Radial Keratotomy in Monkeys

Radial Keratotomy in Monkeys

Radial Keratotomy in Monkeys A One-year Follow-up Report JOHN W. COWDEN, MD, JONATHAN CICHOCKI, MD Abstract: Twelve adult rhesus monkey eyes underwen...

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Radial Keratotomy in Monkeys A One-year Follow-up Report JOHN W. COWDEN, MD, JONATHAN CICHOCKI, MD

Abstract: Twelve adult rhesus monkey eyes underwent a radial keratotomy, consisting of 16, deep evenly spaced radial corneal incisions extending from the edge of a 3 mm central optical zone to the limbus. Peripheral deepening incisions were used. Keratometry and retinoscopy done before and after surgery monthly for one year revealed an average of 1.79 diopters decrease in the corneal curvature and an average increase of 2.49 diopters in the refractive error, which remained stable after one to two months . No significant complications were observed despite four perforations. [Key words: myopia, radial keratotomy, refractive keratoplasty.] Ophthalmology 89:684-686, 1982

The surgical correction of myopia using the method of Fyodorov known as radial keratotomy consists of 16 partial thickness radial incisions in the cornea that result in a central flattening and peripheral corneal bulging, reducing the degree of myopia. 1 This investigation, using the rhesus monkey, determines: 1. The degree of change produced in the refractive error; 2. The amount of change produced in the corneal curvature; 3. The time required for stabilization of the refractive error and the corneal curvature changes; and 4. The surgical and postoperative complications.

MATERIALS AND METHODS Radial keratotomy was performed on 12 eyes of six adult rhesus monkeys. A preoperative eye examination consisting of slit-lamp examination and pachometry using the Haag Streit 900 slit lamp; keratometry, using the A-O keratometer; and cycloplegic From the Kresge Eye Institute of Wayne State University, Detroit. Michigan. Presented at the Annual Meeting of the Association for Research in Vision and Ophthalmology (ARVOl, Sarasota, Florida , April 1981 . Reprints requests to John W Cowden, MD, 3994 John R., Detroit , M148201.

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retinoscopy using 1% cyclogyl. The examinations both before and after operation were performed using ketamine anesthesia. The monkeys were anesthetized with IV penobarbital and topical pontocaine. The center of the cornea was estimated by bisecting the horizontal diameter of the cornea and marking the center with a caliper tip. A 3.0-mm optic zone was outlined with the calipers around the central mark then stained with fluoroscein. A 76A Beaver blade held in a flat-tipped Katena Swiss Blade Holder was set equal to the central corneal thickness using the Bores Corneal depth gauge (0.50 to 0.60 mm in depth). Sixteen, evenly spaced radial incisions were made extending from the edge of the previously marked central zone to the limbus. A peripheral deepening incision of 0.10 mm increased depth was made in the peripheral half of each incision. Antibiotic ointment was instilled, and the animal was returned to its cage. Postoperative examinations that included slit-lamp examination, keratometry, and cycloplegic retinoscopy were done weekly four times, then monthly for one year.

RESULTS The one year follow-up evaluation of 12 nonhuman primate eyes that underwent a standardized radial keratotomy revealed an average change of +2.49 016\-6420/82/0600/684/$00.65

© American

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COWDEN AND CICHOCKI • RADIAL KERATOTOMY

Table 1. Radial Keratotomy Results Monkey

6CR

6K

E019 00 E019 OS E02000 E020 OS E03200 E032 OS E034 00 E034 OS 000200 0002 OS L00200 L002 OS Mean (12)

+2.370 +3.620 +1.750 +2.000 +3.370 +1.870 +2.750 +1.00 +4.500 +4.000 +0.120 +2.500 +2.490

-1.250 -1.000 -1.250 -1.620 -2.750 -0.250 -1.870 -1.620 -3.370 -2.250 -1.120 -3.120 -1.790

LlCR = Change cycloplegic retinoscopy LlK = Change in keratometry

Table 2. Cycloplegic Retinoscopy: 12 Eyes, 12-Months Postop Refractive error (SE) Preop Postop Change

Range

Mean

Median

-6.500 to +5.000 -1.300 -0.500 -2.750 to +7.500 +1.060 +1.750 +0.120 to +4.250 +2.490 +2.370

diopters (D) of refr:;tctive error (Table O. The range of the change of the refractive error was from an increase of +O.12D to +4.25D. The median was +2.37D (Table 2). There was an average change or flattening of -1.79D in the corneal curvature. The range of the change of the corneal curvature was from an increase of +O.25D

to -4.81D (Table 3). The median was -1.62D. The corneal curvature stabilized by four to six weeks after operation and tended to remain stable except for minor fluctuations in all of the eyes examined. The average or mean keratometry reading did not change signifi" cantly after one month (Fig 1). There was no difference in the pre- and post-pachometry readings nor the intraocular pressure. Operative complications included corneal perforation in four cases and the development of a small hyphema in two additional cases not associated with perforations. All of these occurred at the time of the peripheral deepening incisions, however, the operative procedure was able to be completed. All eyes that had a corneal perforation or hyphema healed spontaneously without evidence of synechiae formation or cataract (Table 4). Slit-lamp examination of the corneas between 9 and 12 months after operation revealed no vascularization, excessive scar formation, nor surface irregularity as demonstrated by fluorescein dye in the tear film. The scars in most eyes extended to between one half and three quarters of the thickness of the cornea, although in the cornea that returned to its original curvature, the scars were only one third the depth of the cornea. The corneal scars appeared to be less prominent one year after operation as compared with their appearance one to two months after operation.

DISCUSSION It is apparent from the data presented that radial keratotomy as performed in this study and by others

53.00 52.50 52.00 51.50 Average Keratometry 51.00 Reading 50.50 5000 49.50 49.00

2

3

7

4

8

9

Months Fig 1. Corneal curvature in 12 monkey eyes.

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OPHTHALMOLOGY. JUNE 1982 • VOLUME 89 • NUMBER 6

Table 3. Corneal Curvature: 12 Eyes, 12-Month Postop Keratometry Preop Postop Change

Range 49.91 D to 54.25D 47.50D to 53.50D -O.25D to -4.81D

Mean

Median

52.58D 50.57D -1.79D

53.18D 50.75D -1.62D

Table 4. Complications Perforations Hyphema Synechiae Excessive scarring Vascularization

4 2

o o o

r~sults in a flattening of the cornea curvature that appears to stabilize between one and two months after operation and remains stable for a period of up to one year. 2 There was a change in the refractive error of an average of + 2.49D that did not seem to correlate well with the change in keratometry readings. The amount of change expected in the monkey per diopter of altered corneal curvature may not be the same as expected in humans especially since the normal corneal curvature measured between 48.00D to 53.50D. Further analysis of the data revealed that a greater flattening of the corneal curvature was obtained in the

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eyes in which perforations occurred, although this was not reflected in the retinoscopy findings. There were no significant long-term complications observed despite four perforations at the time of surgery. There were no infections or ulcerations despite only a single application of topical antibiotic and the lack of a dressing. One cornea with superficial incisions and scars returned to its preoperative keratometry readings. The safety, efficacy, and stability of a standardized radial keratotomy procedure done on monkey eyes was investigated. It appears to be a relatively safe procedure, capable of flattening of an average of 1.79D of corneal curvature for at least one year in the monkey. The amount of change in the refractive error or retinoscopy was not as much as expected for the corneal curvature change.

REFERENCES 1. Fyodorov SN, Durnev VV. Operation of dosaged dissection of corneal circular ligament in cases of myopia of a mild degree. Ann Ophthalmol 1979; 11:1885-90 2. Steel D, Jester JV, Salz J, et al. Modification of comeal curvature following radial keratotomy in primates. Ophthalmology 1981:

88:747-54. 3. Cowden JW, Bores LD. A clinical investigation of the surgical correction of myopia by the method of Fyodorov. Ophthalmology 1981; 88:737-41.