Corneal Tissue Addition Techniques (CAIRS and CTAK)

Key points at a glance

• CAIRS is a procedure that improves corneal shape by inserting donor corneal tissue into the stroma.
• Introduced in 2018 by Dr. Soosan Jacob as a biocompatible alternative to synthetic ICRS
• CTAK is a related procedure using gamma-irradiated donor segments from CorneaGen
• Main indications include corneal ectasia, including keratoconus
• Compared to synthetic ICRS, it has lower risk of exposure, melting, and corneal neovascularization
• The AAO PPP also mentions CAIRS as an alternative to ICRS ²⁾

1. Corneal Tissue Addition Techniques (CAIRS and CTAK)

Corneal Allogenic Intrastromal Ring Segments (CAIRS) are a surgical technique that improves corneal shape by inserting stromal segments derived from donor corneal tissue into the cornea for corneal ectasia including keratoconus. First reported by Dr. Soosan Jacob in 2018 ²⁾. Unlike synthetic polymer ICRS (intrastromal corneal ring segments), they are highly biocompatible and integrate naturally with host tissue.

Corneal Tissue Addition Keratoplasty (CTAK) is a related procedure that uses gamma-irradiated donor segments processed by CorneaGen. It differs from CAIRS in that the segments are created and inserted using only a femtosecond laser.

Dr. Jack Parker developed manual insertion techniques and a pre-insertion dehydration method (the “Corneal Jerky” technique), expanding surgical flexibility and accessibility.

CAIRS

Donor tissue: Corneal tissue from eye banks (e.g., KeraNatural®)

Preparation: Manual or femtosecond laser-guided

Features: Arc length, thickness, and taper can be customized intraoperatively. Can be performed in resource-limited settings

CTAK

Donor tissue: Gamma-irradiated segments processed by CorneaGen

Fabrication method: Femtosecond laser only

Features: Additional sterilization steps and precise segment measurement are standardized. Topography-based customization is possible.

Q What is the difference between CAIRS and synthetic ICRS (INTACS, Keraring)?

A

Synthetic ICRS are made of rigid polymer (PMMA) and inserted into the deep corneal stroma, while CAIRS are made from human donor corneal tissue and can be inserted at shallower depths. CAIRS offer advantages such as high biocompatibility, lower risk of exposure, corneal melting, and neovascularization; free customization of arc length and thickness; and fewer optical aberrations (glare, halos).

2. Main Symptoms and Clinical Findings

Indicated Diseases

The main indications for CAIRS and CTAK are as follows:

• Keratoconus: Progressive corneal thinning and conical protrusion causing irregular astigmatism. Cases with insufficient correction by contact lenses.
• Post-LASIK ectasia: Progressive corneal deformation occurring after refractive surgery.
• Failed synthetic ICRS: Cases with complications such as ICRS exposure, intracameral migration, or corneal melting ¹⁾.

Additional eligibility criteria include insufficient correction with glasses or contact lenses, contact lens intolerance, and age 18 years or older.

Subjective Symptoms

Common subjective symptoms of corneal ectasia include:

• Progressive vision loss: Due to increasing irregular astigmatism, vision becomes uncorrectable with glasses.
• Frequent prescription changes: Initially correctable, but corrective effect gradually diminishes.
• Contact lens instability: decentration or loss of hard contact lenses

Clinical Findings

In keratoconus, the central corneal stroma thins and protrudes forward. Characteristic findings include Vogt’s striae (fine vertical lines in the deep stroma), Fleischer ring (iron deposition in the epithelium at the base of the cone), and corneal scarring ²⁾. Corneal topography shows localized steepening and asymmetric patterns.

Keratoconus is usually bilateral, but the severity often differs between eyes. It typically begins in adolescence and tends to stabilize or slow progression around age 30 ³⁾. Without treatment, approximately 20% of cases require corneal transplantation ²⁾.

Contraindications

• Central corneal scarring or opacity
• Extreme corneal thinning (less than 320 μm)
• Active corneal infection or inflammation
• Autoimmune or connective tissue disease
• Pregnancy and breastfeeding

4. Diagnosis and Examination Methods

Preoperative Evaluation

Evaluation Item	Details
Visual Acuity Test	UDVA and CDVA measurement
Corneal topography	Kmax, Kmean, corneal asymmetry
Anterior segment OCT	Corneal thickness and cone location assessment

Slit-lamp examination checks for corneal thinning, protrusion, Vogt striae, and Fleischer ring ²⁾. Corneal tomography (e.g., Pentacam) obtains anterior and posterior curvature, corneal thickness map, and elevation map to evaluate the degree and progression of corneal ectasia ²⁾.

In children and young adults, keratoconus may progress rapidly ³⁾. The KERALINK trial investigated the efficacy of cross-linking in slowing progression in young patients ⁴⁾.

Surgical planning and nomograms

Several nomograms have been proposed for CAIRS surgical planning.

Jacob nomogram: Incorporates cone location, eccentricity, corneal curvature values, and asymmetry pattern to customize the number, arc length, and thickness of segments.

Istanbul nomogram: Planning based on cone location (central/paracentral). Recommends a channel depth of 250 μm, but shallower depths may be effective in steep cones ¹⁾.

Awwad nomogram: Adapted from ICRS nomograms, based on topography, tomography, and clinical judgment.

Currently, there is no universally accepted CAIRS nomogram. Due to the morphological diversity of keratoconus, surgeon judgment and individualization are important.

5. Standard treatment

Donor tissue preparation

CAIRS segments are derived from human donor corneas. There are two main procurement pathways.

Pre-packaged segments from eye banks: Eye banks such as Lions VisionGift provide pre-cut, sterile stromal segments (KeraNatural®). They can be stored at room temperature and eliminate the need for intraoperative customization.

Custom preparation: Manually remove epithelium, Descemet’s membrane, and endothelium from the corneal rim, and create ring segments with a trephine. Arc length, thickness, taper, and curvature can be individually adjusted.

In CTAK, CorneaGen’s preserved and gamma-irradiated tissue is washed with BSS and mounted on a femtosecond laser fixation device.

Channel Creation

Channels are created using a femtosecond laser or manual dissection. Channel depth is set to 35–70% of total corneal thickness (or a fixed depth of 250 μm from the surface). The entry incision is made on the steep axis or superiorly, and channels are created with an inner diameter of 4–6.5 mm and an outer diameter of 6.1–8 mm.

For synthetic ICRS, channels are created in the deep stroma (70–80% depth) ²⁾, while CAIRS can be inserted at shallower depths, potentially providing greater flattening effect ¹⁾.

Hydrated Method

Procedure: Insert the hydrated segment into the channel using forceps or a Y-rod.

Advantages: Maintains natural flexibility and original thickness, gently conforming to corneal curvature.

Cautions: Risk of kinking or twisting due to flexibility. Mark the Bowman’s membrane side with a marker to confirm orientation.

Dehydrated Method "Corneal Jerky"

Procedure: Air-dry the segment for 20–75 minutes before insertion, then rehydrate with BSS postoperatively.

Advantages: Becomes hard and smaller, allowing rapid insertion. Surgical time is reduced by about one-third.

Cautions: Facilitates the use of larger segments for severe keratoconus.

Postoperative Management

• Topical antibiotics: 4 times daily for 1–2 weeks
• Topical steroids: Start 4 times daily, taper over 4–6 weeks
• Bandage contact lens: May be used for a few days after surgery

Postoperatively, corneal shape changes and segment position are evaluated using topography/tomography and anterior segment OCT. UCVA, CDVA, Kmax, and Kmean are tracked over time.

Rescue for Failed Synthetic ICRS

CAIRS is effective as a rescue for failed synthetic ICRS cases ¹⁾. In a 49-year-old woman with ICRS failure (UCVA 20/400), the synthetic ICRS was removed and CAIRS inserted 3 months later, improving Kmax from 68.9 to 61.9 D and UCVA to 20/30 ¹⁾. In a 40-year-old woman with a broken ICRS, CAIRS was inserted alongside the remaining ICRS, improving UCVA from 20/60 to 20/25 and BCVA to 20/20 ¹⁾.

Important Notes

CAIRS is a relatively new procedure; the longest follow-up reported to date is 3 years. Long-term efficacy and stability require further data accumulation. Obtain thorough preoperative explanation and consent.

Q Is the effect of CAIRS permanent?

A

Current reports show that CAIRS maintains stable improvement for up to 3 years. All visual and topographic parameters stabilize by 6 months and are maintained for 3 years. However, longer-term data are limited, and further research is awaited.

6. Pathophysiology and Detailed Mechanism

Mechanism of Action of CAIRS

CAIRS segments are inserted into the corneal stroma, physically flattening the steep area of the cornea. Similar to synthetic ICRS, the arc shortening effect reduces the curvature of the cone and decreases irregular astigmatism.

Synthetic ICRS must be inserted at a deep stromal depth (70–80%), whereas CAIRS can be inserted at shallower depths (35–70%) ¹⁾. Shallower insertion may produce a greater flattening effect. Donor corneal tissue is optically harmonious with the host cornea, unlike synthetic polymers, resulting in less glare and halos from edge effects.

Biocompatibility

Allogeneic segments are implanted into the avascular, low-cell-density corneal stroma. In this environment, fibrous adhesion is minimized, preserving surgical reversibility. The risks of corneal melting, acute stromal necrosis, and corneal neovascularization, which are problematic with synthetic ICRS, are also reduced. CAIRS insertion does not preclude future additional interventions such as DALK.

Although the possibility of rejection theoretically exists, the risk is considered lower than that of DALK (deep anterior lamellar keratoplasty).

TGF-β and Corneal Remodeling

Mechanical insult to the cornea induces a wound healing response via TGF-β signaling. In CAIRS, unlike synthetic materials, the donor tissue is equivalent to ECM components, so foreign body reaction is thought to be minimized.

7. Latest Research and Future Perspectives

Clinical Outcomes

According to a systematic review (Levy et al., 2025), after CAIRS implantation, mean UDVA improved from 0.83 to 0.40 logMAR, CDVA from 0.52 to 0.19 logMAR. Spherical equivalent decreased from −7.09 D to −2.34 D, Kmax from 57.8 to 53.6 D, and Kmean from 49.3 to 45.3 D.

Parameter	Preoperative	Postoperative
UDVA (logMAR)	0.83	0.40
CDVA (logMAR)	0.52	0.19
Kmax (D)	57.8	53.6

In CTAK, Greenstein et al. (2023) reported similar improvements in 21 eyes. Mean UDVA improved from 1.21 to 0.61 logMAR, and CDVA improved from 0.63 to 0.34 logMAR.

Complications

Complications of CAIRS are few and mild. Transient dry eye and intrastromal deposits are most common, but are not clinically significant and are non-progressive. One case of segment displacement was reported, successfully repositioned within one week postoperatively. One case of anterior stromal melting was observed in a patient with severe atopy. Glare and halo were reported in only one patient across all studies, significantly lower than with synthetic ICRS.

CAIRS Rescue for Failed Synthetic ICRS

The complication rate of synthetic ICRS has been reported to be up to 30% ¹⁾. For complications such as ICRS exposure, anterior chamber migration, and corneal melting, successful rescue with CAIRS has been reported in multiple cases ¹⁾. Not only CAIRS insertion after ICRS removal, but also combined CAIRS insertion with ICRS remaining (in a different optical zone) was effective ¹⁾.

Future Perspectives

The most promising advancement in CAIRS is segment customization. By adjusting arc length, thickness, and placement based on individual topography, outcomes for asymmetric or eccentric cones can be improved. Standardization of nomograms, refinement of tissue preparation protocols, and large-scale long-term longitudinal studies are future challenges.

Economically, multiple segments can be created from one donor cornea; the central disc can be used for DALK or DMEK, and the peripheral rim for limbal stem cell harvesting. The manual dissection technique eliminates the need for a femtosecond laser, making it feasible in resource-limited settings.

Medical Disclaimer

The content of this article is intended for informational purposes only and is not a substitute for diagnosis or treatment. Please consult a corneal specialist for the treatment of keratoconus or corneal ectasia.

Q Is concurrent cross-linking (corneal cross-linking) necessary?

A

The decision to perform simultaneous cross-linking varies among surgeons. It is used in cases with confirmed progression or to enhance long-term stability. Concurrent cross-linking is thought to help maintain postoperative corneal flattening, but preoperative cross-linking may stiffen the stroma and reduce the flattening effect of CAIRS. Please discuss thoroughly with your doctor.

8. References

1. AlQahtani BS, Alsulami RA. The role of corneal allogenic intrastromal ring segments (CAIRS) implantation after failed synthetic intracorneal ring segments (ICRS): A rescuer. Am J Ophthalmol Case Rep. 2025;38:102287.

2. American Academy of Ophthalmology Corneal/External Disease Preferred Practice Pattern Panel. Corneal Ectasia Preferred Practice Pattern. San Francisco, CA: American Academy of Ophthalmology; 2024.

3. Meyer JJ, Gokul A, Vellara HR, et al. Progression of keratoconus in children and adolescents. Br J Ophthalmol. 2023;107:176-180.

4. Larkin DFP, Chowdhury K, Burr JM, et al. Effect of corneal cross-linking versus standard care on keratoconus progression in young patients: The KERALINK randomized controlled trial. Ophthalmology. 2021;128:1516-1526.