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Zeri, Fabrizio, Calcatelli, Paola, Donini, Bernardo, Lupelli, Luigi, Zarilli, Lu-ciana, & Swann, Peter G. (2011) The effect of hydrogel and silicone hy-drogel contact lenses on the measurement of intraocular pressure withrebound tonometry. Contact Lens and Anterior Eye, 34(6), pp. 260-265.

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1

Title:

The effect of hydrogel and silicone hydrogel contact lenses on the measurement of

intraocular pressure with rebound tonometry.

Authors

Fabrizio Zeri1*

, Paolo Calcatelli1, Bernardo Donini

1, Luigi Lupelli

1, Luciana Zarrilli

1, Peter

G.Swann2

1 Degree Course in Optics and Optometry, Faculty of Mathematics, Physics and Natural

Sciences -Roma TRE University. Rome, Italy.

2 School of Optometry, Hong Kong Polytechnic University, Hong Kong. School of

Optometry, Queensland University of Technology, Brisbane, Australia.

* Corresponding author

Dr. Fabrizio Zeri

Degree Course in Optics and Optometry, Roma TRE University. Rome, Italy.

Via Galvani, 6 00153 Rome Italy. Tel: +39 3409373736 Fax +39 657133143

E-mail address: zeri@fis.uniroma3.it

*Title Page (with author details and affiliations)

1

1. Introduction

Primary open angle glaucoma is one of the most important causes of blindness [1]. Raised

intraocular pressure (IOP) is a major risk factor for primary open-angle glaucoma [2].

Regardless of the development of sophisticated diagnostic methods, the measurement of

IOP by tonometry remains significant both for the diagnosis and management of glaucoma.

In view of the legal sanctions imposed on optometry regarding the use of diagnostic drugs in

many countries, optometrists have increasingly turned to non invasive methods of measuring

intraocular pressure [3].

Recently several tonometers based on new principles have been introduced [4-6]. A

relatively new tonometer is based on the rebound principle. This type of tonometry was

known many years ago [7] but it was improved and popularized by Kontiola [8,9]. The

measurement of IOP is dependent on processing the rebound movement of a rod probe,

resulting from its interaction with the cornea. These new instruments have the advantage of

being portable and not requiring topical anaesthesia.

It has been shown that the accuracy and reliability of the ICare tonometer (the first rebound

tonometer launched for clinical use) is quite good for clinical purposes especially in the low

to moderate IOP range [10]. Several studies have shown a negligible overestimation of IOP

compared with conventional Goldmann applanation tonometry [11-15] although some

authors consider that this overestimation is excessive [16]. It has also been demonstrated

that there is a good agreement with respect to the NCT Pulsair 3000 [17], and also two

portable tonometers such as the Tonopen and Perkins [18]. Rebound tonometry also gives

reproducible results [12]. Recently a second generation rebound tonometer, the IOPen, has

been introduced. It has been found that this new instrument underestimates IOP when

compared to Goldmann applanation tonometry and the ICare [19].

It has been shown that IOP readings over thin hydrogel and silicone hydrogel CLs can be

successfully and safely undertaken using different kinds of tonometer: the indentation

tonometer [20-21], Goldmann tonometer [20, 22-27], non-contact tonometer [28-33]

Mackay-Marg tonometer [20-21, 34] tonopen [35-38] Draeger tonometer [39], gas

*Manuscript (without author details and affiliations)

2

pneumotonometer [38, 40-41], Ocular Response Analyzer [42], and the Dynamic Contour

tonometer [43]. To our knowledge, this has not been investigated using rebound tonometry.

The aim of this work was to investigate the possibility of performing rebound tonometry

directly over soft contact lenses. This could be very useful in clinical practice because it

allows the measurement of IOP without removing CLs during after-care visits.

2. Methods

2.1 Subjects

Thirty-six subjects (19 male and 17 female) were enrolled in the study. All were optometry

students. Ages ranged from 18 to 43 years (mean 26.3; SD 8.2). Inclusion criteria were

normal corneas (no corneal scarring, corneal pathology or prior corneal surgery), assessed

by slit lamp examination and videokeratoscopy, and corneal astigmatism of not more than

2.50D. All subjects achieved unaided vision or visual acuity of 6/6 or better in each eye.

Informed consent was obtained from each subject after an explanation of the procedure.

2.2 Materials

All tonometry measurements were carried out with a rebound tonometer (ICare; Finland Oy).

The CLs used were bi-weekly replacement hydrogel (Acuvue IITM) and bi-weekly

replacement silicone hydrogel (Acuvue OasysTM). The properties of the contact lenses are

reported in Table 1. Three different spherical powers for each material were used: +2.00D,

-2.00D, -6.00D. We felt this power range reflected that most commonly seen in contact lens

practice.

2.3 Procedure

To evaluate the effect of power and material on the measurement of IOP, a repeated

measurements design was used. In the right eye of each subject, eight measures of IOP

3

were taken. The first (RT1) and the last (RT8) were taken without CLs. The measurements

from the second to seventh were performed over the different materials and powers of the

CLs described above. It has been documented that repeated applanation of the cornea can

affect IOP results [44]. However this has not been demonstrated with the rebound technique.

In order to prevent a possible effect on IOP of the repetition of the measurement or the

insertion and removal of CLs [45], each subject was assigned randomly to one of 12 different

sequences alternated for material and power (Table 2). As ocular accomodation can

influence the value of IOP during measurements [46-47] the left eye (corrected with CLs for

any hyperopic defect) viewed a distance target (6/24) in order to control accommodation.

One investigator assigned each subject randomly to one experimental condition (i.e. different

order of the lenses used from the second to seventh measurements) and fitted all CLs to

each subject. A second investigator, experienced in rebound tonometry, performed all IOP

measurements on each subject for all conditions in order to reduce between–observer bias.

He was blind to which kind of CLs had been fitted. Rebound tonometry was undertaken in

the usual manner as recommended by the manufacturer. For each reading, the instrument

takes six measurements. Two readings were obtained and averaged.

It has been demonstrated that the location of the tonometer on the cornea can affect the

measurement of IOP [48-49], therefore a third investigator checked the position of the

rebound tonometer probe on the cornea during the measurement. If the position was

incorrect, the measurement was rejected. After the measurement the third investigator read

the measure on the display of the tonometer. Measurements of IOP that the instrument

indicated were unreliable were discarded. Thus the measurements were repeated up to the

moment the third investigator had two valid readings. The number of measures required to

achieve two valid readings was recorded. To reduce between-observer and fitter bias, the

three investigators remained the same for the entire experiment. There was an interval of

five minutes between each repeated measure. A new disposable probe was used for each

4

subject. All the measurements were taken between 1.00 and 3.00 pm in order to minimise

the effect of diurnal variation of IOP on the results.

2.4 Analysis

Data were analyzed using STATISTICA (StatSoft Inc., Tulsa, OK, USA) V.6.0 for Windows.

The Kolmogorov-Smirnov test was used to evaluate the results for a normal distribution of

IOP data. All the statistical processing used to analyze the comparison between the

measurements with and without CLs have been performed using a value for the latter that

was the mean of the first (RT1) and last measurements (RT8). The strength of the

relationship between IOP measurements without CLs and with every different kind of CL was

evaluated using a correlation analysis (r of Pearson). A Bland Altman plot was used to

assess the difference in IOP reading without and with every kind of CL as function of IOP

value. A two-way ANOVA for repeated measurement was applied in order to evaluate the

effect of material and power on IOP measurement. Differences between the measurements

obtained without CLs and with each CL were evaluated using a Student’s paired t test.

3. Results

Mean corneal astigmatism of the subjects was -1.00D with a standard deviation of 0.6 (range

-0.1/-2.50). Twenty-nine right eyes had with the rule astigmatism (steepest corneal meridian

90° ± 20°), one had against the rule astigmatism (steepest corneal meridian 180°± 20°) and

six had oblique astigmatism (steepest meridian between 21° and 69° or 111° and 159°). The

results of IOP measurement without CLs and with every material and power of CL are

reported in Table 3. Every single distribution of the measurements obtained in the several

conditions was normal (Kolmogorov-Smirnov test).

The correlations between IOP measurements without CLs and with every different kind of CL

is reported in Table 4. The Bland Altman plots show that no significant trend has been

detected for differences between the two measures of IOP (with and without CLs) as a

function of their mean value (Figures 1-6). A two-way ANOVA for repeated factors (material

5

with 2 levels: hydrogel and silicone hydrogel; power with 3 levels: +2.00D, -2.00D and -

6.00D) was carried out both on the absolute value of IOP (Figure 7) and on the differences

between the IOP measured with and without CLs (Figure 8). The findings show a significant

effect of material and power but no interaction between the two factors (Table 5)

Table 6 reports the paired-samples t-test between the measurements with and without CLs.

All the comparisons between the measurements without CLs and the measurements with

the hydrogel CLs were significant. The comparisons with the silicone hydrogel CLs were not

significant. Figure 9 gives the number of measures performed in order to have two valid

readings as a function of several measuring conditions. A one-way ANOVA didn’t show an

effect of the conditions (F=1.17; p=0.32). However a paired comparison performed by a

Student t test showed a significant difference between RT1 and -6H (p<0.01) and RT8 and

-6H (p=0.05).

4. Discussion

The measurement of IOP is an indispensible and mandatory part of any primary care eye

examination. In the setting of a contact lens aftercare examination, it may be helpful if

patients could continue to wear their soft CLs while IOP was being measured. It has been

extensively shown that different types of tonometry can be accurately performed over soft

hydrogel or silicone hydrogel CLs [20-33]. However the accuracy of the tonometric

procedure is lens power, thickness and material dependent. In applanation tonometry higher

IOP values are obtained for positive, thick or silicone hydrogel CLs [22, 28-33, 39]. These

effects are analogous to those due to corneal properties such as a steeper curvature [50], an

increase in thickness [51] and rigidity [52].

The ICare rebound tonometer is a relatively new development that is reliable, reasonably

accurate, giving a reproducible measurement of IOP and requires neither a local anaesthetic

nor fluorescein. Furthermore it can be used in challenging patients such as children or the

disabled and it is suitable for domiciliary screening.

6

Although we have no knowledge of any study regarding rebound tonometry through a

contact lens, the manufacturer of the ICare tonometer states that it is possible if the CLs are

daily disposable or some soft two-week or monthly lenses.

In our study, we have shown a dependence of IOP measurement on CL material and power,

as previously demonstrated for the non contact tonometer [29, 32-33]. However our results

indicate a different trend in that the underestimation of IOP is greater for positive power

rather than negative power as shown by several authors [29, 32-33]. This is particularly so

for +2.00D hydrogel CLs.

The tonometry readings found after positive soft CL fitting are unexpected. When the

tonometry measurement is taken using a conventional applanation tonometer, especially the

air puff type, positive soft CLs contribute to an increase in the value of IOP [22, 29-33, 39,

53]. In this study we found that IOP taken using a rebound tonometer is lower when a

positive soft CL is fitted. This is true for both hydrogel and silicone hydrogel lenses. In the

latter case, the difference is not statistically significant.

The reason for the inversion of the results trend is not clear. We try to speculate a possible

reason considering that the tonometry result can be influenced by central corneal resistance

[54]. Corneal resistance is influenced by thickness [51; 55] corneal curvature [56] and

corneal biomechanical factors [57]. True IOP will be overestimated in eyes with thick

corneas, a steep corneal curvature and high corneal hysteresis.

When a soft contact lens is fitted the “new” body composed of cornea and contact lens has

a greater central thickness than the cornea alone, a possible different external surface

curvature depending on the CL power and, presumably, different biomechanical

characteristics depending on the lens material mechanical property as Young modulus.

The influence on IOP of thickness and possibly Young modulus of the cornea can be

minimised by new tonometers based on new principles such as the dynamic contour

tonometer [58] and rebound tonometer. Regarding the rebound tonometer, Chui et al (2008)

[59] have found that the result is affected by biomechanical corneal properties but not

corneal thickness. Jorge et al (2008) [60] found that although corneal thickness can play a

7

role in rebound tonometry, individual physiological variations in corneal material properties

such as the elastic and viscoelastic response, may be a more important determinant of the

corneal structural response.

Considering the relevant differences between conventional tonometers and the rebound

tonometers we suggest a possible explanation for the different trend in the results found

using the rebound tonometer. The geometrical variations (increase in central thickness and

a steeper curvature) induced by fitting a positive soft contact lens have a consistently less

effect on the rebound tonometry results while the possible decrease in the value of Young

modulus of the cornea-CL new body could justify the decrease of the tonometric value

versus tonometric values found without a CL.

To elucidate the potential effect on biomechanical properties of the cornea when a positive

soft contact lens is fitted, further investigations should be carried out measuring IOP with

CLs of powers greater than +2.00D and corneal hysteresis in these situations.

Our results suggest that silicone hydrogel CLs have a minimal influence on the

measurement of IOP compared to that without CLs. The differences in IOP are statistically

significant for hydrogel CLs. They are lower than IOP without CLs for all powers.

Furthermore the measurement of IOP by rebound tonometry over CLs does not affect the

quality of the readings or their variability. In fact the number of measures performed in order

to have two valid readings is no different to the normal procedure without CLs.

5. Conclusion

Rebound tonometry can be reliably performed over silicone hydrogel CLs. With hydrogel

CLs, the measurements were lower than those without CLs. However, despite the fact that

these differences were statistically significant, their clinical significance was minimal.

Acknowledgements

8

We wish to thank Johnson & Johnson Vision Care for providing the CLs used in the

experiment, Alfa Intes (the Italian distributor of ICare) for providing disposable probes and

Dr. Donato Errico for some helpful suggestions.

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1

Material Etafilcon A Senofilcon A

BOZR (mm) 8.30 8.40

TD (mm) 14.0 14.0

Modulus (MPa) 0.26 0.72

Dk/t (×10-9) 40 147

Water Content (%) 58 38

Central Thickness (-3.00 D) (mm)

0.084 0.07

FDA Group IV I

Table 1: Properties of the CLs used in the study.

1 2 3 4 5 6 7 8 9 10 11 12

RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1 RT1

+2H +2H -2H -2H -6H -6H +2SH +2SH -2SH -2SH -6SH -6SH

-2H -6H +2H -6H -2H +2H -2SH -6SH +2SH -6SH -2SH +2SH

-6H -2H -6H +2H +2H -2H -6SH -2SH -6SH +2SH +2SH -2SH

+2SH +2SH -2SH -2SH -6SH -6SH +2H +2H -2H -2H -6H -6H

-2SH -6SH +2SH -6SH -2SH +2SH -2H -6H +2H -6H -2H +2H

-6SH -2SH -6SH +2SH +2SH -2SH -6H -2H -6H +2H +2H -2H

RT8 RT8 RT8 RT8 RT8 RT8 RT8 RT8 RT8 RT8 RT8 RT8

Table 2: The 12 sequences of measurements performed on the right eye. (RT1: first measurement without CL. +2H: measurement with hydrogel +2.00D. -2H: measurement with hydrogel -2.00D. -6H: measurement with hydrogel -6.00D. +2SH: measurement with silicone hydrogel +2.00D. -2SH: measurement with silicone hydrogel -2.00D. -6SH: measurement with silicone hydrogel -6.00D. RT8: second measurement without CL)

Table(s)

2

Condition Mean IOP (mm/Hg)

SD Range

Hydrogel

+2.00 D 13.9 3.5 8.0-22.0

-2.00 D 14.3 3.4 8.0-22.0

-6.00 D 14.9 3.7 9.5-23.5

Silicone Hydrogel

+2.00 D 15.3 3.4 9.0-25.0

-2.00 D 15.6 3.8 9.5-27.5

-6.00 D 16.4 3.9 10.5-28.5

Without CLs (Mean RT1-RT8) 15.8 3.8 9.3-24.5

Table 3: Descriptive statistics of IOP measured with and without CLs.

+2H -2H -6H +2SH -2SH -6SH Without CLs

(Mean RT1-RT8) +2H 1 0.84 0.87 0.82 0.86 0.73 0.82

-2H 0.84 1 0.89 0.84 0.85 0.72 0.87

-6H 0.87 0.89 1 0.8 0.86 0.78 0.86

+2SH 0.82 0.84 0.8 1 0.87 0.79 0.87

-2SH 0.86 0.85 0.86 0.87 1 0.89 0.89

-6SH 0.73 0.72 0.78 0.79 0.89 1 0.80

Without CLs (Mean RT1-RT8) 0.82 0.87 0.86 0.87 0.89 0.80 1

Table 4: Correlation Matrix between measurements of IOP. All correlations are significant (p<0.05). (RT1: first measurement without CL. +2H: measurement with hydrogel +2.00D. -2H: measurement with hydrogel -2.00D. -6H: measurement with hydrogel -6.00D. +2SH: measurement with silicone hydrogel +2.00D. -2SH: measurement with silicone hydrogel -2.00D. -6SH: measurement with silicone hydrogel -6.00D. RT8: second measurement without CL)

3

IOP Difference between IOP

with and without CLs

F p F P

Between Materials 26.34 <0.0000 26.34 <0.0001

Between Powers 10.22 <0.0001 10.22 <0.001

Interactions 0.09 0.91 0.08 0.91

Tab.5: Results of 2-way analysis of variance (ANOVA) carried out on the absolute value of IOP and on the differences

between the IOP measured with and without CLs. The F and p values are shown.

Material and power of CLs

Without CLs (Mean RT1-RT8)

t p

Hydrogel

+2.00 D 5.44 <0.000

-2.00 D 5.02 <0.000

-6.00 D 2.74 <0.01

Silicone

Hydrogel

+2.00 D 1.68 0.10

-2.00 D 0.78 0.44

-6.00 D -1.47 0.15

Tab.6: Paired comparison between the measurements in the different conditions with and without CLs.

1

Fig 1: Bland Altman plot between IOP with +2.00 hydrogel CL (+2H) and without CL (RT) (r=0.09; n.s.). The outer dashed lines

represent the 95% confident intervals for the differences.

Figure(s)

2

Fig 2: Bland Altman plot between IOP with -2.00 hydrogel CL (-2H) and without CL (RT) (r=0.17; n.s.). The outer dashed lines

represent the 95% confident intervals for the differences.

3

Fig 3: Bland Altman plot between IOP with -6.00 hydrogel CL (-6H) and without CL (RT) (r=0.01; n.s.). The outer dashed lines

represent the 95% confident intervals for the differences.

4

Fig 4: Bland Altman plot between IOP with +2.00 silicone hydrogel CL (+2SH) and without CL (RT) (r=0.16; n.s.). The outer

dashed lines represent the 95% confident intervals for the differences.

5

Fig 5: Bland Altman plot between IOP with -2.00 silicone hydrogel CL (-2SH) and without CL (RT) (r=-0.08; n.s.). The outer

dashed lines represent the 95% confident intervals for the differences.

6

Fig 6: Bland Altman plot between IOP with -6.00 silicone hydrogel CL (-6SH) and without CL (RT) (r=-0.12; n.s.). The outer

dashed lines represent the 95% confident intervals for the differences.

7

Fig.7: IOP measured over CLs for hydrogel (solid line) and silicone hydrogel (dotted line).

8

Fig.8 The difference in IOP measured over CLs and without CLs for hydrogel (solid line) and silicone hydrogel (dotted line).

9

Fig.9 Number of trials needed to obtain 2 good readings as a function of several measuring condition. RT1: first measurement without CL. +2H:measurement with hydrogel +2.00D. -2H:measurement with hydrogel -2.00D. -6H:measurement with hydrogel

-6.00D. +2SH:measurement with silicone hydrogel +2.00D. -2SH:measurement with silicone hydrogel -2.00D. -6SH:measurement with silicone hydrogel -6.00D.RT8: second measurement without CL)