Corneal Implants for presbyopia, corneal segments and rings

In addition to laser vision correction and intraocular lens replacement, there are also various types of corneal implants and implantable collamer (also contact) lenses (ICL) ^{1 2 3}.

What are corneal implants?

Methods where synthetic materials (implants) are inserted into the cornea to reshape it encompass IntraCorneal Lenses (ICL for presbyopia also known as inlays), Rings (ICR) and Ring Segments (ICRS) ⁴. Unlike Implantable Collamer (Contact) Lenses, also abbreviated as ICL, and Intraocular Lenses (IOL), which are implanted behind the cornea or in the back chamber of the eye, respectively, the implants are inserted into the incision made in the cornea. In case of refractive error between +4 diopters and -8 diopters Implantable Contact Lenses can be used, while the IOL is preferred for cataract. The Implantable collameral (contact) lenses (ICL) have advantages for the correction of myopia and myopic astigmatism due to their predictability, stable refraction, and high efficiency. Studies have shown a low incidence of unwanted symptoms ⁵. In the case of presbyopia, intracorneal rings or lenses may be used. In case of keratoconus corneal segments can be used for therapeutic purposes  ².

Types of corneal implants

For the treatment of presbyopia rings and lenses are implanted into the cornea.

The meaning of the corneal ring is that it improves near vision in the non-dominant eye, but retains distance vision with two eyes ⁶. That is such a ring restricts the flow of light into the eye and is a kind of a narrow pupil.

Presbyopic intracorneal lenses change the curvature of the cornea in the middle, but not change it in the periphery. This creates a multi-vocal effect similar to Presby-LASIK (see Presby-LASIK).

Other intracorneal lenses may have a hole in the middle for distance vision surrounded by a neutral optical zone and refractive power at the periphery of the lens for near vision ⁶.

Benefits of corneal implants

The main advantage of corneal implants is the reversibility of the operation, i.e. they can always be removed if the effect is unsatisfactory ^{1 6}.

However, central corneal opacity may occur. In turn corneal rings restrict light from entering the eye, which can reduce the contrast and quality of night vision, and may also cause optical side effects.

Studies using intrastromal (stroma – the main part of the cornea) segments (ICRS) in keratoconus to correct astigmatism have shown improvement in visual acuity and refraction of the eyes in the first three years ².

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Bibliography

1. Alio JL. Management of Complications in Refractive Surgery. (Alió JL, Azar DT, eds.). Berlin, Heidelberg: Springer Berlin Heidelberg; 2018. doi:10.1007/978-3-540-37584-5
2. Min-Ji K, Yong-Soo B, Young-Sik Y, et al. Long-term outcome of intrastromal corneal ring segments in keratoconus: Five-year follow up. Sci Rep. 2019;9(1):1-7.
3. Higginbotham J. Correcting Presbyopia – Modern Options Head of Clinical Affairs What is Presbyopia ? Birmingham Opt. 2016:44. https://de.slideshare.net/JasonHigginbotham/correcting-presbyopia-modern-options.
4. Shortt AJ ABDS, Evans JR. Laser assisted in situ keratomileusis (LASIK) versus photorefractive keratectomy (PRK) for myopia. Cochrane Database Syst Rev. 2013;(1). doi:10.1002/14651858.CD005135.pub3
5. Packer M. Meta-analysis and review: effectiveness, safety, and central port design of the intraocular collamer lens. Clin Ophthalmol. 2016;10:1059-1077. doi:10.2147/OPTH.S111620
6. Arba Mosquera S, Alió JL. Presbyopic correction on the cornea. Eye Vis (London, England). 2014;1:5. doi:10.1186/s40662-014-0005-z