**1. Introduction**

Keratoconus (KC) is the most common ectatic disease of the cornea. It is characterized by progressive thinning and protrusion of the cornea [1, 2]. Consequently, irregular astigmatism, myopia and a decrease in visual acuity occur. Therefore, the disease has a negative effect on vision-related quality of life. The disease has become an important public health problem due to the economic burden of treatment and vision rehabilitation related processes [3]. KC in children may have negative effects on social and educational development. In this respect, it is necessary to improve the vision in children at an acceptable level [4].

This disease, which mostly starts in young adults, can also be seen in children. It stabilizes in the fourth-fifth decades of life. KC, which usually shows bilateral asymmetric involvement, can be asymptomatic at the beginning, and visual acuity decreases as the disease progresses [1, 2]. Although some systemic involvement of

KC is shown, it is generally known as a local corneal disease [5, 6]. Abnormalities in the corneal epithelium, Bowman's layer and especially the collagen structure of the stroma play a role in the pathogenesis of the disease. Although it is suggested that various biochemical and genetic factors play a role in the etiology, its exact cause is not known exactly. The main diagnostic method of KC is placido disc-based corneal topography [2, 5, 7].

While surgical options in KC management aim to change the natural course of the disease and increase vision, the main goal of non-surgical options is to improve vision without damaging the ocular surface. Classical non-surgical treatment of vision rehabilitation in KC is glasses in a small number of patients and CLs in the majority of patients. In addition, modern surgical options such as intraocular lens implantation, corneal cross-linking (CXL), intra-stromal rings and anterior lamellar keratoplasty are also used in treatment. The common feature of these surgical methods is that they increase visual rehabilitation to a certain level due to residual refraction after surgery and ongoing irregular astigmatism, even if they are performed very successfully. Therefore, CLs are needed for vision rehabilitation after surgical methods [2, 7, 8].

Today, there is a global consensus that CLs play the most important role in the visual rehabilitation of KC patients [8]. Later developments in CL design and materials expanded the application options for KC patients. Considering that CLs cause ocular surface changes even in non-KC individuals, the main purpose of CL application in KC should be to increase visual acuity without compromising the health of the cornea and ocular surface [9]. While the patient should have good vision and comfort with the lens, the practitioner must find a suitable lens fitting that does not compromise the anterior ocular surface health. Therefore, the process is often timeconsuming and difficult for both the patient and the ophthalmologist. Due to the nature of long-term CL use in KC, a careful CL selection should be made considering the physiological needs of the cornea according to the level of ectasia. Since CL movements can cause mechanical effects on the cornea with CL movements during millions of blinking, it is necessary to ensure that CL applies minimal contact and pressure on the cone in KC patients. In addition, since there are stem cells in the limbus region, which are hallmarks of corneal physiology and regeneration, contact with the limbal region should be minimized in order to prevent CLs from damaging the limbal region [2]. Scheimpflug imaging and anterior segment optical coherence tomography, which are frequently used in ophthalmology practice in recent years, can be used to evaluate CL fit. These imaging technologies can be used to reduce the time we spend evaluating CL fitting and to improve guides for CL fitting [10].

In addition to the severity of the KC, it is decided which type of CL will be selected according to the visual demand and comfort of the patient and the CL tolerance. With the latest advances in CL features and design, many CL options have been developed for patients with corneal irregularities, such as large diameter RGP lenses, scleral lenses, hybrid lenses and KC specific soft lenses. New data reveal that special design CLs, new design scleral lenses and hybrid lenses provide better visual acuity as well as better comfort than traditional RGPs [2, 8, 11].
