Physics of the Eye and Intralase Surgery

1
Physics of the Eye and Intralase Surgery
2
Sight and lenses

• The eye is a series of lenses that refract and
  focus light.
• These lenses follow the same laws of physics that
  artificial lenses would.

3
Lenses and focal points

• The cornea and lens of the eye act as two convex
  lenses. In order to understand illness and
  disorders of the eye, we must understand how
  lenses and focal points work.

4
Lenses and focal points
To find the focal point, distance of an image, or
distance to the object you can use the thin lens
equation.
Light bounces off an object from a distance s1.
This light passes through the lens to be focused
on the other side. This image is real, inverted,
and smaller, and is located at a distance of s2
from the lens.
The thin lens equation
5
Lenses and focal points
Focal Point
A convex lens
6
Lenses and focal points

• Changing the distance to an object, or changing
  the focal length of a lens, will result in a
  different focal point.
• Changes of these kind in the eye are the causes
  of visual acuity loss.

7
Sight, its a light thing.

• Light is reflected off of objects and enters into
  the eye.
• Colors are the wavelengths that are not adsorbed
  by the substance.

8
Anatomy and Function of the Eye

• As light enters the eye, it first passes through
  the cornea. Most of the refraction of light
  occurs here. The refractive index of the cornea
  is 1.38.

Refractive index Speed of light in vacuum Speed
of light in substance
9
Anatomy and Function of the Eye

• The eye can control the amount of light that
  enters by the opening and closing pupil aperture
  using the iris.

10
lt- Dilated
Constricted -gt
11
Anatomy and Function of the Eye

• After passing through the cornea, light enters
  and refracts again through the lens of the eye.
  The lens has a refractive index of 1.44.

12
Anatomy and function of the eye
This also changes the bulge of the cornea
somewhat. This also aids to adapt vision into
different situations.

• The eye can change the shape of the lens
• using muscles called ciliary muscles that
• stretch and flatten the lens, or relax and
• allow it to return to normal shape. This is
• called accommodation.

Sometimes this bulging of the cornea is an
appreciable change that can readily be seen by
the naked eye.
13
(No Transcript)
14
Anatomy and Function of the Eye

• The light then is focused onto the fovea, a cone
  and rod dense area on the retina, that produces
  the sharpest visual image.

15
Vision Disorders
Normal eye. Focus hits directly on fovea to
produce sharp image.
16
Vision Disorders
This is known as nearsightedness, or myopia. The
image comes into focus before the retina. Things
that are near to the eye can more easily be seen.
This condition can result from the eye being too
long, the cornea being too round, or the lens
having too much focus power.
17
Vision Disorders
This condition is known as farsightedness, or
hyperopia. The image produced by the lens and
cornea comes into a focal point behind where the
retina is located. Things that are farther away
can be more easily seen. This results from the
eye being too short, the cornea being too flat,
or the focus power of the lens being too weak.
18
Vision Disorders
This is a condition known as astigmatism. The
shape of the lens and/or cornea is irregular
leading to a loss of focal points or, more
commonly, two focal points on the retina.
19
Vision Disorders
This condition is known as presbyopia. The lens
becomes rigid and fails to bend or accommodate to
changing conditions.
20
Treatments

• For centuries people have been using lenses to
  correct or compensate for vision loss.
  Traditional remedies are as follows

21
Treatments
A diverging lens will correct for nearsightedness.
A converging lens corrects for farsightedness.
Bifocal lenses will compensate for presbyopia.
A custom ground lens made specifically for each
eye will correct astigmatism.
22
Treatments

• More recently, doctors have been using surgery to
  correct vision problems. These surgeries employ
  the same physics as corrective lenses would
  outside the eye, only the changes are done to the
  cornea and lens of the eye itself.

23
Treatments

• Radial Keratotomy was one of the original vision
  correction surgeries.
• Small cuts were made in the cornea radiating from
  the center. This was
• used to treat mild cases of myopia.

• Automated lamellar keratoplasty used a
• microkeratome to make a small flap in the cornea.
  This was then folded out of the way, and the
  microkeratome was then used to sculpt the cornea.
  ALK can only correct myopia.

A microkeratome
24
Treatments
Photorefractive keratotomy is the precursor to
LASIK surgery. This type of surgery was
introduced in 1987. It uses a Eximer laser to
reshape the outside of the cornea and thus
correct vision. PRK can correct myopia,
astigmatism, and some hyperopia.
25
Treatments

• In 1989 the FDA approved LASIK (Laser-Assisted in
  Situ Keratomileusis)
• for treatment of
• Nearsightedness
• Farsightedness
• Astigmatism

In LASIK surgery a mechanical cut is made across
the cornea to create a flap of tissue and
exposing the stroma of the cornea.
This flap is moved aside and a cool ultraviolet
laser sculpts and re-shapes the cornea,
correcting acuity problems. This laser allowed
for greater precision and thus greater
flexibility for treatment of a broader range of
symptoms.
26
Intralase Surgery
In 2001, the FDA approved a laser to be used in a
surgery called Intralase.
Before surgery, exhaustive measurements are taken
of each eye that will be operated on.
Measurements of refractivity of the eye tell the
prescription that each cornea and lens is
contributing.
The topography of the cornea and the measurements
of the pupil are also taken.
27
Intralase Surgery
The eye is anesthetized with drops and a suction
ring is placed on the cornea to immobilize the
eye. A guide cone is then placed on the ring to
direct the eye appropriately. The laser will
track the eye in case of accidental eye movement
during the operation.
28
Intralase Surgery
A femosecond laser then makes a very precise
(/- 10 microns) flap across the surface of the
cornea, instead of a microkeratome. (A human hair
is 50 microns in diameter). This reduces rare
complications that LASIK presented, and also
opened corrective eye surgery to previously
ineligible patients.
This laser produces carbon dioxide and water
bubbles in the cornea. These bubbles are what
actually produce the incision to create the
flap. This can also be done vertically to produce
a deeper incision.
29
Intralase Surgery

• After waiting for the cavitation bubbles to
  disappear, the flap is pulled aside and the
  cornea is then sculpted to the needed
  measurements.

30
Intralase Surgery

• The flap is then folded back, and the patient is
  ready to return home.
• A few follow up appointments are done to ensure
  the eyes are moist and that the flaps are closed
  properly.

31
Intralase Surgery

• Intralase and LASIK surgeries can repair
  hyperopia, myopia, and astigmatism.
• By sculpting one eye to be farsighted and one
  to be nearsighted, you can eliminate the need
  for bifocals as well.

32
Intralase Surgery

• Intralase surgery has the same benefits of LASIK,
  but with fewer risks by using a laser to make the
  flap instead of a mechanical cut.
• Greater precision also allows patients that were
  at too high of a risk for complications before to
  now be eligible for surgery.

33
(No Transcript)
34
Bibliography
Pictures

• http//sln.fi.edu/300/images/bifocals.jpg
• plusinfo.jeonju.ac.kr/photos/Art
• http//bnet.org/rbsd/ruhs/academics/science/physic
  s/project2/thierren/convex.jpg
• http//en.wikipedia.org/wiki/Lens_(optics)
• http//www.shokabo.co.jp/sp_e/optical/labo/lens/le
  ns_c2.jpg
• http//www.eyeny.com/images/normal-vision.jpg
• http//www.mcoaicare.com/images/eye.jpg
• http//health.howstuffworks.com/lasik.htm
• http//nate.calc.org/webalbum/fun
• www.peterglass.com/funny20glasses20LG.htm
• www.avclinic.com/rk.htm
• www.institutdelamyopie.com/microkera.htm
• www.focusmedical.ch/aserbehandlung_en.html
• www.refraktives-zentrum.de/behandlung.php
• 141.211.80.200/01regentsreport.html
• www.uvcenters.com/refractive/lasik.html
• www.intralaser.com/
• http//www.optometric.com/article.aspx?article711
  62
• http//files.turbosquid.com/Preview/Content_on_5_8
  _2002_12_54_13/Lightbulb_Idea.jpg764383AE-CDAC-497
  7-AB1FEF08A38CBE19.jpgLarge.jpg

35
Bibliography
Movies

• http//www.ebaumsworld.com/videos/popeyes.html
• http//www.schustereyecenter.com/ts_main.sstg?page
  visx

References

• http//www.physicsclassroom.com/Class/refrn/U14L6a
  .html
• http//www.durrievision.com/index.cfm/news/intrala
  sedec
• http//healthlink.mcw.edu/article/1031002532.html
• http//www.optometric.com/article.aspx?article711
  62
• http//health.howstuffworks.com/lasik.htm
• http//www.fda.gov/cdrh/LASIK/what.htm
• http//en.wikipedia.org/wiki/Lens_(optics)