Intracorneal segments are thin segments placed within the thickness of the cornea, at the periphery of the optical zone, in order to regularise its shape. Long made of a synthetic material, they are now increasingly made of human corneal tissue: that is what the acronym CAIRS designates, for corneal allogenic intrastromal ring segments. The objective remains the same: to bring an over-irregular cornea back towards a shape that a correction can handle again. They address vision, within an indication established beforehand.
Segments change the shape of the cornea; they do not halt its progression. They do not replace stabilisation when progression is documented, and the two procedures are frequently combined, one after the other or at the same sitting — when both questions arise together.
Nor do they render vision independent of any correction. The realistic objective is to bring an over-irregular cornea back towards a shape that glasses or lenses can handle again.
Synthetic segments, in use for several decades, have shown their efficacy on corneal shape, but also limits inherent to a foreign body inserted into the cornea: intolerance, clouding around the segment, and migration or extrusion sometimes leading to its removal.
Corneal tissue segments were developed to answer these limits. The implanted tissue is of the same nature as the cornea that receives it: it integrates progressively rather than remaining a foreign body. Its transparency also allows it to be positioned closer to the optical centre than a synthetic segment permits, which widens the cone shapes accessible to the procedure — notably very decentred or asymmetric cones.
This evolution does not make synthetic segments an invalid technique, and it does not dispense with saying what we do not yet know: the available follow-up on tissue is shorter, and a section of this page is devoted to it.
The segments are placed within the thickness of the stroma — the middle layer of the cornea — at the periphery. There they change the distribution of tension in the corneal wall: the central zone is thereby flattened and regularised, without the procedure bearing directly on the visual axis.
It is an additive procedure: no corneal tissue is removed or destroyed. Material is added to an area of the cornea that lacks it — which distinguishes this approach from surgery that thins the cornea, contraindicated in keratoconus.
The improvement observed does not come from the change in stromal shape alone. The epithelium, the thin surface layer, remodels over the new geometry and contributes to smoothing the surface.
The effect obtained depends on the thickness, the length and the position of the segments, chosen from the map of your cornea. Since the tissue can be cut to the required dimensions, planning adjusts to the shape of your cone rather than to a catalogue of available sizes. It is a procedure planned from your examinations, not a standard model.
The tissue comes from a corneal donation, as for a graft. It is retrieved and prepared by an approved tissue bank, which ensures its health screening and traceability.
Three principles govern this donation in Belgium: it is anonymous — you will not know the donor’s identity, nor their family yours —, it is unpaid, no remuneration being permitted in connection with the donation of an element of the human body, and it is traced, each tissue being followed from retrieval to implantation.
The volume of tissue implanted is far smaller than that of a graft: a single donor cornea allows several segments to be prepared. This reduced volume, the peripheral position of the segments and the absence of vessels in the cornea explain why the risk of rejection is considered very low — without its being possible to call it nil.
The indication rests on visual difficulty that persists despite well-conducted optical correction:
The procedure is performed as a day case, under local anaesthesia with drops. It is brief.
A tunnel is created within the thickness of the cornea, at the planned depth, using a femtosecond laser. The segments, prepared to the chosen dimensions, are then inserted and positioned according to the plan.
When cross-linking is combined, it is carried out at the same sitting or later, depending on the context.
Eye drops are prescribed for several weeks, and check-ups are close together at the start.
The objective is a more regular cornea, and therefore a more effective correction or a better tolerated lens. The result varies from one patient to another and is not precisely predictable: two corneas of similar appearance do not necessarily respond in the same way.
This variability is part of the discussion beforehand. It is also the reason why the objective is framed in terms of correction becoming possible again, and not in terms of vision without correction.
This technique is recent. Described in 2018, it now has published series whose longest follow-up approaches three years. The results reported are encouraging on corneal shape as on corrected vision, but they rest on limited numbers and short follow-up for a disease that accompanies a whole lifetime.
Two questions remain open, and it is honest to name them:
This limited follow-up is not a reason to abstain; it is a reason to discuss it beforehand, and to continue monitoring afterwards.
Follow-up involves close check-ups in the first weeks, then a return to the usual monitoring rhythm for keratoconus. The correction is reassessed once vision has settled.
Because the procedure is additive, it does not compromise later options: an anterior stromal or full-thickness corneal graft remains possible should the disease ever require it.
As for all keratoconus care, monitoring and the precautions concerning eye rubbing remain relevant.
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