Tuesday, September 27, 2005

Chapter 2 - Selecting the Intraocular Lens


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


-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


IOL design considerations


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


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.

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