New approach offers personalised treatment.
Published: Wednesday, November 1, 2023
Placement of Corneal Allogenic Intrastromal Ring Segments (CAIRS) is a technique I first started in 2015 and refers to the placement of allogenic tissue of any source within the host cornea. CAIRS works on the principle of Barraquer’s law of thickness, which states the thickness increasing in the mid-periphery of the cornea results in a flattening of the central cornea and vice versa. Thus, implanting CAIRS in the para-central and mid-peripheral zones gives the desired effect of flattening the cone in an ectatic patient.
Synthetic ICRS have already been used to achieve a central cornea flattening in keratoconic patients. These have been successful in bringing moderate visual improvement. Disadvantages include a limitation to the upper limit of achievable improvement, an inability to treat advanced cases, and a lack of individualised customisation. There is also a higher risk of complications such as anterior stromal necrosis, extrusion, intrusion, and migration, which hinders the use of these synthetic rings, especially in patients with advanced cones, eye rubbers, and those with the potential to progress.
Compared to synthetic ICRS, CAIRS has significant advantages—allogenicity, superior biocompatibility, and a refractive index similar to the rest of the cornea. These reduce the risk of complications, such as anterior stromal necrosis, associated with synthetic segments while allowing the ability to implant in much thinner and steeper corneas. At the same time, though our implantation of CAIRS is most commonly performed at 4.6 mm, we have also implanted it in optic zones up to 4.0 mm without the patient complaining of a ring or halo effect. These advantages allow for harnessing a large amount of flattening effect—even up to 30 D.
CAIRS can also be implanted more superficially than the traditional 70–80% depth for synthetic ICRS. We have been implanting CAIRS at about 50% depth and this superficial implantation—together with the greater thickness and volume of implanted tissue as well as the ability to use small optic zones—allow for a greater effect, thus becoming suitable for very advanced cones as well.
Customisation is probably one of the greatest advancements in CAIRS surgery. Like synthetic segments, CAIRS can be cut to any arc length, thickness or volume, and optic zone. Implementation depth can be changed, and, most importantly, the shape can be customised. Therefore, it can have variable or progressive thickness and/or width.
Progressive thickness synthetic ICRS are available only in fixed arc lengths and optic zones and with a fixed and progressive gradation of thickness or width from one end of the segment to the other or from the centre to the edges. However, we know well that each keratoconic patient is unique concerning the cone pattern, its position and extent, thickness, refractive error, etc.
In addition, the internal gradient of keratometry can vary widely within the cone, so the same type of progressively tapered ICRS does not suit even phenotypically matching keratoconic patients. CAIRS allows exquisite surgeon customisation to exactly match to each patient’s topographic map and can be further customised based on the refractive error.
The taper gradient in customised CAIRS does not need to start at one end and progress smoothly to the other. Instead, the taper can be initiated anywhere within the arc, so there are tapered and non-tapered parts in the same segment—the length and thickness of the tapered part varied per patient. In addition, the taper can be gradual or sudden and can start from the centre towards both sides in a U- or V-shape or from one side and progress to the other. Tapered segments, U- or V-shaped segments, and many other possibilities are possible depending on the patient’s topography.
Surgeons can easily customise on the table using the double-bladed Jacob CAIRS trephine™ (Madhu Instruments, India) and the CAIRS Customizer™ (Epsilon Instruments, US).
Any allogenic tissue source can be used to prepare CAIRS: for example, a donor corneal rim with the epithelium and endothelium removed. The Jacob CAIRS trephine then cuts circular CAIRS from the corneal stromal rim, which is placed on the CAIRS Customizer and aligned along the desired optic zone mark. The plan marked on the patient’s keratometric map is then amended on the CAIRS segment with a fine tipped marker pen. The segment is then placed on a Teflon cutting block and cut. Marks are also placed on the patient’s eye to transfer the plan accurately. The customised CAIRS is then inserted into the patient’s eye using CAIRS inserters. In progressive cases and young patients, this is combined with corneal cross-linking (CXL, Figure 1).
We have more than 600 patients with all grades of keratoconus—from very mild to advanced disease—who have undergone CAIRS. The patient experience has been very positive, with patients reporting a decrease in distortion, improvement in visual acuity, decrease in spectacle power, decreased contact lens dependency, and, when required, improved contact lens fitting. Most patients have bilateral disease and undergo sequential bilateral surgery, and many refer their friends from support groups or other known contacts who suffer from ectasia.
CAIRS has numerous advantages over not just synthetic segments but DALK as well, as a minimally invasive, quick, and easy surgery with a smooth learning curve, quick rehabilitation, and low risk of intra- or postoperative complications, unlike DALK. It is also more advantageous than subtractive procedures such as topography-guided photorefractive or phototherapeutic keratectomy combined with CXL in obtaining a much larger effect and not running the risk of disease destabilisation. Unlike these procedures—which remove corneal tissue—CAIRS adds tissue, thereby increasing corneal thickness and redistributing corneal stress forces, helping decrease progression. Compared to other allogenic technology, CAIRS leaves the visual axis untouched and thus does not carry the risk of decreased vision from haze or rejection. Other synergistic techniques can follow to further decrease spectacle dependency if required.
Dr Soosan Jacob MS, FRCS, DNB is Director and Chief of Dr Agarwal’s Refractive and Cornea Foundation at Dr Agarwal’s Eye Hospital, Chennai, India, and can be reached at email@example.com.