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Chapter 25 – Intrastromal Corneal Ring Segments for Low and High Myopia

Chapter 25 – Intrastromal Corneal Ring Segments for Low and High Myopia

DIMITRI T. AZAR

JONATHAN D. PRIMACK

DEFINITION

• Polymethyl methacrylate (PMMA) arcuate segments that are placed within the peripheral cornea to correct myopia.

KEY FEATURES

• Inner diameter of 6.8?mm.

• Refractive effect is related to the thickness of the segments.

• Initial enthusiasm regarding beneficial effect in low myopia.

• Emerging role as adjunct treatment for high myopia and corneal ectasia.

ASSOCIATED FEATURES

• Surgical steps include intrastromal corneal ring segment channel formation, insertion of segments, and suturing of entry site.

• Surgical techniques are slightly modified for keratoconus and for combined intrastromal corneal ring segments and laser in situ keratomileusis.

• Reversibility and hyperacuity are potential advantages.

INTRODUCTION

In 1949, Barraquer first proposed the use of alloplastic materials as a method to correct refractive errors. Several intracorneal implants, or inlays, made of various materials (hydrogels, polysulfones) have been evaluated in animal and human eyes for the correction of myopia, aphakia, or presbyopia. However, none of them are currently used routinely.[1] Among the materials that have been tested for use as intracorneal implants are polysulfone lenses, small diameter corneal inlays, and hydrogel lenses ( Figs. 25-1 to 25-4 ).

Intrastromal corneal ring segments (ICRSs), or Intacs, are placed in the peripheral stroma at approximately two-thirds depth, outside the central optical zone, to reshape the anterior corneal surface while maintaining the positive asphericity of the cornea. [1] [2] [3] [4] [5] [6] [7] [8] The first-generation design of Intacs was referred to as the 360° intrastromal corneal ring (ICR). The current design consists of two segments, each with an arc length of 150° ( Fig. 25-5 ). Intacs are manufactured from polymethyl methacrylate (PMMA).[1]

Each Intacs segment has a hexagonal cross section that lies along a conic section. With a fixed outer diameter of 8.1?mm and an inner diameter of 6.8?mm, Intacs leave a large, clear central optic zone. Each segment has a small positioning hole at the superior end to aid with surgical manipulation once the segments have been inserted. The two segments are designated as clockwise and counterclockwise to correspond to their orientation within the intrastromal tunnel. [1]

Intacs change the arc length of the anterior corneal curvature. The refractive effect achieved is directly related to the thickness of the product. Placing the product in the periphery of the

Figure 25-1 Slit-lamp photograph of corneal opacity after implantation of a polysulfone intracorneal lens. (Courtesy of Stephen S. Lane, MD.)

cornea causes local separation of the corneal lamellae, which results in shortening of the corneal arc length. This has a net effect of flattening the cornea, thereby correcting for myopia by lowering the optical power of the eye. Increasing the thickness of Intacs causes greater degrees of local separation and increased corneal flattening. Thus, the degree of corneal flattening—or correction—achieved by Intacs is directly related to thickness.[1]

The same effect can be observed by placing a pencil underneath a sheet of paper. With the added bulk of the pencil, the paper is no longer flat and is shorter. In much the same way, when Intacs are placed within the stromal layers of the cornea, they shorten the arc length across the optical zone.[1]

Intacs are available in the United States in three different thicknesses—0.25, 0.30, and 0.35?mm—intended for the reduction or elimination of low myopia ( Table 25-1 ). The initial enthusiasm regarding ICRSs for the correction of myopia has faded for multiple reasons, including a limited range of correction, induced astigmatism, and slow visual recovery. Although the future role of ICRSs in refractive surgery is unclear, they may evolve into an important therapeutic intervention in keratoconus patients.[9] Another potential application of ICRSs may be to minimize the risk of corneal ectasia following laser in situ keratomileusis (LASIK) in patients with high myopia.[10] Intacs are also available in thicknesses of 0.40 and 0.45?mm to correct myopia up to -4.50D.

SURGICAL TECHNIQUE

Topical anesthetic and antibiotic drops are administered preoperatively. The operative eye is prepped with an antiseptic solution, and sterile drapes are placed appropriately. Peri- or retrobulbar anesthesia is unnecessary and could interfere with patient fixation during LASIK.

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Figure 25-2 Schematic of small diameter intracorneal inlay lens placement in cornea. (Adapted with permission of Judy Gordon, MD, and Richard L. Lindstrom, MD.)

Figure 25-3 Slit-lamp photograph of a hydrogel intracorneal lens in position in the patient’s cornea. (Courtesy of Roger F. Steinert, MD.)

ICRS Channel Formation

The center of the cornea is located, and an incision and placement marker (KeraVision, Fremont, CA) is applied to indicate where the PMMA segments and the superior, radial incision will ultimately lie.[1] [8] Ultrasonic pachymetry is performed at the 12 o’clock incision site, and an approximately 1?mm incision of 68% corneal thickness is created with a calibrated diamond knife. A modified Suarez spreader is used to perform a small lamellar dissection at the base of the incision, to create an entry pocket on either side. Next, a vacuum centering guide (KeraVision) is positioned on the globe and stabilized under suction. Specially designed 0.9?mm dissectors are then introduced through the incision (clockwise and counterclockwise) to create stromal tunnels by blunt dissection ( Fig. 25-6 ). Ideally, the channels are located at two-thirds corneal depth. Suction is then released, and the centering guide is removed.

Segment Insertion

Using forceps, the PMMA segments are introduced into the channels. In their final position, the segments are located 3?mm apart superiorly. If necessary, the flap is refloated to eliminate any iatrogenically induced wrinkles. The incision site is hydrated and closed with 10-0 nylon sutures.[8] Figure 25-7 illustrates the appearance of the cornea following this procedure.

Figure 25-4 Intrastromal corneal ring in the corneal stroma. (Original drawing courtesy of Thomas Loarie.)

Figure 25-5 A 150° intrastromal corneal ring segment. (Courtesy of Thomas Loarie.)

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TABLE 25-1 — INTACS FOR THE REDUCTION OR ELIMINATION OF LOW MYOPIA

Intacs Thickness (mm)

Predicted Nominal Correction (D)

Recommended Prescribing Range (D)

0.25

-1.03

-1.00 to -1.63

0.30

-2.00

-1.75 to -2.25

0.35

-2.70

-2.38 to -3.00

Gel Injection Adjustable Keratoplasty

A modification of ICRS surgery is gel injection adjustable keratoplasty. In this procedure, a delaminator is used to separate the stromal lamellae ( Fig. 25-8 ). This is followed by gel injection into the stromal channel. After polymerization, the gel induces central flattening, without significant postoperative inflammation ( Fig. 25-9 ). This procedure may have potential advantages over ICRSs, but it has not been approved by the U.S. Food and Drug Administration.

CLINICAL OUTCOME

The initial reports of ICRSs and ICRs were very encouraging. Studies of ICRs in blind eyes followed for 1 year showed the ring to be safe and effective for the modification of corneal curvature.[11] Schanzlin et al.[8] reported the 1-year results from the phase II clinical trial of the 360° ICR in 81 eyes: uncorrected visual acuity (UCVA) of 20/40 or better in 88% of cases, 73% of patients within 1D of intended correction, 2-month stability, and positive asphericity.

Burris et al.[3] analyzed corneal topography in 74 phase II participants and found that asphericity was preserved and that corneal flattening increased with ring thickness.

The more centrally placed incisions tended to cause more induced astigmatism, whereas more peripherally placed incisions tended to be vascularized. [11] Transient loss of corneal sensation was noted 2 months postoperatively but returned to normal by 6 months.[11]

Durrie et al.[12] reported the potential reversibility of the ICR refractive effect. They showed that ICR explantation resulted in return of corneal curvature and refractive error to preimplant values. Similar results were reported by Davis et al.[13] and Twa et al.[14]

POSTOPERATIVE CARE AND MANAGEMENT

Immediately following surgery, an antibiotic-steroid combination ointment or solution (0.1% dexamethasone–0.3% tobramycin or equivalent) is applied to the operative eye. Small epithelial defects are treated with lubricating drops, and bandage contact lenses are used for large defects. The segment placement and incision closure should be observed using slit-lamp examination. The operative eye is protected with a clear shield, and the patient should be given appropriate postoperative instructions.[1] [15]

Foreign body sensation or “scratchiness” is common during the immediate postoperative recovery period. Symptoms of infection include dull, aching pain or discomfort, with or without photophobia, any time in the postoperative period. During recovery, eyes may feel dry the first 2–3 months. Expect vision to fluctuate during the first month.[1] [15]

ICRSs FOR KERATOCONUS AND AFTER LASIK

ICRSs have been used to treat patients with keratoconus. The results are very encouraging, especially in terms of decreasing astigmatism, increasing topographical abnormalities, and minimizing the risk of further progression of corneal ectasia. Similarly, ICRSs have been used as an adjunct to LASIK surgery.

Figure 25-6 Vacuum centering guide and stromal separator used to create the channel for the intrastromal corneal ring. (Reproduced with permission from Assil KK, Barrett AM, Fouraker BD, et al. One-year results of the intrastromal corneal ring in nonfunctional human eyes. Arch Ophthalmol. 1995;113:159–67.)

Figure 25-7 Slit-lamp photograph of the intrastromal corneal ring segments in position in the patient’s cornea. (Courtesy of Thomas Loarie.)

LASIK and ICRSs differ in several respects. LASIK is a more versatile technique that corrects low to moderately high levels of myopia (<10D) and myopic astigmatism (up to 5D). [16] [17] [18] In contrast, ICRSs are designed to treat only low levels of nearsightedness (up to 3D) without clinically significant astigmatism.[8] [14] The dissimilarity between the underlying mechanisms responsible for the induced central corneal flattening suggests that the procedures could be additive. Using ICRSs as an adjunct to LASIK surgery carries several of the advantages of using ICRSs to treat keratoconus,[9] without further compromising corneal thickness and stability.

Eyes receiving both LASIK and ICRSs as sequential surgeries have been reported. We have implanted ICRSs in patients as a means of treating residual myopia 2 years after LASIK. Further excimer laser surgery might have been unsafe in these patients and may have increased the risk of keratoectasia.[19] Our first patient was a 38-year-old woman with a corneal thickness of 539?µm who underwent LASIK on the right eye (OD) for a refractive error of -8.00 -0.50 × 75 (ablation depth 96?µm). Three months postoperatively, the UCVA was 20/20- OD. Over the following year, however, regression occurred, with a resultant UCVA of 20/40 and a refractive error of -1.00 -0.75 × 18. Given the patient’s low pachymetry value of 439?µm and a theoretical flap thickness of 180?µm, an ICRS procedure was offered as a means of treating the residual myopia. Three weeks after receiving 0.25?mm ICRS OD, the patient’s UCVA was 20/30 and her best spectacle-corrected visual acuity (BSCVA) was 20/20 with 11.75 -1.00 × 118. The second

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Figure 25-8 Surgical concept of gel injection adjustable keratoplasty. A 0.8?mm peripheral corneal incision is made at a selected depth (50–80% of corneal thickness) with a diamond knife. The stromal lamellae are then separated with a blunt spatula, and the lamellar plane guide is placed. The helicoid spatula is inserted below the guide, and the annular dissection is performed. Last, the gel is injected. (Reproduced with permission from Simon G, Parel J-M, Lee W, Kervick GN. Gel injection adjustable keratoplasty. Graefes Arch Clin Exp Ophthalmol. 1991;229:418–24.)

patient was a 50-year-old woman who had undergone LASIK on the left eye (OS) for a refractive error of -6.75 -1.50 × 180. Several months after surgery, the patient regressed to -1.00 -1.50 × 180, with a BSCVA of 20/25+ and a UCVA of 20/150. The patient underwent relifting of the flap and a second laser ablation, which resulted in a UCVA of 20/20. One year later, regression resulted in a UCVA of 20/60 and a BSCVA of 20/20 with -0.75 -0.75 × 175. Orbscan revealed a central pachymetry of 467?µm without signs of ectasia or irregularity. To prevent further corneal thinning, an ICRS procedure was offered as a means of treating the myopia. Six weeks after receiving 0.25?mm ICRS OS, the patient’s UCVA was 20/30- and her BSCVA was 20/20 with 11.50 -1.25 × 090.

Similar reports of sequential treatment have been published. Fleming and Lovisolo[20] reported a patient who received ICRS 10 months following LASIK that left a residual spherical equivalent (SE) of -3.375D. Four months after ICRS placement, the UCVA was 20/20. No flap complications occurred.

Eyes that received LASIK after ICRS have been reported only following explantation of the PMMA segments. Asbell et al. [21] described 10 patients who received LASIK following ICRS explantation. No complications were reported. Davis et al.[13] reported five patients who received LASIK following ICRS explantation for induced astigmatism and intraoperative complications. All patients experienced uneventful LASIK.

COMBINED LASIK AND ICRS

Combined LASIK and ICRS offers several theoretical advantages over other procedures used to correct high myopia. Early cataract

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Figure 25-9 Histological section of a cat cornea 23 months after being subjected to gel injection adjustable keratoplasty. Note the absence of a cellular reaction or scarring in the tissue that lines the gel injection adjustable keratoplasty site. (Courtesy of Jean-Marie Parel, MD, and Gabriel Simon, MD.)

formation, which has been observed with phakic intraocular lenses, may be avoided. Unlike with clear lens extraction, accommodation is preserved. As an extraocular procedure, the combined technique is less likely to incur the risks of intraocular surgery such as uveitis, glaucoma, retinal detachment, and endophthalmitis. Another potential advantage is that surgeons may be able to exchange ring segments to ameliorate presbyopic symptoms. If this change became unsatisfactory, the ICRS could theoretically be re-exchanged to restore the previous refraction.[10] [19]

Patients with relatively thin corneas and a moderate degree of myopia could benefit from combined surgery. Although these patients can often be treated successfully by LASIK alone, potential safety concerns exist. Keratoectasia has been reported in patients who were thought to have adequate remaining corneal thickness (at least 250?µm).[22] Up to 3D of myopia could be treated with ICRSs, sparing precious central stroma and improving corneal stability.

Patients with very low pachymetry values and high myopia are not good candidates for the combined procedure. Even with the ICRS, concerns over residual corneal thickness preclude LASIK ablations deep enough to provide good UCVA. Similarly, patients with thick corneas and lower degrees of myopia may be more suitable candidates for LASIK alone. We have performed simple calculations of the difference between pachymetry/10 and the spherical component of the refractive error at the corneal plane (astigmatism correction in minus cylinder format) to generate the ?10 constant, which we use as a guide for surgical options:

We consider performing LASIK without ICRS when ?10 is greater than 44. When ?10 is less than 42, we discourage keratorefractive surgery for fear of keratoectasia. We generally perform the combined ICRS-LASIK technique if preoperative calculations reveal a ?10 value of 43 ± 1 ( Fig. 25-10 ).

A problem associated with ICRS surgery is corneal flattening in the meridian of the ICRS incision (against-the-rule astigmatic shift). In our patients, induced astigmatism was also against the rule. For patients with high myopia undergoing ICRS-LASIK, we recommend performing 4–5D less of the LASIK ablation before placing the ICRS. This helps avoid a potential ICRS-induced overcorrection that could also result from a greater than expected flattening effect in the thinned stromal bed. One potential remedy for ICRS-induced astigmatism may be to incorporate any ICRS-induced refractive changes into a later LASIK retreatment.

Given the potential shortcomings of the combined procedure, we have narrowed our indications to myopic patients with

Figure 25-10 Combined ICRS–LASIK technique. The first step to prepare the intrastromal tunnel (1a and 1b). The dashed line indicates corneal flattening (1a). Step 2 involves creating a LASIK flap and insertion of the Intacs segments (2a and 2b). The LASIK flap is repositioned (3a and 3b) and can be lifted for retreatment at a later time.

corneal plane corrections of -6 to -12D with a ?10 of 43 ± 1, infrared pupillometry of 6?mm or less, no preoperative glare or haloes, no systemic disease, and no signs of keratoconus on examination and topography. The ICRS-LASIK procedure is limited by numerous factors, the first of which is the need for instruments that may not be available at many surgical facilities. Second, meticulous surgical technique and familiarity with the procedures are important to prevent intraoperative complications such as anteriorly placed intrastromal channels that could preclude the performance of LASIK. Third, the ICRS component provides only an additional 3D of correction. This value could, theoretically, increase in a corneal bed made thinner by a corneal flap and deep LASIK ablation. Therefore, in high myopes, we recommend performing only 70–80% of the LASIK ablation at the initial surgical encounter. Last, astigmatism can be induced by the ICRS. Although this may be amenable to LASIK retreatment, the laser could induce astigmatism that becomes manifest upon ICRS removal or exchange.[19]

We believe that the combined procedure is not applicable to keratoconus patients, because LASIK corrections in such patients have led to ectasia.[22] [23] Colin et al.[9] reported an increase in topographical regularity and decreased astigmatism after ICRS implantation in keratoconus patients. These patients, however, did not undergo lamellar surgery, which may cause corneal instability that is unaffected by the ICRS surgery. We therefore recommend that the combined procedure be avoided unless long-term results convincingly confirm the potential benefits.[19]

In summary, this combined technique is directed toward high and moderate myopes and may especially benefit those with relatively

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thin corneas. This combined procedure is relatively new, and its long-term results are still unknown.[19] Further studies are necessary to establish the efficacy, stability, and safety of ICRS-LASIK.

REFERENCES

1. Colin J, Cochener B. Intracorneal implants. In: Azar DT, ed. Intraocular lenses in cataract and refractive surgery. Philadelphia: WB Saunders; 2001:273–8.

2. Assil KK, Barrett AM, Fouraker BD, et al. One-year results of the intrastromal corneal ring in nonfunctional human eyes. Arch Ophthalmol. 1995;113:159–67.

3. Burris TE, Ayer CT, Evensen DA, et al. Effects of intrastromal corneal ring size and thickness on corneal flattening in human eyes. Refract Corneal Surg. 1991;7: 46–50.

4. Cochener B, Le Floch G, Colin J. Les anneaux intracorneens pour la correction des faibles myopies. J Fr Ophtalmol. 1998;21:191–208.

5. Fleming JF, Reynolds AE, Kilmer L, et al. The intrastromal corneal ring: two cases in rabbits. J Refract Surg. 1987;3:227–32.

6. Fleming JF, Wan WL, Schanzlin DJ. The theory of corneal curvature change with the intrastromal corneal ring. CLAO J. 1989;15:146–50.

7. Nosé W, Neves RA, Burris TE, et al. Intrastromal corneal ring: 12-month sighted myopic eyes. J Refract Surg. 1996;12:20–8.

8. Schanzlin DJ, Asbell PA, Burris TE, et al. The intrastromal corneal ring segments: phase II results for the correction of myopia. Ophthalmology. 1997;104:1067–78.

9. Colin J, Cochener B, Savary G, Malet F. Correcting keratoconus with intracorneal rings. J Cataract Refract Surg. 2000;26:1117–22.

10. Primack JD, Azar DT. A three-step approach combining LASIK and ICRS for correction of high myopia. Presented at the annual meeting of the American Society of Cataract and Refractive Surgery, Philadelphia, 2002.

11. Friedman NJ, Husain SE, Kohnen T, Koch DD. Investigational refractive procedures. In: Yanoff M, Duker JS, eds. Ophthalmology, 1st ed. London: Mosby; 1999:3.7.4–6.

12. Durrie D, Asbell PA, Burris TE. Reversible refractive effect of ICR. Presented at the annual meeting of the American Society of Cataract and Refractive Surgery, Seattle, 1996.

13. Davis EA, Hardten DR, Lindstrom RL. Laser in situ keratomileusis after intracorneal rings: report of 5 cases. J Cataract Refract Surg. 2000;26:1733–41.

14. Twa MD, Karpecki PM, King BJ, et al. One-year results from the phase III investigation of the KeraVision Intacs. J Am Optom Assoc. 1999;70:515–24.

15. Linebarger EJ, Song D, Ruckhofer J, Schanzlin DJ. Intacs: the intrastromal corneal ring. Int Ophthalmol Clin. 2000;40(3):199–208.

16. Lindstrom RL, Hardten DR, Chu YR. Laser in situ keratomileusis (LASIK) for the treatment of low, moderate, and high myopia. Trans Am Ophthalmol Soc. 1997;95:285–306.

17. Salah T, Waring GO III, El Maghraby A, et al. Excimer laser in situ keratomileusis under a corneal flap for myopia of 2 to 20 diopters. Am J Ophthalmol. 1996;121: 143–55.

18. Farah SG, Azar DT, Gurdal C, Wong J. Laser in situ keratomileusis: literature review of a developing technique. J Cataract Refract Surg. 1998;24:989–1006.

19. Primack J, Azar DT. A three-step procedure combining LASIK and ICRS. J Cataract Refract Surg. 2003 (in press).

20. Fleming JF, Lovisolo CF. Intrastromal corneal ring segments in a patient with previous laser in situ keratomileusis. J Refract Surg. 2000;16:365–7.

21. Asbell PA, Uçakhan ÖÖ, Durrie DS, Lindstrom RL. Adjustability of refractive effect for corneal ring segments. J Refract Surg. 1999;15:627–31.

22. Amoils SP, Deist MB, Gous P, Amoils PM. Iatrogenic keratectasia after laser in situ keratomileusis for less than -4.0 to -7.0 diopters of myopia. J Cataract Refract Surg. 2000;26:967–77.

23. McLeod SD, Kisla TA, Caro NC, McMahon TT. Iatrogenic keratoconus: corneal ectasia following laser in situ keratomileusis for myopia. Arch Ophthalmol. 2000;118:282–4.

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One comment on “Chapter 25 – Intrastromal Corneal Ring Segments for Low and High Myopia

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