Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

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

CHAPTER 7 There were no statistically significant correlations one month after surgery. 7.5. DISCUSSION In this chapter we have presented measurements of changes in the anterior and posterior corneal surface in patients that had undergone myopic LASIK refractive surgery and in control eyes. Our results for anterior surface mean pre-operative radius (R ant = 7740 ± 230 m) and asphericity (Q ant = -0.11 ± 0.13) in our group of patients agree well with those reported by Dubbelman and collaborators, who used a customadapted Scheimpflug imaging system (R ant = 7870 ± 270 m, Q ant = -0.18 ± 0.18).(Dubbelman et al., 2002) There is also a good agreement between our mean preoperative posterior corneal radius of curvature (R back = 6440 ± 250 m) and Dubbelman et al’s (R back = 6400 ± 280 m). However, our mean pre-operative posterior corneal asphericity is significantly higher (Q back = 0.18 ± 0.21 in our study and Q back = -0.38 ± 0.27 in Dubbelman et al.’s, for 6 and 7 mm fitted areas respectively). Differences may arise from differences in the refractive state and age of both populations, as our results from the validation using a hybrid model eye indicate an underestimation, rather than an overestimation of the posterior corneal asphericity. We have not found evidence of a systematic influence of LASIK on the posterior surface of the cornea on average. We detected a change in the radius of curvature and asphericity of the posterior corneal surface the first day after surgery, but afterwards this change disappeared. The cause of this change is unclear to us. The fact that changes disappear in a timescale of days-weeks suggests the action of biological processes within that timing. Potential factors affecting posterior corneal shape temporarily may include hydration, keratocyte activity, the stress produced by the suction ring of the microkeratome or the medication. Furthermore, we observed a remarkable similarity between both eyes of each subject. This suggests a physiological mechanism acting bilaterally in the same way, as both corneas of the same patient are expected to have similar biomechanical properties and follow similar biological processes. The bilateral similarity also tends to occur in left and right eyes of the control subjects. We have found that changes in the posterior radius of curvature changes occur primarily in the vertical direction in post-LASIK patients. Possible causes for this effect include the following three: (1) It is possible that surgery affects more strongly the corneal stability in the vertical direction. This might be due to an asymmetrical ablation for correction of astigmatism, but we have not found any statistically significant correlation between astigmatism corrected and radius change. Even with a symmetric ablation corneal stability might be more strongly affected in the vertical direction because of the direction of the flap, which is cut in the vertical direction, leaving a superior hinge. (2) Alternatively, a meridian-independent surgery may cause a greater change in the vertical meridian, if there is a greater mechanical stress of the cornea in that direction. This higher mechanical stress may be caused by the eyelid, which presses on the superior part of the cornea, and has been shown to modify the corneal geometry, having impact on corneal aberrations (Buehren et al., 2003), (3) Interestingly, the intralamellar cohesive strength, studied in human eye bank corneas, has been shown to be lower in the vertical than in the horizontal meridian, suggesting that even a symmetric force applied to the cornea may result in an asymmetric corneal deformation (Smolek, 1993). It is interesting to note that in previous studies (Jain et al., 2007) the vertical posterior radius of curvature was found to be the least repeatable 180
ANTERIOR AND POSTERIOR CORNEAL ELEVATION MAPS AFTER REFRACTIVE SURGERY of the parameters measured by Pentacam, in repeated measurements of post-LASIK subjects. We have found that control subjects also experience statistically significant changes of the same order of magnitude than those found in patients, although the reported differences between vertical and horizontal meridians are unique to patients (see Fig. 7.6(D), see also Fig. 7.5(B)). These differences between patients and control subjects are indicative of some surgical effect on the posterior corneal surface, although the average magnitude of the changes observed in patients is similar to that of the changes observed in control subjects. This suggests that most of the changes observed in patients are normal, perhaps due to changes in the intraocular pressure. In fact, preliminary results of inflation experiments on porcine eyes show that the radius of curvature of the posterior corneal surface changes about 30 m per mm Hg (Perez- Escudero et al., 2008). The changes in posterior radius of curvature that we report in the present study (up to 120 m) are consistent with changes of intraocular pressure of the order of 5 mm Hg, which is the average change in intraocular pressure throughout the day (David et al., 1992). The small changes induced by the surgery are superposed to these physiological changes, and originate the subtle correlations mentioned above. All changes in the radius of curvature of the posterior corneal surface that we found are smaller than 180 m (taking into account an interval of confidence of 98.3%). In an average cornea, this change in radius induces a change in the refractive power of the posterior corneal surface below 0.18 D (Barbero, 2006), too small to be clinically relevant. Therefore, the contribution of the posterior corneal surface to shifts from the attempted refraction is minor. Previous studies using Orbscan reported longterm average changes in posterior radius of curvature up to 400 m (Seitz et al., 2001, Twa et al., 2005), and ectasia measured as forward displacement of the center of the posterior corneal surface up to 40 m (Wang et al., 1999, Baek et al., 2001), much greater than the changes observed by us (lower than 8 m on average, including the 98.3% confidence interval, Fig. 7.3 (C)). The discrepancy may be due to improper correction of the distortion due to the anterior corneal surface in Orbscan (Ueda et al., 2005, Donnenfeld, 2001). Our results are consistent with recent data obtained with the same device (Pentacam) (Ciolino and Belin, 2006). Along with the findings of this study, more experimental data on corneal biomechanical properties and more accurate models of corneal biomechanics will help to better understand the corneal shape response to LASIK surgery. 7.6. CONCLUSIONS In this chapter we have demonstrated that Scheimpflug imaging can reliably assess corneal shape changes following refractive surgery research on real patients, both for the anterior and posterior corneal surfaces. The anterior corneal measuremens (radius and asphericities) obtained using Pentacam agree with those previously reported using other methods and the same laser. We found a transitory and clinically irrelevant change in the shape (radius and asphericities) of the posterior cornea the first day after surgery, but we found no evidence of permanent changes as a consequence of LASIK. The changes in the posterior radius of curvature occur primarily in the vertical direction. In equivalent measurements in control eyes, with no treatment between measurements, we observed changes of the same magnitude, but without directional differences. 181
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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
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METHODS 2Chapter Chapter 2 - Method
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METHODS 2.1. MEASUREMENT OF OPTICAL
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METHODS In both profilometers, cust
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METHODS The measurement method that
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METHODS Fig. 2. 9. Set of points in
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METHODS Slit lamp Scheimpflug Camer
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METHODS representing the deviation
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METHODS 2.2.2.2. Ablation pattern f
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METHODS direction. These calculatio
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METHODS eye through the whole pupil
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METHODS (1993) power thresholds for
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METHODS alignment (frontal-illumina
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METHODS Fig. 2. 24. Example frame f
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METHODS 4) New image processing too
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METHODS Fig. 2. 28. Corrected entra
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METHODS Badal lens, corresponds to
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METHODS Each VA measurement consist
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REFRACTIVE SURGERY PMMA MODEL 3.1.
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CHAPTER 3 optimization of refractiv
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CHAPTER 3 These data demonstrate th
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CHAPTER 3 regression line, since it
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CHAPTER 3 Figure 3.3 shows one of t
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CHAPTER 3 Ablation efficiency facto
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CHAPTER 3 considering the same set
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CHAPTER 3 3.5.3 Impact of correctio
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ABLATION PROPERTIES OF FILOFOCON A
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Page 129 and 130: ABLATION PROPERTIES OF FILOFOCON A
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Page 142 and 143: CHAPTER 5 ablations. The slit-confo
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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
Page 162 and 163: CHAPTER 6 is detected. Therefore, i
Page 164 and 165: CHAPTER 6 no preferential orientati
Page 167: ANTERIOR AND POSTERIOR CORNEAL ELEV
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Page 187: SOFT CONTACT LENS FITTING USING MOD
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Page 194: ON-EYE OPTICAL PERFORMANCE OF RIGID
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Page 212: SOFT MONOFOCAL AND MULTIFOCAL CONTA
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Page 220 and 221: 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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