Comparison of Corneal Biomechanical Characteristics After Surface Ablation Refractive Surgery and Novel Lamellar Refractive Surgery PDF Download
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Languages : en
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Author: Fabio A. Guarnieri
Publisher: Springer
ISBN: 1493917676
Category : Medical
Languages : en
Pages : 151
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Book Description
This book presents a unique approach not found in any other text for those looking to improve the clinical results of refractive surgery by gaining a better understanding of corneal biomechanics and the instrumentation related to it. Written by leading experts in the field, this book provides authoritative coverage of the interactions of the cornea and the bioinstrumentation, such as corneal topography, pachymetry, aberrometers, tonometry and optical coherence tomography. Organized in an easy-to-read manner, Corneal Biomechanics and Refractive Surgery is designed for refractive surgeons and general ophthalmologists alike and describes the biomechanical role of the corneal tissue and how each part is affected in refractive surgery. Additionally, showing what the bioinstrumentation can measure, how models can improve understanding of the interaction between biomechanics, bioinstrumentation, and refractive surgery, and how these models and bioinstrumentation together can improve the refractive results, are also discussed.
Author: C.J. Roberts
Publisher: Kugler Publications
ISBN: 9062998763
Category : Medical
Languages : en
Pages : 328
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Book Description
While lecturing in recent months at a number of prominent institutions, I asked some of the residents and fellows whether and how they might benefit from a book on corneal biomechanics. The typical response was the look of a deer caught in the headlights as they tried to intuit the “appropriate” answer, but had little understanding or insight as to why this would be an important and useful knowledge base for them now, or in the future. I then posed the question differently. “Would a book that explained corneal biomechanical principles and testing devices and their application in detecting eyes at risk for developing keratoconus and post-LASIK ectasia, understanding the biomechanical impact of specific types of keratorefractive surgery and riboflavin UV-A corneal collagen cross-linking, and the impact of corneal biomechanics on the fidelity of intraocular pressure measurement and risk for glaucoma progression be of interest?” Framed in this context, the answer I got was a resounding, “Yes!” Therein lies a fundamental disconnect that highlights both the opportunity and need to educate all ophthalmologists about this nascent field. This comprehensive book is strengthened by the breadth of contributions from leading experts around the world and provides an important resource for ophthalmologists at all levels of training and experience. It gives a panoramic snapshot of our understanding of corneal biomechanics today, bridging the gap between theoretical principles, testing devices that are commercially available and in development as well as current and potential future clinical applications. While there has been a long-held appreciation that all types of keratorefractive surgery have an impact and interdependence on corneal biomechanics and wound healing, the initial finite element analyses that were applied to understand radial keratotomy were limited by incorrect assumptions that the cornea was a linear, elastic, homogenous, isotropic material.1 With the advent of excimer laser vision correction, critical observations indicated that Munnerlyn’s theoretic ablation profiles did not account for either lower or higher order (e.g. spherical aberration) refractive outcomes,2 suggesting that there were important components missing from the equation—e.g., corneal biomechanics and wound healing. In a seminal editorial, Roberts3 pointed out that the cornea is not a piece of plastic, but rather a material with viscoelastic qualities. Since that time, much has been learned about spatial and depth- related patterns of collagen orientation and interweaving, as well as the biomechanical response to different keratorefractive surgeries that sever tension-bearing lamellae, as the cornea responds to and redistributes stress induced by IOP, hydration, eye rubbing, blinking and extraocular muscle forces.3-6 The first reports of post-LASIK ectasia7 highlighted the need to identify a biomechanical signature of early keratoconus as well as corneas at high risk of developing ectasia irrespective of their current topography or tomography. The introduction of two instruments into clinical use—the Ocular Response Analyzer (ORA) and the Corneal Visualization Scheimpflug Technology (Corvis ST)—that allow measurement of various biomechanical metrics further catapulted the field. The availability of these instruments in routine clinical settings allowed the systematic study of the effect of age, collagen disorders, collagen cross-linking, corneal rings, flaps of various depths, contour, sidecut angulation, pockets, and flockets, just to name of few. Future application of biomechanics to the sclera may improve our understanding of the development and prevention of myopia, as well as scleral surgeries and treatments under development for presbyopia. It was appreciated by Goldmann and Schmidt that corneal thickness and curvature would influence the measurement of applanation tonometry. The recent ability to measure some corneal biomechanical metrics have led to IOP measurement that may be more immune both to their influence and the impact of central corneal thickness (CCT). Certain chapters in this book explain how a thin cornea could be stiffer than a thick one and that stiffness is also impacted by IOP, thereby precluding simplistic attempts to adjust IOP measurements using nomograms based upon CCT alone. Also highlighted is how corneal hysteresis, the ability of the cornea to absorb and dissipate energy during the bidirectional applanation response to a linear Gaussian air puff, appears to be an independent risk factor for glaucoma progression and rate of progression.9,10 This comprehensive book starts out with a section devoted to outlining basic biomechanical principles and theories, teaching us the language of what Dupps11 has referred to as “mechanospeak”, thus providing a context and common vocabulary to better comprehend the following chapters. By first defining basic concepts such as stress-strain relationships and creep, this theoretical basis is later applied to explain the pathogenesis of corneal diseases, e.g., explaining how a focal abnormality in corneal biomechanical properties precipitates a cycle of decompensation and localized thinning and steepening, clinically expressed as ectasia progression. These early chapters further detail biomechanical differences between in-vivo and ex-vivo testing, between human and animal corneas and sclera, and between methods of testing. The second section provides a thorough description of two FDA-approved devices to measure corneal biomechanics in the clinic (i.e., the ORA and the Corvis ST), as well as an overview of potential future technologies, including OCT with air puff stimulus, ocular pulse elastography, and Brilloiun microscopy. The third and final section of the book is a thorough treatise on how to interpret the metrics derived from the waveform provided by available clinical devices; their adjunct use in ectasia risk screening; the comparative biomechanical impact of various keratorefractive surgeries and corneal procedures such as PRK, LASIK, SMILE, and corneal collagen cross-linking; the impact of corneal biomechanics on IOP measurement; and potential biomechanical markers of enhanced susceptibility to glaucoma progression. This compendium of our current knowledge of corneal biomechanics, its measurement and application, provides a strong foundation to more fully understand advances in keratorefractive and corneal surgery, diseases, and treatments, all of which are interdependent on and influence inherent corneal biomechanical properties and behavior. Both the robust aspects and limitations of our current understanding are presented, including the challenge of creating accurate and predictive finite element models that incorporate the impact of IOP, corneal thickness, geometry, and scleral properties on corneal biomechanics. This book provides a key allowing clinical ophthalmologists and researchers to grasp the basics and nuances of this exciting field and to shape it as it evolves in the future.
Author: John Charles Casebeer
Publisher:
ISBN:
Category : Science
Languages : en
Pages : 220
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Book Description
In this book, Drs. Casebeer, Ruiz, & Slade go beyond the principles of their own techniques of ALK & LASIK to outline & define the foundation of how these procedures were developed & how they should be performed today. The text closes with a comprehensive look at the ALK & LASIK study results that are available, including the ALK Prospective Trial, the Jeddah ALK Retrospective Review, the Buratto et al Series, the Brint/Slade Summit MKM Report, the Salah et al Pilot Study, the LASIK with Flap Study by Pallikaris & Siganos, & the Bogota LASIK Prospective Study.
Author: Tariq Alhamad
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Understanding the biomechanical properties of the cornea is important in order to develop and improve new reliable standard procedures which can be used effectively to assess corneal behaviour in any disease condition, or before/after any ocular surgery. We believe that the Ocular Response Analyzer (ORA) is the only device that can measures the biomechanical properties of the cornea in vivo. However, it has been used for the first time both in vivo and in vitro. This thesis presents a clinical and laboratory investigation of the biomechanical properties of the cornea before/after LASIK and corneal cross-linking to improve our understanding of the knowledge required in both the laboratory and the clinic. Different machines were used in this project, including an ORA, an Oculus Pentacam, a spectrophotometer and a UV-X Illumination system. Laser in situ keratomileusis (LASIK) is, at present, one of the most well-known operations used to correct refractive errors; however, ocular problems arising from corneal thinning have been reported in some previous studies. Therefore, I looked at the effects of surgery on the central/peripheral thickness and the anterior/posterior curvature, and determined to what extent they affect the biomechanical properties of the cornea. During the past decade, much research has focused on improving and developing a new operation called corneal collagen cross-linking with riboflavin and UVA, which is used to stop the progression of keratectasia in the cornea (which occurs in keratoconus and sometimes follows refractive surgery). In the next phase, a range of experiments were conducted on cross-linking to determine to what extent this operation affects the molecular structure and biomechanical properties of the cornea. This thesis has shown for the first time that it is possible to obtain ORA signals in vitro and this opened up the possibility of examining whole eyes as well as excised corneas. It is also confirmed that the values of CH do not represent only a corneal biomechanical property, but rather depend on the presence of the rest of the eye. These in vitro studies have opened up a number of possibilities the future corneal biomechanical studies.
Author: Ibrahim Seven
Publisher:
ISBN:
Category : Cornea
Languages : en
Pages : 130
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Book Description
The cornea is the outermost layer of the human eye where the tissue meets with the external environment. It provides the majority of the eye's refractive power and is the most important ocular determinant of visual image formation. The refractive power of the cornea derives from its shape, and this shape is a function of the ocular biomechanical properties and loading forces such as the intraocular pressure (IOP). With having the majority of refractive power in the eye, the cornea is the primary tissue of interest for refractive intervention. Globally, the predominant mode of surgical treatment of refractive disorders is photoablation. However, optical power regression over time and under/over correction due to neglected corneal biomechanical properties were still observed following refractive procedures including LASIK, PRK, Astigmatic, Keratotomy etc., especially at high degree corrections. Also, some evolving procedures such as corneal collagen crosslinking (CXL), a collagen stiffening procedure most commonly performed through UVA photoactivation of riboflavin in the corneal stroma, currently lack surgical guidance for optimizing visual outcomes. Thus, there is a need for methods that explore the patient-specific treatment planning strategies for refractive procedures. This work will have a potential impact in translating mechanical principles into corneal surgical planning in order to provide a better guidance and predictive environment to the corneal surgeons.The goals of this thesis are threefold: 1) To develop patient specific models from clinical LASIK cases and to compare the outcomes of these models with clinical outcomes in a patient population. 2) To simulate investigational procedures that utilize CXL. 3) To advance a potential approach to characterize corneal mechanical properties in vivo.
Author: Mengchen Xu
Publisher:
ISBN:
Category :
Languages : en
Pages : 191
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"The biomechanics of the cornea has a significant impact on its optical behavior. Alterations in corneal biomechanics lead to abnormalities in the surface topography and affect ocular aberrations that degrade retinal image quality. The goal of this thesis work is aimed towards investigating the interaction of corneal biomechanical and optical behaviors through development of an individualized corneal model based on the finite element method that accounts for the large variations in corneal geometry and material properties. The goal of the thesis can be divided into four specific aims. First, we investigated the biomechanical and optical behaviors of a healthy normal cornea at various IOPs through numerical simulations based on a widely accepted anisotropic hyperelastic FE model. We conducted a sensitivity analysis based on a powerful experimental/statistical technique, the DOE method. The biomechanical and optical responses of the cornea to IOP elevation as well as the relative contribution of multiple geometrical and material parameters to corneal biomechanical and optical behaviors were evaluated. We found that the radius of curvature of the cornea was the most important geometric parameter that contributes to both biomechanical and optical behaviors of the cornea. For material parameters, corneal apical displacement was influenced nearly evenly by matrix stiffness, fiber stiffness and nonlinearity. However, the corneal optical aberrations were primarily affected by the matrix stiffness and the distribution of collagen fibril dispersion. These findings have important implications for future theoretical and experimental studies of the cornea, especially for the development of an individualized cornea model. Second, we proposed new methods for material characterization of individual corneas. We aimed to characterize a complete set of material parameters for developing an individualized 3-D anisotropic hyperelastic corneal model, which provides accurate prediction of the interrelation between corneal biomechanics and optics of a specific cornea. We proposed novel methods mainly focusing on the individual quantification of three challenging material parameters, including collagen fiber stiffness, collagen fiber nonlinearity and collagen fibril dispersion using optical information of the cornea to overcome the traditional challenges in corneal material characterization. The new material characterization method could also be beneficial for future development of an in vivo individualized biomechanical model of the cornea and the investigation of the impact of corneal biomechanics on patient's visual performance for clinical applications. Third, we evaluated the clinical significance of corneal biomechanical modeling in one of the important clinical applications, laser refractive surgery. An accurate prediction of the biomechanical response of the cornea to tissue ablation would help to predict postoperative surgical outcomes, which can be taken into account in developing new surgical paradigms for obtaining optimal surgical outcomes. The predictive ability of our biomechanical model was evaluated by simulating myopic corrections in PRK surgery. Our findings suggest that both of the spatial variation in collagen fibril dispersion and the depth-dependent extrafibrillar matrix stiffness play a significant role in the postoperative biomechanical and optical outcomes. Characterization of these two material features helps to predict more accurate trend of the HOAs induced by the surgical process. Lastly, we explored a novel method to induce in vivo IOP elevation for potential future development of an in vivo corneal model. Our new material characterization methods require a measurement of corneal optical behavior at varied IOP levels. Therefore, we investigated the potential of developing an in vivo individualized corneal model for clinical applications by developing an efficient and non-contract method to control IOP elevation in vivo. For the first time, we showed that in vivo IOP can be temporarily elevated and controlled in an innovative, safe, non-contact way using an inversion table. The research presented in this thesis helps to gain understandings of the biomechanical and optical responses of individual corneas to various intraocular pressures and to corneal surgery, such as laser vision correction. Furthermore, the capabilities and techniques described in the thesis may be applied to investigate underlying mechanisms, diagnosis and treatments of other clinically important ophthalmic pathologies such as keratoconus, post-refractive ectasia and glaucoma."--Pages ix-xi.
Author: Jesper Østergaard Hjortdal
Publisher:
ISBN: 9788716156815
Category : Cornea
Languages : en
Pages : 23
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Book Description
Author: Jeewan S Titiyal
Publisher: Jaypee Brothers Medical Publishers
ISBN: 9354652131
Category : Medical
Languages : en
Pages : 267
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Book Description
Refractive surgery is eye surgery to improve the refractive state of the eye and reduce or eliminate the need for glasses or contact lenses. The most common type of refractive surgery uses lasers to reshape the curvature of the cornea. This book is a practical guide to the latest advances in refractive surgery procedures. Beginning with an introduction to the evolution of refractive surgery, the next section discusses pre-operative workup, patient evaluation, and decision making. The following sections cover different refractive procedures including corneal ablative, refractive lenticule extraction, lens-based surgeries, enhancements and retreatments, and presbyopia (a condition associated with aging). The final section covers a selection of miscellaneous topics such as bioptics, collagen cross-linking, intracorneal ring segments, incisional surgery, and challenging scenarios. Each section is highly illustrated and provides in depth detail on machines and laser platforms, surgical techniques, outcomes, and potential complications and their management. Key points Practical guide to latest advances in refractive surgery Covers numerous different procedures In depth detail on techniques, outcomes, equipment and complications Highly illustrated with clinical photographs and figures
Author: Dimitri T. Azar
Publisher: Elsevier Health Sciences
ISBN: 0323551165
Category : Medical
Languages : en
Pages : 608
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Book Description
Comprehensive and clinically focused, Refractive Surgery, 3rd Edition, brings you the knowledge and expertise of leading ophthalmology experts and refractive surgeons from around the world. Together they provide practical coverage of the latest advances in this fast-changing field, including improved optics and biomechanics, improved diagnostics, new surgical techniques, and more. Hundreds of high-quality photographs guide you from patient selection to mastery of surgical techniques. Offers step-by-step guidance on a wide range of common refractive procedures. Covers new topics such as management of LASIK complications; SMILE and management of SMILE complications; collagen cross-linking, including instrumentation and applications in keratoconus and ectasia; intracorneal lenses; and multifocal and accommodating IOLs. Contains a new chapter on Preoperative Evaluation of Keratoconus and Ectasia. Uses high-quality surgical video clips to demonstrate a variety of surgical techniques. Addresses initial as well as ongoing management of patients. Includes tables of indications, preferences, hot tips, and management of complications throughout, making guidance quick and easy to access.
