Contact lens verification(raju)
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Contact lens Verification- different techniques
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Contact lens verification(raju)
- 1. RAJU KAITI Optometrist, Dhulikhel Hospital, Kathmandu University Hospital
- 2. ο Checking quality and physical characteristics for- ο Prescription use ο Contact lens fitting sets ο Research purposes ο Contact lens verification undergoes two stages, laboratory and clinical.
- 3. ο Laboratory ο During the final phase of manufacture, an overall parameter check is performed to ensure the lenses do not differ significantly from the parameters ordered by the practitioner. ο Clinics ο Verification of lenses upon receipt, rather than during the dispensing visit, is advisable. ο Patients ο βon-eyeβ fit is another indicator of whether a contact lens has been manufactured to specifications.
- 4. ο Ensure correct lens is dispensed ο Quality of manufacturing(as per quality standard) ο Assess changes in contact lens with wear ο To ensure that proper over-refraction and trial fitting examination has been conducted, the accuracy of trial sets used in the clinic should be determined.
- 5. ο Rigid and soft lenses have similar parameters which require verification by the practitioner. ο Radii of curvature ο Linear parameters ο Edge profile ο Power ο Lens quality ο Rigid and soft contact lenses should be hydrated in a soaking solution for 12 - 24 hours before verification procedures are conducted.
- 7. ο Back optic zone radius ο Back central optic zone radius ο Back peripheral optic zone radius ο Front optic zone radius ο Front central optic zone radius ο Front peripheral optic zone radius
- 8. ο Back optic zone diameter ο Back central optic zone diameter ο Back peripheral optic zone diameter ο Front optic zone diameter ο Front peripheral optic zone diameter ο Total diameter ο Bifocal segment size and position
- 9. ο Central ο Edge ο Lenticular junction ο At any other specified point
- 10. ο Axial and radial edge lift ο Edge shapes
- 11. ο Back vertex power ο Front vertex power ο Near addition ο Prism and base direction ο Cylinder power ο Aberration
- 12. ο Finish ο Polish ο Edge form ο Transitions ο Tint ο Material
- 14. ο Radiuscope ο Keratometer (modified) ο Toposcope ο MoirΓ© fringe deflectometer ο Radius checking device ο Topographical mapping system ο Electrical conductivity method ο Microspherometer
- 17. ο Lens holder is filled with water ο Clean lens is placed centrally on holder, convex surface is in complete contact with water ο Holder is placed on microscope stage and centered ο Microscope eyepiece is correctly adjusted ο By observing through microscope, target is imaged on surface of lens
- 18. ο Dial gauge reading is recorded ο Second focus at centre of curvature of surface is obtained ο Second reading is recorded ο Difference between two dial gauge reading gives radius of curvature of surface ο Procedure is repeated twice and average of 3 reading is taken ο Radius is measured in different point of lens as it may vary
- 19. β’ Lens is measured in the dry state β’ Front surface image eliminated with saline in lens mount β’ Lens is centered concave side-up β’ BOZR=distance between1st and 2nd focal plane
- 20. ο± same procedure for determining the FOZR,except that the lens is centered convex side up on the appropriate lens mount. ο± The scale is reversed so that the aerial image will be focused before the real image.
- 22. ο Air checking, which requires the SCL to be dabbed with a lint- free cloth and measured in a semi-dehydrated state. ο The critical duration of such air- checking is approximately one minute depending on ambient temperature and humidity. ο Requires a wet cell filled with saline solution ο The readings are multiplied by the refractive index of saline to calculate the BOZR IMMERSION
- 23. ο The Keratometer which is used for measuring corneal curvature can also be used to measure the BOZR of a contact lens by using special attachments.
- 24. ο Keratometer set-up is modified with a lens holder and prism or mirror attachment ο Values derived are less than the actual radii ο The same procedure for measuring the cornea is used for contact lenses
- 25. ο Keratometer set-up is modified with a wet cell and prism or mirror attachment ο Values derived are less than the actual radii ο Readings are multiplied by the RI of saline to get the BOZR ο The same procedure for measuring the cornea is used for contact lenses
- 26. The principle of the thick lens system to design the R-C Device whose refractive index is the same as the lens material. The contact lens floats on a liquid interface which has the same refractive index as the lens material. The R-C device is used in conjunction with the focimeter and lens thickness gauge.
- 27. By combining the features of the photokeratoscope and a built-in camera, polaroid images of the cornea are captured and viewed with a computer monitor.
- 28. ο Back vertex power (BVP) ο Front vertex power (FVP) ο Front and back surface radii of curvature. ο Centre thickness. ο Refractive index.
- 29. ο It measures BVP ο Projection focimeter-greater accuracy ο Nakijama-mounted lens in liquid cell and read power of resultant contact lens-liquid lens on projection focimeterBVP of soft contact lens can be measured in air or liquid ο Clarity of focimeter image relates to optical quality of lenses
- 30. ο BVP in air is not equal to BVP in liquid ο If F1=BVP in air ο F2=BVP in liquid ο F1=kF2where k is compensation factor ο K=n2-n/n2-n1 where, ο n=refractive index of air ο ,n1=RI of saline ο .n2=refractive index of Hydrogel material
- 31. ο Lens is cleaned and dried ο Lens must be centered concave side down on the focimeter stop ο Reading is taken off the power drum/scale after focusing the mires ο Back vertex focal length is measured from the plane of the focimeter stop
- 32. ο Power for each meridian is measured ο Cylinder value is derived from measuring the difference in meridional powers
- 33. ο The SCL is measured by air-checking it with the focimeter. ο The lens is dabbed with a lint-free cloth/tissue to remove excess water. ο The SCL is centered on the focimeter support and the procedure used to measure RGP lenses is followed. ο An SCL can also be measured by immersing the lens in saline contained in a wet cell. Because the lens power is measured in saline, the value has to be multiplied by four (approximately) to calculate the true lens power in air.
- 34. ο Procedure is same as for BVP but with lens convex side down ο FVP measurements can be converted to BVP by using a table with known center thickness and back optic zone radius
- 35. ο Lens diameters ο back optic zone diameter (BOZD) ο total diameter ο peripheral curve width (PCW) ο Lens thickness ο centre thickness (ct) ο edge thickness
- 36. ο Diameters and linear parameters ο Measuring magnifier ο V gauze ο Cast, dividers and transparent rule ο Micrometer & spheres Measuring magnifier 20 mm scale: used for corneal lens
- 37. V gauze Scale 6.00 β 12.50mm Cast, dividers and transparent rule Micrometer & spheres: Measure primary optic diameter, sag is determined
- 38. ο Can be measured with all most all techniques ο Indirect method:
- 39. ο Thickness verification ο Dial thickness gauze ο Contek edge thickness gauze & computer ο Radial thickness: perpendicular to front surface of lens ο Axial thickness: parallel to primary axis of lens ο Spectacle lens measure ο Radiuscope: no water used Thickness gauze Contek edge thickness gauze & computer
- 41. ο Lens diameters ο total diameter ο FOZD ο Lens thickness ο centre thickness (ct) ο edge thickness
- 42. ο Instruments: ο Projection magnifier ο MoirΓ© fringe deflectometer ο 10x loupe with graticule ο Electronic thickness gauge ο Pressure controlled gauge ο Electrical thickness gauge ο Radiuscope (modified)
- 43. Instruments/techniques: β’ Edge molding β’ Projection magnifier β’ Ehrmann profilometer β’ Palm test β’ Radiuscope (modified)
- 45. ο Surface defects ο Optical quality ο Lens impurities/deposits
- 46. ο Scratches and lathe marks: ο Can cause: ο deposit build up ο poor wettability ο surface hydrophobicity ο Indicate over polishing during manufacture
- 47. ο Instruments: ο Magnifying 10x loupe ο Projection magnifier ο Contact lens optical quality analyzer (CLOQA) ο Dark field microscope ο Moire fringe deflectometer
- 49. ο White background test ο Variation in thickness
- 50. ο Hydrogel contact lenses are flexible ο If exposed to atmosphere, they dehydrate and alter their contour. Verification in air is inaccurate due to- ο Shrinkage of Hydrogel on dehydration ο Accumulation of surface moisture ο So, artifact liquid cells are used to measure parameters of soft lenses ο But RGP lenses can be measured in air
- 51. ο Mandell 1974 recommend following procedure- ο Lens should be removed from its liquid using sterile spatula or soft plastics protected forceps ο Lens is then placed on lint free tissue and tissue is folded over uppermost convex of lens ο Both surface are blotted dry ο Lens is dried in air with forceps ο Lens surfaces are examined for smudges ο It is preferable to check lens within one minute
- 52. ο %water content = mass of watermass of hydrated lens x 100 ο Water content of Hydrogel contact lenses is measured by sensitive microbalance ο Alternative methods- ο Refractive index ο Refractive indexes decreases as water content increases
- 53. ο On-eye-examination of dispensing lens ο VA assessment before and after over-refraction. ο Always assess ο dynamic and static fit, ο surface wet ability & lens quality, ο corneal integrity.
- 54. ο At the end of the verification process, the real indicator that an accurate and optimal fitting has been achieved is evaluation of the lens in situ.
- 55. ο Ensure that contact lenses dispensed have the correct parameters, are sterile and in good condition. ο Ensure that optimum visual acuity is achieved by the patient with the contact lenses. ο Ensure that the contact lenses fit satisfactorily. ο Provide instruction on care and maintenance.
- 56. THANK