Sunday, October 02, 2005

Chapter 3 - ophthalmic viscoelastic devices (OVD)

There are two basic categories of ophthalmic viscoelastic devices (OVD): [view video]


[click on image for larger view]

Different jobs demand different OVDs

1. Maintain space:

e.g. AC during rhexis
bag during IOL insertion

cohesive best

2. Create space:

e.g. Creating sulcus
shift lens material

cohesive best

3. Sealing off:

e.g. Sealing capsular tear
keeping iris away

dispersive best

4. Coating:

e.g. Protect corneal endothelium
lubricate cornea [view video]

dispersive best

Removal of OVD

Removal

  • Dispersive harder to remove
    • Short molecules don't stick together during I/A
    • but short molecules create less post op IOP spike
  • Cohesive is easier to remove
    • Longer molecules stick together during I/A
    • Longer molecules block the trabecular meshwork for big IOP spike

Adaptive OVD

  • Properties of dispersive OVD at high shear rate (e.g. during phaco)

  • Properties of cohesive OVD at low shear rate (e.g. during IOL placement)

  • Very long fragile chain molecules that break with flow rate

  • One product on market -- Healon 5

  • Form anterior chamber, which will cause intraocular pressure problems if not removed.


Arshinoff Shell


  • Phase I during CCC

    • First place dispersive OVD
    • Then place cohesive OVD just over lens
    • Then dispersive is pushed up to coat endothelium
    • As soon as phaco starts cohesive is aspirated and dispersive coating remains

  • Phase II during IOL insertion

    • First place cohesive OVD in the bag
    • Then place dispersive OVD just inside wound to seal prior to IOL placement
    • When IOL is inserted, dispersive helps to keep cohesive in place; bag formed

Properties of Cohesive & Dispersive OVDs During Key Steps of Phacoemulsification


Step

Cohesive

Dispersive

CCC

Easy to fill AC
Can suddenly lose OVD through wound

Must completely fill AC
Stays in AC

Phaco

Goes away with first vacuum

Stays on endothelium
Particles can stick to endothelium
Increased risk of burn

IOL insertion

Easy to open/maintain bag
Easy to remove material

Hard to remove residual material



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