Chapter 9 Intraocular Lenses

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Chapter 9Intraocular Lenses
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Aphakic IOL Implantation
Page 9.1

• Older IOLs inflexible (e.g. PMMA), so larger
  incision was required
• Larger incision often led to significant (and
  variable) post-surgical astigmatism
• Newer designs are foldable allowing much smaller
  incisions
• Most cataract extractions today are extracapsular
  (capsule remains intact) allows easy insertion
  of foldable implants

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Early Anterior Chamber IOLs (1960s)
Fig 9.1 Page 9.1

• Intracapsular cataract extraction
• PMMA Iris Clip Lens placed in anterior chamber
• Many iris-related problems iritis, pupil
  distortion, corneal endothelial cell loss

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Posterior Chamber IOLs (1977)
Fig 9.2 Page 9.2

• Extracapsular cataract extraction (capsule
  remains intact) now in vogue
• Allowed posterior chamber implantation, initially
  in ciliary sulcus
• Capsular bag soon took over as implant site of
  choice because of problems with ciliary sulcus
  implants (e.g. pigmentary glaucoma)

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Fig 9.3 Page 9.2
Ciliary Sulcus
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Fig 9.4 Page 9.3
Posterior Chamber IOL in Ciliary Sulcus
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Sutured Haptic tied off and knot buried in
conjunctiva
Fig 9.5 Page 9.3
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Fig 9.6 Page 9.4
Capsular Bag Implant
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Fig 9.7 Page 9.4
IOL inside Capsular Bag
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Fig 9.8 Page 9.4
Newer Capsular Bag Lenses
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Phakic IOLs
Page 9.5

• Emergence of phakic IOLs in mid-1980s, as
  biocompatible foldable materials became available
• Phakic IOLs exclude the ciliary sulcus and
  capsular bag as implant sites
• AC IOLs therefore returned
• Had to overcome the previous iris-related
  problems with AC lens
• Advantage over LASIK, PRK etc. ? reversible

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Fig 9.9, Page 9.5

• Iris claw lens (Artisan, 1998)
• Not feasible with AC depth lt 3.2 mm
• This impacted primarily the hyperopic pool
• Unfortunate because hyperopes have lower success
  rate with corneal refractive surgery than myopes
• Complications (e.g. endothelial cell loss, glare,
  etc.) remain but appear to be decreasing with
  newer designs

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Posterior Chamber Phakic IOLs
Fig 9.10 Page 9.6

• Collamer posterior chamber phakic ICL
  (implantable contact lens)
• Implanted between iris and anterior crystalline
  lens
• Contact with anterior lens causes anterior
  subcapsular cataract
• Iris problems also occur
• Best option for hyperopes
• PC location means higher lens power than
  equivalent corneal power change with LASIK

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IOLs and near Vision
Page 9.7

• Multifocal intraocular lenses are the IOL
  equivalent of multifocal contact lenses
• Poor track record until recently

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Fig 9.12 Page 9.8
Array Multifocal Lens

• Alternating distance and intermediate/near zones

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Fig 9.13 Page 9.8
Accommodating IOLs

• Humanoptics aphakic IOL
• Capsular bag-fixated lens
• Four flexible haptics that bend when constricted
  by capsular bag
• Effect ? forward translation of lens
• This increases total ocular power

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Humanoptics Accommodating IOL
unaccommodated
accommodated
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Post-operatively Adjustable IOLs
Page 9.7

• Photosensitive silicone matrix polymerizes with
  UV exposure (a)
• If central region polymerized (b) the chemical
  imbalance causes unpolymerized peripheral matrix
  to diffuse centrally
• (c) result is increased IOL power

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IOL Power Formulae
Page 9.10

• Goal calculate the IOL power required for
  emmetropia
• Early formulae based on two ocular variables
  only axial length and mean corneal power e.g.
  SRK I Formula
• Outcome totally dependent on ultrasonography
  (ax?, or L) and keratometry estimate of total
  corneal power (K mean power)
• Later variant SRK II addressed inaccuracies of
  SRK I at the extremes of axial length

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Intraocular Implant Design

• SRK I Formula

SRK Sanders-Retzlaff-Kraff (developers of
formula)
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Implant Design Example
IOL with A value of 116.5 Patient K _at_ 90
43.75 D K _at_ 180 44.00 D ? mean K 43.875
D Axial length 24.03 mm
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Intraocular Implant Design

• The SRK II Formula allows for errors at the
  extremes of axial length with SRK I
• Makes adjustments to IOL type constant, A

A1 A 3 axial lengths lt 20 mm A1 A 2 axial
lengths between 20 21 mm A1 A 1 axial
lengths between 21 22 mm A1 A axial lengths
between 22 24.5 mm A1 A ? 0.5 axial lengths gt
24.5 mm
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Short Axial Length Example
Same IOL design with A value of 116.5 Patient
K _at_ 90 47.25 D K _at_ 180 48.75 D ? mean K
48.00 D Axial length 20.57 mm ? A1 A 2
(20-21 mm range)
using A ? 21.88 D
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Page 9.10
Limitation of all 2-Variable Formulae

• No allowance for anterior chamber depth
• Example three patients, all with mean K (corneal
  power) 43.05 D and axial length 24.17 mm (?
  standard emmetropic eye)
• Patient 1 AC depth 2.8 mm
• Patient 2 AC depth 3.6 mm
• Patient 3 AC depth 4.4 mm
• Outcome of SRK I formula (II not needed for
  standard axial length) for capsular bag implant
• Patient 2 emmetropic
• Patient 1 (shorter AC depth) is now myopic can
  see OK to read, but distance blurred
• Patient 3 (longer AC depth) is now hyperopic
  cannot see to read cannot see at distance (no
  accommodation)

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Three-Variable Formulae (new variable AC depth)

• SRK/T formula adds an iris location variable, ?
  allow for AC depth
• Effect of IOL location?

Page 9.10

• As AC depth increases, IOL power should increase
• Likewise, IOL location (AC vs. ciliary sulcus vs.
  capsular bag) affects required power
• AC implant longest dcornea ? IOL location ?
  lowest power
• Ciliary sulcus shorter d ? higher power
• Capsular bag another ?0.5 mm shorter again ?
  higher power again

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Example of 3-Variable Formula, allowing for AC
Depth
Fig 9.16, Page 9.11
nIOL