Contact Lens Handouts Compilation PDF

Introduction to contact lenses Definition it is a thin lens considered to be a medical device placed directly on the surface of the eye to – correct vision – cosmetic – therapeutic r...

Introduction to contact lenses Definition it is a thin lens considered to be a medical device placed directly on the surface of the eye to – correct vision – cosmetic – therapeutic reasons  Leonardo da Vinci – first describe the concept of a lens that comes into contact with the eye. (1508) John Herschel – describe a glass contact lens designed to match the shape of the eye (1827) F.E. Muller – fitted a blown glass lens to the eye of a patient whose eye had been surgically removed. (1887)  A. E. Fick – describe the first contact lens intended to correct vision. (1888) Muller and Obrig – constructed the first plastic scleral contact lens (1938) E. Kalt – designed and fitted glass corneal contact lenses (1940’s) – All contact lenses available were scleral types Kevin Touhy – first plastic corneal lens made from PMMA (1947) Otto Wichterle – Developed HEMA According to Material – Hard Non- gas permeable Gas permeable – Soft Conventional disposable According to Design – Monocurve – Bicurve – Tricurve – Aspheric According to Size – Corneal – Semi-corneal – scleral According to Mode of Use Daily Wear – Disposable – Planned replacement Extended – Continuous wear – Flexible wear According to purpose or Use Optical – Spherical – Toric – Presbyopic Therapeutic cosmetic Spherical Contact Lens Used to correct Hyperopia, Myopia, Aphakia and small amount of astigmatism Available in wide range of correction (- 20.00D to +20.00D) Toric Contact Lens For the correction of significant amount of astigmatism Presbyopic Correction Monovision or monofit – One eye is fitted with the proper distance vision power and the other eye is fitted with the power necessary for near vision. Bifocal soft contact lens – Alternating vision Distance viewing is done through the top part of the lens and near viewing is done through the bottom portion of the lens – simultaneous vision The patient looks through both the distance and near portion of the lens at the same time Multifocal Tinted contact lenses visibility tints – Effective on blue and other relatively light colored eyes – Makes handling easier Opaque colored lenses  Lenses that can actually change the apparent color of the eye by making the original color masked while introducing a new color Therapeutic (Bandage) Lenses Used to promote healing for a variety of corneal disease conditions Ortho-K contact lens gas permeable contact lenses that temporarily reshape the cornea to reduce refractive errors such as myopia, hyperopia and astigmatism Hybrid contact lenses large-diameter lenses that have a rigid gas permeable central zone, surrounded by a peripheral zone made of soft or silicone hydrogel material Wink activated telescope contact lens 1.55 millimeter-thick contact lens contains an extremely thin, reflective telescope, which is activated by winks. The wearer winks with the right eye to activate the telescope (2.8x magnification), and with the left eye to deactivate it. Intended for for low vision and age-related macular degeneration Google contact lens contact lens prototype that monitors glucose levels in tears Triggerfish contact lens The SENSIMED Triggerfish® Sensor is a soft disposable silicone contact lens embedding a micro-sensor that measures intraocular pressure iOptik™ contact lens enhances vision and enables visualize their digital world at the same time. It allows light from the display to pass through the center of the pupil, and light from the surrounding environment to pass through the outer portion of the pupil. Each of these sets of light rays produces an image on the retina simultaneously with the other set. They are superimposed to form a single integrated image. Terminator Eyes contact lens show images such as road directions or projecting text messages from smart phones straight to the eye Base Curve (BC)  Radius of curvature of the posterior Zone Optic Zone (OZ)  Posterior central optic portion of the lens  It indicates the size visually usable portion of the lens  SCL : 7-12 mm  RGP: 7-9 mm Lens Diameter (LD)  A measure of the maximum external dimension of a lens  SCL: 13-16 mm  RGP: 8-20 mm Posterior Peripheral Curve Radius (PPCR)  Radius of the peripheral curve Peripheral Curve Width (PCW)  Width of the posterior peripheral curve portion of the lens Sagittal value (sag)  Distance from the line drawn between the outer edges of the lens to the center of the back surface Optic cap (OC)  Central anterior optic portion of a lens Optic cap radius (OCR)  Radius of central anterior optic portion of a lens Anterior Peripheral Curve Zone (APCZ)  Anterior peripheral or lenticular portion of the lens Lens thickness  Central Thickness (CT)  Edge Thickness (ET)  SCL : 0.035 -0.2 mm  SCL : 0.01 – 0.05 mm  RGP : 0.1 – 0.2 mm  RGP : 0.08 – 0.12 mm Oxygen Performance  Lens acts as a barrier to atmospheric oxygen  When eye is deprived of oxygen, corneal physiological processes become compromised  Short term problems  Central Corneal Clouding  Vertical striae  Microcysts  Epithelial and stomal thinning  Long Term Problems  Corneal distortion  Disruption of endothelium  Changes in refractive status of the eye Dk (Oxygen Permeability)  Quality or state of the material that allows the oxygen to move through it.  D – diffusion coefficient  k – solubility coefficient  Higher water content = higher the dk  Low dk = less than 30  Medium dk = 30 – 60  High dk = higher than 60 Dk/t (Oxygen transmissibility)  States how much oxygen goes though the lens  Thinner the lens = higher amount of oxygen can pass through the lens.  25 for daily wear  87 and above for overnight wear EOP – Equivalent Oxygen Performance  describes oxygen flux through a contact lens as if the eye were responding to various amounts of atmospheric oxygen  This is a measurement of the cornea’s oxygen thirst following lens wear  The EOP test is generally conducted using lens materials of various thickness.  EOP allows practitioners to compare different performances of lenses Corneal Swelling  Most valid indicator of real-life clinical performance  It is the measure of the percent increase in the corneal thickness with great precision. Tensile Properties  Four measurement used to evaluate physical tensile properties  Tensile strength  Modulus elasticity  Coefficient of elongation  Tear strength Tensile strength  Measure of how much force can be applied to the material before it breaks  The greater the force, the better the tensile strength and the more durable the material Modulus elasticity  Refers to the flexibility of the material  Materials with low modulus elasticity are more flexible Coefficient of elongation  Refers to how far a lens material can be stretched before it will break  Value is stated by percentage, based on the size of the lens sample tested Tear Strength  Measurement refers to how much force has to be applied to a lens material before it will tear Water Content  The higher the water content lenses generally provide more oxygen to the cornea  High water content materials become dehydrated while on the eye  High water content tend to lose more water than low water content lenses Refractive Index  A physical property of which defines how light rays are affected as they pass through the material  It is important to the lens designer to achieve the proper optical effect on the eye  As water content increases the refractive index decreases  Materials with lower index of refraction will require thicker lens design to achieve the desired optical effect on power Biocompatibility  Contact lens interact with the corneal tissue, therefore it is important to evaluate the physiological balance of the material with the ocular environment  There are number of different tests that are used to evaluate biocompatibility  21 day Rabbit test  Resistance to microbial growth  Finally biocompatibility is confirmed by assessing the compatibility of the lens material with lens care systems Protein deposits  During development process, lens materials are carefully evaluated for protein uptake  High water content lenses absorb more protein than low water content lenses and ionic materials absorb more than non-ionic materials although not totally deposit resistant Dimensional Stability  It is assessed in the laboratory periodically by measuring water content, sagittal value, power, thickness, and base curve to make sure that the lens parameters are not changing.  This ensures that the material can maintain consistent lens performance Hydrolytic stability  Since contact lenses are placed in the tearfilm, which is primarily made up of water, the hydrolytic stability of the material is always assessed prior to placement on the eye.  Only materials that are stable in water can be used for contact lenses Lens Handling  Case of handling is primarily a function of the tensile properties of the lens and the lens design.  The more rigid the lens materials, the easier they are to handle Wettability  Wettability is the characteristic of lens performance that is important to RGP materials.  Since soft contact lens materials are inherently hydrophilic, there is no problem with water adhering to the surface of the lens.  However, some RGP materials with silicone act as water repellant, thus causing problems with comfort, vision, and deposit formation  With RGP lenses, wettability is tested in the laboratory by measuring the wetting angle. The higher the wetting angle, the less wettable the surface is.  The wetting angle of hydrophilic material is considered zero. Introduction  Glass was used exclusively for some yrs  PMMA began to replace glass in 1940s – toughness, optical properties and physiological inactivity IDEAL CONTACT LENS MATERIAL  Meets cornea’s oxygen requirements  Physiologically inert  Excellent in vivo wetting  Resists spoilation  Dimensionally stable  Durable  Optically transparent  Requires minimal patient care  Easily machineable IMPORTANT MATERIAL PROPERTIES  Oxygen permeability  Wettability  Scratch resistance  Rigidity (RGPs)  Flexibility (SCLs)  Durability  Deposit resistance OPTICAL PROPERTIES  Refractive index  Spectral transmission  Dispersion Rigid contact lens  Material used is PMMA  Stable materials  Resists warpage , wets well and clean easily  Lack of permeability to oxygen – tear exchange phenomenon  Backbone of all rigid lens materials  Trial lens Properties  excellent biocompatibility  good optical properties  scratch resistance  good manufacturing properties  Fairly wettable when clean  Easy to care for  Rigid  0.2 - 0.5% water when hydrated fully  Almost zero oxygen permeability  Produces ‘spectacle blur’ Gas permeable lenses  rigid lenses  Material used are  Cellulose acetate butyrate  Silicone acrylate  Fluoropolymers( teflon)  styrene Cellulose acetate  Cellulose is combined with acetic and butyric acids ( 13% acetyl, 37% butyryl and 1-2%free hydroxyl groups)  Low oxygen permeability  dk range of 4-8  Lack of dimensional stability i.e. Warpage, scratching and coating  No longer available Silicone acrylate  Silicone and oxygen are combined to make into siloxane  Combined with PMMA to produce a gas permeable lens  rigid gas permeable material introduced in 1970  Dk value range 12 to 60 are achievable  Negative charge  Tended to become coated with proteinaceous materials from the tears  Scratches easily may cause flexure problems if made thinner  Pure silicone – o2 permeability is high but poor wettability Fluoro-Siloxane Acrylates (FSAs)  Fluorine monomer added to SA material  Lower surface charge  Withstand high heat and chemical attack  O2 permeability is like silicone but more wettable  Dks 40 to 100+ (med-high)  Surface easily scratched  Greater lens flexure Perfluoroethers  consists of: Fluorine, Oxygen, Carbon and Hydrogen  Dk 90+ (high)  Neutral surface charge  Greater flexibility ‘on eye’  Low refractive index  High specific gravity SOFT CONTACT LENS MATERIALS SOFT CONTACT LENS MATERIALS  PHEMA  Incorporation of hydroxyl group into PMMA gives 2- hydroxy ethylmethacrylate and makes it more hydrophilic  close relative of poly(methyl methacrylate)  Water content is approximately 38%  Other variants to improve PHEMA are:  PVP Poly Vinyl Pyrrolidone  MA Methacrylic Acid  MMA Methyl Meth Acrylate  GMA Glyceryl Meth Acrylate  DAA Di Acetone Acrylamide  PVA Poly Vinyl Alcohol  Convenient to consider the polymers that have been used as contact lens materials under four heading: 1.Thermoplastics – capable of being shaped or moulded under heat or pressure  Eg: PMMA  Polyethylene and polyvinyl chloride  Copolymer of tetrafluoroethylene  Poly(4-methyl pent-1-ene)  Cellulose acetate butyrate(CAB)- Synthetic elastomers  Not only fexible but show rubber like behaviour  Intermediate characteristics b/w thermoplastic and hydrogel materials.  Oxygen permeabilities 100x-1000x more than PMMA  Hydrophobic – surface treatment  Ethylene propylene terpolymer(EPT)  Silicone rubber or poly(dimethyl siloxane)  Contact lens materials are divided into hydrophilic and hydrophobic groups  Materials with water content greater than or equal to 10% by weight at ambient temperature are assigned "-filcon" names.  The "-focon" stem is assigned to hydrophobic lens materials with water content less than 10%. Contact lens Groups  Contact Lens Material can be Categorized as Ionic and Non-Ionic  Group 1  Low water / non ionic polymers  Group 2  High water / non ionic polymers  Group 3  Low water / ionic polymers  Group 4  High water / ionic polymers Ionic vs. non ionic contact lens  Based on lens hydration (primarily related to the ionic nature of the material)  non-ionic polymer  can interact with polar molecules, such as water, without resulting in a formal charge.  ionic material  Charged molecule, which increases lens hydration.  Ionic materials are sensitive to changes in pH and osmolality  more sensitive to the components in a lens care system  greater interaction with the ocular environment, such as increased uptake of proteins. Low Water Content Lenses  Provide excellent physiological response for patients with refractive errors in the -1.00 to minus  Compatible with all lens care systems including thermal, hydrogen peroxide, and chemical disinfecting systems  Has lower protein uptake tendencies which contributes to longer lens life  Higher tensile strength  Exhibits good material stability  Since low water content materials don’t absorb preservatives, they don’t have problems with discoloration. High Water Content Lenses  High oxygen permeability, therefore, an excellent choice for the thicker high plus and minus lenses  Lower tensile strength  Not compatible with all disinfecting systems  Higher incidence of acute red eye and lens discoloration  Usually produced with either lathe cut or cast molding process Mid Water Contact Lens  Typically ionic or non-ionic materials  These lenses are an attempt to combine the best of both high and low water materials  Provide good physiology and are produced in thin, comfortable designs  Exhibits increased in protein uptake usually are nor compatible with thermal disinfection techniques Manufacturing Process  Spincasting  Liquid polymer is injected directly into the spinning mold  Lathe Cutting  Pre-polymer is poured into long glass tubes and placed in an oven for specified period of time at a high temperature  Cast Molding  Liquid pre polymer is placed between two molds which are then brought together under high temperature and pressure. DONNIE Y. SALUDES, OD, MAED, FAOC, FILVS NORMAL CORNEAL DIMENSIONS AND TOPOGRAPHY Corneal diameter HVID: 10 – 14 mm ave: 11.7 mm VVID: ave: 10.6 mm smaller than HVID of about 0.5 – 1.0 mm CORNEAL THICKNESS Clinical Studies: 0.50 – 0.65 mm Gullstrand’s Eye No. 1: 0.50 mm VARIATIONS IN CENTRAL & PERIPHERAL THICKNESS CORNEAL THICKNESS MEASUREMENT: PACHOMETERS ultrasonic Beam-splitting device on slit-lamp CORNEAL CURVATURE  CHARACTERISTICS OF THE CENTRAL CORNEAL REGION  WTR astigmatism (early life)  Peripheral portion (limbus)  Limbal topography influences SCL fitting CORNEAL TOPOGRAPHY ASPHERICITY  Cornea is aspheric  Apex – area with shortest radius  Asphericity - the deviation of peripheral curvature from the apical curvature  Prolate (normal)  Oblate (laser surgery or ortho-k) Corneal Topography Application Uses  Estimation of refractive errors  Assessment of pathology  Contact lens fitting  Assessing effects of contact lenses and refractive surgery INSTRUMENTATION FOR CORNEAL TOPOGRAPHY MEASUREMENT  Optical - Photokeratoscope and placido disc - keratometer - Computer assisted tomography  Contact methods - casting and molding - ultrasound - trial contact lenses PHOTOKERATOSCOPE/ PLACIDO DISK PHOTOKERATOSCOPE IMAGES DISADVANTAGES  Corneal astigmatism is not quantified  Absence of central mire reflection  Anatomy of nose or orbit may limit field size KERATOMETERS  Measure radius of curvature of the optic cap  3-4 mm  Total power of the cornea Type  2-position  1-position Refractive index  Uses specific refractive index  Calibration index: 1.3375 Coverting radius to power  D = 337.5 / r  r = 337.5 / D  Where  r = radius  D = power of the cornea Computing for CL Base Curve  Rigid Lenses = 337.5 / D  SCL = (337.5 / D) + 0.7 TOPOGRAPHIC ANALYSIS SYSTEMS  Optics of Contact Lenses DONNIE Y. SALUDES, OD, MAED, FAOC, FILVS  Optically, contact lenses are considered THICK lenses  Their thickness, compared to their short radii of curvature, is optically significant  Radius of curvature  Index of refraction  Thickness Surface Power  D = n – 1 / r  Where:  D = Surface Power  n = index of refraction  r = radius of curvature in meters  An RGP lens made from a material with 1.44 refractive index has a BOZR of 7.8mm. What is the back surface power of the lens?  Nominal Power Dn = D1 + D2 D1 D 2 Dn = (n-1/r1) + (n- 1/r2) An RGP lens made from a material with 1.44 refractive index has an FOZR of 7.8mm and a BOZR of 8.4mm, what is the power of the lens?  Equivalent Power t Deq = Dn – t/n (D1D2)  An RGP lens made from a material with 1.44 refractive index has an OCR of 7.8mm and a BOZR of 8.4mm, if the lens has a central thickness of 0.2mm, what is the power of the lens? Effective Power Measured as:  The position of the second principal focus from the back vertex of Dv = Dn + t/n (D1)2 the lens  An RGP lens made from a material with 1.44 refractive index has an OCR of 7.8mm and a BOZR of 8.4mm, if the lens has a central thickness of 0.2mm, what is the back vertex power of the lens?  Neutralizing Power  Dfv = Dn + t/n (D2)2 CL with very steep curves  An RGP lens made from a material with 1.44 refractive index has an OCR of 7.8mm and a BOZR of 8.4mm, if the lens has a central thickness of 0.2mm, what is the front vertex power of the lens?  Optics of Contact Lens (Correction for Ametropia) DONNIE Y. SALUDES, OD, MAED, FAOC, FILVS Contact Lens Prescription  To determine the power of the contact lens to be prescribed, you must take note of the following:  The manifest refraction result  The amount of the sphere  The amount of the cylinder  The sphere-cylinder ratio  The vertex distance used during refraction Spherical Equivalent  A spherical contact lens can be prescribed if:  The manifest refraction result is sphere  The amount of the cylinder is less than -1.00 D  The sphere to cylinder ration is at least 4:1  Formula:  Spherical Equivalent = Sphere + ½ cylinder Effect of Vertex Distance in Correcting Ametropia Hyperopia Myopia Correction for Vertex Distance  DCL = DSpec / 1 – d(DSpec)  DCL = Power of the Contact Lens to be Prescribed  DSpec = Spectacle Refraction  d = vertex distance in meters Optics of Contact Lenses Contact Lens Magnification Accommodation Convergence DONNIE Y. SALUDES, OD, MAED, FAOC, FILAVS CL Magnification  In comparing spectacle and contact lens image sizes:  Examples with d = 14 mm + 10.00 D, CLM = 0.86 - 10.00 D, CLM = 1.14  Therefore, with contact lenses, hyperopes experience a smaller image size than with spectacles  Similarily, myopes experience a larger image size than with spectacles Relative Magnification  RSM APPLICATIONS  Useful in anisometropia  Aetiology of the ametropia is unknown  If K readings mirror ametropia, major cause is probably refractive  Axialametropia: correct with spectacles  Refractiveametropia: correct with contact lenses  High ametropias usually axial  Axial anisometropia best corrected with spectacles?  Most ametropes are approximately isometropic  Choice of correction is then usually based on other considerations HOW DO SM, CLM & RSM RELATE TO ONE ANOTHER?  SMis a comparison of corrected vs uncorrected retinal image sizes  CLMis a comparison of CL corrected vs spectacle lens corrected, retinal image sizes  RSMcompares image sizes in a corrected ametropic eye and a theoretical emmetropic schematic eye APHAKIA  Aphakia is considered refractive  IfIOLs are not implanted, contact lenses are preferable ASTIGMATISM  Highcorneal astigmatism is classed as a refractive ametropia  Spectaclelenses will cause significant meridional aniseikonia ACCOMMODATION: CLs vs SPECS  INCIPIENT PRESBYOPIA  If a myope is switched from spectacles to contact lenses the change may precipitate the need for a near correction  Ifa hyperope is switched from spectacles to contact lenses the need for a near correction may be postponed NEAR VISION SPECTACLES vs CONTACT LENSES  Prismaticeffect is induced if the line of sight does not pass through the optical centre of a lens  Theprismatic effect can be calculated by Prentice’s Rule:  Prism (D) = Lens Power X Decentration (cm) Convergence in hyperopia and myopia OPTICAL ADVANTAGES OF CONTACT LENSES  No astigmatism of oblique pencils  No distortion  No chromatic aberration  No limitations on the field of view  No spectacle frame diplopia OPTICAL DISADVANTAGES OF CONTACT LENSES  Lens decentration produces ‘ghosting’  When a toric lens rotates, a toric over refraction and decreased vision results  Moving lenses may produce disturbances of vision  In axial ametropia spectacles are better suited?  Routine Preliminary Examination for Contact Lens DONNIE Y. SALUDES, OD, MAED, FAOC, FILVS THE ROUTINE PRELIMINARY EXAMINATION  Slit-lamp examination of the anterior segment  Measurement of ocular dimensions  Assessment of the tears  Spectacle refraction  PRELIMINARY EXAMINATION OF THE ANTERIOR SEGMENT  Check for the following structures  Eyelids  Conjunctiva  Tears  Cornea  Anterior chamber  Iris and lens  Ocular Health Grading System  BrienHolden  Efron Grading System  Ciba Vision BIOMICROSCOPY Performed:  Before CL fitting or CL wear, to establish a ‘baseline’  During trial lens fitting(s) and after-care visits to assess: - lens fit - any anterior eye changes MEASUREMENT OF OCULAR DIMENSIONS  CORNEAL CURVATURE Consider  Central and peripheral keratometry  Corneal topography  Shape regularity  Degree of corneal vs internal astigmatism  Sphere-to-cylinder ratio MEASUREMENT OF OCULAR DIMENSIONS  CORNEA AND PUPIL Cornea: HVID and VVID Pupil: Standard room illumination Low illumination  LIDS  Palpebral Aperture (Interpalpebral Aperture)  The palpebral aperture size and shape of Asians are smaller, with a more acute intra-palpebral angle at the inner and outer canthi. LID POSITIONS/Tension  Lid Tension  As yet no accurate method of measuring lid tension exists. Swarbrick & Holden (1996) measured lid tension by:  1. asking the patient to look down; 2. pulling the upper lid outward by grasping the eyelashes gently; 3. subjectively grading the resistance to pulling  from +3 (very tight) to –3 (very loose). BLINK RATE  Average of 24 / min  Range 10 – 34/min ASSESSMENT OF THE TEAR LAYER  Invasive  Non-invasive ASSESSMENT OF THE TEAR LAYER  Tear flow  Volume  Break-up-time (BUT)  Osmolality  pH ASSESSMENT OF THE TEAR LAYER INVASIVE TECHNIQUES  Break-Up-Time (BUT)  Schirmer test  Phenol-red thread test  Staining  Lissamine Green  Rose Bengal  NON-INVASIVE TECHNIQUES  Non-invasive Tear Break-Up-Time (NIBUT)  Tear prism height SPECTACLE PLANE REFRACTION  Baseline refraction: - Vertex distance - Accommodation - Convergence  Subjective vs objective refraction  Over-refraction SPECIAL CONSIDERATIONS  Refractive  General health  Ocular conditions  Medications/therapeutics  Previous Rxs  Occupational, recreational and environmental factors REFRACTIVE  High myopia  Progressive myopia  High astigmatism  Keratoconus  Anisometropia  Monocular diploma GENERAL HEALTH  Diabetes (moderate to severe; managed with daily insulin  Allergies  Arthritis  Pregnancy  Sinus problems MEDICATIONS/THERAPEUTICS  Ocular  Systemic  Topical CONTACT LENS HISTORY  Current contact lens wearer  Previous contact lens wearer OCCUPATIONAL, RECREATIONAL AND ENVIRONMENTAL FACTORS  Sports  Hobbies  Environmental exposure FINAL EVALUATION  Decision is made on specific lens type to trial fit  Consult with patient to determine specific needs