Three-dimensional modeling of the human eye based on magnetic resonance imaging

PURPOSE. A methodology for noninvasively characterizing the three-dimensional (3-D) shape of the complete human eye is not currently available for research into ocular diseases that have a structural substrate, such as myopia. A novel application of a magnetic resonance imaging (MRI) acquisition and analysis technique is presented that, for the first time, allows the 3-D shape of the eye to be investigated fully. METHODS. The technique involves the acquisition of a T2-weighted MRI, which is optimized to reveal the fluid-filled chambers of the eye. Automatic segmentation and meshing algorithms generate a 3-D surface model, which can be shaded with morphologic parameters such as distance from the posterior corneal pole and deviation from sphericity. Full details of the method are illustrated with data from 14 eyes of seven individuals. The spatial accuracy of the calculated models is demonstrated by comparing the MRI-derived axial lengths with values measured in the same eyes using interferometry. RESULTS. The color-coded eye models showed substantial variation in the absolute size of the 14 eyes. Variations in the sphericity of the eyes were also evident, with some appearing approximately spherical whereas others were clearly oblate and one was slightly prolate. Nasal-temporal asymmetries were noted in some subjects. CONCLUSIONS. The MRI acquisition and analysis technique allows a novel way of examining 3-D ocular shape. The ability to stratify and analyze eye shape, ocular volume, and sphericity will further extend the understanding of which specific biometric parameters predispose emmetropic children subsequently to develop myopia. Copyright © Association for Research in Vision and Ophthalmology.

Ametropia and ocular biometry in a UK university student population

Purpose. The prevalence of myopia is known to vary with age, ethnicity, level of education, and socioeconomic status, with a high prevalence reported in university students and in people from East Asian countries. This study determines the prevalence of ametropia in a mixed ethnicity U.K. university student population and compares associated ocular biometric measures. Methods. Refractive error and related ocular component data were collected on 373 first-year U.K. undergraduate students (mean age = 19.55 years ± 2.99, range = 17-30 years) at the start of the academic year at Aston University, Birmingham, and the University of Bradford, West Yorkshire. The ethnic variation of the students was as follows: white 38.9%, British Asian 58.2%, Chinese 2.1%, and black 0.8%. Noncycloplegic refractive error was measured with an infrared open-field autorefractor, the Shin-Nippon NVision-K 5001 (Shin Nippon, Ryusyo Industrial Co. Ltd, Osaka, Japan). Myopia was defined as a mean spherical equivalent (MSE) less than or equal to -0.50 D. Hyperopia was defined as an MSE greater than or equal to +0.50 D. Axial length, corneal curvature, and anterior chamber depth were measured using the Zeiss IOLMaster (Carl Zeiss, Jena, GmBH). Results. The analysis was carried out only for white and British Asian groups. The overall distribution of refractive error exhibited leptokurtosis, and prevalence levels were similar for white and British Asian (the predominant ethnic group) students across each ametropic group: myopia (50% vs. 53.4%), hyperopia (18.8% vs. 17.3%), and emmetropia (31.2% vs. 29.3%). There were no significant differences in the distribution of ametropia and biometric components between white and British Asian samples. Conclusion. The absence of a significant difference in refractive error and ocular components between white and British Asian students exposed to the same educational system is of interest. However, it is clear that a further study incorporating formal epidemiologic methods of analysis is required to address adequately the recent proposal that juvenile myopia develops principally from myopiagenic environments and is relatively independent of ethnicity.

Predicting changes in measured introcular pressure with non-contact tonometry after LASIK

Purpose: Optometrists are becoming more integrally involved in the diagnosis of and care for glaucoma patients in the UK. The correlation of apparent change in non contact tonometry (NCT) IOP measurement and change in other ocular parameters such as refractive error, corneal curvature, corneal thickness and treatment zone size (data available to optometrists after LASIK) would facilitate care of these patients. Setting: A UK Laser Eye Clinic. Methods: This is a retrospective study study of 200 sequential eyes with myopia with or without astigmatism which underwent LASIK using a Hansatome and an Alcon LADARvision 4000 excimer laser. Refraction keratometry, pachymetry and NCT IOP mesurements were taken before treatmebnt and agian 3 months after treatment. The relationship between these variables anfd teh treatment zones were studied using stepwise multiple regression analysis. Results: There was a mean difference of 5.54mmHg comnparing pre and postoperative NCT IOP. IOP change correlates with refractive error change (P < 0.001), preoperative corneal thickness (P < 0.001) and treatment zone size (P = 0.047). Preoperative corneal thickness correlates with preoperative IOP (P < 0.001) and postoperative IOP (P < 0.001). Using these correlations, the measured difference in NCT IIOP can be predicted preoperatively or postoperatively using derived equations.Conclusion: There is a significant reduction in measured NCT IOP after LASIK. The amount of reduction can be calculated using data acquired by optometrists. This is helpful for opthalmologists and optometrists who co-manage glaucoma patients who have had LASIK or with glaucoma pateints who are consideraing having LASIK.

Synthesis and characterisation of hydrogel polymers for medical applications

A fundamental if poorly understood problem that hydrogels display is the tendency of these contact lens materials to dehydrate, causing certain complications of the corneal epithelium. However, recent studies have indicated that the evaporation rate of water from different hydrogel lenses is the same and the severity of conditions such as corneal staining is controlled by the states of water in the material. A study was therefore undertaken which concluded that increased corneal desiccating staining occurred as the proportion of water existing in the bound state decreased. The possibility of using dehydrated hydrogels as packaging materials with desiccating properties has also been investigated. As hydrogels have a high affinity for water they have adequate ability to function as a moisture scavenger in an enclosed atmosphere. It was concluded that this ability is maximised by a high total water content and an increase in the proportion of this water existing in the bound state for the material when it is fully hydrated. N-vinyl pyrrolidone has a low reactivity in vinyl polymerisation reactions which results in polymers with local domains of the same chemical type which can lead to deposition. As contact lenses comprising of this monomer are susceptible to deposition, a monomer with a higher reactivity in vinyl polymerisations is acryloylmorpholine and its incorporation in favour of NVP is encouraged. Unfortunately a large proportion of high EWC hydrogels are mechanically weak and attempts to increase this property by increasing hydrophobicity or cross-linking results in a decrease in EWC. Monomers with the potential to carry a positive charge were incorporated into a high EWC, AMO-HEMA copolymer and the physical properties were investigated. Although EWC increased, mechanical properties decreased only slightly. Therefore simultaneous incorporation of a positively charged monomer and a negatively charged monomer was investigated. The resulting copolymers showed increased water content and increased initial modulus. A technique for measuring the coefficient of friction of contact lenses during lubrication has been developed.

Towards an improved ocular drug delivery system

The ultimate aim of this project was to design new biomaterials which will improve the efficiency of ocular drug delivery systems. Initially, it was necessary to review the information available on the nature of the tear fluid and its relationship with the eye. An extensive survey of the relevant literature was made. There is a common belief in the literature that the ocular glycoprotein, mucin, plays an important role in tear film stability, and furthermore, that it exists as an adherent layer covering the corneal surface. If this belief is true, the muco-corneal interaction provides the ideal basis for the development of sustained release drug delivery. Preliminary investigations were made to assess the ability of mucin to adhere to polymer surfaces. The intention was to develop a synthetic model which would mimic the supposed corneal/mucin interaction. Analytical procedures included the use of microscopy (phase contrast and fluorescence), fluorophotometry, and mucin-staining dyes. Additionally, the physical properties of tears and tear models were assessed under conditions mimicking those of the preocular environment, using rheological and tensiometric techniques. The wetting abilities of these tear models and opthalmic formulations were also investigated. Tissue culture techniques were employed to enable the surface properties of the corneal surface to be studied by means of cultured corneal cells. The results of these investigations enabled the calculation of interfacial and surface characteristics of tears, tear models, and the corneal surface. Over all, this work cast doubt on the accepted relationship of mucin with the cornea. A corneal surface model was designed, on the basis of the information obtained during this project, which would possess similar surface chemical properties (i.e. would be biomimetic) to the more complex original. This model, together with the information gained on the properties of tears and solutions intended for ocular instillation, could be valuable in the design of drug formulations with enhanced ocular retention times. Furthermore, the model itself may form the basis for the design of an effective drug-carrier.

Comparative performance of analytical techniques for polyhexamethylene biguanide (PHMB) in ophthalmic solutions

Purpose: The use of PHMB as a disinfectant in contact lens multipurpose solutions has been at the centre of much debate in recent times, particularly in relation to the issue of solution induced corneal staining. Clinical studies have been carried out which suggest different effects with individual contact lens materials used in combination with specific PHMB containing care regimes. There does not appear to be, however, a reliable analytical technique that would detect and quantify with any degree of accuracy the specific levels of PHMB that are taken up and released from individual solutions by the various contact lens materials. Methods: PHMB is a mixture of positively charged polymer units of varying molecular weight that has maximum absorbance wavelength of 236 nm. On the basis of these properties a range of assays including capillary electrophoresis, HPLC, a nickelnioxime colorimetric technique, mass spectrophotometry, UV spectroscopy and ion chromatography were assessed paying particular attention to each of their constraints and detection levels. Particular interest was focused on the relative advantage of contactless conductivity compared to UV and mass spectrometry detection in capillary electrophoresis (CE). This study provides an overview of the comparative performance of these techniques. Results: The UV absorbance of PHMB solutions, ranging from 0.0625 to 50 ppm was measured at 236 nm. Within this range the calibration curve appears to be linear however, absorption values below 1 ppm (0.0001%) were extremely difficult to reproduce. The concentration of PHMB in solutions is in the range of 0.0002–0.00005% and our investigations suggest that levels of PHMB below 0.0001% (levels encountered in uptake and release studies) can not be accurately estimated, in particular when analysing complex lens care solutions which can contain competitively absorbing, and thus interfering, species in the solution. The use of separative methodologies, such as CE using UV detection alone is similarly limited. Alternative techniques including contactless conductivity detection offer greater discrimination in complex solutions together with the opportunity for dual channel detection. Preliminary results achieved by TraceDec1 contactless conductivity detection, (Gain 150%, Offset 150) in conjunction with the Agilent capillary electrophoresis system using a bare fused silica capillary (extended light path, 50 mid, total length 64.5 cm, effective length 56 cm) and a cationic buffer at pH 3.2, exhibit great potential with reproducible PHMB split peaks. Conclusions: PHMB-based solutions are commonly associated with the potential to invoke corneal staining in combination with certain contact lens materials. However this terminology ‘PHMBbased solution’ is used primarily because PHMB itself has yet to be adequately implicated as the causative agent of the staining and compromised corneal cell integrity. The lack of well characterised adequately sensitive assays, coupled with the range of additional components that characterise individual care solutions pose a major barrier to the investigation of PHMB interactions in the lenswearing eye.

A biological-inspired support frame for an artificial cornea

Bilateral corneal blindness represents a quarter of the total blind, world-wide. The artificial cornea in assorted forms, was developed to replace opaque non-functional corneas and to return sight in otherwise hopeless cases that were not amenable to corneal grafts; believed to be 2% of corneal blind. Despite technological advances in materials design and tissue engineering no artificial cornea has provided absolute, long-term success. Formidable problems exist, due to a combination of unpredictable wound healing and unmanageable pathology. To have a solid guarantee of reliable success an artificial cornea must possess three attributes: an optical window to replace the opaque cornea; a strong, long term union to surrounding ocular tissue; and the ability to induce desired host responses. A unique artificial cornea possesses all three functional attributes- the Osteo-odonto-keratoprosthesis (OOKP). The OOKP has a high success rate and can survive for up to twenty years, but it is complicated both in structure and in surgical procedure; it is expensive and not universally available. The aim of this project was to develop a synthetic substitute for the OOKP, based upon key features of the tooth and bone structure. In doing so, surgical complexity and biological complications would be reduced. Analysis of the biological effectiveness of the OOKP showed that the structure of bone was the most crucial component for implant retention. An experimental semi-rigid hydroxyapatite framework was fabricated with a complex bone-like architecture, which could be fused to the optical window. The first method for making such a framework, was pressing and sintering of hydroxyapatite powders; however, it was not possible to fabricate a void architecture with the correct sizes and uniformity of pores. Ceramers were synthesised using alternative pore forming methods, providing for improved mechanical properties and stronger attachment to the plastic optical window. Naturally occurring skeletal structures closely match the structural features of all forms of natural bone. Synthetic casts were fabricated using the replamineform process, of desirable natural artifacts, such as coral and sponges. The final method of construction by-passed ceramic fabrication in favour of pre-formed coral derivatives and focused on methods for polymer infiltration, adhesion and fabrication. Prototypes were constructed and evaluated; a fully penetrative synthetic OOKP analogue was fabricated according to the dimensions of the OOKP. Fabrication of the cornea shaped OOKP synthetic analogue was also attempted.

Ocular biometric correlates of early-and late-onset myopia

Myopia is a refractive condition and develops because either the optical power of the eye is abnormally great or the eye is abnormally long, the optical consequences being that the focal length of the eye is too short for the physical length of the eye. The increase in axial length has been shown to match closely the dioptric error of the eye, in that a lmm increase in axial length usually generates 2 to 3D of myopia. The most common form of myopia is early-onset myopia (EO M) which occurs between 6 to 14 years of age. The second most common form of myopia is late-onset myopia (LOM) which emerges in late teens or early twenties, at a time when the eye should have ceased growing. The prevalence of LOM is increasing and research has indicated a link with excessive and sustained nearwork. The aim of this thesis was to examine the ocular biometric correlates associated with LOM and EOM development and progression. Biometric data was recorded on SO subjects, aged 16 to 26 years. The group was divided into 26 emmetropic subjects and 24 myopic subjects. Keratometry, corneal topography, ultrasonography, lens shape, central and peripheral refractive error, ocular blood flow and assessment of accommodation were measured on three occasions during an ISmonth to 2-year longitudinal study. Retinal contours were derived using a specially derived computer program. The thesis shows that myopia progression is related to an increase in vitreous chamber depth, a finding which supports previous work. The myopes exhibited hyperopic relative peripheral refractive error (PRE) and the emmetropes exhibited myopic relative PRE. Myopes demonstrated a prolate retinal shape and the retina became more prolate with myopia progression. The results show that a longitudinal, rather than equatorial, increase in the posterior segment is the principal structural correlate of myopia. Retinal shape, relative PRE and the ratio of axial length to corneal curvature have been indicated, in this thesis, as predictive factors for myopia onset and development. Data from this thesis demonstrates that myopia progression in the LOM group is the result of an increase in anterior segment power, owing to an increase in lens thickness, in conjunction with posterior segment elongation. Myopia progression in the EOM group is the product of a long posterior segment, which over-compensates for a weak anterior segment power. The weak anterior segment power in the EOM group is related to a combination of crystalline lens thinning and surface flattening. The results presented in this thesis confirm that posterior segment elongation is the main structural correlate in both EOM and LOM progression. The techniques and computer programs employed in the thesis are reproducible and robust providing a valuable framework for further myopia research and assessment of predictive factors.

Ocular biometric investigation of anisometropia

The thesis aims to define further the biometric correlates in anisometropic eyes in order to provide a structural foundation for propositions concerning the development of ametropia.Biometric data are presented for 40 anisometropes and 40 isometropic controls drawn from Caucasian and Chinese populations.The principal finding was that the main structural correlate of myopia is an increase in axial rather than equatorial dimensions of the posterior globe. This finding has not been previously reported for in vivo work on humans. The computational method described in the thesis is a more accessible method for determination of eye shape than current imaging techniques such as magnetic resonance imaging or laser Doppler interferometry (LDI). Retinal contours derived from LDI and computation were shown to be closely matched. Corneal topography revealed no differences in corneal characteristics in anisometropic eyes, which supports the finding that anisometropia arises from differences in vitreous chamber depth.The corollary to axial expansion in myopia, that is retinal stretch in central regions of the posterior pole, was investigated by measurement of disc-to-fovea distances (DFD) using a scanning laser ophthalmoscope. DFD was found to increase with increased myopia, which demonstrates the primary contribution made by posterior central regions of the globe to axial expansion.The ocular pulse volume and choroidal blood flow, measured with the Ocular Blood Flow Tonograph, were found to be reduced in myopia; the reductions were found to be significantly correlated with vitreous chamber depth. The thesis includes preliminary data on whether the relationship arises from the influx of a blood bolus into eyes of different posterior volumes or represents actual differences in choroidal blood flow.The results presented in this thesis show the utility of computed retinal contour and demonstrate that the structural correlate of myopia is axial rather than equatorial expansion of the vitreous chamber. The technique is suitable for large population studies and its relative simplicity makes it feasible for longitudinal studies on the development of ametropia in, for example, children.

Dimensional changes in the ageing cornea

The study investigated the central and peripheral corneal characteristics of groups of subjects from 20 to 90 years of age to assist the understanding of ageing changes in the cornea, and to see whether relationships between ocular parameters were revealed. After age 45 the corneal horizontal radius of curvature gradually decreased with age. This trend was shown by the Aston University subjects (group B). The effect was very significant for the hospital patients undergoing biometry before cataract extraction operation (group D). Vertical radius of curvature showed a slight decrease with age after age 45, but similar to corneal eccentricity, this showed no significant age effect. Corneal astigmatism progressed from with the rule towards against the rule, particularly after age 60. The shift seemed mainly due to the decreasing horizontal corneal curvature. In biometry no significant age relation was found for axial length, but a significant relation was found between curvature and axial length in the larger group D. Lens thickness showed a very significant relation to age and to axial length, but no significant relation to corneal curvature. Anterior chamber depth showed a very significant relation to age, lens thickness and axial length, but no significant relation to corneal curvature. A significant age effect was found for corneal thickness decreasing with age for the central, nasal and temporal regions of the right eye. Analysis of the biometry results indicated the influence of two major factors. Firstly, the natural growth of the eye in youth, leading to greater values of axial length, radius of corneal curvature, lens thickness and anterior chamber depth. Secondly, the typical ageing changes where the increasing lens thickness caused a reduction in anterior chamber depth. The decrease in corneal thickness with age shown in some corneal regions may be a sign of ageing changes in the tissue proteins and hydration balance.

Objective methods for anterior ocular grading

This thesis set out to develop an objective analysis programme that correlates with subjective grades but has improved sensitivity and reliability in its measures so that the possibility of early detection and reliable monitoring of changes in anterior ocular surfaces (bulbar hyperaemia, palpebral redness, palpebral roughness and corneal straining) could be increased. The sensitivity of the program was 20x greater than subjective grading by optometrists. The reliability was found to be optimal (r=1.0) with subjective grading up to 144x more variable (r=0.08). Objective measures were used to create formulae for an overall ‘objective-grade’ (per surface) equivalent to those displayed by the CCLRU or Efron scales. The correlation between the formulated objective verses subjective grades was high, with adjusted r2 up to 0.96. Determination of baseline levels of objective grade were investigated over four age groups (5-85years n= 120) so that in practice a comparison against the ‘normal limits’ could be made. Differences for bulbar hyperaemia were found between the age groups (p<0.001), and also for palpebral redness and roughness (p<0.001). The objective formulae were then applied to the investigation of diurnal variation in order to account for any change that may affect the baseline. Increases in bulbar hyperaemia and palpebral redness were found between examinations in the morning and evening. Correlation factors were recommended. The program was then applied to clinical situations in the form of a contact lens trial and an investigation into iritis and keratoconus where it successfully recognised various surface changes. This programme could become a valuable tool, greatly improving the chances of early detection of anterior ocular abnormalities, and facilitating reliable monitoring of disease progression in clinical as well as research environments.

Some clinical aspects of the eye in extended contact lens wear

The limbal vascular response to extended contact lens wear was examined in a group comparative study initially intended to last eighteen months. After six months all patients wearing contact lenses had presented with micro-epithelial cysts. This unanticipated occurrence of the micro-epithelial-cysts necessitated termination of the study, and limited the quantity of data collected. However, sufficient results were available to allow a limited description of •the vascular response to this form of contact lens wear. Interpretations of the date collected ore discussed in relation to suggested vasostimulating factors in the cornea. The micro-epithelial cysts observed after extended wear were classified and their rate of recovery recorded. A further clinical study was undertaken to observe cysts in both contact lens - and non contact lens-wearing eyes. Cysts were observed in every category of patient, although the characteristic patterns varied. These observations of micro-epithelial cysts are discussed with respect to the aetiopathogeneses of corneal epithelial cystic disorders. Subsequently, attempts were made to induce cysts in rabbit corneae by extended contact lens wear. Clinical observations revealed cyst-like appearances. Histological sections did not contain cysts but did exhibit signs characteristic •of cystic disorders of the corneal epithelium. In general, the results from the study indicate that extended wear is subjectively acceptable to contact lens wearers. However, the objective findings of significant vascular changes, micro-epithelial cysts and cases of acute red eye response cast considerable doubt on the recommendation of extended wear contact lenses for purely cosmetic applications.

The effect of intraocular scattered light on the contrast sensitivity function

Intraocular light scatter is high in certain subject groups eg the elderly, due to increased optical media turbidity, which scatters and attenuates light travelling towards the retina. This causes reduced retinal contrast especially in the presence of glare light. Such subjects have depressed Contrast Sensitivity Functions (CSF). Currently available clinical tests do not effectively reflect this visual disability. Intraocular light scatter may be quantified by measuring the CSF with and without glare light and calculating Light Scatter Factors (LSF). To record the CSF on clinically available equipment (Nicolet CS2000), several psychophysical measurement techniques were investigated, and the 60 sec Method of Increasing Contrast was selected as the most appropriate. It was hypothesised that intraocular light scatter due to particles of different dimensions could be identified by glare sources at wide (30°) and narrow (3.5°) angles. CSFs andLSFs were determined for: (i) Subjects in young, intermediate and old age groups. (ii) Subjects during recovery from large amounts of induced corneal oedema. (iii) A clinical sample of contact lens (CL) wearers with a group of matched controls. The CSF was attenuated at all measured spatial frequencies with the intermediate and old group compared to the young group. High LSF values were found only in the old group (over 60 years). It was concluded that CSF attenuation in the intermediate group was due to reduced pupil size, media absorption and/or neural factors. In the old group, the additional factor was high intraocular light scatter levels of lenticular origin. The rate of reduction of the LSF for the 3.5° glare angle was steeper than that for the 30° angle, following induced corneal oedema. This supported the hypothesis, as it was anticipated that epithelial oedema would recover more rapidly than stromal oedema. CSFs and LSFs were markedly abnormal in the CL wearers. The analytical details and the value of these investigative techniques in contact lens research are discussed.

Measurement of ocular component contributions to residual astigmatism in adult human eyes

The aim of this study was to determine whether an ophthalmophakometric technique could offer a feasible means of investigating ocular component contributions to residual astigmatism in human eyes. Current opinion was gathered on the prevalence, magnitude and source of residual astigmatism. It emerged that a comprehensive evaluation of the astigmatic contributions of the eye's internal ocular surfaces and their respective axial separations (effectivity) had not been carried out to date. An ophthalmophakometric technique was developed to measure astigmatism arising from the internal ocular components. Procedures included the measurement of refractive error (infra-red autorefractometry), anterior corneal surface power (computerised video keratography), axial distances (A-scan ultrasonography) and the powers of the posterior corneal surface in addition to both surfaces of the crystalline lens (multi-meridional still flash ophthalmophakometry). Computing schemes were developed to yield the required biometric data. These included (1) calculation of crystalline lens surface powers in the absence of Purkinje images arising from its anterior surface, (2) application of meridional analysis to derive spherocylindrical surface powers from notional powers calculated along four pre-selected meridians, (3) application of astigmatic decomposition and vergence analysis to calculate contributions to residual astigmatism of ocular components with obliquely related cylinder axes, (4) calculation of the effect of random experimental errors on the calculated ocular component data. A complete set of biometric measurements were taken from both eyes of 66 undergraduate students. Effectivity due to corneal thickness made the smallest cylinder power contribution (up to 0.25DC) to residual astigmatism followed by contributions of the anterior chamber depth (up to 0.50DC) and crystalline lens thickness (up to 1.00DC). In each case astigmatic contributions were predominantly direct. More astigmatism arose from the posterior corneal surface (up to 1.00DC) and both crystalline lens surfaces (up to 2.50DC). The astigmatic contributions of the posterior corneal and lens surfaces were found to be predominantly inverse whilst direct astigmatism arose from the anterior lens surface. Very similar results were found for right versus left eyes and males versus females. Repeatability was assessed on 20 individuals. The ophthalmophakometric method was found to be prone to considerable accumulated experimental errors. However, these errors are random in nature so that group averaged data were found to be reasonably repeatable. A further confirmatory study was carried out on 10 individuals which demonstrated that biometric measurements made with and without cycloplegia did not differ significantly.

Evaluation of videokeratoscopy

The present thesis evaluates various aspects of videokeratoscopes, which are now becoming increasingly popular in the investigation of corneal topography. The accuracy and repeatability of these instruments has been assessed mainly using spherical surfaces, however, few studies have assessed the performance of videokeratoscopes in measuring convex aspheric surfaces. Using two videokeratoscopes, the accuracy and repeatability of measurements using twelve aspheric surfaces is determined. Overall, the accuracy and repeatability of both instruments were acceptable, however, progressively flatter surfaces introduced greater errors in measurement. The possible reasons for these errors are discussed. The corneal surface is a biological structure lubricated by the precorneal tear film. The effects of variations in the tear film on the repeatability of videokeratoscopes have not been determined in terms of peripheral corneal measurements. The repeatability of two commercially available videokeratoscopes is assessed. The repeatability is found to be dependent on the point of measurement on the corneal surface. Typically, superior and nasal meridians exhibit poorest repeatability. It is suggested that interference of the ocular adnexa is responsible for the reduced repeatability. This localised reduction in repeatability will occur for all videokeratoscopes. Further, comparison with the keratometers and videokeratoscopes used show that measurements between these instruments are not interchangeable. The final stage of this thesis evaluates the performance of new algorithms. The characteristics of a new videokeratoscope are described. This videokeratoscope is used to test the accuracy of the new algorithms for twelve aspheric surfaces. The new algorithms are accurate in determining the shape of aspheric surfaces, more so than those algorithms proposed at present.