Chapter 17: Refractive surgery Flashcards
What is the refractive error limit to corneal refractive surgery and why?
within the ray +4.00 to -10.00 D
because of greater likelihood of corneal scarring and reduced predictability of outcome with larger refractive errors
What are 3 options for the refractive outcome of refractive surgery?
- emmetropia - clear vision at infinity
- aim for -2.50D if pt wishes to dispense with glases for near
- some surgeons aim -1.00D for reasonable near and distance acuity
What are 4 options for dealing with anisometropia after operating on one ametropic eye?
- contact lens in unoperated eye
- enduring anisometropia until fellow eye operated on
- making first eye less ametropic and second even less so
- monovision
What proportion of patients are unable to adapt to monovision?
as many as 50%
What are 3 things used in vitreoretinal surgery that can have unavoidable effects on post-operative refractive errors?
- scleral buckles
- silicone oil
- intraocular gas
What 2 things must be stable before considering a procedure to change the refractive state of the cornea?
- refractive error
- corneal topography
What is considered the lower acceptable age limit for corneal refractive surgery due to changes happening in terms of refractive error/corneal topography?
21 years
For what time period after the following contact lens types have been worn can it take for corneal warpage to stabilise?
1. PMMA hard lenses
2. gas permeable lenses
3. soft contact lenses
- 15 weeks
- 10 weeks
- 5 weeks
What are 4 common types of refractive correction surgery?
- Photorefractive keratectomy (PRK)
- Laser intrastromal keratomileusis (LASIK)
- Laser-assisted sub-epithelial keratomileusis (LASEK)
- Radial keratotomy
What are 4 common types of refractive correction surgery?
- Photorefractive keratectomy (PRK)
- Laser intrastromal keratomileusis (LASIK)
- Laser-assisted sub-epithelial keratomileusis (LASEK)
- Radial keratotomy
What are 3 methods for the surgical correction of astigmatism?
- Relaxing incisions - transverse and arcuate keratotomy
- Compressive techniques - wedge resection
- Compression sutures
What are 9 additional less commonly used/up and coming surgical methods for refractive correction?
- Intrastromal corneal ring
- Epikeratophakia
- Keratomileusis
- Keratophakia
- Thermokeratoplasty
- Clear lens extraction
- Phakic IOL
- Corneal incisions
- Penetrating keratoplasty (PK)
What type of laser is used in LASIK/LASEK/PRK?
Excimer laser (excited dimer)
What does PRK involve?
surgeon removes the corneal epithelium and uses an excimer laser to apply computer-controlled pulses of light energy to reshape the anterior curvature of the cornea
How does PRK work to correct myopia?
successive concentric applications of increasing diameter are made so more tissue is ablated centrally than peripherally and the surface curvature is reduced
What is a risk of using a smaller diameter of area treated in PRK (3.5-4mm)?
edge may cause haloes to be seen around lights when pupil is dilated
How can haloes around lights from PRK be reduced and what is the disadvantage of this?
wider area of treatment 6-7mm; requires deeper ablation
What is a strategy for PRK to use a wider area of ablation but minimise the depth?
multiple concentrentric treatment zones with transition in between smoothed; only centremost zone provides full dioptric correction
What is PRK more commonly used for, myopia or hypermetropia?
myopia
How can PRK be used to treat regular astigmatism?
reducing surface curvature more in the steepest meridian than in any other, using:
1. slit beam
2. elliptical ablation zone
3. scanning beam
4. ablatable mask
What is an ablatable mask?
plate of PMMA placed in path of laser beam to shield cornea; thinner areas of the mask are ablated first and allow deeper ablation fo the corresponding are of the cornea
What is the sobriquet to remember how LASIK works?
flap and zap
What does the LASIK procedure involve?
- mechanical microkeratome used to dissect through the superficial corneal stroma and fashion a lamellar circular flap of uniform thickness
- dissection not completed - flap remains attached at one point, acts as hinge
- bared **corneal stroma **is reshaped using excimer laser
- flap is replaced
What is the benefit of the LASIK flap further down the line?
if under-correction or regression occurs, flap can be lifted to apply further treatment
What are 4 advantages of LASIK over PRK and LASEK?
- little subepithelial scarring
- less myopic regression
- earlier stabilisation of refraction
- superior predictability in treatment of high myopia
What does the LASEK procedure involve?
- ethanol 20% used to trephine epithelium, then rinses off
- epithelium is scraped
- epithelial (not stromal) flap created by folding epithelium on nasal or superior hinge
- laser ablation performed on bed
- epithelium replaced on stroma
What is an advantage of LASEK over LASIK?
low incidence of dry eye compared to LASIK
What are 3 disadvantages of LASEK vs LASIK?
- more uncomfortable in post-operative period
- higher rates of corneal haze
- higher rates of microbial keratitis
What is radial keratotomy used for?
method to treat myopia by irreversibly flattening the central cornea
How does radial keratotomy work?
- Partial thickness radial incisions placed in the mid-peripheral + peripheral cornea, sparing central zone.
- Wounds gape and the cornea bulges
- Cornea does not stretch so this causes flattening of central cornea.
How is thickness of corneal incisions in radial keratotomy determined?
corneal thickness measured with pachymeter and incisions should be 80-90% thickness (blade has micrometer on it)
What should the diameter of the central zone in radial keratotomy be and why?
3-5mm - any smaller will produce glare, more will have little effect on refraction
When do changes in refractive power stabilise after radial keratotomy?
6 months
What is the problem after RK if patients require further refractive correction in the future?
difficult or impossile to fit with contacts due to altered shape of conea
What are the 2 best options for correcting irregular astgimatism or high degrees of regular astigmatism?
rigid contact lenses or surgery (not spectacles)
What are 4 options for reducing post-graft astigmatism?
- continuous sutures
- interrupted sutures
- combination fo both
- double continuous sutures
When are the key 3 surgical options for correction of astgimatism (arcuate and transverse keratotomy, wedge resection and compression sutures) resorted to?
if simple suture techniques don’t work
How do relaxing incisions i.e. transverse and arcuate keratotomy work to treat astigmatism?
flatten cornea in meridian in which incision is made, and increases curvature at meridian 90 degees to it
What are 2 advantages of a curvilinear incision over tangential incision to correct astigmatism?
- curvilinear is cut in cornea of uniform thickness
- it is also concentric with the visual axis and likely to produce a more regular effect.
What are 2 ways to increase the effect of a cornea incision on astigmatism?
- longer incision
- closer ot the centre
How does a wedge resection work to treat astigmatism?
removes a deep arcuate wedge measuring 60–90° from the graft–host junction in the flattest meridian + then sutured, to increase the curvature in this meridian
How do compression sutures work to treat astigmatism?
tight suture placed across the graft–host junction in the flattest meridian increases the curvature of the cornea and reduces astigmatism
Other refractive surgery techniques 1: what is an intrastromal corneal ring?
PMMA split ring placed in tunnel in mid-peripheral corneal stroma concenric with the limbus, causes flattening of central cornea to treat myopia
Other refractive surgery techniques 2: what is an eipkeratophakia?
creates a new corneal surface with a different surface curvature by attaching a lenticule of pre-shaped donor corneal stroma to the surface of the host cornea
What is epikeratophakia most commonly used for and how?
keratoconus - lenticule of uniform thickness sutured tightly on to conical host cornea to compress it and return ti to more normal contour
Other refractive surgery techniques 3: What is keratomileusis?
microkeratome used to remove a lamella of anterior corneal stroma which is then shaped on a cryolathe before being replaced
Other refractive surgery techniques 4: what is keratophakia?
keratome used to lift lamella of anterior stroma, this is replaced over a shaped lenticule of donor corneal stroma to produce near corneal surface contour
Other refractive surgery techniques 5: what is thermokeratoplasty?
low temp burn induces contraction of colalgen in peripheral cornea and increases corneal curvature (homium:YAG laser used)
Other refractive surgery techniques 6: what is clear lens extraction?
removal of non-cataractous lens with insertion of IOL to correct spherical refractive error
Other refractive surgery techniques 7: what is phakic intraocular lens?
IOL places in anterior or posterior chambers of phakic eyes (i.e. still have a lens) to correct refractive errors
Other refractive surgery techniques 8: How can corneal incisions be used to correct refractive error?
those which are smaller and further from the visual axis are more astigmatically neutral
What must be done after large corneal incisions used for intracapsular and extracapsular cataract surgery?
must be closed by sutures - interrupted or continuous
Which, interrupted or continuous, suture type may cause less astigmatism?
continuous - by distributing tension more evenly along the wound
What is the effect of hte small self-healing incision used in phacoemulsification cataract surgery?
flattens corne ain same meridian
Which part of hte limbus is most astigmatically neutral and why?
temporal limbus - is furthest frmo visual axis
Other refractive surgery techniques 9: what is penetrating keratoplasty?
full thickness corneal transplant procedure
What is the effect of a scleral buckle (used for retinal detachment)?
increases axial length of eye inducing myopia (can also cause astigmatism if asymmetrical compression)
What is the effect of silicone oil in the posterior segment of a phakic eye?
higher refractive index than crystalline lens - changes postetrior surface of lens from converging to diverging interface - hypermetropic shift
What is the effect of silicone oil in the posterior segment of an aphakic eye?
myopic shift - curved anterior surface has higher refractive index of silicone, more strongly converging than crystalline lens
What is the effect of gas in the posterior segment of a phakic eye?
myopic shift
What is the effect of gas in the posterior segment of an aphakic eye?
highly diverging (hypermetropic shift) - almost neutralises refractive power of cornea
When are the key 3 surgical options for correction of astgimatism (arcuate and transverse keratotomy, wedge resection and compression sutures) resorted to?
if simple suture techniques don’t work
