Traditional methods to evaluate the fit of a lens rely primarily on the pattern of fluorescein under the lens. For a soft contact lens, regular fluorescein cannot be used because of permanent staining of the lens surface.12
Evaluation is limited to subjective comfort, lens centration, lens movement, and surface wettability. However, it takes time to evaluate all these parameters. Importantly, these parameters do not provide enough information about the relationship between the lens and the ocular surface that may enable understanding of contact lens–induced complications and discomfort. The interaction between the ocular surface and the contact lens, especially around the lens edge, may play a significant role in lens comfort and in the health of the ocular surface. Characterization of soft contact lens edge fitting may enable understanding of the relationship between the lens properties and the ocular surface, leading to better lens design and improved ocular health. In the present study, we used UHR-OCT to characterize the fit of the lens edge by assessing the conjunctival buildup and the post-lens tear film gap. Evaluation by UHR-OCT may open a new era for objectively evaluating the lens fit to achieve the best possible match between the lens and the ocular surface.
Based on the in vitro eye model, the tip of the edge first touched the ocular surface, which could then induce changes in the lens to comply with the surface shape. The bending of the lens edge could then create another touch point on the cornea, inducing the second tear film gap that was evident in the in vivo UHR-OCT findings. Pressure in the post-lens tear film can be induced by the deformation of the contact lens to the shape of the ocular surface.13,14
If a lens does not match well with the ocular surface shape, localized pressure variations will occur, thus possibly inducing clinical consequences that depend on the magnitude and distribution of the pressure.13,14
Conjunctival buildup occurred around the lens edge and the tear film gaps occurred at the midperipheral cornea and at the limbus. This implies that the mismatch or localized pressured occurred around these locations when fitting the lenses. Higher levels of conjunctival buildup and greater frequencies of tear film gaps occurred in the lenses with rounded edges, indicting that lens edge shape may affect lens fitting. However, these differences may also depend on other lens factors, such as material modulus, central thickness, base curve, and lens diameter. Contact lenses with a higher modulus are less likely to match the shape of the ocular surface. Pressure variations in the post-lens region, combined with elastic forces of the lens, can result in a tight fit or in edge fluting.15,16
This may also be the cause of the different levels of the conjunctival buildup and distribution of post-lens tear film gaps. Lens design parameters, including thickness, diameter, and power, also affect localized pressure14,17
and thus can contribute to the OCT findings investigated in this study. The relationship between these lens factors and OCT findings should be explored in further studies that take into account ocular surface shape, especially at the peripheral cornea and the corneal-scleral junction. It has been shown that for a given contact lens design and material, the main factor that affects lens pressure, and therefore lens fitting, is ocular surface shape.18
Other factors, such as eyelid shape and tension,19
could also play a role in lens fitting.
Tear exchange may be impacted by the interaction between the lens edge and the shape of the ocular surface. We hypothesize that a higher level of conjunctival buildup around the lens edge may reduce tear exchange beneath the lens. The tear exchange may also have an association with the tear film gaps. Another factor related to the tear exchange is movement of the lens,20,21
which may facilitate the tear exchange. In the present study, we did not measure lens movement or tear exchange. Therefore, future studies will be needed to confirm these hypotheses.
The present study demonstrates the feasibility using OCT to characterize the edge fitting of soft contact lenses and paves the way to study the relationship between lens fitting and ocular responses. In vitro simulation clearly predicted the mismatch points between the lens and the ocular surfaces. Based on comparisons between the in vitro and in vivo studies, it is likely that the shape of the lens edge changes when it is placed on the eye. Touch points between the lens and the ocular surface were identified at the corneal apex, midperiphery of the cornea, and the limbus. Given that the apex is the thinnest point of these minus lenses, the other two touch points may be responsible for some clinical signs or mechanical damage to the ocular surface, including conjunctival folds,22
corneal and conjunctival staining,24,25
and possibly epithelial thinning.26,27
Contact lens–induced conjunctival staining and indentation of the conjunctival tissue are commonly seen in tight-fitting soft contact lens wear because of the compression of the lens edge.16,24
Conjunctival folds, a predictor of the dryness in contact lens wearers, is presumed to be the result of the mechanical influence of the lens edge.22
Some studies have reported contact lens wear–induced corneal and conjunctival epithelial thinning because of the overall or localized pressure on the ocular surface.26,27
The touch points may exist around the entire circumference of the eye because of the overall tight fit. If this predication is true, then buckling at the circumferential touch points may cause adverse responses such as staining and superior epithelial arcuate lesions (SEALs).28
The arc shape of many SEALs may be attributed to the interaction of the lid with a poorly fitting contact lens.
Because this was the first attempt to characterize the edge-fitting properties of soft contact lenses and to predict the mismatches between the lens and the ocular surface, the study had some limitations. The UL-OCT was not available at the time of the in vivo study; therefore, we were unable to use it to image the ocular surface of the subjects wearing the lenses. We also assumed that both eyes of each subject were similar; however, this might have introduced some measurement errors because of the different ocular surface shapes of the right and left eyes. Although the in vitro study using the UL-OCT with the eye model provided useful information for generalized prediction, the application of the UL-OCT in vivo will yield more details on the lens fitting at the edge and over the cornea. Interestingly, the simulation of the four study lenses on the surface of the model eye provided some predictions regarding the touch points and resultant tear film gaps. Further studies will be needed to link the ocular surface and the lens for a full range of the fitting evaluation. The relationships between the edge-fitting and overall fitting characteristics will be the subject of future studies. Our results with this small subject population for two visits and a 30-minute study period demonstrated the feasibility of characterizing edge-fitting properties and their implications for successful contact lens wear. A larger sample size will be needed to test the differences between lenses worn for long periods. Frequent ratings of ocular comfort and frequent measurements might also reveal the relationship between ocular comfort and lens edge interactions. Finally, different lens designs for improving ocular comfort, health, and mobility may also be investigated with these OCT techniques.
In summary, this was the first attempt using UHR-OCT and UL-OCT in characterizing lens edge fitting and the interaction between the lens and the ocular surface for a short period of lens wear. Different types of contact lenses presented different levels of conjunctival buildup as well as different frequencies of tear film gaps on the mid-peripheral cornea and at the limbus. The findings by UHR-OCT were predicted in simulations based on UL-OCT of the lenses and a model eye. The evolving technology of OCT may open new methods for designing and evaluating lenses for goodness of fit to the ocular surface.