Optics > 17. Refractive Surgery > Flashcards

17. Refractive Surgery Flashcards

(55 cards)

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1
Q

Ways to alter the refractive state of the eye:

A

refractive power of the ocular media, depth of anterior chamber, axial length of the eye

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2
Q
  • Corneal surgery is limited to correcting refractive errors of
A

+4 to -10D

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3
Q
  • Most patients will be unable to tolerate anisometropia
A

of more than 2.50D

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4
Q
  • Monovision
A

one eye emmetropic for distance, one eye myopic for near

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5
Q

myopes and refractive surgery

A

will prefer to the left mildly myopic post surgery

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6
Q

CL cause

A

corneal warpage and thickening

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7
Q

PMMA hard CL duration

A

15 weeks

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8
Q

Gas permeable duration

A

10 weeks

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9
Q

Soft CL duration

A

5 weeks

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10
Q

Photorefractive keratectomy (PRK)

A
  • Uses an excimer laser to change the anterior curvature of the cornea
  • Each pulse ablates tissue from the surface of the cornea to a depth of 0.4 – 0.5 microns
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11
Q

PRK for myopia

A

o Successive concentric applications of increasing diameter are made
o More tissue is ablated centrall than peripherally  surface curvature is reduced

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12
Q

issues with corrected area

A

o If diameter of treated area 3.5 – 4mm = haeloes
o Wider treatment (6-7mm) makes than less likely but requires deeper ablation to achieve same refractive outcome

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13
Q

ways to overcome issues with corrected area

A

multiple concentric treatment zones and smoothing transition between them

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14
Q

highly myopic eyes and PRK

A

less predictable with increased risk of sub-epithelial scarring

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15
Q

astigmatism and PRK

A

o Harder to correct than spherical error
o Regular astigmatism is corrected by reducing the surface curvature more in the steepest meridian than any other

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16
Q

ways of correcting astigmatism with PRK

A

 Slit beam – widens for successive applications
 Elliptical ablation zone
 Scanning beam
 Ablatable mask

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17
Q

PRK and hypermetropia

A

more difficult
o Not widely used
o More tissue must be ablated peripheraly than centrally to make the cornea steeper

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18
Q

LASIK

A

Laser Intrastromal Keratomileusis

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19
Q

principles of LASIK

A
  • A mechanical microkeratome is used to dissect through the superficial stroma = lamellar circular flap of uniform thickness
  • Bared stroma is then reshapped using an excimer laser
  • The flap can then be replaced over the top  flap and zap
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20
Q

LASIK vs PRK

A

Benefits compared to PRK:
o Little subepithelial scarring and myopic regression
o Earlier stabiliisation of refacion
o Superior predicitbility for high myopes

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21
Q

Radial keratotomy (RK)

A
  • Used to irreversibily flatten the central corneal curvature for myopia
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22
Q

RK steps

A

Partial thickness radial incisions are placed symmetrically in the mid-peripheral and peripheral cornea, sparing the central cornea
o This weakens the cornea  bulges due to IOP
o Adult cornea does not stretch therefore flattens the central cornea
o Depth = 80-90%
o Used diamond knife that prevents too deep incision

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23
Q

Greater effect

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A

o Longer
o Deeper
o More incision
o Smaller central zone

24
Q

Central zone

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A

3-5mm (any less will increase risk of glare, more will have little effect)

25
Number of incision in RK
use 4 or 8 incisions – 4 gives the option of further treatment
26
Best results for RK
myopes <-5.00D o More than ¾ will end of with 1.00D emmetropia o Stable after 6 months
27
Risks of RK
additional tx may be difficult, and won’t be suitable for CLs due to change cornea
28
* Astigmatism may originate from the
cornea, lens, physiological, following surgery or trauma
29
Best way to correct astigmatisim
with rigid contact lenses or surgery
30
correcting astigmatisim
* Very unpredictable o Post-keratoplasty astigmatism is frequently irregular
31
ways to reduce graft astigmatism
* Continuous sutures, interrupted sutures, a combination or double continuous sutures can reduce graft astigmatisim
32
keratoscopy
can allow adjusted intra-operatively
33
later adjustments
can be made by redistributing tension along a continuous suture or removing sutures in the steeper medidia
34
incisions made on the conrea
Incisions made cause the cornea to bulge at that site o This REDUCES surface curvature in the meridian in which the incision is made o It INCREASES the curvature in the meridian at 90degrees to it
35
to reduce pre-exisiting atigmatism
incision made AT THE STEEPEST meridian
36
atigmatism in the conreal graft
o Incising the graft-host junction over 60-90 degrees where it is crossed by the meidian of the steepest curvate o Conserves the wound and is more predictable
37
better incision
incisions are tangential or curvilinear
38
incisions with the greatest effect
* Longer incision or incisions closer to the visual axis have the greatest effect
39
indication for wedge rescection
* High degrees (>10D) of astigmatism post penetrating keratoplasty
40
how is a wedge rescection done
* Removing deep arcuate wedge measuring 60-90 degrees from the graft host junction in the FLATTEST meridian
41
effect of a wedge resecation
reverse of relaxing keratotomy
42
effect of wound suturing in wedge resection
is sutured with non-absorbable merisilne  shortens cornea and steepens in that meridian
43
compression sutures
* A tight suture is placed across the graft-host junction in the FLATTEST meridian to increase the cruvatrure of the cornea * Relaxing incisions can also be combined – placed 90degrees away
44
intra-stromal corneal rings
* Placing a PMMA split ring into the mid-periperal corneal stroma in a concentric fashion around the limbus * Flattens the central corneal curvature to treat myopia up to -4.00 * Central zone is untouched and reversible
45
Epikeratophakia
* Creates a new corneal surface with a different surface curvature by attaching a lenticular of pre-shaped donor cornal stroma to the surface of the host conrea * Reversible, but reciptient cornea must not be changed most commonly usued for keratoncus
46
* Greatest effect for corneal sutrues
longer, deeper or closer to visual axis
47
tight radial sutures
tight radial suture induces corneal astigmatism by increasing the corneal curvature in that meridian
48
continous sutures
* Cotinous sutures can cause less astigmatism as they distubt tension more evenly across the wound
49
small incision in phaco
Phaco will FLATTEN the cornea in that meridian
50
penetraing keratoplasty
* Refracting outcome is determined in part of the donor tissue * Graft whose diameter exceeds that of the tissue removed = myopia * Peadiatric corneas are good for aphakic PKs as they have greater surface curvature
51
refractive outcome for silicone buckle
Increase axial length Esp. if encircles the eye Compressive the eye asymmetrically induce myopia
52
refractive outcome for silicone filled phakic eyes
Higher refractive index than lens, changes posterior surface of lens (more divering) Hypermetropic shift +5.00 to +7.00D
53
refractive outcome for silicone filled aphakic eyes
curved anterior surface and higher refractive index of silicone oil compared with the crystalline lens is more strongly converging and therefore produces a myopic shift. hypermetropia therefore be of the order of only +4.00 to +6.00 D
54
refractive outcome for gas filled phakic eyes
greatly increases the refractive power of the posterior surface of the lens Large myopic shift
55
refractive outcome for gas filled aphakic eyes
makes the posterior corneal surface highly diverging almost neutralises the refractive power of the cornea
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