Astigmatism correction methods Alireza Peyman, MD http://www.drpeyman.ir

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 Incisional methods Soft (toric) RGP orthokeratology Incisional methods Traditional FS assisted full thickness paired incisions Intra-corneal inlays Excimer ablation Toric pIOLs Toric IOLs

Glasses 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 enantiomorphism Impaired proprioception (diabetics in some stages)

Contact lens Always worth try in difficult cases Irreplaceable for irregular astigmatism

Incisional methods AK LRI Induced wound dehiscence Arcuate Straight LRI 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 ICRS Intra-corneal lenses

Excimer ablation Case selection R/O lens problems R/O KC Lens tilt or subluxation Lenticonus R/O KC

Evaluations Inquiry about recent refractive change and FHx of KC are important Check both Placido based topographies and elevations 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.

Measurements 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 measurements 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 (APT) 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 aberrations 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 correction.

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

Drawbacks 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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