Your SlideShare is downloading. ×
REFRACTIVE SURGERY PART 1
VIJAY JOSHI
AN INTRODUCTION
 Surgery to correct refractive errors is becoming very
popular.
 It should be performed after the error ...
CLASSIFICATION
A. Incisional refractive techniques
a. Radial keratotomy
b. Astigmatic keratotomy ( AK)
c. Hexagonal kerato...
CLASSIFICATION CONTD..
C. Laser refractive corneal procedures
a. Photorefractive keratotomy (PRK)
b. Laser in situ keratom...
PATIENT SELECTION & PREOPERATIVE EVALUATION FOR
REFRACTIVE SURGERY
 Patient selection : meticulous patient selection and
...
PATIENT SELECTION & PREOPERATIVE EVALUATION FOR
REFRACTIVE SURGERY
 Following factors should be considered before a patie...
PATIENT SELECTION CONTD..
2. Age of patient:
 Pt younger than 18 years should not be taken for surgery
because of unstabl...
PATIENT SELECTION CONTD..
4. Occupation of the patient:
 Preferably, radial keratotomy(RK), should be avoided in
individu...
PATIENT SELECTION CONTD..
5. Ocular abnormalities:
 That are C/I for keratorefractive sx include the following:
 Absolut...
PATIENT SELECTION CONTD..
 Relative contraindications:
 Blepharitis
 Dry eye
 Glaucoma
 History of uveitis
 Chronic ...
PATIENT SELECTION CONTD..
6. Informed consent:
 Pt should be made aware of the risks and benefits of the
Refractive Sx, t...
PREOPERATIVE EVALUATION
 Preoperative evaluation of each case selected for RSX
should include:
 1. Slit lamp Biomicrosco...
PREOPERATIVE EVALUATION CONTD..
 4. Intraocular pressure (IOP): should be measured using
Applanation Tonometer. Detailed ...
PREOPERATIVE EVALUATION CONTD..
 8. Corneal topography measurements: is an integral
components of preoperative evaluation...
REFRACTIVE SURGERY FOR MYOPIA
 The surgical techniques that have been employed for
correction of myopia over the years in...
REFRACTIVE SURGERY FOR MYOPIA CONTD..
 D. Miscellaneous corneal refractive procedures
 Orthokeratology
 Intacorneal con...
A. INCISIONAL PROCEDURES FOR MYOPIA
 Radial keratotomy
 Radial keratotomy, or RK, as today, refers to making deep
(90% c...
 Radial keratotomy. Partial-thickness incisions result in ectasia of the paracentral cornea
and compensatory flattening o...
RADIAL KERATOTOMY
ADVANTAGES OF RK
 Advantages
 1. Central optical zone of 3-4 mm is spared, so there are
no chances of central corneal ha...
DISADVANTAGES OF RK
 Disadvantages
 1. Corneal is weakened, so chances of globe rupture
following trauma are more after ...
B. LAMELLAR CORNEAL REFRACTIVE PROCEDURES FOR
MYOPIA
 Never been popular and are obsolete.
 Keratomileusis: derived by J...
 Freeze keratomileusis:
 A disc of parallel sides is
resected from the cornea with
the microkeratome.
 After freezing t...
LAMELLAR CORNEAL REFRACTIVE PROCEDURES FOR
MYOPIA
 Epikeratomileusis or epikeratophakia or
epikeratoplasty- developed by ...
C. LASER BASED CORNEAL REFRACTIVE
PROCEDURES FOR MYOPIA
 Photorefractive keratectomy(PRK)
 Laser in situ keratomileusis(...
LASER USED FOR REFRACTIVE CORNEAL PROCEDURES
 Before discussing laser Ref Sx, a brief note about laser
used:
 A. Excimer...
LASER USED FOR REFRACTIVE CORNEAL PROCEDURES
 A. Excimer Laser: most commonly used laser for
refractive corneal procedure...
EXCIMER LASER CONTD..
 This situation makes it attractive for laser action , because
the ground state really doesn’t exis...
EXCIMER LASER CONTD..
 The Excimer family of lasers seems to be most easily
applicable to corneal incisions and reprofili...
EXCIMER LASER CONTD..
 The wavelength of laser use is about 193nm as it
prevents damage of deeper corneal stroma, Desceme...
CLASSIFICATION OF EXCIMER LASER MACHINES
 1. First generation: these machines used a broad
beam, which meant irregularity...
LASERS IN REF SX
 B. Solid State UV Laser: Solid state laser is not an
Excimer laser since the UV laser ablation radiatio...
SOLID STATE UV LASER CONTD..
 BEAM QUALITY: Excimer laser need beam forming
elements i.e mirror in the beam pathway(multi...
SOLID STATE UV LASER CONTD..
 EYE TRACKER SPPED:
 In Excimer laser is approximated 150 Hz while in solid
state laser it ...
LASERS IN REF SX
 C. Infrared femtosecond laser:
 Femtosecond lases have the property of emitting
ultrashort pulses with...
ADVANTAGES OF FEMTOSECOND LASER
 FS laser focus the beam very accurately ( to within the
order of a micrometer) in a tran...
PHOTOREFRACTIVE KERATECTOMY
 PRK is a procedure of photoablation by excimer
laser, which has been in use for the treatmen...
SURGICAL PARAMETERS AND TECHNIQUE OF PRK
 In this technique, to correct myopia, a central optical zone
of anterior cornea...
SURGICAL PARAMETERS AND TECHNIQUE OF PRK CONTD..
 ABLATION ZONE DIAMETER: small ablative zones are
known to cause symptom...
SURGICAL PARAMETERS AND TECHNIQUE OF PRK
CONTD..
 CORNEAL ABLATION:
 Multizone and multi step: procedures are
advantageo...
SURGICAL PARAMETERS AND TECHNIQUE OF PRK
CONTD..
 SCANNING LASER BEAM: surface roughness increases
with small diameter sc...
 Photorefractive keratectomy (PRK). After removal of the corneal
epithelium, the excimer laser is used to reprofile the a...
PHOTO REFRACTIVE KERATOTOMY
POSTOPERATIVE MANAGEMENT PRK
 1. Patching for 24 hrs is usually done with topical
cycloplegic, NSIAD’s and antibiotic oin...
POSTOPERATIVE MANAGEMENT PRK
 3. Night glare and halos: occur with small ablated
zones.
 Under scotopic illumination, di...
 4. Delayed epithelial healing: KCS, topical anti inflammatory
drugs, prophylactic antibiotic therapy and bigger debrided...
 Central islands: With computer assisted topographic analysis
corneas show a central region of higher corneal refractive ...
 10. Subretinal Hge:
 Mechanical stress waves with an amplitude of 100 bars
travel through the eye and might cause disru...
RETREATMENT
 PRK can be re performed safely in cases with marked
under correction. However, most patients don’t require
e...
ADVANTAGES OF PRK
I. No weakening of the globe unlike RK.
II. No night glare and diurnal variation in refractive unlike
RK...
DISADVANTAGES
I. Post operative recovery is slow. Healing of the
epithelial defect may delay return of good vision.
II. Th...
LASER IN SITU KERATOMILEUSIS
 LASIK is a keratorefractive sx that combines the precision
of excimer laser photoablation w...
MILESTONES IN ADVENT OF LASIK
 In 1949, Barraquer developed the myopic
kerratomileusis using freehand disection of half
t...
MILESTONES IN ADVENT OF LASIK
 In 1990, Pallikaris from Greece used microkeratome to
create a 120-160um, corneal cap and ...
INSTRUMENTATION
 Microkeratome: Several automated microkeratomes have
been developed to perform a uniform homogenous
plan...
The suction ring is centered on the eye before suction is applied. This raises
the internal pressure of the eye between 65...
The microkeratome is then placed on the track of the suction ring
The microkeratome applanates the cornea as the blade creates the flap. The
flap is gently lifted and slides into a space w...
The surgeon controls the forward and backward motion of the microkeratome.
As the microkeratome returns the flap is returned to the bed of the cornea
COMPONENTS OF MICROKERATOME
1. Corneal shaper head: is main component.
 It has an oscillating blade which is driven by a ...
PATIENT SELECTION AND EVALUATION
 1. ADEQUATE GLOBE EXPOSURE: is mandatory for
application of suction ring and microkerat...
PATIENT SELECTION AND EVALUATION
 4. PUPIL SIZE: large optical zones are required for
people with large pupils. Young pt ...
ESSENTIAL EXAMINATION
 Following examination and documentation are essential for
a Lasik procedure:
 1. VISUAL ACQUITY: ...
SURGICAL WORK UP AND TECHNIQUE
 1.Broad spectrum antibiotic eye drops: should be sued
four times per day, 3-4 days preope...
SURGICAL WORK UP AND TECHNIQUE CONTD..
 6. Corneal marking: corneal should
be marked with a marker using
gentian voilet.
...
SURGICAL WORK UP AND TECHNIQUE CONTD...
 7. Fixation of Suction Ring: pneumatic suction is fixed in
place on the sclera w...
SURGICAL WORK UP AND TECHNIQUE CONTD..
 8. Preparation of corneal flap: the corneal is moistened
with balanced salt solut...
SURGICAL WORK UP AND TECHNIQUE CONTD..
 Preparation of corneal flap cont..
 A nasal hinge is superior to preserve the co...
RECOMMENDATIONS FOR ACHIEVING A GOOD QUALITY CUT
 In microkeratome new blade should be inserted into the
sharper’s head f...
RECOMMENDATIONS FOR ACHIEVING A GOOD QUALITY
CUT
 The IOP should be between 60 and 65mm Hg during
cutting. IOP lower than...
SURGICAL WORK UP AND TECHNIQUE CONTD..
 9. Stromal Ablation: The corneal flap is retracted along the
hinge and the stroma...
SURGICAL WORK UP AND TECHNIQUE CONTD..
 11. Removal of speculum and drape: is then gently
performed so as not to cause an...
LASIK PROCEDURE
POST OPERATIVE MANAGEMENT
 1. Patching/ dark goggles
 2. Systemic analgesics
 3. Antibiotic eye drops: QID*7-14 days
 ...
FOLLOW UP EXAMINATION
 Slit lamp biomicroscopy examination and visual acquity
testing should be performed on following da...
COMPLICATIONS OF LASIK
COMPLICATIONS
 A. Intraoperative complications
 1. During application of suction ring:
 Conjuctival chemosis may occur ...
COMPLICATIONS CONTD..
ii) Incomplete flap: results due to some foreign
material, debris getting into the microtome gear
me...
COMPLICATIONS CONTD..
 vi) Damage, destruction and dislocation of the flap can
occur due to inadvertent trauma.
 vii) Lo...
COMPLICATIONS CONTD..
ii) Incorrect ablation may result in unpredictable results.
iii Ablation of the hinge may result in ...
COMPLICATIONS CONTD..
 5. Complications during removal of eye speculum:
 Flap can be dislodged during removal of eye
spe...
COMPLICATIONS CONTD..
 B. Postoperative complications
 1. Poor adhesion and anchoring: of the flap with the
stroma
 2. ...
COMPLICATIONS IN LASIK
Air bubble inside flap Dislodged flap
Macrofolds Fibers under flap Microfolds
COMPLICATIONS CONTD..
 3. Infectious keratitis
 4. Diffuse lamellar keratitis or sands of Sahara syndrome:
occurs in ear...
COMPLICATIONS CONTD..
 6. Lost flap postoperatively: results due to incomplete
adherence to the bed.
 Perhaps, there is ...
COMPLICATIONS CONTD..
 8. Undercorrections and over corrections : are not
uncommon.
 These can be treated with repeated ...
COMPLICATIONS CONTD..
 Irregular astigmatism can result due to:
 Eccentric ablation
 Suturing of the flap
 Interface r...
COMPLICATIONS CONTD..
Glare Halos
COMPLICATIONS CONTD..
 12. Haze at the interface: occurs due to cellular response
to toxic substances introduced during s...
COMPLICATIONS CONTD..
 Inherent problems after LASIK
 1. Inaccurate measurements of IOP: tonometer under
estimates the I...
ADVANTAGES OF LASIK OVER RK AND PRK
 1.Minimal or no postoperative pain.
 2.Recovery of vision is very early as compared...
DISAVANTAGES OF LASIK OVER RK AND PRK
 1. LASIK is much more expensive.
 2. It requires greater surgical skill than RK a...
Refractive surgery part 1  Myopia
Refractive surgery part 1  Myopia
Refractive surgery part 1  Myopia
Refractive surgery part 1  Myopia
Refractive surgery part 1  Myopia
Refractive surgery part 1  Myopia
Refractive surgery part 1  Myopia
Refractive surgery part 1  Myopia
Upcoming SlideShare
Loading in...5
×

Refractive surgery part 1 Myopia

3,563

Published on

Refractive surgery part 1: Myopia

Published in: Education, Health & Medicine
0 Comments
9 Likes
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total Views
3,563
On Slideshare
0
From Embeds
0
Number of Embeds
1
Actions
Shares
0
Downloads
0
Comments
0
Likes
9
Embeds 0
No embeds

No notes for slide

Transcript of "Refractive surgery part 1 Myopia"

  1. 1. REFRACTIVE SURGERY PART 1 VIJAY JOSHI
  2. 2. AN INTRODUCTION  Surgery to correct refractive errors is becoming very popular.  It should be performed after the error has stabilized- preferably after 18-20 years of age.
  3. 3. CLASSIFICATION A. Incisional refractive techniques a. Radial keratotomy b. Astigmatic keratotomy ( AK) c. Hexagonal keratotomy B. Lamellar corneal refractive procedures a. Freeze keratomileusis of Barraquer b. Epikeratophakia( Epikeratoplasty) c. Non – freeze keratomileusis d. Keratomileusis in situ ( BKS technique) e. Automated lamellar keratoplasty (ALK)
  4. 4. CLASSIFICATION CONTD.. C. Laser refractive corneal procedures a. Photorefractive keratotomy (PRK) b. Laser in situ keratomileusis (LASIK) c. Laser sub epithelial keratomileusis ( LASEK) d. Epithelial laser in situ keratomileusis (E-LASIK) e. Custom laser in situ keratomileusis ( C-LASIK) f. Inter lase laser in situ keratomileusis( I lasik) g. Refractive lenticular extraction( ReLax) h. Thermal laser keratoplasty (TLK) D. Miscellaneous corneal refractive procedures a. Conductive keratoplasty (CK) b. Orthokeratology c. Intracorneal contact lenses d. Intrastromal corneal ring segments E. Intraocular referactive procedures a. Phakic refractive lenses(PRL’s) b. Refractive lens exchange (RLE)
  5. 5. PATIENT SELECTION & PREOPERATIVE EVALUATION FOR REFRACTIVE SURGERY  Patient selection : meticulous patient selection and evaluation are essential to maximize good outcome from refractive surgery
  6. 6. PATIENT SELECTION & PREOPERATIVE EVALUATION FOR REFRACTIVE SURGERY  Following factors should be considered before a patient is selected for refractive surgery:  1. Patient motivation:  Most important factor, since an unmotivated patient has more chances of being unsatisfied.  The motivation may be based on any of the following factors:  Occupational requirements: eg. In actors and actresses  Desire for an improved cosmetic appearance is the most common motivational factor  Recreational needs  Contact lens intolerance  Patient must understands that while refractive surgical procedure often greatly reduces dependence on optical aids, it rarely eliminates the need for them entirely
  7. 7. PATIENT SELECTION CONTD.. 2. Age of patient:  Pt younger than 18 years should not be taken for surgery because of unstable refraction.  Refractive procedure should be attempted only when the refraction has been stable for at least 1 yr.  There is no upper limit for refractive sx, however, pt older than 45 yrs should be operated sparingly becoz of 2 reasons: 1. A greater risk of over correction. 2. Need for using presbyopic glasses. 3. Refractive error:  Optimal range of RE varies from procedure to procedure.
  8. 8. PATIENT SELECTION CONTD.. 4. Occupation of the patient:  Preferably, radial keratotomy(RK), should be avoided in individuals with following occupations:  Job requiring night driving, because of chances of troublesome and incapacitating glare.  Sportspersons and security personnel, because of danger of more chances of globe perforation following ocular trauma.
  9. 9. PATIENT SELECTION CONTD.. 5. Ocular abnormalities:  That are C/I for keratorefractive sx include the following:  Absolute contraindications-  Keratoconus  Contact lens warpage  Chronic steroid and antimetabolite use for immunosuppression  Glaucoma  Herpes simplex keratitis  Connective tissue disease
  10. 10. PATIENT SELECTION CONTD..  Relative contraindications:  Blepharitis  Dry eye  Glaucoma  History of uveitis  Chronic eye rubbing  Other ocular surface disorders  Diabetes mellitus  Single eyed pt
  11. 11. PATIENT SELECTION CONTD.. 6. Informed consent:  Pt should be made aware of the risks and benefits of the Refractive Sx, the range of complications, and the variable individual response 7. Patient expectation:  Pt should be informed that it is not a full proof procedure and that he or she may need further sx to achieve the desired visual outcome
  12. 12. PREOPERATIVE EVALUATION  Preoperative evaluation of each case selected for RSX should include:  1. Slit lamp Biomicroscopy: of anterior segment to exclude other associated ds.  2. Cycloplegic refraction: may be helpful in avoiding overcorrection, especially in younger patients who may have excess accommodation.  3. Measurement of Pupil size: is essential as the optical function is influenced by the diameter of pupil. The pupil size should be measured under low light( mesopic) conditions(less than 5 lux), preferably with an infrared pupillometer or with an aberrometer.  Ideally the ablation zone diameter should not be less than the diameter of dark-adapted pupil( which is normally 6mm) otherwise the patient may complain of postoperative glare and halos.
  13. 13. PREOPERATIVE EVALUATION CONTD..  4. Intraocular pressure (IOP): should be measured using Applanation Tonometer. Detailed glaucoma investigations may be required to exclude glaucoma in suspected cases.  5. Posterior segment evaluation: with IO should be done in detail, considering that the retinal detachment may occur after refractive sx in Myopia.  6. Keratometry: readings should be obtained, but are of limited value in detecting barely noticeable irregularities.  7. Computerized Videokeratography: has been found to be of help in detecting patients with early Keratoconus, which other wise may go unnoticed and get operated.
  14. 14. PREOPERATIVE EVALUATION CONTD..  8. Corneal topography measurements: is an integral components of preoperative evaluation for refractive sx.  9. Corneal thickness evaluation (pachymetry): is extremely important to rule out the abnormally thin corneas. This may be done prior to, just before or during sx.  Ultrasonic pachymetry is currently the method of choice for corneal thickness evaluation because of its ease of operation, precision, and ability to measure corneal thickness eccentrically.
  15. 15. REFRACTIVE SURGERY FOR MYOPIA  The surgical techniques that have been employed for correction of myopia over the years include:  A. Incisional procedures  Radial keratotomy  B. Lamellar corneal refractive procedures  Freeze keratomileusis of barraquer for myopia( KMM)  Non freeze keratomileusis  Keratomileusis in situ ( BKS procedure)  Automated lamellar keratoplasty (ALK)  C. Laser based corneal refractive procedures  Photorefractive keratectomy(PRK)  Laser in situ keratomileusis(LASIK)  Laser supepithelial keratomileusis (LASEK)  Epithelial laser in situ keratomileusis (E-LASIK)  Custom laser in situ keratomileusis( C LASIK)  Intralase LASIK (I- LASIK)
  16. 16. REFRACTIVE SURGERY FOR MYOPIA CONTD..  D. Miscellaneous corneal refractive procedures  Orthokeratology  Intacorneal contact lenses  Intrastromal corneal ring segments (INTACS)  Gel injectable adjustable keratoplasty  E. Intraocular refractive procedures  Phakic refractive lenses  Refractive lens exchange
  17. 17. A. INCISIONAL PROCEDURES FOR MYOPIA  Radial keratotomy  Radial keratotomy, or RK, as today, refers to making deep (90% corneal thickness) radial incisions in the peripheral part of cornea, leaving about 4mm central optical zone.  These incisions on healing flatten the central cornea, thereby reducing its refractive power.  The most accepted theory holds that normal IOP pushes the peripheral cornea weakened by the incisions, leaving a relatively flatter centre.  This procedure is effective in low to moderate myopia (- 1.5 to -6D)  With the advent of the safer technique LASIK, the RK, because of its disadvantages, has been abandoned now.
  18. 18.  Radial keratotomy. Partial-thickness incisions result in ectasia of the paracentral cornea and compensatory flattening of the central cornea.
  19. 19. RADIAL KERATOTOMY
  20. 20. ADVANTAGES OF RK  Advantages  1. Central optical zone of 3-4 mm is spared, so there are no chances of central corneal haze.  2. RK is much less expensive than PRK and LASIK.  3. Postoperative recovery after RK is much earlier than after PRK.
  21. 21. DISADVANTAGES OF RK  Disadvantages  1. Corneal is weakened, so chances of globe rupture following trauma are more after RK than after PRK.  This point is particularly important for patients who are at high risk of blunt trauma. E.g sportspersons, athletes and military personnel  2. Rarely, uneven healing may lead to irregular astigmatism.  3. The patient may feel glare at night.
  22. 22. B. LAMELLAR CORNEAL REFRACTIVE PROCEDURES FOR MYOPIA  Never been popular and are obsolete.  Keratomileusis: derived by Jose Barraquer in late 1940’s, from greek words for carving or chiselling of cornea  KM is grandfather of all lamellar refractive procedures and is direct ancestor of modern LASIK procedures.  1. Freeze myopic keratomileusis of barraquer (MKM) and hyperopic keratomileusis of barraquer (HKM):  Involves a removal of a thin wafer of superficial corneal tissue with a microkeratome.  The corneal wafer was then graved into a new shape designed to correct the desired myopia by a freezing technique using a cryolathe.  This newly shaped corneal wafer was then sewn back to the patient’s corneal bed.
  23. 23.  Freeze keratomileusis:  A disc of parallel sides is resected from the cornea with the microkeratome.  After freezing the disc, a lenticule of predetermined power is removed from the stromal side with a lathe.  The removed cornea is sutured back in place.
  24. 24. LAMELLAR CORNEAL REFRACTIVE PROCEDURES FOR MYOPIA  Epikeratomileusis or epikeratophakia or epikeratoplasty- developed by Kaufman in 1979.  Non freeze keratomileusis- described by Krumeich and Swinger in 1983.  Keratomileusis in situ (Barraquer- Krumeich-swinger (BKS) technique- developed in 1985.  Automated lamellar keratoplasty- introduced by Reeiz in late 1980s.  The ALK has been a break through for lamellar surgeries.  In ALK, two keratectomies were performed with an automated microkeratome.
  25. 25. C. LASER BASED CORNEAL REFRACTIVE PROCEDURES FOR MYOPIA  Photorefractive keratectomy(PRK)  Laser in situ keratomileusis(LASIK)  Laser supepithelial keratomileusis (LASEK)  Epithelial laser in situ keratomileusis (E-LASIK)  Custom laser in situ keratomileusis( C LASIK)  Intralase LASIK (I-LASIK)  ReLex
  26. 26. LASER USED FOR REFRACTIVE CORNEAL PROCEDURES  Before discussing laser Ref Sx, a brief note about laser used:  A. Excimer laser  B. Solid state UV laser, &  C. Infrared femtosecond laser
  27. 27. LASER USED FOR REFRACTIVE CORNEAL PROCEDURES  A. Excimer Laser: most commonly used laser for refractive corneal procedures.  Term ‘Excimer’ is a contraction of ‘excited dimer’ which includes any diatomic molecule in which the component atoms are bound in the excited state but are not bound in the ground state.  When the two component atoms are electronically excited, they attract each other and forms a stable molecule.  However in ground state the two atoms are usually not bound and indeed are mutually repulsive.  Thus in Excimer laser ,when the excited molecule relaxes from excitations, the molecule decomposes and falls apart.
  28. 28. EXCIMER LASER CONTD..  This situation makes it attractive for laser action , because the ground state really doesn’t exists and there fore one obtains a population inversion as soon as one has the molecules formed that are automatically in the excited state.  The most attractive excimer molecules are rare gas hallides that don’t normally occur in nature but can be easily produced in properly controlled electrical gas discharge.  The choice of gas mixture determines the output wavelength: eg ArF193 nm, XeCl 308nm, Kr 222nm, XeF 351 nm , & KrF 248NM.  In these lasers , the majority of the gas mixture is a buffer gas that mediates energy transfer but doesn’t participate in the laser action
  29. 29. EXCIMER LASER CONTD..  The Excimer family of lasers seems to be most easily applicable to corneal incisions and reprofiling because of the following reasons-  They have extremely short pulse duration(10-15ns) which decreases thermal effect to infinitesimal levels because of the apparent lack of time for thermal diffusions.  Its pulse to pulse energy level is reproducible within acceptable limits and also the repetition rate of the pulse can be varied over a relatively large range, typically 1-50 Hz , etc.  Sufficient energy is available ( upto 450 mJ) so that a large beam can be produced to ablate & reprofile a 4-7 mm diameter portion of central cornea without energy limitation
  30. 30. EXCIMER LASER CONTD..  The wavelength of laser use is about 193nm as it prevents damage of deeper corneal stroma, Descemet’s membrane and corneal endothelium.  Same is not true for longer wavelenght (XeCl 308nm can damage lens, and can reach retina in aphakic eyes)  Also photokeratitis, thermal damage and mutagenic proprerty of UV rays is associated with longer wavelengths(>280nm)  Therefore ArF having 193 nm wavelengths is used, having ability to remove minute amounts of corneal tissue( 0.2-0.3nm ) with no observable thermal damage, also it produces smooth spherical surface in stroma of cornea  Incisions as minute as 20um in width and extending to a depth of 95% thickness of the cornea can be created.
  31. 31. CLASSIFICATION OF EXCIMER LASER MACHINES  1. First generation: these machines used a broad beam, which meant irregularity and hence formation of central islands.  2. Second generation: in these machines the scanning slits was introduced, which reduced the irregularity to a certain extent.  3. Third generation: the scanning spot was introduced in these machines.  4. Fourth generation: the wave front technology and eye trackers were introduced which proved much more beneficial to the patients undergoing treatment.
  32. 32. LASERS IN REF SX  B. Solid State UV Laser: Solid state laser is not an Excimer laser since the UV laser ablation radiation is generated by non- linear frequency conversion of infra red laser light in a laser crystal.  The wavelength of the laser radiation in solid state laser is in range of 208-210 nm  Solid state laser is reported to be ideal for accurate custom ablation( C-LASIK) via-a vis excimer laser because of following features..  A. SPOT SIZE: commonly available excimer laser machines generate a flying spot of 0.8-1.0mm in sixe. The solid state laser generates a flying spot size of 0.2 mm in dia, operating at repetition rate of 1Khz  This very small spot size fits the present requirements for effective custom ablation
  33. 33. SOLID STATE UV LASER CONTD..  BEAM QUALITY: Excimer laser need beam forming elements i.e mirror in the beam pathway(multimode). Solid state lasers produce Gaussian beam, i.e don’t need beam forming elements(single mode).  Due to the accurate overlap made possible by the true Gaussian spot, the solid state laser ensures an extremely homogeneous corneal surface  REPETATION RATE: In Excimer laser is 50-500 Hz while in solid state laser it is 1KHz.  Because of high repetition rate, the energy per pulse is lower in solid state laser than in the Excimer laser.
  34. 34. SOLID STATE UV LASER CONTD..  EYE TRACKER SPPED:  In Excimer laser is approximated 150 Hz while in solid state laser it is more than 1 KHz .The fast eye tracking in solid state laser ensures a reliable centration of ablation for x-y directions as well as the rotation of the eye at high repetition rates.
  35. 35. LASERS IN REF SX  C. Infrared femtosecond laser:  Femtosecond lases have the property of emitting ultrashort pulses with high crest power from low energy pulse.  Femtosecond laser is being tried for ablation within thickness of laser( excimer lasers can only be used for surface treatments)
  36. 36. ADVANTAGES OF FEMTOSECOND LASER  FS laser focus the beam very accurately ( to within the order of a micrometer) in a transparent medium and it produces the local destruction of material without producing any heat damage in the surrounding tissue.
  37. 37. PHOTOREFRACTIVE KERATECTOMY  PRK is a procedure of photoablation by excimer laser, which has been in use for the treatment of myopia, hypermetropia and astigmatism.  PRK has gained maximum success in myopic patients.
  38. 38. SURGICAL PARAMETERS AND TECHNIQUE OF PRK  In this technique, to correct myopia, a central optical zone of anterior corneal stroma is photoablated using excimer laser to cause flattening of the central cornea. Surgical steps as follows:  ANAESTHESIA: PRK can be satisfactorily performed under Topical Anaesthesia.  EPITHELIUM REMOVAL: Deepithelisation methods include mechanical debridement with sharp blade under topical anaesthesia., cocaine or alcohol and photoablative deepithelisation.  An attempt should be made to deepithelize 0.5-1.0mm larger area than the desired ablation zone.
  39. 39. SURGICAL PARAMETERS AND TECHNIQUE OF PRK CONTD..  ABLATION ZONE DIAMETER: small ablative zones are known to cause symptomatic halos while night driving.  The ideal diameter of ablation zone for myopia is 6mm, and for Hypermetropia is 9mm.  FIXATION AND CENTRATION OF ABLATION ZONE: some surgeon use hand held suction ring while others promote the method of self fixation by the patient during ablation.  Fixation light on the microscope should be coaxial with the surgeon’s and patients line of vision  It is better to patch the fellow eye of the patient to prevent inadvertent cross fixation  Laser beam should be aimed at the center of the pupil  Decentration of ablation should never be done.
  40. 40. SURGICAL PARAMETERS AND TECHNIQUE OF PRK CONTD..  CORNEAL ABLATION:  Multizone and multi step: procedures are advantageous, since most cases have a tendency for regression.  A 1.5mm white tapered transition zone bordering the refractive zone of 4mm with overall treatment dia of 6-7mm usually results in better epithelial healing and laser regression.  Fluence and repetition rate: changes in laser fluence and repetition rate affect not only the rate at which tissue is removed and the operation time, but also the surface morphology of the ablated corneal tissue.  Higher fluence rates ablate larger amount of tissue per pulse.
  41. 41. SURGICAL PARAMETERS AND TECHNIQUE OF PRK CONTD..  SCANNING LASER BEAM: surface roughness increases with small diameter scanning PRK because of its influence on the involuntary eye movements, causing increased wound healing response and corneal haze.  ASPHERIC ABLATIONS: planned aspheric ablations are made in high myopia, which avoid central islands, thereby decreasing postoperative spherical aberrations.  CORRECTION OF MYOPTIC ASTIGMATISM: correction is done by ablating the superficial cornea in a cylindrical fashion, known as toric photoablation
  42. 42.  Photorefractive keratectomy (PRK). After removal of the corneal epithelium, the excimer laser is used to reprofile the anterior curvature of the cornea, which changes its refractive power.
  43. 43. PHOTO REFRACTIVE KERATOTOMY
  44. 44. POSTOPERATIVE MANAGEMENT PRK  1. Patching for 24 hrs is usually done with topical cycloplegic, NSIAD’s and antibiotic ointment.  2. Corneal haze: may occur, specially following PRK for high myopia (>6 D).  Patients having severe dry eyes, severe atopic ds, preexisting corneal scar are more prone to develop corneal haze.  Increased no. of activated keratocytes deposit new collagen and proteogycans, which are responsible for the light scatter resulting in corneal haze.  MX:  Topical steroids are useful in resolving the level of haze.  Excimer laser retreatment may be required in cases where haze persists beyond 6 months and is associated with regression
  45. 45. POSTOPERATIVE MANAGEMENT PRK  3. Night glare and halos: occur with small ablated zones.  Under scotopic illumination, dilataion of pupil causes the light rays to pass through midperipheral cornea to reach the posterior pole.  This causes halos, and night driving become difficult.  It is common with ablation zones of 3.5mm, but absent in 5-6mm zones.  This problem can be treated with second ablation by increasing diameter to 6mm.
  46. 46.  4. Delayed epithelial healing: KCS, topical anti inflammatory drugs, prophylactic antibiotic therapy and bigger debrided area are the commonly known causes for delayed epithelial healing.  5. Recurrent epithelial erosions: are known to occur if epithelial defect made before ablation procedure is larger than the ablation zone.  6. Corneal infiltration: are usually focal, but may be multicentric.  These apppear days to week after surgery.  If they are central, they may cause reduction in visual acquity.  7. Corneal ulceration: Pt getting bandage soft contact lenses after PRK are prone for cornel ulcers.  8. Decreased corneal sensations: Pt with high myopia undergoing larger and deeper ablations show reduced sensitivity, more than others
  47. 47.  Central islands: With computer assisted topographic analysis corneas show a central region of higher corneal refractive power compared to the adjacent paracentral cornea.  These are the causes of undercorrection, asphericity and irregular astigmatism.  Many theories have been put forward to explain their formation. These include I. Shock wave formation and ejection of a plume of gaseous and particulate debris which interferes with the subsequent proper delivery of the laser. II. Undesired optics of the laser or variation in beam homogeneity. III. Differential hydration of the corneal tissue postoperatively. & IV. Healing being non uniform leads to greater epithelial hyperplasia centrally.
  48. 48.  10. Subretinal Hge:  Mechanical stress waves with an amplitude of 100 bars travel through the eye and might cause disruption of fragile subretinal vessels, causing these Hg’s  11. Raised IOP:  Is thought to be a result of use of postoperative corticosteroids topically.
  49. 49. RETREATMENT  PRK can be re performed safely in cases with marked under correction. However, most patients don’t require enhancements sx.  PRK should not be repeated under following conditions: I. Slight under correction II. Before 6 months of the initial sx III. Unless steroids have been stopped for more than 3 months. IV. If refraction is not stable. V. When central islands are followed for less than 6-12 months. VI. When corneal haze is not accompanied with regression.
  50. 50. ADVANTAGES OF PRK I. No weakening of the globe unlike RK. II. No night glare and diurnal variation in refractive unlike RK. III. Results are excellent, with an accuracy of 95% in achieving a ±0.5 D correction in patients with a myopia of -2 to -8D.
  51. 51. DISADVANTAGES I. Post operative recovery is slow. Healing of the epithelial defect may delay return of good vision. II. The patients may experience pain or discomfort for several weeks. III. There may occur some residual corneal haze in the center affecting vision. IV. PRK is more expensive than RK.
  52. 52. LASER IN SITU KERATOMILEUSIS  LASIK is a keratorefractive sx that combines the precision of excimer laser photoablation with the advantages of an intrastromal procedure that maintains the integrity of Bowman’s layer and the overlying corneal epithelium.  Can be used to correct upto -13D of myopia and upto -6.0 D of astigmatism.
  53. 53. MILESTONES IN ADVENT OF LASIK  In 1949, Barraquer developed the myopic kerratomileusis using freehand disection of half thickness corneal disc which was later shaped with cryolathe and then sutured on the recepient bed.LASIK is in fact direct descendant of this technique.  In 1983, palner keratomileusis that didn’t require freezing was developed by Krumeich.  Trokel & Srinivasan in 1983 had suggested that excimer laser can be used to ablate the corneal tissue.  In 1986, Ruiz introduced in situ keratomileusis by performing two keratectomies using manual dissection.
  54. 54. MILESTONES IN ADVENT OF LASIK  In 1990, Pallikaris from Greece used microkeratome to create a 120-160um, corneal cap and then ablated the stromal bed using the excimer laser as in PRK and finally sutured the corneal cap.  In 1991, Pallikaris introduced a nasally hinged cornea flap which didn’t need suturing in the end- the basis of LASIK.  He also coined the term ‘LASIK’  In 1992, Buratto from Italy introduced the cap mileusis with excimer laser.  In 1996, Buratto developed the technique of cutting a superiorly hinged flap.
  55. 55. INSTRUMENTATION  Microkeratome: Several automated microkeratomes have been developed to perform a uniform homogenous planar cut on the corneal surface.  The aim is to cut a corneal disc of precisely calculated thickness and diameter with a sufficient hinge to maintain its position and apposition during replacement
  56. 56. The suction ring is centered on the eye before suction is applied. This raises the internal pressure of the eye between 65 to 85 mm of Mercury.
  57. 57. The microkeratome is then placed on the track of the suction ring
  58. 58. The microkeratome applanates the cornea as the blade creates the flap. The flap is gently lifted and slides into a space within the microkeratome
  59. 59. The surgeon controls the forward and backward motion of the microkeratome.
  60. 60. As the microkeratome returns the flap is returned to the bed of the cornea
  61. 61. COMPONENTS OF MICROKERATOME 1. Corneal shaper head: is main component.  It has an oscillating blade which is driven by a motor incorporated in the handle.  The head also has a thickness plate which sets the exact depth of the cut. 2. Suction ring: operated by a suction pump mechanism.  It helps in fixing the globe and maintaining an IOP of 65mm HG. 3. Stop mechanism: ensures that the corneal shaper head stops at a preset distance in the track on the suction ring during the cut to maintain a hinge of the flap. 4. Control unit: contains the electrical sources and a suction pump. The suction pump helps in fixing the suction ring
  62. 62. PATIENT SELECTION AND EVALUATION  1. ADEQUATE GLOBE EXPOSURE: is mandatory for application of suction ring and microkeratome pass.  There fore pt with sunken eyeballs and small palpebral apertures may pose a problem.  CORNEAL THICKNESS: should be at least 450u in the centre.  An adequate handling thickness of 500u provides about 90u for ablation since 180u is the thickness of flap and 250 u stromal base is to be left behind.  therefore, cornea with central thickness of less than 450u is a contradiction for LASIK.  3. INTRAOCULAR PRESSURE: the pt under going LASIK has to tolerate an IOP of at least 65mm Hg for upto several minutes during the procedure.  Therefore pt with glaucoma, retinal vascular ds and systemic vascular ds should be excluded.
  63. 63. PATIENT SELECTION AND EVALUATION  4. PUPIL SIZE: large optical zones are required for people with large pupils. Young pt with large pupil must be informed about the potential glare, halos and night driving problems.  5. CONTACT LENS WEAR: soft contact lens wear should be discontinued at least 2 weeks and in the case of rigid lens at least 3 weeks before the LASIK procedure is undertaken.
  64. 64. ESSENTIAL EXAMINATION  Following examination and documentation are essential for a Lasik procedure:  1. VISUAL ACQUITY: uncorrected and best corrected visual acquity should be noted.  2. REFRACTION: subjective, manifest and cycloplegic refraction should be carried out.  3. BIOMICROSCOPIC EXAMINATION: should be conducted in detail.  4. INDIRECT OPHTHALMOSCOPY: should be performed with dilated pupils with the use of scleral depressor.  5. IOP: should be measure accurately.  6. CORNEAL TOPOGRAPHY AND PACHYMETRY: are most essential before performing LASIK.
  65. 65. SURGICAL WORK UP AND TECHNIQUE  1.Broad spectrum antibiotic eye drops: should be sued four times per day, 3-4 days preoperatively.  2.Pilocarpine1% :which helps to maintain pupillary miosis to assist centration is used by some surgeons.  3.Anaesthesia: topical anesthesia with with proparacaine or xylocaine  4.Cleaning and draping: should be performed meticulously  5.Exposure: is obtained adequately by using a wire speculum
  66. 66. SURGICAL WORK UP AND TECHNIQUE CONTD..  6. Corneal marking: corneal should be marked with a marker using gentian voilet.  An ideal marker is one that has two radial and one para radial mark.  It consists of an inner circle 3.0 mm in diameter, joined by paired para radial lines to an external circle of 10.5 mm.  The inner circle should be placed concentrically with the pupil to aid the centration .  The outer circle aids the concentric placement of suction ring.  The pararadial lines facilitates the correct alignment in the rare free flap situation.
  67. 67. SURGICAL WORK UP AND TECHNIQUE CONTD...  7. Fixation of Suction Ring: pneumatic suction is fixed in place on the sclera with slight decentration towards the side of the hinge.  Then the suction is activated and the IOP elevated to about 65mm Hg.  This is required to obtain a resection of appropriate diameter and thickness of corneal flap.  The IOP may be checked with a Barraquer tonometer.  Four signs indicate that adequate IOP has been achieved: I. Barraquer’s tonometer II. No vision III. Dialation of pupil IV. Reading on the gauge
  68. 68. SURGICAL WORK UP AND TECHNIQUE CONTD..  8. Preparation of corneal flap: the corneal is moistened with balanced salt solution (BSS) to facilitate a smooth movement of the microkeratome.  The microkeratome is then inserted in the track on the suction ring.  Then the forward foot pedal is depressed and the microkeratome makes it pass on the track and cuts the corneal flap/disc.  Imp principle to be remembered is Cut Wet; Ablate Dry.  However most recently it has been a matter of concern that the flap created for LASIK results in the loss of majority of corneal nerve fibres.
  69. 69. SURGICAL WORK UP AND TECHNIQUE CONTD..  Preparation of corneal flap cont..  A nasal hinge is superior to preserve the corneal nerve fibers better than with a superior hinge.  This is because most of the corneal nerve fibers enter the cornea nasally.  The disadvantage with the nasal flap is that there is a small risk of the flap being displaced by the movement of the upper lid.  Once corneal flap is created, the suction should be immediately released immediately so that the period of raised IOP should be as short as possible to avoid damage to the optic nerve head.
  70. 70. RECOMMENDATIONS FOR ACHIEVING A GOOD QUALITY CUT  In microkeratome new blade should be inserted into the sharper’s head for each procedure.  An ideal flap is 130-160 u in thickness.  Speed of the pass affects the quality of the flap. A rapid pass produces a thin cut while a slow pass produces a thicker corneal flap.  Oscillation of the blade affects the flap:  A slow oscillating speed of the blade may produce a rougher surface, which m ay affect the quality of the visual outcome.  A higher oscillating speed of the blade produces greater friction of the movement, resulting in greater heat production and dispersion, which causes damage to corneal tissue.
  71. 71. RECOMMENDATIONS FOR ACHIEVING A GOOD QUALITY CUT  The IOP should be between 60 and 65mm Hg during cutting. IOP lower than this may result in a flap of variable and suboptimal thickness and diameter.  The cornea should be well irrigated with BSS to prevent friction with the thickness plate of the microtome.  Adequate exposure of the eye is very important to achieve a good quality cut.
  72. 72. SURGICAL WORK UP AND TECHNIQUE CONTD..  9. Stromal Ablation: The corneal flap is retracted along the hinge and the stromal bed is dried. The ablation is then carried out with predetermined correction by the excimer laser unit.  10. Reposition of corneal flap: after ablation is done , the irrigation is done under the flap and over the stromal bed to remove any debris.  Excess fluid is then sucked out with the help of cellulose sponge.  The corneal flap is aligned and distened properly.  Once flap is repositioned, it is allowed to air dry naturally for 3-5 min to allow firm adhesion of the flap to stromal bed.  This should be confirmed by the striae test, whereby a dry cellulose sponge is used to depress the corneal periphery near the limbus, producing striae which can be seen radiating on the flap.
  73. 73. SURGICAL WORK UP AND TECHNIQUE CONTD..  11. Removal of speculum and drape: is then gently performed so as not to cause an epithelial abrasion or displacement of flap.  Pt is asked to blink and this should be observed under the microscope by the surgeon.  The flap should appear well adhered and perfectly aligned.  The status of the flap should be reassessed after 1 hr by performing slit lamp biomicroscopy.
  74. 74. LASIK PROCEDURE
  75. 75. POST OPERATIVE MANAGEMENT  1. Patching/ dark goggles  2. Systemic analgesics  3. Antibiotic eye drops: QID*7-14 days  4. Topical steroid eyedrops: QID* 1 month  5. Artificial tears: QID* 1 month  6. Home advice for a LASIK pt is a follows:  Home rest for 24-48 hrs after which pt is allowed routine activities.  Eyes should neither be touched nor rubbed for at least 2 weeks.  No swimming for at least 1 month.  Avoid exposure to strong light.  Avoid overuse of the eye for long periods  Driving should be avoided for 2 weeks.  Report to the doctor in case of pain or marked blurring of vision.
  76. 76. FOLLOW UP EXAMINATION  Slit lamp biomicroscopy examination and visual acquity testing should be performed on following days:  One day after operation  One week after operation  One month after operation  Three months after operation  Six months after operation  One year after operation
  77. 77. COMPLICATIONS OF LASIK
  78. 78. COMPLICATIONS  A. Intraoperative complications  1. During application of suction ring:  Conjuctival chemosis may occur which may occlude the pneumatic ring suction part.  Severe hypotension resulting due to ciliary body shut down because of vacuum created by the suction ring has been reported a case.  2. During preparation of corneal flap: i) Flap of variable and suboptimal thickness and diameter may be formed if:  IOP of 60-65mm Hg is not achieved.  Blade is of poor quality  Speed of the pass is not optimal
  79. 79. COMPLICATIONS CONTD.. ii) Incomplete flap: results due to some foreign material, debris getting into the microtome gear mechanisms, suction loss or motor failure. iii) Tear or button hole in the flap: can occur in steep cornea, due to inadequate suction and poor blade quality. iv) Perforation of the cornea: can occur during microtome use in patients with thin cornea. v) Free cap instead of a hinged flap may result Causes:  Flat cornea  Intraoperative IOP less than 65mm Hg.  Small cornea (< 10.5mm)  Not fixing the keratome stopper.
  80. 80. COMPLICATIONS CONTD..  vi) Damage, destruction and dislocation of the flap can occur due to inadvertent trauma.  vii) Loss of flap can occur rarely.  viii) Irregular stromal bed is a microkeratome induced complication.  ix) Hydration or desiccation of the stromal bed may occur if the time lapse of more than 30 secs occurs between creation of the flap and laser ablation.  x) Intraoperative contamination of the surface/ interface debris may occur.  3. Complications during laser ablation of the stromal bed: i) Decentration of ablation may lead to various complications like monoocular diplopia, loss of best corrected visual acuity and glare.
  81. 81. COMPLICATIONS CONTD.. ii) Incorrect ablation may result in unpredictable results. iii Ablation of the hinge may result in prismatic effect. iv) Interruption of ablation may occur due to technical problems. v) Poor ablation occurs when liquid and/or impurities are present.  4. Complications during flap reposition: i) Irrigation complications include edema of flap and decreased adherence due to excessive irrigation ii) Interface deposits such as lint deposits, metallic deposits, lipoid deposits or blood may occur inadvertently iii) Incorrect replacement of the flap can occur if proper care is not taken iv)Wrinkling or reposition may occur if flap is very thin v) Flap cab be damaged du e to rough handing during reposition.
  82. 82. COMPLICATIONS CONTD..  5. Complications during removal of eye speculum:  Flap can be dislodged during removal of eye speculum.
  83. 83. COMPLICATIONS CONTD..  B. Postoperative complications  1. Poor adhesion and anchoring: of the flap with the stroma  2. Flap striae: seen immediate postoperatively can either be macrofolds or microfolds.  Macrofolds: are easily visualized and are a result of flap slipping. These can cause visual deterioration and can cause full thickness flag tenting.  Microfolds: are within the flap itself in form of wrinkling in the Bowman’s membrane or in epithelial basement membrane.  These are related to flag setting and not flag slipping.
  84. 84. COMPLICATIONS IN LASIK Air bubble inside flap Dislodged flap Macrofolds Fibers under flap Microfolds
  85. 85. COMPLICATIONS CONTD..  3. Infectious keratitis  4. Diffuse lamellar keratitis or sands of Sahara syndrome: occurs in early postoperative period as nonspecfic intrastromal/intralamellar keratitis.  Aetiology is doubtful.  Treatment consists of anti inflammatory drops and antimetabolites.  5.Epithelial ingrowths: under the flap are reported frequently.  If they are in the centre, they can affect visual acquity, and if in the periphery can cause thickening of one edge of the flap and induce or irregular astigmatism.  Rx: flap can be lifted, the epithelium scrapped off and the flap is repositioned.
  86. 86. COMPLICATIONS CONTD..  6. Lost flap postoperatively: results due to incomplete adherence to the bed.  Perhaps, there is no need of lamellar grafts in these cases.  Epithelium grows over the residual stroma and cornea may functin normally with some haze ( Caplen technique)  7. Central islands: also called as induced topographical alteration may result due to intrastromal ablation.  If no sign of resolution is seen ever after 3 months, the central islands can be treated by mobilizing the original lamellar flap.
  87. 87. COMPLICATIONS CONTD..  8. Undercorrections and over corrections : are not uncommon.  These can be treated with repeated procedures within 3 months as the flap can easily be reflected.  9. Regression: of upto 2 D has been reported following LASIK. The refractive Sx stabikizes from the 3rd month onwards in most of the cases.  10.Induced astigmatism: both regular and irregular is reported to occur.  Regular astigmatism can result due to:  Decentration of the ablation zone.  Variation in flap healing and  Presence of epithelial cells under the flap.
  88. 88. COMPLICATIONS CONTD..  Irregular astigmatism can result due to:  Eccentric ablation  Suturing of the flap  Interface remnants  Epithelial islands or  Irregularly reposited thin flap which produces wrinkles.  11. Halos and glare: may be experienced by some patients postoperatively. Common cause are:  High order aberrations, resulting from subclinical decentration  Optical treatment zone smaller than the size of the pupil during mesopic dilation.  Irregular astigmatism due to flap folds.  Topographic abnormalities or irregular epithelial surface.
  89. 89. COMPLICATIONS CONTD.. Glare Halos
  90. 90. COMPLICATIONS CONTD..  12. Haze at the interface: occurs due to cellular response to toxic substances introduced during surgical procedure.  13. Corneal Ectasia: can occur if corneal base is thin due to formation of a thick flap.  14 Dry eyes: occur due to decreased corneal sensation resulting from severing of corneal nerves with subsequent decreased blinking.  15. Loss of contrast sensitivity: is related to oblate shape of the cornea following LAISK.
  91. 91. COMPLICATIONS CONTD..  Inherent problems after LASIK  1. Inaccurate measurements of IOP: tonometer under estimates the IOP after LASIK due to decrease in corneal thickness.  2. Inaccurate intraocular lens power calculation: seen after LAISK sx , the use of mean keratometry in the formula for IOL power calculation results in an inaccurate emmetropic IOL power, afterwards leading too hyperopia.  3. Difficulty in contact lens fitting: occurs due to disturb corneal topography.
  92. 92. ADVANTAGES OF LASIK OVER RK AND PRK  1.Minimal or no postoperative pain.  2.Recovery of vision is very early as compared to PRK.  3.No or little risk of perforation during sx and later rupture of globe due to trauma unlike RK.  4.No residual haze unlike PRK where sub epithelial scarring may occur.  5. LASIK is effective in correcting high myopia of -6 to -3 D.
  93. 93. DISAVANTAGES OF LASIK OVER RK AND PRK  1. LASIK is much more expensive.  2. It requires greater surgical skill than RK and PRK.  3. There is potential risk of flap related complications

×