Effect of applanation tonometry on mean corneal curvature

S. Beatty, FRCOphth, K.K. Nischal, FRCOphth, H. Jones, BSc, E.M. Eagling, FRCOphth

ABSTRACT

Purpose: To evaluate whether applanation tonometry affects corneal curvature and if so, the implications for intraocular lens power calculation.

Setting: Birmingham and Midland Eye Hospital, Birmingham, England.

Methods: Twenty-two patients attending the preoperative assessment clinic were enrolled in the study. Keratometry was performed immediately before, 1 minute after, and 10 minutes after standard Goldmann tonometry. Main outcome mea­sures were mean corneal refractive power and its reproducibility (coefficient of repeatability).

Results: No clinically significant difference was noted between preapplanation and postapplanation readings (P = .6), and reproducibility was not significantly af­fected.

Conclusion: The results indicate that corneal applanation before keratometry does not compromise the prediction of postoperative refraction. J Cataract Refract Surg 1996; 22:970-971

A Pplanation tonometry and keratometry are a rou­

tine part of the preoperative assessment for cataract

surgery. Applanation tonometry flattens a central area of

the cornea (diameter = 3.06 mm). This is achieved by

mechanical contact with the tear film and epithelial sur­

face of the cornea that may lead to transient alteration of

the anterior refractive surface of the eye. Keratometry

measures the radius of curvature of the same central area

(diameter = 3.00 mm). The concern over potential al­

terations in corneal curvature has prompted many oph-

thalmologists to conduct their biometric measurements

either before applanation tonometry or on a separate

visit. A difference in corneal refractive power of

0.55 diopters (D) will result in a 0.50 D discrepancy in the calculated intraocular lens (IOL) power when the SRK 111M formula 1 is applied. This emphasizes the im­

portance of accurate and reliable keratometry readings.

Materials and Methods

From the Birmingham and Midland Eye Hospital (Beatty, Nischal, Eagling) and the University of Birmingham Uones), Birmingham, En­gland.

None of the authors has a proprietary interest in any instrument or formula mentioned.

Reprint requests to S. Beatty, FRCOphth, Senior House Officer, Bir­mingham and Midland Eye Hospital, Church Street, Birmingham B3 2NS, England.

This study comprised 22 patients (15 women) with

a mean age of 64.3 years (range 51 to 78 years). No eye

had had previous intraocular surgery or had pathology

other than cataract.

Automated keratometry (Canon IOL Estimator)

was performed on one eye of each patient immediately before (Ka), 1 minute after (Kb), and 10 minutes after

(Kc) standard Goldmann applanation tonometry. In-

970 J CATARACT REFRACT SURC;~VOL 22, SEPTEMBER 1996

EFFECT OF TONOMETRY ON CORNEAL CURVATURE

traocular pressure was recorded after instillation of one

drop of topical anesthetic (benoxinate hydrochloride

0.4%). All measurements were made by one observer

(S.B.). Five readings were taken each time and the mean

calculated.

The differences between the three sets of readings

(Ka - Kb and Ka - Kc) were calculated. Histograms

were plotted to assess the normality of these differences

and the paired Student's t-test used to investigate the

significance of any changes.

Any difference in the reproducibility of the five

readings composing each overall measurement was then

investigated by comparing the coefficients of repeatabil­

ity for Ka, Kb, and Kc. For each set of measurements, the

mean of the five readings was plotted against their stan­

dard deviation (SD). If no relationship was apparent, the

coefficients were calculated as 2 {2 X pooled within­

subject SD.2 These coefficients indicate how much

change

can be expected between two repeat readings as a result

of the test's variability and may be hypothesized to in­

crease (i.e., show less repeatability) immediately after

tonometry.

To compare our results with those of previous in­

vestigators, we also calculated the coefficient of variation

as an index of reproducibility as follows: SD/mean X 100%.

Results Mean corneal refractive power before applanation

was 44.42 D ± 1.90 (SD). One and 10 minutes after standard Goldmann applanation tonometry, it was

44.45 ± 1.96 D and 44.48 ± 1.90 D, respectively. The differences in mean corneal curvature followed

a normal distribution, and there was no relationship between the mean and the SD of the five readings com­posing Ka, Kb, and Kc. The paired Student's t-test was

therefore used.

Mean changes in corneal refractive power recorded

1 and 10 minutes after Goldmann applanation tonom­

etrywere +0.05 D (Ka - Kb) and +0.02 D (Ka - Kc),

respectively; the changes did not reach statistical signif­

icance (P> 2 and P> 3). One and 10 minutes after applanation, the coeffi­

cient of repeatability for mean corneal curvature

changed by -0.10 and +0.05 D, respectively. The co-

efficient of repeatability was 0.77 D for Ka, 0.67 D for

Kb, and 0.82 D for Kc. The coefficients of variability

were 0.39%, 0.35%, and 0.41 %, respectively.

Discussion Routine preoperative biometry has resulted in im­

proved postoperative refraction and uncorrected visual

acuity? However, IOL calculation can be greatly af­

fected by inaccuracies in biometric measurements.4 Sev­

eral factors influence measurements of corneal curvature

including orbital anesthesia, ocular massage, corneal dis­

ease, and interexaminer variability. 5

In our study the mean corneal refractive power was

not altered in a clinically meaningful way 1 and 10 min­

utes after Goldmann applanation. The differences re­

corded would represent a 0.050 D and 0.002 D

discrepancy in postoperative refraction, respectively,

when the SRK II formula is used.

Butcher and O'Brien6 established the reproducibil­

ity of keratometry in a study in which the reproducibil­

ity of five measurements of corneal curvature was given

by the coefficient of variation of 0.57%. Our results

compare favorably with this finding both before and

after Goldmann applanation. Also, the coefficient of re­

peatability, a measure of the amount of change expected

between consecutive readings resulting simply from the test's variability, was adversely affected by only 0.05 D

after Goldmann tonometry.

In conclusion, applanation tonometry before kera­

tometry does not compromise IOL power calculation.

References

l. Sanders DR, Retzlaff JA, Kraff Me. Comparison of the SRK IITM formula and other second generation formulas. J Cataract Refract Surg 1988; 14:136-141

2. Chinn S. The assessment of methods of measurement. Stat Med 1990; 9:351-362

3. Singh M, Dahalan A. Significance of intraocular lens power calculation. Br J Ophthalmol1987; 71:850-853

4. Richards SC, Olson RJ, Richards WL. Factors associated with poor predictability by intraocular lens calculation formulas. Br J Ophthalmol1985; 103:515-518

5. Zadnik K, Mutti DO, Adams A]. The repeatability of measurement of the ocular components. Invest Ophthal­mol Vis Sci 1992; 33:2325-2333

6. Butcher JM, O'Brien e. The reproducibility of biometry and keratometry measurements. Eye 1991; 5:708-711

J CATARACT REFRACT SURG-VOL 22, SEPTEMBER 1996 971