P 163 In vitro modelling of central island following PRK

Purpt~sc: To design :l rcproduciblc in vitro model to study central island (Cl) formation following PRK. To investignte technical parameters offccting tbc rate and chwxtcristics ofccntrrll islrmd alicr PICK. Methods: We wed PMMA contact lenses to nwdcl the convex quasispherical surface of the cornea. A total of 60 lenses were ablated using 3 different cxcimcr lasers. Lenses were nblated for n thcorcticzd correction ol 6 dioptcrs witb cilhcr ir 5 nmr or 6 mm diiunctcr ;~blation xonc. In xldition. ablation was performed in the presence or in the absence of a surface shock wave absorbing system (SWAS). The formation of Cl wils assessed using slit lamp examination. background shadowing. low power phase contrast microscopy, specular rellection from the gold platted surface, scannning electron microscopy and computerized videotopography. Results: Modilied videotopography using the TMS I systcnl (‘fomcy I:ri,ncc), wils the most rcliablc method to document Cl. The rate of Cl was greater in the absence of SWAS (88% vs 8%), when a spot mode was used as opposed to a scanning mode in the absence of SWAS (I 00 % YS 20 %I. :wd \vhcn :I Ixgcr di;tnrctcr :rbl:~lion xv:is pcrl‘urnxd (56 ‘% vs 44 % for 6 nun and 5 mm respectively). In addition the size and relative refractive power of Cl were correlated to the initial thickness and anterior radius of curvature of the lenses Conclusion: These results demonstrate that Cl may be fully independant of any biological response. This investigative model suggest that shock wave induced deformation may be the primary cause for the formation of Cl.