Tuesday, September 27, 2005

Chapter 2 - Selecting the Intraocular Lens

SELECTING THE INTRAOCULAR LENS (IOL)

Brief History of the IOL

-Harold Ridley placed first lens in 1949, a huge polymethyl-methacrylate (PMMA) Intraocular Lens (IOL) (about the size of the cataract)

-1950s rigid anterior chamber lenses were used with ECCE and ICCE
  • bullous keratopathy was common

  • chronic inflammation led to cystoid macular edema (CME) and glaucoma

-Later iris fixation lenses were used to avoid contact with the angle
  • Some IOLs would suture onto the iris

  • Others would clip on (used today as the Artisan lens)

  • These lenses would frequently dislocate

-Closed loop flexible anterior chamber lens were next and kept PK corneal transplant surgeons in business

-Rare and weird names for IOL: pseudophakos, lenticulus

Today

-Modern open loop flexible anterior chamber lenses are a great success

-The development of viscoelastics (OVDs) allows safe placement
  • Posterior chamber lenses are most commonly used today

  • 3 basic materials – PMMA, acrylic, silicone

  • PMMA is the time tested material but requires a large incision

  • Use the largest optic that can fit incision, e.g., 6.5 or 7 for ECCE

  • Most surgeons use foldable acrylic or silicon lens to allow small incision

-Accomodating IOL has been approved by the FDA

-Multifocal (Restore™, Rezoom™, Array™) and toric lenses (Staar™) are available

IOL material considerations

[CLICK ON TABLE FOR LARGER VIEW]

IOL design considerations


[CLICK ON TABLE FOR LARGER VIEW]


Four things you need to know to calculate correct IOL power:

1) Desired postoperative SE

-Usually -0.50 to -1.00 diopters (D) is the plan. Why?
  • Myopia is better than hyperopia if your calculations are off

  • -1.00 D gets you about 20/40 at far and you can see well at mid distance

  • A spectacle overcorrection of -1.00 will eliminate induced IOL magnification.

  • Sometimes however you may not want a spherical equivalent (SE) of -0.50 to 1.00 D

-the fellow eye has a significant refractive error and must be matched
  • anisometropia > 3.0 D is not well tolerated

-you are confident and want to go closer to plano – good luck

-patients wants this to be a reading eye with goal of -2.00 D or so

2) Axial eye length (AEL)

-Contact probe ultrasound AEL device

  • Contact probe on eye measures distance to fovea

  • Pushing on the eye with probe introduces error (AEL too short)

  • Re-measure when AEL difference between eyes >0.3mm

  • Re-measure when AEL <22 or >25

-OCT (IOL Master)
  • Measure both eyes

  • Less dependant on technician for accuracy

  • Fails in dense nuclear sclerosis (NS) or even mild posterior sub-capsular cataract (PSCP)

-Immersion ultrasound
  • Gold standard when in doubt

  • Significant technician skill required

  • Should get whenever patient is getting B scan anyway for dense cataracts

3) The power of the cornea

-Keratometric measurement of both eyes -- should be about the same
  • Autorefractor

  • IOL Master measures K's for you

  • Keratometer/corneal topography

-Difficult when patient has had refractive surgery

4) The post operative position of the IOL

-The more anterior the IOL the less power the IOL needs

-Goal is to place a posterior chamber (PC) lens
  • These can end up in the bag (best) or sulcus (anterior to ant. capsule)

  • Placement in the sulcus creates about a 0.75 D myopic shift in glasses

-Always plan to have available anterior chamber (AC) lenses
  • These are placed anterior to the iris with haptics that settle into the angle

  • These are used when the capsule is lost and cannot hold an IOL

  • When too small they can tilt and when too large they can hurt

Estimating the IOL power for emmetropia:

-Formulas started with a theoretical model by Fydorov, Collenbrander et all, 1970s

-Based on geometric optics

-Power = N/(AEL-ACD) – N/(N/K-ACD)
  • where: Power is the expected power of IOL for emmetropia post op

  • N is the aqueous and vitreous refractive index

  • ACD is the post operative AC depth of the IOL

  • AEL is the axial eye length as measured via an ultrasound device

  • K is average of the two keratometric axes

-But you don't know ACD or post operative depth of the IOL pre op!

Useful Formulas use regression analysis or other tricks to estimate ACD

[CLICK ON TABLE FOR LARGER VIEW]

Selecting the IOL power for your patient

-The SRK computes the lens power for emmetropia; but, you probably want -0.50 to -1.00 D

-The ultrasound or IOL Master produces a table with IOL power mapped to desired post op. SE
  • Roughly a change in IOL power 1.5 results in spherical equivalent (SE) change of 1.0 D

  • For instance, formula gives 19 D for emmetropia, about 20.5 D will give -1.00 D SE post op.

-If your estimated IOL power is unusual you are probably wrong
  • Double check your calculations

  • Trust what happened with the other eye's IOL if applicable

  • Ask yourself was the patient very hyperopic as a young person (e.g., in the big war)

-Estimate the power for both the AC and the PC lens and compare several formulas

-Then if convinced that the calculations are right, make sure the IOL powers are available

-Be sure that all lenses possibly needed for your case are in OR
  • You will need a posterior chamber lens for the bag

  • A Sulcus Lens: the more anterior Sulcus lens will need 0.5-1.0 less diopters of power and cannot use single piece acrylic (Alcon SA60 lens) in sulcus.

  • AC lenses: typical AC lenses come in 3 diameters: 12.5, 13.0, and 13.5 mm and sized at surgery by adding 1 mm to the “white to white” horizontal limbal diameter.

[Previous] [Next]

0 Comments:

Post a Comment

Links to this post:

Create a Link

<< Home