Contact Lens & Anterior Eye 38 (2015) 194–198
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Low toric soft contact lens acceptance study Sara N. Gaib ∗ , Balamurali Vasudevan Midwestern University, United States
a r t i c l e
i n f o
Article history: Received 12 August 2014 Received in revised form 8 December 2014 Accepted 26 January 2015 Keywords: Toric lens Soft lens Astigmatism Visual acuity Contrast sensitivity Contact lens
a b s t r a c t Purpose: The aim of the study was to evaluate the objective and subjective visual performance of custom toric contact lenses (TL) and their spherical off-the-shelf counterparts (SL) in subjects with low amounts of astigmatism. Methods: Twenty-three habitual soft lens wearers (40 eyes, 25–35 years) manifesting 0.50–1.00 DC and ≤±3.00 DS were recruited. Air Optix Aqua (Lotrafilcon B) was fit using the spherical equivalent of the manifest refraction. Intelliwave toric in Efrofilcon A (Definitive) was fit using the manifest refraction and keratometric data. Comprehensive visual performance tests were done through manifest refraction in a trial frame; in SL; and in TL. A subjective evaluation of quality of vision was also obtained. Results: ANOVA revealed that, at the morning visit (AM), high contrast logMAR distance visual acuity (HCDVA) was significantly better (p < 0.01) in spectacles as compared to SL. A similar trend was noted at the afternoon visit (PM). In addition, at the PM visit, HCDVA was significantly better (p < 0.01) for TL as compared to their SL. ANOVA revealed that, at the PM visit, low contrast distance visual acuity (LCDVA) was significantly better (p = 0.05) in spectacles as compared to SL. None of these differences were clinically significant. In addition, no statistically significant difference (p > 0.05) in subjective vision rating scores was noted between SL and TL. Conclusions: The present investigation found no clinically significant difference in visual performance between spherical and toric soft contact lenses in low astigmats. © 2015 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.
1. Introduction Many patients can benefit from toric contact lens correction. Holden [1] determined the relative percentage of the population that would need toric soft contact lens correction based on their magnitude of astigmatism. If all astigmatism of 0.50 diopters cylinder (DC) or more were corrected, 61.5% of wearers would require toric soft contact lens correction. If only astigmatism of 1.00 DC or more was corrected, 34.8% would require toric correction. Interestingly, there has been a steady increase in toric soft contact lens fitting as a proportion of all soft contact lens fitting for over a decade now. In 2008, toric soft contact lenses represented 34% of all soft contact lens fits [2] that is, astigmatism of 1.00 D or more is being routinely corrected in soft contact lenses. Although toric lenses are used for many patients, the fitting convention for patients with low amounts of astigmatism still involves using spherical lenses in the spherical equivalent refraction. In fact, the lowest cylinder correction available in most off the shelf soft
∗ Corresponding author at: MWU Eye Institute, 19379 N. 59th Avenue, Glendale, AZ 85308, United States. Tel.: +1 623 806 7214; fax: +1 623 806 7240. E-mail address:
[email protected] (S.N. Gaib).
toric contact lenses is 0.75 DC [3]. It is assumed that the spherical equivalent provides adequate vision correction in contact lenses for low astigmats. However, this may not prove to be the case when visual performance is examined between spherical and toric correction. Efron et al. [4] suggest five possible reasons that eye care professionals do not routinely correct refractive cylinder of 0.75 DC or less: (1) The small visual improvement is not offset sufficiently by the increased chair time or potential for variability in vision due to axis mislocation. (2) The belief that higher modulus soft contact lenses mask astigmatism. (3) Patient concern over cost. (4) Limited parameter availability in daily disposable options. (5) Electing for the simplicity of bilateral spherical correction in cases of unilateral indication for toric correction. Interestingly, Holden [1] calculated the proportion of patients having greater than 0.50 DC to be 61.5%. This represents a tremendous potential in toric contact lens fitting if we changed the astigmatic threshold that is commonly considered to be significant. One thought in dealing with low amounts of astigmatism is the use of increased lens thickness or a higher modulus material to mask astigmatism. However, Cho and Woo [5] found that lens thickness did not have a significant effect on visual acuity, in spite of residual astigmatism being lower in the thicker lenses. Similarly, Edmondson et al. [6] did not find a significant effect in
http://dx.doi.org/10.1016/j.clae.2015.01.009 1367-0484/© 2015 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.
S.N. Gaib, B. Vasudevan / Contact Lens & Anterior Eye 38 (2015) 194–198
masking astigmatism using a high modulus silicone hydrogel lens, compared to a low modulus hydrogel lens. Another theory is that using aspheric soft contact lenses corrects low amounts of astigmatism. However, Morgan et al. [2] found that toric lenses do this much more effectively. Therefore, when astigmatic patients are fit in non-toric soft lenses their refractive error is not fully corrected. Several studies have investigated the effect of uncorrected astigmatism on visual function. For example, Guo and Atchison [7] found that 0.28 ± 0.12 DC was necessary for a reduction of 0.1 logMAR of high contrast visual acuity. Similar decrement was observed at low contrast, and at near. In addition, astigmatic blur has also been shown to cause a reduction in contrast sensitivity [7]. Furthermore, the reduction in visual performance has been suggested to translate to functional difficulties as well. Visual performance is usually better with toric soft contact lenses for prescriptions with cylindrical power. Richdale et al. [8] compared the visual acuity and wavefront aberrations in patients fit with spherical, aspheric and toric soft contact lenses and found that the latter yielded the best results. However, subjects had moderate cylindrical power that ranged between 0.75 and 2.00 DC. Interestingly, Lehman and Houtman [9] found that pseudophakic subjects with low levels of postoperative astigmatism benefited from full correction of their astigmatism as compared to spherical equivalent correction. This was evident for both high- and low-contrast visual acuity. More recently, using a wavefront sensor to detect the magnitude of astigmatism, Villegas et al. [10] investigated the effect of low cylindrical power on visual performance. They measured wavefront aberrations on selected patients with less than 0.50 diopters of cylinder. They corrected astigmatism using a cross cylinder device and performed several visual performance tests. The authors found a relative improvement in visual acuity when astigmatism greater than 0.30 DC was corrected. However, this assumed exact axis orientation, something that is not always achieved on the first try when fitting a soft toric contact lens. An error of 10◦ would cause residual astigmatism of 35% with an orientation 40◦ away from the intended axis [10]. Furthermore, there was additional defocus of half of the remaining astigmatism. Snyder [11] found that when a lens is rotated 30◦ , the entire cylindrical power is delivered at an axis oblique to that desired. Gaze direction and gravity may also have an effect on toric lens orientation and visual acuity. McIlraith et al. [12] studied AcuvueTM cOasys® for Astigmatism, Purevision® Toric, Air Optix® for Astigmatism and Proclear® Toric and found that all lenses rotated with change in posture and head position. Rotation ranged from 11◦ to 29.1◦ , causing a consequent visual decrement of 0.05–0.15 logMAR. Improving stability has been an important principle in the evolution of toric soft lens development. In fact, rotational stability has been shown to be the main factor that determines whether a patient is successful in toric contact lenses [13]. Stabilization methodology can influence this, as can the fitting relationship. There are many factors that contribute to the fit of a contact lens, such as palpebral aperture size, lid position, lid tension, inter-canthal angle, horizontal visible iris diameter and corneal topography. Although there have been significant advances in contact lens design, lens rotation and instability continues to be problematic in some cases. Momeni-Moghaddam et al. [14] compared the rotation and rotational recovery in several off-the-shelf lenses: Purevision toric (Bausch + Lomb, Rochester), Air Optix for Astigmatism (Alcon, Fort Worth), Acuvue Advance for Astigmatism (Johnson and Johnson Vision Care, Jacksnoville), Biofinity toric (Coopervision, Pleasanton) and Proclear toric (Coopervision, Pleasanton). Lenses ranked from least to most stable are: Proclear Toric, Acuvue Advance for Astigmatism, Purevision Toric, Air Optix for Astigmatism and Biofinity toric. In the presence of increased stability, it is more likely to correct astigmatism of any magnitude successfully. Practitioners are more
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apt to see the value in correcting low amounts of astigmatism if there is evidence of improvement in visual performance. Thus, the purpose of the present investigation is to compare the objective and subjective visual performance of spherical silicone hydrogel lenses to custom toric silicone hydrogel lenses in patients with manifest astigmatism of 0.50–1.00 DC.
2. Methods A total of 24 subjects (40 eyes) between the ages of 22 and 35 years completed the study. Subjects were recruited from the university population to participate in this study. The research followed the tenets of the Declaration of Helsinki and was approved by the Midwestern University Institutional Review Board (IRB). Potential subjects were briefed on the study, including risks, both verbally and in writing. Informed consent was obtained from every subject. The investigators then performed auto-refraction, autokeratometry, and subjective refraction. The latter performed by the same examiner for all subjects, for consistency. Refractions were examined to ensure that manifest astigmatism was 0.50, 0.75 or 1.00 DC in one or both eyes. The spherical aspect of the refractive error ranged between +3.00 and −3.00, to make the astigmatic component a significant portion of the overall refractive error. Only those patients who met the refractive state requirements either monocularly or binocularly were allowed to participate in the study. The anterior segment health was evaluated to ensure that subjects were free of pathology and had no history of previous corneal surgery. The upper age limit was set to 35 years in an attempt to limit age-related tear film changes, as well as to exclude presbyopic subjects. Once the subject qualified to participate in the study, two sets of soft contact lenses were ordered based on their prescription. One set was made up of multi-packaged spherical silicone hydrogels, as would be customarily used for a subject with this magnitude of cylinder. The second set was made up of custom toric silicone hydrogels. The materials for the spherical and toric lenses were lotrafilcon B (Alcon, Fort Worth, TX) and efrofilcon A (Art Optical, Grand Rapids, MI), respectively. Lotrafilcon B (AIR OPTIX AQUA, Fort Worth, TX) lenses were ordered in the standard 8.6 mm base curve and 14.2 mm diameter, using the spherical equivalent of the subject’s manifest refraction. Efrofilcon A “Definitive” (Contamac, Grand Junction, CO) lenses in the Intelliwave Aspheric Toric (Art Optical, Grand Rapids, MI) design were ordered empirically. This was done using the subject’s keratometry values and manifest refraction. Lenses were ordered for both eyes for equilibrium, regardless of whether one or both eyes met the inclusion criteria. However, data was only collected for qualifying eyes. Once lenses arrived, subjects were scheduled for a dispensing visit. Subjects were randomized to determine whether they were fit into the spherical or toric lenses that day. Lenses were applied to the subjects’ eyes and the following visual performance tests were administered: high and low contrast logMAR (log of the minimum angle of resolution) acuity at distance and near and contrast sensitivity using the CSV-1000 (Precision-vision, La Salle, IL). The fit of the lenses was assessed, judging coverage, centration and movement. If applicable, rotation and stability were documented as well. Rotation was measured using a slit lamp biomicroscope and rotating a bright, narrow optic section to coincide with the toric lens scribe mark. Subjects were asked to wear the lenses for 6 h and return to the clinic for another evaluation. They were asked to subjectively rate their vision in each eye while wearing the lenses on a scale of 0 (worst) to 10 (best). Visual performance tests administered were high and low contrast acuity at distance, high and low contrast acuity at near and contrast sensitivity using the CSV-1000. The fit
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Table 1 Summary of mean and standard deviation (SD) for high or low contrast visual acuity with spectacles (baseline), spherical contact lenses and toric contact lenses. Visual performance
Lens conditions
Mean
SD
HCDVA AM
Baseline Sphere Toric
0 0.03 0.02
0 0.06 0.05
HCNVA AM
Baseline Sphere Toric
0 0.01 0.01
0 0.025 0.03
HCDVA PM
Baseline Sphere Toric
0 0.04 0.01
0 0.06 0.03
HCNVA PM
Baseline Sphere Toric
0 0.01 0.00
0 0.03 0.02
LCDVA AM
Baseline Sphere Toric
0.18 0.23 0.20
0.13 0.14 0.13
LCNVA AM
Baseline Sphere Toric
0.08 0.07 0.07
0.09 0.07 0.09
LCDVA PM
Baseline Sphere Toric
0.18 0.25 0.20
0.13 0.15 0.14
LCNVA PM
Baseline Sphere Toric
0.08 0.07 0.07
0.09 0.09 0.08
of the lenses was re-assessed, and the lenses were removed and discarded. The subjects returned for the crossover portion of the study on a different day. They were either fit in the spherical lenses or the toric lenses, depending on the randomization table. The same protocol was followed as with the initial set of lenses. The permitted interval between trying the first and second set of lenses was 1 week to 1 month, depending on subject availability. 3. Results Of the 40 eyes, for the spherical component of refraction, 3 eyes were hyperopic that ranged between 1 and 1.25 D, 5 eyes were emmetropic that ranged between −0.50 D and 0 D, while the remaining 32 eyes were myopic that ranged between −0.75 D and −3 D. For the cylindrical component of refraction, 24 eyes had −0.50 D, 8 eyes had −0.75 D and another 8 eyes had −1 D astigmatism. Statistical analysis was performed using SPSS (ver. 20). Analysis included ANOVA and post hoc Bonferonni comparison that was performed for each of the visual performance tasks using the manifest refraction in a trial frame (baseline) and each of the two lens designs (spherical and toric), as illustrated in Table 1. Paired t-test (2-tailed) was performed comparing morning to afternoon visual performance in spherical and toric contact lenses. 3.1. Morning visit ANOVA revealed that, at the morning visit, high contrast logMAR distance visual acuity (HCDVA) was significantly better (p < 0.01) at baseline (mean = 0) as compared to spherical contact lenses (mean = 0.03), as illustrated in Fig. 1. There was no statistically significant difference (p > 0.05) between HCDVA at baseline and in toric contact lenses (mean = 0.02), or between spherical and toric contact lenses. ANOVA revealed that, at the morning visit, there was no statistically significant difference (p > 0.05) in high contrast logMAR near visual acuity (HCNVA) between baseline (mean = 0.0) and spherical contact lenses (mean = 0.0), baseline and toric contact
Fig. 1. Plot of mean (±SD) high contrast distance visual acuity (HCDVA) at the morning visit in spectacles (baseline), spherical contact lenses and toric contact lenses.
Fig. 2. Plot of mean (±SD) high contrast distance visual acuity (HCDVA) at the afternoon visit in spectacles (baseline), spherical contact lenses and toric contact lenses.
lenses (mean = 0.0), or between spherical and toric contact lenses. ANOVA revealed that, at the morning visit, there was no statistically significant difference (p > 0.05) in low contrast distance visual acuity (LCDVA) between baseline (mean = 0.2) and spherical contact lenses (mean = 0.2), baseline and toric contact lenses (mean = 0.2), or between spherical and toric contact lenses. ANOVA revealed that, at the morning visit, there was no statistically significant difference (p > 0.05) in low contrast near visual acuity (LCNVA) between baseline (mean = 0.1) and spherical contact lenses (mean = 0.1), baseline and toric contact lenses (mean = 0.1), or between spherical and toric contact lenses. 3.2. Afternoon visit ANOVA revealed that, at the afternoon visit, HCDVA was significantly better (p < 0.01) at baseline (mean = 0.0) as compared to spherical contact lenses (mean = 0.04), as illustrated in Fig. 2. At this visit, HCDVA was also significantly better (p < 0.01) for toric contact lenses (mean = 0.01) as compared to their spherical counterparts. There was no statistically significant difference (p > 0.05) between baseline and toric contact lenses. ANOVA revealed that, at the afternoon visit, there was no statistically significant difference (p > 0.05) in HCNVA between baseline (mean = 0) and spherical contact lenses (mean = 0.0), baseline and toric contact lenses (mean = 0.0), or between spherical and toric contact lenses. ANOVA revealed that, at the afternoon visit, LCDVA was significantly better (p = 0.05) at baseline (mean = 0.2) as compared to spherical contact lenses (mean = 0.3), as illustrated in Fig. 3. There was no statistically significant difference (p > 0.05) between LCDVA at baseline and in
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Table 2 Plot of mean (±SD) contrast sensitivity at various spatial frequencies with spectacles (baseline), spherical contact lenses and toric contact lenses.
Fig. 3. Plot of mean (±SD) low contrast distance visual acuity (LCDVA) at the afternoon visit in spectacles (baseline), spherical contact lenses and toric contact lenses.
toric contact lenses, or between spherical and toric contact lenses. ANOVA revealed that, at the afternoon visit, there was no statistically significant difference (p > 0.05) in LCNVA between baseline (mean = 0.1) and spherical contact lenses (mean = 0.1), baseline and toric contact lenses (mean = 0.1), or between spherical and toric contact lenses. 3.3. Between visits Paired t-test (2-tailed) was performed and revealed no statistically significant difference (p > 0.05) between morning visit HCDVA (mean = 0.03) and afternoon visit HCDVA (mean = 0.0) in spherical contact lenses; between morning visit HCNVA (mean = 0.0) and afternoon visit HCNVA (mean = 0.0) in spherical contact lenses; between morning visit LCDVA (mean = 0.2) and afternoon visit LCDVA (mean = 0.3) in spherical contact lenses; or between morning visit LCNVA (mean = 0.1) and afternoon visit LCNVA (mean = 0.1) in spherical contact lenses. Paired t-test (2-tailed) revealed no statistically significant difference (p > 0.05) between morning visit HCDVA (mean = 0.0) and afternoon visit HCDVA (mean = 0.0) in toric contact lenses; between morning visit HCNVA (mean = 0.0) and afternoon visit HCNVA (mean = 0.0) in toric contact lenses; between morning visit LCDVA (mean = 0.2) and afternoon visit LCDVA (mean = 0.2) in toric contact lenses; or between morning visit LCNVA (mean = 0.1) and afternoon visit LCNVA (mean = 0.1) in toric contact lenses. Paired t-test (2-tailed) revealed no statistically significant difference (p > 0.05) between morning visit contrast sensitivity between spectacle, spherical contact lens or toric contact lens correction at any visit. Results are summarized in Table 2. 3.4. Subjective rating Subjective rating was performed for each visit. Paired t-test revealed no statistically significant difference (p > 0.05) in subjective vision rating on a scale of 1 (worst) to 10 (best) between spherical (mean = 6.6304) and toric contact lenses (mean = 6.3587). 4. Discussion The results of this study indicated that there was no subjective difference in visual quality between spherical and toric contact lenses in low astigmats. HCDVA was statistically higher in toric contact lenses as compared to their spherical counterparts, but only at the afternoon visit. Furthermore, this difference was not clinically significant. Richdale et al. [8] compared the visual acuity of subjects with refractive cylinder between 0.75 and 2.00 D in four different
Conditions and spatial frequency
Mean
SD
Baseline 3cpd Baseline 6cpd Baseline 12cpd Baseline 18cpd Sphere 3cpd AM Sphere 6cpd AM Sphere 12cpd AM Sphere 18cpd AM Sphere 3cpd PM Sphere 6cpd PM Sphere 12cpd PM Sphere 18cpd PM Toric 3cpd AM Toric 6cpd AM Toric 12cpd AM Toric 18cpd AM Toric 3cpd PM Toric 6cpd PM Toric 12cpd PM Toric 18cpd PM
1.57 1.72 1.45 1.08 1.57 1.70 1.45 1.06 1.54 1.68 1.44 1.04 1.43 1.62 1.35 0.99 1.46 1.61 1.35 1.00
0.66 0.73 0.62 0.47 0.68 0.74 0.63 0.47 0.68 0.75 0.64 0.47 0.74 0.90 0.71 0.52 0.76 0.84 0.70 0.53
spherical and toric soft contact lenses. They found that eyes with a low magnitude of astigmatism (≤1.00 DC) gained between 3 and 5.5 letters of acuity with toric contact lenses versus spherical lenses. As expected, eyes with more astigmatism (1.25–2.00 DC) had a higher acuity gain, between 8 and 12.5 letters. The improvement in visual acuity depended on the luminance and contrast test condition. Similarly, Dabkowski et al. [15] found that subjects with 0.75–1.25 DC had better visual acuity in toric versus spherical soft lenses. However, contrast sensitivity between the two groups was relatively similar. In addition, 71% of subjects had a subjective preference for toric lenses. Furthermore, Kurna et al. [16] compared the residual astigmatism and astigmatic neutralization of subjects with 0.75–1.25 DC wearing spherical versus toric soft contact lenses. They found a significant improvement in astigmatic neutralization and decrease in residual astigmatism in toric lenses. In the present study, subjects were fit with the initial pair of lenses ordered. In some cases there was rotation in the toric lenses. This axis mislocation could be responsible for the inferior visual quality. The stabilization system of the Intelliwave Aspheric Toric used in the present study uses traditional prism ballast. Interestingly, Young et al. [17] investigated the various factors that may influence toric soft lens fit. Their aim was to determine predictor variables for lens orientation, using a prism-stabilized, mid-water, cast-molded, back surface toric soft lens. They investigated subject factors and lens fit characteristics and their relationship to both lens rotation and stability. The subject factors with the strongest correlations to lens orientation variables were: the intercanthal angle, palpebral aperture and myopia. A greater incline upward of the temporal lid was associated with more infero-temporal rotation. Lower amounts of myopia and smaller aperture sizes were associated with more stable lenses. The fit factors with the strongest correlations to lens orientation were post-blink movement and lens tightness. Lenses with less movement were more stable and tight lenses re-oriented slower. There were key differences in the material characteristics of the lenses we used in the study. Air Optix Aqua’s® lotrafilcon B material is a low water (33%) silicone hydrogel, while Definitive (efrofilcon A) is a high water (74%) silicone hydrogel. While Air Optix Aqua has a relatively stiff modulus of 1.32 [17], Definitive has a modulus of 0.35 (www.contamac.com). Center thickness is variable between the lathed custom toric and its spherical off-the-shelf counterpart as well. Although water content and modulus have not specifically been studied with respect to their influence on toric lens stability,
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they may affect overall lens fit. Therefore, the differences between the two lens materials used in the present investigation are important to consider when interpreting results. With respect to lens thickness, Cho and Woo [5] found that it did not have a significant effect on visual acuity when using spherical lenses on low astigmats. However, it may still impact toric lens fit. The study was designed to compare the current standard of care – fitting off-the-shelf spherical lenses on low astigmats – to a proposed alternative. While the comparison of a custom sphere to a custom toric made of the same material may be one that imposes fewer variables, it does not examine how our current standard of care compares. In the present study, not only did the use of custom lenses enable low cylinder correction that is not readily available in boxed lenses, it also has the advantage of enabling lens fit customization. While most boxed soft contact lenses only come in one base curve and diameter, custom lenses may have these parameters optimized to best match the subject. The custom toric contact lenses in the present study were ordered using subjects’ keratometry values and spectacle prescription, as this was representative of information that is commonly collected during eye examination. However, it appears as though the fit could be further customized by gathering an additional subject measurement: the horizontal visible iris diameter (HVID). The relationship between the back surface of the contact lens and the front surface of the cornea is determined by the match in sagittal depth between the two. A lens with a larger sagittal depth relative to the cornea fits tighter, while one with a smaller sagittal depth fits looser. The sagittal depth of a contact lens is influenced by several factors, such as base curve and diameter. Douthwaite [18] studied the type of corneal measurement that correlated best with sagittal depth and found that the HVID had the most influence. In contrast, Wong et al. [19] found that HVID did not play a significant role in toric soft lens fit. Further investigation should incorporate the measurement of HVID in the determination of optimal lens parameters. The latter may improve lens fit, minimizing rotation and/or instability. If stable axis mislocation is still found, then compensation using LARS may be used. The combination of fit optimization and rotation compensation will ensure that the toric correction is placed where it is intended. Accurate visual performance evaluation can then be obtained. 5. Conclusion The present investigation found no clinically significant difference in visual performance between spherical and toric soft contact lenses in low astigmats. Axis mislocation may have affected vision through the toric contact lenses. Furthermore, the results are limited to the specific lens designs used in the study.
Funding source Contamac, Grant # 31-1272-2646 UK funded the majority of the study, while Art Optical, USA provided the custom toric contact lenses. Acknowledgement The authors would like to thank Mr. John Tran for data entry and audit. References [1] Holden B. The principles and practice of correcting astigmatism with soft contact lenses. Aust J Optom 1975;58:279–99. [2] Morgan PB, Efron SE, Efron N, Hill EA. Inefficacy of aspheric soft contact lenses for the correction of low levels of astigmatism. Optom Vis Sci 2005;82:823–8. [3] Thompson T. Tyler’s Quarterly. Little Rock, AR. Tyler’s Quarterly. [4] Morgan PB, Efron N, Woods CA, International Contact Lens Prescribing Survey C. An international survey of contact lens prescribing for presbyopia. Clin Exp Optom 2011;94:87–92. [5] Cho P, Woo GC. Vision of low astigmats through thick and thin lathe-cut soft contact lenses. Cont Lens Anterior Eye 2001;24:153–60. [6] Edmondson L, Edmondson W, Price R. Masking astigmatism: Ciba focus night and day vs focus monthly. Optom Vis Sci 2003;80:184. [7] Guo H, Atchison DA. Subjective blur limits for cylinder. Optom Vis Sci 2010;87:E549–59. [8] Richdale K, Berntsen DA, Mack CJ, Merchea MM, Barr JT. Visual acuity with spherical and toric soft contact lenses in low- to moderate-astigmatic eyes. Optom Vis Sci 2007;84:969–75. [9] Lehmann RP, Houtman DM. Visual performance in cataract patients with low levels of postoperative astigmatism: full correction versus spherical equivalent correction. Clin Ophthalmol 2012;6:333–8. [10] Villegas EA, Alcon E, Artal P. Minimum amount of astigmatism that should be corrected. J Cataract Refract Surg 2014;40:13–9. [11] Snyder C. A review and discussion of crossed cylinder effects and overrefractions with toric soft contact lenses. Int Contact Lens Clin 1989;16:113–7. [12] McIlraith R, Young G, Hunt C. Toric lens orientation in non-standard conditions. Cont Lens Anterior Eye 2010;33(1):23–6. [13] Goldsmith WA, Steel S. Rotational characteristics of toric contact lenses. Int Cont Lens Clin 1991;18:227–30. [14] Momeni-Moghaddam H, Naroo SA, Askarizadeh F, Tahmasebi F. Comparison of fitting stability of the different soft toric contact lenses. Cont Lens Anterior Eye 2014;35(5):346–50. [15] Dabkowski JA, Roach MP, Begley CG. Soft toric versus spherical contact lenses in myopes with low astigmatism. Int Cont Lens Clin 1992;19: 252–6. [16] Kurna SA, Sengor T, Un M, Aki S. Success rates in the correction of astigmatism with toric and spherical soft contact lens fittings. Clin Ophthalmol 2010;4:959–66. [17] Young G, Hunt C, Covey M. Clinical evaluation of factors influencing toric soft contact lens fit. Optom Vis Sci 2002;79:11–9. [18] Douthwaite WA. Initial selection of soft contact lenses based on corneal characteristics. CLAO J 2002;28:202–5. [19] Wong MK, Lee TT, Poon MT, Cho P. Clinical performance and factors affecting the physical fit of a soft toric frequent replacement contact lens. Clin Exp Optom 2002;85:350–7.