(History of contact lenses (CL- In 1508, Leonardo da Vinci sketched the first forms of new refracted surface on the cornea.- He used the example of a very large glass bowel filled with water; immersion of the eyes in water theoretically corrected vision (fig.1).
H2OFigure1. A hollow glass semi-spheroid filledwith water
- In 1636, Descartes suggested applying a tube full of water directly to the eye to correct a refractive error. H2O Figure2. Optical apparatus described by Descartes
In 1887, Adolf Fick wasapparently the first tosuccessfully fit contactlenses, which were madefrom brown glass
- In 1887, Dr. Fick a physician in Zurich. He described the first contact lens with refractive power known to have been worn.- The contact lens was made by A.Muller a manufacturer of artificial eyes.
- Early contact lenses were crude disk of ground or blown glass and made spherical although the cornea is not. - Theses lenses because the developer had little knowledge of the metabolic need and physiology of the cornea. Did not conform to the shape of the cornea. Causes corneal abrasions. Wearing time short and frequently painful.
- This led to the development of a larger lenses that rested on the sclera (1888 to 1938) and provided a clearance between the lens and the cornea (fig.4).- However, the edge of the first plastic lenses formed a seal with the sclera, trapping a pool of bathing medium under the lens and precluding vital metabolic exchange and poor tolerance.
The proposed therapeutic uses for contact lensesis to protect and reshape the optical properties ofirregular cornea in disease such as keratoconuse- It become clear that corneal shape is criticalimportance in CL design, and the cornea obtainsthe bulk of its oxygen supply from the air andthat the medium of exchange is the tears.
- As result scleral lenses were abandoned and hard contact lenses were redesigned.- The newer lenses were smaller, thinner and more flexible. They ride on the surface of the tear film, and each blink of the lids provides a flow of oxygenated tear that supplies the cornea the oxygen.
- Even these early suggestions the history of CL did not begin until the 19th century.- In 1964 Dr. Wichterle in Czechoslovakia introduced soft hydrophilic plastic contact lenses.- These lenses had the advantage of comfort and permeability by water and oxygen.
- The main potential in these lens lay in the ability of mass production methods that would bring down the manufacturing costs.- In 1966 Baush & Lomb introduced the spin- cast soft CL on an experimental basis- In 1967, in the USA the first lathe-cut lens was seen.
Contact lens materials- Contact lenses can be classified according to their material as hard or soft contact lenses. Or according to hydrophobic (non-loving water) materials or hydrophilic (loving water) materials.
In general these materials should be Stable. Clear. Nontoxic. Non-allergic. Optically desirable.
Hard contact lenses (HCL(Polymethylmethacrylate (PMMA)- It is organic plastic compounds stable at room temperature and water content 0.5%.- It is easily to work with and has excellent optical clearly, durability, stability, lack of toxicity, resistance to deposit formation.-The refractive index 1.48-1.50.- Disadvantage lack of oxygen permeability.- It is used to produce hard contact lens.
Cellulose acetate butyrate (CAB)- The first rigid gas permeable lenses (RGP) were made from CAB.- CAB has slightly better oxygen permeability than PMMA.- It is strong, durable.- Disadvantage prone to warpage.
Silicone- It is a polymer of dimethyl-silicone, permeable to oxygen and glucose.- It refractive index 1.43.- There are two types of Silicone contact lenses:-a- Silicone rubber lenses their stiffness and rigidity are intermediate between typical rigid gas permeable RGP and they hydrogels.b- Silicone resin lenses are in hard state the stiffness to typical rigid gas permeable materials.
Soft contact lenses (SCL) Hydroxyethlmethacrylate (HEMA) - Soft or hydrophilic contact lenses are characterized by the ability to absorb water, elasticity and flexibility. - HEMA is hydrophilic because it contains a free Hydroxyl group that bounds with water.- The water content range from 38% to 60%.- Its refractive index 1.43.- Example Bausch&Lomb (Soflens)
Oxygen transmission The passage of oxygen molecules and certain other ions and molecules through a contact lens is very important in maintaining normal corneal physiology.- The passage of oxygen is one of the most important aspects of a contact lens material, and much attention is directed to this topic by contact lens practitioners and researchers.
The cornea obtain most of oxygen from the tearfilm. The tear film supply the cornea withoxygen from the atmosphere when the eyes areopen.
-Thecornea has no blood vessels, the oxygensupply necessary for normal metabolism. O2 O2
During sleep, the eyelids block oxygen fromthe atmosphere, and most of the oxygen in thetears diffuses from the blood vessels of thelimbus and the palpebral conjunctiva. Thisreduces the amount of oxygen in the tear filmto approximately one third.
- All contact lenses act as a barrier between the cornea and its oxygen supply.- The oxygen is able to reach the cornea in two different ways: In the form of oxygen dissolved in the tears being pumped behind the lens when the lens moves upon blinking, and By diffusing directly through the lens material.
Contact lens O2 O2 Tear exchangeFig. The pumping mechanisum
- Tear exchange not only provides oxygen and other nutrients to the cornea, but also removes waste products (such as carbon dioxide and lactic acid) and dead epithelial cells.Fig. 11
The pores O2 The contact lens The corneaFig. The diffusion mechanism
- Tear pumping is the major source of corneal oxygenation with PMMA lenses, since these lenses have almost no oxygen permeability.- The tear pump alone is insufficient to provide adequate amounts of oxygen to the cornea.
- The PMMA lenses cause unacceptable levels of corneal hypoxia (lack of oxygen; even in the presence of an active tear pump.- Diffusion significant amounts of oxygen directly pass through the lens that is necessary to provide an adequate oxygen level for normal cornea metabolism.
MESUREMENT OF OXYGEN TRANSMISSION- Direct diffusion is the major source of oxygen transmission with soft lenses,- it is very important to measure this parameter.
Oxygen Permeability- Permeability is the degree to which a substance is able to pass through a membrane other maternal.- Diffusion is the process by which molecules pass through a material (such as a contact lens); the direction of movement is always from the area of higher concentration to the area of lower concentration.
- Permeability is natural function of the molecular composition of the material.- Permeability is affected by concentration, temperature, pressure, and barrier effects.- The permeability of a material is expressed as a permeability coefficient, denoted Dk.
- The diffusion coefficient (D) is the speed with which gas molecules travel (diffuse) through the material (Figure 15).
- The solubility coefficient (k) defines how much gas can be dissolved in a unit volume of the material at a specified pressure (Figure 16).
- In order for oxygen to pass through a contact lens material, the molecules must first dissolve into the material and then travel through it.- Permeability is the product of the diffusion coefficient ( D) and the solubility coefficient (k).
- The Dk value is specified in standard units. The actual testing conditions may vary, but the results must be converted to the standard Dk units.- A typical Dk value, expressed in its standard units: Dk = 8.9 x 10‾¹¹ (cm²/sec)(mlO2 / mL x mm Hg) @ 25°C
- The temperature of the testing conditions should always be noted because Dk increases with increasing temperature.- Because increasing temperature increases the energy of the gas molecules, causing them to travel at a faster rate through the material.
- The oxygen permeability coefficient (the Dk value) of a contact lens material is an inherent characteristic of the material, regardless of its thickness. As a rule, Dk is a constant for a given lens material.
Oxygen Transmissibility- The Dk value of a material is not how much oxygen will actually pass through a given contact lens.- The actual rate at which oxygen will pass through a specific contact lens of a given thickness is called its oxygen transmissibility, denoted Dk/L.
- To calculate the oxygen transmissibility of a given contact lens, the Dk value for the material is divided by the lens thickness, denoted L. - Lens thickness is expressed in centimeters, so care must be taken to convert lens thickness (which is typically expressed millimeters) to the proper units.
- The lens thickness chosen tocalculate Dk/L is usually the centerthickness of a -3.00 D lens, as this istypically the midrange power of theminus lens range for manymanufacturers; +3.00 lenses aretypically used as the midrange of pluslenses.
- The lens thickness chosen to calculate Dk/L is usually the center thickness of a -3.00 D lens, as this is typically the midrange power of the minus lens range for many manufacturers; +3.00 lenses are typically used as the midrange of plus lenses. - It is important to remember that most published Dk/L values represent only -3.00 D lenses.
- It is significant that as lens thickness increases, the oxygen transmissibility decreases. This means that plus lenses (which are thickest at the center of the lens) will have lower calculated oxygen transmissibilities than minus lenses (which are thinnest at the center of the lens) of the same material.
- Dk is a function of water content in hydrogel lenses. As a general rule, this is a linear function with Dk increasing at the same rate as water content.Figure 19.
- Although lenses with higher water content typically have higher Dk values, they often must be made thicker than lower water content lenses for several reasons: They dry out, or dehydrate, more rapidly in thin designs, leading to corneal drying which is observed as corneal desiccation staining. High water lenses are generally more fragile m thin designs.
- The thicker designs of high water lenses oftenresult in Dk/L values that are similar to thinner lenses with lower water content. Table 6
There are 2 main types of water content materials in soft CLs- Low water content materials (Bausch & Lomb, water content 38.6%, trade-name lens Optima 38)- High water content materials (Bausch & Lomb, water content 70%, trade-name lens B&L 70 Minus)
Contact Lens Optics- The general principle of correction of refractive errors with contact lenses is substituting a new refractive surface (contact lens) for the old surface (cornea).- The new surface is uniform, with a different index of refraction and anterior radius curvature. This substitutes for the cornea which may be irregular.
RadiusThe power of the eye is dependent upon- The radius (r) of curvature of the cornea and lens- The index of refraction (n)- The length of the eye.The dioptric value of each surface can becalculated with the formulaD = n2 – n / r;D = dioptric power; n= index of refraction of first medium.n2= index of refraction of second medium
The power of the typical anterior corneal surfaceD = 1.376 – 1.000 / 0.007D = 0.376 / 0.007D = + 48.83 DThe power of the typical posterior corneal surfaceD = 1.336 – 1.376 / 0.0068D = 0.040 / 0.0068=DThe total corneal power in round numbers isD 42.90 = + 5.88 - 48.83+
The typical anterior human lens surface power-D = 1.41– 1.336 / 0.010D = 0.074 / 0.0010D = + 7.40 D The typical posterior human lens surface power-D = 1.376 – 1.000 / 0.0006D = - 0.0074 / 0.0006D = + 12.33 DThe total corneal power in round numbers is-D 19.70 = + 12.33 + 7.40+The total power of the eye is arrived at by use of.the formula for combination of lenses
If the total power of the eyes s 58.00 diopters,then the total length isF = 1/Df = 1/58.00f = 0,017 = 17 mm.Where f = focal length; D = dioptric power
Index of RefractionIf the radius (r ) of the refractive surface and lengthof the eye are constant, then the variations of theindex of refraction (n) (Table 2.4) will change thepower of the refracting surface. Using theformulaD = n2 – n / rassigning a value to r of r = 7.50 mm, then if n isgiven, the amount of change dioptric power can.be calculatedFor example, if n2= 1.33, thenD = 1.33 – 1.00/0.0075D = 0.33/0.0075D = 44.0
- AS the index of refraction (n) increases, the refractive power increases (Table 2.5).
Radius of Curvature The power of the refractive surface is dependentupon the radius of curvature ( r) and the index ofrefraction (n). If n is kept constant and r is changed, the powerwill also changeIf D = n2 – n / r,
n = airn2 = PMMA contact lens (n = 1.49) then the effect of changes in r can becalculated.If r = 7.50D = 1.49 – l-00/0-0075D = 0.49/0.0075D = 65.333 diopters The smaller the radius or the steeper therefractive surface, the greater the refractivepower of the lens (Table 2.b).
Vertex DistanceThe power of a lens is the reciprocal of the focallength, the relative or effective power of acorrective lens changes with the placement of thelens or the distance between the lens and the eye.This relationship is expressed by the formula D= 1 /fWhere D = power in diopters, f = focal length inmeters. Example. In a +10.00 diopter lens, the focallength is 10 cm (0.1 m):D = 1/0.1 D= 10
- The closer a lens comes to the corneal surface and nodal point of the eye, the greater the plus power required and the less the minus power needed to correct the refractive error.- These vertex power changes can be arrived at in four ways: The formula for the change in vertex power of the lens is Δ= D²d Where Δ = change in power due to vertex distance; D = lens power; d = distance lens in meters.
Example 1. An aphakic spectacle correction of+13.00 diopters at 13 mm in from the eye.?What power should the contact lens beΔ = 13² x 0.013Δ = 169 X 0.013Δ =2.197 diopters
The power of the required contact lens corrected for vertex distance is +13.00 +2.20= +15.20 D Example 2. If a -10.00 lens is at 15 lens the power of the required contact is calculated as follows:Δ =D²dΔ = -10² X 0.015Δ = 1.5 D
The total required power is -10.00 + 1.50= -8.50 diopters- The power and position of the correcting lens must be such that the focal point of the lens is conjugate to the focal point of the eye.
General examination1- External examination.- The external examination can be done with a penlight combined with hand magnifier or a slit lamp which is better choice.- CL is contraindicated if there is any active pathology of the eye, e.g. inflammation, injury of the cornea, conjunctiva or lid.
1- Procedure for penlight:- Seat the patient comfortably in room with good light condition.- Direct the penlight illumination at the area to be examined while you look through the magnifier. - Examine the eyelid skin, lid margin (blepharitis marginalis), conjunctiva, cornea (scars), sclera, anterior chamber and iris.- You may need to avert the lids to examine the conjunctiva properly for follicles or papillae.- Note any inflammation or injury of the area examined.
:Procedure for slit lamp- 2Seat the patient comfortably at the slit lamp by-adjusting the patient seat, the slit lamp height. and chine rest or bothExamine all the tissue mentioned above.-Particularly those directly related to contact lens. fittingDiffuse illumination used to examine the-. conjunctiva and the lidsDirect illumination used to examine the cornea. and limbus
- The Patient tear quality and quantity should be tested, because wearing CL on dry cornea can cause poor tear circulation, corneal edema, blurry vision and burning sensation. Therefore, there are two tests commonly used analysis tear. - Tear quality→ Tear breakup time (TBUT) - Tear quantity→ Schirmer test
◊ Tear breakup time (TBUT) - Blinking helps in distributing tear over the cornea, immediately after a blink, evaporation begins and tear film begin to thin. Therefore, the tear breakup time is often used as an index for an abnormal tear formation.- Tear breakup time is the interval time between a complete blink and the first randomly distributed dry spot.
- If an eye is kept open without blinking for 15-34 seconds the tear will show dry spot areas. - When fluoresecin applied these dry areas appear black when examined with ultraviolet light.- Any dry areas occur in less than 10 seconds is considered a negative factor in patient selection for CL fitting.
◊ Schirmer test- It used to evaluate the rate of tear flow. It provides information on hypo and hyper secretion of tears.- A special filter paper (5x35mm) is used; this paper has an indentation at the uppers 5mm of it length.- After 5 minutes the paper is removed and the length moistened by tear is measured with a ruler.- Normal tear secretion moistens 10-15mm of the strip, yet older patient have less reading.
- The corneal diameter has effect on the specification of the CL prescribed especially lens diameter. Because the corneal diameter is assumed to be equal the diameter of the iris.- The actual measurement is made with a P.D. ruler. The pupil diameter can be approximated by using the iris as reference scale.- For older children and adult the iris is usually about 12mm in diameter.
- The palpebral Aperture height is important factor in determining corneal contact lens dimension.- the palpebral Aperture height is measured by instructing the patient to relax and fixating straight ahead, and measurement should be made of the maximum vertical distance when the lids are separated.- This measurement is difficult to obtain since the lid aperture is under voluntary control, so patient tend to squint when ruler is placed near their eyes.
- The refractive error of the patient must be measured and final prescription is written in minus cylinder form for ordering the contact lens.- Three reading of keratometer measurement for the patient is obtained for maximum accuracy, and then the median value of the three is recorded.
Contact fitting procedure lens1- Selection of lens diameter (Dia)- Obtain patients horizontal visible iris diameter (HVID) measurement- CL diameter = HVID + (1 to 3mm, average = 2mm)- Increase or decrease lens diameter in 0.50mm step if necessary during evaluation process.- Lens must completely cover cornea.- Most soft CLs are available from diameter of 13.50mm to 15mm. However, large diameter e.g. 15mm tends to tighten on the cornea and may result in complication e.g. ulcers or neo-vasculariztion.
2- Selection of Base curve (BC)- SCL are usually fitted flatter than the flattest K- The flattest K minus 3.00 diopter BC = flattest K - 3.00D- Convert the diameter value to millimeters using a converting table.- Increase or decrease BC in 0.30mm steps if necessary.- Clinical experience shows that majority of patient can be fitted with an average or median BC. This is usually is the 8.50-9.00mm range.
As a guide and an alternative method of BC:selection. The following table can be used K-reading Soft CL Base Curve Less < 41.00D Flat ( >9.00mm) Between 41.00– 45.00 D Medium (8.00-.00mm) Larger > 45.00D Steep (< 8.00mm)
Selection of lens power- 3 - Refraction prescription must be converted to minus cylinder from:1- If cylinder in refraction is (less than or equal to) less or equal ≤ 0.50D, power = spherical component2 - If cylinder in refraction is 0.750D to 1.00D, the contact lens power = spherical equivalent (spherical component + 1/2 Cyl)- If overall spherical component in 1 and 2 is greater than ± 4.00D, compensate for vertex distance using either method 1 or 2.
Method 1Fc = Fs / 1- d FsWhere Fc = power of CL, Fs = power of spectacle lens (D)d = distance between spectacle lens and CL in meterRef; -5.00 +1.00 x 90 (plus cyl form)-4.00 -1.00 x 180 (minus cyl form)vertex distance = 13mm » 13 / 1000 = 0.013mFc = 4.5 / 1- (0.013 x 4.5)Fc = 4.5 / 1- 0.0585Fc= 4.5 / 0.9415 = 4.249 ≈ 4.25D
Method 2, Add 1/2 of cyl to spheree.g.2)Ref; -5.00 + 1.00 x 90 (plus cyl form) x 180 (minus cyl form 1.00 - 4.00-contact lens power = - 4 + - 0.50 = - 4.50D The contact lens power from the table = - 4.25DThis is greater than ± 4.00D, so compensate forvertex distance e.g. 13mm (by calculation or )using table
Types of Soft contact lensesSoft contact lenses divided into four categories1- Daily ware - These lenses are worn on daily basis for 12 to 14 hours and removed before bedtime for cleaning and disinfection.- Methods of disinfection are heating, chemical and oxidation (hydrogen peroxide).
2- Flexible/Extended ware- Usually corresponds to patients who wear lenses overnight only on an occasional basis such as weekends, and should be cleaned and disinfection upon removal.- Most practitioners now recommend lens wear without overnight removal 3 to 7days.- Methods of disinfection involve chemical and oxidative.- These lenses also available in planned replacement, tinted and toric forms.
3- Planned replacement- These lenses allow the patient to replace lenses weekly or biweekly depend on their preferences.- For example, disposable lenses are worn for a one- week extended wear period and discarded, therefore, a care regimen is not required.
4- TintsThese are usually cosmetic Soft contact lens
Disinfection- The tear film is vital. Not only does it provide oxygen exchange as the lens is moved, but it also passes lysozyme, an antibacterial enzyme that inhibits bacterial proliferation.- Patients with a tear deficiency are more prone to infections and often cannot be fit comfortably with lenses.
- Many complications with soft contact lens wear occur after lenses are successfully fit, when patients care and handle their lenses.- Problems arise due to nature of hydrogel lens materials which vulnerable to contamination by bacteria and fungi.- Routine soft contact lens care including disinfection and cleaning.- There are three methods of disinfection used with SCL: thermal, chemical, and oxidative, each of these methods has advantages and disadvantages which will aid the practitioner in selecting the care regimen best suited for each patient and lens.
Thermal disinfection- This technique is not expensive and most effective system in the short term.- The thermal disinfection technique is contraindicated with lenses containing greater than 55% water.- Thermal care regimen consists of saline, surfactant cleaner, enzymatic cleaner and rewetting or lubricating drops. After soft contact lens removal:-1- The CL should be cleaned with surfactant cleaner.2- Stored in a case filled with saline.3- Enzymatic cleaner should be used weekly.
The advantages of thermal disinfection- Quick (20 min) require very few steps- Preservative-free solution for patients sensitive to preserved solutions- Effective against all form of bacteria such as pseudomonas and AIDS virus.The disadvantage of thermal disinfection- The heat bakes on the deposits so lens not cleaned- The lens life shortened- Not interchangeable with other care systems-Complications such as giant papillary conjunctivitis (GPC), or red eye occurs due to deposited lens.
Chemical disinfection- This method consist of a disinfecting solution that contain preservatives, surfactant cleaner, enzymatic cleaner and rewetting or lubricating drops, and many of these solutions may be used for rinsing and to store of the lenses.- For example, ReNu Multi-purpose solution can be used as a cleaner, saline and with the enzymatic tablets; however disinfection must still be performed following enzymatic cleaning.
The advantages of chemical systems- It can be used for all type of SCL.- Little effect on lens life.- It remove 90% of a measured amount of bacteria.- The solutions number and steps are less make it simple and convenient for patient.The disadvantage of chemical systems- The use of preservatives such as thimerosal and chlorhexidine that are toxic to some patient so it is more likely that the lens will have to be replaced.
Oxidative disinfection- 3- This technique consists of a 3% hydrogen peroxide solution, neutralizing (solution, tablet, and disc), saline, surfactant cleaner, enzymatic cleaner and rewetting or lubricating drops.- Hydrogen peroxide- Effective against bacteria- It can be used in a disinfection cycle of 10min with 10min of neutralization.- Yet longer exposure time is recommended to be effective against fungi.
Disadvantages this system- Large number of solutions and steps- Storing the lens in hydrogen peroxide for lengthily periods may affect the base curve radius of the lens, especially with high-water content lens materials.- The acidity of hydrogen peroxide could cause mild to moderate punctuate keratitis. To prevent this there are many methods to neutralizing it.Advantages of this system- Safe, effective, and preservatives-free.
Examples of Oxidative disinfection1- Allergan Optical has two systems Oxysept and UltraCare both are preservatives-free.A- The Oxysept is two step processes1- The CL is placed in the case containing hydrogen peroxide after proper time interval of disinfection (10min-12hours).2- Place neutralizing tablet in the case to neutralize hydrogen peroxide acidity.
B- The UltraCare is one step process- The CL is placed in the case containing hydrogen peroxide and place UltraCare neutralizing tablet at the same time.- The UltraCare neutralizing tablet is coated with a viscosity agent that prevents activation of tablet for 20-30mins this allow disinfection with hydrogen peroxide to occur prior to neutralization.
3. Ciba Vision has one system called AODisc.- The CL is placed in the case containing hydrogen peroxide after proper time interval of disinfection.- The platinum disc attached to the lens cage begins neutralizing hydrogen peroxide immediately upon contact when the lens cage is placed in the case.- The disc should be replaced after 3 months of daily use.
1- Saline solution - It is necessary part of hydrogel CL care, because the hydrogel CL must stay hydrated. - Saline is non toxic to eye and sterile used to rinse the lens from foreign body as well as to dissolve enzyme tablets.- Distilled water not suitable since it not sterile and easily contaminated.- Saline solution is not capable of disinfecting the lens when used a lone- It available in preserved (with thimersol or sorbic acid) and unpreserved (e.g. aerosol saline) solutions.
2- Surfactant cleaners - It prevents buildup of lens deposition thus it should be used after every lens removal.- It acts as a soap to remove debris, unbound proteins, lipid deposits and some microbial contamination.- The lens placed in the palm of the hand with few drops of the cleaner, the lens rubbed gently back and forth for 20 to 30 seconds, and then the lens rinsed and soaked in disinfection solution.
Alcon introduced three generations of cleaners for hydrogel CL:-- Opticlean (preservative was Thimerosal )- Opticlean II (preservative was Polyquad )- Opti-Free Dialy Cleaner (preservative was Polyquad)
Ciba Vision- Introduce Mira Flow contain among other cleaning ingredients, isopropyl alcohol.- Isopropyl alcohol eliminates the need for a preservative because of its broad-spectrum antimicrobial effects.- It excellent cleaner especially for patients with the tendency toward lipid deposits, but the lens should be rinsed to avoid the risk of parameter changes.
3- Enzymatic cleaner- It is used once a week to break down peptide bonds, allowing protein to be rubbed off mechanically.- The proper care sequence when enzyming hydrogel lenses are cleaning, rinsing, enzymatic cleaning, rinsing, and disinfecting.
4- Lens lubricants/ rewetting- It is optional, but may be beneficial in cases of dry eyes, foreign body sensation, irritations and for morning and evening use in extending wear.- Lens lubricants used directly in the eye with and without the lenses.- It is not suitable to use ophthalmic medication as lubricants because this could cause discoloration and cause toxic reaction.
Fitting evaluationNormal fit- Soft lens should be fitted with what is known as three-point touch.1- The lens should parallel the superior and inferior sclera as well is the corneal apex.2- When the lens rests only on the superior and inferior sclera and jump the corneal apex, the lens is too steep (Fig. 61).
- If the lens rests on the corneal apex and the edges stand off from the sclera, the lens is too flat (Fig. 6-3).
- All soft lenses, regardless of power, size, or manufacturer, should be fitted to obtain this three-point touch (Fig. 6-4).
- A well-fitted lens will show five basic qualities; good centration, adequate movement, stable vision, crisp retinoscopic reflex, clear undistorted keratometry mires, and clear endpoint over-refraction.
1- Good centration.- The lens will center itself well easily after insertion in the eye. After the patient blinks, it will not show more rim of lens on one side of the cornea than on the other side.- Lens decentration requires refitting with either a steeper base curve or a larger diameter (Fig. 6- 5).
2- Adequate movement- The slit lamp is very useful for evaluation of proper movement.- Fitting should be evaluated while the patient looks straight ahead, upward, and laterally. The patient should be asked to blink under slit lamp observation.- Evaluation should then be made clinically as to whether the movement is excessive, negligible, or adequate.
- A standard-thickness lens may show movement of 0.5 to 1 mm on upward gaze after a blink, and it should show no greater movement on lateral gaze.
- If the lens is even with tears and does not move, the person should be switched to a lens with a flatter base curve (Fig. 6-6, A).
If the lens moves excessively, a lens with a-steeper base curve (Fig. 6-7) series or one witha larger diameter should be substituted (Fig. 6-)6, C and D
. Stable vision- 3- When he patient blinks, the vision should remain equally clear before and during the blink and visual acuity should be as sharp as possible (Fig. 6-8). If trial-set lenses re used for fit evaluation, an over-refraction should be performed.- If visual acuity is not adequately sharp after changing the lenses or holding over low-plus or low minus lenses. It is useful to have the patient view an astigmatic clock. If some of the clock lines are significantly blurred, residual astigmatism is present and vision cannot be improved soft lenses.
- Variable vision initially may be caused by a lens that is either too loose or too tight. If the fit is found to be adequate and the patient still complains of fluctuating vision, such factors as dryness of the eye or from the environment, lack of blinking, or excess mucus secretions must be considered as causative factors.- Normally, blinking may be reduced with driving and reading. The patient should be warned of soft lens variable vision. It is easily reduced by a series of blinks or artificial tears.
4- Crisp retinoscopic reflex.- As confirmatory evidence of a good fit, the retinoscope, streak is flashed in all meridians while the patient blinks. When the patient is adequately fitted, the retinoscopic reflex will be sharp and crisp as if no lens were in place, both before and after blinking (Fig. 6-9, A).
- If the lens is steep, there will be a spreading of the streak centrally in the rest position, which will clear after a blink because of ironing out of the apical jump (Fig. 6-9, C).
- If the lens is flat, it may ride low; a position that can be detected by the retinoscopic shadow may be blurry immediately after a blink (Fig. 6-9, B).
5- Clear, undistorted keratometry mires.The mires that are reflected from the keratometer while the person is wearing the soft lens will often indicate if the fit is adequate. With the correct fit, the mires of the keratometer should not be distorted either before or after a blink (Fig. 6-8, A).
- If the mires are blurred, the patient should blink several times; if the mires are still distorted, the lens should be changed (Fig. 6-10, B and C).
Characteristic of steep fitting There are characteristics of steep fitting for soft contact lens:- Little or no movement either on blinking or as the eye change fixation.- Tight fit is quit comfortable, sometimes more so than a correct fit, because a complete immobile lens produces the minimum of lid sensation.- Usually good centration
- The slit lamp may show irritation of the conjunctival or limbal vessels and, with very light lenses an annular ring of conjunctival compression may be seen this often visible when lens removed.- Vision unstable and poor because momentary pressure on the eye during blinking occurred with steep fitting jump the corneal apex.- Subjective refraction is difficult with no clearly defined end point, and more negative power than predicated may be required because of a positive liquid lens.- Retinoscopy and keratometer mires both show irregular distortions these mires improve with blinking.
Characteristic of loss fittingThere are characteristics of loss fitting for soft contact lens:- Easily to diagnose because of poor centration, greater lens mobility on blinking and excessive lag on lateral eye movements.- Very uncomfortable especially on looking upwards, lower lid sensation experienced if the lens drops- Vision and over refraction are variable, but nevertheless may still give satisfactory results.
- The retinoscopy reflex may be clear centrally but with peripheral distorted.- The keratometry mires change according to lens movement.
CorrectionThe following steps should be taken to correct a loose lens:- Either changing the base curve by decreasing it by 0.2 to 0.3 mm OR- Increasing the diameter of the lens by 0.5mm up to 15mm.The following steps should be taken to correct a light lens:- Either changing the base curve by increasing it by 0.2 to 0.3 mm OR - Decreasing the diameter of the lens by 0.5mm.