Transcript
RAJU KAITIOptometrist, Dhulikhel Hospital, Kathmandu University Hospital
Checking quality and physical characteristics for-
Prescription use Contact lens fitting sets Research purposes
Contact lens verification undergoes two stages, laboratory and clinical.
Laboratory 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.
Clinics Verification of lenses upon receipt, rather than during the
dispensing visit, is advisable.
Patients ‘on-eye’ fit is another indicator of whether a contact lens has been
manufactured to specifications.
Ensure correct lens is dispensed
Quality of manufacturing(as per quality standard)
Assess changes in contact lens with wear
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.
Rigid and soft lenses have similar parameters which require verification by the practitioner.
Radii of curvature Linear parameters Edge profile Power Lens quality
Rigid and soft contact lenses should be hydrated in a soaking solution for 12 - 24 hours before verification procedures are conducted.
Back optic zone radius Back central optic zone radius Back peripheral optic zone radius Front optic zone radius Front central optic zone radius Front peripheral optic zone radius
Back optic zone diameter Back central optic zone diameter Back peripheral optic zone diameter Front optic zone diameter Front peripheral optic zone diameter Total diameter Bifocal segment size and position
Central Edge Lenticular junction At any other specified point
Axial and radial edge lift Edge shapes
Back vertex power Front vertex power Near addition Prism and base direction Cylinder power Aberration
Finish Polish Edge form Transitions Tint Material
Radiuscope Keratometer (modified) Toposcope Moiré fringe deflectometer Radius checking device Topographical mapping system Electrical conductivity method Microspherometer
Drysdale’s principle
Lens holder is filled with water
Clean lens is placed centrally on holder, convex surface is in complete contact with water
Holder is placed on microscope stage and centered
Microscope eyepiece is correctly adjusted
By observing through microscope, target is imaged on surface of lens
Dial gauge reading is recorded
Second focus at centre of curvature of surface is obtained
Second reading is recorded
Difference between two dial gauge reading gives radius of curvature of surface
Procedure is repeated twice and average of 3 reading is taken
Radius is measured in different point of lens as it may vary
• Lens is measured in the dry state
• Front surface image eliminated with saline in lens mount
• Lens is centered concave side-up
• BOZR=distance between1st and 2nd focal plane
same procedure for determining the FOZR,except that the lens is centered convex side up on the appropriate lens mount.
The scale is reversed so that the aerial image will be focused before the real image.
Air checking, which requires the SCL to be dabbed with a lint-free cloth and measured in a semi-dehydrated state.
The critical duration of such air-checking is approximately one minute depending on ambient temperature and humidity.
Requires a wet cell filled with saline solution
The readings are multiplied by the refractive index of saline to calculate the BOZR
IMMERSION
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.
Keratometer set-up is modified with a lens holder and prism or mirror attachment
Values derived are less than the actual radii
The same procedure for measuring the cornea is used for contact lenses
Keratometer set-up is modified with a wet cell and prism or mirror attachment
Values derived are less than the actual radii
Readings are multiplied by the RI of saline to get the BOZR
The same procedure for measuring the cornea is used for contact lenses
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.
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.
Back vertex power (BVP) Front vertex power (FVP)
Front and back surface radii of curvature.
Centre thickness. Refractive index.
It measures BVP
Projection focimeter-greater accuracy
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
Clarity of focimeter image relates to optical quality of lenses
BVP in air is not equal to BVP in liquid If F1=BVP in air
F2=BVP in liquid
F1=kF2 where k is compensation factor K=n2-n/n2-n1 where, n=refractive index of air ,n1=RI of saline .n2=refractive index of Hydrogel material
Lens is cleaned and dried
Lens must be centered concave side down on the focimeter stop
Reading is taken off the power drum/scale after focusing the mires
Back vertex focal length is measured from the plane of the focimeter stop
Power for each meridian is measured
Cylinder value is derived from measuring the difference in meridional powers
The SCL is measured by air-checking it with the focimeter.
The lens is dabbed with a lint-free cloth/tissue to remove excess water.
The SCL is centered on the focimeter support and the procedure used to measure RGP lenses is followed.
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 multipl ied by four (approximately) to calculate the true lens power in air.
Procedure is same as for BVP but with lens convex side down
FVP measurements can be converted to BVP by using a table with known center thickness and back optic zone radius
Lens diameters back optic zone diameter (BOZD) total diameter peripheral curve width (PCW)
Lens thickness centre thickness (ct) edge thickness
Diameters and linear parameters
Measuring magnifier V gauze Cast, dividers and
transparent rule Micrometer & spheres
Measuring magnifier
20 mm scale: used for corneal lens
V gauzeScale 6.00 – 12.50mm
Cast, dividers and transparent rule
Micrometer & spheres:Measure primary optic diameter, sag is determined
Can be measured with all most all techniques
Indirect method:
Thickness verification
Dial thickness gauze
Contek edge thickness gauze & computer
Radial thickness: perpendicular to front surface of lens
Axial thickness: parallel to primary axis of lens
Spectacle lens measure Radiuscope: no water
used
Thickness gauze Contek edge thickness
gauze & computer
Lens diameters total diameter FOZD
Lens thickness centre thickness (ct) edge thickness
Instruments:
Projection magnifier Moiré fringe deflectometer 10x loupe with graticule Electronic thickness gauge Pressure controlled gauge Electrical thickness gauge Radiuscope (modified)
Instruments/techniques:
• Edge molding• Projection magnifier• Ehrmann profilometer• Palm test• Radiuscope (modified)
Surface defects Optical quality Lens impurities/deposits
Scratches and lathe marks: Can cause:
deposit build up poor wettability surface hydrophobicity
Indicate over polishing during manufacture
Instruments: Magnifying 10x loupe Projection magnifier Contact lens optical quality analyzer (CLOQA) Dark field microscope Moire fringe deflectometer
White background test
Variation in thickness
Hydrogel contact lenses are flexible If exposed to atmosphere, they dehydrate and
alter their contour. Verification in air is inaccurate due to-
Shrinkage of Hydrogel on dehydration Accumulation of surface moisture
So, artifact liquid cells are used to measure parameters of soft lenses
But RGP lenses can be measured in air
Mandell 1974 recommend following procedure- Lens should be removed from its liquid using sterile spatula or soft
plastics protected forceps
Lens is then placed on lint free tissue and tissue is folded over uppermost convex of lens
Both surface are blotted dry
Lens is dried in air with forceps
Lens surfaces are examined for smudges
It is preferable to check lens within one minute
%water content = mass of water\mass of hydrated lens x 100
Water content of Hydrogel contact lenses is measured by sensitive microbalance
Alternative methods- Refractive index Refractive indexes decreases as water content increases
On-eye-examination of dispensing lens VA assessment before and
after over-refraction.
Always assess dynamic and static fit, surface wet ability & lens
quality, corneal integrity.
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.
Ensure that contact lenses dispensed have the correct parameters, are sterile and in good condition.
Ensure that optimum visual acuity is achieved by the patient with the contact lenses.
Ensure that the contact lenses fit satisfactorily.
Provide instruction on care and maintenance.
THANK
YOU…
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