Myopia following penetrating keratoplasty for keratoconus

The frequent occurrence of spherical myopia after penetrating keratoplasty for keratoconus is partly the result of the excessive dioptric power of the grafted cornea which occurs when the diameter selected for the donor button is greater than the diameter of the host incision. This excessive power could be reduced by eliminating disparity between the diameters of the graft and host. To determine what proportion of the myopia in these eyes would persist as a result of axial myopia the axial lengths of 60 patients grafted for keratoconus and 25 emmetropic controls were compared. A keratometry, objective refraction, and contact probe ultrasonic biometry were performed on all eyes. A comparison of the results with a representational schematic eye indicated that the mean spherical refractive error of the grafted keratoconic eyes (-4.83 dioptres) was the combined effect of steepness of the corneal graft (mean radius of curvature 7.46 mm) and an abnormally great axial length (mean 24.84 mm). The increased axial length was mainly the result of elongation of the posterior segment of the globe with a small contribution from an increased anterior chamber depth. Though axial myopia is common in keratoconus, a further study of 70 keratoconic eyes that had not been grafted showed no statistically significant correlation between the posterior segment length and the severity of corneal ectasia. These data suggest that even if excessive corneal power is eliminated after penetrating keratoplasty for keratoconus the associated axial myopia would still produce a mean spherical refractive error of at least -2.8 dioptres.