1. Clinical Function and Practical Optics
Contact Lenses
Clinical Function and Practical Optics

2. Outline
Basic optics
Soft Lenses
RGP Lenses
Hybrid
Lens Calculations

3. An optical interlude……
The corneal reflex is brighter in a person with contacts- WHY?
The corneal reflex with a contact lens is a superimposition of reflections from the air/tear interface, the anterior and posterior tear/lens interfaces and the tear/corneal reflex. Because RGP lenses have a higher n, the reflex off of these will be greater (2.5% versus 2.3%).
R={n’-n/n’+n}2

4. Air/tear interface
Tear/lens interface
For practical purposes
think of each layer as
a separate lens in air
AIR
CONTACT LENS
CORNEA
AQUEOUS
Prelens tear film
Tear/epithelial interface
Post-lens tear film

5. Nuances of CL power CLS are treated as thick lenses
Anterior and posterior surfaces
Center thickness
Index of refraction
Saggital depth/extreme curvatures

6. Contacts are THICK LENSES (!)
fFVP
fBVP n F n’ F’ t A1 A2 f fT
f’T
Tx this way b/c curves are so great….

7. CL Optics – Effective Power
Remember to consider vertex distance for all powers > +/ D
Myopes need less power in cls
Hyperopes need more power in cls
Also have to vertex the cylinder component
Use optical crosses

8. Vertex of toric Rx’s Rx: +4.00+3.00X180
Fsp= K/ (1+dK)-use this to convert to spectacles
Where d= vertex distance in meters
K= power at the cornea
K= Fsp/1-dFsp- use this to convert from specs to contacts

9. CL Optics- changes in accommodative demand
Hyperopes have more accommodative demand with glasses
Pre-presbyopes love cls! (more plus with CLS)
Myopes have less accommodative demand with glasses
Pre-presbyopes do not do well (more minus to overcome with CLS)

10. CL optics –Changes in accommodative convergence
Myopes = increased accommodation with cls , thus will use MORE accommodative convergence
An esophoric myope will have to use more NEGATIVE fusional vergence
Hyperopes= decreased accom with cls, will use LESS accommodative convergence
An exophoric hyperope will have to use more POSITIVE fusional vergence

11. CL Optics- Prismatic effects
Correctly fitted cls are always centered on the eye, where glasses induce prism
Plus lenses induce BO prism
An esophoric hyperope is at a disadvantage with cls b/c
there is no prismatic effect to counterbalance

12. CL Optics- Prismatic Effects
Minus lenses induce BI prism
An exophoric myope is at disadvantage with cls b/c there is no prism
The lack of prism effect is a benefit for anisometropes.

13. Prescribing prism in cls
BD prism reduces rotation
-toric lenses
- bifocal lenses
The lens thickness is increased toward the base w/o alteration in surface curvature, thus power is more plus towards the base
Exploited in bifocal designs (rgps)

14. CL Optics- cl/eye system
Lenses can alter the shape of the cornea (warpage)
Cornea can alter the shape of lenses (soft cls)
RGP lenses can mask corneal astigmatism
glasses don't do any of this

15. CL optics- Magnification Effects
Occurs because the cl is touching the cornea
The power factor of SM formula includes the distance from lens to entrance pupil, changing this to zero causes a change in magnification
Myope will get larger retinal image
Hyperope will get smaller retinal image
The opposite of what happens in glasses!

16. Magnification Effects
Mag=Original power/vertex power
What is change in RIS switching from glasses to cls for -5.00D at 12mm?
-5/1-.012(5)=-4.72 (vertex)
-5/-4.72=1.06
%change is 6% larger with cl.

17. CL optics –aberrations and field of view
CL wearers have greater field of view
No glasses rim!
Spectacles suffer from oblique astigmatism, curvature of field and distortion
CLS eliminate OA and CF because the lenses are always centered
CLS eliminate D because they are directly on the eye
Wavefront guided contacts are available now!

18. Visual Optics – corneal transparency
The air/tear interface has the most refractive power because of change in index, although optically the TF has no power
The stroma is optically significant
The tear film, epithelium, Bowman’s and Decemet’s are optically insignificant b/c of their CT and parallel surfaces

19. Corneal Transparency
200 lamellar sheets arranged in parallel, stacked in an anteroposterior direction.
Regular spacing 65nm apart in a lattice
Lattice theory states that the spacing between collagen is sufficiently small that light scattered by individual fibers is mutually destroyed by destructive interference

20. Loss of Corneal Transparency d/t CLS
Edema
Infiltrates
Microcysts
Vacuoles
Calcium
Lipid
Dry spots/ dellen
Scarring
Salzmann’s nodules
Vascularization
Dimple Veil
Inclusions (epi)
Trauma
Mechanical
Chemical
Toxic
Osmotic
Hypoxic

21. Alterations to eye optics
Myopic creep –unexplained
edema (rgp < scl)
Steeper k’s and altered pachy readings
Lowered stromal n (more +)
endothelium
Change in anterior corneal curvature
Spectacle blur

22. Lens flexure and warpage
CLS in vivo variably conform to the cornea
Function of material, K’s and the lens/cornea fitting relationship

23. Flexure – soft lenses The most extreme example
As cornea steepens, net minus power increases for all lenses
Topography shows inferior steepening
Soft lenses cannot be assumed to provide the labeled power
OR varies depending on flexure
Dehydration raises n, increasing power
Steepens with dryness, increasing power
Cannot confirm power in office
Tear fluid tonicity/pH/temperature/heat/humidity

24. Flexure RGPs Flex to the steepest meridian
Wtr cornea, lens steepens in the vertical and slightly flattens in the horizontal.
Measured as toricity with over-K’s
Calculated effect is to lessen the minus power of the LL in the steepest meridian

25. Outline Optics of contacts Soft Lenses RGP Lenses Bifocal Lenses
Fitting procedures

26. Types of soft lenses Spherical Toric truncated prism ballast
thin ballast
Aphakic
Extended wear
Bifocal
Bandage lenses
High dK/l
Different polymers
Ionic high water
Ionic low water
Non-ionic high water
Non-ionic low water
Newer silicone FDA proposal:
Same as above plus sihi designation

27. Types of soft lenses Conventional (sphere/toric)
Keep for a year
Bifocals
Higher powers
Disposable (sphere/toric/bifocals)
Quarterly
Monthly
Weekly or Two week
Daily

28. Some optical considerations
Fitted flatter than the cornea
Parallels the periphery and drapes over apex
1 or 2 radii of curvature (base curves) only per type of lens are generally available

29. Why/Why not soft lenses?
Advantages
Comfortable
Available
Easy to fit
Good for social use
Better for internal cyl
Disadvantages
More risk of infection
Dry out
Prone to rotation (toric)
Get dirty quickly
Noncompliance
Poor Oxygenation (older lenses)
GPC

30. Bifocals – soft lenses modified monovision D Simultaneous vision D N N
aspheric
Simultaneous vision

31. Soft CL problem Patient’s Rx is -4.00 -1.00 x 090 all cyl is corneal
Patient cannot afford toric lenses- what do you prescribe?

32. Soft CL problem Patient’s Rx: -6.50+2.00x045
K readings:
-What power CL?
- What bc? (choice is 8.7 or 8.2)
- Say CL OR is – 0.50 x 040 what do
you do?
- what if lens is rotating 20 degrees nasal
(lars)
- what if the OR is x 090?

33. Outline Optics of contacts Soft Lenses RGP Lenses Bifocal Lenses
Fitting procedures

34. Rigid Gas Permeable Sphere Toric Bifocals Bitoric Front surface toric
Back surface toric
Bifocals
Aspheric
Segmented
“Pinhole”

35. Anatomy of RGP CL

36. RGP why/why not? Disadvantages Advantages Adaptation Chair time
More Oxygen
Cheaper
Less surface area
Better for dry eye
Masks corneal cyl
Compliance
Rare sleeping
Rare infection
Superior optics
Disadvantages
Adaptation
Chair time
Misconception

37. Lacrimal Lens An rgp interacts with the tears
Has less effect on the curvature of the cornea (unless poor fit)
As long as rgp maintains it’s bc, the interface between the lens and the tears is spherical (elimiates astigmatism)
Rgp does not affect internal astigmatism!!

38. Lacrimal Lens Steeper contact creates a + power LL
Flatter contact creates a – power LL
For every BC change, and equal and opposite change of power is needed .05mm=0.25D

39. Lacrimal Lens Problem CL parameters: 7.50/-6.00/95
Need to steepen BC .5D what is new power?
Need to flatten BC by .75D what is new power?

40. Fitting Philosophies Lid Attachment Interpalpebral
Fit is under the lid
Moves with blink
More comfortable
Less GPC
Interpalpebral
Wide eyes
Must have good recovery

41. Fluorescein Patterns too flat wtr alignment too steep
                                                                                                                                                          
Alignment pattern Even pattern centrally with slightly greater clearance peripherally
Spherical BC on 3D WTR astigmatic cornea Horizontal bearing and excessive vertical pooling is observed
                                                                                                                                                          
Alignment pattern Even pattern centrally with slightly greater clearance peripherally
Spherical BC on 3D WTR astigmatic cornea Horizontal bearing and excessive vertical pooling is observed
                                                                                                                                                          
Alignment pattern Even pattern centrally with slightly greater clearance peripherally
Spherical BC on 3D WTR astigmatic cornea Horizontal bearing and excessive vertical pooling is observed
      Next  >> More Fluorescein Patterns:     1     2     3 Making Lens Design Changes Fitting Pearls
      Next  >> More Fluorescein Patterns:     1     2     3 Making Lens Design Changes Fitting Pearls
                                                                                                                                                          
2D flatter than "K" Excessive apical bearing, excessive inferior edge standoff
2D steeper than "K" Excessive peripheral sealoff
1D flatter than "K" Excessive apical bearing, excessive inferior edge standoff
1D steeper than "K" Mild peripheral sealoff
Alignment pattern Even pattern centrally with slightly greater clearance peripherally
Spherical BC on 3D WTR astigmatic cornea Horizontal bearing and excessive vertical pooling is observed
                                                                                                                                                          
2D flatter than "K" Excessive apical bearing, excessive inferior edge standoff
2D steeper than "K" Excessive peripheral sealoff
1D flatter than "K" Excessive apical bearing, excessive inferior edge standoff
1D steeper than "K" Mild peripheral sealoff
Alignment pattern Even pattern centrally with slightly greater clearance peripherally
Spherical BC on 3D WTR astigmatic cornea Horizontal bearing and excessive vertical pooling is observed
Fluorescein Patterns
too flat
wtr
too steep
alignment

42.     
Observe apical clearance and insufficient peripheral clearance
On K 95/84
More alignmet with greater
Peripheral clearance
On K 95/76
Good edge, bit narrow
Bicurve design with a 10.0mm PCR; .5mm wide
An increase in edge clearance can be observed
Bicurve design with a 10.0mm PCR; 1.0mm wide
Insufficient edge clearance can be observed
Bicurve design with a 9.0mm PCR; .8mm wide
Observe greater clearance
12mm PCR, .8mm wide

43. RGP Problem
Rx: -8.50DS K:
What power?
What BC?

44. RGP Problem Rx: -3.00 + 6.00 X 090 K: 42.50/47.75@180 What bc?
What power?

45. Therapeutic RGP fits Use topography and SLE to assess K
Goal is to fill in irregular part with tears
Pick steepest K as starting point
Just fit the lens and then OR to get power

46. Bifocal RGP n aspheric aspheric d n
aspheric
aspheric d

47. Bifocal RGP fitting Many different brands Proprietary v custom
Fitting is specific to brand/ type
Many potential changes
Set realistic expectations
Acknowlegde time investment

50. THE BASICS Details of previous cl wear What are the problems?
When/how do you wear them?
Careful refraction (vertex over +/-4D)
Keratometry or topography
Examination of the cornea, lids, lashes
Dryness…..?
Pupil size (dim/light)
Palpebral Apeture/ characteristics –tight/loose etc.
Iris Diamter

51. The Basics Technicians are key to profitability
Insertion/removal training
Lens hygiene teaching
Patient follow up- phone calls
FOLLOW UP IS IMPERATIVE…

52. Seriously? Yes, pretty common so check!