Introduction

Several methods of altering the anterior corneal curvature to modify the refractive status of the eye have been developed.

In 1987, Reynolds began experimenting with implanted rings within the corneal stroma, introducing the concept of an intrastromal corneal ring (ICR) as a refractive device with the potential to correct myopic and hyperopic refractive errors.

Finite element modeling analysis demonstrated that intrastromal rings induce central corneal flattening by acting as a space-occupying element within the corneal stroma. The spacer effectively produces a shortening of the central corneal arc length. Because there is no tissue removal associated with corneal flattening, the normal physiologic prolate shape of the cornea is maintained.

The ICR’s flattening effect has an important role in the management of keratoconus and has been proved to be a safe and effective technique. The types of ICRs most commonly used are Keraring (Mediphacos; Fig. 26.1 ) and Intacs (Addition Technologies; Fig. 26.2 ). Characteristics of these types of ICRs are shown in Table 26.1 . The Intacs SK and Myoring (Dioptex), a 360-degree continuous full-ring implant placed into a corneal pocket, are usually indicated in advanced keratoconus when a more prominent effect is desired.

Keraring intracorneal ring segments implanted in the cornea of a patient with keratoconus.

Intacs intracorneal ring segments implanted in the cornea of a patient with keratoconus.

Results of Clinical Trials With the Intrastromal Corneal Ring

Sighted Eye Studies

After phases I, II, and III and development from 360-degree rings to small arcuate segments to correct corneal astigmatism, a new paired 150-degree arcuate segment device was developed. Titration of the refractive correction was achieved by varying the device thickness.

The results of these studies verified the utility of ICRs as a method to alter the anterior corneal curvature and refractive status of the eye. The ICR material is well tolerated and does not produce deleterious alterations in the nutritional or metabolic activity of the cornea when placed at the appropriate depth. There have been no significant alterations of intraocular pressure (IOP) associated with the ICR. Experimental studies have shown that placement of the ICR device does not interfere with accurate measurement of IOP. In patients, no differences between Goldman and Tonopen tonometry measurements have been noted.

The 150-degree paired arcuate device tested in the clinical trials was approved by the US Food and Drug Administration (FDA) in April 1999, at which time the name Intacs (Keravision) was adopted.

Ferrara ICR segments or Keraring (Mediphacos) are variants of Intacs, which have a triangular cross-section and work like a prism, reflecting away light that hits the rings ( Fig. 26.3 ).

Cross-sectional triangular shape of a Keraring intracorneal ring segment that generates a prismatic effect, reducing the incidence of glare and halos.

(Courtesy of Mediphacos, Belo Horizonte, MG, Brazil.)

The biomechanical corneal effect of ICR implantation has been studied by Piñero et al. through the Ocular Response Analyzer (ORA). A case series of 45 consecutive keratoconic eyes were retrospectively analyzed after the Keraring ICR implantation by femtosecond (FS) laser. Significant changes in ORA parameters were observed 3 months after surgery. Preoperative corneal hysteresis (CH) and corneal resistance factor (CRF) were correlated with postoperative corneal high-order aberration. It was observed that corneal biomechanical changes after ICR implantation are not maintained in all cases in a medium to long-term follow-up. Some corneas may continue to show optical deterioration owing to biomechanical alterations despite ICR implantation.

The effect of Keraring ICR mechanical implantation on the quality of life of patients with keratoconus have been studied by de Freitas Santos Paranhos et al. through the National Eye Institute Refractive Error Quality of Life (NEI-RQL) instrument. The different items in the NEI-RQL scale that have improved include “clarity of vision,” “far vision,” “near vision,” “activity limitations,” “appearance” and “satisfaction with correction.” This study has shown a positive impact on patients’ quality of life after ICR implantation.

There have been some reports of bacterial keratitis after Keraring ICR implantation; some of these patients may require penetrating keratoplasty. Viral keratitis have also been reported in a patient with a history of herpetic keratitis 5 years prior to ICR implantation. Other complications reported after Intacs implantation include corneal thinning and segment exposure (6/20), anterior chamber perforation (1/10), dense corneal infiltrate (1/20), and corneal melting with ICR extrusion. Despite these sight-threatening complications that both patients and clinicians must be aware of, they are encountered infrequently. Tognon et al. analyzed the visual outcomes of 1222 eyes from 1196 patients that received a Keraring ICR, with either manual (mechanical) implantation or FS laser–assisted implantation, for which only 67 patients experienced surgical complications, including: external environment or anterior chamber perforation, late (≥30 days) or early infection, late or early segment extrusion and malposition/movement of the intrastromal corneal ring segments after the procedure.

Studies performed by Piñero et al. through multiple regression analysis have suggested that the monograms for selection of the ICR in keratoconus should not only be based on refraction and subjective appearance of the corneal topographic pattern but rather the corneal should also be considered.

Nonfunctional Eye Study

To evaluate the safety and efficacy of the ICR, a phase I clinical trial was initiated in July 1991. In this study, the ICR was well tolerated in all patients with no complications that necessitated its removal. There were no problems with wound healing after implantation of the device. There were no instances of implant extrusion, undue inflammation, or stromal thinning throughout the 12-month follow-up period.

The ICR was noted to induce a mean keratometric flattening and the retinoscopic spherical equivalence was reduced. These parameters appeared stable throughout the follow-up period.

Sighted Eye Studies

After phases I, II, and III and development from 360-degree rings to small arcuate segments to correct corneal astigmatism, a new paired 150-degree arcuate segment device was developed. Titration of the refractive correction was achieved by varying the device thickness.

The results of these studies verified the utility of ICRs as a method to alter the anterior corneal curvature and refractive status of the eye. The ICR material is well tolerated and does not produce deleterious alterations in the nutritional or metabolic activity of the cornea when placed at the appropriate depth. There have been no significant alterations of intraocular pressure (IOP) associated with the ICR. Experimental studies have shown that placement of the ICR device does not interfere with accurate measurement of IOP. In patients, no differences between Goldman and Tonopen tonometry measurements have been noted.

The 150-degree paired arcuate device tested in the clinical trials was approved by the US Food and Drug Administration (FDA) in April 1999, at which time the name Intacs (Keravision) was adopted.

Ferrara ICR segments or Keraring (Mediphacos) are variants of Intacs, which have a triangular cross-section and work like a prism, reflecting away light that hits the rings ( Fig. 26.3 ).

Cross-sectional triangular shape of a Keraring intracorneal ring segment that generates a prismatic effect, reducing the incidence of glare and halos.

(Courtesy of Mediphacos, Belo Horizonte, MG, Brazil.)

The biomechanical corneal effect of ICR implantation has been studied by Piñero et al. through the Ocular Response Analyzer (ORA). A case series of 45 consecutive keratoconic eyes were retrospectively analyzed after the Keraring ICR implantation by femtosecond (FS) laser. Significant changes in ORA parameters were observed 3 months after surgery. Preoperative corneal hysteresis (CH) and corneal resistance factor (CRF) were correlated with postoperative corneal high-order aberration. It was observed that corneal biomechanical changes after ICR implantation are not maintained in all cases in a medium to long-term follow-up. Some corneas may continue to show optical deterioration owing to biomechanical alterations despite ICR implantation.

The effect of Keraring ICR mechanical implantation on the quality of life of patients with keratoconus have been studied by de Freitas Santos Paranhos et al. through the National Eye Institute Refractive Error Quality of Life (NEI-RQL) instrument. The different items in the NEI-RQL scale that have improved include “clarity of vision,” “far vision,” “near vision,” “activity limitations,” “appearance” and “satisfaction with correction.” This study has shown a positive impact on patients’ quality of life after ICR implantation.

There have been some reports of bacterial keratitis after Keraring ICR implantation; some of these patients may require penetrating keratoplasty. Viral keratitis have also been reported in a patient with a history of herpetic keratitis 5 years prior to ICR implantation. Other complications reported after Intacs implantation include corneal thinning and segment exposure (6/20), anterior chamber perforation (1/10), dense corneal infiltrate (1/20), and corneal melting with ICR extrusion. Despite these sight-threatening complications that both patients and clinicians must be aware of, they are encountered infrequently. Tognon et al. analyzed the visual outcomes of 1222 eyes from 1196 patients that received a Keraring ICR, with either manual (mechanical) implantation or FS laser–assisted implantation, for which only 67 patients experienced surgical complications, including: external environment or anterior chamber perforation, late (≥30 days) or early infection, late or early segment extrusion and malposition/movement of the intrastromal corneal ring segments after the procedure.

Studies performed by Piñero et al. through multiple regression analysis have suggested that the monograms for selection of the ICR in keratoconus should not only be based on refraction and subjective appearance of the corneal topographic pattern but rather the corneal should also be considered.