In order to implant the ICRS into the deep cornea, we need to perform channels within the stroma where the rings will be implanted. For this purpose there are two different surgical options: mechanical and femtosecond laser-assisted technique.

In the mechanical or manual technique, the surgeon must mark the centre of the pupil in order to use it as a reference point during the procedure. Then a calibrated diamond knife is used to create an incision at a depth of 70 % of the corneal pachymetry at the incision point. A suction ring is placed around the corneal limbus in order to fixate the eye during the dissection of the corneal stroma. Then, two semi-circular dissectors are placed through the incision and advanced into the deep stroma in a clockwise and counterclockwise movement aiming to perform the tunnel.

With the femtosecond laser-assisted technique, a disposable suction ring is placed and centred. Afterwards, the cornea is flattened with a disposable aplannation cone, which allows a precise focus of the laser beam thus creating the dissection on the desire depth. Then the tunnel is created at approximately 70 or 80 % of the corneal pachymetry without direct manipulation of the eye. Finally, ICRS are inserted in the created tunnels.

Femtosecond laser produces a more precise and controlled stromal dissection than the manual technique. However, if we are talking about visual and refractive outcomes, most studies that have been conducted concur that both techniques produce similar results in cases of ICRS implantation for keratoconus. On the other hand, femtosecond laser makes the process faster, easier (especially for inexperienced surgeons) and more comfortable for the patient [14–17]. Apart from the safety and efficacy differences between both techniques, Alió and co-workers found that intrastromal segment implantation using femtosecond laser is a method that produces a greater reduction in corneal high order aberrations in eyes with coma aberration >3.0 μm [14, 15].

Regardless of the technique used to make the tunnels in the corneal stroma, the number, thickness, position and arc length of the segments are determined based on the manufacturer’s nomograms. Likewise, rings are chosen from the nomogram taking into account the refractive error and the topographic map of the disease. It should also be noted that the incision guiding implantation of the segments in the tunnel is located on the axis of the steepest meridian of the corneal topography.

It is important to consider that although several authors have reported good results implanting ICRS in keratoconic eyes, the main limitations that nomograms have is that most of them are based in anecdotic clinical data, or variables that are very subjective in patients with keratoconus, such as sphero-cylindrical refraction and topographic pattern of the cone. For instance, it was found that based on the topographic pattern of the keratoconus the best choice was to implant one segment in those cases of inferior steepening and two segments in central cones [18].

Other works published in the literature support that the best location to implant the segments is by placing the corneal incision in the temporal site of the cornea [19–22] or in the steepest meridian of the cornea [23, 24]. There are other works that have reported good results when implanting the ICRS guided by the comatic axis [25]. Recently, Alió and co-workers published a scientific work in which we concluded that the best outcomes for implanting ICRS were observed in those cases where the refractive and topographic cylinder did not differ in more than 15° [12].

As we can see, there are different approaches regarding the guidelines to be used when implanting ICRS. Nevertheless, today the most widespread nomograms that are used in the clinical practice are those developed by the main manufacturers of ICRS: