Mathematical and computer simulation modelling of intracameral forces causing pupil block due to air bubble use in Descemet's Stripping Endothelial Keratoplasty: The mechanics of iris buckling

摘要

Background: Descemet's Stripping Endothelial Keratoplasty has been associated with a steep learning curve. Angle closure post Descemet's Stripping Endothelial Keratoplasty has been reported, either because of air posterior to the iris causing iridocorneal adhesions, or by air anterior to the iris causing pupillary block. The mechanics of floppy iris syndrome and pupil block have not been discussed. Methods: We evaluated the various forces competing within the anterior chamber via mathematical modelling and computational simulation, and considered the influence of floppy iris on pupil block glaucoma. Results: Energy formulae suggest a critical pressure value will maintain normal anterior chamber relationships, above which abnormal iris buckling may occur. This mechanical instability can be influenced intraoperatively by abnormal iris properties and intracameral forces (such as air). This critical value is lowered if the patient has a floppy iris (because of a lower elastic modulus, a mechanical measure of iris rigidity). To demonstrate this mathematical concept, a 3-D computational model was built. Simulations show that, as intracameral pressure increases, the iris ring can buckle into predictable modes of shapes. Conclusion: This model shows how iris buckling could occur with an intracameral air bubble leading to posterior iris displacement and mechanical pupil block. It also shows that abnormal iris behaviour in IFIS is consistent with the expected predicted buckling of an elastic disc.