Below I go through, in order, what CAIRS is, whom I perform it on, whom I do not, the day of surgery, what comes after, and the risks. Every number has the study it came from next to it; where I describe my own practice, I have marked it "in my practice". If keratoconus itself is new to you, start here: What is keratoconus, and what causes it? (in Turkish)
This page does not promise a result. CAIRS is a method that was described in 2018; there is as much we do not yet know about it as there is that we know, and I have written down both.
The short answer
CAIRS is the placement of a thin ring segment, prepared from human donor cornea, into a channel created within the cornea of an eye with keratoconus. The segment changes the shape of the cornea: it flattens the area that has become steep and reduces the irregularity of the surface. It does not halt the progression of the disease; the treatment that aims to do that is corneal cross-linking (CXL). It is not suitable for every patient with keratoconus. The decision is made from corneal topography, the thickness map and the examination.
- It does: regularise the shape of the cornea; published series have reported an average improvement in vision both with and without glasses.
- It does: add tissue to the cornea rather than remove it; the segment can be removed or repositioned if needed.
- It does not: halt the progression of keratoconus.
- It does not: remove the need for glasses or contact lenses in every patient, or make check-ups unnecessary.
If you would like to talk about your own eye: assessment appointment.
What is CAIRS?
What the abbreviation stands for
CAIRS is formed from the initials of Corneal Allogenic Intrastromal Ring Segments. "Intrastromal" means that the segment is placed inside the stroma, the middle layer of the cornea.
What "allogenic" means: the tissue comes from a donor
Allogenic means that the tissue comes from another person, that is, from a donor. The segment used in CAIRS is not taken from the patient's own eye; it is prepared from a donated human cornea. In that sense the procedure is a small-scale tissue transplant: the literature, too, describes it as transplantation of donor tissue into the stroma.1
What it does inside the cornea
The segment adds volume within the cornea, around the cone. The added volume changes the curvature of the cornea in that area; the steepened centre flattens, the surface becomes more regular, and vision benefits from this.1 Synthetic ring segments work on the same principle; the difference is in the material.
Clinical note: The method was described in 2018 by Jacob and colleagues. In the first series, segments cut from donor cornea with a trephine were placed into channels created at mid-depth with a femtosecond laser, and accelerated corneal cross-linking was performed in the same session: 24 eyes of 20 patients, stage 1–4 keratoconus, mean follow-up 11.58 ± 3.6 months (6–18 months).2 The authors call the study a "pilot".
Does CAIRS halt keratoconus?
No. CAIRS reshapes the cornea; the treatment that aims to halt the progression of the disease is corneal cross-linking (CXL). On this site I describe keratoconus treatment with three verbs: stabilise, reshape, replace. What stabilises is CXL. What reshapes is ring segments; this is where CAIRS belongs. What replaces is a corneal transplant, and it is the third step.
I state this distinction at the outset because it is the mistaken expectation I meet most often: "The segment is in, so it will not progress any more." The segment can improve vision; but the cornea's tendency to weaken remains, and monitoring continues.
Together with CXL, or separately?
There is no single rule in the literature. In the first description the two procedures were performed in the same session.2 A 2026 meta-analysis pooling 18 studies (567 eyes of 459 patients) did not show a clear superiority of any approach in the first year, in terms of either the technique used to prepare the segment or the addition of CXL.3 That finding concerns visual and topographic outcomes; it does not mean that CAIRS on its own halts progression.
The sequence in my practice: If corneal cross-linking has not been done before, I perform CAIRS first and CXL at least 6 months later. This is a different choice from the same-session approach of the first description; I am not saying it is the only right way. I have set out my reasoning, and the studies on this question, in a separate article: CXL or CAIRS: read which step you are on.
What this means in practice: If CAIRS has been recommended to you, ask your doctor two separate questions: "What are we doing for the shape?" and "What are we doing for progression?" The answer to the second cannot be CAIRS.
Who is it for?
The typical candidate
The patient with whom I discuss CAIRS is usually in one of three situations: they cannot see clearly with glasses; they cannot tolerate a contact lens on the eye; or they cannot see well enough even with a lens. One published series likewise describes CAIRS as an option for visual rehabilitation, particularly for patients who cannot tolerate contact lenses.4 I do not recommend CAIRS to a patient who sees comfortably and clearly with glasses or lenses simply because they "have keratoconus".
Corneal thickness and the position of the cone
In the area where the segment will sit, enough tissue has to remain above and below the channel. Where the cone lies on the cornea also determines the length and position of the segment.
In my practice: I generally look for at least 350 µm at the thinnest point of the cornea. My reasoning is this: a thinner cornea usually has a scar, and CAIRS does not work properly in scarred tissue. If the cornea has no scar and the thinnest point does not coincide with the area where I will create the channel, I also operate down to 300 µm; 300 µm is my lower limit. I have no fixed threshold for the steepest corneal value (Kmax).
In the literature: in the abstracts of the studies I read, I could not find a single agreed thickness threshold. 350 µm and 300 µm are my own thresholds; they should not be read as guideline values.
Age
In my experience: the youngest patient on whom I have performed CAIRS so far was 14 and the oldest was 62. This is not a limit; it is the range to date.
In the literature: in one series that reported an age range, patients were aged 20–52 (mean 29.5 ± 7.3).5 In my PubMed search I could not find a long-follow-up CAIRS series specific to children and adolescents; in this age group the evidence is even more limited than in adults.
More detail: Your child has keratoconus: is waiting always safe?
Eyes that have already had CXL
Previous corneal cross-linking is not an obstacle to CAIRS. In a retrospective study comparing 67 eyes (30 eyes with prior CXL, 37 eyes without), the flattening effect of CAIRS was seen in both groups; at month 6, the improvement in corrected vision, K1 and mean keratometry was greater in the group without CXL.6 The authors attribute this to the stiffening of the cornea by cross-linking.
Who is it not for?
The headings below describe my own practice: the situations in which I do not perform CAIRS, or postpone it. Another surgeon may draw the line differently.
A central scar that impairs the clarity of the cornea
CAIRS changes the shape of the cornea; it does not restore its clarity. If what impairs vision is a scar lying over the visual axis, correcting the shape will not improve vision to the extent expected.
A cornea too thin to leave room for the segment
If there is not enough tissue in the area where the channel is to be created, there is no physical bed in which to place the segment. In a 52-eye study (26 versus 26) comparing CAIRS with lamellar corneal transplant (DALK) in advanced keratoconus, corrected vision was better in the transplant group (P = 0.027); the authors describe CAIRS as an intermediate option before transplantation in carefully selected cases.7
In my practice, advanced disease is not an obstacle in itself: I also perform CAIRS in patients who have reached the transplant stage, as long as the cornea has no significant scar. What decides it is not the stage but whether the tissue is clear and thick enough.
Active infection, uncontrolled allergy and eye rubbing
I do not perform CAIRS on an eye with an active infection. In a patient with active allergy, or one who continues to rub their eyes, I also wait until these are brought under control first. In the literature, thinning of the corneal tissue over the segment (anterior stromal melt) was reported in a patient with severe atopy, together with a flare of atopy 4 months after surgery.8 It is a single case report; but it is warning enough to bring the allergy under control first.
Other situations in which I postpone
Previous hydrops (sudden swelling of the cornea with fluid), and pregnancy and breastfeeding, are also among the situations in which I do not perform CAIRS or postpone it.
Dry eye is not an obstacle. I do perform CAIRS in patients with dry eye; I am stating this plainly because it is asked so often.
If you are not suitable, which step is discussed
Not being suitable for CAIRS does not mean going without treatment. If the disease is not progressing and your vision is adequate, monitoring is discussed; if it is progressing, corneal cross-linking alone; if the shape is irregular but you do not want surgery, specialised contact lenses; if the cornea has lost its clarity or has become very thin, a corneal transplant. I have explained which of these applies to you in this article: the steps of monitoring, CXL, CAIRS and transplantation.
What this means in practice: The sentence "I will not perform CAIRS on you" is not bad news; most of the time it shows that there is a more suitable step for you on that day. Ask for the reason, and ask for your measurements in writing.
How does it differ from synthetic ring segments (INTACS)?
The principle of the two methods is the same: placing a segment within the cornea to change its shape. The difference lies in the material and in what follows from it. There is no comparison large enough to say which is superior; below I set out the difference, and what is unknown about each.
| Synthetic ring segment | CAIRS | |
|---|---|---|
| Material | A rigid plastic called polymethylmethacrylate (PMMA). | Tissue prepared from human donor cornea. |
| Shaping to the patient | Chosen from among pre-manufactured sizes. | The length, width and thickness of the segment can be adjusted to the eye; individually shaped segments have been described.9 |
| Reported problems | Melting of the tissue over the segment: among 643 synthetic segments implanted at a single centre, the 10-year incidence was reported as 5.5%.10 | The same problem has been reported at case-report level (two cases).8,11 One acute rejection in the 442-eye meta-analysis; displacement of the segment in one patient in a 65-eye series.12,5 |
| Body of data | There are series with ten-year follow-up data.10 | First described in 2018.2 The series with the longest follow-up that I could find is three years.13 |
There is one study that compares the two directly: 34 CAIRS eyes were matched with 34 synthetic segment eyes with similar characteristics (CXL was not added in either group). At the last visit the improvements were similar; the proportion of eyes gaining two or more lines of corrected vision was 60% in the CAIRS group and 31.58% in the synthetic group (P = 0.04). No major complication was seen in either group.14 The study is retrospective and not randomised; on its own it is not enough to say "CAIRS is superior".
Patients who already have a synthetic ring in the eye
In my practice: I do perform CAIRS in patients who have a synthetic ring in the eye. The synthetic ring is removed first, there is a wait of 3 months, and then CAIRS is performed.
In the literature: in a prospective series of 8 patients whose synthetic ring had been removed at least six months earlier, no complications were reported during six months of follow-up after CAIRS.15 In another series, synthetic rings over which the tissue had melted were exchanged for CAIRS in 8 eyes.10 The waiting time in the published reports is not uniform; 3 months is the interval I use in my practice. Both series are very small.
For the history of these two methods: From synthetic rings to CAIRS: the last 10 years.
Where does the tissue come from, and is it safe?
How donor tissue is selected and prepared
In the first description, the segments were cut from donor cornea during surgery with a double-bladed trephine.2 In some of the later series, pre-prepared sterile donor corneal segments were used;13,16 in others the segment was cut with a femtosecond laser.17 In other words, "CAIRS" is not a single product; it is the name of a method.
The tissue I use: screened, sterilised donor corneal tissue that comes from an eye bank.
Can the body reject this tissue?
It can; this has been reported rarely. In the meta-analysis pooling 14 studies and 442 eyes, the only serious adverse event reported was one case of acute rejection (0.2%), and it required removal of the segment.12 It is not correct to say "rejection does not happen"; it is correct to say "it was reported once in 442 eyes". In my practice: I prescribe steroid eye drops for 1 month after surgery.
The worry about "someone else's tissue"
I do not make light of this worry; tissue that comes from another person will be placed in your eye, and you need to make your decision knowing that. At the examination you can ask where the tissue comes from, how it was prepared and what documentation comes with it; these questions do not bother me. If you are not at ease with it, you do not have to choose CAIRS.
What this means in practice: Before surgery, ask this: "Does the segment arrive ready-made, or is it cut during surgery? Where does it come from?" You should get a clear answer.
Assessment: which measurement do I look at, and why?
The decision for CAIRS is not made from a single number. For diagnosis, staging and choice of treatment in keratoconus, the current international consensus document is the basic reference.18 Below I explain which measurement I look at during the examination, and why.
Corneal topography
It shows where the cone is, how steep it is and how irregular the surface is. From this map I plan where the segment will sit and how long it will be. If you have previous topographies, bring them: a single map shows today, a sequence of maps shows the course.
Corneal thickness map (pachymetry)
It shows the thinnest point of the cornea and the thickness in the area where the segment will sit. This map determines whether the channel can be created safely.
Vision, and how far it improves with glasses or lenses
I measure your vision separately without glasses, with glasses and, if you have them, with lenses. The difference between these shows how much of the problem comes from the shape, and what can be expected from surgery.
Before you come: sending your topography for a preliminary opinion
If you have a recent topography, you can send it to me before the examination: send your topography via Instagram. The username is @opdrserkangureser; it is a verified (blue tick) account, so do not confuse it with accounts that have similar names. I will look at it and tell you whether CAIRS is worth discussing in your case. This is a preliminary opinion, not a diagnosis; the decision is made from the examination and the measurements taken here.
Clinical note: During follow-up after CAIRS, Kmax can be misleading: the automatically calculated value is affected by the peripheral steepening over the segment. A 2026 commentary proposes using mean keratometry, the keratometry at the location of the preoperative Kmax, or the maximum flattening on the difference map instead.19
How is the surgery done, and how long does it take?
Here I describe my own practice. The procedure has three steps, takes about 10 minutes, and the patient goes home the same day.
- Numbing the eye. I use topical anaesthesia; the eye is numbed with drops.
- Creating the channel. I create the channel with a femtosecond laser, at a depth of 250 µm. I plan according to the Istanbul nomogram.16
- Placing the segment. The segment prepared from donor tissue is advanced into the channel and brought to the planned position. I do not place sutures; I put a bandage contact lens on the eye, which stays for 1 day.
It would not be correct to say that nothing will be felt during the procedure. In corneal procedures performed under topical anaesthesia, a sensation of touch or pressure and a bright light can generally be noticed; this varies from person to person.
Can both eyes be done on the same day?
In my practice: If both eyes are suitable, I see no objection to operating on both eyes in the same session. If one of the eyes is more advanced or its measurements are different, the plan is made separately for each eye.
Clinical note: Channel depth is not uniform across the published reports. In the first description the channels were created at mid-depth.2 In the published clinical results of the Istanbul nomogram (44 eyes of 32 patients, 6 months of follow-up), the channel is described at a depth of 200 µm;16 the depth I use in my practice is 250 µm, which is different from the published value. In one case in which a thick segment was placed at a depth of 260 µm, melting of the tissue over the segment was reported at month 10; the authors note that the limits for segment and channel dimensions, depth and position still need to be studied.11
What is recovery like after surgery?
The first day and the first week
In my practice: the bandage contact lens stays on the eye for 1 day. You use the steroid eye drops for 1 month. The one rule that does not change is this: do not rub your eye. This applies not only to the first week but for as long as you live with keratoconus.
Check-ups
The schedule in my practice: the first check-up on the morning after surgery, the second check-up at 1 month, and topography at month 3. We agree the interval for later check-ups at the visit.
Returning to daily life
The timings in my practice:
- Driving: after 3 days.
- Returning to work: 3–7 days.
- Water: no water should touch the eye for the first 5 days; after day 5 it may.
- Eye make-up: after 2 weeks.
- Sport: after 2 weeks.
- Swimming: after 2 weeks.
- Contact lenses: we discuss at the check-up when you can wear them again.
When does vision settle?
I cannot give a single timetable; the published reports, too, show a gradual course. In a one-year series of 20 eyes, the reduction in Kmax was measured at month 3 and remained stable until month 12; the thickness of the surface layer of the cornea (the epithelium), on the other hand, reached a steady state after month 6.17 In the three-year series, stromal thickness stabilised at month 6 and epithelial thickness at 1 year.13
What this means in practice: Do not judge the surgery by your vision in the first weeks. In my schedule I look at the shape of the cornea with the topography at month 3; do not miss that check-up.
What to expect from CAIRS: the numbers and the limits of the evidence
Published outcomes
First, a unit: logMAR is a scale that expresses visual acuity; the smaller the number, the better the vision, and a change of 0.1 corresponds to one line on the eye chart.
In the meta-analysis published in 2025, which pooled 14 studies and 442 eyes, the mean changes after CAIRS were as follows:12
- An improvement of 0.37 logMAR in corrected distance vision (with glasses) (95% confidence interval 0.28–0.46; 14 studies, 442 eyes).
- An improvement of 0.43 logMAR in uncorrected distance vision (0.34–0.55; 11 studies, 427 eyes).
- An improvement of 4.59 D in spherical equivalent (3.35–5.84; 12 studies, 430 eyes).
- A reduction of 4.49 D in the steepest corneal value (Kmax) (2.92–6.05; 13 studies, 439 eyes).
A second meta-analysis, published in 2026 (18 studies, 567 eyes of 459 patients), drew a similar picture: an improvement in uncorrected vision of 0.45 logMAR at 1 month and 0.39 logMAR at 1 year; and in corrected vision of 0.36 logMAR at 1 month and 0.34 logMAR at 1 year.3
These are averages; individual eyes are spread on either side of the average. In the first-description series, the gain in uncorrected vision ranged from 0 to 8 lines, and the gain in corrected vision from 0 to 5 lines.2 In other words, there were also eyes that gained no lines at all.
What we do not yet know
- The long term. The method was described in 2018.2 The series with the longest follow-up that I could find is three years: 62 eyes of 49 patients; corrected vision went from 0.72 ± 0.47 to 0.22 ± 0.19 logMAR, and no major complication was reported.13 There are no data showing what happens after ten years.
- Study design. None of the series I read for this page is a randomised controlled trial; they are retrospective or single-arm series, most of them from a single centre. The meta-analyses, too, are the sum of these series.
- Late problems. Melting of the tissue over the segment appeared at month 4 in one case and at month 10 in another.8,11 Series with short follow-up can miss events of this kind.
- The yardstick. Kmax, which most studies use as their measure of success, can be misleading after CAIRS.19
Will I need glasses or contact lenses after CAIRS?
In most patients, yes. CAIRS is not an operation for getting rid of glasses or lenses; its aim is to obtain a cornea that can be corrected with glasses or lenses. The averages in the series show this too: in a 65-eye series the mean spherical equivalent went from −8.82 D to −3.45 D,5 and in a 20-eye series from −6.79 D to −1.88 D.17 The prescription falls, but it does not go to zero. More on lenses: If lenses are enough, is surgery necessary?
Our own study on this subject: I will present the interim results of the eyes in which we have performed CAIRS, as a multi-surgeon series prepared together with three colleagues, as an oral presentation at the KCXL International Experts' Meeting to be held in Zurich on 26–28 November 2026. I will add the results to this page after the presentation.
What this means in practice: Set the goal not as "life without glasses" but as "seeing more clearly and more comfortably with glasses or lenses". If anyone promises you a number of lines, ask which study it is based on.
Risks and possible problems
What to expect in the first days
Stinging, watering, sensitivity to light and blurred vision are complaints that can generally be expected in the first days after corneal procedures; how long they last and how severe they are varies from person to person.
Rare but serious
- Rejection of the donor tissue. One case in the 442-eye meta-analysis (0.2%); the segment was removed.12
- Displacement of the segment. In a series of 65 eyes of 49 patients, in one patient the segment shifted towards the entry site of the channel within the first week and the entry site opened.5
- Thinning of the tissue over the segment (anterior stromal melt). There are two case reports: one in a patient with severe atopy, the other in a patient with no known risk factor. In both, the condition was arrested with drops and medication, and the segment stayed in place.8,11
- Deposits in the channel. In the same 65-eye series, yellow-white deposits were seen inside the channel in five cases after month 6.5
- Infection. As with any procedure on the cornea, it is possible here too. I could not find a rate specific to CAIRS in the abstracts I read; for that reason I am not giving a number.
If you have increasing pain, increasing redness, discharge or a sudden drop in vision, do not wait for your check-up day. I give the patients I operate on a number on which they can reach me directly in situations of this kind.
If the expected improvement does not come: repositioning or exchange
Not every eye gets the expected result from the first placement. In a series of 13 eyes of 13 patients, in eyes whose vision or topography fell short of expectations after the first surgery, the arc length, width, thickness, position or optical zone of the segment was readjusted. After the first surgery 2 patients had lost lines of uncorrected vision and 5 patients lines of corrected vision; after the adjustment no patient was left with a loss of lines.20 It is a small series; but it shows that a second intervention is possible.
In my experience: I have performed a revision in 2 patients; I added an additional segment.
What CAIRS does not change
- The tendency of the disease to progress. That needs a separate decision (CXL).
- The effect of eye rubbing on the cornea. Having a segment in place does not make rubbing harmless.
- The need for monitoring. Topography check-ups continue after surgery as well.
What this means in practice: Before you sign the consent form, you should know the answers to these three questions: "What is the most likely problem in my case? If it happens, what will we do? How will I reach you?"
Can it be undone?
In CAIRS no tissue is removed from the cornea; tissue is added. The added segment can be removed: in the rejection case in the meta-analysis, the segment was removed.12 The position and dimensions of the segment can also be changed afterwards.20
But I use the word "reversible" with care. A channel has been created within the cornea, and in the publications I read I could not find a CAIRS series showing how far the cornea returns to its preoperative shape after the segment is removed. The correct sentence is this: the segment can be removed or exchanged; I cannot say that the cornea returns to the state of one that has never been operated on.
In my experience: I have not removed a segment to date.
Frequently asked questions
I have already had corneal cross-linking (CXL); can CAIRS still be done?
Yes, previous corneal cross-linking is not an obstacle to CAIRS. In a 67-eye study, the flattening effect of CAIRS was seen both in the 30 eyes that had previously had CXL and in the 37 that had not; the effect was more limited in the eyes that had had CXL. The decision is still made from your topography and thickness map.
Is CAIRS enough on its own, or is CXL needed afterwards as well?
CAIRS changes the shape of the cornea; it does not halt the progression of the disease. The treatment that aims to halt progression is corneal cross-linking. The sequence in my practice: if CXL has not been done before, CAIRS first, then CXL at least 6 months later.
I have a synthetic ring in my eye; can I switch to CAIRS?
Yes. In my practice: the synthetic ring is removed first, there is a wait of 3 months, and then CAIRS is performed. There are examples in the literature too: in a series of 8 patients whose synthetic ring had been removed, no complications were reported during six months of follow-up after CAIRS. That series is very small, and the 442-eye meta-analysis excluded eyes that had previously had a ring implanted.
What will I feel during the surgery?
I perform the surgery under topical (eye-drop) anaesthesia; the procedure takes about 10 minutes. It would not be correct to say that nothing will be felt: in corneal procedures performed under topical anaesthesia, a sensation of touch or pressure and a bright light can generally be noticed. This varies from person to person; ask whatever is on your mind at the examination.
When can I go back to work, screens and driving?
The timings in my practice: driving after 3 days, returning to work in 3–7 days. No water should touch the eye for the first 5 days; for eye make-up, sport and swimming the wait is 2 weeks. We discuss at the check-up when you can go back to contact lenses.
Is CAIRS performed in patients under 18?
In my experience: the youngest patient on whom I have performed CAIRS so far was 14; this is not a limit, it is the range to date. In the literature, in one series that reported an age range, patients were aged 20–52; in my PubMed search I could not find a long-follow-up CAIRS series specific to children and adolescents. In this age group the decision is made together with the family, at the examination.
Does CAIRS rule out laser, an intraocular lens or a transplant later on?
In the literature CAIRS is treated as a step before transplantation: in a comparison carried out in advanced keratoconus, the authors describe CAIRS as an intermediate option before transplantation in selected cases. Laser or an intraocular lens after CAIRS is assessed individually once the cornea has settled; we discuss this at the examination.
What does the price of CAIRS depend on?
I am not giving a figure here. The main items that affect the price are the supply of the donor tissue, the use of the femtosecond laser, and whether the procedure is done on one eye or both. Exact information is given after the examination and measurements.
I am travelling from another city; how does the process work?
The assessment is made with the same measurements; in patients I find suitable, the examination and the surgery can be done on the same day.
More detail: CAIRS for patients travelling from out of town
Travelling from another city? CAIRS for patients travelling from out of town
Assessment appointment
Let us assess together, with your topography and an examination, whether you are suitable for CAIRS.
You can send your topography via Instagram before you come; I will give a preliminary opinion, which is not a diagnosis.References
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- Keskin Perk FFN, Tanriverdi C, Karaca ZY, Tran KD, Kilic A. Long-Term Results of Sterile Corneal Allograft Ring Segments Implantation in Keratoconus Treatment. Cornea. 2025;44(4):475-482. PMID 38900741
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