Comparative biomechanical analysis of combined treatments for keratoconus M. A. del Buey 1, E. Lanchares 2, J. A. Cristóbal 1, B. Calvo 2, M. Doblaré 2 Aragon Institute of Engineering Research (I3A) University of Zaragoza, Spain Department of Ophthalmology “Lozano Blesa” Clinical University Hospital, Zaragoza, Spain The authors have no financial interest in this work. The authors gratefully acknowledge the research support of the Spanish Ministry of Education and Science through the research project DPI

 KERATOCONUS is a degenerative disorder of the eye in which structural changes within the cornea cause it to thin and change to a more conical shape than its normal gradual curve.  Treatments : - Intrastromal ring segments (ISRS) - Collagen Cross-linking (CXL) - Keratoplasty INTRODUCTION To compare the effects of two combined treatments for keratoconus: corneal CXL and ISRS insertion, using a biomechanical model of the keratoconic cornea. PURPOSE: 2

METHODS Finite Element Model FE model of a healthy cornea (~ spherical) FE model of a keratoconic cornea (Thinnest ≈ apex) Apex Thinnest IOP = 15 mmHg Surroundings  4mm Cornea Thinnest The pathologic tissue is weaker than the healthy one. The CXL-treated tissue is ~ 300% stiffer (G. Wollensak & E. Iomdina 2009) 3

METHODS Simulation of CXL, ISCR ISCR: O.Z.= 6 mm length= 160º t= 0.3 mm CXL: UVA: 3 mW/cm 2 Exposure time: 30 min  beam 9 mm Depth: 300  m 4

Mean valuesPREOPPOSTOPp ACD3.38 ± ±0.33< 0.05 K ± ± 4.17< 0.05 Sim K 5.95 ± ± 2.20< 0.05 Clinical results Decrease of the ACD caused by the implantation of ICRS RESULTS  ACD=0.04 5

RESULTS Numerical simulation ICRS insertion Stress distribution Displacement distribution (deformed shape) 6

Stress distribution Displacement distribution (deformed shape) 1 st CXL 2 nd ICRS RESULTS Numerical simulation 7

Stress distribution Displacement distribution (deformed shape) RESULTS Numerical simulation 1 st ICRS 2 nd CXL 8

RESULTS Effect of the ICRS insertion The insertion of ICRS causes the applanation of the central area of the cornea (hyperopic effect) and a decrease in the ACD. Num.Sim.ICR 1 st CXL 2 nd ICRS 1 st ICRS 2 nd CXL Stress (  1 ) MPa2 MPa0.03 MPa  ACD mm mm mm Close to clinical estimation (after ICRS insertion):  ACD=0.04  ACD 9

CONCLUSIONS 1.Our clinical results show that the ACD can be used to quantify the hyperopic effect caused by the insertion of ICRS. 2.Since the combination of both corneal CXL and ICRS insertion techniques leads to a regularization of the pathological cornea regarding morphology (shape) and biomechanics (tissue behaviour), the sequence of the treatments was analyzed by numerical simulation. 10

CONCLUSIONS 3.The highest value of stress corresponds to the 1 st CXL-2 nd ICRS treatment. This is due to the increased stiffness of the previously CXL-treated tissue. 4.The outcomes show a higher effect of the ICRS implantation (↓ACD, ↑  1 ) when a CXL treatment is previously performed. Thus, biomechanical simulation suggests that ICRS have less effect in advanced keratoconus, due to the weakness of the tissue. María A. del BueyElena Lanchares 11