Journal of Refractive Surgery

Surgical Correction of Hyperopia: A New Experimental Approach Using X-Incision and Suture Technique in Rabbits

Jin H Lee, MD; Kiho Park, MD; Kyung J Lim, MD

Abstract

ABSTRACT

BACKGROUND: A new experimental surgical technique to correct hyperopia by steepening the central corneal curvature was performed on the eyes of rabbits.

METHODS: After X-shaped incisions at four quadrants of the paracentral cornea, four deep, transverse interrupted sutures were placed across the incisions and the knots were tightened to compress the corneal tissue in the direction of the suture. Twenty-three eyes of 12 albino rabbits were divided into group A (7 eyes) with a 4-millimeter diameter clear zone, group B (9 eyes) with a 6-millimeter diameter clear zone, and control group C1 consisting of 7 contralateral eyes of group A without any incision or suture.

RESULTS: There was significant (p < .01) steepening of the central cornea as measured by the keratometer at postoperative 8 weeks in group A (+1.88 diopters) and group B (+1.68 D) compared with that of the control group C (-3.44 D). The difference in effect of hyperopic correction between groups of A and B was not statistically significant.

CONCLUSIONS: This procedure serves only as an experimental model of corneal response to incisions and sutures and is not for use in human corneas. (Refract Corneal Surg 1992; 8:389-393.)

Abstract

ABSTRACT

BACKGROUND: A new experimental surgical technique to correct hyperopia by steepening the central corneal curvature was performed on the eyes of rabbits.

METHODS: After X-shaped incisions at four quadrants of the paracentral cornea, four deep, transverse interrupted sutures were placed across the incisions and the knots were tightened to compress the corneal tissue in the direction of the suture. Twenty-three eyes of 12 albino rabbits were divided into group A (7 eyes) with a 4-millimeter diameter clear zone, group B (9 eyes) with a 6-millimeter diameter clear zone, and control group C1 consisting of 7 contralateral eyes of group A without any incision or suture.

RESULTS: There was significant (p < .01) steepening of the central cornea as measured by the keratometer at postoperative 8 weeks in group A (+1.88 diopters) and group B (+1.68 D) compared with that of the control group C (-3.44 D). The difference in effect of hyperopic correction between groups of A and B was not statistically significant.

CONCLUSIONS: This procedure serves only as an experimental model of corneal response to incisions and sutures and is not for use in human corneas. (Refract Corneal Surg 1992; 8:389-393.)

Hyperopia can be surgically corrected by lamellar refractive keratoplasty techniques such as keratomileusis, epikeratoplasty, and intracorneal lenses; by refractive keratotomy such as hexagonal - transverse; and by thermokeratoplasty,1"8 techniques that steepen the central corneal curvatures.

Figure 1 : The surgical procedures in group A. (A) The marking of a 4-millimeter clear zone was done at four locations and (B) X-lncision marking was done at tour locations with a 1.5millimeter incision length. After incision, (C) four transverse interrupted mattress sutures were performed across the Xincision with nearly full depth to Descemet' s membrane. The stitch width was 2.5 mm, and it was tightened to 1 .5 mm of width in group A.

Figure 1 : The surgical procedures in group A. (A) The marking of a 4-millimeter clear zone was done at four locations and (B) X-lncision marking was done at tour locations with a 1.5millimeter incision length. After incision, (C) four transverse interrupted mattress sutures were performed across the Xincision with nearly full depth to Descemet' s membrane. The stitch width was 2.5 mm, and it was tightened to 1 .5 mm of width in group A.

Table

Table 1Preoperative and Postoperative Average Keratometry Readings (Diopters)Figure 2: The time-related changes of average keratometric readings after four sutured x-incisions. Group A = 4 mm central clear zone; group B = 6 mm central clear zone; group C = contralateral unoperated eyes of group A. Groups A and B steepened initially, flattened between 2 to 3 weeks, and then showed slow flattening similar to group C which gradually flattened as part of normal growth of the rabbit cornea.

Table 1

Preoperative and Postoperative Average Keratometry Readings (Diopters)

Figure 2: The time-related changes of average keratometric readings after four sutured x-incisions. Group A = 4 mm central clear zone; group B = 6 mm central clear zone; group C = contralateral unoperated eyes of group A. Groups A and B steepened initially, flattened between 2 to 3 weeks, and then showed slow flattening similar to group C which gradually flattened as part of normal growth of the rabbit cornea.

Experimentally, a new technique was designed using four transverse interrupted mattress sutures with the underlying idea that circumferential shortening of the paracentral cornea by tightening the sutures would induce a forward bulging of the central cornea. In a preliminary study (unpublished data), we found an initial 4.00 to 5.00 diopters of increase in keratometry readings, but this lasted only 1 or 2 weeks postoperatively, so we added corneal incisions to increase and maintain the suture effect. The purpose of this study was to evaluate the effect of a new experimental surgical technique using corneal sutures and incisions for the correction of hyperopia in rabbits.

MATERIALS AND METHODS

Twenty-three eyes of 12 immature albino rabbits (1.8 to 2.4 kg) were divided into three groups. Group A included 7 eyes with a 4-millimeter clear zone, group B included 9 eyes with a 6-millimeter clear zone, and group C (as the control group) consisted of 7 contralateral eyes of group A. After anesthesia with an intramuscular injection of 30 mg/kg ketamine hydrochloride, keratometry with an American Optical Keratometer and central pachometry with a DGH 2000 pachometer were performed.

Table

Table 2Postoperative Change in Keratometry Readings (Diopters)Table 3Preoperative and Postoperative Astigmatism (Diopters)

Table 2

Postoperative Change in Keratometry Readings (Diopters)

Table 3

Preoperative and Postoperative Astigmatism (Diopters)

Figure 3: The corneal photograph of group A at postoperative 8 weeks shows clear optical zone area. Arrows indicate intrastromal suture placement (10-0 Mersilene).

Figure 3: The corneal photograph of group A at postoperative 8 weeks shows clear optical zone area. Arrows indicate intrastromal suture placement (10-0 Mersilene).

In group A, optical-center marking was done on the proptosed globe with a caliper tip. Then clearzone marking was done at the superior, inferior, temporal, and nasal location (Fig IA). X-shapedincision marking was done with a caliper tip at four locations with 1.5 mm of incision length (Fig IB). An X-incision was done to the depth of 80% of corneal thickness (X-shaped incision will hereafter be referred to simply as "X-incision"). Four transverse interrupted sutures were performed across the Xincision nearly full depth to Descemet's membrane. 10-0 Mersilene (Ethicon, W1775) with an advanced micropoint spatular needle (diameter: 150 µm, length: 6 mm) was selected as the suture material for its resistance to elastic deformation. The length of the bite was 2.5 mm and the first tie was made with three throws then tightened to 1.5 mm of length (Fig IC). Then the knot was completed with three additional single throws. The excess was cut off with a #11 Bard-Parker blade and the suture rotated to bury the knot in the suture tract.

In group B, the diameter of the clear zone was 6 mm and the X-incision length was 2 mm. Sutures were performed with a bite length of 3 mm which was tightened to 2 mm. Other procedures were the same as in group A. In group C, neither incision nor suture was done. Postoperatively, gentamicin 20 mg subconjunctival injection and kanamycin ointment were applied.

Keratometry was performed at 2, 3, 4, and 8 weeks postoperatively. Results were expressed as mean diopter changes in keratometry readings and analyzed by Wilcoxon rank sum test. Two rabbits of groups A and B were sacrificed at postoperative 8 weeks. After hematoxylin-eosin staining, the corneas were examined under light microscopy.

RESULTS

In group A, the average keratometric measurements increased to 56.14 D at 2 weeks, which decreased to 54.04 D at 4 weeks and 52.50 D at 8 weeks after surgery (Fig 2). In group B, the average postoperative keratometric measurement was 55.01 D at 2 weeks, 53.01 D at 4 weeks, and 51.70 D at 8 weeks. In group C, the keratometric measurements gradually decreased to 49.36 D at 2 weeks, 48.82 D at 4 weeks, and 46.91 D at 8 weeks (Table 1).

There were significant decreases in the effect between 2 and 3 weeks in groups A and B compared with control group C (p < .05). But the slope had flattened out considerably after 3 weeks, like that of control group C. There was no significant difference in the amount of keratometric measurement decrease in groups A and B compared with group C during the period from 3 to 4 weeks (p > .05) and from 4 to 8 weeks (p > .05).

Figure 4: Light micrograph of a cornea (group A) 8 weeks following surgery. The histologic change was meager, except for a mild thickening of the epithelium, which grew over the upper suture material (arrow) and mild fibrous thickening around the lower suture material (arrowhead), with a few lymphocytes (hematoxylin and eosin, original magnification x 100).Figure 5: The underlying mechanism of this technique. By applying and tightening the mattress sutures across the Xincision, tissue shortening can be made in the direction of the suture, and thus transverse shortening of the paracentral corneal tissue will induce a forward bulging of the central cornea.

Figure 4: Light micrograph of a cornea (group A) 8 weeks following surgery. The histologic change was meager, except for a mild thickening of the epithelium, which grew over the upper suture material (arrow) and mild fibrous thickening around the lower suture material (arrowhead), with a few lymphocytes (hematoxylin and eosin, original magnification x 100).

Figure 5: The underlying mechanism of this technique. By applying and tightening the mattress sutures across the Xincision, tissue shortening can be made in the direction of the suture, and thus transverse shortening of the paracentral corneal tissue will induce a forward bulging of the central cornea.

There were significant changes (p < .01) in the postoperative keratometric measurements at 4 weeks in groups A ( + 3.16 D) and B ( + 2.71 D), compared with control group C (-1.53 D). There were also significant changes (p < .01) in keratometric readings after 8 weeks of follow up of group A ( + 1.88 D) and group B ( + 1.68 D), compared with group C (-3.44 D). There was no significant difference (p > .05) in the effect between groups A and B (Table 2). There were no significant postoperative astigmatism changes in any group (p > .05) (Table 3). At postoperative 8 weeks, the central cornea was clear and no gross corneal surface irregularity was found (Fig 3).

Light microscopic evaluation of the cornea (group A) 8 weeks following surgery revealed that foreign body reaction to the suture material was nearly absent except for few lymphocytic infiltrations (Fig 4).

DISCUSSION

Various attempts to create corneal steepening have been reported, but these still remain at the initial stage. Hexagonal keratotomy1"3 steepens the cornea with a proposed mechanism in which creating a hexagonal transverse incision around the visual axis would allow the central cornea to balloon forward "like an umbrella," a technique now supplemented by transverse incisions - "Hex-T." The purse-string intrastromal suture technique4 and circling keratorrhaphy5 steepen the cornea by placing a suture in the corneal stroma circumferentially around the visual axis, thus creating a ring of traction forces inside the cornea that acts from its midperiphery to its center. Even though it is a rather destructive procedure, radial thermokeratoplasty68 with a hot needle may be one of the means which induces the increase in corneal curvature and is now done with the holmium-YAG laser.

As described in the introduction, the preliminary study was performed using only four transverse sutures without incision. This idea came from the compression sutures913 which was used with relaxing incisions after corneal transplantation for correction of astigmatism. Although the compression suture had a radial direction compared with the transverse direction of this study, it had the same mechanism of tissue shortening by compression.

Generally, intersecting corneal incisions are avoided in human corneal surgery, but, for the experimental purpose to potentiate the effect of compression sutures, four X-incisions were added which exaggerated the tissue shortening effect of the suture compared with the suture-only technique and produced little irregular astigmatism. By applying and tightening the suture across the X-incision, tissue shortening was made easily in the direction of the suture, and thus circumferential shortening of the paracentral corneal tissue induced a forward bulging of the central cornea (Fig 5). The advantages of this procedure are that it has no risk of complication occurring in lamellar refractive surgery, and that the effect can be adjusted by controlling the strength of the suture tightening. It also has the potential to control postoperative astigmatism simply by removing and reapplying one or two of the four sutures.

Although the changes in corneal curvature of group A ( + 1.88 D) and group B ( + 1.68 D) were relatively small, the curvature keratometric measurements of the control group decreased ( -3.44 D), so that the net amount of correction was approximately 5.00 D. We used immature rabbits, which resulted in a spontaneous decrease of corneal curvature as a natural effect of growth, but we do not think this invalidates the effect of the surgery.

The early postoperative corneal flattening between 2 and 3 weeks probably resulted from the gradual cutting of the thread through the tissue. The equilibrium among the cutting power of the suture, the resistance of the corneal tissue, and wound healing at the incision site will determine the time of stabilization of the correctional effect.

As the results of group A and B showed no statistical difference in the effect, the procedures of group B could be preferred to those of group A because they are technically easier and a larger clear zone probably has less irregular astigmatism. This study was intended merely as a preliminary experiment in a new surgical technique to demonstrate the response of corneal curvature to sutures and incisions. It requires further study. Intersecting incisions without sutures cause wound gaping and are avoided in human surgery. Since we sutured the X-shaped incisions, this procedure could have human application if further experimental results demonstrate feasibility.

REFERENCES

1. Yamashita T, Schneider ME, Fuerst DJ, et al. Hexagonal keratotomy reduces hyperopia after radial keratotomy in rabbits. Journal of Refractive Surgery. 1986;2:261-264.

2. Gilbert M, Friedlander M, Aiello JP, et al. Hexagonal keratotomy in human cadaver eyes. Journal of Refractive Surgery. 1988;4:12-14.

3. Neumann AC, McCarty GR. Hexagonal keratotomy for correction of low hyperopia: preliminary results of prospective study. J Cataract Refract Surg. 1988;14:265-269.

4. Starling JC, Hofmann RF. A new surgical technique for correction of hyperopia after radial keratotomy: an experimental model. Journal of Refractive Surgery. 1986;2:9-14.

5. Krasnov MM. Circling keratorrhaphy: a new approach to surgical correction of aphakia (preliminary communication). Ann Ophthalmol. 1987;19:423-427.

6. McDonnell PJ. Radial thermokeratoplasty for hyperopia. I. The need for prompt prospective investigation. Refract Corneal Surg. 1989;5:50-52.

7. Neumann AC, Sanders DR, SaIz JJ Radial thermokeratoplasty for hyperopia. II. Encouraging results from early laboratory and human trials. Refract Corneal Surg. 1989;5:50-54.

8. Feldman ST, Ellis W, Frucht-Pery J, et al. Experimental radial thermokeratoplasty in rabbits. Arch Ophthalmol. 1990;108:997-1000.

9. Troutman RC. Corneal wedge resection and for postkeratoplasty astigmatism. Int Ophthalmol Clin. 1983;23:161-168.

10. Roper-Hall MJ, Atkins AD. Control of astigmatism after surgery and trauma: a new technique. Br J Ophthalmol. 1985;69:352-359.

11. Mandel MR, Shapiro MB, Krachmer JH. Relaxing incisions with augmentation sutures for the correction of the postkeratoplasty astigmatism. Am J Ophthalmol. 1987;103:441-447.

12. McCartney DL, Whitney CE, Stark WJ, et al. Refractive keratoplasty for disabling astigmatism after penetrating keratoplasty. Arch Ophthalmol. 1987;105:954-957.

13. Limberg MB, Dingeldein SA, Green MT, et al. Corneal compression sutures for the reduction of astigmatism after penetrating keratoplasty. Am J Ophthalmol. 1989;108:36-42.

Table 1

Preoperative and Postoperative Average Keratometry Readings (Diopters)

Figure 2: The time-related changes of average keratometric readings after four sutured x-incisions. Group A = 4 mm central clear zone; group B = 6 mm central clear zone; group C = contralateral unoperated eyes of group A. Groups A and B steepened initially, flattened between 2 to 3 weeks, and then showed slow flattening similar to group C which gradually flattened as part of normal growth of the rabbit cornea.

Table 2

Postoperative Change in Keratometry Readings (Diopters)

Table 3

Preoperative and Postoperative Astigmatism (Diopters)

10.3928/1081-597X-19920901-12

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