Finite Element Modelling of Soft Contact Lenses on Eye

When fitting soft contact lenses, it is impossible to visualise the tear layer below the lens in white light. In addition, being permeable, soft lenses absorbs normal fluorescein and use of high molecular fluorescein is not sensitive enough to identify subtle changes in fit. This study provides a software tool based on a Finite Element Model of the human eye, developed over a period of more than 15 years at both Dundee University and Liverpool University, that can demonstrate the fit of a known contact lens design on a particular subject’s eye through computer simulation. Using this new technique, thickness maps for the tear layer under contact lenses can be created. These maps give important feedback to the contact lens fitting and design process and have the potential to enable the full customisation of contact lenses. In addition, the model itself can demonstrate how the lens settles onto the eye during the blink process, together with rebound from the corneal surface directly after the blink

Effect of Correcting Non-Orthogonal Astigmatism in Corneas with Novel Optical System

For normal eyes, astigmatism is assumed to have orthogonal axes between its optical power meridians. Irregular (non-orthogonal) astigmatism is defined as having axes with less than 90° between them. The eye condition Keratoconus generally results in non-orthogonal (NO) astigmatism and this cannot be fully corrected by conventional orthogonal optics. Objects viewed by people with significant NO astigmatism can present as multiple images or appear severely ghosted or distorted. All ophthalmic spectacle lenses and toric contact lenses assume astigmatism has orthogonal axes, making it difficult to correct NO astigmatism optically. Additionally, topography machine software imposes orthognal axes on their power map outputs, so it is not possible to easily assess the extent of NO astigmatism present in any individual eye. An investigation was undertaken to attempt to correct NO astigmatism with an appropriate optical system and assess the ffect on viausl acuity. Raw data was taken from scanning topography machines and processed to detect the natural maximum and minimum power meridians of the anterior and posterior cornea. Axial and tangential maps were created as well as power maps achieved through Light Ray Tracing A means of creating spectacle trial lenses with NO power axes was developed and three sets made to use as refraction trial lenses. The axes of each set were orientated at 80°, 70° & 60° respectively and cyl powers -1.00DC to -6.00 DC in 1.00DC steps plus an additional -0.50DC lens. Three subjects were chosen to be refracted by these lenses: two with mild keratoconus and one with longstanding, non strabismic ambylopia. They were refracted with each set of lenses, using a standard LogMAR Chart. The chart was changed randomly in between testing each set. For each subject, the refraction starting point was taken from the orthogonal spectacle prescription. The subjects were refracted with NO cyls from each set of lenses and wre asked to locate the point of optimal acuity by rotating the lens. This was independently checked by the examiner. To ensure that the subject was not experiencing a placebo effect, the lenses were randomly “flipped” during the examination. Unlike conventional cyl lenses, which present the same power meridians whichever way the lens is presented to the eye, NO lenses will present meridians at different axes when flipped which should cause an obvious difference in VA

Early term results of the stress-strain index in patients with keratoconus

Purpose To test the early term effect of corneal cross-linking (CXL) on the stress-strain index (SSI) in patients with keratoconus. Materials and Methods Medical records of 31 patients undergoing CXL were retrospectively evaluated before and at least 4 weeks after the procedure. All patients underwent complete ophthalmic examination including Corvis ST (OCULUS Optikgeräte GmbH; Wetzlar, Germany). The main outcome measures were SSI and other dynamic corneal response (DCR) parameters. Results The mean follow-up time was 53 ± 42 days (28 – 196). The SSI was significantly increased from 0.82 ± 0.17 at the postoperative to 0.91 ± 0.24 at the postoperative (p=0.002).Among the DCR parameters integrated inverse radius (IIR), time to reach the first aplannation (A1 time) and corneal deflection during this time (A1 Deflection) were significantly reduced (p<0.05). Central corneal thickness (CCT) was significantly reduced, -17.54 ± 15.17µm (p<0.001), while intraocular pressure estimates provided by the device, was significantly increased (1.03 ± 2.53mmHg, p=0.030), there was a smaller change in the biomechanically corrected intraocular pressure for soft corneas (bIOPs, 0.67 ± 1.27mmHg, p=0.004) Conclusion Corneal stiffening induced by crosslinking could be directly observed by means of SSI at the early postoperative of patients submitted to crosslinking

Inflation Experiments and Inverse Finite Element Modelling of Posterior Human Sclera

The complexity of inverse finite element modelling methods used in ocular biomechanics research has significantly increased in recent years in order to produce material parameters that capture microscale tissue behaviour. This study presents a more accessible method for researchers to optimise sclera material parameters for use in finite element studies where macroscale sclera displacements are required. Five human donor sclerae aged between 36 and 72 years were subjected to cycles of internal pressure up to 61 mmHg using a custom-built inflation rig. Displacements were measured using a laser beam and two cameras through a digital image correlation algorithm. Specimen specific finite element models incorporating regional thickness variation and sclera surface topography were divided into six circumferential regions. An inverse finite element procedure was used to optimise Ogden material parameters for each region. The maximum root mean squared (RMS) error between the numerical and experimental displacements within individual specimens was 17.5 μm. The optimised material parameters indicate a gradual reduction in material stiffness (as measured by the tangent modulus) from the equator to the posterior region at low-stress levels up to 0.005 MPa. The variation in stiffness between adjacent regions became gradually less apparent and statistically insignificant at higher stresses. The study demonstrated how inflation testing combined with inverse modelling could be used to effectively characterise regional material properties capable of reproducing global sclera displacements. The material properties were found to vary between specimens, and it is expected that age could be a contributing factor behind this variation

Three-dimensional non-parametric method for limbus detection

Purpose To present a novel non-parametric algorithm for detecting the position of the human eye limbus in three dimensions and a new dynamic method for measuring the full 360° visible iris boundary known as white-to-white distance along the eye horizontal line. Methods The study included 88 participants aged 23 to 65 years (37.7±9.7), 47 females and 41 males. Clinical characteristics, height data and the apex coordinates and 1024×1280 pixel digital images of the eyes were taken by an Eye Surface Profiler and processed by custom-built MATLAB codes. A dynamic light intensity frequency based white-to-white detection process and a novel three-dimensional method for limbus detection is presented. Results Evidence of significant differences (p<0.001) between nasal-temporal and superior-inferior white-to-white distances in both right and left eyes were found (nasal-temporal direction; 11.74±0.42 mm in right eyes and 11.82±0.47 mm in left eyes & superior-inferior direction; 11.52±0.45 mm in right eyes and 11.55±0.46 mm in left eyes). Average limbus nasal-temporal diameters were 13.64±0.55 mm for right eyes, and 13.74±0.40 mm for left eyes, however the superior-inferior diameters were 13.65±0.54 mm, 13.75±0.38 mm for right and left eyes, respectively. No significant difference in limbus contours has been observed either between the nasal-temporal direction (p = 0.91) and the superior-inferior direction (p = 0.83) or between the right (p = 0.18) and left eyes (p = 0.16). Evidence of tilt towards the nasal-temporal side in the three-dimensional shape of the limbus was found. The right eyes mean limbus contour tilt around the X-axis was -0.3±1.35° however, their mean limbus contour tilt around the Y-axis was 1.76±0.9°. Likewise, the left eyes mean limbus contour tilt around the X-axis was 0.77±1.25° and the mean limbus contour tilt around the Y-axis was -1.54±0.89°. Conclusions The white-to-white distance in the human eye is significantly larger in the nasal-temporal direction than in the superior-inferior direction. The human limbus diameter was found not to vary significantly in these directions. The 3D measures show that the limbus contour does not lay in one plane and tends to be higher on the nasal-inferior side of the eye

Simulated Optical Performance of Soft Contact Lenses on the Eye

Purpose: To evaluate the impact of soft contact lens eye-fit on optical power by computational modelling and to produce correction maps for reversing this impact during the design process. Methods: Finite element models of spherical and toric hydrogel contact lenses at varying nominal powers of -20 D to +20 D, base curves radii (R1b) of 8.2, 8.5, 8.8 mm, and overall diameters (d3) of 14.5, 15.0, 15.5 mm were generated. Lenses were fitted to computational eye models generated with human eyes’ topography data. Combined eye-lens simulations were run under the boundary conditions of the tears’ surface tension between the contact lens and the eye in addition to the eyelid blink pressure. Lens optical zone power changes were calculated through computational light-ray tracing methods following each simulation. Results: Effective power changes (EPC) were affected negatively for all toric simulated lenses with power varying from 20 D to +20 D. Spherical lenses demonstrated similar behaviour, however with some positive EPC over the power range from 20 D to -10 D for spherical power (SPH) lenses. EPC assessment was between +0.25 D and -0.5 D for most lenses, however, lenses with prescriptions from +10 D to +20 D incurred EPC outside this range. The spherical lenses showed a maximum effective power change of +1.046 ± 0.338 D (Average Eye), and a minimum of -3.278 ± 0.731 D (Steep Eye). Similarly, the toric lenses showed a maximum of +1.501 ± 0.338 D (Average Eye), and a minimum of -3.514 ± 0.731 D (Steep Eye). EPC trends, along with minimum and maximum power, generally increased negatively as nominal lens prescription increased positively. Contact lens base curve selection affected the assessed effective power change for both spherical and toric lenses. The effect from lens total diameter for spherical lenses was less substantial than that for toric lenses. Conclusions: This study considered the impact of soft contact lens design parameters on effective optical power changes (EPC) after eye-fit. Spherical lenses experienced more EPC of clinical significance (>0.25 D) than toric lenses. Both types of lenses, spherical and toric (simple astigmatism), demonstrated similar trends in EPC on fitting from -20 D to +20 D, with lenses in the extremely positive and the extremely negative prescriptions demonstrating the highest EPCs. The lens base curve impacted the extent of EPC observed, with flatter base curves experiencing less power change. Diameter proved to impact toric lenses more than spherical ones, however generally the diameter has less effect on power change than base curve selection

Simulated Optical Performance of Soft Contact Lenses on the Eye

Purpose: To evaluate the impact of soft contact lens eye-fit on optical power by computational modelling and to produce correction maps for reversing this impact during the design process. Methods: Finite element models of spherical and toric hydrogel contact lenses at varying nominal powers of -20 D to +20 D, base curves radii (R1b) of 8.2, 8.5, 8.8 mm, and overall diameters (d3) of 14.5, 15.0, 15.5 mm were generated. Lenses were fitted to computational eye models generated with human eyes’ topography data. Combined eye-lens simulations were run under the boundary conditions of the tears’ surface tension between the contact lens and the eye in addition to the eyelid blink pressure. Lens optical zone power changes were calculated through computational light-ray tracing methods following each simulation. Results: Effective power changes (EPC) were affected negatively for all toric simulated lenses with power varying from 20 D to +20 D. Spherical lenses demonstrated similar behaviour, however with some positive EPC over the power range from 20 D to -10 D for spherical power (SPH) lenses. EPC assessment was between +0.25 D and -0.5 D for most lenses, however, lenses with prescriptions from +10 D to +20 D incurred EPC outside this range. The spherical lenses showed a maximum effective power change of +1.046 ± 0.338 D (Average Eye), and a minimum of -3.278 ± 0.731 D (Steep Eye). Similarly, the toric lenses showed a maximum of +1.501 ± 0.338 D (Average Eye), and a minimum of -3.514 ± 0.731 D (Steep Eye). EPC trends, along with minimum and maximum power, generally increased negatively as nominal lens prescription increased positively. Contact lens base curve selection affected the assessed effective power change for both spherical and toric lenses. The effect from lens total diameter for spherical lenses was less substantial than that for toric lenses. Conclusions: This study considered the impact of soft contact lens design parameters on effective optical power changes (EPC) after eye-fit. Spherical lenses experienced more EPC of clinical significance (>0.25 D) than toric lenses. Both types of lenses, spherical and toric (simple astigmatism), demonstrated similar trends in EPC on fitting from -20 D to +20 D, with lenses in the extremely positive and the extremely negative prescriptions demonstrating the highest EPCs. The lens base curve impacted the extent of EPC observed, with flatter base curves experiencing less power change. Diameter proved to impact toric lenses more than spherical ones, however generally the diameter has less effect on power change than base curve selection