Part 2 Ocular correction of ametropia 2021.pdf -...

Ocular correction of Ametropia
Course coordinator: Dr. Maitreyee Roy
Presenter: Associate Professor David Pye
School of Optometry and Vision Science,
UNSW, Sydney
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By the end of this lecture, you should
be able to understand optical
corrections required for:
• Surgical modification of the cornea
• Implant lenses
• The exploded method
• The eye under water
Learning outcomes
By the end of this lecture, you
should be able to understand optical
corrections required for:
• Surgical modification of the cornea
• Implant lenses
• The exploded method
• The eye under water
• Contact lenses
Learning outcomes

Surgical Modification of the Cornea
• In refractive surgery, the front surface
power of the cornea is altered by
changing the radius of curvature of the
front surface
• Knowing the front surface radius before
surgery we can calculate the radius of
curvature required after surgery to
provide correction
Surgical Modification of the Cornea
Before
After
Finite Element Analysis Dr Svyatoslav Fyodorov

EyeSys System 2000

Surgical Modification of the Cornea
Surgical Modification of the Cornea
• LASIK (Laser assisted in situ
keratomileusis) is one of a group of
procedures used to correct ametropia
• In this procedure, a microtome or
femtosecond (FS) laser is used to create
and lift a corneal flap of epithelial layer.
• Then the underlying stroma is sculpted
using a laser beam to correct the
refractive error
• The surgery is performed in a region of
about 5 to 7 mm diameter at the centre
of the cornea.
Surgical Modification of the Cornea

Surgical Modification of the Cornea
• To provide correction for a myopic eye the cornea is
flattened by removing more tissue from the central
region than from the peripheral region
cornea
ablation zone
cornea
flat corneal centre
Laser correction
Surgical Modification of the Cornea
• To provide correction for a hyperopic eye, the cornea is
steepened in the centre by removing more tissue from the
periphery

Surgical Modification of the Cornea - Pachymetry Surgical Modification of the Cornea - Pachymetry
Surgical Modification of the Cornea
• Sag at radial distance y; OB = OC-BC;
• BC = 𝒓
𝟐
− 𝒚
𝟐
; OC = r
• So, sag = r- 𝒓
𝟐