Biometry for Cataract


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  • The ultrasound probe placed directly on the corneal surface. Quick & easy Variable corneal compression Operator dependant Common reason for IOL power error Anterior lens capsule, retina spikes high and steeply rising The scleral echo should easily be identified 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
  • 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. 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.
  • The immersion technique prevents corneal compression and the two-dimensional B-scan display helps guide the superimposed vector A-scan for measurements directly to the fovea. Horizontal axial B-scan taken. Center the cornea and lens echoes in the echogram while simultaneously displaying the optic nerve void near the center. The A-scan vector is then adjusted so as to pass through the middle of the cornea as well as the anterior and posterior lens echoes. Such alignment assures that the vector will intersect the retina in the region of the fovea. This technique is particularly important when the macula lies on the sloping wall of the staphyloma. Most posterior staphylomata are located in the peripapillary region, adjacent to, but not centered at, the macula. When the fovea is situated on the sloping wall of the staphyloma, it may only be possible to display a high quality retinal spike when the sound beam is directed eccentric to the fovea, toward the rounded bottom of the staphyloma. This will result in an erroneously long axial length reading. Paradoxically, if the sound beam is correctly aligned with the refractive axis, measuring to the fovea may result in a poor quality retinal spike and inconsistent axial length measurements.
  • Immersion biometry is preferred because the two corneal spikes will aid in beam alignment
  • the patient is seated upright and there is no corneal contact Not only will it do axial length measurements with great precision, but it will also measure the central corneal power by automated keratometry Because the IOLMaster is an optical device, measurements may not be possible in the presence of significant axial opacities, such as a central corneal scar, mature cataract, vitreous hemorrhage, or dense PSC plaque.
  • Use both axial length and K value to predict ELP Effective lens position ELP: Effective Lens position ACD: Anterior chamber depth AL: Axial Length
  • This information effectively works as a pattern-recognition system to estimate the Effective Lens Position ( ELP )
  • If the silicone oil is to remain in the eye for an extended period of time after cataract surgery, an adjustment to IOL power must be made. Holladay and others have recommended that these patients receive a PMMA convex-plano lens, with the plano side oriented so it is facing towards the vitreous cavity, preferably over an intact posterior capsule. This approach prevents the silicone oil from altering the refractive power of the posterior surface of the IOL.
  • Double K method. Both pre lasik and post lasik k is used
  • The corneal diameter is less than 11.00 mm.
  • A second person should repeat the axial length measurements, keratometry readings, and re-run the IOL power calculations for both eyes if:
  • The gain setting controls the width of the sound beam and the overall sensitivity in detecting peaks.
  • Biometry for Cataract

    1. 1. Biometry Dr. Devin Prabhakar MS, MNAMS,FRCS Divya Prabha Eye Hospital Trivandrum
    2. 2. <ul><li>Axial length 1 mm error = 3 D </li></ul>
    3. 3. A-scan techniques <ul><li>Applanation A Scan </li></ul><ul><li>Immersion A- Scan </li></ul><ul><li>Immersion Vector A/B Scan </li></ul><ul><li>IOL Master </li></ul>
    4. 4. . a: Initial spike (probe tip and cornea) b: Anterior lens capsule c: Posterior lens capsule d: Retina e: Sclera f: Orbital fat Applanation A Scan
    5. 5. Immersion
    6. 6. Immersion vector A / B-scan
    7. 7. Looking For Post. Staphyloma. <ul><li>Suggestive Signs </li></ul><ul><ul><li>a long axial length </li></ul></ul><ul><ul><ul><li>< 26.5 mm Uncommon, </li></ul></ul></ul><ul><ul><ul><li>> 33.5 mm 70% of eyes </li></ul></ul></ul><ul><ul><li>inconsistent axial length readings </li></ul></ul><ul><ul><li>indistinct retinal spike </li></ul></ul>
    8. 8. Localization of the macula <ul><ul><li>The technique is as follows: </li></ul></ul><ul><ul><li>Identify the optic disc . Center the probe on the cornea with the sound beam aimed slightly nasally, toward the optic disc. </li></ul></ul><ul><ul><li>Shift the sound beam slightly temporally to locate the macula. When the scleral and orbital spikes appear behind the retinal spike, the beam is aligned on the macula. </li></ul></ul>
    9. 9. <ul><li>Uses a partially coherent light source </li></ul><ul><li>Advantages </li></ul><ul><ul><li>Can use with specs to ensure adequate fixation. </li></ul></ul><ul><ul><li>high degree of precision, including extreme refr. errors, aphakes, pseudophakes, silicone oil </li></ul></ul><ul><ul><li>&quot; all-in-one &quot; IOL power calculation device . </li></ul></ul><ul><li>Optical device- measurements may not be possible in axial opacities </li></ul><ul><li>Haigis-L formula for post myopic refr. surgery eyes calculation is performed with currently measured values </li></ul>The IOL Master
    10. 10. SRK Regression Formula <ul><li>SRK I </li></ul><ul><li>P = A - 0.9 K - 2.5 L </li></ul><ul><li>P : IOL power for emmetropia K : corneal refractive power (K-reading) L : axial length A : A-constant </li></ul><ul><li>SRK II </li></ul><ul><li>P = A1 - 0.9 K - 2.5 L </li></ul>A I Axial Length A + 3 <20 A + 2 20-21 A + 1 21-22 A 22-24.50 A - 0.5 >24.5
    11. 11. Gen III 2 variable formulas <ul><li>The third-generation formulas do not account for effective lens position </li></ul><ul><li>The assumption that the ACD was a proportion of the AL and not a true measurement can cause IOL surprises with post-refractive patients. </li></ul>>26 mm SRK/T 24-26 mm Holladay I <22 mm Hoffer Q
    12. 12. How SRK II Varies With Gen III Formulas AL/ K 40 41 42 43 44 45 46 47 30 0 -.5 -1.0 -1.5 -2.25 -3.0 -4.0 -5.0 29 -.5 -.5 -1.0 -1.75 -2.25 -3.0 -3.5 -4.5 28 -.5 -1.0 -1.0 -1.75 -1.75 -2.5 -3.0 -3.5 27 -.5 -1.0 -1.0 -1.25 -1.75 -2.5 -2.5 -3.0 26 -.5 -.5 -1.0 -1.25 -1.75 -2.0 -2.5 -3.0 25 0 0 0 -.75 -.75 -1.5 -1.5 -2.0 24 .5 .5 .5 -.25 -.25 -1.0 -1.0 -1.5 23.5 1.0 .75 .75 0 0 -.5 -.75 -1.0 23 1.5 1.5 1.0 .75 .25 0 -.5 -1.0 22.5 2.25 1.75 1.75 1.0 1.0 .5 .25 -.25 22 2.5 2.5 2.5 1.75 1.75 1.0 1.0 .5 21.5 3.5 3.25 2.75 2.5 2.0 2.0 1.75 1.0 21 4.0 4.0 3.5 3.25 3.25 2.5 2.5 2.0 20.5 5.0 4.75 4.5 4.0 4.0 4.0 3.25 3.0 20 5.5 5.5 5.5 5.25 5.25 4.5 4.5 4.5
    13. 13. HAIGIS <ul><li>Real measurement of ACD </li></ul><ul><li>The three constants derived from a statistical analysis of postoperative results of 50 or more patients for a given IOL </li></ul><ul><li>With no individualization, the Haigis formula is as good or bad as the other theoretical formulas </li></ul><ul><li> </li></ul>
    14. 14. Holladay II Formula <ul><li>Derived in 1998 </li></ul><ul><li>Seven different variables </li></ul><ul><ul><ul><li>white to white ACD </li></ul></ul></ul><ul><ul><ul><li>lens thickness patient's age </li></ul></ul></ul><ul><ul><ul><li>preop Rx axial length </li></ul></ul></ul><ul><ul><ul><li>corneal diameter </li></ul></ul></ul>
    15. 15. Silicone Oil <ul><li>Additional IOL power (D) </li></ul><ul><li>= ((Ns - Nv) / (AL - ACD)) x 1,000 Ns = refractive index of silicone oil (1.4034). Nv = refractive index of vitreous (1.336). AL = axial length in mm. ACD = anterior chamber depth in mm. </li></ul><ul><li>Easy Calculation when vitreous cavity filled with silicone oil </li></ul><ul><li>For an eye of average dimensions </li></ul><ul><li>Convex-plano PMMA IOL </li></ul><ul><li>Additional power needed is +3.5 D </li></ul><ul><li>With IOL Master </li></ul><ul><li>relatively easy: Under the menu heading, Axial Length setting select Silicone Filled </li></ul>
    16. 16. IOL Calculation after LASIK
    17. 17. Bag Vs. Sulcus Power -1.50 +28.50 to + 30.00 -1.00 +17.50 to + 28.00 -0.50 + 9.50 to + 17.00 Correction for sulcus placement IOL Power for in bag insertion 0.00 + 5.00 to + 9.00
    18. 18. Scleral Buckle <ul><li>Scleral buckle increases axial length by 0.75 to 1.25 mm without changing the ACD </li></ul><ul><li>IOL power calculated has to be reduced by 0.50 if the IOL power is > +14.00 </li></ul>
    19. 19. Validation Guidelines A Scan <ul><li>Axial length < 21mm or > 26 mm </li></ul><ul><li>Difference in axial Lenghts > 0.3 mm </li></ul><ul><li>AL corresponds poorly with clinical data </li></ul><ul><li>IOL power b/w eyes > 1.00 D </li></ul><ul><li>Difficulty in obtaining measurements </li></ul>
    20. 20. Validation Guidelines- K <ul><li>The corneal power is < 40 or > 47 </li></ul><ul><li>If there has been prior keratorefractive surgery. </li></ul><ul><li>The average corneal power difference between the two eyes is > 1.00 diopter . </li></ul><ul><li>The amount of corneal astigmatism by keratometry correlates poorly with the most recent manifest refraction. </li></ul>
    21. 21. Validation Guidelines- IOL Calc <ul><li>• The IOL power for emmetropia is greater than 1.00 diopters different than anticipated </li></ul><ul><li>• There is a difference in IOL power > 1.00 D between the two eyes </li></ul><ul><li>• If the patient has had prior keratorefractive surgery and the calculated IOL power is not +20.0 D to +23.0 D . </li></ul>
    22. 22. Tips <ul><li>Patch the other eye for better fixation </li></ul><ul><li>Scan both eyes </li></ul><ul><li>Gain Setting </li></ul><ul><ul><li>too high , too much noise may result in a short reading </li></ul></ul><ul><ul><li>too low , the scleral spike may be mistaken for the retinal spike, leading to long readings.   </li></ul></ul>
    23. 23. Advances in IOL technology makes accurate biometry more important than ever before. In order to make the leap into refractive cataract surgery and lens exchange optimization, adoption of third-generation formulas is necessary, and use of fourth-generation formulas is preferable. The time spent optimizing your formula of choice and i mproved results with immersion biometry more than repays your effort to learn it.
    24. 24. Thank You <ul><li>[email_address] </li></ul><ul><li> </li></ul>