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 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• 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 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

پوزش عرض ضمن

باالی حجم در LECTUERبدلیل نمیباشد پذیر امکان اسالیدها ادامهآموزشی مرکز بصری و سمعی واحد به لطفا ادامه به نیاز صورت

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