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Rigid lens principals and products

Rigid lens principals and products. Week 9. Rigid gas perm lens is a hard material that allows 02 to pass through the material to the cornea. It is smaller than a soft contact lens. It is fit differently and more precisely. It “reshapes” the cornea to eliminate the astigmatism.

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Rigid lens principals and products

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  1. Rigid lens principals and products Week 9

  2. Rigid gas perm lens is a hard material that allows 02 to pass through the material to the cornea. • It is smaller than a soft contact lens. • It is fit differently and more precisely. • It “reshapes” the cornea to eliminate the astigmatism. Soft CL VS gas perm CL

  3. Gas perm lenses are made from SA (silicone acrylate) and FSA (fluorinated silicone acrylate). • These material allow O2 to permeate the lens to the cornea, reducing edema and increasing wear time for the patient. • It must be fit more precisely because it reshapes the cornea. • Hard lenses,PMMA • (polymethylmethcrylate) have poor O2 transmission. • 1st generation “hard” lens. • Long lasting and very durable. Rigid lens principals

  4. Dk Value • Diffusion coefficient is the value of O2 permeability of the lens material. • The higher the Dk value the higher the O2 permeation to the cornea. • A higher Dk value will lower the chance of corneal edema. • Dk can be expressed in the center thickness (L)of the material by the equation DkL. • This will define the O2 transmissibility of the material through the center thickness. Rigid lens principals

  5. Two corneal shapes • Spherical • Aspherical (toric) • Spherical Aspherical • 44.00/44.00@90 46.00/42.00@90 Rigid lens principals

  6. Spherical Cornea • Implies that this corneal has minimal topographic variation. • The K readings will be very similar horizontally and vertically. • There will be minimal corneal astigmatism. • Aspherical Cornea • Implies that the cornea has a variance in corneal curvature and power. • K readings will be different horizontally and vertically. The more the readings vary, the more astigmatism there will be. • This is corneal astigmatism. Rigid lens principals

  7. Aspherical corneas • Aspherical corneas will has corneal astigmatism. • With the rule astigmatism • Against the rule astigmatism • Lenticular astigmatism • Oblique astigmatism Rigid lens principals

  8. Base Curve (BC) • The primary curve of the lens. • It has to be designed to contour the anterior corneal surface and tear film. Rigid lens principals

  9. The BC’s relationship to the cornea is referred to as: • Bearing • Alignment • Clearance • These are adjectives used to describe how the CL fits on the cornea. Rigid lens principals

  10. Bearing • Suggests that the lens is fitting flatter on the cornea than the keratometric readings. • If the lens is bearing centrally, it is flat centrally, pushing the tear film to the periphery. • If the lens is bearing peripherally, there will be no tear film peripherally. Rigid lens principals

  11. Clearance • Suggests that the lens is fitting steeper on the cornea than the keratometric readings. • If the lens has clearance centrally, the lens fits steeper than K centrally and the tears will pool in the center. Rigid lens principals

  12. Alignment • Suggests that the lens perfectly contours the cornea and the tear film is evenly distributed on the corneal surface. • The lens will not be bearing or have clearance. • Usually used for patients with little or no corneal astigmatism, fit on “K”. Rigid lens principals

  13. The base curve is selected based on the “K” readings. • Rigid lenses are fit in three ways. • On “K” • Steeper than “K” • Flatter than “K” • This is ALWAYS based on corneal astigmatism, not refractive astigmatism. • When fitting any of these, use the flattest “K” readings taken for that eye. • Example…….. • OD = 39.00/41.75@90 • You want to fitter steeper than “K”. (even though it is a flat cornea) • Use 39.00 as the starting “K” for this eye. • Fit steeper than 39.00 depending on the amount of corneal astigmatism. Rigid lens principals

  14. On “K” (alignment) Flatter than “K” • Implies that the curvature is equal to the flattest “K” reading. • This method is appropriate for low cylinder patients. • Implies using a base curve flatter than the flattest”K” meridian. • It will exhibit a bearing effect on the corneal cap allowing increased lid interaction. Bearing will cause a minus tear layer effect requiring a plus power compensation. • (FAP) flatter, add plus. Rigid lens principals

  15. Steeper than “K” Tear film layer • Implies using a base curve steeper than the flattest “K”. This allow central clearance of the lens. The lens will position centrally, along with an increase of tear film. • This tear film will cause a plus lens effect, requiring a minus power compensation. • (SAM) Steeper add minus. • The tear film created by fitting steeper or flatter than “K” re shapes the cornea and the tear layer fills in the gaps to help correct for the cylindrical power not put into the CL. Rigid lens principals

  16. Tear film layer • Fitting Flatter than “K” causes a minus tear layer effect. • The tear layer is pushed to the periphery creating a minus lens effect. • This will have to compensated with flatter add plus when figuring the lens power. Rigid lens principals

  17. Tear film layer • Fitting steeper than “K” will cause a plus tear layer effect. • The tear layer will pool in the center of the lens and this will need to be compensated by steeper add minus to the power of the lens. Rigid lens principals

  18. Determining diameter Horizontal visible iris diameter • The CL diameter should be 2.0-2.5 mm smaller than the visible iris diameter or 1.2 – 1.5 mm from the limbus to the edge of the lens. • Normal rigid lens D is 8 to 10.5 mm, much smaller than a soft CL. Rigid lens principals

  19. Determining diameter Corneal Alignments fit, flatter than “K” • Large diameters will be fit flatter than “K”. • This is called corneal alignment fit or Korb method. • Corneal alignment fit rides high on the cornea and fit UNDER the upper lid. Sometimes referred to lid attachment fit. • FAP rule applies. Rigid lens principals

  20. Determining diameter Apical fit, steeper than “K” • Smaller diameter corneas are fit steeper than “K” • This is called an Apical lens design or interpalpebral lens design and fits in between the lids, centrally on the cornea. • SAM rule applies. Rigid lens principals

  21. Determining diameter Don’t worry, there’s a chart for everything!!! • Corneal alignment fit • Flatter than “K” • Large diameter • Apical fit • Steeper than “K” • Smaller diameter Rigid lens principals

  22. Let’s review what you learned today!!! Rigid lens principals

  23. Rigid lens principals review

  24. Rigid lens principals review

  25. Bearing means? • Alignment means? • Clearance means? • FAP means? • Why do we need to use FAP? • SAM means? • Why do we need to use SAM? • What type of fit is steeper than “K”? • Is that for a large or small diameter corneal? • What does on”K” mean? Rigid lens principals review

  26. Great job! See you next week for rigid lens fitting. Test in 2 weeks. Rigid lens principals

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