Page 9 - Advances in Biomechanics and Tissue Regeneration

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CHA PTE R

Personalized Corneal Biomechanics

´
,†
Miguel Angel Ariza-Gracia* , Julio Flecha-Lescu ´n*,
‡
Jos eF elix Rodrı ´guez Matas , Begon ˜a Calvo Calzada* ,§
†
*Instituto de Investigacio ´n en Ingenierı ´a de Arago ´n, Universidad de Zaragoza, Zaragoza, Spain Institute for Surgical
‡
Technology and Biomechanics, Universit€at Bern, Bern, Switzerland Chemistry, Materials and Chemical Engineering
§
Department “Giulio Natta”, Politecnico di Milano, Milan, Italy Centro de Investigacio ´n Biom edica en Red en el a ´rea
tema ´tica de Bioingenierı ´a, Biomateriales y Nanomedicina (CIBER-BBN), Madrid, Spain

1.1 INTRODUCTION

About 90% of incoming information reaches the brain through the eyes. According to the World Health Organization
(WHO),about285 million people are visually impairedworldwide.Globally,the first cause of visualimpairmentis uncor-
rectedrefractiveerror:myopia,hyperopia,astigmatism,and age-relatedpresbyopiarepresent 43% ofthe total (notinclud-
ing presbyopia). Cataracts, with 33%, and glaucoma, with 2%, are the second- and third-leading causes of visual
1
impairment. Refractive errors in Western Europe and the United States affect one-third of people over 40 years old.
Nowadays, refractive surgeries are applied to change the curvature of the corneal surface and to modify its optical
power. Despite the surgical breakthroughs over the last decades (radial keratotomy [RK], photorefractive keratotomy
[PRK], and laser in situ keratomileusis [LASIK]), the unpredictability of the surgical outcomes remains. This unpre-
dictability is manifested inside effects that can lead to unexpected results in visual acuity after an intervention. Some-
times, undercorrection ( 11.9%) or overcorrection ( 4.2%) may occur and a second “enhancement” procedure is
required. In many cases, additional surgery may be used to refine the result. According to the Food and Drug Admin-
istration, close to one million LASIK procedures are performed annually in the United States, positioning it as one of
the most common surgeries.
2
Regarding ectatic disorders, Keratoconus (KC) shows major incidence in the general population (1–430/2000), but
official statistics do not include those who have been misdiagnosed or lately diagnosed. KC has a negative impact on
the patient’s life because it decreases visual acuity and has a lasting negative impact on all aspects of a patient’s life.
Keratoconus affects three million patients worldwide with a higher prevalence among females [1]. Also, South Asian
ethnicity has an incidence probability 4.4 times higher than Caucasians, and they are also more prone to be affected
earlier [2]. Not only that, but advanced Keratoconus can cause corneal blindness, which is responsible for 40,000 people
needing a corneal transplant in Europe every year. 3
The corneal shape is the result of the equilibrium between its mechanical stiffness (related to the corneal geometry
and the intrinsic stiffness of the corneal tissue), intraocular pressure (IOP), and the external forces acting upon it such as
external pressure. An imbalance between these parameters, for example, an increment of IOP, a decrement of the cor-
neal thickness induced by refractive surgery, or a corneal material weakening due to a disruption of collagen fibers, can
produce ocular pathologies (ectasias) that seriously affect a patient’s sight.

1
World Health Organization, Visual Impairment and Blindness; Fact Sheet No. 282 (http://www.who.int/mediacentre/factsheets/fs282/en/
index.html, WHO link to digital version).
2
The Global Keratoconus Foundation (http://www.kcglobal.org/, the Global Keratoconus Foundation link to digital version).
3
See http://cordis.europa.eu/news/rcn/32213_en.html, CORDIS Europe link to digital version.

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