Myopia following penetrating keratoplasty for keratoconus Stephen

Abstract

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. (BrJ Ophthalmol 1992; 76: 642-645) Moorfields Eye Hospital, City Road, London EC1V 2PD S J Tuft R J Buckley Institute of Ophthalmology, Judd Street, London WC1H 9QS FW Fitzke Correspondence to: Dr S Tuft. Accepted for publication 21 May 1992 Penetrating keratoplasty is reserved for the minority of patients with keratoconus who cannot attain a satisfactory visual acuity with spectacles or contact lenses. Though a first graft to an eye is usually technically successful in terms of clarity and survival,'-3 astigmatism and residual myopia may prevent full visual rehabilitation.3-5 This has prompted interest in the origins of postoperative refractive error and methods for its reduction." A disparity between the shape of the wound margins ofthe host and recipient is thought to be the primary cause for post-keratoplasty astigmatism,9" but the origin of spherical myopia following keratoplasty for keratoconus is the subject of debate. It has been demonstrated that the use of an oversized donor corneal button produces a steeper graft contour which results in an excessive corneal dioptric power,8 12 but Wilson et al observed that grafted keratoconic eyes have a greater postoperative myopic spherical error than eyes grafted for Fuchs' dystrophy, even if the differences between diameters of the host and donor trephines (025 mm) are the same in the two groups,8 and suggested a contribution by factors in addition to the graft contour toward the final refraction. The importance of the axial length of the globe was highlighted by Brooks et al who reported that the mean axial length of eyes with keratoconus was greater than emmetropic controls and that there is a correlation between the spherical myopic error following keratoplasty and the axial length of the eye.5 To characterise the origin of myopic error associated with keratoplasty for keratoconus we have compared the axial length and keratometry of grafted eyes and emmetropic control eyes. To assess if the grafted eyes could have been rendered emmetropic by eliminating overpower of the cornea we have used a representational schematic eye to determine the contribution of the axial length to the final refraction. To determine if there is a relationship between the severity of corneal disease in keratoconus and an elongation of the rest of the globe we studied the eyes of patients with keratoconus who had not undergone surgery. Patients and methods We examined the right eye of 60 phakic patients who had a clear penetrating keratoplasty for keratoconus and who had all sutures removed for at least 6 months. No eye had been grafted more than once. Patients were examined consecutively as they presented for review in clinic, but four eyes that had undergone refractive surgery for astigmatism were excluded from the study. Surgery had been performed by a number of different surgeons, but in the majority of cases the donor button was cut from the endothelial surface with a trephine 0 25 to 0 50 mm larger than the host and secured with a single 10/0 running suture. The follow-up period was defined as the interval between the removal of graft sutures and the observation point. The 642 group.bmj.com on November 7, 2017 Published by http://bjo.bmj.com/ Downloaded from

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@inproceedings{TuftMyopiaFP, title={Myopia following penetrating keratoplasty for keratoconus Stephen}, author={J Tuft and Fred W. Fitzke and Roger J Buckley} }