As a bike rider, I always compelled to think about my eye health, when riding in high speed and in extreme cold. I frequently visit to ophthalmologists not only for the treatment but to get scientific c and satisfactory causes, prevention...
moreAs a bike rider, I always compelled to think about my eye health, when
riding in high speed and in extreme cold. I frequently visit to ophthalmologists not only for the treatment but to get scientificc and satisfactory causes, prevention and control measures of my eye problems. My curiosity raised several queries, however, hardly answered. I heartily thank Dr. Dil Bahadur Gurung, my re- search supervisor, to make me able to answer my queries by myself.
Two wheeler users are increasing day by day and proportionally eye problem holders in our country. According to different medical theories, the main factors behind dry eye, eye irritation, refractive errors, excess tearing, cataract etc. are the environmental temperature fluctuations, wind, pollution, ultraviolet(UV)/infrared(IR) rays, electromagnetic waves etc. In the sense of thermo-regulation all these factors either increase or decrease eye tissue temperature, which indirectly induce such abnormalities.
In this thesis, study have been carried out for the temperature variation
in human eye tissues due to several internal and external factors. Internal factors include blood perfusion, metabolism, blinking, tearing, thermal conductivities. External factors include different ambient temperatures, evaporation rates, and air speeds. Some medical therapies or treatments like laser surgeries, hyperthermia, cryosurgery are used to treat several eye abnormalities like refractive errors,
glaucoma, cataract, diabetic retinopathy, eyelid carcinoma etc. The treatment procedure is either heating or cooling eye tissues by external means. We end up the thesis by presenting the impact of two laser refractive error treatment procedures: laser in situ keratomileusis (LASIK) and laser thermal keratoplasty (LTK) in temperature distribution of cornea.
The thesis is divided into six chapters. Chapter-wise cameo description of the present study is as follows.
Chapter 1 deals with the general introduction on human eye anatomy and physiology. It covers the general heat transfer mechanisms in human eye namely conduction, convection, metabolism, perfusion and evaporation. The derivation of Pennes bio-heat equation and the signicance of using it in human eye has been discussed. At the end, we present the review of literature.
Chapter 2 deals with the mathematical theory of finite elements. It covers the basic theory of finite elements, calculus of variations, weak formulations, finite element solution procedure of elliptic boundary value problems and heat equation. The solution procedure of bio-heat equation in one and two dimensions in human eye discretization has been discussed. In thermoregulation of human body, about 50-80% of heat ow in the tissue is carried in or out by blood ow. In human eye only few layers retina, choroid, iris, and ciliary body, constitutes only 5% of entire volume, having blood
ow.
Chapter 3 deals with comparative study of temperature distribution in
human eye with and without taking the effects of blood
ow in these tissues. Numerical calculations are carried out using different tear evaporation rates, blood temperatures, ambient temperatures and lens thermal conductivities. Both one
and two dimensional steady and transient state cases are included. Convergence of temperature values by varying the mesh size has been carried out. The numerical results are compared with past experimental and similar simulation results. Eyelids are the thin fold of skin that cover and protect the anterior surface of the eye globe from local injury. Blinking is regarded as the continuous interrupted eyelid closure or opening and it's thermal effect will compromise between
these two. During blinking heat loss via convection, radiation, and tear evaporation from cornea is prevented, warm tear is layered across corneal surface and the vessels of palpebral conjunctiva provide heat to anterior eye. Chapter 4 deals with the steady and transient temperature distribution of human eye in case of eyelid closure, opening and blinking. Eyelid is considered as a heating/cooling source of anterior cornea and model as a part of ocular component. Besides the various functions of eyelid blinking, heating and cooling mechanism play important role in maintaining anterior eye temperature. The eye temperature using
different values of evaporation rates, blood temperatures, ambient temperatures, lens thermal conductivities, blinking rates, blood perfusion rates and their impact on eye diseases like presbyopia, cataract etc. are investigated. The results so obtained are compared with past results.
High air speed makes thinner the thermal boundary layer between cornea and environment. This accelerates evaporation and heat transfer. Eyelid blinking increases with increased air ow speed into the eye. Increased blinking increases corneal temperature when drops below normal level. In cold climatic condition, high speed wind causes rapid temperature drop. Most often eye injuries caused by cold exposure occur in individuals who try to force their eyes open in cold weather in combination with wind such as two wheeler rider. Chapter 5 investigates the temperature changes in two wheeler rider's cornea in his/her different speed and their impact on eye health. The numerical calculations are carried out during eyelid opening, closure and blinking with appropriate forced convection coefficient at different air speeds and ambient temperatures.
Refractive errors are some of the most common ophthalmic abnormalities worldwide. Refractive surgeries were developed so that people could enjoy good vision with no or reduced dependence on glasses or contact lenses. Two types of laser surgical techniques: lamellar and thermal are available to reshape the corneal curvature. Ultraviolet (UV) emitting argon fluoride (ArF) excimer laser
is used to sculpt cornea in lamellar procedure, whereas, infrared (IR) emitting holmium yttrium aluminum garnet (Ho:YAG) laser is used to shrink cornea in thermal procedure. Tissue heating is common in all types of laser surgical techniques. Chapter 6 deals with the temperature distribution of cornea in different laser refractive surgeries. Characteristics of optical and thermal processes and influence of radiation and tissue parameters on the results of laser action are investigated. The results of mathematical modeling in different surgical techniques are discussed, compared and validated with experimental results.