Comparison of Ultrasonic Corneal Thicl<ness Measurements Before and During Surgery irt the Prospective Evaluation Of Radial. Keratotomy {PERK) Study RICHARD A. VILLASENOR, MD, VICKI RICE SANTOS, MA, MS, KERRY C. COX, MD, DELAWARE F. HARRIS, II,. MD, MICHAEL LYNN, MS, GEORGE 0. W ARlNG, III, MD, FACS

Abstract: Central corneal thickness, determined by an ultrasonic pachymeter, was studied in one eye each of 395 patients enrolled in the Prospective Evaluation of Radial Keratotomy (PERK) Study, to determine the changes in central corneal thickness betvveen preoperative and intraoperative measurements. There was no statistical difference between the preoperative and the intraoperative mea­surement. However, 9.4% of the corneas were 0.03 to 0.08 mm thinrier intra­operatively and 3.3% were 0.03 to 0.06 mm thicker intraoperatively, indicating that clinically meaningful changes in corneal thickness occurred between the preoperative and intraoperative measurements in some eyes. We conclude that the most reliable way to use ultrasonic pachymetry readings to set the length of the knife blade for radial keratotomy surgery is to measure the corneal thickness intraoperatively. [Key words: cornea, corneal thickness, myopia, pachymetry, radial keratotomy, refractive keratoplasty.] Ophthalmology 93:327-330, 1986

During radial keratotomy, radial incisions are made in the anterior cornea to induce flattening of the central cor­nea, resulting in a reduction of the refractive power and

the amount of myopia. The proper setting of the blade length is dependent upon precise measurements of the corneal thickness, usually made using an ultrasonic pachymeter. 1•2

From the PERK Coordinating Centers, Emory University, Atlanta.

Presented at an Annual Meeting of the American Academy of Ophthal­mology.

Funded by the National Eye Institute Grant Nos. EY03761, EY03752, and EY03765.

Reprint requests to Richard A Villasenor, 11645 Wilshire Blvd., Suite 620, West Los Angeles, CA 90025.

Ultrasonic measurements of corneal thickness can be taken in an examination room, before surgery, or in the operating room, during surgery. Taking measurements in the examination room before surgery eliminates the need to procure a pachymeter for the operating room, and shortens the time of surgery. When measurements are made during surgery, the surgeon is more certain of the thickness reading, since it is taken immediately before the

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incisions are made. The surgeon can also measure the thickness reliably in. paracentral and peripheral areas, which is more difficult to do in the outpatient clinic. Re7 liable measurements decrease the probability of corneal perforation, while permitting maximum incision depth.

Using the protocol in the Prospective Evaluation of Radial Keratotomy (PERK) Study, 2 the central corneal thickness readings taken with an ultrasonic pachymeter during the preoperative clinical examination were com­pared with those taken intraoperatively with the same pachymeter.

MATERIALS AND METHODS

Details of the clinical methods and patient population of the PERK Study have been previously published,2•3

and we summarize the relevant information here. The central corneal thickness was measured using an Accu­tome ultrasonic pachymeter with a 3 mm diameter probe tip, and the speed of sound was set at 1640 m per second. 4

Measurements were taken at each of the nine clinical cen­ters in the PERK Study. The same ultrasonic pachymeter was used preoperatively and intraoperatively. All mea­surements were taken using topical 0.5% ponticane anes­thesia. The 3 mm tip of the pachymeter was filled with a balanced salt solution and calibrated on a test block. The readings in the outpatient clinic, obtained by the clinical coordinator, were taken in the central cornea over the center of the pupil. Three separate readings were recorded on the data sheet. The average of these three readings was considered to be the preoperative corneal thickness. The intraoperative readings, obtained by the surgeon, were taken over the visual axis mark5 that was also over the pupillary zone. Readings were taken until two identical sequential readings were achieved, and this was recorded as the intraoperative central corneal thickness.

The preoperative and intraoperative measurements may not have been taken in exactly the same location, because the center of the pupil was used to locate the pachymeter probe preoperatively, and the visual axis mark was used intraoperatively. However, the center of the pupil and the intersection of the visual axis mark are very close to each other, certainly within the area covered by the 3 mm diameter of the probe tip. Therefore, similar areas of the cornea were measured preoperatively and intra­operatively. Using the same pachymeter for both mea­surements eliminated a variation in measurement from one machine to another. Calibrating the instruments on a test block insured that the machines were reading prop­erly. Using an ultrasonic pachymeter, rather than an op­tical pachymeter, eliminated errors intrinsic in observer interpretation.

Measurements were taken in 435 eyes, one eye of each patient.3 We excluded 22 eyes for various reasons, in­cluding violations of the PERK protocol. These exclusions are stated in detail in an earlier paper. 3 We excluded mea­surements for 18 patients because 16 had their initial out­patient readings taken with the pachymeter speed of sound set at 1550 mjsec, the setting used early in the study for

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outpatient measurements, but not for the intraoperative measurements; one patient had no preoperative pachy­metry measurement; and one patient had no intraoper­ative central pachymetry measurement recorded. Were­port our findings fcir 395 eyes. These eyeswere similarly distributed among the nine centers, with each center con­tributing between 9% and 12% of the data.

The difference between the preoperative and intra­operative pachymetry measurements was obtained by subtracting the preoperative from the intraoperative value, so that positive changes indicate an increase in corneal thickness, and negative changes represent a decrease in the corneal thickness at the intraoperative reading. A dif­ference of .more than 0.02 mm was considered a true thickness difference, since the accuracy of the pachymeter is ±0.01 mm.

All data were forwarded to the PERK Statistical Co­ordinating Center at Emory University, and were entered and maintained in a computer using the Dictionary Driven Datasystem.6 Statistical analyses were made using the Biomedical Computer Programs, BMDP.7

Summary statistics for the preoperative and intraoper­ative thickness measurements, and the change between the two measurements, are reported. The difference be­tween the two measures was examined statistically, by comparing the regression line relating the preoperative and intraoperative measures with the line of equality.8

The regression equation: I = a + b (P) relates the pre­operative (P) and intraoperative (I) values. If the intercept, a, is 0 and the slope, b, is 1, then I = P, and the intra­operative and preoperative measurements are not statis­tically different. A correlation coefficient was computed to evaluate the relationship between the change in corneal thickness and the amount of elapsed time between the two measurements.

RESULTS

The time between the preoperative and intraoperative measurements ranged from 6 days to 18 months. Ofthese measurements, 113 were taken 6 days to 1 month apart, 113 were taken after 1 month but before 21f2 inonths, and 113 were taken after 21h months up to 18 months.

The average central corneal thickness measured pre­operatively was 0.54 mm (SO = 0.03) with a range of 0.44 to 0.64 mm, and intraoperatively was 0.53 mm (SO = 0.04) with a range of0.41 to 0.65 mm. The preoperative and intraoperative measurements were not statistically different (Fig 1 ). The regression equation relating the pre~ operative (P) and intraoperative (I) measurements was: I = 0.02 + 0.96 P. The intercept, 0.02, was not statistically different from 0 (t = 1.39, P > 0.05) and the slope of the regression equation, 0.96, was not statistically different from 1 (t = -1.71, P > 0.05). The coefficient of deter­mination, R2, was 0.78, and the mean squared error was 0.0003.

However, the distribution of change between the pre­operative and intraoperative measurements (Table 1) in­dicates that the thickness measurements for 12.7% of the

VILLASENOR, et al • CORNEAL THICKNESS MEASUREMENTS

corneas changed by more than 0.02 mm. At the intra­operative reading, 7.1% were thinner by 0.03 to 0.04 mm, 1.5% were thinner by 0.05 to 0.06 mm, and 0.8% were thinner by 0.07 to 0.08 mm. However, a few became thicker, with 2.3% thicker by 0.03 to 0.04 mm, and 1% thicker by 0.05 to 0.06 mm at the intraoperative reading.

Differences in the preoperative and intraoperative measurements were found at all of the nine centers, al­though all centers were not equally represented. There were two to four corneas that changed by more than 0.02 mm from five centers, five to eight corneas from two cen­ters, and 9 to 11 corneas from two centers.

The amount of change between the preoperative and intraoperative measurements was not related to the amount of time that had elapsed between the two mea­surements (correlation coefficient, -0.11; number of eyes, 395). The time interval between the two measurements ranged from 16 days to 12 months for the 50 eyes that changed more than 0.02 mm.

DISCUSSION

We conclude from these data that there was not a clin­ically meaningful difference between the preoperative and intraoperative central corneal thickness measurements for the majority, 87.3%, of the patients in the PERK Study. However, there was a clinically meaningful difference for 12.7% ofthe patients.

We believe our results represent real thickness differ­ences, and are not the result of measurement error, since several precautions were taken to minimize this possibility. Both the clinical coordinator and the surgeon who mea­sured corneal thickness were highly trained and certified, prior to collecting this data for the PERK Study.2•3 Mul­tiple readings were taken, to ensure accuracy. The same pachymeter was used both preoperatively and intraoper­atively, which eliminated machine variation. The pachy­meters were calibrated on a test block, to make sure that the machines read properly. An ultrasonic pachymeter, which has been shown to be highly reliable,9 rather than an optical pachymeter, was used to eliminate errors due to observer interpretation.

We have shown that differences in corneal thickness were not unique for a limited number of the clinical cen­ters, but were found at all nine clinical centers. Thus, the differences cannot be attributed to some unusual practice at one or two centers, but, more likely, represent real dif­ferences that would be encountered among practicing ophthalmologists.

One of the major variables determining the outcome of a radial keratotomy operation is the depth of the in­cision in the cornea. 10 Most surgeons think that a deeper incision gives greater effect, and there is experimental ev­idence to support this belief. 11 The depth of the incision is controlled by many factors, including the thickness of the cornea, the length of the knife blade, the sharpness of the knife blade and its configuration, the intraocular pres­sure during surgery, the surgical technique used to make each cut, and, possibly, by intrinsic factors within the cor-

0.70 0.70 ~~

.tt •'

0.65 ~~·

e 0.65

!. ., ., CD 0.60 0.60 c

.:< .., :<: 1-ii ; : CD 0.55 i

: 0.55 E : 0 i i i ()

CD

~ 0.50 0.50 !!! CD a. 0

~ .: 0.45 0.45

0.40+---~---.------.---~---+ 0.40 0.40 0.45 0.50 0.55 0.60 0.65 0.70

Preoperative Corneal Thickness (mm)

Fig 1. Scattergram displays the pachymeter measurement of corneal thickness obtained in an examination room before surgery, compared with the pachymeter measurement obtained on the operating table. The oblique line indicates equal measurements before and during surgery.

neal stroma. Of all these variables, the length of the knife blade is the most important, and its setting is derived from corneal thickness measurements.

Our findings suggest that it is possible to use preoper­ative central corneal thickness readings to set the length of the knife blade in radial keratotomy, with the following rejoinders:

1. The surgeon realizes that some of the eyes may have clinically meaningful changes from the preoperative to the intraoperative measurement. In our study, 12.7% of the eyes changed by more than 0.02 mm.

2. The surgeon sets the knife blade on the basis of cen­tral corneal thickness measurements, and not on the basis of paracentral or peripheral measurements. If the surgeon uses paracentral readings adjacent to a clear zone mark, or peripheral readings to set the length of the knife blade, it is more reasonable to use the intraoperative readings, since it is difficult to know exactly where one is reading

. in the outpatient clinic.

Table 1. Change in Central Corneal Thickness between Preoperative and Intraoperative Measurements taken with an Ultrasonic

Pachymeter in 395 Eyes

Amount of Change in mm

Thinner-0.07 to 0.08 Thinner-0.05 to 0.06 Thinner-0.03 to 0.04. Changed-0.02 or Less Thicker-O.Q3 to 0.04 Thicker-0.05 to 0.06

No. of Eyes (%)

3 (0.8) 6 (1.5)

28 (7.1) 345 (87.3)

9 (2.3) 4 (1.0)

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3. Factors inducing intraoperative thinning or thick­ening of the cornea are minimized. Factors that induce thinning of the cornea during surgery include the duration of the time the eye is open and the induction of evapo­ration of tears and stromal fluids by air currents and heat from the microscope lights. Villasenor and colleagues showed an average thinning of the cornea during surgery of 10%, with a range of7 .8% to 15%. 12 Factors that induce thickening of the cornea include damage to the barrier function of the corneal epithelium by topical anesthetics or preparation solutions and by abrasions of or incisions through the epithelium. Marked changes in intraocular pressure intraoperatively might also influence corneal thickness.

Measurements of corneal thickness taken intraopera­tively with an ultrasonic pachymeter allow more accurate mapping of the corneal thickness in different areas, and obviate changes in corneal thickness that might occur be­tween measurements in the outpatient clinic and mea­surements at the time of surgery.

REFERENCES

1. Thornton SP. A guide to pachyrneters. Ophthalmic Surg 1984; 15: 993-5.

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2. Waring GO Ill, Moffitt SD, Gelender H. et al. Rationale for and design of the National Eye Institute Prospective Evaluation of Radial Kera­totomy (PERK) Study. Ophthalmology 1983; 90:40-58.

3. Waring GO Ill, Lynn MJ, Gelender H. et al. Results of the Prospective Evaluation of Radial Keratotomy (PERK) Study one year after surgery. Ophthalmology 1985; 92:177-98, 307.

4. Coleman OJ, Lizzi FL. Jack RL. Ultrasonography of the Eye and Orbit. Philadelphia: Lea & Febiger, 1977; 114.

5. Steinberg EB, Waring GO Ill. Comparison of two methods of marking the visual axis on the cornea during radial keratotomy. Am J Ophthalmol 1983; 96:605-8.

6. Blumenstein BA. Dictionary based data storage and extraction system. In: Proc Statis Comp Sect of Am Statis Assn. Washington, DC: Am Statis Assn, 1976; 103-4.

7. Dixon WJ, ed. BMDP Statistical Software. Berkeley Cal: University of California Press, 1981.

8. Neter J, Wasserman W. Applied Linear Statistical Models; Regression, Analysis of Variance, and Experimental Designs. Homewood Illinois: Richard D. Irwin, 1974; 60-2.

9. Salz JJ, Azen SP, Berstein J, et al. Evaluation and comparison of sources of variability in the measurement of corneal thickness with ultrasonic and optical pachymeters. Ophthalmic Surg 1983; 14: 750-4.

10. Deitz MR, Sanders DR, Marks RG. Radial keratotomy: an overview of the Kansas City Study. Ophthalmology 1984; 91:467-77.

11. Jester JV, Venet T, Lee J, et al. A statistical analysis of radial keratotomy in human cadaver eyes. Am J Ophthalmol1981; 92:172-7.

12. Villasenor RA, Salz J, Steel D, Krasnow MA. Changes in corneal thick­ness during radial keratotomy. Ophthalmic Surg 1981; 12:341-2.