Astigmatism correction
Alireza Peyman, MD
• One of the troublesome aspects of refractive surgery
What is astigmatism
• Regular
• Irregular
Regular astigmatism
Presbyopic with the rule in near vision
Source of astigmatism
• Cornea-tear film
• Crystalline lens
• Including tilt
• Posterior segment
Measurement of astigmatism
• Auto-refraction and retinoscopy
• Subjective refraction
• Astigmatic dial
• Cross cylinder
• Wavefront PPR
• Keratometry
• Automated or manual
• ORA could be calculated
Correction methods
• Glasses
• Contacts
• Soft (toric)
• orthokeratology
• Incisional methods
• Traditional
• FS assisted
• full thickness paired incisions
Intra-corneal inlays
Excimer ablation
Toric pIOLs
Toric IOLs
• Easy and difficult!
• Cause distortion of images and depth due to dissimilar meridional
magnification in eyes
Easy cases
• Persons that have had astigmatic glasses for years or from childhood
• Minor vertical or horizontal astigmats
• Monocular patients, and children
Most difficult ones
• New glasses with > 2.5 diopters of oblique astigmatism and
• Impaired proprioception (diabetics in some stages)
Contact lens
• Always worth try in difficult cases
• Irreplaceable for irregular astigmatism
Incisional methods
• AK
• Arcuate
• Straight
• Induced wound dehiscence
• After PKP or improperly sutured wounds
• Compression sutures & wedge resection
• Paired full 3.2 incision
• FS assisted
• Incisional methods mostly used during or after a major intra-ocular
surgery like cataract extraction or PKP
Corneal inlays
• Intra-corneal lenses
Excimer ablation
• Case selection
• R/O lens problems
• Lens tilt or subluxation
• Lenticonus
• R/O KC
• Inquiry about recent refractive change and FHx of KC
are important
• Check both Placido based topographies and
• In Pentacam check
4 map
Front & Back elevations in detail
Belin enhaced ectasia map
Refractive map for KC indices
Toric ellipsoid fixed reference body
• Use front and back Pentacam elevation maps with “toric ellipsoid fixed” reference if you have
decided to proceed to surgery.
• Always look at autorefraction
• Check subjective refraction and BCVA
• Consider keratometric astigmatism
• Amount
• Axis
• Check PPR and optical aberrations
• Decide for the amount and axis of the correction seeing all
• Under-correct the power for at least 5% to decrease induced
astigmatism due to angle of error of corrections.
• Check, check, and recheck the numbers at each stage.
Determine ablation protocol
• Conventional (Plano-scan)
• Tissue Saving
• Aspheric
• Customized WF guided
WF guided ablation
• Best for moderately aberrated corneas
• Not suitable for highly aberrated eyes
• Removes much higher amount of tissue
• Post-op hyperopia may arise
• Not appropriate for patients with non-corneal
• Crystalline lens opacities
• Cloudiness of vitreous
• No benefit in eyes with low aberration
Errors of angle of correction
• Exact alignment of measured angle of astigmatism with angle of
correction is of paramount importance for best results in astigmatic
Basis of error in angle alignment
• Position of head and eyes are different in upright measurement phase
and supine correction stage.
• Incorrect position of head compared to body in operation cradle.
• Misaligned and unlucked operating bed.
Only 5 degrees of tilt
make difference
Head tilt in upright position
• This type of rotation does not occur in supine position.
• This phenomenon cause error even if the amount of tilt were similar in
upright and supine positions
Rotational registration
• Manual
• Mark 90, 180, and 270 in upright
• Re-align with axes in operating bed
• Automated
• Iris image registration
Automated Iris registration
• Takes iris image in sitting position
• Takes another image immediately before Sx and compensate rotation
comparing two images
Iris registration tips
• Add another image taken in exam room with room lights on
• Turn off lights in OR
• Align with pupil center exactly
• Don’t move head until beginning of ablation
Tips (cont.)
• If registration unsuccessful:
• Turn off all lights even of monitor and red green target lights
• Use both of two LED IR light sources
• I prefer to remove epithelium before registration for quick continuing
of the surgery.
Toric pIOLs & IOLs
• Available options:
Toric phakic artisan
Toric Artiflex
Toric ICL
Toric IOLs of multiple brands
Toric supplement IOLs for sulcus
• Cost
• Availability
• Imaginable complications with intra-ocular surgery
• Problems with stability of lens
• Occasionally Difficult pre-op marking
• Sometimes difficult intra-operative alignment
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