Background: Autologous ipsilateral rotating keratoplasty is a special form of keratoplasty in which a nonprogressive opacification of the centre of the cornea is rotated towards the limbus and the clear peripheral cornea is rotated into the optical axis of the eye. This study was performed to find an equation for calculating the best size and location of the trephine for this special kind of keratoplasty.
Methods: Geometrical calculations were used to derive the formula.
Results: We arrived at the following equation for the best diameter of the trephine: diameter(trephine) = 3/4 diameter(cornea)-1/2e(e = preoperative distance between corneal centre and nearest edge of opacity covering corneal centre). The postoperative diameter of the optical zone is: 2 x diameter(trephine)-diameter(cornea). A postoperative clear optical zone of half the corneal diameter is achieved if the opacity just touches but does not extend beyond the corneal centre. For a postoperative optical zone of at least 30% (40%) of the corneal diameter, the opacity is preoperatively not allowed to extend beyond the corneal centre for more than 20% (10%) of the corneal diameter.
Conclusion: This equation can be used for calculating the optimum size and location of the trephine for autologous ipsilateral rotational keratoplasty. For that purpose it is advisable to take photographs preoperatively.