact lens fit characteristics to predict overall lens movement was determined with stepwise multiple regression (step-up method) modelling. The difference between contact lens fit characteristics with lens curvature were compared by t-test (with Bonferroni correction of significant levels for multiple test) and materials were compared with analysis of variance.
Fig. 2. Example images of excursion lag in up, down, left and right gaze.
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3. Results Lens fit characteristics across all lenses and subjects were averaged. Lens centration was 0.06 0.42 mm (superior) vertically and 0.07 0.14 horizontally (temporal) compared to the centre of the cornea. There was no significant difference between vertical and horizontal centration (p = 0.87). Movement on blink was significantly lower (p < 0.001) in up-gaze (0.15 0.20 mm) than in primary-gaze (0.22 0.14 mm). Lag was 0.61 0.38 mm in temporal-gaze, 0.41 0.25 mm in nasal-gaze, 0.21 0.27 mm in up-gaze and 0.70 0.36 mm in down-gaze excursions, each being significantly different from the others (F = 49.33, p < 0.001). The contact lens rarely moved towards the corneal centre on excursion gaze (1.2–3.8%) except for in up-gaze when the lens often drifted downwards (23.8%). Push-up test recovery speed was 1.32 0.73 mm/s, ranging from no movement to 6.0 mm/s. Vertical contact lens centration was correlated with superior and inferior directions of gaze excursion lags, whereas horizontal centration was only correlated with temporal gaze lag (Table 2). Centration was not related to movement on blink or push-up recovery speed as expected. Movement on blink in up-gaze was correlated to movement on blink in primary gaze (p < 0.005), however the latter was also correlated to push-up recovery time and horizontal centration (Table 2). Lag on nasal excursion was related to temporal excursion and inferior excursion as well as push-up recovery speed. Lag on inferior gaze was correlated with nasal and temporal gaze lag as well as vertical centration. Overall lens movement was objectively modelled as being the average percentage difference from the maximum movement for each of the blink, lag and push-up recovery lens fit characteristics. The direction of movement was ignored as any movement is an indicator to lens mobility and is likely to result in tear exchange. The only lens-cornea combination showing excessive movement (6 mm/s recovery push-up test recovery speed) was removed from the analysis, so more movement was always a positive characteristic. Stepwise multiple regression (step-up method) identified that lag on temporal gaze was the strongest factor, accounting for 62.9% of the variance, and a combination of lag on horizontal excursions (nasal and temporal), movement on blink in up-gaze and push-up test recovery accounting for 90.9% of the variance. The remaining factors accounted for only between 2.3 and 3.5% of the variance each (Fig. 4). If the range was divided by three grades for lag on horizontal excursions (nasal <0.35 mm, 0.35–0.70 mm and
Fig. 4. Comparison of overall contact lens movement parameter with movement on blink, excursion lag and push-up recovery speed fit characteristics. Each point represents one of the 10 subjects wearing one of the 8 lenses. Each of the contact lens fit parameters were converted to a percentage of that parameter compared to the range of that parameter experienced in the study to allow the overall contact lens movement parameter to be calculated and for comparison between parameters, n = 79.
>0.70 mm, and temporal <0.66 mm, 0.66–1.32 mm and >1.32 mm), movement on blink in up-gaze (<0.24 mm, 0.24– 0.48 mm and >0.48 mm) and push-up test recovery (<2 mm/s, 2– 4 mm/s and >4 mm/s), 81.7% of the variance in the overall lens movement was still accounted for. Objective push-up recovery speed was not significantly correlated with subjectively assessed ease of contact lens displacement on push-up (r = 0.02, p = 0.87), and if the former was replaced by the latter, the variance accounted for in the overall lens movement, in conjunction with grading of horizontal lag and movement on blink in up-gaze, was reduced to 72.3%. Steeper lens base-curves (Purevision 8.3; Acuvue Advance 8.3; 1 Day Acuvue 8.5) did not have a significant effect compared to flatter base-curves (Purevision 8.6; Acuvue Advance 8.7; 1 Day
Table 2 A comparison of objective lens fit characteristics, n = 10. Centration
Movement on blink
Excursion lag
Horizontal
Up-gaze
Primary-gaze
Nasal-gaze
Temporal-gaze
Superior-gaze
r = 0.097 p = 0.394 r = 0.087 p = 0.444
r = 0.179 p = 0.111 r = 0.497* p < 0.000
r = 0.325* p = 0.003
r = 0.246 p = 0.028 r = 0.063 p = 0.576 r = 0.349* p = 0.002 r = 0.422* p < 0.000
r = 0.177 p = 0.117 r = 0.496* p < 0.000 r = 0.051 p = 0.654 r = 0.217 p = 0.055
r = 0.138 p = 0.224 r = 0.028 p = 0.806 r = 0.244 p = 0.029 r = 0.050 p = 0.662
r = 0.309* p = 0.005 r = 0.274 p = 0.014 r = 0.062 p = 0.585 r = 0.251 p = 0.025
r = 0.400* p < 0.000 r = 0.070 p = 0.539 r = 0.722* p < 0.000
r = 0.014 p = 0.901 r = 0.492* p < 0.000
r = 0.256 p = 0.023
r = 0.084 p = 0.458
r = 0.099 p = 0.382
r = 0.106 p = 0.349
r = 0.290* p = 0.009
r = 0.361* p = 0.001
r = 0.173 p = 0.124
r = 0.169 p = 0.134
Vertical Centration
Movement on blink
Vertical Horizontal
Up-gaze Primary-gaze
Excursion lag
Nasal-gaze Temporal-gaze Superior-gaze Inferior-gaze
Push-up recovery speed *
Inferior-gaze
r = 0.046 p = 0.687
Bonferroni corrected level of significance of p < 0.006.
r = 0.247 p = 0.028
Push-up recovery speed
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Fig. 5. Contact lens fitting characteristics with lens material, n = 80. Error bars = 1S.D. *Significance at p < 0.05;
Acuvue 9.0) on lens centration. movement or excusion lags (p > 0.05). However, steeper base-curves showed a trend towards small lens lag on down-gaze excursion (0.65 0.33 m vs 0.77 0.39 mm, p = 0.02), but conversely a greater movement on blink in up-gaze (0.17 0.21 mm vs 0.09 0.21 mm, p = 0.007) compared to flatter lenses (Bonferroni correction for multiple t-tests requires a p < 0.0056 level for statistical significance). Comparing lens material affects on contact lens fit, vertical centration was generally higher and less central in the HEMA than the silicone hydrogels, but not with the Focus Dailies lenses (F = 2.67, p < 0.05; Fig. 5). Lag was significant in up (F = 5.94, p = 0.001) and downward (F = 2.94, p < 0.05) excursions with the Focus Dailies more prone to be located higher than the other lens materials. Push-up speed of recovery was significantly greater with the silicone hydrogel than the HEMA material contact lenses (F = 3.34, p < 0.05).
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**
significance at p = 0.001.
was well described by a combination of lag on horizontal excursions (nasal and temporal), movement on blink in up-gaze and push-up test recovery. This was the case even if objective measures were replaced by three point grading. This finding is similar to that of Young [19] on retrospective analysis of subjective records, although push-up had been assessed by ease of displacement rather than recovery speed and movement on blink was only assessed in primary gaze. A comparison of observer rated ease of displacement (three point grade) with objective speed of recovery in this study did not show a significant correlation between these two measures, and the former weakened the predictive ability to assess overall contact lens movement, so support is given to the assessment of push-up speed of recovery rather than ease of pushup displacement. Therefore, to complete the description of lens fit, a grade (+ quick or large; 0 medium; or slow or minimal) should be recorded for B (movement on blink in up-gaze; B+ = >0.5 mm, B0 = 0.25–0.50 mm, B = <0.25 mm), L (horizontal
4. Discussion This study aimed to determine the most important parameters of soft contact lens fit characteristics to promote the appropriate recording of the assessment of lenses, but taking the shortest possible time to maximise compliance. To achieve this, the study is the first to extensively assess contact lens movement objectively. Despite the lack of published evidence for the proposed damage caused by contact lenses repeatedly crossing the limbal area, the change in surface curvature at the corneo–limbal junction, and the physiological importance of the underlying limbal blood vessel arcades and stem cells highlight the importance of recording contact lens centration and coverage. A decentred lens may also compromise the optimal visual performance. A simple way to denote this is by drawing a horizontal and vertical crossing line to indicating the corneal centre, with a circle overlaid to show the relative contact lens position (Fig. 6). Any incursion between the lens edge and limbus can be marked at the appropriate position on the lens circumference (Fig. 6). Minimally mobile contact lenses have been shown to have a negative impact on ocular physiology, causing fluorescein staining, bulbar and limbal hyperaemia [4] and potentially less tear exchange [5]. In this study, a model of overall lens movement
Fig. 6. The proposed simplified method of recording soft lens fit of trialed lenses in clinical practice. The lens schematic depicts a soft contact lens in the right eye of the subject, which is displace slightly up and temporal, with limbal incursions occurring nasally, large movement on blink in up-gaze, medium excursion on horizontal lag and a quick push-up recovery time.
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lag; L+ = >1.0 mm, L0 = 0.5–1.0 mm, L = <0.5 mm) and P (push-up test; P+ = >4 mm/s, P0 = 2–4 mm/s, P = <2 mm/s; Fig. 6). In addition to optimising the recording of soft lens fit, the study confirmed that the relationship between corneal curvature and lens back optic zone radius does not affect lens movement, perhaps counter to expectation and some studies on early hydrogels [21]. However, this has previously been demonstrated by with thinner, higher water contact lens designs [22,23], with the hydrodynamic squeeze effect predicted to be less significant between lenses [11]. The Etafilcon A Acuvue material movement did not correlate to movement on blink in primary-gaze (r = 0.13, p = 0.59) and was inversely linked to blink in up-gaze (r = 0.29, p = 0.21) as has been previously suggested [20]. Silicone-hydrogels were generally more mobile than HEMA contact lenses as would generally be expected from their higher stiffness. Interestingly, the Focus Dailies lenses performed slightly differently to other materials, perhaps due to its geometry, material physical characteristics or because of its polyvinyl alcohol release properties [24]. In conclusion, together with centration and limbal incursions, horizontal lag, movement on blink in up-gaze and push-up recovery speed should be recorded to adequately describe soft contact lens fit. The described simple diagrammatic representation of lens fit can be used to minimise the time to record this information and to standardise the technique between practitioners, enhancing practitioner compliance and the service provided to patients. Conflicts of interest None. Acknowledgment The authors thank Richard Armstrong for his statistical support with the data analysis. References [1] McMonnies CW, Chapman-Davies A, Holden BA. The vascular response to contact lens wear. Am J Optom Physiol Opt 1982;59:795–9.
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