Reader and Industry Forum
Rethinking Central Ks, Base Curve Radius, and Edge Fitting
BY AGUSTIN GONZALEZ, OD, FAAO
Preventing and predicting corneal and eye health issues is a cornerstone of contact lens fitting and the reason why in vivo eye evaluation of a contact lens is important. Traditional contact lens fitting is based on the base curve radius-to-central cornea relationship; however, in clinical practice, the peripheral contact lens-to-cornea fitting relationship is much more essential, as it dictates comfort level, tear exchange, and physiological response of the ocular surface (Muntz et al, 2015). A poorly fitted contact lens can be uncomfortable for the wearer and can alter ocular physiology, resulting in a range of symptoms that can include irritation, stinging sensation, and eye pain. Furthermore, extended use of poorly fitted contact lenses is documented to cause both physiological and physical changes that can reduce visual acuity and alter normal corneal physiology (Holden et al, 1985).
Contact Lens Fitting and Evaluation Methods
Traditional Methods Common methods used for contact lens fitting include keratometry (K) readings and direct observation of in vivo contact lens-to-cornea interaction. K readings help to determine the central corneal curvature, while direct observation allows evaluation of contours, pupil, and iris size (Holden et al, 1985). However, traditional K readings are interpreted to primarily match with a contact lens base curve radius (BCR) and are primarily focused on aiding and predicting lens centration, movement, and lens-to-cornea interaction. Traditional contact lens philosophy states that the BCR is usually selected 0.6mm to 0.8mm flatter than the average corneal curvature for soft lenses, which is often determined using a keratometer (Künzler, 1996). This method aims to be predictive of central corneal contact lens interaction, but it fails to be predictive of limbal and post-peripheral cornea and edge interaction.
These traditional measurements provide little information of the entire coverage area and may not be able to predict the interaction between the larger ocular surface and a contact lens, primarily the area around the contact lens edge, which can play a vital role in both contact lens comfort and, more importantly, ocular health.
Therefore, traditional predictive tests and extrapolation to contact lens BCR may prove insufficient to understand the underlying causes of discomfort and complications arising from contact lens wear, which often are related to contact lens edge and ocular surface interaction.
Recent Advances More recently, corneal topography has given clinicians the ability to evaluate corneal contours beyond the central cornea and, therefore, to predict more midperipheral and peripheral cornea-to-contact lens interaction. Advances in optical coherence tomography (OCT) may be more beneficial compared to traditional keratometry and the BCR-to-cornea fitting relationship to understand subtle but important characteristics of contact lens edge and conjunctiva interaction (Liu and Pflugfelder, 2000).
OCT is a non-invasive imaging method used in biomedical optics and medicine. OCT can perform cross-sectional, real-time imaging of the micro-structure of biological tissues by measuring the echoes and magnitude of backscattered light. In eyecare practice, OCT has proven to be a beneficial imaging modality as it can generate high-resolution cross-sectional images of the layers of the retina. Recent advances in OCT technology have improved its resolution on the micrometer scale, which has enabled imaging of anterior segment ocular structures; this aids in evaluating contact lens edge fitting over the cornea (Maeda, 2011).
Recently, OCT technology has been used by contact lens fitters to take ultra-fine measurements of a contact lens fit, which can help in determining the precise designs for contact lenses over the cornea and making adjustments, if needed (Shen et al, 2011). OCT also provides a magnified view with accurate measurements of the lens-to-ocular-surface fitting relationship for optimized design adjustments. OCT technology can be an accurate imaging method to determine optimal contact lens fitting and is often associated with scleral lens fitting (Maeda, 2011; Shen et al, 2011).
Effects of Lens Wear on the Ocular Surface
Modulus Soft contact lens polymers and designs have evolved from mostly hydrogel lenses in the 1990s to more silicone hydrogel polymers in use today (French, 2007; Morgan et al, 2016). Modulus is a term descriptive of the rigidity of the material. In silicone hydrogel materials, it is a property characterized by the silicone or combination of silicones and their molecular binding and is dependent on the arrangement of polymers.
A thinner lens made with a low-modulus material will drape the cornea, often resulting in a mechanical interaction with even pressure distribution on the ocular surface (French, 2007; Künzler, 1996). Conversely, a thick lens may be relatively stiff even when made from a low-modulus material and may be more dependent on the lens parameters (BCR, overall diameter, sagittal depth) for on-eye comfort, with less corneal “draping” and more uneven pressure points on the ocular surface. Lower-modulus architecture allows the lens-to-cornea relationship to be less dependent on these contact lens parameters (French, 2007).
Tear Exchange Contact lenses interfere with the relationship between the cornea and the tear film. Tear exchange is important during contact lens wear to maintain the physiological and functional integrity of the cornea. The tear film functions to protect, nourish, and lubricate the cornea to ensure optimal health. When a contact lens is placed on the ocular surface, it divides the tear film into two layers: the pre-lens tear film and the post-lens tear film. The circulation of fluid between these two layers is termed “fluid exchange” and is functionally critical to corneal integrity.
An important function of tear exchange is to reduce post-lens debris, which consists of metabolic byproducts that can accumulate between the contact lens and cornea. Accumulation of such debris can trigger adverse reactions by reversing the epithelial barrier function. Therefore, proper tear exchange beneath the contact lens is essential to facilitate corneal health and prevent adverse effects (Snyder, 2007).
Conjunctival Blood Vessels Contact lenses can cause distension of the conjunctival blood vessels, resulting in hyperemia or conjunctival redness. Although mostly asymptomatic, eye redness is an adverse reaction of contact lens use. It can have various etiologies, such as direct mechanical effect of the contact lens on the conjunctiva, interference of the normal physiological metabolic processes of the cornea and conjunctiva by the contact lens, or a local chemical or toxic effect of contact lens use (Snyder, 2007).
Soft Contact Lens Edge Fitting Using OCT
The relationship between contact lens fitting and ocular responses is important. If there is a mismatch between the contact lens and the ocular surface shape, localized pressure changes can occur, leading to the formation of conjunctival buildup and tear film gaps (Shen et al, 2011). This device/eye interaction primarily depends on the material modulus and the lens thickness/diameter and is less dependent on BCR (French, 2007).
Contact lenses manufactured in materials that have a higher modulus are less likely to match with the ocular surface shape and can cause a tight fit; this can result in compression of the contact lens edge, leading to conjunctival staining and indentation of the conjunctival tissue. Furthermore, such contact lens fitting can also result in conjunctival folds, which are a predictor of eye dryness. It is therefore important to understand the edge fitting properties of a contact lens and its interaction with the ocular surface shape (Szczotka-Flynn et al, 2011)
A study conducted by Shen et al (2011) characterized edge fitting of soft contact lenses by assessing conjunctival buildup and post-lens tear film gap. To assess these characteristics, ultra-high-resolution OCT (UHR-OCT) and ultra-long-scan-depth OCT (UL-OCT) were used. The study consisted of 20 participants (11 men and nine women) who had a mean age of 32.3 years. During two separate visits, four different types of soft contact lenses were randomly fitted to both eyes of each subject. After 30 minutes, using UHR-OCT, the horizontal meridians of the corneal center, midperiphery, and limbus of each contact lens were imaged. UL-OCT was used to image each contact lens in vitro and the ocular surface of a physical model eye. The findings of the study reported the following:
• Compared to round-edged lenses, angle-edged lenses resulted in significantly less conjunctival buildup.
• Limbal post-tear film gap was observed in 42% of the eyes, with the highest being in the round-edged lenses (68%).
• Corneal midperipheral tear film gaps were observed in 47% of the eyes, with the highest percentage observed in round-edged lenses (75%).
• The in-vitro simulation clearly showed the mismatch points between the lens and ocular surface.
The study concluded that soft contact lens edge fitting is characterized by conjunctival buildup and tear film gaps, which can be observed in different levels and frequencies in different types of contact lenses (Figure 1). The study demonstrated that for a given contact lens design and material, the primary factor that affects contact lens fitting is ocular surface shape. The study concluded that OCT technology can be an effective technique to evaluate lens fit to achieve the best possible match between the contact lens and ocular surface.
Figure 1. OCT images of four different contact lens brands showing edge fitting characteristics with different edge profiles.
Rethinking Central Ks and Base Curve
Contact lens material science and design, imaging technology, and our understanding of contact lens corneal physiology has expanded in the last decade, yet traditional ways of fitting contact lenses based on base curve and the central cornea have remained unchanged. Maybe it is time to stimulate a discussion on redefining the way in which we use traditional contact lens information. We might just have to reclassify BCR and central corneal curvature as outdated empirical predictors of contact lens fitting success. Should we be promoting a conversation on the use of OCT imaging and modulus in lens edge design as a predictive factor for success in soft contact lens-to-cornea interaction?
With corneal measuring and imaging technology becoming more pervasive in clinical practice, perhaps it is time to rethink how we perceive the traditional BCR and central corneal interaction and to start having a conversation about how modulus as a lens polymer characteristic interacts with the entire cornea. In addition, we need to consider how the lens edge/conjunctival interaction is perhaps more relevant to corneal health than is the more traditional central cornea and BCR conversation.
Incorporating technology, and newfound ways of using it, will provide us with a much clearer “north” to our clinical compass as we seek to predict and manage contact lens complications. We have many tools to evaluate soft contact lens fitting in new ways; perhaps it is time to use this technology to redefine the way we think and talk about soft contact lens fitting parameters.
As clinicians, we must be sensitive to and acknowledge the changes in technology and rethink the way we use traditional physical descriptive contact lens parameters, such as BCR and diameter, while encouraging product manufacturers to develop new descriptive parameters, such as modulus and edge design, that may be more representative of the new technology and useful in predicting cornea-to-contact lens interaction. CLS
To obtain references for this article, please visit http://www.clspectrum.com/references and click on document #249.
Dr. Gonzalez is a graduate from the Inter American University School of Optometry in Puerto Rico (IAUPR) and has been in practice in Texas since 1995. He is a recognized industry expert in adoption and usage of ophthalmic medications by optometrists and is a board certified Clinical Medical Optometrist who has lectured extensively, including at AOA’s Optometry’s Meeting and the American Academy of Optometry.