Soft Contact Lenses PDF

OPT 030: Contact Lens 1 Soft Contact Lenses FDA Groupings...

OPT 030: Contact Lens 1 Soft Contact Lenses FDA Groupings Group 1 = low water, non-ionic Parameters Group 2 = high water, non-ionic Base curve Group 3 = low water, ionic Back vertex power Group 4 = high water, ionic Lens diameter, optic zone diameter Center thickness, edge thickness Wear Schedules Lenticulation / Edge design Daily wear Water content - Wear during waking hours only FDA groupings - No overnight wear Wear schedule approval Extended wear Replacement modality - Wear overnight up to 7 days in a row Tints - Any lens approved for EW Inversion markers Continuous wear Manufacturing technique - Wear continuously up to 30 days in a row - Only for silicone hydrogels Base Curve - Also referred to as Sagittal depth (sag) or Vault Replacement Modality - Same as for RGPs Conventional - Tend to be much flatter than K due to large diameter Frequent replacement - Typical range: 8.00 to 9.20 mm Disposable Power Conventional - Back vertex power - Lens is replaced when it needs replacing - Determined by BC, front curve, CT, n - Typically 12 months for a SCL - Effective power not altered by lacrimal lens - Patient receives one pair of lenses at a time - Assume that LL is plano for most SCLs - If a lens is lost or damaged, a replacement must be purchased - More lens care required Lens Diameter / Optic Zone - More deposit related complications - Larger than RGPs in general - Increases stability and comfort Frequent Replacement - Decreases tear exchange - Also called Programmed Replacement - Typically, 13.5 to 15.0 mm - Lens is replaced on a regular schedule, 2 weeks to 3 months - Creates greater sag, resulting in need for flatter BC’s - Lens is worn on a DW or EW basis - Lens is reused after removal and disinfection Center and Edge Thickness - Fewer complications - Thinner than RGPs due to decreased fragility of material (greater flexibility) - Still requires lens care - Min CT = 0.03 mm, increases for higher water content materials - More costly - Affects Dk/L - Confusion with various replacement schedules - Edge profile may be more important to consider Disposable Lenticulation / Edge Design - One time use - Due to large diameter, all SCLs are lenticulated to maintain thin profile - Daily wear: 1 day - Edge designs vary - Extended wear: 1 week - Continuous wear: 1 month Water Content - Greatest convenience - Water is bound within a hydrophilic polymer matrix to provide flexibility and oxygen - No lens care permeability - Most costly - The amount of water determines material characteristics - Range: 25 to 74% Tints - Low 50% (FDA) - Handling / Visibility - Enhancing Low High - Opaque cosmetic Good durability and handling Higher Dk - Prosthetic masking Fewer deposits Used more often for extended wear Lower Dk More deposits Handling / Visibility Tints - Light tints to aid in lens handling Can be made thinner Poorer durability and handling - Can see more easily in case, sink, or counter Used more often for daily wear Must be made thicker - Not meant to change eye color Exception: SiHy - Can change eye color in light blue irises - Virtually all current lenses have handling tints - For silicone hydrogels, less water content means higher silicone content, and therefore higher Dk Color Enhancing Tints - PureVision: 36% water, Dk = 110 Change eye colors of light irised patients - Focus N&D: 24% water, Dk = 140 - Blue, green, hazel eyes - Will only darken brown eyes Silicone Hydrogels Combines with natural eye color to create a new color SOFT CONTACT LENS 1  OPT 030: Contact Lens 1 Opaque Tints - Flat fit: clear-blur-clear - Tints that change dark irises to other colors: “Brown eyes to blue” - Note: blurring of the mires can also be due to lens surface drying - Mask underlying iris color Retinoscopy reflex - May see some iris through clear pupil - Steep fit: central distortion - Patient may notice some blur around the edges - Flat fit: inferior distortion Conjunctival drag: Movement of vessels with tight or adhered lens Prosthetic Tints - DDX: Loose conjunctiva Mask underlying disfiguration of the eye Bubbles at limbus indicate tight fitting lenses - Corneal scars Vessel blanching and engorgement with tight lenses - Iris defects Lens imprint after removal with tight lenses Can be clear or occluder pupil lenses Can be painted any color Soft Lens Optics Very expensive and long production time Vertexing Ciba: Special Eyes Program Be sure to vertex any meridian +4.00 or over. CooperVision: Prosthetic lenses If taking the spherical equivalent, vertex each meridian first, then calculate the SE: Adventure in Colors Crystal Reflections Ex: SR -5.50 -1.25 x 180 Inversion Markers SRv -5.25 -1.00 x 180, SE = -5.75 Tell a patient if a lens is inside-out The CLP is usually equal to the vertexed refraction or its spherical equivalent. Tells you if the lens is inside-out on the eye Many manufacturers now put letters/numbers on their lenses Lacrimal Lens Assume that for most soft spherical lenses, the LL = plano, regardless of BC or K-reading Soft Lens Fit Characteristics Ex: Cibasoft 8.60 (39.25 D) / 13.8 / -3.00 K-reading 43.50 sphere Lens position and Corneal coverage LL = plano, NOT -4.25! Vertical and lateral assessments Therefore, if SR = -3.00, the above lens is optimal. - Superior -Central -Inferior - Temporal -Central -Nasal Predicting the OR Full corneal coverage is necessary at all times. Since there is no LL, the OR simply the difference between the refraction and the CLP Assessment is made in primary gaze. POR = SRv –CLP - Eye lids in normal position. Ex: SR = -3.75 -1.00 x 180 - Head in normal position. CLRx = 8.6 / 13.8 / -4.00 Factors affecting lens position POR = (-3.75 -0.75 x 180) –4.00 sph - Sagittal depth = +0.25 -0.75 x 180 - Lens diameter - Eye lid interaction Effect of lens fit on optics Steep lens: poor draping, poor VA between blinks Blink Movement in Primary Gaze Flat lens: poor draping and excessive movement, poor VA right after the blink - An assessment of lens movement induced by the normal blink. It is assessed in primary Dry lens: poor hydration and optical quality, poor VA between blinks as lens dries gaze. - Acceptable blink movement ranges from 0.5 to 2.0 mm of vertical excursion. At no time Aspheric Designs should the inferior cornea be exposed during this movement. A few aspheric SCL designs are now available - Aspheric front surfaces reduce spherical aberration and may increase depth of focus Upgaze movement - Improves optical quality, esp for higher powers - The amount of lag movement on upgaze is assessed. Acceptable amounts of upgaze lag - May improve vision for low astigmats and early presbyopes movement range from 0.5 to 2.0 mm. Ciba Choice AB - The amount of lens movement on blink while the patient is looking up should also be made. Coopervision Frequency Aspheric This is usually greater than blink movement in primary gaze. Movement up to 2.0 mm is acceptable as long as the inferior cornea is not exposed. Lateral lag movement - An assessment of lag movement induced by lateral eye excursions. - Lateral lens movement on lateral eye excursions should be 0.5 to 2.0 mm. - A tightly fitting lens will not have lateral lag movement. - A loosely fitting lens will lag excessively, exposing some of the cornea. - This is a supplemental observation. Push-up movement - The lower lid can be used to manually push upward on the lens to determine if the lens is freely floating on the cornea. - This test is supplemental Factors affecting lens movement Base curve Lens diameter Eye lid interaction Lens thickness Lens modulus Lens hydration/dehydration - Dehydration effects on soft lens parameters: ▪ Steeper base curve ▪ Smaller diameter ▪ Slight decrease in lens power Auxiliary Observations Over-keratometry mires - Steep fit: blur-clear-blur SOFT CONTACT LENS 2