The Accuracy of Dynamic Contour Tonometry
Over Soft Contact Lenses
Fabrice Gogniat*, Daniela Steinegger*, Daniela S. Nosch†, Roland Joos‡, and Michael Goldschmidt§
ABSTRACT
Purpose. Dynamic contour tonometry (DCT) has been shown to measure the intraocular pressure (IOP) independently of
corneal thickness. This study aimed to investigate if DCT remains accurate when the IOP measurement is taken over soft
contact lenses (CLs) of different thicknesses and material characteristics.
Methods. This was a prospective clinical study that included 42 patients. Subject age was 22 to 59 years (26.5 T 6.3 years).
Intraocular pressure and ocular pulse amplitude (OPA) measurements were taken under topical anesthesia without CLs and
over various daily disposable CLs with j0.50, +5.00, and j5.00 diopters (D) in hydrogel (Nelfilcon A) and in silicone
hydrogel (Narafilcon A) materials.
Results. No statistically significant differences were found when comparing the IOP measurements obtained using either of
the different CL powers ofj0.50 orj5.00 D, irrespective of which CL material was being used. However, the difference of
0.62mmHg observed when theNelfilcon Awith a power of +5.00Dwas used turned out to be highly statistically significant
(p = 0.0002), whereas the Narafilcon Awith the same power of +5.00 D, with a small difference ofj0.16 mm Hg, was not.
Regarding OPA measurements, no significant differences were found between measurements with and without CL neither
for different materials nor for change in dioptrical power (F = 0, p = 1.000).
Conclusions. This study showed good reliability of IOP and OPA measurements over CLs with varying thickness profiles
and different soft materials when using theDCT.Only a small but statistically significant difference of 0.62mmHgwas found
for the IOP measurement with the hydrogel CL of +5.00 D compared with ‘‘no CL.’’
(Optom Vis Sci 2013;90:125Y130)
Key Words: dynamic contour tonometer, cornea, intraocular pressure, ocular pulse amplitude, topical anesthesia,
hydrogel, silicone hydrogel, contact lens
Goldmann applanation tonometry has long been consid-ered to represent the gold standard for intraocular pres-sure (IOP) measurement.1,2 Its underlying principle is to
estimate the necessary force to flatten the central corneal area within
a diameter of 3.06 mm. Unfortunately, this makes IOP measure-
ments dependent on corneal properties, such as thickness, rigidity,
curvature, and axial length.3Y6 It is calibrated for a central corneal
thickness of 520 Km and hence overestimates the IOP of eyes with
thicker corneas and underestimates the IOP of eyes with thinner
corneas.5 Furthermore, there are practical challenges during IOP
measurement when using traditional Goldmann applanation to-
nometry, such as aligning the two prism mires accurately.
The dynamic contour tonometer ([DCT] Ziemer Ophthalmic
Systems, Port, Switzerland) is believed to be largely independent
of corneal properties. Boehm et al.7 compared DCT readings with
intracameral IOP measurements and found the DCT measure-
ments to be accurate.
In certain clinical situations, it may be advantageous or indeed
necessary to measure the IOP over a contact lens (CL) in situ.
Obviously, the use of anesthetic drops can be avoided. It is also
helpful in cases where the patient is wearing a bandage CL because
lens removal may impair corneal epithelial healing. For eyes with
corneal pathology such as bullous keratopathy, abrasions, and
ulcers, a lens can serve as a buffer and hence protect the cornea
during the IOP measurement. In some countries (such as those of
continental Europe), optometrists are not registered to use diag-
nostic drugs.
Several studies have already shown the IOP measurements to be
accurate over soft CLs using various other tonometry methods,
and they have been discussed previously.8 Using the DCT, IOP
measurements have been shown to remain accurate if they were
1040-5488/13/9002-0125/0 VOL. 90, NO. 2, PP. 125Y130
OPTOMETRY AND VISION SCIENCE
Copyright * 2013 American Academy of Optometry
ORIGINAL ARTICLE
Optometry and Vision Science, Vol. 90, No. 2, February 2013
*BSc
†MSc, MCOptom
‡PhD
§OD
University of Applied Sciences Northwest of Switzerland, Institute of Op-
tometry, Olten, Switzerland.
Copyright © American Academy of Optometry. Unauthorized reproduction of this article is prohibited.
carried out over a thin daily disposable hydrogel CL (j0.50 di-
opters [D]; Filcon IV) on normal corneas with regular topo-
graphic maps.8 Because the DCT takes a different approach to
measure IOP, it was considered interesting to explore the effect
of the CL thickness profile and its rigidity on the accuracy of
its measurements.
The aim of this study was to determine the reliability of the
DCT over soft daily disposable CLs of different dioptric powers
and different soft materials (hydrogel and silicone hydrogel).
The Dynamic Contour Tonometer
The underlying principles of the DCT have been previously
explained elsewhere.8,9 Briefly, the DCT has a specifically designed
tip with a concave contact surface that matches the contour of the
cornea and hence allows the cornea to assume the shape that it
naturally assumes when the IOP is equal to the transverse pressure
applied to its anterior surface and its distortion is minimal. A digital
pressure sensor integrated in the DCT contoured surface allows a
direct IOP measurement across the cornea. Correct positioning of
the tip is indicated by an audible signal that changes in pitch with
detected variations in pressures. The tip generates an electric sig-
nal that is proportional to the IOP level. Rhythmic oscillations
corresponding to the pressure signal are detected during 6 to 10
heartbeats. The IOP is calculated automatically and displayed on a
digital liquid crystal screen. The quality score of the measurements
ranges from excellent (score of 1) to poor (score of 4 or 5) and is
simultaneously displayed on the screen.
The main advantage of the DCT is its ability to measure IOP
independently of physiologic variables such as corneal thickness,
corneal radius, astigmatism, and corneal rigidity. Hence, IOP does
not need to be corrected for particularly thin or thick corneas.
The Ocular Pulse Amplitude
The DCT also measures the ocular pulse amplitude (OPA). Its
value is expressed in millimeters mercury. The mean value for a
normal OPA value is 3 mm Hg, with a mean difference of 0.4 mm
Hg between the two eyes. It represents the difference between the
mean systolic IOP and mean diastolic IOP.10 The pulsatile
character of the IOP is thought to be caused by the blood volume
that is pumped into the eye with each cardiac cycle. In this way,
these pulsations might reflect the ocular blood flow.11Y13 During
the past decades, evidence has grown that vascular factors also
contribute to the pathogenesis of glaucoma, and hence, the
measurement of OPA may help in monitoring the clinical course
of glaucoma.11,13Y18
METHODS
This was a prospective clinical study, which included 42 white
patients (21 right and 21 left eyes), of which 26 were female. The
age varied from 22 to 59 years (mean, 26.5 T 6.3 years).
The inclusion criteria were corneas with a regular topographical
shape (i.e., no degeneration or dystrophy affecting corneal thickness
or shape) and no presence of ocular pathology (assessed by slit
lamp examination). All participants were volunteers from the
University of Applied Sciences in Olten. They signed an informed
consent, which also included a comprehensive explanation of
possible adverse events of topical anesthesia. Two clinicians were
taking measurements (D.S. and F.G.). To exclude any effect of
patient anxiety resulting from the first IOP measurement, three
additional IOP measurements were carried out with a noncontact
tonometer (Nidek NT-530) and two additional DCT measure-
ments were carried out beforehand that were not included for
analysis. The following different dioptric powers for the CLs were
used: j0.50, +5.00, and j5.00 D. Two different soft materials
were used: one hydrogel (Nelfilcon A, CIBA VISION/Alcon) and
one silicone hydrogel (Narafilcon A, Johnson, Johnson & Vision
Care). One published study showed a small effect of topical an-
esthesia on IOP.19 It was therefore decided to cancel out any
possible effects this may produce by applying the topical anesthetic
oxybuprocaine 0.4% SDU Faure every 15 minutes during the
duration of the experiment. This means that if there was a bias
caused by anesthesia, it would have been present in all measure-
ments and therefore can be neglected as far as measurements with
and without CLs are compared.
In the event of adverse reactions, the trial would have been
aborted and the patient would have been sent to the nearby eye
hospital. All subjects enrolled for this study completed their study
participation and could be included for statistical analysis.
One drop of oxybuprocaine 0.4% SDUFaure was instilled in the
examined eye. After the three preliminary noncontact tonometer
measurements and the two preliminary DCT measurements, IOP
and OPA measurements were carried out with ‘‘no CL’’ and six
different CLs as specified above. The order of all measurements, the
choice of right or left eye, and the examiner (F.G. or D.S.) were
randomized and balanced. Onlymeasurements of high-quality level
of 1 or 2 (out of 5 levels) were included for analysis. If an inferior
quality appeared, the measurement was repeated until the necessary
quality was achieved. This study was conducted in accordance with
the ethical commission of Aargau, Switzerland.
During the course of a pilot study in preparation of this clinical
study, interobserver reliability between the two examiners F.G. and
D.S. was measured. This was done by looking at Bland-Altman
plots and estimating the confidence limits of the measurements on
14 eyes. No clinically significant difference, neither for IOP nor for
OPA, could be found. The IOP and OPA measurements could
hence be considered to be independent of the examiners.
Statistical Analysis
All statistical analysis was carried out using the statistical soft-
ware R.20 A power calculation was applied to detect a difference of
0.75 mm Hg, with an SD of 1.11, > = 0.05 and 1-A = 0.8, which
determined a minimum number of 35 subjects. From the pilot
study mentioned before, the SD was estimated with a confidence
interval of 1.03 to 1.18 mm Hg and a mean of 1.11 mm Hg. The
value 0.75 mm Hg was adopted as a clinically significant differ-
ence because this amount represents one-half of the value of the
SD of DCT. For these conditions, a sample size of 32, 35, and 40,
respectively, was calculated. With respect to this and for a better
balanced statistical analysis, it was decided to enroll 42 subjects for
this clinical study. A linear mixed-effects model was used, which
has been shown to be appropriate for this kind of correlated
data.21 In the linear mixed-effects models, either IOP or OPA was
126 Dynamic Contour Tonometry Over Soft Contact LensesVGogniat et al.
Optometry and Vision Science, Vol. 90, No. 2, February 2013
Copyright © American Academy of Optometry. Unauthorized reproduction of this article is prohibited.
the dependent variable. Candidates for fixed factors in the model-
building process were a combination of lens power and lens type
called ‘‘power lens type’’ (PLT), a factor with seven levels (i.e., ‘‘no
CL,’’ Nelfilcon A,j5.00D,j0.50D, and +5.00 D;Narafilcon A,
j5.00 D, j0.50 D, and +5.00 D), sex, age, investigator, and test
sequence (14 different test sequences had been defined to get a
balanced design).
Test subjects were treated as random factors. The model-
building process was started by including all measured possible
factors. Model reduction was aided by applying StepAIC from
package MASS, by which nonsignificant variables could be elimi-
nated one after another.22 Each factor eliminated was further shown
to be not significant by an appropriate F test. Diagnostic plots and
a Shapiro-Wilk test were applied to examine if the measurement
error distribution showed normal distribution.
As a result of this procedure, an ordinary regression table was
obtained, showing which factors are statistically significant. The
model’s covariance matrix was used to estimate intervals of con-
fidence; where appropriate Bonferroni correction for multiple
comparisons was applied.
RESULTS
The results obtained for IOP andOPAwere compared in relation
to the different dioptric CL powers (j0.50,j5.00, and +5.00 D)
and the two soft CL materials (Nelfilcon A and Narafilcon A) used
for this study, summarized by the factor PLT. Table 1 shows the
mean values and SDs obtained for IOP and OPA for the overall
group of subjects, and Table 2 shows the mean values for individual
material and power combinations. In this study, all mean OPA
values were lowerwithout andwith variousCLmaterials and powers
tested than the 3 mm Hg one would normally expect.
To keep the presentation of results concise, only one linear fixed
model is being discussed in more detail. First, the reduction of
nonsignificant factors leads to a model where only one fixed effect
term remained, that is, the PLT. The analysis of variance table of
this model shows that besides the constant term, this factor was
highly significant (F540 = 6.0; p G 0.0001). From this, it could be
concluded that thematerial and power combination was statistically
significant. In the regression table for this model (Table 3), the
combination of ‘‘Power 0 D’’ and ‘‘no CL’’ serves as a baseline and
is represented in the ‘‘Intercept’’ term. Further differences for other
PLTs with respect to this reference value are listed in Table 3, along
with SEs, number of degrees of freedom, and p value. As this table
contains multiple comparisons, the significance level was corrected
by the method of Bonferroni; as there are six comparisons, sig-
nificance level was lowered to 0.05/6 = 0.00833 (Table 4).
The Effect of CL Power and Material on IOP
Figs. 1 and 2 show that the IOP values obtained with different
lens powers and materials were very similar. Table 3 shows that no
statistically significant differences were found when comparing the
IOP measurements obtained using either of the different CL
powers ofj0.50 andj5.00 D, irrespective of which CL material
was being used. However, the difference of 0.62 mm Hg observed
when the Nelfilcon A with a power of +5.00 D was used turned
out to be highly statistically significant (p = 0.0002), whereas the
Narafilcon A with the same power of +5.00 D with a small dif-
ference of j0.16 mm Hg was not.
The Effect of CL Power and Material on OPA
Fig. 3 shows very little difference for any OPA measurements
for neither lens power nor CL material. The OPA values were not
influenced by any CL power or material used in this study: no
TABLE 1.
Means and SDs for IOP and OPA for the overall group without CL
Mean value and SD, mm Hg
IOP 15.08 T 2.10
OPA 2.05 T 0.69
TABLE 2.
Means and SDs for the IOP and OPA for different materials and powers
CL material
Power,
D
Mean IOP and SD,
mm Hg
Mean OPA and SD,
mm Hg
None 0.00 15.08 T 2.10 2.05 T 0.69
Nelfilcon A j5.00 14.91 T 1.77 2.05 T 0.68
j0.50 15.20 T 2.11 1.99 T 0.77
+5.00 15.71 T 2.08 2.04 T 0.67
Narafilcon A j5.00 14.95 T 2.22 2.00 T 0.72
j0.50 14.98 T 1.92 2.01 T 0.69
+5.00 14.93 T 2.18 2.03 T 0.69
TABLE 3.
Regression table for the linear mixed-effects model
Term
Value,
mm Hg
SE, mm
Hg df t p
Intercept 15.08 0.31 540 48.97 0.0000
Nelfilcon A, j0.50 D 0.11 0.17 540 0.67 0.5030
Nelfilcon A, j5.00 D j0.18 0.17 540 j1.06 0.2886
Nelfilcon A, +5.00 D 0.62 0.17 540 3.73 0.0002
Narafilcon A, j0.50 D j0.11 0.17 540 j0.63 0.5260
Narafilcon A, j5.00 D j0.14 0.17 540 j0.83 0.4086
Narafilcon A, +5.00 D j0.16 0.17 540 j0.95 0.3434
TABLE 4.
Confidence intervals for various lens type and lens power combinations
Comparison
Lower,
mm Hg
Estimate,
mm Hg
Upper,
mm Hg
No CL vs. Nelfilcon A,j0.50 D j0.33 0.11 0.55
No CL vs. Nelfilcon A,j5.00 D j0.62 j0.18 0.26
No CL vs. Nelfilcon A, +5.00 D 0.18 0.62 1.06
NoCLvs.NarafilconA,j0.50D j0.55 j0.11 0.33
NoCLvs.NarafilconA,j5.00D j0.58 j0.14 0.30
NoCL vs. Narafilcon A, +5.00D j0.60 j0.16 0.28
Confidence levels are 1-0.05/6, accounting for multiple com-
parisons (Bonferroni).
Dynamic Contour Tonometry Over Soft Contact LensesVGogniat et al. 127
Optometry and Vision Science, Vol. 90, No. 2, February 2013
Copyright © American Academy of Optometry. Unauthorized reproduction of this article is prohibited.
FIGURE 1.
Box plots representing IOP measurements for each lens power.
FIGURE 2.
Box plots representing IOP measurements dependent on lens power and CL material.
128 Dynamic Contour Tonometry Over Soft Contact LensesVGogniat et al.
Optometry and Vision Science, Vol. 90, No. 2, February 2013
Copyright © American Academy of Optometry. Unauthorized reproduction of this article is prohibited.
significant dependency of the OPA from the lens-type power-
variable PLT (F = 0, p = 1.000) was obtained by the linear mixed-
effects model.
DISCUSSION
For this study, the reliability of the DCT was evaluated when
IOP and OPA measurements were carried out over CLs with
various thickness profiles (different dioptric powers) and different
soft materials (hydrogel and silicone hydrogel).
Our results showed that the reliability of IOP and OPA mea-
surements over CLs with varying thickness profiles and different
soft materials remained good. Only a small but statistically sig-
nificant difference was found for the IOP measurement with the
hydrogel CL of +5.00 D (Nelfilcon A). One possible explanation
for this finding could be that the silicone hydrogel material may
have provided the DCT sensor tip a more stable surface for its
measurement because it has a higher elasticity modulus than the
hydrogel material and consequently is stiffer. It is also possible that
the hydrogel CL of +5.00 D had in fact a higher center thickness
than its silicone hydrogel counterpart, which could have given rise
to a higher IOP value in itself. However, this would also mean that
the DCT’s ability to measure independently of thickness would
have to be questioned. Clearly, further studies are required to
investigate this issue in more detail.
To cancel out any possible additional effects the topical anes-
thesia itself could have had on any IOP or OPA results, the in-
stillation of oxybuprocaine was repeated every 15 minutes for the
period during which DCT measurements were carried out for the
purpose of this study. This means that if there was a bias caused by
anesthesia, it would have been present in all measurements and
therefore can be neglected as far as measurements with and
without CLs are compared.
Interestingly, the mean values for OPA obtained in this study
were lower than the expected 3 mm Hg, irrespective of the CL
material or power or if a CL was worn at all. There has been some
suggestion for a negative correlation between OPA and axial
length,23Y25 but unfortunately, the refractive status was not re-
corded for the subjects participating in this study. Hence, this
finding could unfortunately not be further explored.
CONCLUSIONS
This study showed good reliability of IOP and OPA mea-
surements over CLs with varying thickness profiles and different
soft materials when using the DCT. No statistically significant
difference was found for the IOP measurement when using daily
disposable silicone hydrogel CLs of either power used in this
study. A small but statistically significant difference of 0.62 mm
Hg (p = 0.0002) however was noted when using the daily
FIGURE 3.
Box plots representing OPA measurements dependent on lens power and CL material.
Dynamic Contour Tonometry Over Soft Contact LensesVGogniat et al. 129
Optometry and Vision Science, Vol. 90, No. 2, February 2013
Copyright © American Academy of Optometry. Unauthorized reproduction of this article is prohibited.
disposable hydrogel CL of +5.00 D. Nevertheless, this difference
would not be considered to be of clinical relevance. Regarding
OPA, no significant difference was found for any power or mate-
rial of any CL used for this study. In other words, in this study,OPA
measurements were not affected by CL power or CL material.
ACKNOWLEDGMENTS
None of the authors have any financial or personal relationships with the
manufacturer of the DCT used in this study. All authors declare that there is
no conflict of interest.
Received March 16, 2012; accepted October 7, 2012.
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Daniela S. Nosch
Institut fu¨r Optometrie
Fachhochschule Nordwestschweiz
Aarauerstrasse 30
4600 Olten, Schweiz
e-mail: daniela.nosch@fhnw.ch
130 Dynamic Contour Tonometry Over Soft Contact LensesVGogniat et al.
Optometry and Vision Science, Vol. 90, No. 2, February 2013
Copyright © American Academy of Optometry. Unauthorized reproduction of this article is prohibited.