fitting RGP lenses

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fitting RGP lenses

  1. 1. In the name of God RGP Lens Design, Fitting and Evaluation By : hossein alimirzaie optometrist
  2. 2. Methods to correct vision: <ul><li>Optical: </li></ul><ul><li>spectacles </li></ul><ul><li>Contact lenses </li></ul><ul><ul><ul><ul><ul><li>Refractive surgery </li></ul></ul></ul></ul></ul>
  3. 3. Optical methods: <ul><li>Spectacles: </li></ul><ul><li>High index lenses </li></ul><ul><li>Light weight frames </li></ul>
  4. 4. Optical methods: <ul><li>Rigid gas permeable </li></ul><ul><li>Soft contact lenses: </li></ul><ul><li>conventional </li></ul><ul><ul><li>Frequent replacement </li></ul></ul>Contact lenses :
  5. 6. Disadvantages of Soft Contacts vs.. Hard <ul><li>Less durable than hard lenses </li></ul><ul><li>Unable to correct astigmatism completely in some cases </li></ul><ul><li>Tendency for deposit formation </li></ul><ul><li>Cannot be modified as hard lenses can </li></ul><ul><li>Requires significant maintenance </li></ul><ul><li>Lenses can cause decreased vision when dry or dirty </li></ul>
  6. 7. BENEFITS OF RGP LENSES <ul><li>1. Good Vision </li></ul><ul><li>  Automatically corrects most astigmatism  </li></ul><ul><li>Excellent optical quality  </li></ul><ul><li>Good surface wettability  </li></ul><ul><li>Good bifocal lens designs </li></ul>
  7. 8. 2. Excellent Eye Health <ul><li>  On average 2-3 times more oxygen provided to the cornea as compared to most soft lenses  </li></ul><ul><li>Smaller size allows tear exchange and debris removal  </li></ul><ul><li>Fewer deposits due to surface wettability </li></ul><ul><li>3. Durability </li></ul><ul><li>  GP lenses last longer and don't need to be replaced as often as soft lenses  </li></ul><ul><li>Lower long-term cost </li></ul>
  8. 9. 4. Ease of Handling <ul><li>  Smaller size lenses are easier to insert </li></ul><ul><li>5. Ease of Care </li></ul><ul><li>  Simple cleaning and disinfecting systems can be used due to wettable surfaces and deposit-resistant surfaces  </li></ul><ul><li>Lower annual cost </li></ul>
  9. 10. 6. Slow Down Myopia Progression <ul><li>Studies have shown that GP lenses slow down myopia progression in young people </li></ul>
  10. 11. APPLICATIONS <ul><li>1. Critical Vision Needs </li></ul><ul><li>2. Astigmatism Correction </li></ul><ul><li>3. Children and Teenagers </li></ul><ul><li>4. Presbyopia - bifocal correction  </li></ul><ul><li>5. Irregular Cornea - keratoconus, trauma, corneal warpage  </li></ul><ul><li>6. Soft Lens Failures - poor vision, eye health compromise </li></ul><ul><li>7. Orthokeratology/Corneal Refractive Therapy </li></ul>
  11. 12. EMPIRICAL FITTING <ul><li>there are good reasons to consider empirical methods today. Empirical fitting is time-efficient and also quite accurate. </li></ul><ul><li>corneal topography instrumentation has included sophisticated contact lens design software with the ability of accurately simulating the fluorescein pattern based upon topography while recommending a lens design which results in a high first-fit success rate. </li></ul>
  12. 13. FLUORESCEIN EVALUATION <ul><li>The application of fluorescein to the tear film is essential for proper evaluation of the lens-to-cornea fitting relationship </li></ul>
  13. 14. Omitting this step can result in any one or more of the following problems : <ul><li>Peripheral sealoff : tear exchange is not possible; therefore, debris removal and additional oxygen supply is eliminated. </li></ul><ul><li>Areas of bearing or contact of lens against cornea : corneal distortion can result over time. </li></ul><ul><li>Regions of excessive edge clearance : more lid awareness, corneal dryness or desiccation and lens decentration can result. </li></ul><ul><li>Corneal compromise : areas of coalesced cornea staining can be present. </li></ul>
  14. 15. Alignment fit : <ul><li>A fluorescein pattern showing the lens evenly contouring the cornea with only light tear pooling between the lens and cornea is termed an alignment fit. </li></ul><ul><li>A steep central fit with excessive fluorescence or tear pooling centrally is termed apical clearance . </li></ul><ul><li>Apical bearing represents the opposite form of pattern or direct contact of the lens against the central cornea </li></ul>
  15. 16. One should always strive for an alignment fitting relationship <ul><li>The only exceptions would pertain to cases in which is an alignment fit can not be achieved (i.e., irregular corneas) or is not desired (i.e., corneal reshaping or orthokeratology). </li></ul><ul><li>Excessive apical bearing should be avoided as corneal molding can occur, possibly resulting in warping or distortion </li></ul>
  16. 17. Figure 1 : An apical bearing fitting relationship
  17. 18. Figure 2 : An apical clearance, low edge clearance fitting relationship
  18. 19. Figure 3 : A high edge clearance fitting relationship
  19. 21. Fluorescein evaluation of the lens-to-cornea fitting relationship should be performed at both the fitting and all progress evaluation visits.
  20. 22. <ul><li>The ability to accurately assess fluorescein patterns comes only after constant and frequent evaluations. It is not uncommon to have a lens that centers optimally but could induce corneal compromise over time due to the absence of an alignment fluorescein pattern </li></ul>
  21. 23. LENS DESIGN PHILOSOPHY <ul><li>It is important to note that – regardless of the design philosophy – the most important factor is fluorescein pattern evaluation of the fitting relationship. </li></ul><ul><li>1. Overall/Optical Zone Diameter. </li></ul><ul><li>The optical zone diameter is often approximately 1.4mm less than the overall diameter. </li></ul>
  22. 24. The overall diameter and optical zone diameter of a rigid lens are typically obtained by considering the following variables: <ul><li>(a) palpebral aperture size, </li></ul><ul><li>(b) pupil size, </li></ul><ul><li>(c) corneal curvature. </li></ul>
  23. 27. a. Palpebral Aperture Size(PAL) . Normal PAL values range between 9mm-10.5mm. In most lid-to-cornea relationships there is, at minimum, slight overlap of the lids over the limbus.
  24. 28. Figure 4 : A lid attachment fitting relationship
  25. 29. c. Corneal Curvature . <ul><li>Steeper-than-average corneal curvatures (i.e., >45D) often necessitate a smaller-than-average overall diameter to maintain centration on a cornea that typically flattens at a higher rate than normal. </li></ul><ul><li>An average overall diameter/optical zone diameter of 9.4/8.0mm should be successful in most cases, particularly when a lid attachment fitting relationship is desired. 0 </li></ul>
  26. 30. 2. Base Curve Radius . <ul><li>The base curve radius (BCR) to be selected for a given patient depends upon several factors, including corneal curvature, lid-to-cornea relationship and fluorescein pattern evaluation. </li></ul><ul><li>. As the cornea is aspheric and tends to flatten at a greater rate at an increasing distance from the center, the selection of a base curve radius that is slightly flatter than “K” should result in an alignment fitting relationship. </li></ul>
  27. 32. <ul><li>Gas permeable lenses will always move along the steeper meridian of the cornea as well as to the steepest region of the cornea (i.e., apex). </li></ul><ul><li>A simple rule of thumb is to flatten the BCR by 0.25D for every increase in overall diameter of 0.5mm and to steepen the BCR by 0.25D for every decrease in overall diameter of 0.5mm. </li></ul>
  28. 33. 3. Peripheral Curve Radii and Width . <ul><li>The purpose of the peripheral curve radii is to allow good tear flow while compensating for an aspheric corneal shape. </li></ul><ul><li>The distance from the edge of the lens and the cornea is termed the edge clearance. </li></ul>
  29. 34. 4. Center Thickness . <ul><li>Overall lens thickness is dependent on a number of variables, but primarily lens power and overall diameter </li></ul><ul><li>The center thickness is greater and the center of gravity more anterior for plus lenses, while the edge thickness is greater and the center of gravity more posterior for minus lenses. </li></ul>
  30. 35. When making lens design changes, it is important that this change is significant enough to result in an improvement in the lens-to-cornea fitting relationship
  31. 36. Power <ul><li>. After obtaining an optimum lens-to-cornea fitting relationship, accurate determination of the final lens power is important. </li></ul><ul><li>Vertex Distance . </li></ul><ul><li>Another important consideration in predicting the final lens power is the effective power at the corneal plane for patients exhibiting a high amount of myopia or hyperopia </li></ul>
  32. 38. به امید موفقیت وسربلندی شما

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