INCISIONAL CORNEAL SURGERY ( AK , LRIs ) M . NOURI FESHARAKI MD
In modern phaco surgeons no longer seek to avoid inducing ast. but rather must address to reduce significant pre-existing cylinder. Patients have now come to judge the success of their surgery largely by their refractive result.
Several different methods may be employed to achieve this goal . 1- Placing the incision upon the steep meridian 2- Varying size and design of incision . 3- Relaxing incisions . 4- Toric intraocular lens . 5- Bioptics approach (excimer laser or other keratorefractive modality).
In conjunction with cataract surgery Incisional procedures can be used to reduce or eliminate astigmatism.( > 0.75 D ) The most common incisional surgery for the correction of astigmatism associated with cataract surgery is the limbal relaxing incisions (LRIs). LRIs for the correction of mixed ast. can be very safe , inexpensive , useful technique for surgeons experienced in incisional surgical techniques.
Arcute incisions and transverse incisions are both placed in the steep meridian of the cornea. Both techniques flatten the steep meridian of the cornea and cause steepening of the opposite meridian (coupling). Although the coupling ratio can be quite variable,in general the meridian of the incision will flatten approximately 2 to 3 times as much as the opposite meridian will steepen.
Pre - op Evaluation • Topography –keratometry to detect steep meridian • Evaluation of tear film (no incisional keratotomy in moderate to sever dry eye • Evaluation of fellow eye • R/0 keratoconus
Arcuate Keratotomy • %95 depth in 7 mm optical zone • Depend to age –length – number of incision • 2/3 effect with first symmetric incision • Lesser effect in young patient ( < 30 y decreased 0.2 D/year • Coupling effect .
30 -90 degree AK CR =1 stable SE no change IOL power . AK<20 CR >1 hyperopic shift AK>90 CR < 1 myopic shift.
At study in eyes undergoing phaco , evaluated the efficacy of paired intraoperative arcuate keratotomy combine with 3.5 mm limbal steep axis clear corneal incision. Arcute incisions were performed in the steep axis at 7 mm optical zone based on lindstrom nomogram ( after 8 weeks). Arcute group 2.281.02 D (1.26 D) S Arcute group 2.041.55 D (0.48 D)
Limbal Relaxing Incisions The notion of Astigmatic relaxing incisions in cataract procedures : Osher mid .1980. For many years corneal incisions done at 7 mm optical zone . Although effective , causes irregular ast. More recently , a number of authors have recommended moving these incisions out to the peripheral cornea .Indeed these LRI are proving to be a less demanding and more forgiving approach to Ast.
LRIs advantages -Less likely to cause a shift in the resultant cylinder axis (deminished need to centration). - Technically , they are easier to perform . - Patients generally report less discomfort. - Coupling ratio ,(1:1 ratio) negligible change in spheroequivalent (no need to adjust the IOL power) - Peripheral incisions are less powerful but are still capable of correcting up to 3-4 diopters. For these reasons more refractive surgeons now prefer LRIs for all patients.
Incision Strategy Because of decrease incision sizes under 3.5 mm in single plane or beveled fashion causes astigmatically neutral incision. Most authors would agree that mild residual with-the-rule ast. is desirable since most patients will drift against-the-rule over their lifetime, and such residual ast. may enlarge the conoid of sturm , increasing the depth of focus. Keratometry tends to provide an accurate determination of Ast. axis , topography also can be helpful.
LRIs Surgical technique - At the onset of surgery . - One exception (against-the-rule Ast.) - Most surgeons placing an orientation mark at 6 or 12 o clock limbus in an upright position . - Furthermore , it is helpful to identify the steep maridian intraoperatively by using keratoscopy. - The LRIs are placed at the most peripheral extent of clear corneal tissue , just inside the true surgical limbus irrespective of the pannus - Empiric blade depth setting :600 µm.
Trapezoid shape blade • Back –cutting better in LRIs • Front – cutting better in AK
Post – PkArcuate Keratotomy AK in graft –host interface or in graft 45 - 90 degree , > 6 mm o . Z Evaluation by lindstromnomogram and keratoscopy In high astig , Have high response Pre –op 10 D astigmatism decreased to 3 .3 post –op Misallignment 15 degree % 50 decreased effect 15-30 degree shift of axis and no effec > 30 degree increased astig
Complications Infection - Weakening of the globe - Perforation (BCL –Suture ) - Decrease corneal sensation – Dry eye - Induced irregular astigmatism - Misalignment/ axis shift - Glare - Wound gape and discomfort(in long –horizontal ) Central – corneal epitheliopathy (more in horizontal meridian )