James S. Lewis, MD I have no financial interest in the subject matter of this poster Mathematical Analysis of 150- Degree Femtosecond Reverse Bevel ASCRS 2011

Motivation for this Study Inverted Bevel-In Side Cut Inverted bevel-in side cut promotes flap stability and positioning. 1,2 Enhanced biomechanical stability of the post-LASIK cornea Promotional literature on Femtosecond Vendor’s Website

Geometric Implications Assuming this is a standard 8.5 mm corneal flap with a thickness of 100 microns

Geometric Reality Assuming this is a standard 8.5 mm corneal flap with a thickness of 100 microns

Using Computer Assisted Design Assuming 50 microns of epithelium and 110 micron flap

Assessment of True Geometry

True Representation of the Geometry

Total Region of Enhanced Support

Greater Support 1.5% overhang Average Manhole cover has 3.5 % overhang Total area of peripheral contact correlates to wound strength 150 degree bevel increases wound strength only 20%

Another Problem with the Bevel Myopic 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 flap Consequently, there is as much as 100 microns of “excess” flap length. Two point fixation (hinge and bevel apex) allows for no degrees of freedom A 150 degree bevel will generate striae from poor flap- stromal bed apposition* *the formal proof is being prepared for submission