17. Refractive Surgery Flashcards
Ways to alter the refractive state of the eye:
refractive power of the ocular media, depth of anterior chamber, axial length of the eye
- Corneal surgery is limited to correcting refractive errors of
+4 to -10D
- Most patients will be unable to tolerate anisometropia
of more than 2.50D
- Monovision
one eye emmetropic for distance, one eye myopic for near
myopes and refractive surgery
will prefer to the left mildly myopic post surgery
CL cause
corneal warpage and thickening
PMMA hard CL duration
15 weeks
Gas permeable duration
10 weeks
Soft CL duration
5 weeks
Photorefractive keratectomy (PRK)
- 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
PRK for myopia
o Successive concentric applications of increasing diameter are made
o More tissue is ablated centrall than peripherally surface curvature is reduced
issues with corrected area
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
ways to overcome issues with corrected area
multiple concentric treatment zones and smoothing transition between them
highly myopic eyes and PRK
less predictable with increased risk of sub-epithelial scarring
astigmatism and PRK
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
ways of correcting astigmatism with PRK
Slit beam – widens for successive applications
Elliptical ablation zone
Scanning beam
Ablatable mask
PRK and hypermetropia
more difficult
o Not widely used
o More tissue must be ablated peripheraly than centrally to make the cornea steeper
LASIK
Laser Intrastromal Keratomileusis
principles of LASIK
- 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
LASIK vs PRK
Benefits compared to PRK:
o Little subepithelial scarring and myopic regression
o Earlier stabiliisation of refacion
o Superior predicitbility for high myopes
Radial keratotomy (RK)
- Used to irreversibily flatten the central corneal curvature for myopia
RK steps
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
Greater effect
o Longer
o Deeper
o More incision
o Smaller central zone
Central zone
3-5mm (any less will increase risk of glare, more will have little effect)
Number of incision in RK
use 4 or 8 incisions – 4 gives the option of further treatment
Best results for RK
myopes <-5.00D
o More than ¾ will end of with 1.00D emmetropia
o Stable after 6 months
Risks of RK
additional tx may be difficult, and won’t be suitable for CLs due to change cornea
- Astigmatism may originate from the
cornea, lens, physiological, following surgery or trauma
Best way to correct astigmatisim
with rigid contact lenses or surgery
correcting astigmatisim
- Very unpredictable
o Post-keratoplasty astigmatism is frequently irregular
ways to reduce graft astigmatism
- Continuous sutures, interrupted sutures, a combination or double continuous sutures can reduce graft astigmatisim
keratoscopy
can allow adjusted intra-operatively
later adjustments
can be made by redistributing tension along a continuous suture or removing sutures in the steeper medidia
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
to reduce pre-exisiting atigmatism
incision made AT THE STEEPEST meridian
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
better incision
incisions are tangential or curvilinear
incisions with the greatest effect
- Longer incision or incisions closer to the visual axis have the greatest effect
indication for wedge rescection
- High degrees (>10D) of astigmatism post penetrating keratoplasty
how is a wedge rescection done
- Removing deep arcuate wedge measuring 60-90 degrees from the graft host junction in the FLATTEST meridian
effect of a wedge resecation
reverse of relaxing keratotomy
effect of wound suturing in wedge resection
is sutured with non-absorbable merisilne shortens cornea and steepens in that meridian
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
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
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
- Greatest effect for corneal sutrues
longer, deeper or closer to visual axis
tight radial sutures
tight radial suture induces corneal astigmatism by increasing the corneal curvature in that meridian
continous sutures
- Cotinous sutures can cause less astigmatism as they distubt tension more evenly across the wound
small incision in phaco
Phaco will FLATTEN the cornea in that meridian
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
refractive outcome for silicone buckle
Increase axial length Esp. if encircles the eye
Compressive the eye asymmetrically
induce myopia
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
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
refractive outcome for gas filled phakic eyes
greatly increases the refractive power of the posterior surface of the lens
Large myopic shift
refractive outcome for gas filled aphakic eyes
makes the posterior corneal surface highly diverging
almost neutralises the refractive power of the cornea
