Microkeratome-Assisted Posterior Keratoplasty

We concur with the views of Azar and coauthors1 that microkeratome-assisted posterior keratoplasty is a potential alternative to penetrating keratoplasty (PKP) in the management of corneal endothelial failure. However, there are areas that we want to discuss. A postoperative manifest refraction of +16 diopters is an unpleasant surprise. The authors postulate that this could result from a thin posterior button.1 The posterior button, in accord with the way it was produced, should be thicker instead of thinner. It has been shown that there is differential swelling of the post-mortem cornea,2,3 and there is relatively more post-mortem hydration present in the posterior than the anterior cornea. After the anterior flap is created, the residual posterior cornea could therefore be thicker than normal. It would be of interest to measure the actual thickness of the posterior button with confocal microscopy or optical coherence tomography.4 The excessive flattening may not be related to a thin posterior button. One possible contributing factor to excessive corneal flattening is a relatively undersized donor button. Although both the donor and recipient buttons were trephined at a diameter of 6.0 mm, the effective diameter was not the same because the donor button was endothelial side up while the recipient button was stromal side up. In routine PKP, the donor button is prepared endothelial side up and is usually 0.25 mm or 0.50 mm larger than the recipient button,5 except in cases such as keratoconus in which a same size button is used to reduce the amount of myopia. A relatively undersized donor graft, especially when the diameter is small, could have caused a moderate amount of hyperopia. The extreme hyperopic shift in the report, however, cannot be adequately explained by this reason alone. The flap created in the recipient cornea could well be thinner than the planned 130 μm. There have been numerous reports of the variable thickness produced by different microkeratomes, and there is a tendency to produce thinner flaps than planned.6,7 Furthermore, endothelial failure of the recipient cornea will make the cornea thicker than normal because of stromal hydration and swelling. A 130 μm flap will eventually undergo deturgense and become thinner when endothelial function returns after a successful transplant. A thicker flap would be more desirable, as a thin flap could be associated with more problems such as flap wrinkling. For these reasons, we propose that one consider using a slightly oversized donor graft as well as creating a thicker flap with the 160 or 180 μm microkeratome head. Emily F.Y. Yeung FRCS C. C. Chi FRCS Jun Li MD Gary H.F. Yam PhD C. P. Pang DPhil Dennis S.C. Lam FRCS, FRCOphth aHong Kong, People's Republic of China