Soft contact lens 345.PdF

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					Soft contact lens
      345
    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).
                                 H2O




Figure1. A hollow glass semi-spheroid filled
with 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 was
apparently the first to
successfully fit contact
lenses, which were made
from 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 lenses
is to protect and reshape the optical properties of
irregular cornea in disease such as keratoconuse


- It become clear that corneal shape is critical
importance in CL design, and the cornea obtains
the bulk of its oxygen supply from the air and
that 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.
Tear layer




  The cornea
- 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 tear
film. The tear film supply the cornea with
oxygen from the atmosphere when the eyes are
open.
-Thecornea has no blood vessels, the oxygen
supply necessary for normal metabolism.


                                  O2




                                   O2
During sleep, the eyelids block oxygen from
the atmosphere, and most of the oxygen in the
tears diffuses from the blood vessels of the
limbus and the palpebral conjunctiva. This
reduces the amount of oxygen in the tear film
to approximately one third.
       Palpebral conjunctival blood vessels
                                              Eyelid


                                               O2




                                              O2


Fig.
                                  Limbal capillaries
- 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 exchange

Fig. 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 cornea
Fig. 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 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.
- 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 often
 result 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.
                      Radius
The 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 be
calculated with the formula
D = n2 – n / r
D = dioptric power;
n = index of refraction of first medium;
n2= index of refraction of second medium.
       The cornea

                    Air =n1
                     Light




The lens
The power of the typical anterior corneal surface
D = 1.376 – 1.000 / 0.007
D = 0.376 / 0.007
D = + 48.83 D

The power of the typical posterior corneal surface
D = 1.336 – 1.376 / 0.0068
D = 0.040 / 0.0068
D=

The total corneal power in round numbers is
+ 48.83 - 5.88 = + 42.90 D
- The typical anterior human lens surface power
D = 1.41– 1.336 / 0.010
D = 0.074 / 0.0010
D = + 7.40 D

- The typical posterior human lens surface power
D = 1.376 – 1.000 / 0.0006
D = - 0.0074 / 0.0006
D = + 12.33 D
- The total corneal power in round numbers is
+ 7.40 + 12.33 = + 19.70 D
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 is
F = 1/D
f = 1/58.00
f = 0,017 = 17 mm
Where f = focal length; D = dioptric power.
               Index of Refraction
If the radius (r ) of the refractive surface and length
of the eye are constant, then the variations of the
index of refraction (n) (Table 2.4) will change the
power of the refracting surface. Using the
formula
D = n2 – n / r
assigning a value to r of r = 7.50 mm, then if n is
given, the amount of change dioptric power can
be calculated.
For example, if n2= 1.33, then
D = 1.33 – 1.00/0.0075
D = 0.33/0.0075
D = 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 dependent
upon the radius of curvature ( r) and the index of
refraction (n).
If n is kept constant and r is changed, the power
will also change
If D = n2 – n / r,
n = air
n2 = PMMA contact lens (n = 1.49)
then the effect of changes in r can be
calculated.
If r = 7.50
D = 1.49 – l-00/0-0075
D = 0.49/0.0075
D = 65.333 diopters
The smaller the radius or the steeper the
refractive surface, the greater the refractive
power of the lens (Table 2.b).
                 Vertex Distance
The power of a lens is the reciprocal of the focal
length, the relative or effective power of a
corrective lens changes with the placement of the
lens or the distance between the lens and the eye.
This relationship is expressed by the formula
                      D= 1 /f
Where D = power in diopters, f = focal length in
meters.
Example. In a +10.00 diopter lens, the focal
length 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 be?
Δ = 13² x 0.013
Δ = 169 X 0.013
Δ =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 =D²d
Δ = -10² X 0.015
Δ = 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 examination

1- 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.
2- Procedure for slit lamp:
- Seat the patient comfortably at the slit lamp by
adjusting the patient seat, the slit lamp height
and chine rest or both.
- Examine all the tissue mentioned above.
Particularly those directly related to contact lens
fitting.
- Diffuse illumination used to examine the
conjunctiva and the lids.
Direct 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 lens
1- Selection of lens diameter (Dia)
- Obtain patient's 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)
3- Selection of lens power
 - 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
  component
2 - 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 1
Fc = Fs / 1- d Fs
Where Fc = power of CL, Fs = power of spectacle
  lens (D)
d = distance between spectacle lens and CL in
  meter
Ref; -5.00 +1.00 x 90 (plus cyl form)
-4.00 -1.00 x 180 (minus cyl form)
vertex distance = 13mm » 13 / 1000 = 0.013m
Fc = 4.5 / 1- (0.013 x 4.5)
Fc = 4.5 / 1- 0.0585
Fc= 4.5 / 0.9415 = 4.249 ≈ 4.25D
Method 2
Add 1/2 of cyl to sphere,
e.g.2
Ref; -5.00 + 1.00 x 90 (plus cyl form)
- 4.00 - 1.00 x 180 (minus cyl form)
contact lens power = - 4 + - 0.50 = - 4.50D
The contact lens power from the table = - 4.25D
This is greater than ± 4.00D, so compensate for
vertex distance e.g. 13mm (by calculation or
using table)
         Types of Soft contact lenses
Soft contact lenses divided into four categories

1- 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- Tints
These 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.
3- Oxidative disinfection

- 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 disinfection

1- Allergan Optical has two systems Oxysept and
  UltraCare both are preservatives-free.

A- The Oxysept is two step processes

1- 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 evaluation
Normal 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 with
a larger diameter should be substituted (Fig. 6-
6, C and D)
3- Stable vision.
- 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 fitting

There 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.
                     Correction
The 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.

				
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