Biomechanics of Intrastromal Ring Segments

FABIO A GUARNIERI

Biomecanica da Córnea

Lugar: Belo Horizonte; Año: 2006;

Introduction The aim of supervision relies on a thorough understanding of corneal biomechanics in order to predict refractive surgery outcome. Changes in stress and elasticity after corneal shape changes by adding or removing tissue are very important in order to obtain reliable procedures. As in RK, ICRS refractive outcome relies directly on corneal biomechanics. In PRK and LASIK, biomechanical effects are undesired but present, esp. for higher corrections [Guarnieri et al ARVO 2001]. The concept of a ring-shaped device that could be introduced into the cornea through a single, peripheral, radial incision originated with Reynolds in 1978. This implant would alter the anterior corneal curvature by expansion of the device’s diameter resulting in flattening (myopic correction) or by constriction to steepen the cornea (hyperopic correction). The insertion of segments without expansion also resulted in corneal flattening. Silvestrini proposed the “arc shortening theory” based on the tissue separation by the inserts producing a flattening in the anterior corneal surface. This mechanism was described thoroughly by Pinsky et al. using a corneal model by the Finite Element Method [chapter in book not published]. While the Silvestrini mathematical model defined a linear relationship between ring thickness and flattening, Pinsky model allows predicting effect variations in physical and surgical parameters as thickness, width, diameter, cone angle, arc length, and materials. This model is used to predict different designs for myopia, astigmatism and hyperopia.  Further modeling of the cornea has been developed in order to account its material nonlinearity, viscoelasticity and anisotropy [Guarnieri et al. ARVO 2001, Guarnieri, doctoral thesis INTEC 1999]. This model has been used to simulate the behavior of the Ferrara Ring segments in astigmatic and further in keratoconic eyes. The mechanical behavior of this implant is different since its triangular cross section combined with its nonconformal curvature to the curve of the cornea alters differently the anterior corneal surface. Moreover, in keratoconus, the corneal biomechanics is altered and both geometrical and material changes should be accounted in order to have reliable and predictable results in ICRS surgery.