NEW PACHYMETER TO MEASURE CORNEAL THICKNESS

HERNANDO CARDONA, M.D., j

New York,

Measurement of the corneal thickness is of clinical importance in several conditions, including keratoconus when periodic deter­minations of the apical thickness is a practi­cal method of establishing the degree of thin­ning of the corneal tissues. In cases of cor­neal edema it serves to correlate the loss of vision and the edematous condition of the cornea. The thickness of the cornea should also be established in eyes affected with bul-lous keratopathy to determine if it exceeds 1.2 mm, in which case the prognosis is fa­vorable for either a keratoplasty or a pros-thokeratoplasty.

To this end, we have developed a new pa­chymeter* which makes possible the determi­nation of the thickness of a clear cornea with an accuracy of within .02 mm. A totally opaque cornea cannot be measured.

The calibration of this new pachymeter is based on a refraction index of 1.376, the same as that of the normal cornea. This rep­resents an entirely new approach. A search of the literature has revealed no other model of pachymeter calibrated on the normal cor­neal refraction index of 1.376.

Gunnar von Bahr,1 basing his pachymeter on principles similar to those used by Blix to construct his ophthalmometer, mounted two planes of plate glass, one in front of the con­densing lens and the other in front of the microscope of the slit lamp. When these

From the Edward S. Harkness Eye Institute of Presbyterian Hospital and the Department of Oph­thalmology, Columbia University, College of Physi­cians and Surgeons, New York, New York. This investigation was made possible by Grant NB-04968-03 from the National Institutes of Health, Bethesda, Maryland, and by a Fight for Sight Grant-in-Aid from the National Council to Combat Blindness, Inc., New York, New York.

Reprint requests to Hernando Cardona, M.D., Department of Ophthalmology, Edward S. Hark­ness Eye Institute, New York, New York 10032.

♦This instrument is manufactured by H & I In­strumentation, 293 Carlton Terrace, Teaneck, New Jersey 07666.

ND A. GERARD D E V O E , M.D.

New York

plates were rotated, the reflection of the ray on the endothelium could be displaced and made to coincide with the reflection of the ray on the epithelium. The thickness of the cornea was calculated from the angle of rev­olution, the thickness and index of refrac­tion of the glass plate and the corneal radius.

Maurice and Giardini2 modified this method. They used a single one-fourth inch thick persplex plate in the illuminating beam from the slit lamp traversed by a horizontal cut covered by a thin strip of colored cellu­loid. Their apparatus can be adjusted so a white reflection from the posterior surface and a colored reflection from the anterior surface of the cornea are seen in the micro­scope at the same time. These reflections are aligned by rotating the persplex plate and the angle of rotation gives the corneal thickness.

In 1932, Goldman3 described a new ocular to measure the transparent media of the an­terior pole of the globe. It is mounted inside the right tube of the microscope of the old Haag-Streit slit lamp. The angle is formed by the beam of light set at 60° from the mi­croscope and a system of gears is adapted to provide the reading.

DESCRIPTION OF NEW PACHYMETER

Basically, the instrument consists of a double mirror (Fig. 1, see A and B) . One mirror is movable, replacing the usual mir­ror in the Haag-Streit 900 slit lamp. Thus, separate images are obtained from the ep­ithelial and endothelial surfaces. These sur­faces can be measured separately by means of a calibrated gearing system.

The beam emitted by the slit lamp strikes the cornea surface (Fig. 2) at B and reaches C (the endothelial surface), which is viewed with the microscope through point A. The corneal thickness is determined when the beam from the upper slit lamp mirror strikes the corneal surface at point A (Fig. 2).

128 AMERICAN JOURNAL OF OPHTHALMOLOGY JULY, 1971

Fig 1 (Cardona and DeVoe). The instrument consists of two arms united at an angle of 90°. One side is the slide rest, which fits into the mirror hous­ing of the lamp. This arm has two front surface mir­rors, one fixed superiorly (A) and mounted on two crossed axes, and the other fixed inferiorly (B). The other arm is a SO mm lever which multiplies the reading x40. The calibrated dial (C) which synchronizes the two corneal images at "0" position adjustment is at one end. The fixation screw (D) fastens the pachymeter rigidly to the slit lamp.

A special method was used to calibrate this instrument. It eliminates the logarithmic error in calibration obtained with a plastic lens (1.495). The figure obtained with this calibration method very closely approximates the thickness of the human cornea.

When the corneal refraction index sur­passes 1.376, a new calibration can easily be

performed for research purposes by adjust­ing the refraction index of the variable thickness chamber to the value desired.

This calibration was chosen for practical purposes. A base curvature of 7.75 and a re­fraction index of 1.376 were selected as average values for the present calibration. The instrument was checked and the equiva­lent values are shown in Table 1.

MEASUREMENT OF CORNEAL THICKNESS

A total of 204 healthy corneas from 102 patients of different races, age, and sex were measured. Two zones were selected, the cen­tral and peripheral, because they afforded great precision. No appreciable changes were noted with varying refractive errors except two eyes with high myopia (—9.50 to 16.00). In these cases the central corneal thickness was 0.535 mm, or approximately .024 mm below our average finding of 0.651 mm. The average corneal thickness at the periphery was 0.745 + mm.

SUMMARY

A new simple instrument to measure the thickness of the cornea is designed for use with the Haag-Streit 900 slit lamp. Its cali­bration is entirely based on a chamber filled with a solution which has 1.376 index of re­fraction similar to the human cornea.

Fig. 2 (Cardona and DeVoe). Diagram showing the pattern of the beam and the physical bases used in corneal thickness determination. The formula is: A + (Cos — Sine) n' — sine* -f- (Cos + Sine) Sine.

Plexiglass Thickness

in mm (N-1.49)

0.25 o.so 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00

TABLE 1

EQUIVALENT VALUES (IN MM)

Human Cornea (N-1.375)

Inside Outside Radius Radius

8.00 8.25 7.75 8.25 7.50 8.25 7.25 8.25 7.00 8.25 6.75 8.25 6.50 8.25 6.25 8.25 6.00 8.25 5.75 8.25 5.50 8.25 5.25 8.25

Equivalent Thickness of Human

Cornea

0.22 0.45 0.68 0.90 1.13 1.35 1.58 1.80 2.03 2.25 2.48 2.70

VOL. 72, NO. 1 NEW PACHYMETER 129

This instrument is primarily used to study the thickness of the corneal before insertion of a keratoprosthesis. The corneas must meas­ure at least 1 mm in thickness. If the cornea is thin, a graft is advisable in order to increase the amount of collagen support as a safety measure.

ACKNOWLEDGMENT We thank Dr. S. Mishima who kindly reviewed

the mathematical portions of this manuscript.

REFERENCES

1. Von Bahr, Gunnar: Corneal thickness: Its measurement and changes. Am. J. Ophth. 42:251, 19S6.

2. Maurice, D. M., and Giardini, A. A.: A simple optical apparatus for measuring the corneal thick­ness and the average thickness of the human cor­nea. Brit. J. Ophth. 35:169, 1951.

3. Goldman, H.: Etude des dimensions de la chambre anterieur de l'oeil humain. Ophthalmolo-gica 142:650, 1961.

4. Smelser, G. K., and Chen, D. K.: Physiologi­cal changes in cornea induced by contact lenses. Arch. Ophth. 53:676, 1955.

OPHTHALMIC MINIATURE

Graefe's entropion forceps. Lawrence, W.: A Treatise on the Dis­eases of the Eye (Hayes, Isaac, ed.). Philadelphia, Lea and Blanchard, 1847.