Astigmatic considerations in corneal graft.

Some fictions about astigmatism should be dispelled: 1) Sutures determine astigmatism. They do not. 2) The size of the graft is contributory to astigmatism. It is not contributory. 3) Anterior or posterior cutting of the graft makes no difference. It does make a difference. 4) Same size trephine can be used to cut graft and recipient opening. Not when graft is cut from posterior surface or significant flattening will result. 5) A small cutting error is of no consequence. A small cutting error can cause a great deal of astigmatism. Tissue factors are paramount. Sutures must maintain wound apposition primarily. Posterior cutting requires a larger diameter trephine. A one-half-mm cutting error can cause an astigmatism of five diopters. Excessive corneal astigmatism, when it does occur, can be controlled by corneal wedge resection. The surgical keratometer identifies and limits surgical and suture-induced errors. Success in microsurgical keratoplasty can no longer be defined only as a clear graft. It must include also minimal or no residual optical error.