Mathematical Analysis of 150-Degree Femtosecond Reverse Bevel
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Mathematical Analysis of 150-Degree Femtosecond Reverse Bevel. James S. Lewis, MD I have no financial interest in the subject matter of this poster. ASCRS 2011. Motivation for this Study. Inverted Bevel-In Side Cut Inverted bevel-in side cut promotes flap stability and positioning. 1,2 - PowerPoint PPT Presentation
Transcript of Mathematical Analysis of 150-Degree Femtosecond Reverse Bevel
Slide 1
James S. Lewis, MD
I have no financial interest in the subject matter of this posterMathematical Analysis of 150-Degree Femtosecond Reverse BevelASCRS 2011Motivation for this StudyInverted Bevel-In Side CutInverted bevel-in side cut promotes flap stability and positioning. 1,2Enhanced biomechanical stability of the post-LASIK cornea
Promotional literature on Femtosecond Vendors Website
Geometric ImplicationsAssuming this is a standard 8.5 mm corneal flap with a thickness of 100 micronsGeometric Reality
Assuming this is a standard 8.5 mm corneal flap with a thickness of 100 micronsUsing Computer Assisted DesignAssuming 50 microns of epithelium and 110 micron flap5Assessment of True GeometryTrue Representation of the GeometryTotal Region of Enhanced SupportGreater Support1.5% overhangAverage Manhole cover has 3.5 % overhangTotal area of peripheral contact correlates to wound strength
150 degree bevel increases wound strength only 20%
Another Problem with the BevelMyopic ablation reduces the length of the path (non-Euclidean distance) from one end of the stromal bed to the other.There is no such foreshortening of the flapConsequently, there is as much as 100 microns of excess flap length.Two point fixation (hinge and bevel apex) allows for no degrees of freedomA 150 degree bevel will generate striae from poor flap-stromal bed apposition**the formal proof is being prepared for submission
116 Microns
30 degrees
110 Microns 50 Microns60 Microns
220 Microns
191 Microns
101 MicronsFLAPRecipientCornea
116 Microns of stroma to
stroma contact
101 Microns
116 Microns
30 degrees
110 Microns 50 Microns60 Microns
220 Microns
191 Microns
60 / b = sin 30b = 120 microns
60 / a = tan 30a = 104 microns
bA
60
101 Microns
116 Microns
30 degrees
110 Microns 50 Microns60 Microns
220 Microns
191 Microns
60 / b = sin 30b = 120 microns
60 / a = tan 30a = 104 microns
120
10460
FLAP Re
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FLAP Re
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d22 60
d22 50
