From theoretical laser ablation design to real outcomes: implications for optimized corneal refractive surgery

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2 Wavefront Sensing Meeting. Whistler,Canada, Feb 04 From theoretical laser ablation design to real outcomes: implications for optimized corneal refractive surgery Susana Marcos, PhD Instituto de Optica, CSIC, Madrid, Spain

3 Ablation pattern design Laser energy delivery Desired corneal shape?

4 Total Corneal Pre-LASIK RMS=0.54 mm RMS=0.35 mm Post-LASIK RMS=1.95 mm arcos et al. IOVS (2001) RMS=1.93 mm ahcoi

5 Why corneal spherical aberration / asphericity increases after standard LASIK? Due to the design of the profile? Due to discrepancies in the laser energy delivery? Due to corneal biomechanical effects?

6 Ablation pattern design Theoretical predictions of the standard Munnerlyn pattern and its parabollic approximation Simulated POST corneal height Munnerlyn ablation depth Experimental PRE corneal height - = Using actual topographic and clinical data from the patients

7 Laser energy delivery Theoretical predictions of laser efficiency changes across the cornea Experimental ablation model in PMMA: Flat and spherical surfaces

8 Theoretical computations Evaluation of ablation profile Contribution of laser efficiency changes across the cornea Patients outcome Effects of ablation profile, laser efficiency and corneal biomechanics on post-operative corneal shape PMMA model

9 ubjects and urgery 13 eyes from 7 subjects mean age: 28±5 Spherical equivalent: -2.5 thru 13 D No ocular or retinal condition Flying spot Technolas 217-C (B&L surgical) Hansatome microkeratome Optical zones: mm. Transition zone: 9 mm Marcos et al. IOVS 2001

10 Computer surgery Pre-operative corneal topographies Munnerlyn and parabollic ablation patterns Clinical data of radii of curvature, correction and ablation zone. Programmed in Matlab Marcos, Cano, Barbero. JRS 2003; Cano, Barbero & Marcos, JOSA A 2004

11 PMMA model and surgery Flat and spherical surfaces Radii of curvature:7-8 mm Different types (cast/extruded) of PMMA tested Spherical corrections from 3 D to 12 D Flying spot Technolas 217-C (B&L surgical) Optical zones: mm. Transition zone: 9 mm

12 xperimental measurements in patients Corneal topography before (< 1 month) and after (>1 month) LASIK refractive surgery Ray tracing Corneal radius and asphericity Corneal spherical aberration

13 Computer surgery: ablation depth Munnerlyn equation Parabollic approximation y 2 4Dr t ( r) = DS 3 2 S/2 R 1 R 2 R 1= Initial radius of curvature R 2= Intended radius of curvature S=Optical zone D= dyopter change

14 Munnerlyn and parabolic approximation More tissue removed Less tissue removed ablation depth (mm) MUNNERLYN PARABOLIC -3, ,5 r (mm) Correction = - 4D

15 Computer surgery: laser efficiency changes a Munnerlyn equation + laser efficiency t Mun= tmun(y) K(ay) Parabollic equation + laser efficiency Reflection losses Larger spot size in the periphery Ka=1+1,14 ln[(cos a(1-r)] a = angle of incidence R = reflection factor t Par= tpar(r) K(ar) Mrochen & Seiler, 2001 Jiménez et al., 2002

16 unnerlyn with and w/o efficiency changes More tissue removed Less tissue removed ablation depth (mm) CONSIDERING EFFICIENCY ,5 0 3,5 r (mm) Correction = - 4D NO CONSIDERING EFFICIENCY

17 orneal shapes after computer surgery Corneal topography before (< 1 month) and after subtraction of theoretical ablation patterns Ray tracing Corneal radius and asphericity Corneal spherical aberration

18 sphericity 5 asphericity PRE POST MUN PMUN Eye # Cano, Barbero & Marcos. JOSA A 2004

19 Corneal spherical aberration spherical aberration (microns) Pre-op spherical error (D) Cano, Barbero & Marcos. JOSA A 2004 Post real Post Munnerlyn Post Parabollic Munnerlyn F= 4.4 mm

20 verage asphericity Asphericity 1,2 1 0,8 0,6 0,4 0,2 POST Munnerlyn + efficiency Parabollic Munnerlyn + efficiency 0-0,2-0,4 PRE Munnerlyn Parabollic Munnerlyn Cano, Barbero & Marcos. JOSA A 2004

21 xperimental measurements in PMMA surfaces Flat surfaces Contact profilometry (Dektek 3000) Correcting for tilts Applying scale factor 2- D profiles Pre-op cornea+ profile

22 blated PMMA flat surfaces Ablation depth (mm) Ablation profile -3 D -6 D Surface location (mm) Measured ablation depth (microns) Measured (mm) Ablation depth slope=0.4 Slope= Nominal ablation depth (microns) Nominal (mm)

23 blated PMMA flat surfaces Ablation profile Correction Ablation depth (mm) -3 D -6 D Surface location (mm) Measured power change (D) Measured ablation depth (microns) slope=0.39 Slope= Nominal ablation depth (microns) Nominal correction (D)

24 verage asphericity 1,2 1 POST Asphericity 0,8 0,6 0,4 0,2 Parabollic Experimental Flat PMMA profile* 0-0,2-0,4 PRE Munnerlyn (*) After application of scale factor

25 xperimental measurements in PMMA surfaces Spherical surfaces Contact profilometry (Talysurf) Corneal topography 3- D profiles Post-operative corneal Good correspondence topography

26 MMA spheres data processing Problems Errors in ablation centration Decentration of topography measurement Inaccuracy of corneal topographer at the periphery Inaccurate center estimation Inaccurate pre-op shape

27 MMA spheres data processing Custom software Uses measured sphere before ablation Asymmetric in ablation zone Estimates center of topography Performs virtual rotation of ablation Symmetric within ablation zone

28 blated PMMA spherical surfaces 3- D profiles Ablation depth -3 D -6 D Measured ablation depth (microns) Measured (mm) slope= Slope= Nominal ablation depth (microns Nominal (mm)

29 blated PMMA spherical l surfaces 3- D profiles Correction -3 D -6 D Measured Measured power change (D) (D) Slope= Nominal correction (D) Nominal correction (D)

30 blated PMMA spherical l surfaces 3- D profiles Correction -3 D -6 D Measured power change (D) Measured power change (D) Flat surface Spherical surface Nominal correction (D) (D)

31 Asphericity 5 Clinical data Asphericity in cornea Munnerlyn prof. Parabolic prof Flat PMMA prof.* Parabolic + eff. Spherical PMMA* Myopic correction (D) (*)After application of scale factor

32 Corneal aberrations Post-op PMMA sphere* Post-op real cornea -6 D -5.5 D *After alignment and application of scale factor

33 onclusions The Munnerlyn ablation profile itself should not increase asphericity. A parabolic profile increases it slightly Experiments on PMMA show a larger contribution of the laser efficiency changes across the cornea to the increase of asphericity than previously published Ablation of PMMA spherical surfaces replicates the findings on real corneas, leaving a small contribution to biomechanical effects, once the scale ablation factor is taken into account Relevant for customized corneal ablation

34 Funding Grant BFM Spanish Ministry of Science and Technology Grant CAM08.7/0010.1/2000 Madrid Regional Government, Atlanta, GA Alcon Research Labs Carl Zeiss, S.A., Spain

35 VISUAL OPTICS & BIOPHOTONICS LAB. Instituto de Optica, CSIC Sergio Barbero Patricia Rosales Carlos Dorronsoro Lourdes Llorente Elena Garcia de la Cera Daniel Cano Susana Marcos Sergio Ortiz

36 in PMMA spheres Munn + absorption (in PMMA) Parab + absorption (in PMMA) Ablated spheres NEW Myopic correction

37 Measured ablation depth (microns) PMMA spheres PMMA flat surf Nominal ablation depth (microns)

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