Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

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$XXX Congress of the European Society of Cataract and Refractive ...$

CHAPTER 3 considering the same set of corrections). This change in the corneal ablation threshold affects only slightly the predictions of the theoretical factor (0.29±0.53) (not shown). Asphericity 6 4 2 0 Clinical With experimental efficiency factor (Fth=60) With theoretical efficiency factor No efficiency effects (flat corneas) -2 0 3 6 9 12 15 Correction (D) Fig. 3.8. Asphericities of simulated postoperative corneas (central radii 7.8 mm, asphericity -0.2) as a function of nominal correction, in comparison with clinical data (red diamonds) on real patients operated with the same laser system. Predictions include asphericities obtained from post-operative corneas after subtracting the corneal ablation pattern obtained from flat PMMA, i.e. with no efficiency effects (green diamonds), as well as this pattern multiplied by the experimental (orange diamonds) and the theoretical efficiency factor (blue triangles). In summary, the predicted corneal asphericities from the experimental efficiency factor are higher than those obtained with the theoretical factor, although for the highest corrections they are still lower than the clinical data on patients. 3.5. DISCUSSION 3.5.1. Ablation pattern programmed into the laser The comparison of the theoretical ablation patterns with the experimental ablation pattern (all shown in Fig. 3.3) suggests that the actual pattern programmed into the laser is close to the theoretical equations (Munnerlyn and parabolic). In addition, corneal asphericities induced purely by the experimental ablation pattern (see open diamonds in Fig. 3.8), without taking into account ablation efficiency changes and corneal biomechanics are close to those induced by the Munnerlyn pattern and show almost no change with correction, as reported before from analytical (Anera et al., 2003, Gatinel et al., 2001) and computational simulations (Marcos et al., 2003, Cano et al., 2004) of the Munnerlyn algorithm. Those asphericity predictions are far from the post-operative asphericities in real patients, indicating that the ablation profile per se is not the cause for the increased asphericity found clinically. 3.5.2. Experimental estimates of the ablation efficiency correction factor As opposed to ablation patterns obtained from ablated flat PMMA surfaces, ablation patterns calculated from ablated spherical PMMA surfaces are affected by changes in 112
REFRACTIVE SURGERY PMMA MODEL ablation efficiency, i.e. the ablation depth per laser pulse decreases as the spot moves from the apex to the periphery, due to variations in the angle of incidence of the laser radiation. Measurements on both flat and spherical PMMA surfaces have allowed us to obtain a direct estimate of the ablation efficiency factor on PMMA, and a prediction of the ablation efficiency factor for the cornea based on those measurements. Previous estimates of the ablation efficiency factor had been obtained mathematically (Anera et al., 2003, Jimenez et al., 2002), using approximations as a truncated polynomial series expansion, and assuming only reflection losses using Fresnel equations and non-polarized light, along with nominal values for laser fluence and corneal refractive index and ablation thresholds. However, there are other effects that potentially affect the changes in laser efficiency. These include spot shape, beam divergence changes from the center to the periphery, beam scanning effects, polarization, defocus, etc. Unlike the theoretical factor, the experimental ablation efficiency factor K PMMA and K cornea estimated from K PMMA include these effects. While K PMMA is a direct measurement of the ablation efficiency changes on PMMA and does not rely on any assumption, the estimates of the experimental K cornea from the experimental K PMMA rely on three assumptions: 1) that the Beer-Lambert’s law applies to the cornea as well as on PMMA. 2) that the reflectivity factor f(R) is similar in PMMA and corneal tissue, which is true provided that the index of refraction at the laser wavelength is the same on cornea and PMMA and 3) that the laser fluence and the ablation thresholds for PMMA and corneal tissue are known. The fact that the Beer-Lambert´s law appropriately describes photoablation of corneal tissue is fairly well established (Pettit and Ediger, 1996, Fisher and Hahn, 2004). We have also tested assumption 2 by computing the difference in the reflectivity factor when changing the index of refraction by 1.52 instead of 1.49 and found no appreciable change. For PMMA, an ablation threshold of 80 mJ/cm 2 seems to be an accepted value. For the cornea however, while many studies use 40 mJ/cm 2 (Berns et al., 1999), some authors have recently reported (Pettit et al., 2005) 60 mJ/cm 2 . The ablation efficiency factor that we present represents therefore a conservative estimate. We repeated the computations using a corneal ablation threshold of 60 instead of 40 mJ/cm 2 and found predicted corneal asphericities closer to the clinical estimates, as shown in Fig. 3.8 (gray solid diamonds). Tissue hydration, that causes changes in reflectivity and absorption, among other effects, during corneal ablation (Manns et al., 2002b), could also modify the laser ablation rate in corneas, and the cornea/PMMA factor. Our experimental ablation efficiency factor K cornea , although slightly stronger, agrees well with the theoretical K cornea from Jimenez et al.’s (Fig. 3.6). However, the impact of this slight difference is important, and the predicted corneal asphericities using the experimental K cornea are much closer to the clinical findings (Fig. 3.8). A similar effect was found on PMMA (Fig. 3.7). The calibration protocols that we describe here can be generalized to any laser system and any material (with a refractive index similar to the cornea at the ablation wavelength), even if the numerous assumptions and simplifications of the theoretical model do not hold. This systematic calibration (Marcos et al., 2005b) will potentially be more effective than ablation efficiency factors empirically obtained by adjusting recursively the algorithm, which require prior treatments on real patients at different stages of the ablation algorithm refinement. 113
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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
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INTRODUCTION contact lenses the fie
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INTRODUCTION 1.10.1. Tear studies A
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INTRODUCTION there are still many u
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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 104 and 105: CHAPTER 3 These data demonstrate th
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 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
Page 154 and 155: CHAPTER 5 Both the ablation algorit
Page 157: HYBRID PORCINE/PLASTIC MODEL 6.1. A
Page 160 and 161: 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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