The theoretical effect of trephine tilt on postkeratoplasty astigmatism.

Most corneal surgeons use a free-hand technique to perform trephination. Of the factors thought to be associated with postkeratoplasty astigmatism, surgeons agree that the process of trephination has the greatest effect on astigmatism. We describe the use of a new trephine that, while affixed to the donor and/or recipient by suction, applanates the cornea to eliminate the oblique incision and undercutting that occur with currently available suction trephines. By mathematical analysis we demonstrate that a 5 degrees to 10 degrees tilt of the trephine can induce significant astigmatism. In theory, the use of the guided trephine system described here will eliminate this complication. This new instrument should be capable of producing identical incisions in the donor and recipient, thereby allowing easier wound coaptation with sutures. This new technology has the potential to eliminate trephination errors as a major cause of postkeratoplasty astigmatism.