Stabilisation of refractive power after penetrating keratoplasty.

Abstract

Sir, In the December 2000 issue of Acta Ophthalmologica, Dr. Peter Isager discussed the stability of graft refractive power after penetrating keratoplasty (Isager et al. 2000). Penetrating keratoplasty (PK) was performed using a same-size graft and recipient bed. On an average, spherical equivalent power was found to be 41.9 D at 1 month and it stabilized at 3 months (42.7 D). The authors have recommended cataract extraction (if required) after 3 months, as corneal refractive power stabilises by this time and the adjustment in intraocular lens (IOL) power can compensate for post PK ammetropia. We would like to report our experience with the same sized graft and with a 0.5 mm oversized donor button and graft clarity after second stage cataract surgery. The donor corneal trephing from the endothelial side results in a graft size which is 0.25 mm smaller (Troutman 1979; Olson 1979) than the desired size. This smaller graft along with the wound compression by sutures produces corneal flattening.To overcome this problem, we prefer 0.25 to 0.5 mm oversized grafts .In unpublished data collected from follow-up of 25 patients, the average spherical power of patients with similar size host and graft of 7.5 mm was found to be 41.2 D at 1 month and 41.8 D at 3 months. The average spherical power was calculated from keratometry done by Baush and Lomb keratometer. When a 7.5/8.0 mm graft was used, the average spherical power was 42.6 D and 43.30 D at 1 and 3 months, respectively. Thus, in our experience a similar size graft produces more flattening. The authors have recommended cataract surgery after 3 months of penetrating keratoplasty. This takes into account a new corneal power while calculating IOL power and helps adjust for any gross ammetropia. Thus, it makes post operative refraction and best corrected visual acuity more predictable. However, cataract surgery in patients with operated PK carries an additional risk to graft survival (Bourne et al. 1994; Zacks et al. 1990; Binder 1989). Thus, triple procedure (combined cataract extraction + IOL + PK) in patients undergoing PK and who have pre-existing cataract may obviate the need for cataract surgery later on. In addition, the incidence of posterior capsular opacification is less with triple procedure than with primary cataract extraction (Dangel et al. 1994). In our series of second-stage uncomplicated cataract surgery in 16 patients done at an average interval of 4.3 months [range 3 to 8 months], we found a higher risk of graft failure. All the patients had graft clarity of 4+ (Grade 4+: Iris details very clearly seen, Grade 3+ Iris details seen but hazily, Grade 2+: Iris and pupil seen, but iris details could not be made out, Grade 1+: pupil can be made out hazily, Grade 0: pupil cannot be seen at all). All patients underwent uncomplicated extra-capsular cataract extraction and posterior chamber polymethyl methacrylate (PMMA), single piece 6.5 mm optic IOL. A decrease in corneal clarity was seen in 10 cases (62.5 %) after 6 months of follow-up. At 6 months of follow-up, graft clarity of 8 patients dropped to 1–2+, while 6 patients had graft clarity of 4+ and 2 of 3+. This suggests an increased risk of graft failure after cataract surgery. Therefore we recommend triple procedure in patients undergoing penetrating keratoplasty and having pre-existing cataract. Sudarshan Khokhar, MDHarinder Singh Sethi, MD andRajeev Sudan, MD