Photoelasticity Of The Human Cornea - Buzard.info

Photoelasticity Of The HumanCorneaKurt Buzard MD FACS

• Photography:• Scott Hunt• Machine Tooling:• Craig Cerminara

Morris Klatskin PE• 1945-Adapted Retiniscope WithPolarized Filters To Find Sliver Of SteelIn Own Eye• 1946-Raytheon-Manhattan ProjectPolarizing Filters To Show Stress InPlastic Insulators• 1972-Rand Project To Detect GlaucomaOptically.....Unable To Correlate EffectTo Increased IOP

Polarizing Filters• Normal White Light Is Like A WaveVibrating In All Directions• Polarizing Filters Allow Vibration In OnlyOne Direction• If Polarized Light Is Observed With ASecond "Crossed" Polarizing Filter............ No Light Is Passed

Polarizing Filters

Birefringent Materials• Any Material Which Can Change ThePolarization Of Light• Placed Between Crossed PolarizedFilters, Causes Bands Of Colored Light• Used In Pathology To Identify CertainBiologic Materials: Amyloid In LatticeCorneal Dystrophy

Stressed Hard Plastic

Photoelasticity• Force Applied At A Point On A GlassPlate Generates A Series OfPhotoelastic Stress Lines• Viewed Through Crossed PolarizingFilters• Internal Material Stress Can Be InferredFrom These Pictures

PhotoelasticityPhotoelasticStresLinesApliedForce

Non-Parallel Surfaces

IOL with Stress Defect

Experimental Setup• Cadaver Eye Was Bisected, UvealContents Were Eviscerated• Mounted On Plexiglass Tube Filled WithFluid• Polarized Light Directed ThroughCornea• Observed Through Crossed PolarizedFilter

Experimental Setup

Experimental Setup

Light Microscopy• Low Power Light Microscopy ShowsFibrillar Structure Of Cornea• Unfortunately Little Additional Detail IsRevealed Concerning Collagen Fibrils• Question Exact Length Of IndividualCollagen Fibrils• Question Anchor Points of CollagenFibrils

Corneal Structure

Electron Microscopy (TEM)• The Higher Power Of ElectronMicroscopy Shows More Details• Alternating Direction Of Collagen FibrilLayers• Questions Remain About Length AndAnchoring Of Collagen Fibrils

Corneal Structure (TEM)FibrilsPerpendicularFibrilsParallel

Collagen Fibril Distribution• Theories Of Collagen Fibril DistributionUsually Assume Fibrils Run FromLimbus To Limbus• Little Evidence To Support TheseTheories

Collagen Fibril DistributionTheoriesSchachar, Black,HuangKeratorefraction1980Pinsky, Datye: Ref &Corneal Surg8;192p164-172

Experimental Results• Unexpected Pattern Observed ThroughCrossed Polarized Filters• Corneal Fibrils Appear To Be Arranged InPeripheral "Leaves" Rather Than EvenLayers• Suggests ThatCollagen Fibrils MayNot Extend FromLimbus To Limbus

Orientation Of Crossed Filters• The Pattern Observed Seems To Change InIntensity And Color By Rotating RelativePosition Of Crossed Polarizing Filters• Corneal Collagen Distribution Seems MoreComplicated Than Originally Thought• Why Certain LayersOf Collagen FibrilsRespond To PolarizedLight Not Known

Corneal Support ZonesCorneal Astigmatism Mosby-Yearbook 1992

Corneal Support Zones• Polarized light examination of thecornea supports the concept ofperipheral support zones• Structural differences separate thesupport zones and these differenceshave clinical correlation

Arcuate Keratotomy• Enhanced Visualization Of Incisions• No Basic Change In Color Pattern

Arcuate Incisions - Coupling• Arcuate incisions are performed with anoptical zone of 7 mm ... In the peripheralsupport zone• T-cuts performed between 5-6 mm OZ,in the intermediate support zone, couplefairly accurately to maintain sphericalequivalent• Coupling of arcuate incisions is muchmore erratic, perhaps related tostructural factors

Mini-RK• Lindstrom Mini-RK has minimizedpenetration intoperipheral supportzone• Slightly less effect• Perhaps betterlong term stabilityOZCenteredOnVisual AxisLimbus

Mini-RKOZ3.5m4-Cut RKAvg5CadaverEyesCorrection Percentage (D)9 1008 .1 181007 80 7 3 .75 .9 8656043 9 .83 .2 3 4032 2010 02 mm 3 mm Limbus 2 mm 3 mm LimbLength Of RK Length Incisi OSabates, Friedlander, Buzard/ SubmitedForPublication

Corneal Pressure• Preexisting Pattern Seen With CrossedPolarizing Filters Disrupted By SimplePressure Applied With Forceps

Corneal Suture• Disruption Of Natural Pattern Seen WithCrossed Filters• Stress Lines Can Be Seen Both Under TheSuture and At The Ends Of The Suture• This Effect Can ProvideVisual Feedback WithRegards To SutureTension

Probe In Corneal Incision• Corneal Stress Can Be Induced With AHook In Corneal Incision• Note "Wake" Of Induced Corneal StressLines

Slit Lamp Adaptation• Adaptation Of Slit Lamp Can ProvideClinical Application Of CrossedPolarizing Filters• Filter Placed On Slit Beam And CrossedFilter Placed On Optics Of Slit Lamp• Polarized Light Is Reflected From IrisThrough Cornea

Slit-Lamp AdaptationPolarizingFilterCrosedPolarizingFilter

Basic Pattern• Much Different Color Pattern With Slit LampAdaptation• Typically 3-4 Peripheral Colored BandsSeparated Into 4 Quadrants• Perhaps Bands Related To Rectus Muscles• Colored Bands May Be Related To ChangesIn Corneal Thickness• Similar Patterns Seen InPre-stressed SpectacleLenses

Stromal Fine Structure• Polarizing Filters Eliminate SurfaceGlare• Internal Stromal Structure BecomesMore Visible...Corneal Nerves,Keratitis,Etc.• Lens InternalStructure AlsoMore Apparent

Tight Corneal Suture• Disruption Of Color Banding Pattern• Similar Stress Lines As Seen InExperimental Setup• May Provide A Real-Time Method OfAssessing Suture Tension In OperatingRoom

Circular Corneal Suture• Good Visualization Of Suture• No Disruption Of Basic Color Pattern• Note Disruption Of Pattern At LocationOf Suture Knot

Radial Keratotomy• No Change In Basic Color Pattern• Incisions Become Much More Apparent• Improved Determination Of IncisionDepth• Eye-Bank Screening For RK

Keratoconus

Keratoconus

Keratoconus

Summary• Corneal Stromal Structure MoreComplex Than Previously Theorized• Improved Visualization Of CornealIncisions At Slit Lamp...ImprovedDetermination Of Incision Depth• Eye-Bank Screening For RK

Summary• Suture Tension Is Seen By Distortion OfBasic Pattern And Colored Stress Lines• Possibly Useful As An Adjunct ToCorneal Topography• Keratoconus Shows Distortion Of BasicPattern• Future Work Will Focus On EnhancingEffect With Different Combinations OfFilters