Dr. Rupal ShahThe author is a consultant to Carl

Zeiss Meditec and has also received travel support from Carl Zeiss

Meditec

• Is an important property of the cornea• Can be measured using the Corneal

Hysteresis (CH) and Corneal Resistance Factor (CRF) using the Ocular Response Analyzer (ORA, Reichert)1

• Has been shown to reduce significantly following LASIK2

• Reduced Biomechanical Strength of the Cornea may be implicated in the development of Post-LASIK ectasia

A mechanical or femtosecond microkeratome is used to make a hinged flap at about 120 microns thickness

The hinged flap is lifted to one side An Excimer Laser is used to ablate a

lenticule from the cornea, corresponding to the desired refractive error correction

The flap is replaced in its place

• All Femtosecond Procedure

• A Femtosecond Laser is used to first separate the posterior part of the lenticule from the stroma

• The Femtosecond Laser is then used to separate the anterior part of the lenticule from the stroma, which also serves as the flap

• The flap side cut is created, and the lenticule is physically removed from the cornea, after lifting the flap

• The flap is replaced

1

2

3

To study whether LASIK performed using the FLEx technique would result in a change in the biomechanical strength parameters of the cornea relative to standard LASIK

There are differences between the two techniques in the thickness variation of the flaps, the flap size and the incision width (i.e. the side cut width)

12 patients with myopic refractive errors with spherical equivalent between -1 Diopters and -10 Diopters were selected

All patients had approximately the same refractive error in both eyes

One eye of each patient was randomly assigned to the FLEx and Standard LASIK groups

Flap thicknesses in both groups were in 100 micron range

• One eye of each patient underwent “All in One” LASIK (FLEx) with the Visumax Femtosecond Laser (Carl Zeiss Meditec)

• One eye of each patient had standard microkeratome LASIK with the BD K-4000 microkeratome and MEL 80 (Carl Zeiss Meditec) excimer laser

• ORA readings of each patient were taken immediately prior to LASIK and one week post-LASIK. Four ORA readings deemed acceptable by the operator were taken each time, and the average CH and CRF values were obtained and used for analysis

FLEx Standard LASIKTotal Eyes Treated 12 12 Male 4 4 Female 8 8 Spherical Equivalent Pre-operatively

Mean=-4.92 DStandard Deviation=1.92DRange=-2 to -8.25 D

Mean=-5.02 DStandard Deviation=2.32 DRange=-2.25 to -8.5 D

Spherical Equivalent Post-Operatively

Mean =+0.17DStandard Deviation=0.13DRange -0.25 to +0.5

Mean=-.12DStandard Deviation=0.11DRange=0 to-0.5D

CH CRF CH-CRF 2

Mean -1.90 (LASIK)-1.84 (FLEx)

-2.49 (LASIK)-2.66 (FLEx)

0.58 (LASIK)0.83(FLEx)

Variance 0.66 (LASIK)1.54 (FLEx)

0.95 (LASIK)1.84 (FLEx)

0.84 (LASIK)0.63 (FLEx)

t -0.18 0.59 -1.32

p (null difference) 0.42 0.28 0.10

• Both FLEx and Standard LASIK result in a decrease in corneal biomechanical stability as measured by the Corneal Hysteresis (CH) and Corneal Resistance Factor (CRF)

• There is no statistical difference in the corneal biomechanical properties when eyes are treated with either of the two techniques

• FLEx is at least as safe in terms of biomechanical stability as standard LASIK

• More studies are needed, including at longer time intervals, because there is some evidence that CH and CRF values change over several weeks post-LASIK4

1. Assessment of the Biomechanical Properties of the Cornea with the Ocular Response Analyzer in Normal and Keratoconic Eyes., Shah S, Laiquzzaman M, Bhojwani R, Mantry S, Cunliffe I., Invest Ophthalmol Vis Sci. 2007 Jul;48(7):3026-31.

2. Correlations Between Corneal Hysteresis, Intraocular Pressure, and Corneal Central Pachymetry. Touboul D, Roberts C, Kérautret J, Garra C, Maurice-Tison S, Saubusse E, Colin J. ,J Cataract Refract Surg. 2008 Apr;34(4):616-22.

3. Corneal biomechanical properties in normal, post-laser in situ keratomileusis, and keratoconic eyes., Ortiz D, Piñero D, Shabayek MH, Arnalich-Montiel F, Alió JL.,J Cataract Refract Surg. 2007 Aug;33(8):1371-5.

4. Corneal Hysteresis, Resistance Factor, Topography, and Pachymetry After Corneal Lamellar Flap, Gatinel D, Chaabouni S, Adam PA, Munck J, Puech M, Hoang-Xuan T. J Refract Surg. 2007;23.