Flexure of thin rigid contact lenses.

The flexure of spherical rigid contact lenses was measured on the eyes of 10 young subjects using a videokeratoscope. Five subjects had little or no with-the-rule astigmatism (<0.75 D) andfive had moderate levels of with-the-rule astigmatism (1.00-2.00 D). Two lens materials (polymethylmethacrylate [PMMA] and Boston XO) in three centre thicknesses (0.05, 0.10 and 0.15 mm) were used in the study. No significant difference in the amount of flexure was found between the two materials tested. The degree of regular astigmatism on the lens front surface was found to increase as the centre thickness of the contact lens decreased. For the astigmatic group, the lenses with centre thicknesses of 0.05 mm had levels of front surface astigmatism similar to those of the underlying cornea. On spherical corneas the level of regular astigmatism can exceed that of the cornea for thinner lenses. When sphero-cylinder variations are accounted for, residual higher order aberration (root mean square) levels were found to approach those of the cornea when the lens thickness was reduced to 0.05 mm.

Hyperopia is predominantly axial in nature.

PURPOSE: Myopia has been found to be predominantly axial in nature, i.e. myopic eyes have longer than normal axial lengths, with corneal radius variations having only a small influence on the magnitude of the refractive error. In this study we assess whether a similar relationship exists for hyperopia. METHODS: Biometric data were collected on 57 subjects with either emmetropic or hyperopic refractive errors ranging in magnitude from -0.37 D to +17.25 D. Our main analysis concentrated on subjects with less than +10 D of hyperopia (group 1, n = 53), as subjects with +10 D of hyperopia or more (group 2, n = 4) exhibited marked differences in their biometric characteristics. RESULTS: Analysis of group 1 data revealed a significant relationship (r2 = 0.611, p = 0.0001) between the degree of hyperopia and the measured axial lengths. A weak but statistically significant relationship (r2 = 0.128, p = 0.009) was also found between mean corneal radius measures and mean spherical refractive errors, with the mean corneal radius flattening with increasing hyperopia. In group 2, three of the four subjects exhibited much steeper corneal characteristics than predicted from the group 1 data. CONCLUSIONS: Our results suggest that hyperopia, like myopia, is predominantly axial in nature, although the corneal radius also plays a role in determining refractive error magnitude. These results have implications for refractive surgery and visual performance in hyperopic eyes.

Corneal topography and myopia. A cross-sectional study.

PURPOSE: Central corneal curvature is known to vary with refractive error, but the relation between corneal topography and ametropia is less clear. The current study was conducted to determine whether a relation exists between corneal asphericity and myopia. Associations between corneal asphericity and each of the components of refraction also were examined. METHODS: Corneal asphericity and apical radius of curvature were determined for 113 eyes (spherical equivalent refractive error +0.25 diopter [D] to -9.88 D) by fitting a conicoid equation to videokeratoscopic data. Computerized videokeratoscopic images were recorded using a Topographic Modeling System. Keratometry also was performed on each eye. Anterior chamber depth, lens thickness, vitreous chamber depth, and axial length were measured with a hand-held biometric ruler. RESULTS: A low but statistically significant positive correlation was found between corneal asphericity (Q) and spherical equivalent refractive error (r = 0.275, P < 0.01). Significant relations also were observed between Q and vitreous chamber depth (r = 0.17, P < 0.1) and between Q and axial length (r = 0.24, P < 0.05). The association between Q and corneal radius of curvature was found not to be significant. Eyes with higher levels of myopia had steeper central corneal curvatures, deeper anterior and vitreous chambers, and greater axial lengths. CONCLUSIONS: A tendency for the cornea to flatten less rapidly in the periphery with increasing myopia was shown. Decreased peripheral corneal flattening also was observed in association with increasing vitreous chamber depth and increasing axial length. These findings have implications for refractive surgery outcomes, schematic eye modeling, contact lens design, and ocular aberration analysis.

Rigid lens dynamics: lid effects.

PURPOSE: Lid architecture, lid tension, blink action, and blink rate may all influence rigid lens centration and stability. The aims of this study were to assess the nature of the relationship between lid geometry and lens position and to examine the influence of lens-lid interactions on the association between lens position and lens center of gravity and mass. METHODS: Eight subjects (four with high riding lenses and four with low riding lenses) participated in the study. Each subject was fit with 12 lenses-six designs in each of two materials. Lens center of gravity was calculated and lens mass was measured in every case. For each lens, the following four lens dynamics variables were assessed over a five blink cycle for both the vertical and horizontal meridians: 1) initial lens position; 2) settled lens position; 3) amount of lens movement; and 4) rate of lens movement. Lower lid position and palpebral aperture height were measured for each subject. RESULTS: Graphical analysis showed that a low upper lid position and small palpebral aperture promoted superior lens decentration, while a high upper lid position and wide palpebral aperature predisposed an individual to inferior lens decentration. Significant lid-lens overlap was a common finding with high riding lenses in both the initial and settled lens positions. The results also suggested that while a rigid lens tended to become more stable as the lens center of gravity shifted further behind the corneal apex, this gravitational effect was reduced for lenses that interacted with the upper lid. CONCLUSIONS: Lid geometry can influence rigid lens centration and stability by modifying the effects of lens design and lens mass. We recommend that consideration be given to the nature of potential lens-lid interactions prior to lens fitting.

The influence of center of gravity and lens mass on rigid lens dynamics.

PURPOSE: Center of gravity and lens mass have both been shown to influence rigid lens centration and stability. The purpose of this study was to investigate the relative contributions of these two factors to rigid lens dynamics. METHODS: Eight subjects (four with high-riding lenses and four with low-riding lenses) participated in the study. Each subject was fit with 12 lenses-six designs in each of two materials. Center of gravity and lens mass were recorded in every case. For each lens, four lens dynamics variables were assessed over a five-blink cycle for both the vertical and the horizontal meridians: 1) initial lens position; 2) settled lens position; 3) amount of lens movement; and 4) rate of lens movement. RESULTS: Multiple regression and correlation analysis showed that center of gravity influences initial and settled lens position, in both the horizontal and the vertical meridians. Mass alone was found not to be a significant predictor of rigid lens dynamics. The effect of center of gravity on settled vertical lens position was only apparent when high rider and low rider subgroups were analyzed separately, with a posterior movement of the center of gravity being associated with a lowering of the settled lens position for high rider subjects, and a raising of the settled lens position for low rider subjects. The results suggest that a rigid lens will become more stable as the center of gravity is shifted further behind the lens vertex, but this effect is reduced as lens mass is increased. CONCLUSIONS: The location of the center of gravity of a rigid lens influences its on-eye centration and stability more so than does lens mass.

The relation between corneal and total astigmatism.

Using computer-assisted videokeratoscopy we measured corneal astigmatism and compared these results over a range of corneal zone diameters with total ocular astigmatism derived by subjective refraction. Videokeratoscopes permit a more detailed analysis of the power distribution within a given corneal surface area, enabling comparison to the total astigmatism for equivalent aperture sizes. Although there were significant individual variations, the group average data supports the traditional view of a linear relation between corneal and total astigmatism. This was true across the range of apertures tested from 2 to 7 mm, with the coordinates of the relation being consistent with that of the modified Javal's rule; namely a slope of 1 and an intercept of approximately 0.50 D against-the-rule residual astigmatism.

Predicting success with orthokeratology lens wear: a retrospective analysis of ocular characteristics.

PURPOSE: Orthokeratology procedures suffer from lack of predictability in the response of individuals. To identify factors contributing to this, we have retrospectively studied a range of ocular parameters in patients with varying outcomes from orthokeratology lens wear. EXPERIMENTAL DESIGN: Three groups were studied: an experimental group (9 subjects wearing Contex OK-3 design orthokeratology contact lenses), and 2 control groups [10 rigid gas permeable (RGP) contact lens wearers and 10 non-contact lens wearers]. Three categories were identified among the orthokeratology group: those responding well, moderately, or poorly to orthokeratology lens wear. Measurements included subjective refraction, intraocular distances, corneal thickness, ocular rigidity, and epithelial fragility. RESULTS: When comparing the three orthokeratology categories, there was no significant difference for central and peripheral epithelial fragility and ocular rigidity. There was also no significant difference for any of the biometric characteristics measured. The prefitting spherical equivalent power was found to be significantly different between categories of responders (p = 0.0228), with the poor responders having the highest initial level of myopia. None of the measured characteristics differed significantly among the orthokeratology group and the two control groups. CONCLUSIONS: In this pilot study, the success of orthokeratology lens wear was not related to ocular biomechanical or biometric attributes, but it was related to prefitting refractive error.

Using corneal oxygen demand to optimize rigid contact lens design.

To evaluate the effects of base curve radius, overall diameter, and axial edge lift on rigid contact lens tear pump efficiency, corneal oxygen uptake rates were measured on six eyes under three conditions: normal open eye, after 5 min of static (without blinking) wear, and after 5 min of dynamic (with blinking once every 5 s) wear. The three parameters were varied one at a time from a standard lens design. Differences in corneal oxygen demands between the static and dynamic rates provided quantitative measures of the tear pump efficiency for each lens design. Tear exchange was found to be most sensitively related to changes in base curve radius, followed by overall diameter changes (66.9% as effective) and axial edge lift changes (64.6% as effective). Design equivalencies (to produce identical tear exchange to a 0.05-mm flattening of base curve toward alignment) were 0.07-mm steepening of the base curve toward alignment, 0.35-mm decrease in overall diameter, or 0.037-mm increase in axial edge lift.

Rigid contact lens design equivalencies of overall diameter and axial edge lift: individual variations.

We compared the effects on rigid contact lens tear pump efficiency of changes in lens overall and optic zone diameter and of changes in lens axial edge lift by measuring corneal oxygen uptake on the right eyes of six human subjects after 5 min of both static and dynamic contact lens wear. Two sets of five lenses were worn by each subject. In set 1, overall diameter varied from 8.2 to 9.4 mm in 0.3-mm steps, with the optic zone diameter being 1.4 mm smaller and a 0.09 mm axial edge lift. In set 2, axial edge lift varied from 0.05 to 0.13 mm in 0.02-mm steps; overall diameter was 8.8 mm, and optic zone diameter was 7.4 mm. For the pooled data, a 0.3-mm decrease in overall diameter was approximately equivalent to a 0.03-mm increase in axial edge lift as a means of enhancing tear pump efficiency. However, considerable intersubject differences were exhibited, with the following increases in axial edge lift being equivalent to the 0.3-mm reduction in overall diameter: 0.01, 0.01, 0.04, 0.06, 0.09, and > 0.20 mm. This illustrates that subject factors (i.e., palpebral aperture size, lid-to-cornea geometry, corneal toricity, peripheral corneal topography, lid tension) influence lens positioning, movement, and tear pump efficiency.

Influence of lens design on the tear pump efficiency associated with plus-power rigid contact lenses: a comparison of single cut, lenticular, and minus-carrier designs.

In order to determine the tear pump efficiencies associated with three plus-power rigid contact lens designs (single cut, lenticular, and minus-carrier), we measured corneal oxygen uptake rates on the right eyes of six human subjects under three conditions: (1) normal open eye, (2) after 5 min of static (without blinking) wear of the contact lens, and (3) after 5 min of dynamic (with blinking once every 5 s) wear of the same lens. The differences among the corneal oxygen uptake rates measured under static and dynamic conditions, relative to those measured for the normal open eye, served as an index of tear pump efficiency. Under both static and dynamic conditions, no significant differences were found among the corneal oxygen uptake rates associated with the three plus-power lens designs. In addition, no significant differences were found in the differences among the static and dynamic condition data, an indication of tear pump efficiency, associated with the three plus-power lens designs. This was true for both the population data and for the data from each subject. Nevertheless, there were significant differences among subjects in the static condition, dynamic condition, and difference data.

A comparison of the TMS-1 and the corneal analysis system for the evaluation of abnormal corneas.

We used two computerized corneal topography instruments (EyeSys Laboratories' Corneal Analysis System and the Topographic Modeling System (TMS-1) from Computed Anatomy, Inc.] to evaluate 18 eyes (ten patients) with abnormal corneas. The diagnoses included keratoconus, corneal scars, and residual postoperative astigmatism following refractive surgery. The patients were randomly selected from a busy corneal practice to include those with significant corneal refractive error whose management, it was believed, would benefit from corneal topography imaging. Images were taken with both systems, and comparisons were made of the ease of operation, the accuracy of corneal readings, and the usefulness of generated data. Three millimeter zone (Corneal Analysis System) and Simulated Keratometry (TMS-1) values were also compared to each other and to standard keratometric readings. We found the two systems to be very similar in their application and results, although certain differences were apparent in terms of operation, patient acceptance, and the ability to obtain useful data from the more distorted corneas. Neither system worked ideally for severely irregular surfaces.

Influence of power changes in single cut rigid contact lenses on tear pump efficiency.

Differences between corneal oxygen uptake rates measured after 5 min of static (without blinking) and 5 min of dynamic (with blinking once every 5 s) non-gas permeable (polymethyl methacrylate) contact lens wear, referenced to the oxygen uptake rates of the normal open eye, were used as indications of tear pump efficiencies associated with each of seven contact lens back vertex powers (-9.00 to +9.00 D, in 3.00 D steps). Measurements were made in vivo on six human corneas showing with-the-rule (0.25 to 0.87 D) toricity. Lens overall diameter was 8.8 mm, optic zone diameter was 7.4 mm, and base curve radius was fitted "on K." Lens center thickness was 0.14 mm for all minus-powered lenses, and 0.19, 0.26, 0.34, and 0.41 mm for the plano, +3.00, +6.00, and +9.00 D lenses, respectively, maintaining an edge thickness of 0.11 mm. Under both static and dynamic wearing conditions, no significant differences were found among the corneal oxygen uptake rates associated with the various contact lens back vertex powers; however, the difference between the static and dynamic condition data, an indication of tear pump efficiency, was greatest for the -3.00 D lens. The difference values associated with the +9.00 and +6.00 D lenses were significantly lower than those associated with the minus power lenses.

Influence of rigid contact lens base curve radius on tear pump efficiency.

The effects of rigid contact lens base curve radius changes on tear pump efficiency were determined by measuring oxygen uptake on six with-the-rule corneas (with toricities between 0.37 and 1.00 D) under three conditions: (1) normal open eye, (2) after 5 min of static (without blinking) wear of polymethyl methacrylate (non-gas permeable) contact lenses, and (3) after 5 min of dynamic (with blinking once every 5 s) wear of the same lenses. The difference between the oxygen uptake rates measured under static and dynamic conditions provides an index of tear pump efficiency. Each subject wore a series of five lenses: fitted on K and 0.05 or 0.10 mm steeper and flatter than the flattest corneal meridian. The overall diameter of all lenses was 8.8 mm, the optic zone was 7.4 mm, the axial edge lift was 0.09 mm, and all other parameters were constant. The contact lens fitted on K was associated with the greatest central corneal oxygen debt under static conditions, whereas the same lens also provided for the best tear exchange with the blink. In addition, better tear exchange was found to be associated with larger palpebral aperture sizes.

Visual performance after aphakic epikeratoplasty.

While good visual acuity and refractive results after epikeratoplasty for aphakia have been reported, particularly for adults and older children, limited detailed information on the nature of the resulting vision is available. We have evaluated the visual performance of seven aphakic subjects corrected by epikeratoplasty by measuring contrast sensitivity, with and without the presence of glare. These results were compared with those for other aphakic corrections, namely spectacles (n = 5 eyes), contact lenses (n = 5 eyes), or intraocular lens implantation (n = 5 eyes). There were statistically significant differences among these four aphakic correction types (p = 0.0330), with a consistent trend for diminished visual performance after epikeratoplasty. Threshold elevations occurred in the presence of glare, but they were not statistically different between the groups (p = 0.1631). Based on these visual assessments, epikeratoplasty does result in statistically significant visual losses. Despite this, it may still offer an acceptable alternative to other managements of the aphakic patient when those others are contraindicated.

Effects of rigid contact lens edge lift changes on tear pump efficiency.

The effects of rigid contact lens axial edge lift changes on corneal oxygenation and tear exchange with the blink were studied for lenses of five axial edge lifts (0.05 to 0.13 mm in 0.02-mm steps). The overall and optic zone diameters were maintained at 8.8 and 7.4 mm, respectively, and all other parameters were held constant. Oxygen uptake rates were measured for the normal open eye after 5 min of static (without blinking) and dynamic (with blinking once every 5 s) contact lens wear. As expected, no significant differences were found in the oxygen uptake rates associated with the static wear of the contact lenses; however, under dynamic conditions, increasing edge lift was associated with a reduction in oxygen demand. The differences between data obtained under static and dynamic conditions increased with increasing axial edge lift, and subjects with small palpebral aperture sizes seemed to benefit more from increasing axial edge lift.

Visual losses after myopic epikeratoplasty.

The vision of seven patients who had undergone epikeratoplasty for myopia was evaluated by measuring their contrast sensitivity functions without and with the presence of a glare source. Compared with findings for a normal control population, these epikeratoplasty patients were found to have statistically significant contrast sensitivity losses. To evaluate the clinical relevance of these losses, the results were also compared with those for two other patient groups with abnormal corneal optics, seven radial keratotomy patients and seven keratoconus patients wearing rigid contact lenses. Contrast thresholds for the epikeratoplasty and keratoconus patients were similar and usually statistically indistinguishable, and their losses were far more profound than those of the radial keratotomy patients. The addition of glare did not alter this pattern of losses. The results document the extent of vision losses in these patients and suggest that increased intraocular light scattering is not a major contributor to the observed contrast sensitivity decrements.

Rigid lens tear pump efficiency: effects of overall diameter/base curve combinations.

The effect of changes in rigid contact lens overall and optic zone diameters on tear pump efficiency were studied using a polarographic technique. Six overall diameters ranged from 7.6 to 10.6 mm in 0.6-mm steps, with the optic zone diameter always being 1.4 mm smaller. Base curve radii were made flatter as lens overall/optic zone diameters increased in order to maintain a constant tear layer thickness. All other parameters were held constant. Oxygen uptake rates were measured for the normal, open eye, as well as after 5 min of static (without blinking) and dynamic (with blinking once every 5 s) wear of each lens design. Under both static and dynamic conditions, oxygen uptake rates increased with increase in lens overall/optic zone diameter. The differences between the static and dynamic condition data served as an index of tear pump efficiency for each lens. These differences decreased with increase in lens overall/optic zone diameter.

Rigid contact lens design: effects of overall diameter changes on tear pump efficiency.

The effects of rigid contact lens overall diameter changes on tear pump efficiency were studied for lenses of 7 overall diameters (8.2 to 10.0 mm in 0.3-mm steps) while maintaining all other parameters (including a 7.4-mm optic zone diameter) constant. Oxygen uptake rates were measured in the normal, open eye and after static (without blinking) and dynamic (with blinking once every 5 s) wear of oxygen-impermeable (polymethyl methacrylate) contact lenses. Differences between uptake rates obtained under static vs. dynamic conditions served as an index of tear pump efficiency for each lens. Tear pump efficiency increased with the use of progressively smaller lens diameters and subjects with large palpebral aperture sizes were also found to have better tear exchange (r = 0.934, p = 0.006 for the 9.1-mm overall diameter reference lens).

Management of keratoconus: comparative visual assessments.

We compared the visual performance of keratoconus patients whose vision had been corrected with one of the following: RGP lenses alone, penetrating keratoplasty, or epikeratoplasty. Despite similar Snellen visual acuities with optimal correction, significant differences in visual performance were found in the three groups. Visual performance was assessed by measuring contrast thresholds for electronically-generated sinusoidal gratings and by measuring contrast thresholds in the presence of a glare source. These measurements were carried out on six bilateral keratoconus patients such that three eyes were corrected with RGP lenses alone, five eyes were corrected by penetrating keratoplasty, and four eyes by epikeratoplasty. There were statistically significant differences in the visual responses for the three correction types (ANOVA, F2,45 = 5.755, P = 0.0059), with the epikeratoplasty procedure providing the poorest results. Visual performance was further affected in the presence of a glare source. Statistically significant group differences were again found (ANOVA, F2,45 = 7.593, P = 0.0014), with the penetrating keratoplasty eyes being the most affected. These differences were spatial frequency dependent (p = 0.0293), the effect being most marked at medium spatial frequencies. These findings provide a ranking of the quality of vision provided by each management strategy.

Responses to oxygen deprivation: variations among human corneas.

The cornea's interface with the atmosphere makes it uniquely accessible among avascular tissue for the study of local oxygen demand and proximal reservoir dynamics. Corneal oxygen-uptake rates obtained under normoxic vs hypoxic conditions are not predictive of each other. Proximal tissue reservoirs appear to play a role in moderating oxygen demands following periods of hypoxic stress: as the ratio of epithelia/stromal thickness rises, so does oxygen demand (r = +0.932, P = 0.01). The thickness of the stroma was most closely related to the oxygen uptake rates associated with the normal, open eye (r = +0.773, P = 0.07), whereas the thickness of the epithelium was more closely related to rates obtained after the cornea had been deprived of oxygen (r = +0.783, P = 0.07).