17. Refractive Surgery

Question 1

Ways to alter the refractive state of the eye:

Answer 1

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

Question 2

• Corneal surgery is limited to correcting refractive errors of

Answer 2

+4 to -10D

Question 3

• Most patients will be unable to tolerate anisometropia

Answer 3

of more than 2.50D

Question 4

• Monovision

Answer 4

one eye emmetropic for distance, one eye myopic for near

Question 5

myopes and refractive surgery

Answer 5

will prefer to the left mildly myopic post surgery

Question 6

CL cause

Answer 6

corneal warpage and thickening

Question 7

PMMA hard CL duration

Answer 7

15 weeks

Question 8

Gas permeable duration

Answer 8

10 weeks

Question 9

Soft CL duration

Answer 9

5 weeks

Question 10

Photorefractive keratectomy (PRK)

Answer 10

• 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

Question 11

PRK for myopia

Answer 11

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

Question 12

issues with corrected area

Answer 12

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

Question 13

ways to overcome issues with corrected area

Answer 13

multiple concentric treatment zones and smoothing transition between them

Question 14

highly myopic eyes and PRK

Answer 14

less predictable with increased risk of sub-epithelial scarring

Question 15

astigmatism and PRK

Answer 15

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

Question 16

ways of correcting astigmatism with PRK

Answer 16

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

Question 17

PRK and hypermetropia

Answer 17

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

Question 18

LASIK

Answer 18

Laser Intrastromal Keratomileusis

Question 19

principles of LASIK

Answer 19

• 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

Question 20

LASIK vs PRK

Answer 20

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

Question 21

Radial keratotomy (RK)

Answer 21

• Used to irreversibily flatten the central corneal curvature for myopia

Question 22

RK steps

Answer 22

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

Question 23

Greater effect

Answer 23

o Longer
o Deeper
o More incision
o Smaller central zone

Question 24

Central zone

Answer 24

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

Question 25

Number of incision in RK

Answer 25

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

Question 26

Best results for RK

Answer 26

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

Question 27

Risks of RK

Answer 27

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

Question 28

* Astigmatism may originate from the

Answer 28

cornea, lens, physiological, following surgery or trauma

Question 29

Best way to correct astigmatisim

Answer 29

with rigid contact lenses or surgery

Question 30

correcting astigmatisim

Answer 30

* Very unpredictable o Post-keratoplasty astigmatism is frequently irregular

Question 31

ways to reduce graft astigmatism

Answer 31

* Continuous sutures, interrupted sutures, a combination or double continuous sutures can reduce graft astigmatisim

Question 32

keratoscopy

Answer 32

can allow adjusted intra-operatively

Question 33

later adjustments

Answer 33

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

Question 34

incisions made on the conrea

Answer 34

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

Question 35

to reduce pre-exisiting atigmatism

Answer 35

incision made AT THE STEEPEST meridian

Question 36

atigmatism in the conreal graft

Answer 36

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

Question 37

better incision

Answer 37

incisions are tangential or curvilinear

Question 38

incisions with the greatest effect

Answer 38

* Longer incision or incisions closer to the visual axis have the greatest effect

Question 39

indication for wedge rescection

Answer 39

* High degrees (>10D) of astigmatism post penetrating keratoplasty

Question 40

how is a wedge rescection done

Answer 40

* Removing deep arcuate wedge measuring 60-90 degrees from the graft host junction in the FLATTEST meridian

Question 41

effect of a wedge resecation

Answer 41

reverse of relaxing keratotomy

Question 42

effect of wound suturing in wedge resection

Answer 42

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

Question 43

compression sutures

Answer 43

* 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

Question 44

intra-stromal corneal rings

Answer 44

* 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

Question 45

Epikeratophakia

Answer 45

* 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

Question 46

* Greatest effect for corneal sutrues

Answer 46

longer, deeper or closer to visual axis

Question 47

tight radial sutures

Answer 47

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

Question 48

continous sutures

Answer 48

* Cotinous sutures can cause less astigmatism as they distubt tension more evenly across the wound

Question 49

small incision in phaco

Answer 49

Phaco will FLATTEN the cornea in that meridian

Question 50

penetraing keratoplasty

Answer 50

* 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

Question 51

refractive outcome for silicone buckle

Answer 51

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

Question 52

refractive outcome for silicone filled phakic eyes

Answer 52

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

Question 53

refractive outcome for silicone filled aphakic eyes

Answer 53

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

Question 54

refractive outcome for gas filled phakic eyes

Answer 54

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

Question 55

refractive outcome for gas filled aphakic eyes

Answer 55

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