Contact Lens Fitting Guide Ajay Kumar Bhootra
INDEX
A
Altering SCL parameters 32
Alternating or translating vision lens design 67
Aspheric multifocal lenses 68
B
Back vertex power of the trial lens 27
Bi-weekly disposable lenses 2
Blink rate 23
C
Changing back optic zone radius (BOZR) 32
Color cosmetic contact lenses 4
Concentric bifocal lenses 68
Contact lens case cleaning 34
Contact lens design variables 4
base curve 6
blending 8
center thickness 6
edge design 7
front and back surface 5
lenticular zone 9
optical zone 7
overall diameter 5
peripheral curves 6
sagittal height 8
Contact lenses
classifications 1
introduction to 1
Contact lens fitting
contraindications for 25
Contact lens power 9
Corneal curvature 21
Corneal diameter 21
Corneal irregularities 64
Corneal sensitivity 24
D
Daily disposable lenses 1
Diffractive lenses 68
E
Enhanced monovision 67
F
Fitting disposable soft contact lenses 39
Fitting methods for soft contact lenses for presbyopic correction 69
Fitting silicone hydrogel lenses 35
Fitting soft contact lenses
fitting methods 26
initial data gathering 27
trial lens selection 28
Fitting soft toric lenses 41
fitting methods 42
rotational behavior 43
I
Instructions for RGP lens insertion 59
Instructions for RGP lens removal 60
Instructions for soft lens insertion 39
Instructions for soft lens removal 40
L
Lens delivery 31
Lens movement 29
Lens ordering 56
Lens rotation 44, 45
Lens tightness 30
Lid tonicity 24
M
Monovision correction 66
Monthly disposable lenses 2
N
Nontranslating or simultaneous vision lens design 67
O
Ocular disorders that can be treated with bandage contact lenses
bullous keratopathy 62
corneal erosion 62
corneal injuries 63
corneal perforations 63
corneal ulcer 62
exposure keratitis 62
trichiasis 63
Orthokeratology 64
Orthoptics 63
P
Palpebral aperture 22
Photophobia 64
Pre-contact lens fitting eye examination 11
conjunctiva 16
cornea 15
eyelashes 14
eyelids 13
lid eversion 17
lid margin 14
Presbyopia and contact lens fitting 66
Presbyopic contact lens fitting
indications for 69
Pupil diameter 22
R
Refraction 24
RGP lens care and maintenance 59
RGP lens parameters change and lens fitting
BOZD 56
BOZR 56
Rigid gas permeable lenses 3
S
Schirmer test 19
Silicone hydrogel lens care system 38
Silicone hydrogel lenses 3
Soft contact lens care system 32
Soft contact lens fitting in dry eyes 39
Soft hydrogel lenses 3
Spherical RGP lens fitting
fitting methods 48
Static fit
central fit 53
edge clearance or lift 54
midperipheral fit 53
T
Tear break-up time 20
Tear film 18
examination of tear film 19
Tear lens 9
Tear prism height 19
Tear thinning time (TTT) 20
Therapeutic contact lens
care 65
maintenance 65
Therapeutic contact lens fitting 61
Traditional lenses 2
Trial lens selection 51
U
Unsightly eyes 64
V
Vision stability 31
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Chapter Notes

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Introduction to Contact LensesChapter 1

 
INTRODUCTION
Contact lenses are small disks of plastic that allow you to see without the stogy look of glasses. In many cases, contact lenses are used as a substitute for glasses. Contact lenses may also be used to treat certain eye diseases or to change the apparent color of the eyes. Ideally contact lens should be fitted by a contact lens specialist practitioner, in a clinical set up and after a complete evaluation of the eyes. The successful contact lens wear requires a strong partnership between the professional skills of the practitioner and the wearer's discipline approach.
 
CLASSIFICATIONS
Based on wearing modalities contact lenses can be classified as under:
 
Daily Disposable Lenses
Daily disposable lenses are used once and when they are removed from the eyes in the night, they are thrown away in the trash. When it comes to putting something into eyes, everybody would like to be conscious about his ocular health and in that respect shorter is always better. The convenience of these lenses makes them popular with lots of people. The wearer of daily disposable lenses enjoys following benefits:
  1. Excellent vision throughout the day
  2. No ocular health compromise
  3. More comfort at the end of the day.
  4. 2No maintenance
  5. Spare lenses always available
  6. Eliminates risk of potential ocular problems
  7. Lower incidence of visit to practitioner.
 
Bi-weekly Disposable Lenses
Some people may find it difficult to afford the cost of daily disposable lenses. They may find it more economical to use two–weeks contact lens modality instead. As the name suggests, these contact lenses are worn for two-weeks before being discarded. Remember they need to be removed each night, therefore, cleaning, rinsing and storing in solution filled case is mandatory. The wearer has to keep the track of dates on which a new pair was put on the eyes and the date on which day he has to throw away the same. The wearer of bi-weekly disposable lenses enjoys following benefits:
  1. Simpler lens care
  2. Back up lens available
  3. Less dryness
  4. Increased comfort
  5. Better vision
  6. Better overall satisfaction.
 
Monthly Disposable Lenses
Monthly disposable modality is the most popular modality and are the first lens of choice by millions of people around the world. These contact lenses can be worn for one month or as directed by the practitioner. This means that at the end of everyday, you remove your lenses, store them in solution and clean before reinserting them.
 
Traditional Lenses
Contact lenses may be worn for a year or longer on daily wear basis. A traditional or conventional contact lens wearer must clean them on a daily basis and store them in proper protective lens case. Daily cleaning, rinsing, disinfection, protein tablet deproteinizing are all very critical to ensure trouble-free wearing.
Contact lenses can also be classified on the basis of material type:
 
3Soft Hydrogel Lenses
Soft contact lenses are made with a stable, solid polymer component that can absorb or bind water. Poly HEMA (Hydroxy Ethyl Metha Acrylate) is used to manufacture soft hydrogel lens. The lenses are made in their dry state and then hydrated in saline solution where they absorb water. The water so absorbed gives the lens its softness and makes them comfortable and pliable. Water content also increases its oxygen permeability. On an average water content varies from 38–80%. Higher water content makes a lens less durable.
 
Silicone Hydrogel Lenses
Silicone hydrogel lens material is a combination of silicone rubber and hydrogel polymer. The silicone phase facilitates oxygen transmission and the hydrogel phase allows good lens movement and fluid transport. Silicone hydrogel lens materials are very different from other class of contact lens material and within the silicone hydrogel class itself major difference exists as well. The hydrogel lens materials are more of a single material family. They behave more homogenously and in that increased water content results in improved oxygen permeability. On the other hand, silicone hydrogel materials that have been introduced so far contain a variety of polymer chemistries, surface treatments and material properties that result in less predictable eye-lens relationships.
 
Rigid Gas Permeable Lenses
Rigid gas permeable contact lenses are made of rigid plastic material and contain no water. RGP lenses permit oxygen to pass directly through the lens to the eye. Because they transmit oxygen, through the material these lenses are referred to as gas permeable. They are more durable and resistant to deposits, and generally provide a crisper vision. They tend to be less expensive over the life of the lens as they last longer than soft contact lenses. They are easier to handle and less likely to tear. However, they are not as comfortable initially as soft contacts and it may take a few weeks to get used to wearing RGP lenses.
 
4Color Cosmetic Contact Lenses
Colored contact lenses are used to change the eye color. One can change the color of his/her eyes to match facial make up. Sometimes they are also used for therapeutic reasons. They are available in wide range of colors, crazy patterns and appeal. They are basically worn to cater the mood and aesthetic values. Colo-red contact lenses are also used for masking the corneal opacities. Patients with Albinism or Aniridia are fitted with Pinhole contact lenses and Amblyopic patients with dark pupil lenses.
Contact lenses can also be classified as under:
 
Daily Wear
Daily wear lenses are used on daily basis during the daytime. They are removed before sleeping in the night.
 
Flexi Wear
Flexi wear lenses are used on daily wear basis with occasional overnight wearing.
 
Extended Wear
Extended wear lenses are worn upto 7 days and 6 nights at a stretch without being removed from the eyes.
 
Continuous Wear
Continuous wear lenses are worn upto 30 days and 29 nights at a stretch before removal.
 
CONTACT LENS DESIGN VARIABLES
zoom view
Figure 1.1:: Contact lens design variables
5The lens design refers to the shape, size, thickness and curve profile of the lens (Fig. 1.1). Design variables are very important aspects while manufacturing contact lenses to make a lens that will drape the cornea very gently, giving maximum comfort right from the start and at the same time ensuring peak visual performance. Each lens design variable is a part of contact lens anatomy and plays a critical part in how it functions on the eye. All of them are important and are affected by each other. These variables are:
  1. Front and back surface
  2. Overall diameter
  3. Base curve
  4. Peripheral curves
  5. Center thickness
  6. Optical zone
  7. Edge design
  8. Blending
  9. Sagittal height
  10. Lenticular zone.
 
Front and Back Surface
A contact lens has two surfaces, i.e. a back surface which is in contact with the cornea and a front surface which is the surface over which the eyelids pass. The back surface of the lens is shaped to have desired relationship to the overall shape of the patient's cornea. The front surface is curved to alter the refraction of light as required for the patient's refractive error. The front surface construction may affect the action of the eyelids.
 
Overall Diameter
The linear measurement of the greatest distance across a lens from one outside to another outside edge, specified in millimeter is the total lens diameter. This is an important variable to determine the centration of the lens on the cornea. The corneal lenses are interlimbal or smaller in diameter, whereas corneal-scleral lenses are intermediate in size and goes beyond the corneal diameter into the sclera. Several factors determine the size of the corneal and corneal-scleral lens to be fitted. The most important is the hardness of the material. The less rigid final lens has to be bigger in size for stability of fit. The minimum functional lens diameter should be used while fitting a lens that allows adequate movement and proper position of the lens.
 
6Base Curve
The central posterior surface curve of the contact lens is called the base curve of the contact lens and is abbreviated as BC. The other terms for the base curve include back optic zone radius (BOZR), back central optical radius (BCOR), base curve radius (BCR) and the central posterior curve (CPC). This curve is always concave and is expressed in millimeters of radius. A longer radius of curvature produces a flatter base curve and a shorter radius produces a steeper curve. The BC is the curvature from which all the curves of a lens is determined in arriving at the final dioptric power. Radiuscope is the most commonly used instrument to measure base curve of the contact lenses.
The base curve may be spherical or aspherical. A spherical base curve is specified by one radius and has same curvature at all points along the curve. An aspheric curve exhibits a gradual lengthening or shortening of the radius from the center towards the edge of the lens. The aspheric curve is specified by e-value or numerical eccentricity. The larger e-value implies greater rate of flattening or lengthening from center to edge.
 
Peripheral Curves
The curves surrounding the base curve on the posterior lens surface are called peripheral curves. The first curve outside the base curve is called the secondary curve; and if there is another peripheral curve, it is termed as the tertiary curve. The secondary curve is also known as intermediate posterior curve. Contact lenses are designed with one or more peripheral curves that are deliberately intended to lift away from the cornea. Each peripheral curve is typically flatter than the preceding curve in order to fit better and provide proper clearance from the gradually flattening shape of the anterior cornea. Another important thing to understand is the peripheral curve width. Peripheral curve width is the overall diameter of the lens minus the optical zone. The size of peripheral curve width depends upon the size of optical zone.
 
Center Thickness
The center thickness of a lens is the distance between the anterior and posterior surface of the lens measured at the 7geometric center of the lens. It determines the optical power and the fit of the lens on the cornea. It is measured in millimeter using a thickness gauge. Manufacturer usually quotes the center thickness of a lens for −3.00D. The center thickness and overall thickness profile of the lens affects the fitting characteristics, oxygen transmissibility, comfort and handling of the lens.
In general, larger diameter requires greater thickness to have adequate dimensional stability of the lens. If the center thickness is reduced, structural stability of the lens is decreased. Increasing the thickness of the lens will cause the lid to have a great effect on the lens and will therefore tend to fit more loosely. On the other hand decreasing the thickness of the lens will cause the lens to be less affected by the upper lid and consequently, it will fit more tightly. Decrease in center thickness results in reduction of weight of the lens and pressure on the cornea, but it leads the gravitational force to move behind the lens which increases the lens adherence to cornea, resulting in less lens movement.
 
Optical Zone
Optical zone(OZ) is the central zone of the lens, the function of which is to provide the lens with an accurate and stable refractive power. On the front surface of a single curve lens, the OZ is the entire front surface of the lens. In a lenticular cut lens, OZ is the front surface minus the carrier width. On the back surface of the lens, OZ is the total diameter of the lens minus the width of the intermediate and the posterior peripheral curve. The size of the OZ is specified in millimeter which varies with lens design and lens power. In a corneal-scleral lens the size of the OZ varies between 6–12 mm. The size of the optical zone should completely cover the pupillary size at all times. If front optical zone diameter is too small, the edge of the optical zone will move into the pupillary area and will result in flare.
 
Edge Design
The edge denotes the extreme periphery of the lens where the front and the back surface meet. It is verified by using profile analyzer or stereomicroscope. The shape of the edge is one of the most important parameters in terms of patient comfort and acceptance of contact lenses. There are two important factors to be considered as far as edge design is considered:
  1. 8Edge profile
  2. Edge thickness.
Thicker edge thickness may make it difficult for the upper eyelids to fold over the lens edge. Round edge profile both anterior and posterior provides more comfort than other edge profile.
 
Blending
When the junction between two curves is altered either by the addition of a very narrow curve or a series of some intermediate radius, the lens is said to be blended. Specifications of the blend present is difficult because the beginning and the end of the blended area is not easily delineated and the radius of the blending curve cannot be determined. Customarily the blend is specified as either, medium or heavy. With light blending, it is possible to see the area of the blend but with heavy blending it is usually impossible to determine the width of the blend.
 
Sagittal Height
Sagittal height of a lens is the perpendicular distance measured in millimeter between the apex of the back optic zone radius and the plane of the lens edge. It is an important parameter to alter the fit of the lens and is determined by the base curve of the lens and total diameter of the lens. If the base curve is constant, increasing the diameter of the lens increases the sagittal depth and therefore the lens fit is steeper. If the diameter is constant, increasing the base curve in millimeters decreases the sagittal height and therefore, the lens fit is flatter (Fig. 1.2).
zoom view
Figure 1.2:: Sagittal depth
 
9Lenticular Zone
The front surface may be divided into two zones in case of a lenticular lens design. The central zone contains the optical portion that corrects the patient's refractive error and the front peripheral zone is called the carrier. The main reason for using lenticular lens design is to reduce the thickness and weight of the lens. The carrier portion has a radius of curvature flatter than the central portion.
 
Contact Lens Power
Contact lenses are usually less than 0.2 mm thick and it may seem surprising that they need to be thought of as thick lenses. The thin lens formula explains the lens power as the simple addition of the two surface powers. The separation of the surfaces, i.e. center thickness is assumed to be negligible and is ignored.
The contact lenses are meniscus lenses with very high surface powers. The refractive power of the lens depends on the refractive index of the material, front and back surface radii, and the lens thickness. If the lens front radius is shorter than the back surface radius, it will have plus refractive power. If the front surface radius is longer than the back surface radius, the lens has minus refractive power. The refractive power of the lens can be calculated by knowing the dimensions, but, clinically a lens power is determined directly by using the lensometer. This instrument gives the power of the contact lens in air.
 
Tear Lens
When a flexible lens is placed on the cornea, the ‘tear lens’ under the contact lens is very thin. It has no dioptric power due to the conformity of the lens to the shape of the cornea. If a rigid lens is used, the tear lens depends on the relationship between the curvature of the lens back surface and the cornea (Fig. 1.3).
If the rigid lens decenters, the tear lens will acquire a prismatic component in addition to the spherical or spherocylindrical optics dictated by the fitting relationship.
Steepening the lens fit produces plus effect. So to maintain the same BVP of the system (CL, tear lens and eye) a compensating element must be added to the BVP of contact lens while ordering. Flattening the lens fit produces minus effect. So to maintain the 10same BVP of the system (CL, tear lens and eye) compensating element must be added to the BVP of contact lens while ordering.
zoom view
Figure 1.3:: Tear lens
 
SUMMARY
Optically, contact lenses are no different as compared to other vision correction devices. However, contact lens designing is not an easy task. Fewer degrees of freedom in lens design are available. This is because the back surface of the contact lens must relate to the anterior geometry of the eye. Narrow range of refractive indices is available as it is determined by the material selected, which is usually a clinical rather than an optical decision. The lens designer often has no choice of refractive index. The range of indices within a lens type is also relatively narrow. The optical surface shape of flexible lenses is determined by conformance to corneal shape, as well as lens profile and material physical properties. The optics of rigid gas permeable lenses, while more predictable, may involve noncoaxial optics due to lens movement and decentration. This means the exact effects are difficult to predict or calculate.