Contact Lens Midterms PDF

CONTACT LENS 1 - LECTURE Bernadeth Mea M. Casao | OD4A MODULE 3.1 ❖ Demonstrated the principal of the telescope, not the contact lens I. HISTORY...

CONTACT LENS 1 - LECTURE Bernadeth Mea M. Casao | OD4A MODULE 3.1 ❖ Demonstrated the principal of the telescope, not the contact lens I. HISTORY OF CONTACT LENSES ❖ Rene Descarte’s water filled tube used to ❖ The early “contributors” were not describing demonstrate the principle of the telescope contact lens applications by enlarging the retinal image ❖ Contributors – created principles but not created the contact lens PHILIP DE LA HIRE (1685) ❖ Attempted to show how various optical LEONARDO DA VINCI (1508) phenomena in myopes were corrected with spectacle glasses applied to the cornea ❖ Telescopic or image magnifying device ❖ Not a contact lens THOMAS YOUNG (1801) ❖ Used a water filled tube to study the accommodative process ❖ Not a contact lens JOHN HERSCHEL (1801) ❖ Sketches of a SCHEMATIC EYE ❖ Discussed the correction of IRREGULAR ❖ Explained the behavior of lights or how the CORNEA by: light enters the eye o Applying in contact lens with the ❖ Describes the mechanism of image formation surface of the eye some transparent from the cornea to the optic nerve animal jelly contained in a spherical o Which Rene Descartes supported capsule of glass; or whether an actual ❖ INDEX OF REFRACTION VARIATION = IMAGE mold of the cornea might not be taken CHANGE and impressed on some transparent ❖ Some drawings may represent the idea of a medium contact lens ❖ Describe the following: o Need to correct irregular cornea RENE DESCARTES (1637) o Contact with the eye o Eye impressions o Need for a transparent medium o Combining a soft material with rigid overlying material WILLIAM WHITE COOPER (1859) ❖ Recommended the insertion of a “glass mask” filling the fornices, in order to prevent formation of symblepharon following lime burns of the eye ❖ Understood that better vision could be ❖ Possibly the first to suggest use of a achieved by enlarging the retinal image therapeutic contact appliance o inspired by Leonardo Da Vinci ❖ Sketched an elongated tube filled with water placed against eyeball  CONTACT LENS 1 - LECTURE Bernadeth Mea M. Casao | OD4A XAVIER GALEXOWSKI (1886) DE SULZER (1892) ❖ First person to apply a therapeutic contact ❖ Observed that a ‘fluid lens’ acted as an device artificial refracting medium ❖ Used gelatin square soaked in mercury ❖ SHELL PARAMETERS chloride o Diameter: 14.00 – 15.50 mm ❖ first drug delivery system o Corneal Radius: 8.00 mm o Scleral Radius: 12.00 mm II. EMERGENCE OF CONTACT LENSES HENRY R DOR (1892) ADOLF E FICK (1888) ❖ First to replace the glucose solution used by ❖ First optical lens covered entire cornea, later Fick with normal saline added a scleral flange: o Diameter: 14.00 mm THOMAS LOHNSTEIN (1896) o Scleral Flange: 3.00 mm ❖ Bilateral keratoconic o Corneal Curvature: 8.80 mm ❖ Produced a spectacle with saline – filled lens o Scleral Radius: 14.00 mm cups – ‘water spectacles’ o Tint: Black with Stenopaic Slit ❖ Cups could be worn for 1 – 1.5 hours ❖ Attempted to develop a contact lens ❖ Device was known as the hydrodiascope corrective device o First contacts lens is scleral and 1896 - 1912 made up of glass ❖ Precorneal space filled with 2% glucose ❖ During this period, very little progress in the ❖ First published paper on a practical clinical contact lens field attempt to correct visual problems ❖ ELSCHNIG’s criticism of Fick: ❖ Glass ‘contractbrillen’ on irregular corneal o Contact lenses only usable in primary surfaces position ❖ Shells were fitted to SIX patients with highly o Difficult to manufacture irregular corneal surfaces due to scarring o difficult to insert o cause mechanical irritation EUGENE KULT (1888) III. THE PIONEERING CONTACT LENS COMPANIES ❖ Devised the first contact lens for keratoconus ❖ Wanted to remodel corneal curvature by FA MULLER & SONS (1887) using glass contact shells as splints ❖ Farther: Friedrich Anton Muller (1862-1939) ❖ Resulted in substantial visual improvement ❖ Son: Albert Carl Muller (1864 – 1923) ❖ Artificial eye makers in Wiesbaden, Germany AUGUST MULLER (1889) ❖ In 1887, they fitted a protective glass shell to ❖ Used the term “HORNHAUTLINSEN” or the eye of a patient who had partial lid corneal lens removal ❖ Noted that a steeper corneal radius needed a ❖ Made lenses from brown glass larger dioptric correction which he attributed ❖ Very regular curvature, no sharp edges at the to the lacrimal meniscus corneo – scleral junction ❖ Experienced and described subjective and ❖ Generally better tolerated than lathe cut objective signs of corneal oedema lenses ❖ Observed that tears had a metabolic function ❖ Designed a lens for ptosis correction ❖ Made unsuccessful attempts to take mold of living eyes  CONTACT LENS 1 - LECTURE Bernadeth Mea M. Casao | OD4A CARL ZEISS OF JENA (1911) MULLER – WELT CONTACT LINSEN (1947) ❖ Muller – Welt Contact Linsen firm was founded in 1947, Stuttgart by Adolf A Muller – Welt ❖ Initially, devoted to the advancement of glass, fluid lens sclera contact lenses ❖ Established a Toronto and Detroit laboratory in 1949 IV. CONTACT LENS MATERIALS DEVELOPMENTS ROHM & HAAS (1930s) ❖ Produced an acrylic resin called Plexiglass for ❖ Made lathe cut lenses from molds aviation industry ❖ Lathe cutting resulted in a better optical *the DEVELOPMENT OF PMMA* performance ❖ First commercially available trials lenses J CRAWFORD & R HILL (1934) ❖ Complete trial lens set contained 21 lenses ❖ Developed and patented the material poly ❖ Lenses were afocal (methyl methacrylate) (PMMA) ❖ Fitting determined by fluorescein and white o First plastic RGP material light ❖ Trade name of Perspex (Latin for transparent) ❖ PARAMETERS OF FIRST SET o Diameter: 20.00 mm PMMA FOR SCLERAL CONTACT LENSES o Scleral radius: 12.00 mm ❖ PMMA rapidly became the material of choice o Corneal Radius: 6.50 mm, 7.10 mm & for the construction of cornea – scleral lenses 9.00 mm ❖ PMMA replaced glass as the material of ❖ PARAMETERS OF COMPLETE SET choice for contact lens manufacture o Diameter: 20.00 mm ❖ ADVANTAGES o Scleral Radius: 11.00 mm, 12.00 mm, o Low specific gravity which meant & 13.00 mm lighter lenses = less prone to riding o Corneal Radius: 5.00, 6.00, 7.00, low on the eye 8.00, 9.00, 10.00 & 11.00 mm o Ease of manufacture, allowing thinner LEOPOLD HEINE (1930s) design to be produced, lens modification by optometrist in their ❖ Improved the Zeiss lens by enlarging the own practices series of trial lenses ❖ Used a supplementary curve between the PMMA CONTRIBUTORS corneal and scleral component ❖ WILLIAM FEINBLOOM (1936) JOSEPH DALLOS (1930s) ❖ ERNEST MULLEN (1938) ❖ THEODORE OBRIG (1938) ❖ Observed that scleral lenses with more ❖ ISTVAN GYORRFY (1938 movement were better tolerated ❖ Suspected that tear flow behind the lens was DENNIS C ENGLAND (1946) very important ❖ Patent rejected on the first PMMA corneal ❖ Added a supplementary limbal curve and contact lens fenerstration  CONTACT LENS 1 - LECTURE Bernadeth Mea M. Casao | OD4A o Instead of making scleral contact ❖ Synthesized a hydrogel material that was lens, they started making a smaller compatible with the body diameter which is known as the ❖ First lenses fitted to human eyes in 1956 but corneal contact lens we have today were unsuccessful due to their heavy weight and fragility KEVIN TUOHY (1946) ❖ Lens was patented as the first corneal O WICHTERLE & D LIM (1961) contact lens ❖ The Czech Ministry of Health ceased support of the project GEORGE H BUTTERFIELD (1950) ❖ Wichterle and his wife continued their research and succeeded in spin casting ‘four very good lenses’ ❖ Patented technique in 1961 SOFT CONTACT LENSES DEVELOPMENT ❖ National Patent Development Corporation and Dr Robert Morrison bought spin – casting patent rights – 1964 ❖ Bausch and Lomb acquired license to manufacture spin – cast lenses – 1966 ❖ US FDA classified soft contact lens as a ‘drug’ ❖ Tuohy’s corneal contact lens design was a – 1968 monocurve which had to be fitted 1.50 D o Today, contact lenses is classified as flatter than the central corneal curvature. This medical device was improved in the concept of a multicurve ❖ Bausch and Lomb obtained FDA approval to design which was first described by market Soflens – 1971 Butterfield under US Patent No. 2,544,246 SILICONE ELASTOMER (Mandell, 1988). The flatter posterior peripheral curves approximated the ❖ Walter Becker commenced his silicone nonspherical corneal shape and thus elastomer contact lens patent – 1956 anticipate the modern concept of fitting rigid ❖ Joe Breger acquired the Becker patent – 1959 corneal contact lenses. ❖ The Dow Corning Company acquired Breger’s technology – 1972 V. HYDROGEL AND SILICONE CONTACT LENSES SILICONE ELASTOMER MANUFACTURERS DIFFERENCES ❖ Dow Corning received FDA approval for ❖ PMMA is hard silicone in 1981 ❖ HYDROGEL is water and gel (softer) ❖ Bausch & Lomb acquired Dow Corning’s o Started making more comfortable silicone technology in 1981 contact lenses since RGP or hard contact lenses may not be JOHN de CARLE (1970) comfortable as soft contact lenses ❖ Pioneered the development of continuous O WICHTERLE & D LIM (1954) wear soft contact lenses ❖ Developed PermalensTM ❖ O Wichterle (Otto Wichterle) & D Lim (Drashoslav Lim)  CONTACT LENS 1 - LECTURE Bernadeth Mea M. Casao | OD4A 1970’s DAILY DISPOSABLE LENSES (1994) ❖ Rapid development in soft contact lens ❖ First developed and marketed by Vistakon, design and manufacturing Johnson & Johnson ❖ Uses the automated continuous flow process VI. RIGID GAS PERMEABLE CONTACT LENSES technique J TEISSLER (1937) MODULE 3.2 ❖ Made scleral shells from cellulose acetate I. SOFT LENS DESIGN FACTORS butyrate (CAB) – still hard o First gas permeable lenses ❖ Design matters o Poor optical quality o Most with physiologically poorer materials THE FIRST RIGID GAS PERMEABLE LENS o Least with better materials ❖ Silicone Acrylate SOFT CONTACT LENS DESIGN FACTORS ❖ ‘Polycon’ patented in 1972 by Norman Gaylord ❖ Geometric Centre Thickness (tc) o how thick is the center CONTRIBUTORS IN THE DEVELOPMENT OF CL ❖ Lens Diameter (total diameter, TD, ∅T) ❖ Norman O Stahl, Leon A Reich, & Edward o HVID Ivani (1974) ❖ Back Optic Zone Radius (BOZR, r0) ❖ These people help facilitate and created a ❖ Back Surface Design study that explain and justifying the ❖ Front Optic Zone Radius (FORZ, ra0) development in contact lenses ❖ Front Surface Design ❖ Radial Edge Thickness (tER) VII. DISPOSABLE SOFT CONTACT LENSES o Mas makalap ang edge, mas masakit ORLANDO BATTISTA (1978) ❖ Edge Design o Can round, tapered, pointed, square ❖ Developed collagen contact lenses ❖ Material physical/mechanical properties ❖ Conceived the concept of throwaway or ❖ Material physiological properties disposable lenses (daily’s contact lens) o functional ❖ Peripheral junctional thickness if MICHAEL BAY (1980s) transitions exist (tpj) ❖ Developed DanalensTM as the first o Junction = connector commercially available disposable lens ❖ In 1984, the Dana disposable lens from DESIGN Denmark was purchased by Vastakon ❖ DIAMETER o Greater than HVID COMMERCIAL SUCCESS WITH DISPOSABLE LENSES ❖ THICKNESS o Overall profile, center, mid-periphery ❖ CURVATURE ❖ 1987 o Variation in radius across lens, o Vistakon released the Acuvue lens on curvature in center a limited basis in the USA ❖ DESIGN ❖ 1988 o Front/back surfaces o Vistakon launched Acuvue ❖ RELATIONSHIP WITH THE EYE o B & L launched Seequence o CIBA Vision launched NewVues  CONTACT LENS 1 - LECTURE Bernadeth Mea M. Casao | OD4A o Ks cf lens back surface, total diameter o Lenses < -3.00 D often made thicker cf. HVID 1and /or with larger FOZD to improve handling II. MATERIAL PROPERTIES o Lenses > -5.00 may have FOZD MATERIAL PROPERTIES decreased to reduce mid – peripheral thickness ❖ Material properties are significant in soft contact lens design PLUS LENS SERIES ❖ Water contents of 25 – 79% - mean material ❖ Select first junction thickness, tpb1 properties vary greatly ❖ Select FOZD (∅a0) o more water content = more ❖ Center thickness reduction by FOZD permeable the material is reduction is limited by vision issues ❖ Significance of material properties leads o Used of ultrathin designers to develop material specific ❖ No degrees of freedom remain contact lens series ❖ Now tc is a function of BVP only o If dry eye, don’t used hydrogel cl since it always need water and can have IV. WATER CONTENT AND THICKNESS pervaporation so cl can be shrink. Used silicone cl para di magka WATER CONTENT problem sa pervaporation LOW WATER 20 – 40 % WITH A THIN, FLEXIBLE SOFT LENS MATERIAL, MEDIUM WATER 41 – 60 % HIGH WATER > 60% DESIGN IS ALMOST IRRELEVANT ❖ The higher water content is equal to higher ❖ Since soft cl kaya mahulma kung ano man permeability (means mas nakakahinga) shape ng cornea (di ko ma english, sorry) ❖ The lower water content is equal to less ❖ It is only relevant to rigid or hard contact permeability lenses TRANSMISSIBILITY (Dk/t ) III. CENTER THICKNESS ❖ Dk ∞ H2O content CENTER THICKNESS CONSIDERATION ❖ O2 ad CO2 transmissibilities ∞ ❖ Therefore, corneal respiration is best served ❖ Dk/t by a thin high-water lens. However, o thicker lens = less permeable pervaporation can then occur ❖ PERVAPORATION PREVENTION o Thicker lens = harder permeability so, PERVAPORATION input high water content however, ❖ If lens too thin, corneal dehydration may higher water content = more result pervaporation o Due to bulk flow of water through lens ❖ FITTING CONSIDERATIONS and instability of water flow at surface ❖ LITTLE OR NO MOVEMENT o Subject to individual variation DESIGN CONSIDERATIONS o Worse with higher water content o Results in corneal dehydration from MINUS LENS SERIES loss to air via lens ❖ Select material o Dehydration produces epithelial ❖ Select a practical FOZD (∅a0) desiccation staining – pervaporation ❖ Select a center thickness for lenses of about staining -3.00 D and greater  CONTACT LENS 1 - LECTURE Bernadeth Mea M. Casao | OD4A HIGH WATER CONTENT LENSES ON EYE OPENING ❖ Lose more water than low water lenses (% of ❖ Corneal deswelling total) on eye ❖ More O2 available – better condition of the ❖ Lose water even when worn in a high- eye humidity environment o Hydrogel 8% (Dk/t) ❖ Experience on eye lens shrinkage which o RGP 10-11% affects TD and BOZR ▪ Better than hydrogel because o These affect fit and need to be taken of the permeability into account o (Dk/t and tear pump) V. OTHER DESIGN CONSIDERATIONS PREVENTING OEDEMA SCL DESIGN OTHER CONSIDERATIONS ❖ HOW MUCH O2 IS NEEDED? o DW LENSES ❖ CENTRATION ▪ 9.9% o Vision, comfort, mechanical ▪ (Dk/t = 24) ❖ MOVEMENT o EW LENSES o Debris clearance, comfort ▪ 17.9% ❖ 4C’s ▪ (Dk/t = 87) o Coverage o Centration HYDROGEL LENSES o Comfort ❖ All lenses currently available causes >8% o Clarity overnight edema VI. PHYSIOLOGICAL CONSIDERATIONS o Chronic hypoxia ❖ Intermittent extended wear INTENTED LENS USAGE IS RELEVANT o 1 -2 nights per week is maximum ❖ For extended wear (EW), cornea’s minimum advisable requirement must be met LIMITATION OF CENTRE THICKNESS IN o Longer wear = more problem in TRANSMISSIBILITY CONSIDERATIONS oxygen ❖ Lesser open eye requirements apply to daily ❖ LIMITATIONS: wear (DW) o tc for minus lenses overestimates ❖ EW require always > DW Dk/t OEDEMA CYCLE (LENS WORN) o tc for plus lenses underestimate Dk/t o best estimate is average thickness ❖ Whenever you wear CL, oedema comes after because lack of oxygen TEAR MIXING ❖ Overnight edema ❖ studies have shown: ❖ Incomplete deswelling during day o little tear mixing under soft contact ❖ Persistent chronic residual daytime oedema lenses OVERNIGHT LENS WEAR o corneal swelling at any point is related to Dk/tlocal’ , hence local thickness is ❖ Corneal Oedema the only relevant dimension ❖ Less O2 available ▪ thickness is one of the factors ❖ Dependent on Dk/t in ordering CL o Depends on thickness o Thicker lens = lesser oxygen  CONTACT LENS 1 - LECTURE Bernadeth Mea M. Casao | OD4A REASONS FOR POOR TEAR MIXING UNDER A ELASTIC FORCES SOFT LENS ❖ Hydrogel lenses are deformed by the lid during blinking ❖ Conformance of lens to eye ❖ Lens alignment more closely with anterior eye o Soft that hugs the cornea topography ❖ Lens thickness and profile ❖ Visco-elastic forces are induced in the lens ❖ Material Properties ❖ After blink, lens relaxes but process lags o High or low behind the retreat of lids ❖ Lack of lens movement ❖ Forces to move lens α 1/tear film thickness o Can create four mixing under soft CL LENS LAG VII.A. FITTING CHARACTERISTICS - BOZR ❖ Visco-elastic properties of the lens prevent SOFT LENS FITTING PPHILOSOPHY an instantaneous response ❖ Lens relaxation may involve movement of, and/or a change in volume of, the post-lens tear film ❖ If post – lens tear film is very thin it is mainly viscous mucin and lipids. Lens movement is “damped” ❖ Magnitude of lid forces also influences the apparent fit SOFT LENS BOZR SOFT LENS BOZR ❖ For low rigidity lenses, changes of BOZR are less effective in altering lens fit ❖ BOZR – based curve ❖ BOZR IS LESS SIGNIFICANT THAN RGPs VII.B. FITTING CHARACTERISTICS - FOZR because: o A different fitting philosophy is used SOFT LENS FOZR o More flexible material producing ❖ The upper lid covers more of a soft lens than greater conformity and thinner post an RGP lens post-lens tear film ❖ This influences: o Larger changes required for clinically o Lens resting position (static position) significant alteration to on-eye ▪ Static: not moving or on behavior primary gaze only; not blinking o Lenses are more environmentally ▪ Dynamic: moving; blinking susceptible and the effect less o Movement induced by a blink predictable o Front surface design needed to o Visco-elastic forces induced on optimize lens position and blink- decentring aid lens self-centring induced movement o Initial fitting relationship is lost due to ▪ Always have decentration conformity, osmotic equilibration, lid when blinking pressure and elastic forces induced ❖ Sensation is from lid o Lens shape most dependent on o Thick lens means more awareness anterior eye topography  CONTACT LENS 1 - LECTURE Bernadeth Mea M. Casao | OD4A VIII. ALTERING LENS FIT IX.A. DESIGN FACTORS SAGITAL HEIGHT BACK SURFACE DESIGN ❖ Sigle curve (moncurve) o Mono = single o From the central to periphery is equal ❖ Bicurve , second curve often 0.8-1.0 mm flatter than BOZR about 0.5-0.8mm o Steeper from the center o Flatter in the periphery ❖ Blended multiple spherical curves (multicurve) ❖ SAME SAG, SAME DIAMETER ❖ Aspheric ❖ But DIFFERENCE DESIGN = DIFFERENT BACK PERIPHERAL CURVES BEHAVIOUR ❖ Sag gives fitting ❖ Presence or absence of back peripheral o ↑ SAG = Steeper curves is insignificant physiologically o ↓ SAG = Flatter ❖ Changes in back peripheral curves, especially ❖ Larger LD = steeper radical edge lift, affect lens movement ❖ Smaller LD = flatter; loose substantially o Periphery usually not needed SOFT LENS FITTING PHILOSOPHY ▪ Used for lens stability ❖ Center is the OZD FRONT SURFACE DESIGN ❖ Front surface design is also important to: o Lens fit o Comfort ❖ Front surface design somewhat dependent on manufacturing process ❖ If xerogel (hydrogel) part of fabrication process, spherical xerogel becomes aspheric after hydration – swelling is anisotropic ALTERING SOFT LENS FIT FRONT OPTIC DIAMETER ❖ Increasing sagittal height (sag) ‘TIGHTENS’ lens fit o steepens ❖ Decreasing sag ‘LOOSENS’ fit o flatter ❖ Decreasing lens diameter ‘LOOSENS’ fit (sag height decreased) o ↓LD = ↓ SAG means it FLATTENS ❖ Increasing lens diameter ‘TIGHTENS’ fit (sag height increased) o ↑LD = ↑SAG means it STEEPER ❖ ↑POWER = ↑THICKNESS OF THE LENS  CONTACT LENS 1 - LECTURE Bernadeth Mea M. Casao | OD4A o Means lack of water FRONT SURFACE DESIGN ❖ Commonly bicurve, periphery chosen to -> thin edge ❖ Intersection of FOZR and peripheral curve defines FOZR ❖ Multiple peripheral spherical curves ❖ Continuous aspheric is uncommon ❖ Front surface may also include bifocal or multifocal components such as: o Continuous aspheric surface ❖ y2 = 2r0x – x2 (1 – e2) where: o Concentric bifocal o e = eccentricity = √(1 – b2 / a2) o Flat-top segment o b = major diameter of section o a = minor diameter IX. B. EDGE DESIGN ECCENTRICITY VALUES OF CONIC SECIONS SOFT LENS EDGE DESIGN AND THICKNESS ❖ CIRCLE ❖ Edge positioned under both lids o e=0 ❖ Edge has relatively little effect on comfort ❖ ELLIPSE ❖ Edge design may be limited by o 0 < e < 1.0 manufacturing/patent issues ❖ PARABOLA ❖ Thickness governed by durability o e = 1.0 considerations rather than comfort/physiology concerns ECCENTRICITY X.A. ASPHERIC OFT LENSES ❖ as eccentricity increases, peripheral flattening increases exponentially ASPHERIC SOFT LENSES ❖ p = (1 – e2), p = shape factor, an index of ❖ Here ‘aspheric’ means a conicoid peripheral flattening or steepening ❖ A mathematically regular non-spherical WHY ASPHERIC surface ❖ Based on conic sections ❖ aspherics attempt to optimize the ❖ While a circle is a conic it is treated here as a lens/cornea relationship special case ❖ aspherics reduce local bearing pressure due to peripheral curve/transition zone CONIC SECTIONS discontinuities FIRST ASPHERIC SOFT LENSES ❖ Con-O-Coid, elliptical back surface ❖ 2 eccentricities offered ❖ Spreres and torics CORNEA IS NOT SYMMETRICAL ❖ Asymmetry is usually not large ❖ Cornea is considered an ellipsoid (first-order approximation)  CONTACT LENS 1 - LECTURE Bernadeth Mea M. Casao | OD4A ❖ Eccentricities (e) of the cornea: Spin casting Only back surface o H = 0.53, V = 0.58 (Holden, 1970) design o Flat = 0.41, Steep = 0.44 (Kiely, et al, Molding & Lathing Lathing limitations 1984) Spin-casting & Lating Lathing Limitations o H and V = 0.44 (Guillon, et al, 1986) XI.A. SOFT LENS SPECIFICATIONS AVAILABLE ASPHERICS RANGE OF SOFT LENS PARAMETERS ❖ Few aspheric soft lenses are marketed ❖ Most have continuous aspheric back curve ❖ Production easier with CNC lathes or molding SOFT ASPHERICS ❖ Aspherics require fewer back curve steps to cover range fits ❖ Attractive to ‘stock lens’ companies ASPHERIC ADVANTAGES ❖ Better lens/cornea-peri-limbal fitting relationship ❖ Fewer base curve steps required LENS MASS ❖ Lens fit less sensitive to lens iameter changes ❖ Depends on: ❖ Increased lens movement o Thickness ❖ Bearing pressure more uniform o BVP ASPHERIC DISADVANTAGES o Water content o Diameter ❖ Lens shape not optically optimal ❖ Adverse visual effects of decentred lens XI.B. THE FUTURE greater than with spherical lens SOFT LENS MATERIAL ❖ Visual acuity may not be optimum ❖ More difficult to manufacture ❖ New and novel materials ❖ More expensive to manufacture ❖ More complex design ❖ Not a readily available HYDROGEL LENSES TO AVOID RESIDUAL OEDEMA ❖ Perceived to be more complex ❖ May decenter and move more that spherical ❖ Aim to reduce thickness without dessication design ❖ Experimental hydrogels incorporating X.B. LENS DESIGN - LIMITATIONS silicone, fluorocarbon etc. under investigation MEAN OCULAR SURFACE DRYING TIME (SECS) MANUFACTURING PROCESS MAY LIMIT LENS DESIGN: ❖ NORMAL EYE METHOD LIMITATIONS o 25 Lathing Simple designs only ❖ SOFT LENS Molding - Anhydrous Few, but anisotropic expansion on hydration o 7 changes lens shape ❖ RIGID LENS Molding -Wet Almost none o 4 stabilized  CONTACT LENS 1 - LECTURE Bernadeth Mea M. Casao | OD4A LENS SURFACE MODIFICATION ❖ Make more wettable ❖ Mimic nature (epithelium/tears) ❖ Durability of modified surface must match expected lens life THE PERFECT LENS DESIGN MATERIAL COMBINATION IS YET TO BE CREATED