Contact Lenses

1. Contact Lenses Clinical Function and Practical Optics

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

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 CORNEA CONTACT LENS AIR 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 (!) H H’ fFVP fBVP n F’ n’ F t A1 A2 f f’T fT Tx this way b/c curves are so great….

7. CL Optics – Effective Power • Remember to consider vertex distance for all powers > +/- 4.00 D • Myopes need less power in cls • Hyperopes need more power in cls • Also have to vertex the cylinder component • Use optical crosses www.rgpli.org

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

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

26. 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 Types of soft lenses

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 http://oculuseyehospital.com/images/toric.jpg

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. 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 Why/Why not soft lenses?

30. N D D I N Bifocals – soft lenses modified monovision D 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: 45.00/46.00@135 -What power CL? - What bc? (choice is 8.7 or 8.2) - Say CL OR is -1.00 – 0.50 x 040 what do you do? - what if lens is rotating 20 degrees nasal (lars) - what if the OR is -1.00-1.00 x 090?

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

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

35. Anatomy of RGP CL

36. Advantages 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 RGP why/why not?

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 • 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 too steep alignment

42. Observe apical clearance andinsufficient peripheral clearance More alignmet with greater Peripheral clearance On K 95/84 On K 95/76 Bicurve design with a10.0mm PCR; .5mm wide Bicurve design with a10.0mm PCR; 1.0mm wide An increase in edge clearancecan be observed Good edge, bit narrow Bicurve design with a9.0mm PCR; .8mm wide 12mm PCR, .8mm wide Insufficient edge clearancecan be observed Observe greater clearance

43. RGP Problem • Rx: -8.50DS • K: 44.50/46.00@78 • 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 http://www.hroptical.com/images/bifocal-contact-design.jpg n aspheric aspheric d

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

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