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Optics > 17. Refractive Surgery > Flashcards

17. Refractive Surgery Flashcards

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

A

o Longer
o Deeper
o More incision
o Smaller central zone

24
Q

Central zone

A

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

25
Q

Number of incision in RK

A

use 4 or 8 incisions – 4 gives the option of further treatment

26
Q

Best results for RK

A

myopes <-5.00D
o More than ¾ will end of with 1.00D emmetropia
o Stable after 6 months

27
Q

Risks of RK

A

additional tx may be difficult, and won’t be suitable for CLs due to change cornea

28
Q
  • Astigmatism may originate from the
A

cornea, lens, physiological, following surgery or trauma

29
Q

Best way to correct astigmatisim

A

with rigid contact lenses or surgery

30
Q

correcting astigmatisim

A
  • Very unpredictable
    o Post-keratoplasty astigmatism is frequently irregular
31
Q

ways to reduce graft astigmatism

A
  • Continuous sutures, interrupted sutures, a combination or double continuous sutures can reduce graft astigmatisim
32
Q

keratoscopy

A

can allow adjusted intra-operatively

33
Q

later adjustments

A

can be made by redistributing tension along a continuous suture or removing sutures in the steeper medidia

34
Q

incisions made on the conrea

A

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
Q

to reduce pre-exisiting atigmatism

A

incision made AT THE STEEPEST meridian

36
Q

atigmatism in the conreal graft

A

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
Q

better incision

A

incisions are tangential or curvilinear

38
Q

incisions with the greatest effect

A
  • Longer incision or incisions closer to the visual axis have the greatest effect
39
Q

indication for wedge rescection

A
  • High degrees (>10D) of astigmatism post penetrating keratoplasty
40
Q

how is a wedge rescection done

A
  • Removing deep arcuate wedge measuring 60-90 degrees from the graft host junction in the FLATTEST meridian
41
Q

effect of a wedge resecation

A

reverse of relaxing keratotomy

42
Q

effect of wound suturing in wedge resection

A

is sutured with non-absorbable merisilne  shortens cornea and steepens in that meridian

43
Q

compression sutures

A
  • 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
Q

intra-stromal corneal rings

A
  • 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
Q

Epikeratophakia

A
  • 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
Q
  • Greatest effect for corneal sutrues
A

longer, deeper or closer to visual axis

47
Q

tight radial sutures

A

tight radial suture induces corneal astigmatism by increasing the corneal curvature in that meridian

48
Q

continous sutures

A
  • Cotinous sutures can cause less astigmatism as they distubt tension more evenly across the wound
49
Q

small incision in phaco

A

Phaco will FLATTEN the cornea in that meridian

50
Q

penetraing keratoplasty

A
  • 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
Q

refractive outcome for silicone buckle

A

Increase axial length Esp. if encircles the eye
Compressive the eye asymmetrically
induce myopia

52
Q

refractive outcome for silicone filled phakic eyes

A

Higher refractive index than lens, changes posterior surface of lens (more divering)
Hypermetropic shift +5.00 to +7.00D

53
Q

refractive outcome for silicone filled aphakic eyes

A

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
Q

refractive outcome for gas filled phakic eyes

A

greatly increases the refractive power of the posterior surface of the lens
Large myopic shift

55
Q

refractive outcome for gas filled aphakic eyes

A

makes the posterior corneal surface highly diverging
almost neutralises the refractive power of the cornea

Optics (18 decks)

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