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

Levelling the Eye to Provide a More Accurate Ocular Shape for Comparative Analysis and Contact Lens Fitting

Topography maps play an important, increasing role in contact lenses design and fitting, particularly for large diameter lenses. Whenever an eye is measured using a topography machine, the fixation point is required to be in the machine head, around 2 to 10 cm in front of the eye. This practice induces rotation of the eye which can result in tilted maps, affecting measurement accuracy. Additionally, this measurement is necessarily around the visual axis whereas the geometric axis is more important for ocular shape measurements. It is not a simple matter to compensate for this induced error, as the eye itself has few, if any, identifiable characteristics to facilitate correct orientation. This study developed methodologies to level the eye topography data around its geometrical axis, thus providing more accurate information regarding ocular shape

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

Improvements in or Relating to Contact Lenses

Disclosed is a contact lens the back surface of which has a shape such that the sagittal depth of a vertical meridian in the non-central surface differs from, and preferably is greater than, the sagittal depth of a vertical meridian in the non-central surface by an amount in the range 50μm to 500μm

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

Determination and mapping of corneal stiffness in keratoconic corneas

Purpose This research addresses an unmet clinical need by making it possible to estimate corneal biomechanical stiffness in vivo and how it varies across corneal surface in keratoconic corneas. Conclusion With this technology, clinicians can achieve a number of important goals: (1) estimate magnitude and distribution of stiffness in a KC cornea and use them to optimise the CXL treatment, (2) accurately quantify the disease progression and decide when intervention was needed, and (3) estimate magnitude and distribution of stiffness post-CXL to determine the effectiveness of the treatment. Materials and Methods While it is well known that keratoconus (KC) leads to tissue softening, the degree by which this softening takes place cannot still be determined clinically and hence cannot be used in the optimisation of collagen cross-linking (CXL) treatment. An added difficulty is the expectation that KC would not have a uniform effect on the cornea, but rather the softening would be concentrated at and around the KC cone. This research addresses these two challenges and presents technology that can estimate the magnitude and distribution of mechanical stiffness across corneal tissue. The technology is based on representative numerical modelling, leading to an algorithm for estimating the full stress-strain relationship for corneal tissue, which can then provide values of the tangent modulus at any stress or intraocular pressure level. The next step relies on the proven link between the distribution of collagen fibrils in corneal tissue and the distribution of mechanical stiffness. It uses microstructure maps of both healthy and KC corneas to translate the stiffness value obtained from the algorithm into a map of stiffness across corneal surface. Results In preliminary results, a clear difference is observed between normal and keratoconic eyes. A relative reduction of up to 70% of the total collagen fibrils is present inside the cone area. The cone position and dimensions can be evaluated with this method

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