Intracorneal Ring Segments for Keratoconus

Cheryl Guttman Krader reports.

The use of intracorneal ring segments (ICRS) to reduce corneal steepening and astigmatism associated with keratoconus has grown since first proposed by Joseph Colin MD in 2000. Performed worldwide in more than 500,000 eyes, several companies are now marketing intracorneal rings/ring segments for treating keratoconus.

“Experience confirms the procedure is safe, and the rings are very well-tolerated in the long-term. In addition, a meta-analysis of published reports of more than 4,500 eyes found ring implantation has a large positive effect for improving both uncorrected and corrected distance visual acuity. Currently, ring insertion cannot be considered as a refractive surgery procedure for keratoconus,” said David Touboul MD, PhD.

“Still, further technological refinements and combined strategies are pending to make this modality even more reliable and efficient for targeting emmetropia. More and more often, ring implantation is being combined with cross-linking, phototherapeutic keratectomy, phakic IOLs, or IOLs to limit progression of keratoconus and treat anisometropia.”

Mechanisms and outcomes

Ring insertion into the deepest layers of the corneal stroma induces a local curvature inflexion, resulting in corneal flattening. This effect amplifies with the device’s increasing thickness and is inversely proportional to the corneal diameter at the implantation site.

Although corneal biomechanics would be expected to change with the corneal curvature induced by the rings, biomechanics parameters measured with various elastographic devices are unchanged.

“Stress redistribution should occur and could explain a trend seen in some studies for the rings to inhibit keratoconus progression,” Prof Touboul said. “However, inhibition of eye rubbing by patients who fear the manipulation will cause ring extrusion may also play a role in limiting disease progression.”

He noted the keratometric outcome of the surgery is more predictable in eyes with low keratoconus stages than in those with more advanced disease. Functionally, however, eyes with stage 2 and 3 keratoconus derive greater benefit since eyes with forme fruste or stage 1 have better visual function at the time of surgery and greater risk for BCVA loss.

Surgical planning

Nomograms for ICRS implantation are mostly based on anterior topographic pattern and refraction, which are controversial and imperfect for failing to consider many parameters.

“The most important things to know are corneal flattening is increased by symmetrical implantation, greater ring thickness, narrower channels, more superficial implantation, and the smaller diameter in the cornea where the device is implanted decreases,” he explained. “Astigmatism is reduced by making the incisions on the steepest meridian, inserting rings on the flattest meridian, and using the smallest arc length.”

He noted symmetrical implantation also addresses defocus and aspheric aberration, while asymmetrical implantation addresses comatic aberration. Combining ring insertion with topoguided photokeratectomy can also dramatically decrease coma if enough tissue is available. Additionally, newer ring designs are useful for correcting coma and astigmatism.

Satisfactory safety

Research so far indicates the overall rate of complications with ring insertion is less than 5%. The problems include keratitis, epithelial ingrowth, extrusion, hydrops, migration/decentration, and breaks.

“To the best of our knowledge, no large studies had follow- up longer than 5 years after implantation. But to date, there is no epidemic of ring exchange, migration, or extrusion after almost 20 years of use worldwide,” Prof Touboul said.

Extrusion risk factors include superficial ring implantation, excimer laser enhancement, eye rubbing, and atopy. Although late extrusion is very rare, he cautioned it is a concern.

“Extrusion can occur spontaneously with few symptoms, and it must be treated quickly with ring removal. Corneoplastic patching has been tried, but it is not easy,” he said.

Ring implantation for keratoconus has also been associated with photic optical aberrations, especially in patients with large pupils implanted with rings at a small diameter in the cornea. Patients usually describe the symptoms as very different from the halos or glare experienced the night before ring surgery and typically tolerate them well, thanks to neuroadaptation.

Literature reports the removal of less than 5% of implanted rings. The procedure is easy if done within the first few months after implantation and allows for exchange with or without a new channel. Later removal is more challenging but usually beneficial, even though fibrosis around the ring may prevent complete restitution of the previous corneal shape.

“Ring placement will not compromise the ability to perform deep anterior lamellar keratoplasty if it becomes necessary,” he added.

Prof Touboul presented at the 40th Congress of the ESCRS in Milan.

David Touboul MD, PhD is an ophthalmologist at the University Hospital of Bordeaux, France. david.touboul@chu-bordeaux.fr