12 Why Femtosecond Laser for Intracorneal Rings?
Why femtosecond laser (FSL) for intracorneal rings? The advantages of FSL technology in intracorneal ring (ICR) implantation and keratoconus can no longer be ignored. The first implantations of ICR were performed by mechanical method which were manual and depended on the skill of the Surgeon. However, there were many important factors which had significant importance on the success of the treatment such as centralization, depth, in/out diameter, etc. The demand of the precision on such factors motivated FSL technology development on ICR implantations. The main advantages of ICR implantation using FSL include the following: less epithelial defects and therefore decreased discomfort after operation, infection control due to disposable cones used in every eye, less vacuum time and pressure compared to the mechanical method, perfect centration, excellent depth stabilization, error-free incision, and reliable effectiveness. Additionally, effectiveness can be increased by creating a narrower channel because of the option to change the inside and outside diameters. Endothelial perforation is the most important complication. Incomplete tunnel formation is the most frequent. Moreover, complications such as galvo error during incision, incorrect entry of the channel, and vacuum loss can be observed.
Keywords: femtosecond, intracorneal ring, keratoconus
The corneal cross-linking (CCL) method has been known for stopping progression of keratoconus and also plays a slight role in corneal regulation. The treatment of keratoconus is proved with publications around the world. However, the CCL treatment cannot be considered for increasing visual acuity. ICR’s history is much longer than that of CCL. Prof. Barraquer started the first intracorneal inlay implantation for correction of myopia in 1950. ICR is safer and more efficient in order to increase vision in keratoconus. In this regard, Dr. Ferrara’s work in Brazil in 1995 and the first paper published by Prof. Jeseph Collin in 1997 have to be mentioned. This successful method was started using the mechanical method. While for INTACS implantation (Addition Technology, Inc., Lombard, IL, A company of AJL Ophthalmic, S.A, Araba, Spain) channel separation is conducted using a suction ring capable of vacuum, the KeraRing ICRS (KeraRing, Mediphacos Ltda., Belo Horizonte, Brazil) is implanted after manual mechanical separation (▶ Fig. 12.1). The mechanical method has some disadvantages like high vacuum pressure, centration errors, problems of stable depth and correspondingly perforation into the anterior chamber, epithelial defects, and discomfort. These disadvantages of mechanical separation led to complications like segment migration (▶ Fig. 12.2) and vascularization in the wound location (▶ Fig. 12.3), corneal melting (▶ Fig. 12.4), segment extrusion (▶ Fig. 12.5), and serious infection (▶ Fig. 12.6), which led many doctors to stop performing this kind of surgery.
Fig. 12.1 Manual mechanical technique separators set.
Fig. 12.2 Segment migration.
Fig. 12.3 Vascularization in wound location.
Fig. 12.4 Melting in wound location.
Fig. 12.5 Extrusion.
Fig. 12.6 Endophthalmitis.
FSL, discovered in the 2000s, were designed for laser-assisted in situ keratomileusis (LASIK). However, FSL was the solution to all problems in ICR implantation.
12.2 For the Infection
Fig. 12.7 Disposable suction ring and cone.
Fig. 12.8 Disposable suction ring and cone.
12.3 Low Vacuum Pressure and Less Time
Additionally, the FSL also uses a low vacuum. Approximately 35 mm Hg of vacuum is applied for 8 seconds, whereas in the mechanical technique, the vacuum pressure exceeds 50 mm Hg for a couple of minutes (▶ Fig. 12.9).
Fig. 12.9 Vacuum is applied 8 seconds.
12.4 Good Centration
Another big advantage of the FSL for intracorneal ring segment implantation is its predictable centration (▶ Fig. 12.10), which is important especially for rings with 5-mm optical zones. Centration may be more easily achieved using a marker before applying the laser (▶ Fig. 12.11). After placing the vacuum ring and applanation to the cornea, the surgeon can choose the central point at the exact desired place. Some ring companies advise that the central point be the anatomical limbus center; other companies advise the “Purkinje reflex” as the center (▶ Fig. 12.12).
Fig. 12.10 Intracorneal ring implantation in predictable centration.
Fig. 12.11 Use of a marker before applying the laser.
Fig. 12.12 Purkinje reflex and pupil center.
12.5 Customizing Tunnel Size
Customization of the tunnel is possible with an FSL; the depth of the channel as well as the inner and outer channel diameters is digitally changed with ease in FSL techniques (▶ Fig. 12.13). If the surgeon decreases the inner diameter of the channel after ring implantation, the ring pulls the channel distally. This effect may increase with narrower channels. Rabinowitz et al 1 showed that narrower ring segment channels produced a greater improvement in visual acuity and refraction (▶ Table 12.1). A similar study was presented, at the XXIV Meeting of the European Society of Cataract and Refractive Surgeons, London, on September 11, 2006. In this study, three groups were evaluated according to inner and outer diameters. 2 Group II had a narrower channel and group III had the same channel width as group II, but the inner diameter was smaller. Outcomes showed that a smaller inner diameter and narrower channel resulted in more effective results versus the other groups. In this study, INTACS real ring parameters were 6.77 to 8.1 mm (inner and outer). In group I, it was 6.77 to 8.1 mm; in group II, it was 6.6 to 7.4 mm; and in group III, it was 6.5 to 7.3 mm (▶ Table 12.2). Group III had the narrowest channel in this study (▶ Fig. 12.14). Keratometric changes in steep axis were measured with Orbscan. The keratometric changes were 4.25 D in group I, 25.59 D in group II, and 37.08 D in group III. The narrowest channel group III demonstrated maximum changes in steep axis after ring implantation (▶ Fig. 12.15). Preoperative changes compared to postoperative changes in refraction in groups I, II, and III were 1.17, 2.54, and 4.64 D, respectively (▶ Fig. 12.16). Preoperative changes compared to postoperative changes in uncorrected visual acuity (UCVA) in groups I, II and III were 0.26, 0.23, and 0.29 D, respectively. Changes in best spectacle–corrected visual acuity (BCVA) in groups I, II and III were 0.1, 0.13 and 0.16 D, respectively (▶ Fig. 12.17). The results showed that the narrowest channel group III offered the maximum effect after ring implantation.
Change from preoperative to postoperative
Wide ≥ 8.0 mm outer diameter
Narrow ≤ 7.6 mm outer diameter
Abbreviations: BCVA, best spectacle–corrected visual acuity; UCVA, uncorrected visual acuity.
Tunnel size (mm)
Eyes of patients
53 (n = 37)
14 (n = 9)
10 (n = 7)
29 (n = 21)
Fig. 12.13 The depth of the channel