Astigmatism correction methods Alireza Peyman MD http www

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