Soft contact lens 345

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Soft contact lens 345
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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).

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H2O
Figure1. A hollow glass semi-spheroid filled
with water
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• - In 1636, Descartes suggested applying a tube
  full of water directly to the eye to correct a
  refractive error.
• Figure2. Optical apparatus described by
  Descartes

H2O
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In 1887, Adolf Fick was apparently the first to
successfully fit contact lenses, which were made
from brown glass
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• - 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.

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• - 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.

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• - 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.

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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.
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• - 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.

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Tear layer
The cornea
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• - 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.

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• - 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.

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• 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.

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• In general these materials should be
• Stable.
• Clear.
• Nontoxic.
• Non-allergic.
• Optically desirable.

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• 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.

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• 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.

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• 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.

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• 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 BauschLomb (Soflens)

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• 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.

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• 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.

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• -The cornea has no blood vessels, the oxygen
  supply necessary for normal metabolism.

O2
O2
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• 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.

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Fig.
Palpebral conjunctival blood vessels
Eyelid
O2
O2
Limbal capillaries
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• - 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.

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Fig. The pumping mechanisum
Contact lens
O2
O2
Tear exchange
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• - 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

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Fig. The diffusion mechanism
The pores
O2
The contact lens
The cornea
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• - 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.

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• - 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.

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• MESUREMENT OF OXYGEN TRANSMISSION
• - Direct diffusion is the major source of oxygen
  transmission with soft lenses,
• - it is very important to measure this parameter.
  

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• 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.

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• - 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.

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• - The diffusion coefficient (D) is the speed with
  which gas molecules travel (diffuse) through the
  material (Figure 15).

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• - The solubility coefficient (k) defines how much
  gas can be dissolved in a unit volume of the
  material at a specified pressure (Figure 16).

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• 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).

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• - 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) _at_ 25C

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• 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.

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• - 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.

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• 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.

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• - 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.

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• - 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.

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• - 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.

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• - 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.

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• - 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.

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• - 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.

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• - 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

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• 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 BL 70 Minus)

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• 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.

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• 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.

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The cornea
Air n1
Light
The lens
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• 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

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• - 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.

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• 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.

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• 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

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• - AS the index of refraction (n) increases, the
  refractive power increases (Table 2.5).

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• 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,

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• 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).

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• 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

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• - 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
• ? D²d
• Where ? change in power due to vertex
  distance D lens power d distance lens in
  meters.

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• Example 1. An aphakic spectacle correction of
  13.00 diopters at 13 mm in from the eye. What
  power should the contact lens be?
• ? 13² x 0.013
• ? 169 X 0.013
• ? 2.197 diopters

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• 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
• ? D²d
• ? -10² X 0.015
• ? 1.5 D

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• 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.

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• 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.

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• 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.

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• 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.

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• - 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

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• ? 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.

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• - 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.

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• ? 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.

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• - 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.

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• - 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.

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• - 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.

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• 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.

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• 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.

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• As a guide and an alternative method of BC
  selection. The following table can be used

Soft CL Base Curve K-reading
Flat ( gt9.00mm) Medium (8.00-.00mm) Steep (lt 8.00mm) Less lt 41.00D Between 41.00 45.00 D Larger gt 45.00D
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• 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.

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• 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

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• 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)

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• 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).

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• 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.

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• 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.

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• 4- Tints
• These are usually cosmetic Soft contact lens

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• 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.

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• - 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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.

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• 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)

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• 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.

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• 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.

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• 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.

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• 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).

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• - If the lens rests on the corneal apex and the
  edges stand off from the sclera, the lens is too
  flat (Fig. 6-3).

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• - All soft lenses, regardless of power, size, or
  manufacturer, should be fitted to obtain this
  three-point touch (Fig. 6-4).

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• - 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.

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• 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).

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• 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.

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• - 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.

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• - 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).

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• - 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)

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• 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.

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• - 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.

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• 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).

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• - 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).

130

• - 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).

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• 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).

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• - 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).

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• 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

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• - 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.

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• 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.

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• - The retinoscopy reflex may be clear centrally
  but with peripheral distorted.
• - The keratometry mires change according to lens
  movement.

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• 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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