**3. The role of new imaging technologies in contact lens fitting in keratoconus**

First of all, the data on the radii of curvature obtained in the corneal topography can be helpful in determining the initial base curve when placing the RGP. By evaluating the size and localization of the cone in the KC with the help of tangential maps in the topography, a more appropriate RGP diameter and base curve can be selected [69]. It has been reported that these data in the topography are also useful in hybrid lens fitting in KC [70]. These systems also include CL fitting simulation software to model the possible effects of lens designs and changes in parameters on the fitting. Rigid lens fluorescein simulations are based on corneal elevation data modeled on tangential maps. There are also studies showing that the video keratoscopic system gives successful results from standard methods in RGP fitting when compared to standard procedures. It was determined that the virtual sodium fluorescein staining pattern created based on the data from the CL simulator in the corneal topography and the actual staining pattern observed in the slit lamp were found to be highly matched. These findings show the importance of video keratoscopic virtual applications in CL management in KC patients and they have the potential to reduce the time we spend for CL [23, 71].

Previously, corneal clearance could roughly be estimated by comparing it with the thickness of the cornea. Today, with new technological devices such as anterior segment optical coherence tomography (AS-OCT), the amount of corneal clearance can be measured much more accurately (**Figure 2**). It is stated that the vault changed over time after the S-CL was inserted. It is important to follow this with

#### **Figure 2.**

*Anterior segment-optical coherence tomography image showing corneal clearance in a hybrid contact lens wearer.*

*Eyesight and Imaging - Advances and New Perspectives*

sclera can be observed. Today, S-CLs with quadrant-specific peripheral designs can be produced for these KC patients with scleral asymmetry. This increases the comfort and lens wearing time of patients [56]. With the advances in CLs, S-CLs are also available today for elderly KC patients to rehabilitate near vision [8, 57]. However, studies on these are limited. Production of these specially designed lenses is still quite difficult, as they require special equipment and training and high cost [54]. Since the S-CL fits on the bulbar conjunctiva, minimal tear change occurs under the lens. The generally accepted minimum diameter for the cornea and limbal area to be unpressurized is 16 mm. Optical correction in these lenses is provided by the liquid under the well centralized lens. Therefore, anterior optical aberrations of the keratoconic cornea are neutralized. Front surface eccentricity in S-CLs aims to correct the optical quality and vision by compensating the back surface anomalies in the KC. Front surface eccentricity is zero in a spherical lens. Higher front surface eccentricity values indicate that the lens flattens rapidly from the center to the periphery [2, 27, 54, 58]. Providing continuous lubrication of the whole corneal surface ensures the stabilization of visual acuity [59]. S-CLs eliminate high grade aberrations and provide good centering and improve the visual quality. The complexity of the usage procedures and the poor comfort in long-term use limit their use [54]. S-CLs are generally not the first CLs to be applied in KC. They are preferred when tolerance problems are experienced with other CLs (SCL, RGP, PBCLs) or when acceptable vision cannot be obtained [49, 53, 54, 59]. S-CLs are indicated in RGP intolerance, very advanced and decentered cones, cornea staining at 3–9 o'clock, vascularization with PBCL, advanced KC. The fact that it is indicated in the presence of ocular surface disorder and in severe dry eye further expands the areas of use in the KC [60]. Corneal vaulting, centralization and perfect comfort have led to the preference of S-CLs in less severe cases, thus widening the indication for use of S-CLs in KC. S-CL designs are generally preferred after all corneal surgeries in the liver (CXL, intracorneal ring, keratoplasty). In such cases, higher Vault may be preferred if the ring or graft junction or sutures are to be protected [61–63]. If success is not achieved with these lenses, surgical methods are used. Contraindications are corneal edema due to decreased endothelial count, hydrops, and previous filtration surgery. Scleral lenses show success in extremely irregular and steep corneas because of their large diameters. Therefore, the role of treatment is increasing in advanced ectatic corneas where there is no option other than surgery. In addition, due to their large diameter and vaults, they are more comfortable than RGPs since they do not directly contact the cornea, which has much more innervation than the sclera. In recent years, new S-CL

designs have expanded the scope of CL use in KC patients [11, 13].

reduce dry eye signs and symptoms [60, 66].

Miniscleral lenses have less corneal opening than full scleral lenses. Small diameter lenses tend to adhere to the cornea due to the suction vacuum, which may cause difficulties for the practitioner [54, 64]. It has been shown that S-CLs reduce the need for keratoplasty and patients are successfully treated with S-CL instead of keratoplasty [65]. When the effect of CL on quality of life was evaluated in liver patients, it was seen that RGP, hybrid, soft CL had a similar effect. S-CLs are more comfortable than these lenses, but midday fogging continues to limit the quality of life in these lenses. In addition, unlike these lenses, S-CLs have been reported to

Haptic and vault are evaluated under biomicroscope in S-CLs. An acceptable fitting is defined by a corneal clearance, no air bubbles underneath, and no compression of the conjunctiva veins. After obtaining the appropriate fit, a trial use of 4–6 hours is required to evaluate the KC patient's comfort and visual quality. A 400–600 micron Vault is acceptable for scleral lenses. However, a slightly higher vault may be prescribed due to the detection of a decrease in the vault after four hours of use and also considering that KC may progress over time. A convenient

**70**

#### **Figure 3.**

*Anterior segment-optical coherence tomography image showing the interaction between the contact lens corneal and the conjunctival surface in a hybrid contact lens wearer.*

AS-OCT in progressive diseases such as KC. Because, in patients with KC, with the advancement of the cone and the decrease of the Vault, it may cause the touch between the cornea and the lens, corneal scarring and decreased vision. Therefore, the idea (owing to AS-OCT) that lenses can be used for a long time by increasing the vault has emerged in KC patients [54, 72]. The fact that the anterior segment AS-OCT provides in vivo information that cannot be obtained with videokeratoscopy and standard methods in CL applications of KC patients has led to an increasing interest in AS-OCT in CL practitioners. AS-OCT helps to examine the corneal midperiphery, the limbus region, the border structure of CL [73, 74]. Although OCT can also help evaluate scleral curvature, which will be useful in peripheral designs of S-CLs, it is not yet possible to measure scleral shape. OCT also helps to accurately evaluate the interaction between the anterior corneal and conjuctival surface and CL (**Figure 3**). It can measure the central and peripheral tear film clearance under the CL and thus provides information about the fitting [75, 76]. Central and peripheral vaults of hybrid, scleral and miniscleral lenses can also be measured with OCT. This helps us to examine in detail the relationship between asymmetric cornea and CL in KC. With using AS-OCT in CL practice, the maximum central cone vault values required to prevent edema due to hypoxia in the cornea under the scleral lens have been suggested. OCT also plays a major role in defining the relationship between CL and tears [8, 77].
