Abstract
Purpose: :
The biomechanical properties of the cornea in-vitro are usually measured from strip extensiometry or inflation methods. We developed a 2D-flap extensiometry technique, which combines the advantages of both methods, and applied it to the measurement of the effect of UV-Riboflavin cross-linking (CXL).
Methods: :
Corneal flaps (13 pig / 4 rabbit) from the anterior stroma (100 um) were obtained from enucleated eyes after de-epithelization using a mechanical microkeratome. (Carriazo Pendular, SCHWIND Kleinostheim). Flaps were mounted on a custom chamber, consisting of a BK-7 lens, a reflective retina and two reservoirs (filled with Riboflavin and silicone oil respectively). A water column controlled hydrostatically the pressure in the riboflavin chamber, stretching the corneal flap and thus changed the power of the system. Zernike aberrations of this artificial eye were monitored with a commercial ray-tracing aberrometer (iTrace) as a function of pressure. The pressure was in/decreased in 5 cycles (0-30mmHg). Pig-flaps were treated in-situ and measured before and immediately after treatment. Rabbits were treated in-vivo following clinical procedures. Rabbit flaps were evaluated 1-month post-operativelly. Contralateral eyes served as controls. An analytical model was applied to estimate the Young Modulus from the change in surface (strain) as a function of pressure (stress). The applied force was sufficiently small to assume elastic deformation only.
Results: :
In porcine flaps treated in-situ no immediate changes in the mechanical response were observed after CXL (p=0.76). In 1-month post-op rabbit flaps curvature changed significantly less (p=0.026) after CXL than in untreated corneas (19.3 vs 6.36 mD/mmHg). Moreover the difference between horizontal and vertical meridians decreased (-0.22 D). Young modulus was estimated 2.29MPa in porcine corneas, 1.98 MPa in untreated rabbit corneas and 4.83Mpa in CXL rabbit corneas (1-month post-op).
Conclusions: :
Flap extensiometry allows estimating corneal elasticity in-vitro, unaffected by corneal thickness while preserving the integrity of the cornea more than strip extensiometry. The absence of immediate changes after CXL in porcine corneas might be associated to intra-collagen layer bonds being limited in the in-situ treatment. The method proved the efficacy of CXL in increasing corneal rigidity after 1 month.
Keywords: imaging/image analysis: non-clinical • refractive surgery: other technologies • cornea: basic science