Occular Implants

1
Occular Implants

• Anatomy
• Vision correction
• Corneal implants

http//www.keratoconusinserts.com/
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The Anatomy of the Eye

• Aqueous Humor- fluid component-pressurization
• Retina-receives light rays and sends signals to
  brain
• Schlera-tough wall of the eye
• Lens-located behind iris and helps to focus
  light.
• Cornea- vital part of focusing through curvature

http//cvm.msu.edu/courses/AP/cataract/vocab/anat2
.htm
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Structure of the Cornea

• Epithelium - Blocks passage of foreign material.
  Absorbs oxygen and cell nutrients from tears.
• Bowman's Layer -Fibrous collagen
• Stroma - Comprised of water (78) and collagen
  fibres. Nutrient transport.
• Endothelium -Primary task is to pump this excess
  fluid out of the stroma.
• Properties of Cornea
• Nonlinear
• Viscoelastic
• Anisotropic

http//cvm.msu.edu/courses/AP/cataract/vocab/anat2
.htm
4
Corneal Disorders

• About 120 million people in the U.S. wear
  eyeglasses or contact lenses to correct
  nearsightedness, farsightedness or astigmatism.
• Refractive errors occur when the curve of the
  cornea is irregularly shaped (too steep or too
  flat). When the cornea is of normal shape and
  curvature, it bends, or refracts, light on the
  retina with precision.

http//www.nei.nih.gov/health/cornealdisease/
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Keratoconus

• "Keratoconus is a condition of the eye. The name
  is based on the Greek word "kerato," for cornea,
  and "conus," for cone.
• Keratoconus causes the cornea to become cone
  shaped and thinner, disrupting the visual
  function. Clinical treatments include Intacs and
  corneal implants.

• What Is Keratoconus?
• Keratoconus is a progressive non-inflammatory
  disorder that causes a characteristic thinning
  and cone-like steepening of the cornea. This
  steepening results in distortion of vision,
  increased sensitivity to glare and light and an
  associated reduction in visual acuity. These
  symptoms usually appear in the late teens and
  early twenties. Keratoconus may progress for
  10-20 years and then can slow or even stabilize.
  Each eye can be affected differently. This can
  result in a dramatic decrease in the ability to
  see clearly even with corrective lenses.

Caused by herniation or weakening of the cornea
http//www.keratoconusinserts.com/
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Clinical treatment

• Cornea acts like a camera lens to transmit and
  focus images on the retina.
• With age or disease, the eye muscles weaken and
  causes the shape of the cornea to alter.
• Semi-stiff rings can be used correct curvature--.
  
• PMMA is a candidate material. Good optical,
  mechanical and biocompatibility properties.

Intacs reshape the curvature of the cornea from
within, enhancing the natural shape of the eye to
correct mild nearsightedness. Because no tissue
is removed, natural optics are enhanced and adds
to the structural integrity of the cornea.
http//health.ucsd.edu/specialties/eyes/visioncorr
ection/intacs.htm
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Device Mechanism and Limitations

• Flattens central part of cornea
• Moves focal point posteriorly-- only corrects
  nearsighted vision
• Foreign Body response limits life of implants

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Corneal Replacement

• Disease and progressive degradation of the cornea
  is common cause of blindness
• Cornea becomes cloudy. Affects optical
  refraction. Light cannot penetrate the eye to
  reach the retina
• Keratoprostheses (KP) or artificial corneas can
  correct vision. A corneal transplant involves the
  removal of the central portion of the diseased
  cornea and replacing it with a donor cornea.

http//www.opt.indiana.edu/lowther/html/keratoconu
s_transplant.htm
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Mechanical Requirements

• Mechanical strength to maintain the
  structure/shape of the globe (pressure)
• Resistance to tearing
• Elasticity to accommodate the movement of the eye
  and provide a mechanism for energy absorbency
• Viscoelasticity to allow changes in eye curvature
  over time
• Lubricity

10
Mechanical Behavior of Cornea

• Intraocular pressure is often modeled by
  bi-axial loading
• Tensile strength of human cornea is 4 MPa

www.cornea.org
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Biomaterials for corneal reconstruction
PolyHEMA Poly (2-hydroxyethyl methacrylate)

• Thermo-hydrogel
• High water content
• Oxygen permeability
• Stable in both alkaline and acidic environments
• Crosslinking affects its ability to swell in H2O
• Used in contact lenses, drug delivery

HEMA monomer
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Mechanical Properties of pHEMA

• pHEMA under uniaxial loading
• E2.43-3.79 MPa for strain
• E1.68-1.87 MPa for larger strains
• E 4.5 MPa (human cornea)
• Strength is 0.53-0.71 MPa
• 3.81 MPa (human cornea)
• Elongation at failure for pHEMA is 32.

Chirala, T.V., Biomaterials, 22, 3311 (2001)
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Poly HEMA Devices

• Pore size important for biocompatability
• calcium deposition and tissue in-growth
• Porous pHEMA
• pores 5-80 ?m
• 45 water by weight
• Optically transparent
• monomeric solution
• less than 30 water by weight

Chirala, T.V., Biomaterials, 22, 3311 (2001)
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Keratoplasty Implantation of Device

• Cornea button must have significant amount of
  remaining host tissue to tightly fit the device
• Attach the implant with 12-16 sutures (resorbable)

http//www.opt.indiana.edu/lowther/html /keratocon
us_transplant.htm
15
Biocompatability

• Porous sponge allows tissue ingrowth of skirt and
  normal wound healing-- decrease chances of tissue
  necrosis, implant extrusion.
• Problems include calcium deposition which leads
  to brittle behavior, shelf life and stability,
  prevention of cellular growth due to immune
  cells, and neovascularization

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Improving Biocompatability

• Both the posterior and anterior
  surface of the artificial cornea must stimulate
  migration and attachment of corneal cells
• Thin glycoprotein coating stimulates epithelial
  cell growth

http//www.opt.indiana.edu/lowther/html/keratoconu
s_transplant.htm
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Facial Implants
http//www.plasticsurgery.org/public_education/ pr
ocedures/FacialImplants.cfm
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RhinoplastyHistorical perspective

• Indian culture nose cut off as a form of
  punishment
• Indian tales of city without a nose
• Need to find ways to replace tissue-- the nose
  was taken as a sign of respect

cpmcnet.columbia.edu/dept/ dental/mpc/nasalpt.html

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Rhinoplasty

• Detailed in 600 BC by Sushruta
• Dissect skin from patients cheek and turn it
  back to cover the nose
• Insert two tubes of castor-oil plant to keep
  nostrils open and nose elevated
• Sprinkle powder (made of licorice, red
  sandal-wood, barberry plant) and covered it with
  cotton and apply sesame oil constantly

http//www.bbc.co.uk/education/medicine/nonint/pre
hist/dt/prdtcs4.shtml
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European Procedure

• Use flap from forehead, arm - Voltaire wrote a
  satirical poem about using flap from the buttocks
  
• Started seeing use of synthetic material
• Substitutes made out of gold, silver, or ivory
• Tycho Brahe had a nose made out of gold, silver,
  and wax after having losing his nose over a duel
  involving who was the better mathematician

http//www.fencingsucks.com/gallery/images/duel.gi
f
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Modern Implants

• Autologous own soft tissues cartilage (from
  ear, rib, septum), bone grafts.
• Injectable collagen made out of cow collagen.
• Synthetic silicone and polymers (e-PTFE, HDPE)

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Functional Requirements

• Provide an aesthetic shape of nose
• Withstand the loadings due to impact, sneezing
• Maintain open air passageways for breathing
• Compliance match

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Mechanical Requirements

• Impact with object
• Cyclic loading
• Compressive, tensile, bending, and shear forces
• Contact loading, interface wear

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Corresponding Failure Modes

• Fracture
• Wear
• Fatigue
• Yielding
• Loosening

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Problems with Silicone Rubber

• Migration
• Extrusion
• Encapsulation by scar tissue
• No tissue ingrowth
• Bone resorption beneath implant area
• Silicone has a poor reputation resulting from
  silicone breast implant lawsuits

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Alternate Synthetics

• Expanded poly-tetra-flouro-ethylene (ePTFE)
• Porous High Density Polyethylene (PHDPE)
• Coralline Hydroxyapatite
• Injectable Products

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E-PTFE

• Expanded PTFE (GoreTex)-porous ingrowth
• Subcutaneous Augmentation Material (S.A.M.)
• Sheets Carved and shaped by surgeon to fit the
  patient
• Pre-formed parts (nose, chin, etc)

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ePTFE Comparison

• ADVANTAGES OVER SILICONE
• Porous - Tissue Ingrowth
• Less Resorption
• Biocompatible
• MAJOR DISADVANTAGE
• Less rigid than silicone less form

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Facial Augmentation Trends

• In 2002, the nose job was the most common
  cosmetic procedure 354,327
• American Society of Plastic Surgeons Expects sees
  association of numbers of cosmetic procedures
  with national economy and financial security
• Surgery as an investment steady demand