1. Moderator: Niraj Dev Joshi Presenter: Arjun Sapkota B.Optometry, Final Year 2. Introduction 3. Condition of refractive error: ametropia myopia hyperopia astigmatism presbyopia Eye’s refractive power determined by 3 variables – 1. Power of the cornea(43D) 2. Power of the lens(16D) 3. Length of the eye •Change in axial length 1mm—3 D •Change in radius of corneal curvature 1mm—6 D 4. Pupil size & centration Rays of light from a point source are refracted by the area of cornea overlying the entrance pupil. This area is c/a the corneal optical zone. Entrance pupil- Virtual image of anatomical pupil formed by magnifying effect of the cornea – larger & closer to cornea. Optical zone in a keratorefractive procedure : “The area of central cornea that bears the refractive change caused by the surgery”. 5. Alter refractive state of eye to enable patient to see without visual aids Aim of refractive surgery 6. Classification REFRACTIVE SURGERIES CORNEA BASED LENTICULAR BASED COMBINED(BIO PTICS) Incisional -R.K. Lamellar-EPK,ALK Laser ablation; Conductive Keratoplasty Corneal Inlays and rings PRK LASEK E-LASIK LASIK C-LASIK Phakic refractive lenses (PRLs) Refractive lens exchange (RLE) Combination of the two 7. Keratorefractive Procedures LOCATION ADDITION SUBTRACTION RELAXATION COMPRESSI ON SUPERFICIAL 1.EPIKERATOP HAKIA 2.SYNTHETIC EPIKERATOPHA KIA 1.PRK 2 LASEK 3.Epi-LASIK ---------- CORNEAL MOLDING INTRASTOMAL 1.KERATOPHAK IA 2.INTRACORNE AL LENSES 3.PINHOLE APERTURES 1.LASIK 2.WAVEFRONT GUIDED LASIK 3.IntraLASIK (IntraLase) LAMELLAR KERATOPLASTY PERIPHERAL INTRACORNEAL STROMAL RING WEDGE RESECTION 1.RADIAL KERATOTOMY 2.ARCUATE KERATOTOMY 1.THERMOKE RATOPLASTY. 2.COMPRESSI ON SUTURES 8. Preoperative evaluation and patients selection Motivation Age- better age(18-45 years) Refractive error Occupation night drivers, sportsperson,security persons Informed conscent Patient expectation 9. Ophthalmic Examination Visual acutiy – Distance & Near : with & without correction Refraction – Current spectacle correction - Manifest refraction - Cycloplegic refraction (1% cyclopentolate ) External examination – Ocular dominance - Ocular motility - Gross external examination Slit-lamp examination – Fluorescein & vital stain IOP measurement Fundus examination 10. Keratometry: Normal range ( 40D-48D) The normal difference between K max and the steep less than 1.00 D. Flat myopia The rule -1.00 D correction reduces the flat K by 0.75 D. The final flat sim K should not go below 34.00 D Steep Hyperopia The rule each 1.00 D correction increase the K max 1.20 D The final K max should not exceed 49.00 D 11. 4 map Anterior (Axial) sagittal Corneal thickness map Anterior elevation map Posterior elevation map Computerized Videokeratography :central 3 mm corneal curvature)(only way to uncover early KC) In each map, both shape and parameters should be studied 12. 13. Thickness Rules Ectasia is the dreaded complication in any refractive procedure altering the central thickness LASIK -480µ, PRK- 450µ Minimum residual thickness in the bed post ablation = 250 microns Total central thickness post ablation = 300 – 350 microns Excessive ablation in PRK : Haze 14. Thickness calculation Roughly defines the depth of ablation required to achieve a specific amount of correction For 1 D correction depth of ablation required (in microns) one-third of the square of diameter (in mm) So each spherical equivalent diopter of correction performed at a 6mm optical zone will ablate 12 microns of tissue MUNNERLYN EQUATION 15. When Scotopic pupil >5.5mm Ablation OZ = 6.5 mm So 1D Ablate 14um. When Scotopic pupil <5mm Ablation OZ = 5.5 mm 1D Ablate 10um In small thickness save tissue The AD should be at most 20% of the original corneal thickness at the TL 16. CORNEAL THICKNESS CALCULATION - TOO THIN Patient's refractive error: -14.00 diopters Amount to ablate: 14 x 12 = 168 micrometers Thickness of patient's cornea: 510 micrometers LASIK flap thickness: 160 micrometers Minimum residual stromal bed thickness: 250 micrometers Available tissue for ablation: 510 - 160 - 250 = 100 micrometers So in this example, the patient needs to ablate 168 micrometers from 100 micrometers of remaining corneal stromal tissue. CORNEAL THICKNESS CALCULATION - THICK ENOUGH typical LASIK flap thickness of 160 micrometers, and for a 6 millimeter diameter pupil, every 1 diopter of prescription power requires the removal of 12 micrometers of corneal tissue. Patient's refractive error: -8.00 diopters Amount to ablate: 8 x 12 = 96 micrometers Thickness of patient's cornea: 580 micrometers LASIK flap thickness: 160 micrometers Minimum residual stromal bed thickness: 250 micrometers Available tissue for ablation: 580 - 160 - 250 = 170 micrometers So in this example, the patient needs to ablate 96 micrometers from 170 micrometers of corneal stromal tissue. 17. Contraindications Ocular Disorders that may be exacerbated by PRK - HZO (if active during last 6 months) - Glaucoma Dry eye – Keratoconjunctivitis sicca, Exposure keratitis, Lid disorders Abnormal corneal shape - Shape changes induced by contact lens - High irregular astigmatism - Corneal ectasias : Keratoconus, Keratoglobus, Pellucid marginal degeneration Uveitis, Lenticular changes, Progressive retinal ds., myopic degeneration, Diabetic retinopathy, RP, RD 18. Diabetes mellitus ( if corneal sensation is not intact ) Pregnancy/lactation Autoimmune / connective tissue disorders(RA,SLE,PAN etc)/ Immunodeficiency Abnormal wound healing-Marfans,Ehler-Danlos Systemic Infection-(HIV,TB) Drugs-Azathioprene,Steroids(Slow wound healing) -Antihypertensives, Antipsychotics, Antiparkinsonian drugs (induce dry eye) Systemic 19. Radial Keratotomy Series of 4-8 deep(90%corneal thickness), radial corneal stromal incisions in peripheral part of the cornea Incisions on healing-flatten the central cornea Normal IOP pushes the peropheral cornea Weakened by incisions & flatten the central cornea I/C- keratoconus - astigmatism - myopia 20. For mild to moderate myopia Good results Weakens cornea 21. Complication of RK Infection Weakening of the globe Decreased corneal sensation Irregular astigmatism Misalignment / axis shift Wound gap & discomfort Incorrect number of incisions & incorrect orientation of incisions Overcorrection Undercorrection Diminished night vision Non healing wound 22. PERK 10 years study -2.00D- -8.75 D myopia Eight radial incisions were used 10 years after 53% --20/20 45%--20/40 2%--loss of previous vision Most important finding in this PERK study was the continuing long term instability of the procedure. A hyperopic shift of 1.0 D or greater was found in 43% of eyes between 6 months and 10 years postoperatively. More susceptible to rupture in the event of an injury. 23. Arcuate keratotomy Limbal relaxing incision Different incision 24. Laser ablation corneal refracive procedures 25. Types of lasers used Excimer : for corneal stromal ablation Non-Excimer solid state lasers : for flap creation IR femtosecond laser 26. Excimer Lasers Excited dimer of two atoms -an inert gas(Argon) -a Halide(Fluoride) releases ultraviolet energy at193nm Reshapes corneal surface by removing anterior stromal tissue Process – Non-thermal Ablative Photodecomposition 27. Laser delivery patterns : 1) Broad-beam lasers- deliver a large diameter beam of laser – starts small & expands as the laser is delivered ADV. Less operative time DISADV. Creation of central islands ( difficulty to maintain uniform consistency over a larger diameter beam) 28. 2) Scanning excimer lasers: - Scanning-slit laser - Flying spot laser Provide smoother ablation than the old broad- beam lasers 29. Advantage of Non-Excimer solid state lasers- No toxic excimer gases Wavelength closer to absorption peak of corneal collagen—less thermal and collateral damage Better pulse to pulse stability Not absorbed by air,water,tear fluid-so less sensitive to humidity or room temperature No purging with inert gases required. 30. Intralase keratome employs solid state laser instead of gas based excimer laser i.e. Diode pump,Neodymium-glass laser Wavelegth: infrared (1053 nm) Spot size < 3μ Speed of laser: 15,000 to 60,000 pulses per second Low energy,high peak power Increase speed-decrease suction time Stromal ablation done with excimer laser Femtosecond laser 31. Intralase can be used for • Creating corneal flaps for LASIK • Lamellar keratoplasty • Creating a channel for INTACS • Penetrating keratoplasty. 32. Advantage of Femtosecond Laser • Greater accuracy of flap creation • Thin yet stable flaps • Fewer flap complications • Better centration • Stronger flap adherence • Less IOP rise during suction • Fewer high order aberrations • Less dry eyes • Less epithelial ingrowth • Better contrast sensitivity Disadvantages of Femtosecond Laser • Suction breaks • Incomplete flaps • Transient light sensitivity • Persistent bubbles – stroma / anterior chamber • Harder to lift flaps • Granular bed 33. PRK First widely used procedure with the excimer laser (1987) Outer layer of cornea is removed then laser is applied No microkeratome involved No flap created Ultimate visual results similar to LASIK Longer recovery period (> 2 weeks) 34. Indication Myopia: -1 to -8D Hypermetropia upto +4D Astigmatism upto 3 D Flat corneas(<40D) Steep corneas Thin cornea with bead thickness(< 250µm) Epithelial irregularities/ dystrophies LASIK complications in contralateral eye Predisposition to trauma Dry eyes 35. Epithelium is ablated more irregularly than stroma ;so epithelium removed mechanically before ablating stroma Optical zone :7-8 mm in Myopia 9-10 mm in Hypermetropia 36. Consideration against PRK Slower visual recovery than LASIK or phakic IOL surgery. Not recommended for people with significant ocular disease of any type, especially corneal disease. Not recommended for people with significant skin or systemic disease that could adversely affect healing. Not recommended for people with unstable or changing refractive error. 37. Complication(PRK) Recurrent erosion Corneal infiltrates & infectious keratitis Central island: localized elevated area within corneal treatment ablation zone>1.5 mm in diameter & over 3D in height or 1.5 D & 2.5 mm on corneal topographical mapping Eccentric ablations & decentration Irregular astigmatism Undercorrection: failure to achieve within 0.75 D of the intended correction by 6-12 weeks postoperatively Overcorrections Haze , scarring & regression Night glare & halos 38. LASEK Surface ablative procedure Epithelium is loosened with alcohol & replaced after laser ablation Indication Low to moderate Myopia & astigmatism Risk of ocular trauma Thin cornea Steep cornea Flat cornea 39. LASEK Metal handle as alcohol reservoir Epithelial flap formation With Vannas scissors Ablation with excimer laser Epithelial sheet is repositioned Bandage contact lens is placed 40. Why LASEK ? Eliminated post-PRK pain Reduced corneal haze Provides good visual and refractive outcomes Can be used instead of LASIK in case of a thin cornea and steep corneas Less dry eye. 41. Even if infection occurs it is superficial easy to treat since the infection is not covered by a stromal flap. No surface flap striae causing irregular astigmatism with loss of vision. Epithelial folds or striae in LASEK will simply smooth out by themselves in contrast to the stromal flap of LASIK which are very hard or may be impossible to eliminate. No epithelial ingrowth is possible with LASEK. 42. Complication of LASEK Pain (greater than LASIK in 80% of patients) Corneal scarring/haze (<1-2%) Epithelial defects Keratitis (0.5-1%) Dry eye syndrome associated with recurrent erosions Overcorrection (1%, incidence similar to LASIK and PRK) Undercorrection (10-15%, incidence similar to LASIK and PRK Irregular astigmatism 43. Epi -LASIK Blunt, plastic oscillating blade Instead of the alcohol (in LASEK ) the plastic blade, called an epithelial separator or epikeratome is used to separate the sheet from the eye more appropriate for people with less steep corneas (low myopia). epithelial flap made it is lifted sculpt the underlying corneal tissue laser epithelial flap is placed back contact lens is placed on the eye to keep the flap in place Advantage over LASEK: Less pain, Faster healing, Less corneal haze 44. LASIK Most commonly performed refractive surgery Combines lamellar corneal surgery with accuracy of the excimer laser Excimer laser ablation of corneal stroma beneath a hinged corneal flap that is created with a mechanical femtosecond laser microkeratome 45. Why LASIK? Around 80% of patients no longer need corrective eye wear. Very little pain. Vision is corrected nearly immediately or by the next day Recovery is quick and usually no bandages or stitches are required after LASIK laser eye surgery. Adjustments can be made years after LASIK laser eye surgery to further correct vision. 46. Patient selection Patients need to be fully informed about potential risks,benefits and realistic expectations Age should be above 18 years Refractive status should have been stable for at least 1 year. Current FDA approval- Myopia-upto -15D Hyperopia –upto +6D Astigmatism-upto 6D 47. CCT such that minimum safe bed thickness left(250-270µ).Post op Corneal thickness should not be <410µ. Extreme keratometric values ( flatter than 41.00 or steeper than 47.00) avoided Videokeratoghic clues to a KC suspect: K value > 47.2 D, Inferior steepening of > 1.4 D, difference of > 1.9 between K values of both eyes Contact lens free period before examination : 3-4 wks for rigid contact lens wearers 2 wks for soft contact lens wearers 48. Basic Mechanism Normal cornea – prolate shape ( greater curvature centrally ) Myopic correction – create an oblate shape by central corneal laser ablation Hyperopia- Excimer laser ablation at mid-periphery steepening of central cornea Mixed astigmatism – 1)Bitoric LASIK technique – flattening the steep meridian with paracentral ablation over the flat meridian 2)Cross-cylinder technique – dividing cylinder power into 2 symmetrical parts – half of the correction is treated on the positive meridian & half on the negative meridian 49. Operative Procedure 5mg Diazepam 5-10 min before procedure Verification of entered computer data before starting procedure Topical anasthesia-Proparacaine 0.5%, Lignocaine 4%. Surgical Painting and draping Lid speculum with aspiration Proper centration over pupil & maintenance by the aid of Tracking systems & iris registration 50. Corneal marking with ink Adequate placement of suction ring using bimanual technique Suction engagement by foot control 1st step - Creation of flap 51. Adequate IOP (>65mmHg) which is necessary for the microkeratome to create a pass and resect the corneal flap verified by BARRAQUER TONOMETER confirmed by patient – temporary loss of visualization of fixation light 52. 2nd step - Resection of corneal flap Artficial tear drops instilled MICROKERATOMES 1) Steel Microkeratome -Uses Disposable blades -Blade Plate can be set at 120µ,140µ,160µ and180µ. -Nasal or superiorly hinge flaps can be created. -Eg.Hansatome,ACS,Carriazo Barraquer, Moria. 2) Waterjet Keratome -Less debris & collateral damage than blade keratomes - Raised IOP not needed to create flap 53. 3) Laser Keratome (IntraLase) - Solid-state laser - 1053 nm wavelength - 3 µm spot size- high precision - Uses brief Femtosecond laser pulses to cause disruption in a lamellar plane - Needs lower vacuum & any hinge can be made - Can make flaps as thin as 100µ(Sub Bowmans Keratomileusis) 54. - Flap has vertical edges –so reduced epithelial ingrowth. - Steel Microkeratome flap thicker in periphery and thinner in the centre. Not so with Intralase(Planar). 55. 3rd Step-Delivery of Laser- After flap is lifted,assessment of residual corneal bed thickness usin USG pachymetry laser is applied to the stroma according to the ablation profile calculated by the machine. Excimer Laser beam is delivered by the following ways depending on the machine- Beam Delivery Broad Beam Scanning Slit Beam Flying Spot 56. Most machines employ a flying spot to deliver laser with the help of incorporated eye tracker or iris registration. 57. 4th step-Reposition Of the Flap- After irrigating interface ,flap repositioned Sweeping movements with a wet cellulose sponge From the hinge towards the periphery of flap Adhesion verified – stretching the flap towards gutter Topical antibiotic, steroid & lubricant instillation transparent plastic shields 58. Wavefront-Guided (Customized) EXCIMER LASER To correct higher-order aberrations in addition to lower- order sphero-cylinder corrections - LOWER ORDER Nearsightedness Farsightedness Astigmatism - HIGHER ORDER Spherical aberration Chromatic aberration Diffraction Curvature of field Coma Trefoils Quadrifoils 59. Higher order aberrations occur in visually significant manner in 10-15% of population Cannot be corrected with spherocylinder lens or conventional laser refractive surgery Correction – Hard contact lenses - Wavefront-guided customized laser refractive surgery 60. Complications In LASIK Intraoperative microkeratome-related complications Incomplete flap Thin flaps Buttonhole flap Free cap Full thickness resection : corneal perforation Laser-related complications : Central island Decentration Epithelial defects: prevention :-one drop LASIK Loose epithelium Keratectasia 61. Postoperative complications Flap striae Dislodged flap Infection Diffuse lamellar keratitis (Sands of Sahara syndrome): interface inflammation Epithelial ingrowth Errors in refractive outcome : undercorrection , overcorrection Keratectasia Central island Night vision disturbance: halos& glare Irregular astigmatism Dry eye 62. Complications of LASIK Diffuse intralamellar keratitis Epithelial ingrowth Flap striae Button hole Incomplete flap Free cap Epithelial defect Dislodged flap 63. GlareHalos Decentered flap & ablation Bacterial keratitisDry eye Non healing flapPerforated cornea Loose epithelium 64. Disadvantages of LASIK Changes made to the cornea cannot be reversed Corrections can only be made by additional LASIK laser eye surgeries. Expensive Technically complex. Can cause a loss of "best" vision with or without glasses. 65. Thin corneas < 480 um Steep corneas > 47D RST< 250 um (residual stromal thickness) Epithelial basement membrane dystrophy Poor candidates for Lasik 66. LASIK vs PRK LASIK Less postop pain Correctable range(1-30D) Flap complication Microkeratome is used Better surgical skill Early visual outcome Stable refraction-within 1 month No stromal haze PRK More pain 1-12D No flap complication MK is not used Less Skill Comparativelty late Stable refraction- 4-8 months Present 67. Differences Flap & Surface Surgery Flap surgery lasik Surface surgery lasek, epi-lasik Eye Pain after Surgery Minimal (may last up to 12 hours after surgery) Moderate to Severe (may last up to 72 hours after surgery) Functional Vision Recovery Earlier (Less that 24 hours) Later (3 to 7 days) Stable Refraction Earlier (1 to 6 weeks) Later (3 weeks to months) Corneal Scarring Risk Minimal (Less than 1%) Greater (1 to 2 %) Dry Eyes Symptoms More risk (may last more than 6 months) Less risk (lasts for 1 to several weeks) Risk of Complications More risk Flap issues: Flap wrinkles, Epithelial ingrowth, Flap melt Less risk In general, safer than LASIK. Best For Most patients Patients with thin corneas or large pupils, contact sports 68. LTK (Laser Thermokeratoplasty) Treatment of hypermetropia +0.25 to +2.75 D laser beam uses heat to shrink & reshape cornea : contraction of collagen “non-permanent” procedure. Target temperature: 65 - 75º C 69. Current technique: Contact holmium :YAG laser 8 Spots in circle Optical zone of 7.0 mm – correct 1.3D Double concentric ring at 7 & 9mm – 3.0 D Laser is perpendicular to corneal surface 70. Mainly for hyperopia and presbyopia Heat is applied directly to the corneal surface Steepen the cornea with a ring of collagen constriction 71. Conductive Keratoplasty Application of low-energy, high frequency radiofrequency current to heat & shrink peripheral & paracentral stromal collagen resulting in steepening of central cornea Used for hyperopia (1 – 2.25D), hyperopic astigmatism and presbyopia FDA approved 2002 Provides better stability than the previously used procedure Laser Thermal Keratoplasty (LTK) 72. CORNEAL RESPONSE TO HEAT 55 – 58 ˚C collagen shrinkage (disruption of H bonds of tertiary collagen structure) 65 – 78 ˚C collagen relaxation > 78 ˚C collagen necrosis 73. Hyperopia Lower corrections : 8 spots at 6mm optical zone & 8 spots at 7mm optical zone Greater corrections : 24 spots applied ( 8 additional (+2 to +2.50D ) spots at 8mm optical zone) Even greater corrections : 32 spots Hyperopic astigmatism Peripheral heat spots along a single (flatter) meridian Effect decreases with time : production of new collagen by corneal fibroblasts 74. Intrastromal Corneal Ring Segments ( INTACS ) Thin arc-shaped segments of polymethylmethacrylate(PMMA) Used for mild myopia (< 3 diopters) & minimal astigmatism (< 1 diopter). Maintains a central, clear, optical zone as 2 segments are placed in periphery 75. Intracorneal Ring opening is made on corneal edge ,at 12 o’clock with diamond flap 2 tunnels are created using Pocket hook ,stromal spreader Under vacuum centering guide Insertion of INTACS Inserted in pairs through a 1.8mm radial corneal incision at 12o’ clock into the peripheral corneal stroma at 2⁄3 depth 76. Complication of INTACS Localized incision-related epithelial defects Epithelial plug formation Wound dehiscence Superficial neovascularization Surgically induced astigmatism Infiltrates in the channel Transient decreased corneal sensation Delayed infectious keratitis Haze & deposits around intrastromal channel. Infiltrates in tunnel Epithelial plug 77. Had been advocated for high myopia-16 to -18D Involvs lens extraction by phacoemulsification with appropriate IOL Indication -refractive error a/w lens opacities -high refractive error, myopia of >10D & hyperopia of >5D in patients above 40 yrs of age are ideal for RLE. Patients having retinal diseases and are of occupational night drivers are considered contraindications for RLE Refractive lens exchange 78. Traditional monofocal IOLS Multifocal IOLS Pseudoaccommodative IOLS Accommodative IOLS Choice of IOL & Power calculation Iol power calculation - Immersion biometry - Optical inteferometry based biometry - IOL power for myopia- SRK/T and for hyperopia-HolladayII 79. Phakic Intraocular Lenses Artificial lenses implanted in the anterior or posterior chamber in the presence of the natural crystalline lens to correct refractive errors Intraoperative iridectomy or preoperative Nd:YAG laser iridotomies – necessary to avoid post-op pupillary block glaucoma 80. 3 types Anterior chamber-angle supported PIOL AC iris-fixated Posterior chamber PIOL Early models – PMMA Newer models – foldable (more safe & efficacious) 81. Criteria for Implanting Phakic IOLs Age above 18 years Stable refraction (< 0.5D change for 6 months) Ammetropia not suitable for Excimer laser surgery (high powers or thin cornea) AC depth >= 3.2mm for iris-claw lens >= 2.5mm for pc PIOLs Minimum endothelial cell density > 3500 cells/mm² at 21 yrs age > 2800 cells/mm² at 31 yrs age > 2200 cells/mm² at 41 yrs age > 2000 cells/mm² at 45 yrs age No other ocular pathology (corneal disorders, glaucoma, uveitis, cataract) 82. Indications High Myopia December 2004, FDA approved 1st PIOL : Verisyse/ Artisan ‘iris-claw’ lens Myopia -5 to -20 D Astigmatism upto 2.5 D December 2005, FDA approved a 2nd PIOL : Visian ICL(Implantable Contact Lens) Myopia -3 to -20 D Astigmatism upto 2.5 D High Hyperopia Upto +3.0 D 83. Diam of lens = w-w dist + 0.5 to 1.0 mm(For both angle & sulcus- supported PIOLs) 84. AC iris-fixated PIOLs Artisan/ Verisyse Most commonly used phakic IOL 85. ADVANTAGES Most stable & predictable refractive method Newer designs – improved safety & efficacy Reversible Significant gain of postoperative BCVA in myopia – reduction in image minification No loss of contrast sensitivity (as seen in LASIK) 86. COMPLICATIONS Haloes & glare Pupillary ovalization ( Angle supported PIOLs) Endothelial damage Elevation of IOP Uveits (iris trauma during surgery) Cataract (mostly nuclear) 87. Anterior chamber inflammation/ pigment dispersion – repeated traumatic attempts at iris enclavation (Iris-fixated PIOLs) Iris atrophy & IOL dislocation (Iris-fixated PIOLs) Hyphaema (Iris-fixated PIOLs) Decentration / Dislocation into vitreous cavity (PC PIOLs) 88. BIOPTICS Concept of first implanting a phakic IOL to reduce the amount of myopia, then fine tuning the residual correction with LASIK I/Cs –extremely high myopia - high astigmatism - lens power not available Combination has expanded the limits of refractive surgery 89. Refractive surgery Section 13 AAO 2015 Yanoff and duker –Ophthalmology Third edition(2009) Various internet journals Optics and refraction- AK khurana Borish,s clinical refraction(part C) References 90. Thank you 91. Special celebration At Seminar Hall (A ) 12:30 pm