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Contact Lens Exam 1 Study Guide _Alana_.docx

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  • pg 1
									CORNEA + CONTACT LENS

 LENS ANALYSIS + FABRICATION

ANALYSIS INSTRUMENTS

  -   Overall diameter (OAD): widest portion of CL
        o Always measure diameter in 2  meridians
        o V-channel gage: slide lens concave side down down the channel; where it stops is msmt of OAD
        o Hand measuring magnifier (7X): place CL on reticule; hold magnifier up to light to read diameter
                   Zero out by opening the magnifier and twisting until reticule comes into focus
  -   Back and front vertex power
         o Lensometer
         o Measure both front and back vertex powers w/ higher prescriptions > +10D
                   LAB: measurements are made off front vertex power
                   OD: measurements are made off back vertex power
  -   Base curve radius
         o Radiuscope
                 SET-UP: lens placed in middle of well atop drop of water
                       Make sure CL is relatively clean before analyzing
                       Water is important to give clear image
                 MIRES
                       Zeroing mire: real image that is projected off the lens appears when stage and
                          objective are closest together
                       Measuring mire: aerial image appears when stage and objective are furthest apart
                       Coil: appears btwn the two mires
                       Mire pattern should be centered w/in FOV
                 BC msmt = difference btwn zeroing and measuring mires (real & aerial images
         o AO radiuscope (no longer made)
                 Scale runs internally along right-hand side of your view
                 Stage stationary; objective moves up and down
                 Center the measuring mire via built-in centering lines
         o Marco radiuscope
                 Scale is external to the instrument
                 Objective stationary; stage moves up and down
                 Center the measuring mire by placing it in the middle of your FOV
         o Warped lenses are due to heat or vigorous cleaning
                 See one line sharply in focus and other lines of mire pattern are faint
                 2 BC msmts must be made
  -   Center thickness
         o Lens always placed gently btwn the calipers, concave side up
         o Marco radiuscope: lift up the pin atop the dial scale and place CL btwn pin & pole
                 Do not drop the pin on the CL
                 Only the big hand must be zeroed
                 Measurements are in 0.00 form (hundredths of a mm)
  -   Edge contour
         o Shadowscope: shape of lens projected onto a screen as a shadow
                 Measures OAD (not as accurate) and edge contour
         o Edge can be in form of round, (+), knife, butt, V, A
FABRICATION

  -   Sequence of fabrication
         o OAD reduction using a cut-down tool
         o Cut 3rd (peripheral) curve to decrease edge mass using diamond tool
         o Cut 2nd (secondary) curve
         o Polish 2nd curve
         o Recut 3rd curve
         o Polish 3rd curve
         o Identify edge contour
         o Create knife edge
         o Plus knife edge
         o Final F1/F2 polish
  -   Fabrication tools
         o Polishing compounds: polish and water only
         o Cut-down tool: shaped like a V
                 Used to cut down bulk of lens until you reach desired OAD
                 V cut-down cool creates a V-edge
         o Diamond radius tool: silver on top + shiny
                 Only use WATER for diamond tools  polish will ruin the tool
                 Used to cut 3rd curve and 2nd curve
         o Brass radius tools
                 Tape placed on top so tool can be used in conjunction with POLISH
                 Used to polish the curves
                 Must use same brass tool as your diamond tool (10.0 diamond tool used w/ 10.0 brass tool)
         o Sponge tools
                 Used to re-edge the lens to make it comfortable for the patient
  -   Modification units
         o 2 parts: bowl + motor
         o Diamond, cut-down V, brass, sponge tools can be set on the motor unit inside the bowl
         o Motor units run counterclockwise
         o Vector forces
                 Run lens in opposite direction (clockwise) to remove plastic from lens
                 Vector forces depend on where you position the lens, orientation of lens, which way sponge is
                   sitting, and how much plastic you want removed
                 At 6:00 vector forces will hit F2 surface of lens
                 At 12:00 vector forces will hit F1 surface of lens
                 At 3:00 vector forces will hit profile of lens

 INTRODUCTION

CONTACT LENS PROFILE

  -   CL services at ECC: prosthetics, pediatric CL, ortho-K, refractive surgery co-mgmt, keratoconus
  -   DIAMETERS: OAD, OZD, TCw
         o Overall diameter: widest part of the lens (anywhere from 8.5mm – 9.4mm – 11mm – 18mm)
         o Optic zone diameter: optical zone where patient is viewing
         o Measured with 7x
         o SCw not specified because it can be calculated from other 3
                 SCw = (OAD – OZD – 2TCw) / 2
  -   CURVATURES
         o Tri-curve lenses
                 Central base curve: optical curve
                 Secondary curve: fitting curve; involved w/ exchanging oxygen
                 Third curve: fitting curve
         o Base curve is steeper than secondary curve which is steeper than third curve
       o   Base curve
               Radius expressed in mm / curvature expressed in diopters
               Conversion formula: D = 337.5/mm
               Measured easily with a radiuscope (can also use keratometer)
               Recorded in 0.12D or 0.01Dmm increments
       o   Peripheral curves: secondary curve (SCr) + third curve (TCr)
               Patient feels the edge of lens with eyelids
               Blended lens: curve placed btwn BC and SC to soften the junction and reduce pain to cornea
                     EX: BC 7.50mm SC 8.50  8mm curve used to soften junction btwn BC and SC
                     Traditionally do not blend bc it is not necessary
                            o Non-blended lens is easier to analyze AND modify
                     Common blends
                            o Light: 0.1mm wide (1, A blend)
                            o Medium: 0.2mm wide (2, B blend)
                            o Heavy: 0.3mm wide (3, C blend)
                     More blend = wider junction btwn curves
               Peripheral curves are more difficult to measure
                     Moiré images used to measure peripheral curves
                     Diamond tools used to measure peripheral curves
                               o   If scratches across entire surface = radius of curvature
                               o   If scratches on inside edge: flatter radius
                               o   If scratches on outside edge: steeper radius
-   EDGE
      o  Edge = comfort (what patient’s feels most)
      o  Bad edge: butt or knife or chipped
      o  Good edge: plus or sometimes round
      o  Judge quality of edge with 7X or shadowscope (or patient’s response)
             If edge too sharp or too round  change the edge; plus the edge
-   POWER
       o Front vs. back vertex power
             FRONT: power of concave surface
             BACK: power of convex surface
             Communicate F/B vertex power with fabricating laboratory
       o Measured using lensometer
       o Warpage caused by heat
             2 different BC readings but will not give you a different power
             Sphere still read on spherical lens
-   THICKNESS
       o Measured with thickness gage or radiuscope
       o O2 transmissibility more important with soft lenses than rigid lenses
                 PMMA: plastic rigid lens impermeable to O2
                 Dk/L = transmissibility where Dk = permeability of material and L = thickness
       o   As lens gets thicker, center of gravity moves further from cornea  lens begins to fall w/ gravity
       o   Edge thickness is the issue (not center thickness)
                te must be approximately 0.10mm
                Lenticular lenses used if lenses are high + or high –
-   TINT
       o Measured using white piece of paper as background
       o Important to patient
             RIGHT EYE: gReen
             LEFT EYE: bLue
       o #1, #2, #3 tints
SCCO ORDER FORM

  -   EMR order form: parameters listed randomly & in unorganized manner
  -   Paper order form: logical order of thinking wrt parameters
         o RGP order parameters
                BC (D & mm)  Power  OAD  OZ  SCr  TCr  TCw  t  Tint
         o RGP analysis parameters
                BC (D & mm) power  OAD  OZ  TCw  edge
                Difficult to measure SCr, TCr
         o Soft lens order parameters
                BC  power  diameter  quantity (ind/box)  color

 DA CORNEA

  -   Elliptical shape
          o CURVATURE
                   Steep to flat from central to peripheral cornea
                        Radius of curvature is longer more peripherally
                   Back of contact typically spherical (radius of curvature is constant)
          o SAGITTAL HEIGHT: defined by curvature + diameter
                   Central cornea has smaller sagittal height than peripheral
                   Change sagittal height by changing core diameter OR radius
  -   Visible Iris Diameter (VID)
          o Tough to measure  borders of cornea are vague so don’t spend too much time measuring
                   PD stick
                   Limbus-to-limbus topography (medmont)
                             Most instruments measure central 8mm rather than limbus-to-limbus
                   OCT (optical coherence tomography): measures corneal thickness, sagittal height (front surface
                    of eye to back of iris), curvature of sclera
         o Range: ~10.5 – 13.5mm
                 Average: VIDH=11.5mm and VIDV=10.6mm
                 Females: 0.1mm less than average
                 One year olds: 0.6mm less than average
         o Measured VID would be helpful in:
                 Determining fit/BC of soft CL (a better way but hardly used)
                          Soft CL dependent on diameter covered as opposed to curvature covered
                 Choosing OAD of GP lenses
                 Fitting infants (due to their smaller corneas)
                 Fitting patients with microcorneas
                 Fitting scleral GP lenses
                 BUT, typically use OAD to judge fit
  -   Sagittal height
         o Sagittal height is what we are doing when we fit contact lenses but is not measured clinically
         o Sagittal height is on average 2.59mm
         o CL fitting: trying to contour spherical BC of CL to aspheric front surface of cornea
         o CL parameters that alter sagittal height and change the fit
                 Curvature (BCr, PCr)
                 Diameter (OAD, OZD)
         o Changes to CL back-surface (F2) alters inside sagittal height whereas changes to the CL front-surface
            (F1) do not alter inside sagittal height
         o Plastic is subtracted from the lens  steeper to flatter fitting relationship
                    Decrease SH = flatter fitting relationship as plastic is subtracted from the lens
                    Flatter radii = flatter fitting relationship
-   Corneal thickness
       o Measured via:
               Pachymetry (ultrasound or optical)
               Orbscan topography
               Interferometry
       o Measurements
               Thinnest centrally: 536
                     95% of msmts btwn 473 - 595
               Thickest peripherally: 700-1200
       o Central corneal thickness (CCT)
               Differences wrt different ethnicities
                        AA: 535 (thinner corneas) / Asian: 550 / Caucasian: 553 / Hispanic: 551
              Differences affect GAT readings bc GAT calibrated for specific CCT
-   LASIK: corneal thinning procedure
       o Pre-LASIK measurements
              Start w/ pre-surgical thickness of 5xx microns
                      Subtract flap thickness of roughly 160
                      Subtract vaporization of 12 for every diopter of correction
              Need post-surgical corneal bed thickness of 250
              EX: 536 - 160 flap - 250 bed = 126 is max amt of cornea to remove (up to ~10D myope)
              Consider PRK (surface ablation) if patient has borderline thickness
                      No flap cut allows removal of slightly more cornea
       o Post-LASIK
              Thinned corneas are not structurally stable  susceptible to trauma + ectasia (keratoconus)
              Thinned corneas cause underestimation of true IOP which can lead to delay in diagnosis +
                 establishing the wrong target reading for glaucoma tx
                        Many KC pts IOP msmts are single digits
-   Corneal curvature
       o Average: 43.00D (7.85mm)
       o Born w/ steeper, smaller corneas and curvature flattens from infancy to age 5
       o Curvature stable from age 5 to 95 unless corneal injury, surgery, or CL wear occurs
-   Types of corneas
       o Prolate: steeper centrally and flattens peripherally (normal cornea)
               Spherical lens design
       o Oblate: flatter centrally and relative steepening peripherally (corneal surgery)
               Post-LASIK for myopia (laser assisted in situ keratomileusis)
               Post-OK (orthokeratology or CRT)
               Post-RK (radial keratotomy)
               Penetrating keratoplasty (corneal transplant)
-   Corneal eccentricity
       o Eccentricity: rate of flattening from center to periphery
               Eccentricity varies among individuals (cornea elliptical in shape)
               Lower eccentricity = more spherical cornea
       o Average: 0.5 eccentricity (0.48 + 0.11)
       o Steepness of cornea and rate of flattening are independent  steeper corneas don’t necessarily
          have higher rates of flattening in periphery
-   Keratometer vs. topography
       o Not done on every patient
       o Both instruments very repeatable on central cornea
               Topographer = simulated K  0.50D or better
               Keratometer = K  0.50D or better
       o Both instruments less accurate/repeatable on peripheral cornea
       o Steel ball measured to calibrate the instruments
               However, cornea ≠ steel ball due to patient fixation and tear film
  -   Keratometer tips
         o Focus eyepiece from too much (+) to (-) to control accommodation
         o Make sure instrument is calibrated (especially at ECC and with RGP pts)
         o Patient position
                 Forehead tight against headrest and keep head still
                 Patient should not be talking/chewing
                 Set a little low to have patient lean forward
                 Don’t have patient stoop or hang  uncomfortable and inaccurate
         o Axes are not welded to axis 90 and 180
                 90 and 180 are most common but will not fit every patient
  -   Off-the-scale K readings  extend the range
         o TOO STEEP (>52D) = keratoconus patients
                 Extend range by taping +1.25D spectacle trial lens on pt side of k’meter
                         Add 8 – 9D to drum reading
                 Extend range with +2.25D trial lens
                         Add 16D to drum reading
         o TOO FLAT (<38D) = post-refractive surgery pts
                 Extend range with -1.00D spectacle trial lens
                         Subtract 6D from drum reading
  -   Traditional K shape theories
         o Spherical corneal cap: central 4mm of maximum (steep) and constant (sphere) curvature
                 Cap surrounded by annular (sphere or ellipse) portion
         o Ellipse: flattening occurs at constant (or not) rate

 CORNEAL TOPOGRAPHY

  -   Keratometry: used to fit regular corneas w/ GPs (not used for soft CL)
  -   Topography: used to understand the contour of irregular corneas
         o Not used so much for fitting  actual curvature numbers are iffy
         o Useful in orthokeratology to determine if treatment area is centered and corneal surface is smooth
  -   Measurement of K curvature
         o Keratometer: manual or automated
                 Quantitative: K readings
                 Qualitative: mire image quality; 1+ to 4+ mire distortion based on corneal distortion
                        Can be used to measure NITBUT
         o Placido’s disk
                 Qualitative measurements only  observe reflections of disc on patient’s cornea
                        Looking for distortion of mires
                 Set-up: pt looks thru center of disc and you observe mires on cornea in lighted conditions
         o Klein keratoscope
                 Qualitative only  looking for distortion of mires thru magnifying lens
                 Set-up: small, battery-powered instrument w/ smaller disc
         o Gene Reynolds corneascope
                 Polaroid camera used to take picture of images on cornea
                 Measure separation of images on cornea in different meridians to calculate corneal curvature
                 Modern topographers have same technology with attached computer to do calculations
         o Videokeratoscope
                 Video to capture image
                 Computer to interpret image
  -   Mire distortion grading scale (don’t memorize)
         o Scale subject to personal interpretation  develop your own scale
         o Grade 0: no distortion
         o Grade 1: slightly wavy mires; 1/4th of the mire pattern
         o Grade 2: moderate waviness; ½ of mire pattern
         o Grade 3: major waviness; 3/4th mire pattern
         o Grade 4: extreme waviness; missing parts of pattern
-   Videokeratoscopy
       o Instrument projects rings or grid pattern on to cornea  placido images
              Rings are narrow and more in number  image separation from ring to ring
              Rings are wide and fewer in number  edge detection + distance btwn edges of rings
       o Pattern detected/measured using edge detection or image separation
              Rings closer together represent steeper cornea
       o Computer enhances pattern, interprets data, + produces topographic contour map
              Algorithms set up + calibrated for normal patients  accurate data for normals
              Computer interpretations can be inaccurate on irregular corneas
       o Qualitative AND quantitative data is presented
-   INDICES
       o Simulated keratometry value (sim k)
              Readings are taken 3 mm apart or at every 3rd ring
              Not in great agreement w/ k’meter  ballpark
       o Surface asymmetry index (SAI): symmetry of cornea from L to R, top to bottom
       o Surface regularity index (SRI): regularity of cornea or how distorted the image is
       o Potential visual acuity (PVA): not accurate
       o Corneal eccentricity index (CEI): indicates whether cornea is prolate (+ value), oblate (- value), or
          spherical (zero)
-   HOW TO INTERPRET
       o COLOR MAPS
              Cool colors (blue) = flatter areas (less curvature)
              Hot colors (red, orange) = steeper areas (more curvature)
       o COLOR SCALES
              Absolute: fixed scale; compared universally
                       Range: 35 – 52D (normal corneal curvatures)
                              o   May be different on various instruments
                       Same curvature is always same color for everyone
                       Lower resolution due to larger increments
                       Can change absolute scale into normalized scale by reducing range + increment size
                        for more detailed results
                  Normalized: individual scale; specific to patient
                       Range: encompasses extremely flat/steep K values (irregular corneal curvatures)
                       Colors can be different curvatures depending on the individual’s cornea
                       More detail due to smaller increments + range
       o SHAPE
             Round: spherical cornea
             Oval (hourglass/dumbbell): astigmatic cornea
                   ATR = horizontal oval (cornea steeper in 180; axis 90)
                   WTR = vertical oval (cornea steeper in 90; axis 180)
                   Symmetric = steepness concentrated equally on either side of center  “mirror image”
                   Asymmetric = steepness concentrated more heavily on one side of center
      o EXTENT
             Most topographers do not go limbus to limbus
             Astimastism/toricity is primarily central  may extend into periphery near edge of display
             Typically, mid-periphery is more spherical in nature
-   DATA ANALYSIS: smoothing + extrapolating
      o Sagittal map = axial
             Most common topographer setup
             Values relative to visual axis
                   Center to 1st ring; center to 2nd ring; center to 3rd ring; etc
                   Downplays any  in corneal curvature bc compares everything back to same point
             Extreme values are averaged
             More smoothing = less noise + less detail  not as accurate for distorted corneas
                   Keratoconus contour map will appear more decentered and larger
       o  Tangential map = instantaneous rate of curvature
              Values are obtained at foci  measures point to point
              Less smoothing = more detail  more accurate for distorted/peripheral corneas
                       Keratoconus contour map will appear fairly centered + small
       o Elevation = height data
              Height data is relative to cpu-generated best-fit reference sphere line
                       Central cornea is often steeper than best-fit sphere + peripheral cornea is often flatter
                         than best-fit sphere
              Colors: RED = higher elevation / BLUE = lower elevation
              Elevation maps not used in clinic
       o Topography classification system
              Used to classify topographies as round, oval, symmetric bowtie, asymmetric bowtie, irregular
              Not used in clinic; just look at the picture
       o Scaling: changes appearance of topographic map
              Smaller scale intervals: more details of what is actually going on; picks up minor distortions
              Larger scale intervals: less details; smooth
       o Fixation
              Alignment + focus more critical if capture distance is shorter
              Keratoconic map can be changed to resemble WTR astigmatism just by changing pt’s fixation
              Most current instruments are foolproof  decide when to take image and auto-focus image
-   Topography display possibilities
       o Power maps
       o Difference maps: compares how a msmt changed from baseline msmt
              Pre- & post-LASIK difference map
                       Difference map should be centered, smooth, round or oval
                       Difference map shows you difference in Ks (how many diopters of curvature/myopia
                         was removed during the surgery)
                       Not extremely useful bc cannot reverse the surgery
                       More useful for ortho-K because it is reversible
              Pre- & post-RK difference map
                       RK: incisions made in cornea to flatten cornea + reduce myopia
                       Good results initially
                       Over time, incisions stressed + stretched causing large refractive errors to return
                             o More stress superiorly indicates eye lid pressure causes stretching
                       Diurnal pressure w/in eye changes refraction significantly throughout the day
              Post-corneal scar difference map
                       Flattening is seen at the wound
       o Screening indices: likelihood of keratoconus
              Helpful to know if patient has keratoconus for care reasons + refractive surgery reasons
                       Refractive surgery + keratoconus are both a corneal thinning condition
              Rabinowitz: compared the difference btwn a point a few mm superior to central cornea & a
                 few mm inferior to central cornea to develop a template for keratoconus
                       Keratoconic activity occurs centrally and inferior to central
                       Pt’s cornea is steeper inferiorly than same area superior to central
       o Contact lens design
              Topography image will give you different curvatures at different points away from center
              FP does not always show what the picture shows
              More practical to go with what the human eye shows than what the proposed picture shows
-   Fluorescein pattern > topography for assessing CL fit
       o CL doesn’t ever touch cornea due to tear layer
       o NaFl fills in gaps btwn cornea + RGP to assess fitting relationship btwn lens and cornea
              If lens too steep  tear layer thick in center  green
              If lens too flat  tear layer thin in center black
  -   INDICATIONS for topography
         o All patients  not necessary; might do as baseline or wow-factor
         o All CL patients  yes in private practice
                All RGP patients  yes in private practice
                All soft CL pts  not as necessary; indicated for patients with thick lenses due to high Rx
                      Thick lenses = localized swelling + some distortion
         o Orthokeratology  absolutely necessary
         o Refractive surgery patients  useful in pre- and post- difference maps
         o Diseased cornea  depends; useful for anterior surface dz
         o Irregular/distorted corneas  necessary (fit using FP but helpful to have a baseline)
  -   WHAT’S NEW in topography?
         o Price reductions + smaller sizes
                Competition has driven the price down
                Smaller spaces have decrease the instrument size
                Technology hasn’t changed much bc there is no money to pump into new designs
                Mature market  not sold often bc there’s one in the office already
         o New data capture techniques
                Most instruments still use rings
                ORBSCAN: topography + scanning device (gold standard)
                      Regular topography images, posterior corneal contour, corneal thickness
                      Corneal thickness useful for LASIK potentials + keratoconus patients
                            o Keratoconus pts at risk of corneal rupture
         o Hand-held instruments + instruments that attach to laptop
         o Foolproof  don’t require expertise to run
         o Limbus-to-limbus measurements
                Important for fitting scleral lenses
                Not many people need/wear scleral lenses

 VERTEXING

  -   Vertexing = conversion btwn powers at spectacle and corneal plane using basic effectivity formula
         o Fc = Fs / (1 – dFs) where Fc is pwr at corneal plane, Fs is pwr at spectacle plane, and d is vertex distance in m
         o Phoropter vertex distance = 13mm for calculations
         o Each meridian of cylindrical component must be vertexed separately
  -   Indication: vertex subjective refraction or over refraction if power ≥ +4.00D
         o EX: -4.00 becomes -3.75; +4.00 becomes +4.25
         o EX: -10.00 becomes -8.87 (higher rx’s have bigger difference in spec vs. CL powers)
  -   Rounding: to nearest 0.125D step (prior to placing in spherocylinder format)
  -   Example
         o SRx: -5.00 -4.00 x180 with d = 12mm
                     Power in 180 = -5.00  vertex
                     Power in 90 = -9.00  vertex
          o   v180: Fc = (-5.00) / (1 – (0.012)(-5.00)) = -4.7169 = -4.75
          o   v90: Fc = (-9.00) / (1 – (0.012)(-9.00)) = -8.1227 = -8.12
          o   SRv: -4.75 -3.37 x180

 TEAR LENS CALCULATIONS

  -   Review of corneal eccentricity
         o SPHERE: same radius of curvature  e = 0
                 Spherical surfaces described by only 1 variable: radius of curvature
                 If cornea was spherical, GP alignment BC = cornea’s K-value and alignment is achieved with
                    plano tear lens
         o ASPHERIC: radius of curvature continually changes as you move along the curve  eccentricity =
            rate of flattening from center to periphery  e ≠ 0
                 Eccentricity varies: e < 1 (ellipse: central cornea), e = 1 (parabola), e > 1 (hyperbola:
                    peripheral cornea)
                   Elliptical surfaces are described by 2 independent variables: radius of curvature + eccentricity
                   Cornea is an ellipse with e ~ 0.5  GP alignment BC ≠ cornea’s K-value and alignment is
                    achieved w/ -0.75 tear lens
  -   Fitting RGPs
          o RGP is spherical (e = 0) but CORNEA is elliptical (e ~ 0.5)
                 Radius of curvature gets larger and cornea gets flatter as we move from center to periphery
                 To perfectly contour the cornea, you would need a GP that also changed BC
                 GP lenses have a constant BC  cannot contour cornea perfectly
          o GOAL: contour RGP to wide surface to create an even plane of bearing
                 IDEAL: small lens just fitting over apex of cornea would need same BC as central cornea
                 HOWEVER: typical RGPs OZ is greater than 3mm central area that K-readings are taken from
                 SO: larger lens/OZ needs flatter BC than curvature of apex
          o When spherical CL rests on aspheric cornea, 1 or 2 or 3 points of touch result
                 3 points of touch = apical alignment
                 2 points of touch = apical clearance (RGP BC steeper than K)
                 1 point of touch = apical touch (RGP BC flatter than K)
  -   Tear lens
          o To achieve alignment, BC must be 0.75D flatter than central K or Kmean
                 Assuming e = 0.5 & OZ = 7.6mm, TL needed = -0.69  round up to -0.75
          o Tear lens: tear layer formed by posterior BC surface of the CL and anterior surface of the cornea
                 TL = BC - K
                 Toric corneas: find TL in each meridian and average
                 Steeper corneas need more (+) TL  TL values approach plo
                 Flatter corneas need more (-) TL  TL values get more negative
                 Larger OZ lenses need bigger TL values
          o TL is created when RGP BC ≠ corneal K
                 Negative TL created since BC flatter than K
          o TL contributes power to the system
                 Negative TL contributes (-) power to the system  CLP more (+) than spectacle power bc TL
                   adds addn’l minus needed
                 Examples
                           K = 43.00, SR = -3.00DS  RGP BC = 42.75 for AA, CLP = -2.25DS
                           K = 44.50/43.75 x180  Kmean = 44.12  RGP BC = 43.37 for AA
         oFluorescein patterns
               GREEN: thicker tear layer / BLACK: thinner tear layer (not enough tears to fluoresce)
               Apical touch: black area in middle of FP surrounded by plenty of green  tear layer thin
                 where RGP touches apex of cornea
               Apical clearance: green in middle of FP surrounded by black ring  tear layer thicker in
                 center of cornea bc RGP vaults apex and thicker more peripherally where RGP touches
  -   Summary
        o TL: area btwn cornea + BC of lens that contributes power to system
        o Apical alignment: -0.75 TL  BC 0.75D flatter than K (CL contours the cornea)
        o Apical clearance: TL more (+) than -0.75  BC steeper than K (CL vaults the cornea)
        o Apical touch: TL more (-) than -0.75  BC flatter than K (CL touches apex of cornea)
                   CL doesn’t actually touch but space btwn cornea + lens can be as small as 5  not large enough to detect on
                    fluorescein patterns

 GP PARAMETER PRESCRIBING

  -   GPs have been called hard CL, rigid CL, and RGPs
  -   BASE CURVE: km + FP
         o BC should be 0.75 flatter than mean k’metry reading
         o FP should show apical alignment
         o Should have w/in 0.37D agreement btwn Km and FP
                 More than 0.37D difference indicates bad data
       o   If Km and FP don’t agree:
                K’meter not calibrated
                Incorrect K-reading
                GP BC analyzed incorrectly
                FP interpreted wrong (common error)
                          FP is higher level piece of information  more difficult to interpret; requires expertise
              Patient’s cornea doesn’t have eccentricity of 0.5
       o Use most compelling data
              FP > K data if you are confident in FP interpretation
              K data > FP if you are NOT confident in FP interpretation
-   CLP: contact lens power
       o CLP = DxCL CLP + OREDS
              Power only applicable for the same BC as DxCL power
              Over-refraction
                          CYLINDER: EDS cylinder if <0.75; include cylinder in toric lens if >0.75
                          VERTEXING: only vertex ORs > 4.00D
                 Example: 43.00 BC / -2.00DS
                          DxCL = -2.00DS and OR = +0.50DS
                          CLP = -1.50 @ 43.00 BC only
       o   Altering BC: must add (+) or (-) power by same amount that BC is changed
               SAM: steep add minus (if BC gets steeper, add (-) to power)
               FAP: flatter add plus (if BC gets flatter, add (+) to power)
               Examples:
                      43.00 BC / -2.00DS; FP = AT by 0.37 (BC too flat by 0.37)
                               o    Steepen BC by 0.37 to achieve AA
                               o    SAM: add -0.37 to CLP bc we made TL more (+) by making BC steeper
                               o    Final parameters: 43.37 / -1.87
                          44.00   BC / -5.00 DS; FP = AC by 0.25 (BC too steep by 0.25)
                               o    DxCL = -5.00
                               o    OR = +1.00 -0.25 x090  EDS = +0.875
                               o    CLP = -4.12 @ 44.00
                               o    Flatten BC by 0.25 to achieve AA  FAP: add +0.25 to CLP
                               o    Final parameters: 43.75 / -3.87
       o   CLP should be correlated to SR
               Predicted CLP should be w/in 0.37D of the actual CLP used on patient
                     Predicted CLP = SRv EDS power + 0.75 to allow for -0.75 TL
               If predicted + actual CLP do not agree:
                     SR or OR are overminused
                     Incorrect lens analysis of BC or CLP
               Example: SR = -2.50 -0.75 x180
                          EDS = -2.87
                          Predicted CLP = -2.12
                          Actual CLP = -1.75 to -2.50
-   OAD: overall diameter
      o Factors to consider:
              Palpebral fissure + fitting philosophies
                     Intralimbal/intrapalpebral: fit CL btwn the 2 limbals, btwn the lids
                     Lid-attached: fit CL underneath upper eyelid
              Lid apposition to globe: flaccid vs. tight
              Diameter of habitual CL: patient will notice if OAD s
              Lens position on cornea: does CL sit relatively centered
              OZ size: look at where OZ settles on the cornea
      o Fitting philosophies: (1) Measure VID or (2) slap on a 9.2 and decide if you need bigger or smaller
      o Default OAD = 9.2mm
              Smaller: covers less corneal surface; more O2 availability to cornea; less mass
                          O2 permeable lens materials allow us to go larger on OAD
                 Larger: enhances initial comfort; enhances centration (?); less interaction btwn lid + lens edge
                         BUT larger lens tend to be thicker and might move COG further from eye
       o   If change   OAD, must change OZD by same proportion (keep 1.60 difference btwn OAD and OZD)
-   OZD: optic zone diameter
       o Factors to consider:
              Pupil size: larger pupil will require larger OZ
              Lens position on the cornea: if lens centers well on cornea, smaller OZ can be used
              OZ of habitual lens: if patient is used to a smaller OZ, don’t  it by much
                     OZ used to be smaller to  O2 availability to cornea
              Adaptation to glare: smaller OZ will increase glare
       o Default OZD = 7.6
              Too small: glare (esp at night)
              Too large: poorer tear exchange in pts w/ higher O2 needs
-   SCr: secondary curve radius
       o Default = 0.7mm flatter than BCr
              SCr is flatter than BCr bc we calculate out edge lift (edge of CL comes away from peripheral
                 cornea by 0.1mm)
              Example: BC = 44.00D (7.67mm)  SCr = 8.37mm
                         Flatter: 8.5 v. steeper: 8.25
       o   When is SCr NOT 0.7mm flatter?
               FP shows AC  SCr must be flattened
               Lens fit is too tight  SCr must be flatter
                      Fitting a steep lens wrt lid attachment  SCr must be flattened
                             o Fitting steep lens req’s flat peripheral curve system to maintain tear exchange
                             o If peripheral curve system is too steep, the fit is too tight
               Lens fit is too loose  SCr must be steeper
                      Labs typically make SCr 1.00mm flatter  too loose for SCCO fitting philosophy
-   TCr: third curve radius
       o Default = 12.00mm (range from 10.00 – 13.00)
       o TCr is NOT a fitting curve, rather an edge treatment on posterior surface
               TCr is the start of the posterior edge
-   TCw: third curve width
       o Default = 0.2mm
       o TCw is kept narrow bc third curve NOT used as a fitting curve
               TCw is wider if used as fitting curve
               Wider TCw will allow more mvmt of the lens
-   CT: center thickness
       o CT = [(0.023) x (CLP)] + 0.19
       o Effect of power on CT
               CLP = plo  CT = 0.19
               CLP = (–)  CT = thinner than 0.19
               CLP = (+)  CT = thicker than 0.19 (+ lenses have 2x CT compared to – lenses)
       o Arbitrary SCCO minimum CT = 0.12mm
               0.12mm is thinnest we can make lens
               High minus lenses calculated to be thinner than 0.12 are cut off at 0.12mm
               Typically tell lab to make lens ATAP: as thin as possible
       o When is CT thicker than it needs to be?
               Flexure
                      Tight lids + thin lens can cause lens to bend + cause OR cylinder power to 
                      Extra thickness prevents lens from bending
               Different OAD / SCr
                      Larger lenses must have thicker centers/edges  CT = 0.023(CLP) + 0.21
                      Smaller lenses must have thinner centers/edges  CT = 0.023(CLP) + 0.17
       o Edge thickness is more important but hard to measure
               Typically order CT to give us a 0.10 edge thickness (SC gives clearance of 0.10)
                      Edge thickness must be ~0.10mm to minimize edge chipping
               Verify lens when it comes back from lab
       o Remember: changing CT by a certain amount changes edge thickness by same amount
               CT + ET have 1:1 relationship
-   EDGE CONTOUR
       o Edge = comfort
       o Most comfortable edge is (+) edge
               (+) edge tucks back twd eye  less discomfort
                      Edges bend back twd cornea by making edge thin & SC close to the cornea
               (+) edge makes it easier for lids to slide over lens/cornea  prevents dryness + 3-9 staining
       o When is edge not a (+)?
               Fitting lid attachment  want edge to be contoured like a minus edge so there is something
                for lid to hold on to
               Difficult removing lens  round edge easier to remove in pts w/ flaccid lids
               Prescribing lenticular lens design  edge thinner for high (-) pts; center thinner for high (+)
               (+) edge is uncomfortable  re-edge/round the edge
-   TINT
       o Default = #1 blue
               Easy to find in case and on top of counter
               Doesn’t  appearance of eye (doesn’t change eye color)
       o gReen used for right lens; bLue used for left lens
       o Lenses are smaller than the cornea  move around upon blinking
       o Patients really care about the tint  tell pt the tint before prescribing (gives pt a chance to tell you
         they want something else)
-   UV INHIBITOR
       o Does NOT replace sunglasses  augments the function of sunglasses
       o Does NOT help pingueculae bc does not cover conjunctiva
       o All aphakic patients should have UV inhibitor to protect retina
       o UV inhibitor protects lens, macula, retina
-   PLASMA-TREATED GPs
       o Plasma treatment is a surface treatment, not a coating
       o GP lens placed in vacuum  low temperature ionized gases remove organic contaminants  surface
         becomes hydrophilic  wetting angle decreases
       o Treatment loses effect in 6 months + sooner if lenses are polished or used w/ abrasive cleaners
-   MATERIAL
       o Default = FluoroPerm 60 (Dk value = 60)
       o Higher Dk values are used for special cases
               Extended wear or flexible wear (Dk > 100)
               Scleral CL pts
               Overnight ortho-K
               Compromised cornea (PKP, keratoplasty)  lack of O2 can cause vessel growth + graph rejection
               Wettability
       o Pros/cons
               Higher Dk: lens too gentle; can warp with heat/modification
               Lower Dk: better wettability; easier to modify
-   BLEND
       o Blending typically not done at SCCO
               Non-blended junctions do NOT scrape corneal epithelium  no real advantage
       o Blending done for:
               Keratoconus pts: cut a curve ½-way btwn BC and SCr
               Patients w/ sx of glare or flare: blend jxn btwn OZ and SCr to make more aspheric
       o Blends
               Light (A, #1) = 0.1mm wide
               Medium (B, #2) = 0.2mm wide
               Heavy (C, #3) = 0.3mm wide
       o Make sure to specify size you want OZ after the blend  lose some of OZ size when blending
  -   GP PARAMETER ORDER
         o ORDER (n=11): material / BC / CLP / OAD / OZD / SCr / TCr / TCw / CT / edge / tint
         o EXAMPLE: FP 60 / 44.00(7.67) / -5.00 / 9.2 / 7.6 / 8.37 / 12.00 / 0.2 / 0.12 / (+) / #1 blue
         o NOTES
               BC = 0.75 flatter than K
               CLP = SRv, EDS, OR
               SCr = 0.7 flatter than BCr
               TC minimum = 0.12mm
         o ROUNDING
               Radii of curvature (BC, SCr, TCr): hundredths (0.00)
               Diameters (OAD, OZD, TCw): tenths (0.0)

 DIAGNOSTIC CL SELECTION

  -   BASE CURVE
        o Important K-values: (1) K (2) Kmean
        o Base curve: 0.75D flatter than Kmean
        o Fudge FLAT  always fudge toward flatter BC
               Flatter BC will yield apical touch fluorescein pattern  AT easier to quantify
               If your choices are btwn a lens that is 0.125D steeper and a lens that is 1-2D flatter, choose
                 the 0.125D steeper lens
               Example: Kmean = 43.12  BC = 42.37 (AA)
                           42.37 is not in your trial set
                           42.00 v. 42.75  choose the flatter BC (42.00)
  -   CONTACT LENS POWER
        o Add 0.75 to the SRv EDS power to achieve a tear lens of -0.75
              Example: SRv = -2.75 -0.75 x180  EDS = -3.12  CLP = -2.37
        o Fudge PLUS  always fudge toward more (+) CLP
              More plus CLP allows for a minus OR
                    More plus CLP yields 20/40ish VA  pt will accept the challenge w/ (-) bc VA will
                       increase  prevents overminus
                    More plus CLP with a minus OR will control pt’s accommodation
              More minus CLP yields 20/15 VA  pt won’t accept the challenge w/ (+) during OR  pt
                overminused
              Example: CLP = -2.37  FIT -1.50  minus OR (patient will desire 0.87 addn’l minus)

								
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