EyeWitness

Winter 2017

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• When the contact lens is not moving sufficiently, the optic zone diameter can be decreased to promote greater movement. • For improved comfort, consider in- creasing the overall diameter of a GP. A larger lens has fewer direct interactions between the eyelids and the lens edge. GP contact lenses can provide ex- cellent vision and are available in a wide variety of designs. Setting realistic ex- pectations and making patients aware of the initial lens adaptation period can influence overall success. Fitting guides and lens calculator are available to help select an initial trial lens. If fitting or vision issues are experienced, do not hesitate to speak to your GP consultant – they know their lens best! ANATOMY OF A SOFT CONTACT LENS Sft contact lenses are made to fit the sagittal depth of the cornea. There are two overall measurements that impact sagittal depth: keratometry reading (for lens base curve) and horizontal vis- ible iris diameter or HVID (for overall lens diameter). Similarly to GP contact lenses, soft contact lenses have an over- all diameter, optic zone diameter, and multiple peripheral curves. The optic zone carries the power of the contact lens and the peripheral curves are nat- urally made flatter as they move out towards the periphery to mimic the shape of the peripheral cornea while aligning with the sclera. For inventory (aka "stock" or "off-the-shelf ") lenses, the overall diameters range from 13.8- CLSA EyeWitness Winter 2017 23 14.5mm, base curves range from 8.2- 9.0mm, and optic zone diameters on average range from 8.0-10.0mm. Recently, many contact lens man- ufacturers have returned to discussing lens edge profiles and ocular surface interaction, especially how this relation- ship affects the fit of the lens. Currently, several edge designs exist in the market: knife edge, round edge, semi-round edge, and chisel edge; however, there is no consensus on the best edge design. Luckily, soft contact lenses are for- giving due to their material properties blood vessel growth (neovasculariza- tion), corneal swelling, or problems with corneal cell healing. High plus lenses may cause similar issues central- ly at the patient's visual axis. Sphere power options of inven- tory lenses on average range from +6.00 to -8.00DS and cylinder options typically come in -0.75 to -2.25DC in 0.50D steps. Recently, more and more toric contact lenses come in axis ranges of 10˚ to 180º in 10˚ increments; how- ever, it is always a good idea to double check a parameter guide. While inventory lenses work for a lot of patients, there is a population of pa- tients who may require a more custom- ized lens choice: post-LASIK/RK, ker- atoconic, post-penetrating keratoplasty, and post-ocular trauma. In these cases, custom soft lenses are an excellent option. Custom soft lenses allow for variability in diameter, base curves, and powers (in- cluding astigmatic changes in one degree increments and multifocal capabilities). Contact a custom soft contact lens man- ufacturer with a patient's glasses prescrip- tion, keratometry values, and HVID to know more about what they can offer! COMMON QUESTIONS TO ASK Dry eye, corneal trauma, Herpes Simplex Virus keratitis, other ocular infections Diabetes, Sjögren's syndrome, rheumatoid arthritis Hay fever, seasonal or chronic allergies, medications, chemicals Average wear time, wear time today, vision and comfort rating out of 10, replacement schedule, contact lens case replacement, and contact lens care system used Ocular history: Medical history: Allergies Contact lens history It is important to communicate to patients that contact lenses are not a fashion accessory. They are a medical device which requires monitoring and regular follow-up to ensure that health and vision are maintained and are able to be fit on a majority of patients. There are three types of soft contact lens materials on the market: hydrogel, silicone hydrogel, and a hy- brid of the two called "water gradient silicone hydrogel." A vast majority of the contact lenses on the market are currently silicone hydrogel. The bene- fit of a silicone hydrogel over tradition- al hydrogels is the increase in oxygen permeability (Dk) and oxygen trans- missibility (Dk/t) to the cornea. With higher lens powers, this makes a differ- ence in the overall health of the cor- nea. With high minus lenses, the thick edges of the contact lens may suffocate the corneal cells and limbal stem cells at the corneal periphery, causing new Choosing a Soft Contact Lens There are two main considerations when choosing a soft contact lens to fit – parameter availability and ocular health. A patient with higher amounts of astigmatism or who needs a mul- tifocal toric will be limited to only a handful of inventory lens options. It is, again, always a good idea to keep in mind that custom soft contact lenses are a possibility. Typical replacement schedules for inventory lenses include daily disposables, two-week disposables, and monthly disposables. Choose a contact lens modality based on the patient's needs. To know what the patient's needs are, always have a con-

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