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Empirical fitting of soft or rigid gas-permeable contact lenses for the correction of moderate to severe refractive astigmatism: A comparative study Langis Michaud, O.D., a Cynthia Barriault, O.D., b Annie Dionne, O.D, b and Peter Karwatsky, O.D. b a Universite ´ de Montre ´al, E ´ cole D’optome ´trie, Montreal, Canada; and b Private Practice, Montre ´al, Canada. KEYWORDS Astigmatism; Contact lenses; Visual acuity; Toric fit Abstract BACKGROUND: This study aims to validate the fitting of contact lenses derived from refractive and keratometric values in a group of subjects with moderate (2.50 diopters [D] to 5.00 D) to severe (.5.00 D) astigmatism. It aims also to show whether soft or rigid gas-permeable (GP) contact lenses offer the best correction and to determine which modality is preferred by subjects. METHODS: Twenty subjects were randomly fitted with both soft and GP lenses. Group A was first fit- ted with soft lenses then switched to GPs and vice versa for group B. For each type of lens worn, low- and high-contrast visual acuity (VA) and stereoscopy were evaluated at both near and far.Each subject was asked to select the lens type of choice and to rate quality of vision in day-to-day activities through a questionnaire. RESULTS: There was no significant difference in objective binocular VA between current spectacles and empirically calculated soft toric lenses and GP toric/bi-toric contact lenses at all distances. That was also true for stereoscopy. Subjectively, most of the subjects preferred rigid contact lenses because of the quality of their vision in day-to-day activities, and a majority of them decided to continue with that modality. CONCLUSION: Empirical fitting of toric soft or GP contact lenses leads to very good results, which suggests that each type of lens might constitute a viable option. Optometry 2009;80:375-383 Astigmatism is a common refractive anomaly affecting from 4% 1 to 30% of the population, depending on the studies consulted. Asians 2 and Native Americans 3 seem to show a higher prevalence compared with other ethnic groups. Caucasian and European populations present a higher inci- dence during childhood and are affected to a lesser extent later in life. 4 Prevalence varies according to the value used to define astigmatism. Some investigators consider astig- matism to be present only at 1.00 diopter (D) and higher, whereas others take all levels of ametropia into account. Overall, in North America, 39% of people wearing glasses are being corrected for astigmatism. 5 For those wearing con- tact lenses, however, only 18% to 22% wear toric correction, showing that many practitioners tend to mask low levels of astigmatism using spherical equivalence or are reluctant to propose or to correct higher amount of astigmatism with contact lenses. One reason for this behaviour is that toric lenses are more complex to fit and have variable results. Recommendation for toric lenses could be limited because Corresponding author: Langis Michaud, O.D., E ´ cole d’optome ´trie de l’Universite ´ de Montre ´al, 3744, Jean-Brillant, Suite 190-70, Montre ´al (Que ´bec), Canada, H3T 1P1. E-mail: [email protected] 1529-1839/09/$ -see front matter Ó 2009 American Optometric Association. All rights reserved. doi:10.1016/j.optm.2008.11.008 Optometry (2009) 80, 375-383

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Page 1: Empirical fitting of soft or rigid gas-permeable contact lenses for the correction of moderate to severe refractive astigmatism: A comparative study

Optometry (2009) 80, 375-383

Empirical fitting of soft or rigid gas-permeable contactlenses for the correction of moderate to severe refractiveastigmatism: A comparative study

Langis Michaud, O.D.,a Cynthia Barriault, O.D.,b Annie Dionne, O.D,b andPeter Karwatsky, O.D.b

aUniversite de Montreal, Ecole D’optometrie, Montreal, Canada; and bPrivate Practice, Montreal, Canada.

KEYWORDSAstigmatism;Contact lenses;Visual acuity;Toric fit

AbstractBACKGROUND: This study aims to validate the fitting of contact lenses derived from refractive andkeratometric values in a group of subjects with moderate (2.50 diopters [D] to 5.00 D) to severe(.5.00 D) astigmatism. It aims also to show whether soft or rigid gas-permeable (GP) contact lensesoffer the best correction and to determine which modality is preferred by subjects.METHODS: Twenty subjects were randomly fitted with both soft and GP lenses. Group A was first fit-ted with soft lenses then switched to GPs and vice versa for group B. For each type of lens worn, low-and high-contrast visual acuity (VA) and stereoscopy were evaluated at both near and far. Each subjectwas asked to select the lens type of choice and to rate quality of vision in day-to-day activities througha questionnaire.RESULTS: There was no significant difference in objective binocular VA between current spectaclesand empirically calculated soft toric lenses and GP toric/bi-toric contact lenses at all distances. Thatwas also true for stereoscopy. Subjectively, most of the subjects preferred rigid contact lenses becauseof the quality of their vision in day-to-day activities, and a majority of them decided to continue withthat modality.CONCLUSION: Empirical fitting of toric soft or GP contact lenses leads to very good results, whichsuggests that each type of lens might constitute a viable option.Optometry 2009;80:375-383

Astigmatism is a common refractive anomaly affectingfrom 4%1 to 30% of the population, depending on the studiesconsulted. Asians2 and Native Americans3 seem to show ahigher prevalence compared with other ethnic groups.Caucasian and European populations present a higher inci-dence during childhood and are affected to a lesser extentlater in life.4 Prevalence varies according to the value used

Corresponding author: Langis Michaud, O.D., Ecole d’optometrie de

l’Universite de Montreal, 3744, Jean-Brillant, Suite 190-70, Montreal

(Quebec), Canada, H3T 1P1.

E-mail: [email protected]

1529-1839/09/$ -see front matter � 2009 American Optometric Association.

doi:10.1016/j.optm.2008.11.008

to define astigmatism. Some investigators consider astig-matism to be present only at 1.00 diopter (D) and higher,whereas others take all levels of ametropia into account.

Overall, in North America, 39% of people wearing glassesare being corrected for astigmatism.5 For those wearing con-tact lenses, however, only 18% to 22% wear toric correction,showing that many practitioners tend to mask low levels ofastigmatism using spherical equivalence or are reluctant topropose or to correct higher amount of astigmatism withcontact lenses. One reason for this behaviour is that toriclenses are more complex to fit and have variable results.Recommendation for toric lenses could be limited because

All rights reserved.

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376 Optometry, Vol 80, No 7, July 2009

of their higher price or presumed discomfort, or they couldbe perceived as unprofitable in view of the chair time theyrequire to fit them. Some practitioners will consider toriclenses only if the patient reports unsatisfactory vision inspherical contact lenses.6 Few of these would prescribespherical equivalent lenses in glasses; however, it remainstrue that some prefer to do so for contact lenses, even if71% of the patients may prefer toric over spherical lenses.7

It was also proven that toric lenses improved visual acuityversus the spherical lens for both low- and moderate-astig-matic patients.6,7 Furthermore, it was found that even pa-tients with 0.75 D of astigmatism, considering the limit forusing toric contact lenses,8 had a clinically meaningful im-provement of up to one full line of visual acuity.6

It is obvious that no one would consider compensatingmoderate (2.50 D to 5.00 D) or severe (.5.00 D) astigmatismwith spherical contact lenses. However, the average practi-tioner finds it even more daunting to fit contact lenses on suchpatients, especially gas-permeable (GP) designs. This is whyan empirical method that proved to be reliable to design thelenses would ease the process and could increase the numberof patients whose vision is corrected with appropriate contactlenses. Consequently, many practitioners would be lessreluctant to propose contact lenses proactively for theirpatients with moderate to high astigmatism, thus maximizingthese lenses’ potential benefits for the wearer.

Contact lens correction for astigmatism

Contact lenses for astigmatism (toric lenses), soft or GP, areconsidered to be more difficult to design because moreparameters must be selected during their fit than forspherical contact lens correction.8 In soft contact lenses,the problems occur when a lens rotates on the surface ofthe eye, lacks stability, or does not fully compensate for am-etropia. Moreover, in rigid GP lenses, some designs couldinduce oblique astigmatism that is difficult to troubleshoot.

Any type of contact lens offers a certain degree ofrotation (misalignment) induced by many factors such asthe lids, palpebral anatomy, or the type of refractive error.This misalignment represents a critical component of toriccontact lens fit especially with higher amounts of astigma-tism where the negative crossed-cylinder effects alteringvisual acuity are amplified.9 Of all the factors being consid-ered, eyelids should be identified as the main obstacle totoric lens rotational position and stability. The upper lidmoves mainly in vertical mode and shows very little hori-zontal movement during the blinking process, whereas thelower lid (showing almost no vertical movement) does havelimited horizontal movement. These differences inducerotational movement in any contact lens placed on theocular surface, for which it is difficult to fully compensate.Previous works proved that the distribution of rotations, inthe nasal or temporal direction, does not follow a standardcurve and is skewed to the nasal side.10 To counteract thiseffect, contact lenses need to be designed with specific

parameters allowing them to remain in a consistent positionon the surface of the eye. For soft lenses, the use of prismballast is the most popular approach toward stabilizing thelens. In fact, this old design, with smaller prism zones, doesprovide 2 points of contact with the lids, with the lens act-ing as a pendulum. Theoretically, this type of lens is not sostable and can provide rotation up to 30� under certain cir-cumstances, mainly when the head position is not verticalor if the patient is sleeping or lying on a couch, for exam-ple. Instability also occurs in extreme peripheral vision,namely in superonasal or inferotemporal excursions.11

Looking at the mirror while driving or to read a newspaperfrom top left to the bottom right are good examples of suchoccurrence. Methafilcon B lens (F55 Toric/Toric XR�;CooperVision, Fairport, New York), used in this study, ismanufactured based on a prism ballast design with eccen-tric lenticulation to help stabilize the lens. This type oflens was selected because of its extended range of parame-ters that are not available in other designs.

Modern lens designs aim to minimize the detrimentaleffects of the lids on lens rotation. Their design includes awider prism base, providing a wider area of contact withthe lower lid. These lenses are considered to be more stable.Other designs offer differential thickness along the verticalmeridian to minimize the lids’ impact on the contact lens.The most recent approach involves the use of two(lotrafilcon B, Air Optix toric�; CIBA Vision, Duluth,Georgia) or 4 active zones (galyfilcon A, Acuvue Advancefor Astigmatism�; Johnson and Johnson Vision Care,Jacksonville, Florida) in the interpalpebral area, lockingthe lens between the lids. This type of design also has theadvantage of being the quickest to stabilize on the corneaand may contribute to an increased stability of the lens.This could lead to improving the chances of first-timesuccess with empirically fitted lenses and to a reduction inthe chair time associated with toric contact lens fit.11

Correcting astigmatism with rigid GP lenses may beachieved by means of spherical lenses, a back-toric design, afront-toric design, or a bi-toric design. Spherical lenses cancompensate for corneal astigmatism up to 3.00 D, although it isknown that using them on corneas showing more than 2.00 Dof toricity may be inappropriate.12 A spherical lens will alwaysmove in the direction of least resistance:13 this implies that alens that is restricted along the vertical meridian and free alongthe horizontal meridian will move freely up and down. Con-versely, if there is restriction along the horizontal meridian,the lens will de-center on the nasal or temporal side. Becauseof this, it is recommended that spherical fittings should be ac-complished using Korb’s model, where the base curve of thelens is equal to or flatter than the cornea’s flattest meridian.The limit of this approach is reached on highly toric corneaswhere the lens may alter the tear film flow, leading to periph-eral desiccation of the cornea known as 3-9 o’clock staining.This is why a spherical GP lens should be used with extremecaution on a cornea presenting toricity greater than 2.00 D.At this time, it is recommended to rely on a topographicmap of the cornea to determine the optimal parameters.

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Michaud et al Clinical Research 377

Back-toric design implies that the back surface of thelens is toric and its front surface spherical. This design isthe most successful when corneal toricity represents twothirds of the refractive astigmatism. Back-toric GP lensesare not designed with prism ballasts; their stabilization ismainly achieved by the correspondence between the backcurves and corneal toricity. However, because this type oflens is designed with only 66% corneal toricity, it is quitecertain that a rotation of the lens will be in play. Clinically,this translates to induced astigmatism that can disturb thevisual outcome of contact lens correction should its valuebe significant and exceed 0.75D.12

Front-toric designs are designed with the astigmaticcorrection put on the front surface of the lens, leaving theback surface spherical. They are stabilized with a base-down prism and represent the best choice for correctingrefractive against-the-rule (ATR) astigmatism, in which thecornea is almost spherical.

A bi-toric lens offers both surfaces as toric. The backtoric surface curves are designed to be aligned with thecorneal toricity. Optically speaking, this generates anovercorrection of the refractive astigmatism. This is whya front toric surface is needed to compensate the power ofthe lens. Bi-toric design is the most popular toric design inGP contact lens practice and represents the best choice ifcorneal toricity equals or exceeds refractive astigmatism.

In the optometric community, rigid GP lenses aregenerally felt to provide a better alternative for correctinghigher refractive astigmatism because they provide sharperand better visual acuity than do soft contact lenses. In fact,the only studies done in this field have found that GP lensesshould be considered a better option for correctingspherical ametropia14 or irregular astigmatism.15 However,empirical fitting was proved to be an efficient way to pre-scribe soft toric contact lenses,16 with 50% to 80% ofpatients being fitted successfully with this method.9,17

To our knowledge, no study has been done that specificallyaims to compare the use of soft and GP lenses with visuallycorrect moderate to high refractive astigmatism within asingle group of subjects.

This study aims to show whether soft contact lenses orrigid GP lenses offer the best correction of visual acuity andto determine which modality is preferred by a group ofsubjects with moderate to severe refractive astigmatism. Italso aims to validate the empirical fitting of contact lensesfor subjects with moderate (2.50 D to 5.00 D) to severe(.5.00 D) astigmatism.

Methods

Subjects

This project was presented and approved by the ethicscommittee for Health Sciences of Universite de Montreal.Candidates for this study were recruited from amongpatients at the Clinique Universitaire de la Vision. The first

20 subjects meeting the entrance criteria (see Table 1) andwho were able to provide informed consent were recruited.

Each subject recruited was evaluated during an initialvisit where the following tests were performed: first,corneal topography was done (Medmont topographer),and refraction was evaluated at a vertex distance of 13mm. With this correction in place, the following testing wasperformed: high- and low-contrast visual acuity at distance(Bailey-Lovie chart) and stereoscopy at far (Brewsterstereoscope) and at near (Randot Stereo Test). Cornealand pupillary diameters were measured as well as interpal-pebral heights with a rule, in dim light condition. Lastly,anterior segment ocular health was evaluated through a slitlamp examination.

Lens parameters

Empirical fitting implies that the parameters of the contactlenses to be fitted are designed based on theoretical andmathematical aspects instead of being selected after a trialprocess.

During this study, the soft lens parameters we needed todetermine were the base curve, the power of each meridian,and the axis of correction. Base curve selection was madeby flattening the subject’s mean K value by 4 D.18 BecauseFrequency 55 Toric lenses are available in 8.4 mm and 8.7mm base curves, the value closest to this theoretical valuewas ordered. Practically speaking, the 8.8 base curve wasselected for corneal mean K flatter than 42.50 D. Powerswere determined for each meridian from the refraction,vertex compensated. The closest parameters in the powerand axis ranges available for Frequency 55 Toric or ToricXR lenses were then ordered.

For the GP lenses, we selected the type and parametersof the most appropriate lens using a 4-step approach,known as the two thirds rule19:

Step 1: Calculation of two thirds of the refractive cylin-der (vertex compensated).

Step 2: Calculation of K1.K1 of the lens was determined by flattening thesteepest meridian of the cornea according to Re-mba’s rule (see Table 2).20

Table 1 Entrance criteria

� 18-40 years of age� Having a valid prescription issued less than 12 months

before recruitment; to be validated for a vertex distanceof 13 mm during the first visit� Presenting best-corrected visual acuity of 6/9 or better

in each eye� Having refractive myopia ,10.00 D with corneal

astigmatism .1.75 D� Able to wear soft and GP contact lenses for at least

5 days/week, 8 hours/day;� In good ocular health

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378 Optometry, Vol 80, No 7, July 2009

Step 3: K2 for the lens was determined.K2 was determined by applying the value inStep 1, to calculate the flattest meridian.

Step 4: Calculation of lachrymal lens power and in-duced astigmatism (IA) using Pesky crosses.This type of astigmatism is induced by theback surface toricity of the lens caused by thedifference in refractive index of the lens mate-rial and tear film. If this IA value was ,1.00D, a back-toric design was ordered becauseIA was not considered as a contributing factoraffecting the visual acuity. At this time, a spher-ical equivalence was made to determine the fi-nal power of the back-toric lens. However, ifthe IA value was equal to or exceeded 1.00 D,a bi-toric design was then selected becausethe IA should be corrected by the addition ofplus cylinder power to the front surface of thelens.

Moving from a back-toric design to a bi-toric one meansabandoning the two thirds rule. The lens parameters had to berecalculated using another method because bi-toric lenscurves are more aligned with the cornea. In this study, theMandell-Moore model was used.21 This model is based onlow-toric simulation, mimicking the fitting of a sphericallens on a cornea with 0.50 to 0.75 D of astigmatism, whichis considered the best fit to alleviate nasal-temporal desicca-tion To do this, the steepest meridian of the lens was flattenedby 0.50 to 0.75 D, depending on the value of the corneal cyl-inder; the flattest meridian was also flattened by a factor vary-ing from 0.00 to 0.25 D. Powers were vertex compensatedand adjusted for the fitting factor in each meridian.

Other parameters of GP lenses were designed as in Table3, according to the inside-out approach as proposed byCaroline.13

When both types of lenses (soft and GP) were received,they were checked for accuracy of parameters, and subjectswere then scheduled for delivery appointments. They wereselected in an equally randomized way using a standardrandomization table in 2 groups: group A (10 patients) wasfitted first with the soft lenses, and group B (10 patients)was fitted with GP lenses first.

Delivery appointment (V2)

After 15 minutes of wear, the following tests wereperformed for each group:

Table 2 Usable values according to Remba’s rule

DK cornea Flattening of the steepest meridian

2.0 D 0.25 D3.0 D 0.50 D4.0 D or more 0.75 D

� Slit lamp evaluation of the soft lenses’ position, move-ment and rotation; the latter being precisely measuredusing the scale at the base of the biomicroscopelamp housing. Any rotation exceeding 30�,22 eitherclockwise or counter clockwise, is considered clini-cally unacceptable and constituted an exclusion crite-rion. For the GP lenses, position, movement, andfluoresceine pattern were carried out using blue cobaltand yellow (Kodak Wratten no.12) filters.

� High- and low-contrast visual acuity using Bailey-Lovie chart at far

� Stereoscopy at far (Brewster stereoscope at 20 feet [6 M])and at near (Randot Stereo Test at 16 inches [40 cm])

Insertion and removal of the lenses were revisited, andthe subject was educated in recommended care and follow-up.Opti-free Express solution (Alcon Labs, Fort Worth, Texas) wasprovided for soft lens subjects, and Boston original formulacleaner and conditioning solution were provided for GPwearers. Both groups were given formal instructions on rubbingand rinsing the lenses before soaking them overnight. For eachpatient, a progressive schedule of wear was recommended forGP lenses starting with 6 hours/day increasing by 2 hours/dayfor a maximum of 14 hours/day. No particular schedule wasrecommended for subjects used to the wear of soft lenses. Fornew wearers the same progressive schedule was proposed. Afollow-up examination was set for a month later.

Follow-up: 4 weeks later (V3)

On the third visit, a careful history was conducted as well asa subjective evaluation of lens behavior during several day-to-day activities, using a questionnaire filled out by eachsubject (Table 4). The position and movement of each typeof lenses and rotation of the soft ones were evaluated througha slit lamp examination. Visual acuity and stereoscopy testswere measured. Lastly, upon removal of the lenses, each sub-ject’s ocular health was assessed with and without fluores-ceine, by means of a slit lamp examination. Cornealstaining, conjunctival hyperemia, and upper papillary ap-pearance were evaluated using CCLRU (Cornea and ContactLens Research Unit, Sydney, Australia) grading scale.

Table 3 GP lens parameters

Optic zone Mean K of the lens in mm

Diameter Optic zone 11.4 mm, calibrated with thecorneal diameter

Peripheries Spherical peripheries: standard design ofthe local lab that produced the lenses;R1: 0.5 mm flatter than flattest BC x0.2 mm; R2: 1.5 mm flatter than flattestBC x 0.2 mm; R3: 2.5 mm flatter thanflattest BC x 0.2 mm 1 blend 0.1 mm5 0.3 mm

Material Boston ESThickness According to the power of the lens

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Michaud et al Clinical Research 379

Table 4 Day-to-day activities questionnaire

Please answer each question by placing a vertical mark in each of the following boxes. The scale varies from 0 (very poor) to 5(very good).

Evaluate your vision at far during the day:—————————————————————————0 5Very Poor Very good

Evaluate your vision at far at night:—————————————————————————0 5Very Poor Very good

Evaluate your vision while driving during the day:—————————————————————————0 5Very Poor Very good

Evaluate your intermediate vision at the computer:

—————————————————————————0 5Very Poor Very good

Evaluate your near vision while reading:—————————————————————————0 5Very Poor Very good

FINAL QUESTIONNAIREIdentify your overall preference for vision:

SOFT , RGP ,Identify the type of lenses you wish to continue to wear in the future:

SOFT , RGP ,

Thereafter, the other types of lenses were provided: GPlenses for group A and soft lenses for group B. A singledrop of proparacaine 1% was applied before insertion if thesubject was new to GP lenses. Position, movement, andfluoresceine pattern (GP) were evaluated through slit lamp(with 2 filters) after 30 minutes. Visual acuity and stereos-copy tests were performed. Proper instructions were givenregarding insertion, removal, care of the lenses, andrecommended schedule of wear. The next follow-up exam-ination was set 4 weeks later.

Final visit (V4)

At this visit, a careful history was conducted, and subjectswere asked to complete an evaluation questionnaire, just asthey had done for their previous lenses. Visual acuity andstereoscopy were again assessed. Lenses and ocular healthwere evaluated through slit lamp examination with filters.At the end of the visit, subjects were asked to identify theirlenses of choice, on the basis overall vision, as well as thelenses they intended to wear on a regular basis.

Statistical analysis methods

Three measurements are obtained to test visual acuity foreach patient. High and low contrasts are added as variablesfor vision at far. These results are analyzed using a repeatedmeasures analysis of variance (ANOVA) with 2 factors:

contrast with 2 levels (high and low) and the mode ofcorrection with 3 levels (glasses, soft contact lens, and rigidGP contact lens).

Questionnaire results (subjective data) are consideredcontinuous variables based on the scale used in this study.This scale of the questionnaire has limited values andis similar to pain scales used in medicine or Likert scalesused in psychometric studies. In such cases, it is consideredstandard procedure to use paired t test to analyze these typesof data.

Results

Demographics, refraction, and ocularcomponents

The initial group of 20 subjects consisted of 12 women and8 men. At the end of the study, it was decided to excludefrom the analysis the results of one of the subjects based onametropia. This subject was one of the first enrolled in thestudy but was the only one with high hyperopia andastigmatism. (He was part of group B, selected to receiveGP lenses to start.)

For the remaining 19 subjects, the average age was 34.5years (64.5 years). Spherical mean refraction was 24.50(62.37) D and astigmatic correction was 23.62 (61.87) Don average. Most of the subjects (12) showed with-the-rule

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380 Optometry, Vol 80, No 7, July 2009

astigmatism (180� 6 20�), whereas the remainder (7)presented with oblique astigmatism. None of the subjectshad against-the-rule astigmatism.

At the beginning of the study, 2 subjects were wearingGP back-toric lenses, 10 were fitted in soft toric lenses, and7 had negative experiences with contact lens wear and didnot wear any type of lenses.

Corneal diameter was 11.7 mm (60.4) on average.Pupillary diameter was 6.5 mm (61.1 mm), and palpebralheight reached 7.8 mm (60.6 mm) on average. There was nosignificant difference in these aspects among the subjects.

Group effect

Even if the sample size (10 subjects in group A and 9 ingroup B) limits the power of statistical analysis, it seemsthat the results are not affected by any group effects.Consequently, the following results are presented consid-ering the 19 patients as a whole, no matter what type of lenswas assigned to the subjects to start.

Lens designs and rotations

The design of the study implies the use of the lenses asordered empirically. Bi-toric contact lenses were fitted on16 of 19 subjects; the remaining 3 subjects wore back-toricdesigned lenses. As for soft lenses, all the lenses displayeda rotation of less than 10�, both at delivery and at follow-upappointments. Most of the lenses were considered stableover blinking, with some showing a slight rocking withinthe acceptable range of rotation (,15�). Consequently,each patient remained with the original lenses ordered forthem. No reorder was made. It is also important to note thateach patient was able to wear GP and soft lenses for at least

Table 5 Visual acuity at far provided by glasses, rigid GP,and soft lenses

Initial (glasses) Soft lensesRigid GPlenses

Far – highcontrast

O.D. 20.02 (60.12) 0.03 (60.14) 20.04(60.11)

O.S. 20.04 (60.14) 0.01 (60.12) 20.05(60.12)

OU 20.10 (60.11) 20.08 (60.11) 20.10(60.11)

Far – lowcontrast

O.D. 10.21 (60.12) 10.25 (60.11) 10.20(60.12)

O.S. 10.18 (60.12) 10.24 (60.11) 10.21(60.13)

OU 10.11 (60.10) 10.15 (60.09) 10.13(60.11)

8 hours per day, 5 days per week as required during theentire study. There was no dropout, and all the subjectscompleted the study.

High- and low-contrast visual acuity

Visual acuity was measured in LogMar units. Table 5 showsthe results for each eye and for each condition. Near visionwas optimal for all subjects. Monocularly and binocularly,each subject reached the 0.37 M (J1) level. No statisticaldifference was found for this particular point (ANOVA:F(2,36) 5 1,377 ; P 5 0.265). For vision at far, there is nointeraction between contrast and the mode of correction(ANOVA: in the right eye [O.D.], F(2,36) 5 0.277; P 5

0.760; in the left eye [O.S.], F(2,36) 5 0.960; P 5 0.392; inboth eyes [OU], F(2,36) 5 0.762; P 5 0.461). Taken alone, thereis also no effect for the mode of correction (ANOVA: O.D.,F(2,36) 5 2.427; P 5 0.103; OS, F(2,36) 5 2.002; P 5 0.150;OU, F(2,36) 5 0.971; P 5 0.389), but there is a strong correla-tion with contrast (ANOVA: F(1,18) 5 405.105; P 5 0.000;OS, F(1,18) 5 935.247; P 5 0.000; OU, F(1,18) 5 337.379;P 5 0.000). That means that visual acuity was measured asreduced under low contrast versus high contrast, but therewas no difference in the comparative results between glassesor soft or rigid contact lenses under each condition. Theseresults are similar monocularly and binocularly.

Stereoscopy

Stereoscopy at far was measured with the Brewster stere-oscope, and results showed the number of targets identified.A perfect score is 12. For near stereoscopy, the results aregiven in seconds of arc. For this element, the lower thescore, the better it is, with a perfect score being 20 secondsof arc. Table 6 indicates the results for stereoacuity. Thereis no statistical difference between glasses, GP lenses, orsoft lenses for stereoscopic visual acuity at all distances.(ANOVA: stereo at far, F(2,36) 5 2,086; P 5 0.139; stereoat near, F(2,36) 5 0.965; P 5 0.391).

Slit lamp findings

Slit lamp results analysis found no significant findingssecondary to the wear of soft lenses. Considering the wearof GP lenses, a majority of patients (14 of 19) showed agrade 1 (CCLRU grading scale) peripheral corneal stainingafter a month of wear. One patient was identified with a

Table 6 Stereoacuity

Initial (glasses) Soft lenses Rigid GP lenses

Far (/12) 10.8 (61.9) 10.0 (62.5) 11.2 (61.7)Near (seconds

of arc)36.5 (647.5) 38.2 (644.5) 31.3 (629.2)

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grade 2 diffuse punctuate corneal staining related to contactlens solution sensitivity. Few of the patients (4 of 19)showed grade 1 conjunctival hyperemia, and the samepatient that showed increased staining was identified with agrade 2 diffuse bulbar hyperemia. No papillary reactionwas identified on the upper palpebral conjunctiva. Thesesigns are all considered not clinically significant.

There was no adverse sign noted for patients when theyused soft lenses. There was no significant corneal stainingidentified for all the patients at the end of a month of wear.Three patients showed trace staining (,grade 1) in theinferior portion of the cornea. This finding was seen onboth eyes for 2 subjects and on the left eye only for theother one.

Subjective questionnairedday-to-day activities

Table 7 shows the results of the questionnaire. Each item wasrated on a scale from 0 (very poor) to 5 (very good). Correla-tion between subjects’ answers and their visual acuity wasmeasured using Pearson test. Paired t tests were done oneach question of the subjective questionnaire to evaluateany difference between soft and GP lenses.

There was no correlation between subjects’ answers andtheir visual acuity level (see Table 8). This means that the ob-jective results do not match the subjective evaluations of thepatients. The analysis of each element tested through thequestionnaire shows that computer work or reading was sta-tistically better with GP lenses compared with soft lenses (seeTable 9). However, there was not such a difference for otheritems (vision at far during the day or the evening and for driv-ing). These results are confirmed with both statistical tests.

Subjective questionnairedlens of choice

Of the 19 subjects, only 5 preferred the soft lenses forvision quality. This means that 14 subjects selected GPlenses for vision. This preferred choice is statisticallysignificant (Wilcoxon test, P , 0.05). Eleven patientsidentified GP lenses as those they would keep in the future.This implies that 8 patients would remain in soft lenses;most of them (6) were already fitted in that modality, andthe remaining (2) were without lenses before the study.

Table 7 Subjective questionnaire results

Soft lenses(of 5)

Rigid GPlenses (of 5)

Far vision (day) 3.5 (61.2) 3.8 (60.8)Far vision (night) 2.9 (61.1) 3.5 (60.8)Driving 3.3 (61.4) 4.1 (60.8)Computer work

(intermediate)2.6 (61.3) 3.8 (60.8)

Near vision (reading) 2.6 (60.8) 4.1 (60.8)

Discussion

Because the previous contact lens wear experience was notthe same for all participants, to eliminate any bias, the orderof the dispensed lens materials was randomized. Therefore,no washout period was needed between the wear of eachtype of lens; most were exposed to the same experience atthe end of the study.

The first result for analysis is the fact that all the lensesthat were empirically designed and fitted for moderate tosevere astigmatism were considered appropriate fordelivery and for wearing. There was no need to reorder alens. In fact, the majority of these lenses provided goodposition, movement, and aligned fluoresceine patterns andoffered good tear exchange under their surfaces. Visualacuity was, on average, better than 20/20 at far and J1 atnear with the first lens delivered. Therefore, one importantfinding of this study is that there was no significantobjective difference in binocular VA between currentspectacles and empirically calculated soft toric lenses andGP toric/bitoric contact lenses.

Some practitioners would expect to see this type ofresult for GP lenses, but the fact that soft lenses do notshow any statistical difference in this matter shows that thedesigns may be accurate, even though the prism ballastlenses used in this study date from a previous generationthat is known to offer less stability compared with thenewest or newer products. In this matter, improvedmanufacturing from modern technology is a key factor to

Table 8 Correlation between subjective answers andcorresponding visual acuity findings

Pearson correlation Sig (2-tailed)

Vision at far – visualacuity, high contrast

0.196 0.422

Vision at far – visualacuity, low contrast

0.087 0.732

Driving – visualacuity, highcontrast (far)

0.452 0.190

Computer – visualacuity (near)

20.341 0.153

Reading – visualacuity (near)

20.306 0.203

Table 9 Subjective answers – correlation soft versus GPlenses

t Sig (bilateral)

Vision at far (day) 21.063 0.303Vision at far (night) 21.844 0.084Driving 21.325 0.234Computer 23.997 0.001Reading 22.953 0.009

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382 Optometry, Vol 80, No 7, July 2009

increasing the performance of soft toric lenses comparedwith the results obtained with empirical fitting made usingprism-ballast design and Metafilcon A lenses in the past.8,15

As to levels of visual acuity, stereoscopy presents similarresults for glasses, soft lenses, and GP lenses. These resultsadd to the results of a previous study suggesting that well-fitted hydrogel toric lenses can provide visual acuity andvisual performance equivalent to that with spherical GPlenses.23 Globally, the effectiveness of toric contact lenses,compared with glasses, is proven if the former are fittedappropriately. We reiterate that the parameters for contactlenses in this study were determined by calculation andwithout previous trials. This implies that the empiricalmethod used is of value, contributes to reducing chairtime, and may be considered whenever a practitioner doesnot have the appropriate trial lenses on hand, as is thecase for moderate to severe ametropia.

A second major finding of this study is that practitionersshould feel free to propose GP toric lenses for their subjectsbecause they tend to prefer them over soft lenses, primarilyfor visual acuity during important day-to-day tasks. Thiscould be explained by the fact that a significant differencewas found in visual acuity at near or intermediate distancefor reading and computer use. One explanation is that GPlenses could compensate more accurately for astigmatismeven if the level of visual acuity does not differ from softlenses. To measure visual acuity on a chart and to completea task during several minutes, at near or intermediatedistance, is not the same. Astigmatism is a type ofametropia that alters the fine definition of the objectsseen, and its correction adds sharpness to the images andincreases their definition. This is important mostly forprecise tasks such as reading or using a computer. If wetake the questionnaire’s results into account, it becomesobvious that for such tasks GP lenses provide bettersubjective results with less variability among subjects. Atfar, differences could be unnoticed or more difficult toevaluate because of the size of the targets and the nature ofthe visual tasks at this distance.

The above are the key reasons subjects selected GPlenses as their favorites. In a word, subjects were better ableto see with GP lenses during some of the important day-to-day activities that matter in modern life.

Finally, it is reassuring that slit lamp results indicate thatthe lenses used in this study were worn without inducingany ocular health problem. It was recommended to the onlypatient showing grade 2 corneal staining and conjunctivalhyperemia to switch from Boston original formulationsolutions to Boston Simplus and to use nonpreserved salinefor rinsing instead of tap water (which solved the problem).

Conclusion

This study shows that both soft and GP toric contact lensesare effective in correcting moderate to severe astigmatismin a given population. Visual acuity and stereoscopy

provided by either type of contact lenses were equivalentto correction by glasses. GP lenses outperform soft lensesfor reading and computer use. It also shows that it is easyand efficient to determine lens parameters using calcula-tions, rather than undergoing trials. For the design of GPlenses, we found the Mandell-Moore formula to be accurateand user-friendly. This could help practitioners save chairtime and be more efficient in their fittings. Finally, thisstudy illustrates that GP lenses could be very well acceptedas a viable option for the correction of moderate to severeastigmatism. Subjects preferred GP lenses for visual acuity,especially at near and intermediate distance. The results ofour day-to-day activities questionnaire suggest that a sub-ject’s perception of enhanced, more stable vision whileaccomplishing precise tasks explains this preference.

Acknowledgments

The authors thank Cooper Vision Canada (Patrick Boutinand Jacques Lecavalier) and Laboratoires Blanchard (JeanCastonguay, Gilles Castonguay, and Jean Blanchard) fortheir support in providing advice and the lenses needed forthis study.

The authors deny any financial arrangement with anycompany whose products were used in this study or with acompetitor company.

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