The Influence of Corneal Thickness and Corneal Curvature on Tonometry Readings after Corneal

Refractive Surgery M. KOHLHAAS*, R-CH. LERCHE*, J. DRAEGER*, M. KLEMM*, N. EHLERSt, J. HJORDTALt, H. OLSENt,

C. BARRAQUER:j::, J. I. BARRAQUER:j::, D. FLiCKER:j::, F. RIVERA:j:: and C. CARRIAZO:j::

·Department of Ophthalmology, Hamburg, Germany, tDepartment of Ophthalmology, Aarhus, Denmark and tlnstituto Barraquer, Bogota, Colombia

M. Kohlhaas, R.-Ch. Lerche, J. Draeger, M. Klemm, N. Ehlers, J.lljordtal, H. Olsen, C. Barraquer, J. I. Barraquer, D. Flicker, F. Rivera and C. Carriazo. The Influence of Corneal Thickness and Corneal Curvature on Tonometry Readings after Corneal Refractive Surgery. Eur. J. Implant Ref. Surg., 1995; 7: 84-88.

INTRODUCTION

The intraocular pressure (lOP) is a fundamental parameter of ocular health and disease. Not only is lOP important in the diagnosis and management of glaucomatous conditions, but its assessment is important in the postoperative management of corneal, lenticular and vitreoretinal diseases. In recent decades the most common way to assess lOP is with Goldmann-style applanation tonometry.

Hans Goldmann has given an equation correlat­ing the defined mechanical stability of the intact cornea as a membrane of finite thickness and stiffness, and the liquid between cornea and the applanation surface developing capillary adhesion forces. This is valid for a diameter of the flattened area between 3 and 3.5 mm. In cases of such a defined applanation, the increase of lOP caused by the compressed volume of less than 0.5 III can be neglected [1, 2].

It has been reported that measurements of lOP using the Goldmann applanation tonometer are influenced by corneal thickness and probably cor­neal curvature. Normally hydrated, thicker corneas cause elevated readings, and thinner corneas cause lower readings. The clinical significance of these observations is demonstrated in studies which have shown decreased corneal thickness in some cases of apparent low-tension glaucoma, and increased

Correspondence to: Dr. Markus Kohlhaas, Department of Ophthalmology, MartinstraBe 52, 20246 Hamburg, Germany.

0955-3681/95/020084 + 05 $08.0010 84

corneal thickness in some refractory cases of glau­coma and ocular hypertension [3-5].

Ehlers et ai. [3, 4] reported that an oedematous cornea would result in applanation tonometer read­ings which are much too low. This is significant in that an oedematous cornea would be measured as thick by ultrasonic or specular pachometry, and could lead the clinician to perceive lOP as being lower than its true value. The obvious danger is the under diagnosis of ocular hypertension and glaucoma.

Mark [6] reported that many of the uncertainties in tonometry arise from variable physical parame­ters of the individual globe. One of these factors is the curvature of the cornea to which the tonometer is applied. Theoretically, one would expect that greater force would be needed to flatten a structure with greater curvature. In applanation tonometric terms, this theory suggests that, all else being equal, tonometric readings might be lower in flatter cor­neas, and vice versa.

Draeger investigated the effect of contact lenses of 32 and 80% water content on the accuracy of appla­nation tonometry. He found that a 0 D 80% water content lens produced a reduction in the measured lOP varying between 2 and 2.6 mmHg within the lOP range 12-39.6 mmHg. A + 12 D lens produced an overestimation varying between 3.8 and 8.65 mmHg within the lOP range of 6.2-28.3 mmHg [4].

This must be taken into consideration even more when the intraocular pressure is taken on corneas after corneal refractive microsurgery.

© 1995 W.B. Saunders Company Limited

The Influence of Corneal Thickness and Corneal Curvature on Tonometry Readings 85

Table 1

Eyes Men

Aarhus Group 1 (40 normal probands) 60 26 Group 2 (32 excimer patients) 44 16 Group 3 (36 RK patients) 71 26

Bogota Group 4 (20 eryo-keratomileusis patients) 31 8 Group 5 (50 keratomileusis in situ patients) 91 14

Table 2

Group 2 Group 1 Preoperative Postoperative

Visus (ee) 1.12 (0.3) 0.94 (0.2) 0.92 (0.27) Sphere (D) -2.4 (3.26) --6.77 (2.32) -1.44 (1.87) Cylinder (D) -0.43 (0.49) -0.79 (2.9) -0.84 (0.69) Paehymetry (mm) 0.53 (0.025) 0.52 (0.03) 0.5 (0.034) Axial length (mm) 24.4 (1.6) 25.8 (0.86)

Corneal curvature K 1 (mm) 7.9 (0.23) 7.79 (0.25) 8.57 (0.37) K2(mm) 7.68 (0.22) 7.6 (0.2) 8.29 (0.32)

Tonometry (mmHg) Goldmann 14.0 (2.19) 14.0 (3.1) 14.1 (2.1) Draeger 14.0 (2.02) 14.09 (1.94)

However, after refractive surgery for the correc­tion of myopia, the corneal stiffness might be altered, the superficial corneal curvature is flat­tened and corneal thickness decreases or increases depending on the surgical technique.

Several methods of measuring lOP are accurate in healthy eyes with a normal corneal surface. These include the Schi6tz indentation tonometer, the Gold­mann applanation tonometer, the McKay-Marg tonometer, the Tonopen tonometer, and the non­contact and contact pneumotonometers, all of which have some limitations [8-12]. This study demon­strates the effects of corneal thickness and corneal curvature on Goldmann applanation tonometry in patients after refractive corneal surgery (radial keratotomy, superficial photorefractive keratectomy and keratomileusis) for correcting myopia.

PATIENTS AND METHODS

In combined studies with N. Ehler, Department of Ophthalmology in Aarhus and C. Barraquer, lnsti­tuto Barraquer in Bogota, the lOP was measured with different tonometers (Goldmann applanation tonometer, I)raeger handheld applanation tonom­eter) to investigate if there is any influence of the above mentioned parameters (Table 1).

Eur J Implant Ref Surg, Vol 7, April 1995

Women Age (mean (± S. D.)) Examination (postop)(months)

14 33.27 (8.5) 16 28.73 (6.94) 12.32 (9.94) 10 30.88 (7.02) 32.9 (15.33)

12 28 (10) 8.55 (5.46) 36 28 (7.4) 8.1 (4.7)

Group 3 Groups 4 and 5 Preoperative Postoperative Preoperative Postoperative

1.03 (0.16) 1.08 (0.18) 0.69 (0.53) 0.54 (0.19) -3.13 (0.55) -0.84 (0.99) -10.85 (5.4) 0.05 (3.66) -0.63 (0.55) -0.61 (0.68) -1.69 (1.16) -2.07 (1.22)

0.53 (0.03) 0.53 (0.026) 0.55 (0.034) 0.49 (0.039) 24.75 (0.79)

7.9 (0.25) 8.45 (0.66) 7.92 (0.34) 9.45 (0.92) 7.74 (0.24) 8.23 (0.35) 7.63 (0.31) 9.15 (0.95)

14.0 (2.5) 13.8 (2.56) 14.19 (3.01) 13.8 (2.73) 9.67 (3.16)

Forty probands (group 1) with intact corneas as a control group, 32 patients after excimer ablation (group 2), 36 patients after radial keratotomy (group 3) and 70 patients after keratomileusis in situ (group 4) or cryo-keratomileusis (group 5) were examined (Table 1).

The best-corrected visual acuity was recorded, and the cornea was examined for any abnormalities of the epithelium, stroma, or endothelium. Patients with abnormalities of any layer of the cornea were excluded. Keratometry was then performed using the Zeiss keratometer, the calibration of which was checked before beginning the study. The central corneal thickness was obtained by using the Haag­Streit Depth Measuring Attachment for the Haag­Streit slit lamp. The axial length was measured by A-scan ultrasonography. Before measuring the lOP with the Goldmann applanation tonometer or with the Draeger handheld applanation tonometer the precorneal film was stained with fluorescein solution.

RESULTS

Table 2 demonstrates the results obtained from the above-mentioned 5 groups.

In our control group of normal and intact corneas the lOP readings measured with the Goldmann

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applanation tonometer and the Draeger handheld tonometer are identical in mean value and S.D. This is well known, but a handheld tonometer is in general much more practical for laboratory work than a slit lamp.

The linear regression line demonstrates a slight correlation of an increasing corneal thickness and corneal curvature on the tonometry readings (Fig. 1).

The corneal thickness decreases after photore­fractive keratectomy by about 0.02 mm, the corneal curvature flattens by about 1. 7 mm radius. The lOP readings of the 2 different tonometers are identical compared with the control group.

The corneal thickness in the RK group increases in the centre after surgery by about 0.01 mm. This increased corneal thickness could be explained with a loss of endothelial cells after surgery or with a disturbed or decreased barrier function of Bowmans membrane. The corneal curvature flattens about 1.5 mm radius.

Although the corneal thickness is increased the mean value of the obtained lOP readings measured with applanation tonometry are statistically 0.2 mmHg lower than before the operation. The linear regression lines of the RK group demonstrate only a slight influence of the increased corneal thickness for the applanation readings (Fig. 2).

Only a slight influence of the flattened corneal curvature on applanation tonometry readings could be noticed after excimer ablation (Fig. 3).

20 b:D 18 · · ::c:~ 16 . . . • · S 1:1 14

. . ; -S@ .

12 0 . ~S 10 1:1'1:1 0- 8 .... 0 000 6 Regression line (n = 58) nonnal probands I:I~

4 ~ 2 0 0.46 0.48 0.5 0.52 0.54 0.56 0.58 0.6

Corneal curvature (mm)

Fig. 1 Regression line concerning corneal thickness and actual measured lOP of healthy subjects.

0:; 25 '" IlD

'" '" 20 ... 8 b:D 15 ::c: j 10

1:1 5 .S rfJ 1:1 0 ~ 7.5

•• Regression line (n = 45) RK

. . :. .... .. • ... " .. . . .. . . . • u. o

.... .. o • • ••• - u •

Corneal curvature (mm)

9.5

Fig. 2 Regression line concerning corneal curvature and actual measured lOP of RK-patients.

M. Kohlhaas et 81

With regard to the 70 patients after cryo-kerato­mileusis and keratomileusis in situ the corneal thickness was decreased postoperatively by about 0.06 mm, the minimum thickness was 0.39 mm, the corneal curvature was flattened up to a maximum of 11.8 mm radius (Table 2).

The linear regression line of this group again demonstrates the influence of the corneal thickness on applanation tonometry. The thicker a cornea is the higher the lOP reading will be (Fig. 4). However, applanation tonometry is influenced too by the flatter corneal curvature (Fig. 5).

The preoperative lOP readings taken with the Goldmann tonometer were 14.2 mmHg, the post­operative readings taken with the handheld ton­ometer were significantly lower with only 9.7 mmHg.

The regression coefficients at Table 3 will demon­strate the statistical relation between lOP measure­ments with corneal curvature and corneal thickness.

DISCUSSION

The dimensions of the Goldmann tonometer head were determined not on the basis of the Imbert-Fick law, but on the basis of empirical experimentation by Goldmann. He found that in the eye preparations he

-;:; '" 20 IlD

'" Regression line (n = 45) Excimer '" 18 I-... 3 .. .. • •

16 l- • . • b1 ... • • • •• :0 14 l-S • . . § 12 - .. • · .. 1:1 10- • • S 00 I I I I 1:1 8 ~ 7.5 8.0 8.5 9.0 9.5 :-<

Corneal curvature (mm)

Fig. 3 Regression line concerning corneal curvature and actual measured lOP of excimer patients.

0:;

'" 18 IlD • • '" Regression line '" 16 • • ... 8 14 • . b:D 12 • · . • .. · · . . ] 10 · . • · . · . . 8 • · . · • · . • · E 6 • • • • • 1:1 4 • .S 2 • 00 1:1 0 ~ 0 .. 35 0.4 0.45 0.5 0.55 0.6

Corneal thickness (mm)

Fig. 4 Regression line concerning corneal thickness and actual measured lOP of keratomileusis patients.

Eur J Implant Ref Surg, Vol 7, April 1995

The Influence of Corneal Thickness and Corneal Curvature on Tonometry Readings 87

-;:; '" bll 18 '" Regression line Ol 16 ... •• 8 14

b:o 12 .. . . . 10 • .. .

:I: . . . •• . . . . . e 8 • . .. . .. -5 . . • • .

6 • • . .. . • .: 4 .S 2 • rn .: 0 ~ 7.3 7.8 8.3 8.8 9.3 9.8 10.3 10.8 11.3 11.8

Corneal curvature (mm)

Fig. 5. Regression line concerning corneal curvature and actual measured lOP after keratomileusis of keratomileusis patients.

Table 3 Regression coefficient

Tonometry-corneal thickness

Goldman Draeger

Group 1 0.03 0.028 Group 2 0.007 0.019 Group 3 0.1 0.06 Groups 4 and 5 0.032

Tonometry-corneal curvature

Goldman Draeger

Group 1 0.0045 0.001 Group 2 0.0483 0.028 Group 3 0.0678 0.0629 Groups 4 and 5 0.165

studied, an applanating area of between 3 and 3.5 mm diameter gave the best results because with this area the corneal resistance to indentation was balanced by the capillary attraction of the pre­corneal tear film for the tonometer tip [1, 2].

When solids are wettable and are in contact with a liquid there is an adhesive force between the solids and liquid. This force depends not only on the surface tension of the fluid and the wettability of the solids that the fluid contacts, but also on the curvature of the liquid in contact with the solids.

The greater the surface tension, the greater the capillary attraction between the tonometer tip and the cornea. Hence, a higher than average surface tension of the precorneal film will pull the tonometer head toward the cornea more than expected and cause underestimation of the lOP due to the surface tension pulling the tonometer head toward the cornea [13].

Compared with our control group of normal corneal curvatures almost all applanation tono­metry measurements on patients with flatter cor­neal curvatures showed lower lOP readings as mentioned. Whitacre and Stein reported that in theory, the steeper the corneal curvature, the more

Eur J Implant Ref Surg, Vol 7, April 1995

the cornea must be indented to produce the stan­dard area of contact [13]. Therefore, more force must be applied against a steep cornea than against a flat cornea, increasing the indicated value of the lOP. In addition, when producing the standard area of contact more fluid is displaced from under a steep than flat cornea, which increases the contribution of a high ocular rigidity in overestimating the lOP.

Mark performed applanation tonometry and kera­tometry on 400 eyes and found that the mean lOP had a positive correlation with increasing corneal curvature. It is known that a change of corneal curvature of 1 D could cause a change in measured lOP of 0.34 mmHg and a range of 40-49 D of curvature could be responsible for a 3 mmHg variation in the tonometer readings [6].

Goldmann knew that the cornea possessed a rigidity that opposed indentation, and assumed the surface tension that draws the tonometer tip to the cornea would equal and offset the resistance to indentation offered by the cornea. This assumption, however, is not true for all patients. Studies of the accuracy of applanation tonometry in vivo have disclosed that corneal thickness has a large influ­ence on the accuracy of Goldmann applanation tonometry [14]. Ehlers et aZ. studied the accuracy of applanation tonometry in 29 eyes in vivo and found a statistically significant relationship between cor­neal thickness and the disparity between applana­tion tonometry readings and manometrically con­trolled lOPs. They also found a decline in central corneal thickness with age. A declining corneal thickness with age might render older eyes more susceptible to underestimation of the lOP by appla­nation tonometry [15, 16].

Several studies have noted a relationship between corneal thickness and lOP. Because of the influence of lOP on the corneal swelling pressure, increased lOP will tend to cause corneal thinning and decreased lOP will tend to cause corneal thickening. The influence of these pressure-induced changes on the accuracy of applanation tonometry is unknown [17-19] .

Table 4 Correction table for applanation tonometry

Physiological values lOP correction

Corneal thickness: 0.52 mm

Corneal curvature: 7.8 mm

:!: 1 mmHg per 0.05 mm increase/decrease :!: 1.5 mmHg per 1 mm increase/decrease

Bohnke et aZ. investigated a group of 50 patients with myopic epikeratophakia [20]. Two different applanation diameters were applied. The patients

88

were seated for the first 2 measurements, then positioned horizontally for 5 min, and the applana­tion force measured again with both applanation tonometers. With an applanation diameter of 3.06 mm, readings were obtained in all measurements. With an applanation diameter of 5 mm, the force required could not be obtained in all cases due to a limitation in the scale available. Myopic and emme­tropic eyes serve as controls. Comparing operated and non-operated myopic eyes in a subgroup of 15 patients, a reduction in the applanation force with the 3.06 mm tonometer head was found in the former group. They concluded that in myopic epiker­atophakia, a small systematic error in tonometer readings may occur.

In summary our results can be explained by 3 reasons: (1) the flatter the corneal curvature is, the earlier the desired applanation diameter is reached the more also due to the increased capillary adhesion forces of the tear film. (2) Thin corneas will produce an underestimation and thick corneas will produce an overestimation of the lOP. However, corneal stromal and epithelial oedema will cause the appla­nation tonometers to underestimate the lOP. (3) Mechanical stability of the corneal tissue is the other important detail in Goldmann's hypothesis. Loss of stability again results in an underestimation of pressure readings. Therefore in general tonometry readings are lower after myopic refractive proce­dures compared with base line data collections.

One technical solution to measure the actual lOP might be the use of bigger applanation diameters to consider the influence of decreased corneal thickness and corneal curvature and increased capillary adhe­sion forces of the tearfilm [21, 22]. But this of course needs recalculation of Goldmann's formula, the corre­lation force also needs to be altered. Only then is precise applanation tonometry, in any case of refrac­tive change, possible. Otherwise one has to keep in mind possible misreadings, in general underestima­tion of the IOP. This has to be taken into considera­tion when patients are treated after refractive cor­neal surgery, who can suffer either from steroid induced glaucoma or true glaucoma, both more often found in myopic patients than in others.

For these reasons we suggest a correction table for applanation tonometry on corneas with intact Bow­mans membrane to compensate deviations of corneal curvature and corneal thickness from physiological values. For each increase or decrease of corneal thickness of 0.05 mm from the physiological corneal thickness one may add or substract 1 mmHg. On the other hand, for each increase or decrease of corneal curvature of 1 mm one may add or substract 1.5 mmHg from the obtained IOP (Table 4). These suggestions are not valid for patients after radial

M. Kohlhaas et at

keratotomy and excimer ablation. These surgical techniques destroy Bowmans membrane and desta­bilize the entire structure of the globe which will lead to an altered corneal bending force [23].

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2 H Goldmann, Th Schmidt. Weiterer Beitrag zur Applanation­stonometrie. Ophthalmologica, 1961; 141: 441-456.

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23 KA Buzard. 1992 Introduction to Biomechanics of the cornea. Refract. Corneal. Surg., 1992; 8: 127-138

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