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Intra Ocular Lens (IOL) Design ,Material and Types

Type of IOL

IOL

An Intra-ocular lens (IOL) is an artificial aid surgically implanted in the living eye to restore sharpness of vision, frequently after removal of a cataract.

History of IOL  

  • Casaamata – (1795)- attempted Intraocular implant 
  • Harold Ridley (1949) -- first successful human intraocular lens implant
  • During World War II, noticed intraocular foreign bodies of acrylic fragments from airplane canopies were tolerated. 
  • Performed on November 29, 1949, at St. Thomas Hospital in London. 
  • High incidence of complication - discontinued his original design
  • Pioneering work of Cornelius Binkhorst in Holland and E. Epstein in South Africa – interest in this modality rekindled.
  • Charles Kelman - Phacoemulsification - early 1970.
  • A major turning point of IOLs acceptance - 1974. 
  • In 1976, Steven Shearing - first compressible posterior chamber lens

Pseudophakic vs. Aphakic vision

IOL eye - optically similar to the phakic condition, Compared to aphakic with spectacle correction, pseudophakic patient has
  • Enlarged visual field,
  • Less magnification,
  • Better stereopsis,
  • Rapid visual  rehabilitation

Types of intraocular lens

  1. Anterior chamber IOL, 
  2. Iris-fixated IOLs,
  3. Posterior chamber IOLs, 
  4. Sulcus-fixated IOLs.

Ideal implant material

Properties
  • High optical quality
  • High index of refraction
  • Lightweight
  • Durability
  • Ease of  manufacture
  • Lack of inflammatory reaction
  • Lack of antigenicity
  • Lack of carcinogenicity
  • Ease of sterilization

OPTIC Materials

1. PMMA  (polymethyl methacrylate)
  • Can be used to create multi-focal lenses
  • Surface modification - reduce the immediate postoperative inflammation in high-risk eyes.
  • Graft Hydrogel to its surface
  • Heparin coating
  • Fluorocarbon or Teflon coating of the lens.
  • Surface passivation
Drawback
  • PMMA is not totally inert
  • Hard, rigid material
  • Monomer - toxic to  eye
  • Nd: YAG laser has potential to damage  PMMA optic.
  • Hydrophobic - corneal endothelial cells adhere to its surface.
  • Larger incision required

 PMMA IOL

2. New Materials:

Avoiding problems associated with rigid lenses - use softer polymeric materials for lenses.
  • Silicone
  • Hydrogels
  • Foldable Acrylics

Foldable IOL Advantage

Cellular response:
  • Less than PMMA lenses.
Optical quality:
  • Silicone IOL no finishing or polishing is required.
  • Have transmissions of >90% in 400-800nm range.
  • Excellent surface properties that reduce reflection and glare.
Bio-stability and chemical purity:
  • Resistant to hydrolytic and oxidative degradation.
  • Harmful chemicals fully extracted during the making.
  • Low adhesion - low pigmentary and inflammatory deposits.
Sterilization and laser effects:
  • Autoclaved safely avoiding harmful gases or chemicals.
  • Exposure of silicone IOL to LASER doesn’t release any harmful chemicals.

Foldable IOL disadvantage:

Biocompatibility of materials other than silicones:
  • Silicone has been proved to be biocompatible.Mechanical strength:
Tensile strength considerably lower than that of PMMA 
  • care must be taken while handling soft IOL.

optic and haptic 

Loop and Haptic materials

Broadly divided into
  • One piece with PMMA haptic or soft biomaterials
Multi-piece consisting of an optic supported by one or more loops (haptics).
  • Polymide
  • Prolene
  • Mersilene

IOL Suitable for implantation 

Quality Control
  • Must be within 0.25D of stated power
  • Proper shape and configuration within 0.25mm
  • Uniform, smooth surfaces, and edges
  • Chemically pure without any residual monomer, ethylene oxide, or contaminants
  • Clean of surface debris; and be sterile.
Sterilization 
For PMMA: 
  • Ethylene oxide most commonly use
  • Alternative method Sodium hydroxide
Autoclave for Acrylic

Design characteristics, optic size, and shape

  • IOL optic size ranges from 4.5-7.5mm
  • Optic size of 6.5-7.0mm is popular among surgeons performing planned ECCE.
  • Lenses with round optic of 5.5-6.0mm are currently favored by many phacoemulsification surgeons.
  • Some surgeons- prefer oval lens  5mm x 6mm optics.
Large optic size IOL
  • Less likelihood of decentration.
  • Less optical aberration.
  • Reduced likelihood - pupillary capture.
Small optic IOL
  • Most popular in phacoemulsification and continuous tear anterior capsulotomy.
  • Minimal displacement in a bag- favored by most phaco surgeons.
  • Potential for optical aberration from lens edges.

Shape of IOL

The biconvex lens seems to be the soundest design.
Advantages:
Simulates natural lens and provides good optical quality.
Capable of retarding opacification from Elschnig pearls.

IOL Lens Design

  • AcrySof® Natural IOL
  • Piggy back IOL
  • Phakic IOL
 piggyback iol
phakic iol
Multifocal IOL design:
  • Toric IOL
  • Accomodative IOL
  • Smart IOL
multifocal iol
smart iolfocal iol 
References:
  • Cataract surgery and complications- Norman F Jaffe (2nd Edition)
  • Duane’s ophthalmology- CD ROM -2005
  • Albert Jakobeic CD ROM (5th Edition)
  • AAO Series- Lens And Cataract (2004-2005)
  • Ophthalmology Monographs- Intraocular Lenses(1993)

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