Study of Phacodepth Required for Safe and Effective Chop During Phacoemulsification for Cataract.

Overview

There is a correlation between phacodepth required for full thickness nuclear crack and the various grades of cataract

BACKGROUND:- Phacochop techniques have been categorized as either horizontal or vertical. These approaches are thought to be among the best techniques for managing hard cataracts. In vertical chop technique proper placement of the phaco tip is crucial. It must be embedded deeply into the centre of the nucleus , pointing toward the optic nerve , with high vacuum and burst mode.Inadequate penetration of phacotip may result in partial thickness nuclear crack with residual posterior plate and overzealous penetration may result in posterior capsular rupture.This may be avoided if some estimate can be made preoperatively of the depth of penetration required to achieve full thickness crack. With this in mind the study intends to make a preoperative assessment of the depth required to reach the centre of the nuclei and observe the actual depth of penetration required for a safe and effective vertical chop. MATHEMATICAL BASIS :- The cross section of a lens nucleus is ellipsoidal . A geometrical estimate of the radius of the ellipse (x )traversed by the phacotip to reach the centre can be made preoperatively.The calculation would depend largely on three parameters. The major axis of the ellipse ( namely the lens diameter\[b\] ) The minor axis of the ellipse (namely lens thickness\[a\] ) The point of entry on the surface of the lens.This defines an angle Ø which this radius makes with the vertical axis and in turn can be approximated by the length of the arc from centre of lens surface to entry point\[y\]. Adult human crystalline lens varies in thickness from 4.2+/-0.5 mm and has a diameter of 9.6+/- 0.4mm. Assuming the lens diameter (b) to be from 9.2 mm to 10.0mm , the lens thickness ( a) varying from 3.7mm to 4.7mm and the point of entry of phacotip 2.0mm to 3.0mm from centre of anterior lens surface ( y ) , this distance (x) would vary from 2.59mm to 3.2mm.(using Online Wolfram calculator) in thinnest to thickest nuclei. This would be increased with increase in ( a ) as well as ( y ) .In cataractous lens the harder a nucleus the deeper it may be required to penetrate to achieve a full thickness crack. If the investigators make a note of all three variables i.e lens thickness , nuclear hardness and point of entry at the time of surgery and correlate with the actual depth required at the time of surgery the investigators may be able to recommend an effective and safe phacodepth for achieving full thickness nuclear crack in various grades of hard cataract. Till now it was not possible to measure the length of penetration of phacotip during surgery. The investigators have designed a calibrated phacotip with visible markings on 0.2mm scale from 2 to 3. 4mm to enable measurements and recordings. Thus the investigators aim to identify a safe and effective phacodepth required in each grade of cataract {3.0 - 6.9 according to Lens Opacities Classification System( LOCS-III )}to achieve successful nuclear crack . At the same time any correlation between different nuclear grades and the effective phacodepth would be analysed to find any association between them.

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Outcomes