Corneal ring segments and CAIRS for keratoconus: what the evidence shows

Ring segments are small curved implants placed inside the cornea to flatten it and make its shape more regular, and published studies report that vision improves on average after surgery. They have not been shown to stop keratoconus from getting worse, and most of the evidence comes from small studies with no comparison group.

What ring segments are

Synthetic ring segments are made of a medical plastic (PMMA). A surgeon places them in a channel away from the centre of the cornea. The added volume flattens the centre of the cornea and makes its curve more even [1]. The segments can be removed or exchanged later if needed [1].

CAIRS stands for corneal allogenic intrastromal ring segments. The segment is cut from donated human corneal tissue and placed in a similar channel. The technique was first described in 2018 [2].

Who they are considered for

In a 2026 systematic review of one widely used synthetic segment, the usual reason for surgery was mild to moderate keratoconus with falling vision and difficulty tolerating contact lenses [1]. Several studies in that review only treated corneas at least 400 micrometres thick and free of central scarring [1]. Eyes with very steep corneas or scarring tended to gain less [1].

CAIRS studies have included eyes from early to advanced keratoconus [3]. One study in a recent review found that eyes with extremely steep corneas (above 75 dioptres) did not gain a significant improvement in vision [3].

What studies report for synthetic segments

A meta-analysis pooled 95 case series covering 4,560 patients. Vision, astigmatism and corneal steepness all improved on average. The authors described the quality and strength of the underlying data as questionable [4].

The 2026 review (17 studies, 544 eyes) found that unaided vision improved by about two to four lines on the eye chart. Vision with glasses improved by about half a line to two lines [1]. One five-year study in that review found the gain in unaided vision faded over time and moved back towards the starting level [1].

Complications are uncommon but real. They include a segment moving or working its way out, inflammation around the segment, and infection. Some segments need to be removed [1].

A meta-analysis of 115 studies found similar visual results with a hand-made channel and a laser-made channel, with higher complication rates for the hand-made channel [5].

What studies report for CAIRS

Two meta-analyses have pooled the CAIRS studies:

• One covered 18 studies and 567 eyes. At one year, vision with glasses had improved by about three lines on average and the steepest part of the cornea was about 3.7 dioptres flatter [3].
• The other covered 14 studies and 442 eyes and found similar average gains. It recorded one serious complication, a rejection of the donor segment that required its removal [6].

A Brisbane case series of 85 eyes reported an average gain of four lines of unaided vision and two lines with glasses. Five eyes lost one or two lines of vision with glasses. No segment moved out of place or caused corneal melting during an average follow-up of seven and a half months [2].

Every study in the larger review was an uncontrolled case series. Follow-up was limited to one year, and the surgery was done by a small number of experienced surgeons [3]. The Brisbane authors note that the longest published follow-up for CAIRS is three years, so long-term safety and stability are unknown [2].

Do ring segments stop keratoconus progressing?

The evidence does not show that they do. The 2026 review states that synthetic segments do not halt keratoconus and are often combined with cross-linking for that reason [1]. A small study followed 14 children and teenagers for about six years after synthetic segments. Corneal steepness tended to drift upwards again, and the authors concluded that segments did not stop progression [7].

For CAIRS, reviewers say it remains uncertain whether the procedure slows progression, particularly in younger patients [3].

When segments and cross-linking are both planned, a 2026 meta-analysis of seven studies found similar vision and safety at 6 to 12 months whether they were done on the same day or in stages [8].

Will I still need glasses or contact lenses?

Often, yes. The 2026 review reports that a substantial proportion of patients still need contact lenses after synthetic segments. It adds that a more regular cornea may make lenses easier to tolerate, and that scleral lenses can be used over segments [1].

One early study compared CAIRS with a partial-thickness transplant (DALK) in 52 eyes with advanced keratoconus. Both groups improved. Vision with glasses was better after the transplant. The authors suggest CAIRS may be an intermediate step before a transplant in carefully selected eyes, and they call for larger and longer studies [9].

Talking to a surgeon

Whether a ring segment suits you depends on your corneal thickness, shape, scarring and whether your keratoconus is stable. A corneal surgeon makes that assessment. Dr James McKelvie's clinic describes the keratoconus surgery options offered at Eye Surgeons NZ.

This article is general information. It does not replace an examination or personal advice.

Choosing a surgeon: in the Waikato, corneal cross-linking is offered by Dr James McKelvie at Eye Surgeons NZ, Dr Michael Merriman at Hamilton Eye Clinic and Mr Chris Murphy. You are free to choose any of them, or another surgeon, and we will send your results to whoever you choose.

Care at Rose Optometry is provided by our optometrist team: Jagrut Lallu, Jacqueline Rowe, Emilie Lawson, Jason Shen, Jessica Wood, Stella Wong and Anjali Hira.

References

1. Czuj-Porębska, 2026, J Clin Med. DOI PubMed 42278940
2. Gunn, 2026, Clin Exp Ophthalmol. DOI PubMed 41823568
3. De Clerck, 2026, Medicina (Kaunas). DOI PubMed 41901604
4. Benoist d'Azy, 2019, Transl Vis Sci Technol. DOI PubMed 31211003
5. Struckmeier, 2022, Cornea. DOI PubMed 34839335
6. Friedrich, 2025, Am J Ophthalmol. DOI PubMed 40157443
7. Abreu, 2018, Int Med Case Rep J. DOI PubMed 29445305
8. Bonajmah, 2026, Int Ophthalmol. DOI PubMed 41874765
9. Kara, 2026, Cornea. DOI PubMed 41915699

This article is general information, not personal medical advice. Most sources were checked against the full published papers in October 2026.