Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

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CHAPTER 3 These data demonstrate that videokeratoscopy produces reliable data on postablated surfaces, at least within the ablation zone. For this reason, all data on pre- and post-ablated spherical surfaces presented in this study are based on videokeratoscopy. 3.3.5. Assessment of the ablation pattern generated by the laser system 3.3.5.1. Flat surfaces The surface profiles on ablated flat surfaces should represent the actual ablation profiles produced by the laser. The support of the samples in the profilometry system was not necessarily perfectly parallel to the translation of the probe tip, causing some tilt in the measured profiles. The measured profiles were corrected numerically for this tilt, until the untreated areas appeared flat (Section 2.2.2.1). 3.3.5.2. Spherical surfaces The ablation patterns for spherical surfaces were obtained by subtracting the surface heights measured by videokeratography after ablation from the sphere with the preablation curvature (as measured from videokeratography) (Section 2.2.2.2). As the reference axis for the videokeratographer (the line passing though the surface apex) may not coincide with the laser beam direction during ablation, the surface apex during topography measurements may not correspond with the ablation center. This may happen even if both ablation and measurement were well centered with respect to the surface circular limits. We wrote routines in Matlab (Mathworks, Nantick, MA) to rotate the measured corneal elevation profiles to bring the center (maximum depth) of the measured ablation to the apex, before subtraction. This results in a more precise (and symmetric) ablation pattern. In most cases, post-operative corneal elevation maps were repeated, tilting the cornea by the angle obtained from the programs, to align the ablation and measurement axes. This was achieved placing the base of the ablated model cornea on a tip-tilt table, and performing an iterative process until the estimated tilt and subsequent decentration were less than 0.2 deg and 0.05 mm respectively. 3.3.6. Ablation efficiency estimates Ablation patterns on spherical surfaces are altered by variations in ablation efficiency due to changes in the angle of incidence of the laser beam as a function of surface location. In contrast, ablation efficiency is constant on flat surfaces, where laser incidence is always perpendicular. The ablation efficiency factor K() takes values between 0 and 1. From our measurements on PMMA, we will estimate K() by dividing the ablation profile measured on spherical surfaces by the ablation profile measured on flat surfaces, for the same conditions. This function describes the laser efficiency effects reducing the ablation depth at the periphery, and therefore the adjustment factor by which the laser ablation profile is multiplied to give the final ablation produced on curved surfaces. From a design point of view, K() represents the adjustment factor by which the intended profile should be divided to compensate for the changes in ablation efficiency on curved surfaces. It should be noted that K() depends on the refractive index for the laser wavelength and the ablation threshold, and therefore if will be different for a material with different n and F th . Following application of the Beer-Lambert law (Jimenez et al., 2002): = 1 + a · f (R) (3.1) 104
REFRACTIVE SURGERY PMMA MODEL where a=1/ln(F 0 /F th ), with F 0 the laser fluence and F th the ablation threshold, and f (R) is a reflectivity function, depending on the surface location and material index of refraction, given by Fresnel equations. While the refractive index at 193 nm is similar in cornea and in PMMA (n cornea = 1.52 (Pettit and Ediger, 1996); n PMMA =1.49), and therefore we will assume f (R) to be the same for both materials, the ablation threshold differs (F th, cornea = 40 mJ/cm 2 (Berns et al., 1999) or F th, cornea = 60 mJ/cm 2 (Pettit et al., 2005) ; F th, PMMA = 80mJ/cm 2 (Srinivasan, 1986)). The ablation efficiency factor for the cornea cornea will be estimated from the experimental estimates of PMMA using the following expression: cornea () = 1 + (a cornea /a PMMA ) · ( PMMA () – 1) 3.3.7. Post-operative asphericities In order to estimate the asphericities produced by the actual ablation patterns when the effects of reflection losses and non-normal incidence are not present, we computed the post-operative asphericities produced on an spherical PMMA surface (radius = 8 mm) by the laser ablation profiles measured on flat surfaces. We used fitting routines written in Matlab, as described in a previous publication (Cano et al., 2004). To consider efficiency changes, these surfaces were multiplied by the measured K PMMA before calculating their asphericity. Central radii of curvature and asphericities were also directly computed from the surface elevation maps measured on PMMA ablated spheres. Corneal ablation patterns were calculated by converting PMMA ablation depths into corneal ablation depths by using a PMMA-cornea ablation factor to account for the different ablation rates (material removed per pulse) in PMMA and in cornea. The simulated postoperative corneas were calculated by: 1) applying an overall depth offset to the ablation profile, to account for a different transition zone; 2) multiplying by the estimated K cornea , to account for the effects of ablation efficiency changes; 3) subtracting the resulting corneal ablation patterns from the simulated preoperative corneas. In all cases, the preoperative cornea was simulated as a conic surface with a typical human corneal shape (Radius of curvature = 7.8 mm; asphericity = -0.2). Corneal asphericity was computed as described above. 3.4. RESULTS 3.4.1. PMMA-cornea correction factor The ablation rate is different on corneal tissue than extruded PMMA. A conversion factor was estimated, to account for the difference in the ablation depth per laser pulse between PMMA and cornea. Figure 3.1A shows central ablation depths (for different refractive corrections) in both flat and spherical PMMA samples versus the nominal ablation depth displayed on the laser system. There is a good correlation between nominal central ablation depths in the cornea and those measured on PMMA, with a slope of 1.8 m on cornea /1 m on PMMA. Despite the good correlation, which was expected since the amount of material removed by each laser pulse must be the same in all samples, we considered that the ablation PMMA-cornea correction computed from the regression line may not sufficiently accurate for several reasons: 1) there appears to be an offset in the 105
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Doctoral thesis Corneal Ablation an
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Contents Corneal Ablation and Conta
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2.2.1. Fitting surfaces............
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5.3.3. Ablation efficiency factors
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10.5. DISCUSSION...................
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We thank José Antonio Sánchez-Gil
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BOZR Back Optic Zone Radius BCVA Be
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INTRODUCTION 1.1. MOTIVATION The fr
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INTRODUCTION 1.2. THE OPTICAL SYSTE
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INTRODUCTION Myopia is a very commo
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INTRODUCTION a point focus: the dif
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INTRODUCTION system, and tilt repre
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INTRODUCTION term Z 0 2 stands for
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INTRODUCTION which image was shadow
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INTRODUCTION The defocus Zernike te
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INTRODUCTION 1.4.6.3. Internal Aber
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INTRODUCTION 1.5.2.3. Alternating v
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INTRODUCTION and further-more, at l
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INTRODUCTION alcohol is used to loo
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INTRODUCTION changes in reflectivit
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INTRODUCTION Marcos et al. (Marcos
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INTRODUCTION 1.7.3. Ablation effici
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INTRODUCTION corneal surface may be
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INTRODUCTION Algorithm design: The
Page 53 and 54: INTRODUCTION contact lenses the fie
Page 55 and 56: INTRODUCTION 1.10.1. Tear studies A
Page 57 and 58: INTRODUCTION there are still many u
Page 59 and 60: INTRODUCTION 6. Evaluation and unde
Page 61 and 62: METHODS 2Chapter Chapter 2 - Method
Page 63 and 64: METHODS 2.1. MEASUREMENT OF OPTICAL
Page 65 and 66: METHODS In both profilometers, cust
Page 67 and 68: METHODS The measurement method that
Page 69 and 70: METHODS Fig. 2. 9. Set of points in
Page 71 and 72: METHODS Slit lamp Scheimpflug Camer
Page 73 and 74: METHODS representing the deviation
Page 75 and 76: METHODS 2.2.2.2. Ablation pattern f
Page 77 and 78: METHODS direction. These calculatio
Page 79 and 80: METHODS eye through the whole pupil
Page 81 and 82: METHODS (1993) power thresholds for
Page 83 and 84: METHODS alignment (frontal-illumina
Page 85 and 86: METHODS Fig. 2. 24. Example frame f
Page 87 and 88: METHODS 4) New image processing too
Page 89 and 90: METHODS Fig. 2. 28. Corrected entra
Page 91 and 92: METHODS Badal lens, corresponds to
Page 93: METHODS Each VA measurement consist
Page 99: REFRACTIVE SURGERY PMMA MODEL 3.1.
Page 102 and 103: CHAPTER 3 optimization of refractiv
Page 106 and 107: CHAPTER 3 regression line, since it
Page 108 and 109: CHAPTER 3 Figure 3.3 shows one of t
Page 110 and 111: CHAPTER 3 Ablation efficiency facto
Page 112 and 113: CHAPTER 3 considering the same set
Page 114 and 115: CHAPTER 3 3.5.3 Impact of correctio
Page 117: ABLATION PROPERTIES OF FILOFOCON A
Page 121 and 122: ABLATION PROPERTIES OF FILOFOCON A
Page 133: ABLATION PROPERTIES OF FILOFOCON A
Page 137: OPTIMIZED LASER PLATFORMS 5.1 ABSTR
Page 140 and 141: CHAPTER 5 pulses. Fluence, repetiti
Page 142 and 143: CHAPTER 5 ablations. The slit-confo
Page 144 and 145: CHAPTER 5 (Anera et al., 2003): d S
Page 146 and 147: CHAPTER 5 asymmetries also appeared
Page 148 and 149: CHAPTER 5 5.3.4. Ablation patterns
Page 150 and 151: CHAPTER 5 a) b) : 6.5 mm c) Fig. 5.
Page 152 and 153: CHAPTER 5 achieve proper reflection
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CHAPTER 5 Both the ablation algorit
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HYBRID PORCINE/PLASTIC MODEL 6.1. A
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CHAPTER 6 6.3.1. Hybrid porcine-pla
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CHAPTER 6 is detected. Therefore, i
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CHAPTER 6 no preferential orientati
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ANTERIOR AND POSTERIOR CORNEAL ELEV
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SOFT CONTACT LENS FITTING USING MOD
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SOFT CONTACT LENS FITTING USING MOD
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ON-EYE OPTICAL PERFORMANCE OF RIGID
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SOFT MONOFOCAL AND MULTIFOCAL CONTA
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SOFT MONOFOCAL AND MULTIFOCAL CONTA
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CORRELATION BETWEEN RADIUS AND ASPH
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CONCLUSIONS Conclusions This thesis
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CONCLUSIONS 5We have developed a pr
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CONCLUSIONS LIST OF METHODOLOGICAL
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CONCLUSIONS FUTURE RESEARCH LINES T
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CONCLUSIONS LIST OF PUBLICATIONS AN
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REFERENCES References AIZAWA, D., S
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REFERENCES PHOTODECOMPOSITION (APD)
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REFERENCES DORRONSORO, C., CANO, D.
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REFERENCES GISPETS, J., ARJONA, M.
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REFERENCES KOPF, M., YI, F., ISKAND
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REFERENCES MARCOS, S., DORRONSORO,
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REFERENCES NOVO, A., PAVLOPOULOS, G
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REFERENCES SAKIMOTO, T., ROSENBLATT
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REFERENCES WANG, L., DAI, E., KOCH,
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RESÚMENES pacientes reales fue com
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RESÚMENES experimentales de cirug
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RESÚMENES cada punto de las cornea
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RESÚMENES plástico medidas anteri
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RESÚMENES 8 Evaluación óptica de
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RESÚMENES 9 Prestaciones ópticas
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RESÚMENES 10 Calidad óptica y cal
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RESÚMENES A Correlación entre rad
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CONCLUSIONES clínicos. La descripc
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CONCLUSIONES 12 Las superficies ocu
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CONCLUSIONES IMPLICACIONES DE ESTA
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CONCLUSIONES contribuyen a la compr
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(y muchísimo más difícil). Me da
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