The present study will evaluate the effect of two silicone hydrogel contact lenses with varying sagittal heights on the cornea of 25 normal subjects. On two non-consecutive days, subjects will be asked to wear the same Lotrafilcon A soft contact lenses with two different base curves (8.4mm and 8.6mm) for a period of eight hours, bilaterally in randomized order. Corneal topography and corneal epithelial thickness will be compared from baseline measures at 4 hours and 8 hours post-insertion. Based on published research, we suspect that the soft contact lens with the higher sagittal height (8.4mm) will compress the cornea more than the lens with the lower sagittal height (8.6mm), and therefore we will see a greater change in the corneal topography and an increased epithelial thickness after 8 hours of wear. The instruments used in this study are the slit lamp, for cornea inspection and lens centration and movement, the Medmont corneal topographer, the Eaglet Eye ESP eye profiler, and the Optovue anterior segment OCT, the last three ones being used to map the ocular surface. The present study aims to help determine whether the sagittal height of a contact lens should be taken under greater consideration when initially fitting contact lenses on a patient. Fitting a patient with a contact lens that will minimize mechanical impact may contribute to reduce discomfort associated with lens wear and could help limit contact lens wearers dropout.
1. INTRODUCTION
Over the past two decades, vast improvements have been made regarding contact lens design, material, wettability, and oxygen transmissibility (Dk/t). In late 1990s, silicone hydrogel contact lenses were introduced to the market, which helped to lower the incidence of hypoxia related to contact lens wear. However, despite all these advancements in recent years, the dropout rate still seems to be just as high as it was prior to these new technologies, and studies have yet to offer a valid alternative to improve patient’s experience (1).
One of the obstacles to the growth of the Soft Contact Lenses (SCL) industry is the high discontinuation (or “dropout”) rate of wearers. Studies have shown that the number of contact lens wearers who dropout are between 12% and 51%, with contact lens discomfort (CLD) being the leading cause (2). In fact, CLD accounts for nearly 27% of all contact lens dropouts, the other two primary reasons being dryness and red eyes, accounting for 16% and 11% of all dropouts, respectively (3, 4). Poor vision is also a major cause of contact lens dropout, especially in patients over 40 years old (3.8% to 17.5%) (5)(6). It is also recognized that CLD is multifactorial in nature, and must be addressed accordingly. One of these contributing elements relates to the way lenses are fitted nowadays (7-9).
An inflammatory response can be triggered during contact lens wear due to mechanical factors such as trauma (tear or break in the lens), poor lens fitting, especially a tight-fitting lens limiting the oxygen flow to the cornea, and immunologic factors linked to poor cleaning or replacement habits (10). This inflammation can cause symptoms of dryness, and can lead to contact lens dropout (11, 12).
In the past, soft contact lens fitting was based on the evaluation of the central corneal curvature. Lens selection was made in order to get a flatter lens base curve, around 4D in average, compared to the cornea, namely to improve a regular tear exchange during lens wear. Before the era of frequent replacement lens modality, contact lenses were manufactured with several bases curves and a few diameters, which helped practitioners to select the most appropriate combination to achieve an optimal fitting. It is no longer the case, except for niche customized lenses. To this day, in the disposable lens world, lenses are manufactured with a single base curve and a fixed diameter, to the exception of a few products offered with a steeper and a flatter version. Consequently, there is no real possibility to really adapt lenses on a given ocular surface.
A few authors suggested, in the last years, that base curve basis should be revisited, on the fact that most lenses are not produced with a single back curvature, and that the value given to a particular lens does not represent the reality (13, 14). For them, it is an accepted fact that in order to properly fit a patient in contact lenses, the sagittal height of the contact lens must be considered. Recent studies have demonstrated that sagittal height, as opposed to base curve radius, is a far more precise measure to consider when adapting a patient in contact lenses (15). However, corneal sagittal height is hardly ever measured clinically, and contact lenses don’t indicate the sagittal height on their boxes, opting instead to use the central radius of curvature as their index of choice. Finally, because it is a new argument put in the field, there is no real consensus on how an optimal lens should match or vault the corneal height.
Despite this innovative concept, contact lens fitting is still largely based on keratometry readings and horizontal visible iris diameter (HVID)(16). Once the measures have been taken and an initial trial lens is chosen, a diagnostic evaluation is performed: applying the trial lens on the patient’s eye, evaluating the fit through a slit lamp, and modifying the lens if need be. The keratometry readings, although providing some insight into the nature of the central cornea (and therefore aiding in the initial lens selection), do not shed light on the required sagittal.
The sagittal height is defined as the distance between the apex of a circular arc and the center of its base, or chord. Two main factors influence sagittal height: radius of curvature and diameter. Corneal shape factor is also to consider, but at a lesser extent. Considering a constant diameter, as the radius of curvature increases, so does the sagittal depth. This direct correlation is also found between the diameter and the sagittal. As such, if we compare two corneas (or contact lenses) with the same central radius of curvature, we will find that the one with the greater diameter will have a superior sagittal height (17).
Figure 1. Influence of corneal diameter on sagittal height (18)
As for the ocular sagittal height, there are mathematical models that allow us to theoretically determine this value. According to this formula, the total ocular sagittal is equal to S1 + S2 – S3, in which S1 and S2 are the corneal and scleral sagittal height, respectively (Figure 2). We may calculate the theoretical value of the ocular sagittal height using the following equation:
in which r = corneal or scleral radius of curvature, p = corneal or scleral shape factor (where p = 1-e2), and d = corneal or scleral diameter, accordingly.
Figure 2. Model for the calculation of total ocular sagittal height (19)
Sagittal height vs base curve
There are significant differences in sagittal height between commercially available frequent replacement silicone hydrogel lenses, according to studies at the University of Maastricht. The same labeled base curve value of different lens brands also shows important differences in actual sagittal height, which could possibly have a clinically significant influence on lens behavior in the eye (7, 15) (17).
Sagittal height and lens fitting
A flat soft lens fit, with an insufficient sagittal height, may be uncomfortable due to excessive movement, may be decentered on the flatter conjunctiva, and may exhibit edge misalignment, called “flutting”. A loose fitting lens will also result in fluctuating vision, most noticeable immediately post-blink. In this case, two options present themselves in order to better fit the cornea of the patient: either steepen the base curve radius, or increase the overall lens diameter. Both of these options will result in a greater sagittal height, thus tightening the fit on the cornea (20).
A steep soft lens fit, meaning a lens with a sagittal height too large for the ocular surface, on the other hand, will display little to no movement on the blinking eye. This can also be evaluated by the push-up test: the lens will be difficult to dislodge. Other indications of a tight fitting lens include conjunctival indentation, conjunctival redness, low grade inflammation, and limbal vessel constriction. A fluctuation in the patient’s vision can also be noted, with the vision improving post-blink. Although a steep fitting SCL may initially present a greater degree of comfort for the patient, the lack of tear film exchange beneath the lens surface will result in an increased discomfort over the long term. In this case, we will want to either flatten the base curve radius or decrease the overall lens diameter in order to improve the fit of the lens (20) .
Lens fitting and ocular health
Lens fitting can influence the patient’s comfort, the ocular physiology, by penalizing or enhancing tear exchange and the flush of toxins and debris which accumulates under its surface during lens wear, or the entrapment of chemicals and biocides, released in the first hours after lens soaking overnight in chemical solutions (21).
Lens fitting can also induce mechanical stress to the cornea, with unknown long-term effects on the immune system answer (22). This stress can become visible through corneal topography. Corneal topography is a useful technique to evaluate the quality of soft lens fit on the ocular surface (5). there seems to be a consistent correlation between corneal topography and soft contact lens fit regarding centration and corneal sagittal height(17).
Changes occur in silicone hydrogel soft lens wear regardless of lens power, replacement frequency, lens manufacturer and design (20). Corneal deformations are often a result of suboptimal (inadequate) soft lens fitting.
More specifically, it has been shown that soft contact lens wear affects the normal physiological structure of the cornea, especially the corneal thickness (23). In fact, it was observed that the corneal epithelial thickness decreases with long-term soft contact lens wear (24). The corneal epithelium serves as first protective barrier of the cornea and presents significant regeneration and repair capacities. It has a role in preventing microorganism invasion and it is an essential media for optical transduction and refraction.
Again here, there are paucity of data related to the clinical outcome on topography from contact lens wear. This is why this study will try to evaluate the corneal modification, at epithelial level, occurring after the wear of lenses fitted with 2 different sagittal heights.
2. OBJECTIVES
● MAIN
To evaluate the changes made to corneal topography, using the Medmont corneal topographer, following four hours and eight hours of wear of silicone hydrogel contact lenses with 2 different base curve radius.
● SECONDARY
To evaluate changes in corneal epithelial layer thickness after four hours and eight hours of wear of silicone hydrogel contact lenses with 2 different base curve radius.
To determine whether the steeper or looser fitting soft contact lens generates more of an impact on corneal topography and epithelial thickness.
3. HYPOTHESIS
1. According to preliminary findings, we expect the soft contact lens with the lower base curve radius (higher sagittal height) to inflict, on a normal subject, a greater compression of the cornea than the contact lens with the higher base curve radius (lower sagittal height). Therefore, we expect the contact lens with the 8.4mm base curve radius to induce a greater change in the simK value compared to baseline (time=0) on the Medmont topographer. We expect a greater change from baseline to be visible after 8 hours compared to 4 hours of wear.
2. The lens with the most compression of the cornea of a normal subject will result in a thicker epithelium layer following contact lens wear due to inflammation of the epithelial cells. We expect a greater change from baseline after 8 hours compared to 4 hours of wear.
4. MATERIALS AND METHODS
This is a non randomized prospective study divided in six sessions (3 per day) over 2 days (non consecutive). Excluding the session for reading the consent form, asking questions etc.
5.1 Inclusion criteria
1. Must be between 18 and 40 years of age.
2. Must be familiar with soft lens handling.
3. Must have a refractive error between -0.25D and -6.00D (to alleviate biased based on thicker edges from lenses > -6D)
4. Must present normal ocular health.
5. Must not have worn contact lenses over the past 48 hours.
6. Must be available for two non-consecutive days, for a total of 6 sessions of 30 minutes each (morning, noon, afternoon).
7. Must be legally able to consent to participate in this study.
5.2 Exclusion criteria
1. Must not have an abnormal ocular surface (including corneal ectasia)
2. Must not have worn soft contact lenses in extended wear (part-time or full-time) over the last 6 months.
3. Must not have an active ocular infection at the moment of the clinical trials.
4. Must not be currently using topical medication.
5. Must not have a known hypersensitivity or allergy to the products used during this trial.
6. Must not be a gas permeable lens wearer.
5.3 Instruments
Medmont E300 Corneal Topographer
The Medmont topographer (Medmont International Pty Ltd, Nunawading, Australia) is a computerized videokeratometer. It is able to map the surface of the human cornea using the projection of Placido rings on the tear film and offers different representations (displays) of the results captured. It measures up to a 11-12 mm corneal diameter depending on anatomy of the eyelids and the inter-palpebral opening. It is frequently used in current optometric practice for its excellent accuracy and reproducibility (25, 26).
In this study, several maps will be used from the topographer. Details of the corneal capture will allow us to estimate the corneal height at a chord of 11 mm. Then an extrapolation will be made to establish the ocular sagittal height at 14,0 mm (200 um added for each 0.5 mm exceeding 11 mm). An axial map will be used to determine the normal corneal profile, at baseline. The tangential (true power) map, using a modified scale of 0.1D, will be used to assess corneal changes associated with the lens wear at t+4h and t+8h. Finally, a differential map will be used to compare results after 4h and 8h of lens wear and baseline. The scale of each type of map will be customized to be sensitive enough to display small differences in curvatures and height.
Eaglet Eye ESP, Eye Surface Profiler
The Eye Surface Profiler (Eaglet Eye, Houten, the Netherlands) uses a double projection infrared system to project moiré fringe patterns onto the surface of the anterior segment of the eye to measure corneal and scleral topography up to 20mm diameter area, 360 degrees around. It is primarily used to measure overall sagittal height and for the mapping of scleral shape to aid in scleral lens fitting. The instrument is able to acquire 350 000 points of data for direct measurement of anterior surface height (of the cornea, the sclera, as well as the junction between the two) using a dual light source system, instead of only one like the traditional topographical imaging systems. Fluorescein must be instilled in the eye of the patient prior to image capturing. The analysis is based on Fourier advanced transformation profilometry (27, 28). The double projection moiré profilometry allows for a 3D “height maps” of the cornea and sclera. The instruments projects two orthoscopic grids to generate a diffused image on the fluorescein, and images of high resolution are instantly captured by a built-in CMOS camera.
http://www.sumipro.nl/eye-surface-profiler/
From the maps generated after ocular surface assessment, several data will be extracted and used in this study, namely visible corneal diameter and sagittal height of the entire ocular surface at a 14mm chord.
Optovue iVue anterior segment OCT
The anterior segment optical coherence tomography (ASOCT) is a simple and non-invasive technique allowing the quantitative analysis of ocular parameters using high resolution cross-sectional imaging. The instrument used in this study is the Optovue iVue SD-OCT (Clarion Medical Technologies, Cambridge, Canada). It is a high definition non-contact spectral domain OCT used for retina, glaucoma and anterior segment imaging. It uses a scan beam wavelength of 840±10nm and offers a resolution of 5um for a highly detailed analysis. It provides a direct measure of the ocular surface in contrast with the Medmont topographer which provides an indirect measure. It can provide accurate measures of pachymetry and corneal epithelial thickness mapping (ETM) over a 6mm diameter as well as angle measurement, retinal mapping and macula, optic disc, retinal nerve fiber layer (RNFL) and ganglion cell assessment (30){Optovue, N/A, Avanti RTVue XR}.