High Precision Biometry
- 1. High Precision Biometry Dr Vibha Dr Shekhar
- 2. Several values are required to calculate IOL Power · Accurate Corneal power · Actual axial length · Accurate prediction of estimated lens position (half a mm shift in lens position can have a dramatic effect on final vision) · Desired post op refraction · A good understanding of the various IOL Power calculation formulas is also required . .
- 3. Keratometery Keratometery by—Manual Topography Autokeratometer IOL master
- 6. Source of keratometry errors Unfocused eye piece Failure to calibrate unit Poor patient fixation Dry eye Drooping eye lids Irregular cornea
- 7. Repeat Keratometery If Corneal curvature more than 47D or less than 40D The difference in corneal cylinder is more than one diopter between eyes.
- 8. A-Scan biometery/laser inferometery A-Scan ultrasound by applanation method by immersion method Laser inferometery IOL Master
- 9. A-scan facts 50% of a surgeon post operative surprises are A-scan errors (olsen). Error of 2.0D or more are always A scan related (Holladay). All A-scan unit make mistake in eco interpretation.
- 10. Applanation A-scan Biometry A-scan biometry by applanation requires that the ultrasound probe be placed directly on the corneal surface. This can either be done at the slit lamp, or by holding the ultrasound probe by hand. Even in the most experienced hands, some compression of the cornea is unavoidable; this typically being 0.14 mm - 0.28 mm.
- 11. Applanation A-scan Biometry. a: Initial spike (probe tip and cornea) b: Anterior lens capsule c: Posterior lens capsule d: Retina e: Sclera f: Orbital fat
- 12. Applanation A-scan Biometry When echoes b through d are high and steeply rising, the ultrasound beam is most likely on axis. The scleral echo should easily be identified and the orbital fat echoes should descend quickly and at a steep angle. If there are no scleral or orbital fat echoes visible, the ultrasound beam is most likely aligned with the optic nerve rather than the macula.
- 13. The five basic limitations of applanation A-scan biometry are: 1. Variable corneal compression. 2. Broad sound beam without precise localization 3. Limited resolution. 4. Incorrect assumptions regarding sound velocity. 5. Potential for incorrect measurement distance.
- 14. Immersion A-scan Biometry. . a: Probe tip. Echo from tip of probe, now moved away from the cornea and has become visible. b: Cornea. Double-peaked echo will show both the anterior and posterior surfaces. c: Anterior lens capsule. d: Posterior lens capsule. e: Retina. This echo needs to have sharp 90 degree take-off from the baseline. f: Sclera. g: Orbital fat.
- 15. Immersion A-scan Biometry The immersion technique requires the use of a Prager Scleral Shell .
- 16. Immersion A-scan Biometry When the ultrasound beam is properly aligned with the center of the macula, all five spikes (cornea, anterior and posterior lens capsule, retina and sclera) will be steeply rising and of maximum height.
- 17. NON CONTACT The Zeiss IOLMaster . A non- contact optical device that measures the distance from the corneal vertex to the retinal pigment epithelium by partial coherence interferometry, the IOL Master is consistently accurate to within ±0.02 mm or better.
- 18. Accuracy of axial length by different machine +/- .01mm +/- 0.12mm +/- 0.24mm IOL Master Immersion A-scan Applanation A -scan
- 19. Do not throw away old ultrasound machine Yes Yes Yes No No No No IOL master Difficult Difficult Variable Yes Yes Yes Yes Posterior staphyloma Silicone oil Pseudophakia 4++brunescent lens Central PSC plaque Vitreous hemorrhage Central corneal scar Immersion ultrasound
- 20. IOL FORMULA Ist generation Most are based on regression formula developed by Sander ,Retzlaff & Kraff Known as SRK formula. P--A-2.5(L)-0.9(K) P=lens implant power for emetropia L= Axial length (mm) K=average keratometric reading (diaopters) A= lens constant
- 21. IOL FORMULA 2 nd generation SRK formula – work well for average eyes. less accurate for long, short eyes SRK II formula modification of SRK work on ELP
- 22. IOL FORMULA 3 rd generation Third generation formulas- SRK/T -very long eyes >26mm Holladay -long eyes 24-26 mm hofferQ -Short eyes<22mm
- 23. IOL FORMULA 4 th generation Holladay2 Haigis formula- d = a0 + (a1 * ACD) + (a2 * AL) ACD is the measured anterior chamber depth AL is the axial length of the eye The a0, a1 and a2 constants are set by optimizing a set of surgeon- and IOL-specific outcomes for a wide range of ALs and ACDs.
- 24. SRK/T formula — uses "A-constant" Holladay 1 formula — uses "Surgeon Factor" Holladay 2 formula — uses "Anterior Chamber Depth" Hoffer Q formula — uses "Anterior Chamber Depth"
- 25. When capsular tear does not allow bag placement of the lens change IOL power for sulcus placement >=28.5 D Decrease by 1.5 D +17 To 28 D Decrease by 1.0 D +9 To 17 D Decrease by 0.5 D <+ 9 D No change
- 26. Summary Use IOL master or immersion ultrasound for most accurate axial length measurement Use fourth generation IOL formulas Examine and reevaluate your result periodically
- 27. THANK YOU